How to connect the output of a low power amp to a second high power amp - Page 1
Forum section: electronics / projects

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 04, 2013, 09:23:05 pm
content: Hello EEVblog forum!This is my first post, so please forgive me if I've done something wrong! :pI am a 16 year old sixth form student in the UK learning electronics as a hobby, with a hope to do electronic engineering at university. I am working on a project that seems to have put me in a rather unique situation from what I can tell from my google'ing etc. I have a device with a built in class D amplifier capable of outputting 1.8 watts to a 4 ohm speaker. This 'unit' if you like cannot be changed, I cannot modify it in any way. Now here's my challenge, this isn't enough, and I need to somehow further amplify the output of this device, cause it just ain't beefy enough. Too a newbie like me, the world of analogue electronics, audio, impedance etc is very bewildering, so I apologise for my incompetence. Apparently I need an "impedance matching transformer" with the primary connected to the output of the first amp and secondary connected to the input of the second. I have no idea what type though. Input impedance, output impedance, turns ratio etc, its all a complete blurIs this the correct angle to approach? If so how do I go about it? Any help would be greatly appreciated.Thank youMichael

author: Unknown
author_rank: Unknown
timestamp: July 04, 2013, 09:29:51 pm
content: A quick google search turned this up:http://www.bcae1.com/trnimpmt.htmwhich might be helpful.A good rule to follow is, do a quick google search first, and post if it doesn't turn up anything useful. But other than that, don't worry about doing anything wrong here, as long as you can avoid being a dick I think most people here are friendly and helpful.

author: c4757p
author_rank: Super Contributor
timestamp: July 04, 2013, 09:37:09 pm
content: I think you're barking up the wrong tree. What exactly do you mean by "ain't beefy enough"? I can think of two things: 1) The signal level is too low. 2) The amplifier isn't powerful enough and the signal clips.Additional amplifiers will only increase the signal level, not the drive capability. If your problem is actually #2, you really need to use a different amplifier.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 04, 2013, 09:47:35 pm
content: Quote from: duskglow on July 04, 2013, 09:29:51 pmThank youI have done extensive google searching, and did come across that article which was helpful, but I was still left a little confused. I understand using transformers to match amplifier outputs to speakers, but matching amplifier outputs to amplifier inputs without it sounding awful is what is getting me stuck. I have done some experimenting connecting the output of the amplifier directly to the input of another, and it just sounds crap.Quote from: c4757p on July 04, 2013, 09:37:09 pmI believe what I want is an increased signal level. Basically I am trying to increase the volume.Thanks,Michael

author: c4757p
author_rank: Super Contributor
timestamp: July 04, 2013, 09:49:51 pm
content: OK. No power is actually transmitted from the amplifier input to its output, so by feeding a big power amplifier into another amplifier you're wasting all of its power; only the signal transfers. What you need is a preamplifier, which just boosts the signal without any drive strength. That can be fed right into another amplifier.What exactly is your setup?

author: madshaman
author_rank: Frequent Contributor
timestamp: July 04, 2013, 09:50:06 pm
content: 1.8 Watts isn't much, and *most* amps will drive impedences from 4ohms to 16ohms with decent efficiency (it's better to avoid 4ohms if you're not sure about your amp, it will draw more current and run hotter).The biggest impediment to turning electrical energy into audio energy is actually the sensitivity (read: efficiency) of the driver/speaker.You'll likely get bigger gains by replacing the speaker with a more efficient one.Try googling: "speaker efficiency"

author: madshaman
author_rank: Frequent Contributor
timestamp: July 04, 2013, 09:51:47 pm
content: Quote from: c4757p on July 04, 2013, 09:49:51 pmI'll further this by saying that if there's an impedence or voltage level mismatch, it's more likely to be occuring at your signal input.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 04, 2013, 10:12:03 pm
content: Quote from: c4757p on July 04, 2013, 09:49:51 pmMy setup: I have designed a circuit that takes in a 0-24V PWM wave from the motor outputs of a high current (120amp, thats high-ish right?) motor controller used in the "model engineering" hobby for ride on locomotives, opto-isolates it, through a low pass filter to get the mean voltage and into a micro controller, for which I have written code that generates the DCC waveform, and with the help of a H-bridge, I am able to send speed commands to a DCC decoder with a sound output, so I get engine sounds in sync with the speed of the locomotive (I couldn't be bothered to work out how to do all of that sound looping stuff, decoder does it for you). Sort of hard to explain which is why I didn't include it in the original post. The decoder (a Loksound v4.0) has an inbuilt 1.8W amplifier, designed for driving a tiny 20mm speaker for use in miniature trains, but I want a higher power output, more volume, bigger speakers etc?I am much more comfortable with basic microcontrollers, programming etc than analogue stuff.I hope this makes what I want more clearThanksMichael

author: madshaman
author_rank: Frequent Contributor
timestamp: July 05, 2013, 01:20:05 pm
content: Seriously, look for a high efficiency 20mm speaker as a replacement (I only know guitar amp and PA speakers, so not sure where to source it from, digikey?)The senstivity of your current speaker is probably in the 85db range (poor), you should look for something 98db+ if possible (but I don't know what's available in that size)You can easily double the perceived loudness or more without drawing any more power with a more sensitive speaker.Also, that tiny speaker will also have a maximum power handling ability, you'll melt the voice coil if you drive it too hard.

author: tjw
author_rank: Contributor
timestamp: July 05, 2013, 02:20:03 pm
content: Hi,you said that you cannot modify the Loksound V4.0 in any way - does this include soldering a wire to the output of the D/A converter or the input of the D-class amp. chip ?......The LokSound V4.0 works with 8 polyphonic channels, i.e. that the decoder is able to play up to 8 different sounds simultaneously, using the 16-Bit mixer and the nearly noiseless D/A convertor......If you can tap the sound before it is amplified you could then feed it into a standard 10 ~ 20 Watt amplifier module to drive a 4" speaker for some real grunt.Electric r/c aeroplane modelers use devices similar to the Loksound V4.0 however the amplifiers are at least 2x20W driving 4" speakers. You may have seen this site -http://www.benedini.de/Home_E/Products_E/products_e.htmlThe Locksound site recommends 4 Ohm speakers be used - otherwise you may damage the class D amp. on the board. So your idea about impedance matching is correct. The amp. must drive a 4 Ohm load in place of the speaker and that's where the transformer is required however finding a suitable transformer will be difficult.For a low-fi solution you could make a cone to fit over the 20mm speaker and fit an electret microphone insert into the cone - effectively coupling the speaker output to the electret microphone insert. Then connect the electret microphone to a preamp/amp of your choosing.Have a look at the "Universal Mono Pre-Amplifier Kit" here -http://www.velleman.co.uk/contents/en-uk/d21_Amplifiers.html.It can be setup to use an electret microphone insert and then you could feed the output to a 7W mono or 2x30W stereo kit shown on the same page. No doubt ebay is full of these pre/power amp boards from China as well.Regards, tjw.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 05, 2013, 02:36:20 pm
content: Quote from: madshaman on July 05, 2013, 01:20:05 pmThank you for your help. I will look for a more sensitive speaker tonight. I know the tiny speaker will have a max power handling ability, the idea was to chop it off and use bigger speakers (at least 2). I thought I would need to amplify it further to get a decent volume, but I will look into the more sensitive speakers. ThanksQuote from: tjw on July 05, 2013, 02:20:03 pmReally, yes. I was trying to avoid doing that at all costs, as it voids the warranty etc, and I'm not sure I trust myself doing that with such small components on what I consider a quite dense board. Plus they aren't at all cheap. Yeah I had come across that site in my research. The output of those is more what I'm after, some real grunt.The use of a microphone & preamp had crossed my mind, but considering the quality of that tiny little speaker, I assumed the end result would be pretty horrible.If I were to go down the transformer route, am I correct in thinking I would need one of which the primary coil has an impedance of 4 Ohms? If so, how would I go about determining the impedance of the secondary coil? Would it have to be the same as the input impedance of the second bigger power amplifier?Thanks and kind regards,Michael

author: tjw
author_rank: Contributor
timestamp: July 05, 2013, 03:19:15 pm
content: Hi,... If I were to go down the transformer route, am I correct in thinking I would need one of which the primary coil has an impedance of 4 Ohms? If so, how would I go about determining the impedance of the secondary coil? Would it have to be the same as the input impedance of the second bigger power amplifier?Yes, you are right. The primary would be 4 Ohms and the secondary would need to be higher. Look for audio matching transformers however most I have seen are high -> Low impedance like 600 Ohms to 4 or 8 Ohms. The transformers also have poor frequency response and I think a better quality small 4 Ohm speaker + microphone would be better in terms of sound quality.However you would have to be careful with the microphone/speaker coupling since it will pickup external sounds unless isolated. I am guessing you are going to use this device on a ride-on model train ?Regards, tjw.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 05, 2013, 03:26:12 pm
content: Quote from: tjw on July 05, 2013, 03:19:15 pmYup, with plenty of vibration, noise from motors, rattling, clatter, squealing etc. Got myself in a bit of a pickle here :/ Could the transformer be used 'back to front', with the primary being used as secondary and vice versa? Alternatively, any other methods of coupling the 2 amps together?ThanksRegardsMichael

author: MasterOfNone
author_rank: Regular Contributor
timestamp: July 05, 2013, 03:30:30 pm
content: Quote from: MichaelKavanagh on July 05, 2013, 03:26:12 pmA few years back before the modern in-car entertainment systems, some young folk used to add an additional amplifier stage to their car stereos to make the louder. These amplifiers were simply referred to as boosters, as far as I remember.I did a quick search for “car audio amplifier booster ic” and got some results for amps with hundreds and thousands of wattsBut there was also a really simple designed based on the LM2896-2:http://www.electroschematics.com/3011/lm2896-audio-booster/Unfortunately this IC is now obsolete but you can still find it on Ebay. Anyway you might be able to find something better if you spend some quality time with the Google (or one of it’s friends).

author: AlfBaz
author_rank: Super Contributor
timestamp: July 05, 2013, 03:48:07 pm
content: This should probably work...Replace the speaker with 2 x 10 ohm 1/2 watt resistors in series, use the center tap to feed your other amp. Thats assuming your other amp has a line input which are usually rated at 1V pk-pk

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 04:01:12 pm
content: Quote from: AlfBaz on July 05, 2013, 03:48:07 pmAt last! I read through lots of posts waiting for someone to suggest this obvious approach.The output of an audio amplifier is basically an AC voltage source with a reasonably low impedance. Instead of feeding the output to a speaker you can feed the output into a potential divider (a tapped resistance) and pick a suitable voltage from the tap to use as the input to a separate power amplifier.You will be wasting the original amplifier, but if you can't or don't want to break into that amplifier to find the signal path then picking off the output signal for further amplification is the only way.Using an adjustable potentiometer instead of fixed resistors may help you to pick off the right signal level, but watch the power rating. The first stage amp will be trying to dump its power into that resistor and it will get warm. You could try higher total resistances up to say 100 ohms to reduce the power dissipation.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 05, 2013, 04:04:38 pm
content: Quote from: MasterOfNone on July 05, 2013, 03:30:30 pmHmm, that is interesting, I presume you connect the amplifiers output straight to the input of that circuit? From a quick look it seems identical to the schematic given in the "Typical Applications" section of the datasheet. Very interesting. No idea how that works though.Quote from: AlfBaz on July 05, 2013, 03:48:07 pmPotential divider? Not quite sure what that would achieve other than reduce amplitude?Cheers

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 05, 2013, 04:08:00 pm
content: Quote from: IanB on July 05, 2013, 04:01:12 pmWhoops, beat me too it! Thank you for the explanation. I'll have to experiment I guess. It does not matter what the total series resistance of the two resistors is then? As long as it exceed's 4 ohms I guess?ThanksMichael

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 04:16:50 pm
content: Yes, as long as it exceeds 4 ohms. The original amplifier won't really mind driving a high impedance load, it will just (for example) think it is driving an earpiece or headphones. You need the resistance to be high enough that you can develop the full output voltage across it, but low enough that it provides a suitable input signal to the external amplifier.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 05, 2013, 05:46:57 pm
content: Just tried the potential divider, but nothing really happened, only decreasing the level of the output signal. Both with the output of amp 1 directly connected to input of amp 2, and output of amp 1 connected to input of amp 2 via the potential divider, they sounded exactly the same. Pretty awfulI suppose my only option now is to try and source a transformer?ConfusedThanksMichael

author: lewis
author_rank: Frequent Contributor
timestamp: July 05, 2013, 05:55:30 pm
content: Sounds like you just need to bang another audio amplifier on the arse end of the Loksound decoder. 1.8W into 4 ohms is about 2.6V which should be low enough to drive an audio amplifier IC directly. Check out some ICs available here:http://www.rapidonline.com/Electronic-Components/Audio-power-ICs-67347.Decide how much power you want and choose an IC accordingly, the datasheet for the IC will give you an application circuit that you can simply copy. Have a play with it while you're at it, there's lots of potential to learn from doing something like this.If you want to go really silly, you can build an amp out of discrete transistors. The Maplin MOSFET amp circuit will give you about 150W:http://www.schematicsunlimited.com/m/maplinQuote from: MichaelKavanagh on July 04, 2013, 10:12:03 pmThat's damn impressive from a 16-year old, much respect. Keep it up! ALWAYS play and experiment. You'll be a pro in no time at all.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 06:13:06 pm
content: Quote from: MichaelKavanagh on July 05, 2013, 05:46:57 pmWell, OK, there are two questions to consider:1. What is the intrinsic sound quality of the output from the LokSound device?2. What is the quality of the second amplifier you have tried?I don't think a transformer is going to help you. There is nothing more linear than a resistor. So anything more complicated than a resistor can only introduce more distortion, not less.If the sound you are getting is "pretty awful" it means that something in your signal chain is introducing horrible distortion. There are only three places that distortion can come from:1. The sound generator feeding the first amplifier,2. The first amplifier3. The second amplifierA typical way of investigating this would be to probe the signal from the earliest point it is accessible and follow it along until you first hear distortion.Can you improvise any tools to help you with this? For example, do you have any kind of regular audio amp of known good sound quality, like a regular home stereo amp with a line input jack? What you might do is wire up a probe to this jack and try connecting it first to the input of the first amplifier, then to the output of the first amplifer (perhaps through the potential divider to avoid overloading the line input, which will introduce distortion of its own).It could be that the intrinsic sound quality of the LokSound is poor. If it is only driving a tiny little speaker this lack of quality might not be noticed until you try connecting a better quality amplifier. Good amplifiers have a tendency to magnify any problems in the original signal source.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 06:17:30 pm
content: I just thought of one more thing: a class D amp is not a regular analog amplifier. It may be relying on the speaker to smooth out the digital noise inherent in the output signal. So possibly you need to introduce some kind of filter between the output of the first amp and the input of the second amp. Or else you should try to tap directly into the source signal before it goes through the first amp.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 06:25:39 pm
content: I'm becoming more convinced the lack of an output filter on the class D amp is the problem. Since it is compact and only driving a tiny speaker at low power, the designers may have thought an output filter was not necessary. But the result of this will be terrible if it is fed into a further high quality amplifier stage. You can find out if there is an existing filter by looking inside the LokSound and tracing back from the speaker output. There would be an LC filter with components of significant size, an inductor and a capacitor. If these are not present (if the speaker output comes straight from an amplifier chip) you are in trouble.

author: lewis
author_rank: Frequent Contributor
timestamp: July 05, 2013, 08:38:54 pm
content: Quote from: IanB on July 05, 2013, 06:17:30 pmI'd been thinking the same thing, it's almost certainly a filterless class D to save cost, EMI wouldn't be much of a concern at these power levels. Simple RC filter'll be enough to smooth it out, maybe10K + 1uF1K+100nF right before the input to the second amplifier.The class D amp is almost certainly running in full bridge configuration too (unless of course one end of the speaker goes to ground externally from the LokWosname) so you need to only use one of the output terminals to go to the second amp. Using both will short out half the class D amp to ground unless you configure the amp with a differential input. Come to think of it, you might need to run differentially anyway because the filterless architecture can have some weird modulation. A quick google found this:http://pdfserv.maximintegrated.com/en/an/AN3977.pdf.A quick poke about with a scope will reveal what the class D amp is doing.This is getting complicated! Not quite as simple as we thought...Edit - I can't math either.

author: c4757p
author_rank: Super Contributor
timestamp: July 05, 2013, 08:41:14 pm
content: Quote from: lewis on July 05, 2013, 08:38:54 pm10k * 1uF = 10ms! How about something more like 100R + 470nF? That gives you an fCof 3.4 kHz.Edit: I can't math.

author: lewis
author_rank: Frequent Contributor
timestamp: July 05, 2013, 08:44:53 pm
content: Quote from: c4757p on July 05, 2013, 08:41:14 pmProbably only needs a couple kHz bandwidth, it ain't hi-fi! But yes, it is a bit heavy!

author: c4757p
author_rank: Super Contributor
timestamp: July 05, 2013, 08:49:01 pm
content: 10k, 1uF has a bandwidth of 16 Hz!

author: lewis
author_rank: Frequent Contributor
timestamp: July 05, 2013, 08:54:38 pm
content: Quote from: c4757p on July 05, 2013, 08:49:01 pmYep, didn't think that through.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 05, 2013, 09:28:13 pm
content: Quote from: lewis on July 05, 2013, 05:55:30 pmThanks! That's the plan hopefully!Quote from: IanB on July 05, 2013, 06:13:06 pmOkay, "intrinsic sound quality" I'd say is pretty decent. I have hooked up a larger speaker just to listen to the "natural" sound a bit better, and it sounded half decent. The second amplifier is a good quality, for testing purposes I just hooked it up to the input of my Audioengine A2 speakers. If a transformer is not required then that's great. Simpler, cheaper, etcI was sort of reluctant to do that, although its beginning to look like its not an option. The decoders come wrapped in this transparent sort of heatshrink. Thats not too much of an issue. The trouble is finding the right components to probe. I think I have found what looks to be the amplifier, but I'm not too sure. The only markings on it are "MMAB" and "SSSB". It is a 8 pin device. Could this be it? Not too great with the whole SMD thing. Have had basically no experience with it.Quote from: IanB on July 05, 2013, 06:17:30 pmThis had also crossed my mind during my research. Didn't really know what to do with the info though.Quote from: lewis on July 05, 2013, 08:38:54 pmYou lost me as soon as you mentioned bridge configuration!Will have to do some extensive googling! Not as simple as I first thought either. When I first started I just thought "hey, just stick another amplifier on the 'arse end' of it!". Oh ignorance...So whats the general opinion on what to do now then? LC filter on output? or try and probe around the circuit? Or both?CheersMichael

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 10:00:17 pm
content: If the sound quality is OK when the output is connected directly to a speaker, but sounds poor when fed through a second amplifier, then there is an indication that the speaker is being expected to provide some filtering.You don't absolutely need an LC filter, an RC filter will be OK as others have suggested above. You want a low pass filter that rolls off high the high frequencies coming from the class D output. A corner frequency somewhere around 10 kHz should do it. Try c4757p's suggested values.

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 10:06:16 pm
content: BTW, I think "bridge configuration" means that both speaker wires from the amp are "live" and the pair of terminals produces an AC signal that swings positive and negative relative to the amplifiers ground (this reversing AC voltage helps to drive the speaker cone back and forth). But because both wires are "live" you don't want to connect either of them directly to any ground that the amplifier knows about as it will be like a short circuit. I'm not sure about the details, but I think you can use a capacitor to AC couple the output to the input and avoid this problem. Maybe someone else can chime in on that?

author: IanB
author_rank: Super Contributor
timestamp: July 05, 2013, 10:10:28 pm
content: Quote from: MichaelKavanagh on July 05, 2013, 09:28:13 pmCan you take a picture of the circuit board and post it? Someone may be able to help decode it.

author: lewis
author_rank: Frequent Contributor
timestamp: July 05, 2013, 10:31:25 pm
content: Quote from: IanB on July 05, 2013, 10:06:16 pmThat's right. BTL (bridge tied load) configuration is essentially two antiphase amplifiers, one driving each end of the speaker. With this technique you can get quadruple the output power from given supply rails compared to the single-ended (one end of the speaker grounded) configuration because you've doubled the voltage across the voicecoil.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 06, 2013, 07:18:48 am
content: Crikey, 3 page thread. Who'd have thought it?https://dl.dropboxusercontent.com/u/5612078/loksound/IMG_0416.JPGhttps://dl.dropboxusercontent.com/u/5612078/loksound/IMG_0419.JPG^^ Pictures I took of the decoder. Not sure how much they'll help. And only taken with my iPhone and the help of a LED torch so apologies if they're not very helpful. No idea why the first one is so darkQuote from: IanB on July 05, 2013, 10:00:17 pmOkay, will doQuote from: lewis on July 05, 2013, 10:31:25 pmRight, that would make some sense. How would I connect the filter though? 1 wire from amp -> resistor -> capacitor -> second wire back into amp? Then take the output from the usual between the resistor and capacitor? Where would I connect the "ground" of the second amp?Also, whilst we're talking about capacitors. Does anyone know of somewhere that lists all the different types of cap, and what they should and shouldn't be used for, and why etc? Currently I've used electrolytic caps for the input/output sides of the voltage reg, however I have no idea whether this is the right sort to use?Thanks for all your helpMichael

author: IanB
author_rank: Super Contributor
timestamp: July 06, 2013, 07:47:12 am
content: Can you post the color key for the wires? Especially which wires go to the speaker?It looks like the dark side of the board with the Atmel chips on it is the signal processing side and the other side looks more like power supply bits.I'm not an electronics expert and I can't immediately pick out what might be the audio amplifier part. There is a possibility the Atmel micro is sending a PWM signal directly to the speaker output through buffer transistors and there is no actual amplifier chip on the board. In which case your best option would be to piggy back on the speaker wires as you have been trying to do so far.I don't know how to connect to the second amp to avoid problems. I think that if it has a completely independent power supply and doesn't share a common ground with the signal source then you should just be able to connect the two wires.For the filter you place the resistor and capacitor in series as a potential divider across the output of the first amplifier. You then take the input to the second amplifier from the terminals of the capacitor.Can you redo the "dark" picture with better lighting?

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 06, 2013, 06:21:45 pm
content: Quote from: IanB on July 06, 2013, 07:47:12 amIs this picture any better? Sort of hard trying to balance my iPhone, LED torch and focus and press the shutter button and not shake it :phttps://dl.dropboxusercontent.com/u/5612078/loksound/IMG_0426.JPGWiring Key:Red and black: from the rails, aka DCC signal in & powerOrange and grey: traction motor output - these aren't used in our scenario.White, green, blue, yellow and violet: known as function outputs for lights and other things. Again these aren't usedBrown: Speaker outputsMaybe this diagram from the instruction manual would make more sense?http://cl.ly/Q6kwInstruction manual is here if needed:www.southwestdigital.co.uk/PDF/LokSound/51972_LokSound_V40_ESUKG_EN_User-Manual_Edition-4_eBook_03.pdfI reckon its a 4 layer board....I think that if it has a completely independent power supply...Thats a problem, this whole thing will share the power supply with everything else.ThanksMichael

author: IanB
author_rank: Super Contributor
timestamp: July 06, 2013, 07:44:00 pm
content: I see. The manual is interesting.It looks like you would be best sticking to the brown wires and not trying to get into the board.I don't know precisely how you would connect the brown wires to the input of another amplifier without causing any electrical problems. You might try to use a multimeter to see if either of the brown wires is common to either of the red or black power supply terminals. This might give you a shared ground with the other amp. Otherwise I think you can put capacitors in series with the output wires to block any DC current.Unfortunately you are not getting responses here from more knowledgeable people on the forum. Perhaps your thread title is the problem? Try to pick a thread title that concisely states the question you have. For example, "How to connect the output of a low power amp to a second high power amp for more sound volume?"

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 06, 2013, 08:47:49 pm
content: Thank you IanB, you have been a great helpWould have been nice to get some other responses as well, maybe the title wasn't a great idea. Can't change that now thoughAnd there isn't a way to change the topic title. What a shameThanks for the advice anyway.Oh well, maybe some more googling should help me.Thanks once again,Michael

author: IanB
author_rank: Super Contributor
timestamp: July 06, 2013, 09:00:03 pm
content: I think you can change the topic title. Just click Modify on the first post and edit it.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 06, 2013, 09:09:54 pm
content: Quote from: IanB on July 06, 2013, 09:00:03 pmCrikey, you're a genius!

author: madires
author_rank: Super Contributor
timestamp: July 06, 2013, 09:42:06 pm
content: I wouldn't use the output of a class D amp as input for another amp since class D amps got a high THD. But you could try it to check if it would be ok for you, it's not a HiFi application :-) Add a voltage divider (output about 0.5-1V) and a small pass capacitor.Another way is to take the audio signal (before it enters the class D amp) itself and feed it into another amp. But the big question is if the audio signal output of the MCU is analogue or PWMed already for the class D amp. When analogue you could just take the signal and feed it into an additional audio amp. If the signal is PWMed you would have to build a simple class D amp (driver, two MOSFETs and a LC low-pass output filter) like on the decoder PCB but beefier.

author: lewis
author_rank: Frequent Contributor
timestamp: July 07, 2013, 12:19:27 am
content: Quote from: IanB on July 06, 2013, 07:44:00 pmIt looks like the amplifier on the board is a filterless class D type in full bridge configuration. If it was single-ended, the DCC would need a bipolar supply or a large coupling capacitor in series with the speaker. If the amp is filterless, which I'm almost certain it is, the signal going out of the speaker terminals will be full of high frequency rubbish.In light of this, it looks like you need a differential RC filter feeding a differential amplifier. See below. I'd usually post one of my infamous hand drawn schematics, but MS autoshapes will have to do because I'm not in the office with the scanner until Monday.I just threw this together (well actually it's taken hours because I'm using fucking autoshapes), it's 1am and I'm knackered, haven't thought about it that much, I'm sure there's room for improvement. Play with the values!Here's a very brief circuit description, there should be some titbits to put you on the right track in google!Working from left to right - the 100R resistor provides a bit of load for the class D amp to keep it stable. The two 220R resistors form the RC filter with the 100nF capacitor, and the other 220R forms a potential divider with the first two to provide some attenuation. The two 100uF non-polarised caps provide AC coupling into the input of the differential amplifier (you could probably get away with 10uF). The 10nF caps provide a bit of common-mode RFI filtering, and the 100K resistors set the input DC bias of the diff-amp to 1/2 VCC (more on that in a minute).The four 22K resistors together with one half of a dual op-amp form a unity-gain differential amplifier. This circuit subtracts one input from the other, leaving the difference between them. This difference signal is then AC coupled into the TDA2030 power amp with the 10uF cap, with yet another 100K resistor setting the DC bias of the power amp to 1/2 VCC. The 47K and 4.7K feedback resistors provide about 10x AC gain, and the 2.2uF capacitor ensures the gain drops to unity at DC. The 470uF cap across the power amp's supply terminals provides a bit of local decoupling. The 100nF and 2.2R at the 2030's output is called a Zobel network (or Boucherot Cell if you're gandalf) and helps maintain stability into reactive loads, and the two 1N4007 diodes ensure that any transient voltages returned by the speaker do not damage the TDA2030.The big 2200uF series capacitor allows us to power the whole thing from a single supply - it blocks DC going to the speaker. Make sure you get this the right way round. You might be able to get away with 1000uF or 470uF. When the TDA2030 is quiescent (not doing anything) the output terminal, pin 4, is at 1/2 VCC DC level. When the TDA2030 is doing something, the audio output signal wobbles about this level. So the signal now has two components - the DC part and the audio wobbly part. The 2200uF capacitor ensures only the audio wobbly part gets to the speaker, otherwise the speaker's voicecoil will be cooked.I keep banging on about 1/2 VCC. This whole circuit is designed to run from a single supply, like +12V from a battery. Because audio is AC, we need to make the signal wobble about some point. If we make it wobble about ground, half the time the signal will go negative which we can't have because there is no negative supply. So instead, we make the AC signal wobble about 1/2 VCC. What's that? Literally half the supply voltage. If you run this from a 12V battery, the audio signal will wobble about 6V DC. (VCC and +V mean the same thing on my diagram - it's the positive supply voltage).This 1/2 VCC level is generated by the bottom op-amp. The two 10K resistors form a potential divider that produces half the supply voltage at the op-amp's non-inverting (+) input. The op-amp buffers this voltage giving the same voltage at its output but at much greater current (yes, much lower impedance). This goes on to bias the circuit so the whole thing operates around the 1/2 VCC level right up until the positive terminal of the 2200uF speaker coupling capacitor.I'm makingso manyassumptions about the operation of your DCC thingybobwossisface that this whole circuit may explode in a massive ball of flames. But try it on abreadboardand see what happens. You'll either learn a hell of a lot from buggering about with the values, or to never trust idiots on internet forums. Either way, a positive experience.

author: IanB
author_rank: Super Contributor
timestamp: July 07, 2013, 01:22:57 am
content: Wow, nice work, but that's complicated!Wondering, couldn't you just take the two lines feeding the two 22 k resistors and feed that as a buffered input signal to the input of some other black box amplifier like a home audio system?I'm now wondering what kind of signal any given home audio amplifier expects on its red and white input terminals, and I'm going to have to start Googling...

author: justinjja
author_rank: Regular Contributor
timestamp: July 07, 2013, 03:06:53 am
content: How much power are you looking for?And what are your size constraints?You could just buy a cheap amplifier designed for a car,Most of them have High level inputs. (As in you could connect the 2 amps directly) And they run on 12V DC

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 07, 2013, 08:54:45 am
content: Wow lewis, that is fantastic work, thank you so muchQuote from: lewis on July 07, 2013, 12:19:27 amWell actually, the way DCC works is that it is a bipolar wave on each rail, one rail being the antiphase of the other. Not sure how this would affect the decoder, since I assume its shoved through a bridge rectifier to get some sort of DC to power the micro etc.Ha, autoshapes. Its horrible. You have done a very nice job though, it must have taken you foreverThe description is great thank you. I shall need to do a lot of googling to find out what it all means thoughI get some of it, but op-amps confuse me, they always have. Probably because I've never bothered to learn about them properly!Power supply is also interesting. Would it have to be regulated to a known voltage? I have up to 24V available (2x12V batteries in series), slightly more if they're fully charged. The datasheet says up to 36V, so it would be okay to run at 24V regulated supply?Anyway, I feel another parts order accumulatingWill let you know how I get on.

author: lewis
author_rank: Frequent Contributor
timestamp: July 07, 2013, 11:24:53 am
content: Quote from: IanB on July 07, 2013, 01:22:57 amYes, absolutely. But unfortunately not if the amplifier shares the same power supply (well, ground reference) as the DCC, you need to convert from full bridge to single ended to get the ground references the same.A much much easier way to implement the differential amplifier is to use a 1:1 audio isolation transformer like this one:http://www.rapidonline.com/Electrical-Power/PCB-Audio-transformer-Open-82389Quote from: justinjja on July 07, 2013, 03:06:53 amThat is a very good idea!Quote from: MichaelKavanagh on July 07, 2013, 08:54:45 amDoes the DCC need two supplies? You mention that there are 2x 12V batteries in series, do you need to connect the centre-point of this series connection to the DCC?Quote from: MichaelKavanagh on July 07, 2013, 08:54:45 amThe golden rule of an op-amp is this: it will do whatever it can to its output to make the inputs identical. Sounds a bit weird. But if you put components from the output of the op-amp back to one of the inputs, forming a feedback loop, you can get the op-amp to do some exciting stuff like provide gain (multiply), add/subtract voltages, perform integration and differentiation, all sorts of things. Op-amps are essential to know about, definitely get reading!Quote from: MichaelKavanagh on July 07, 2013, 08:54:45 amDoesn't need to be regulated, the op-amps and the TDA2030 will take upto 36V supply as you say, so as long as your battery voltage never exceeds this you'll be fine. Might be a good idea anyway to regulate the supply to the op-amp, but I wouldn't bother with the TDA2030.When I was starting out in electronics, I had the full set of the 'Engineer's Mini Notebook' by Forrest M Mims III:http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=Engineer's%20Mini%20-%20NotebookI got the set when I was 7, read the whole set ragged, and loved them to bits. I would thoroughly recommend them, the op-amp one is available for 1p!

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 07, 2013, 08:26:57 pm
content: Quote from: lewis on July 07, 2013, 11:24:53 amNope, the 24V from either end of the series connected batteries regulated down to 12v, thats fed into the H-bridge. I know it sounds inefficient, but in this application the battery voltage soon dips below 12V, in which case the whole circuit will cease to work.Thanks for all the help and advice. I shall build that circuit and report back. You all have been an amazing help. Thank you so much :')RegardsMichael

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 11, 2013, 08:06:12 pm
content: Hello all,Ordering the components today, and I've come across a bit of a problem. I can't seem to source any of the 100uF or 10uF non-polarised capacitors. I can get electrolytics, but they're polar.Thanks and kind regardsMichael

author: IanB
author_rank: Super Contributor
timestamp: July 11, 2013, 09:59:23 pm
content: I think non-polarized capacitors are hard to obtain in large sizes like 10 uF or 100 uF. I was surprised when that was mentioned.I think you can possibly find non-polar electrolytics, or you can construct one by putting two ordinary electrolytic capacitors in series back to back (in other words in opposing polarities). For instance putting two 200 uF capacitors in series will give a 100 uF capacitor.I'm no expert on this, so maybe someone has a better idea?

author: lewis
author_rank: Frequent Contributor
timestamp: July 11, 2013, 10:06:39 pm
content: You can put polarised capacitors in reverse-series to make a non-polar cap, get two 220uF caps, put them in series with their negatives connected together, and you've got yourself a 110uF non-polar cap. Which is close enough. Or 2x 22uF in reverse series = 11uF.There's some 100uF non-polar caps here:http://www.rapidonline.com/Electronic-Components/Non-polarised-85-deg-C-radial-capacitors-78276Voltage ratings of all capacitors need to be at least that of the power supply.

author: SeanB
author_rank: Super Contributor
timestamp: July 12, 2013, 03:30:28 pm
content: Funny I have no problem getting 100, 200uF film capacitors, though they are the size of a cola can and are marked for motor start applications. Voltage rating is 125V Dc continuous or 275VAC for 30 seconds with 6 starts per hour.

author: IanB
author_rank: Super Contributor
timestamp: July 12, 2013, 04:34:31 pm
content: Quote from: SeanB on July 12, 2013, 03:30:28 pmThey have come down in size a lot thenIn the mid-70s I recall the unpolarized ballast capacitors in fluorescent tube fittings were also the size of a cola can and were only about 8 uF (they were spec'd for 250 V AC continuous).

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 12, 2013, 05:38:33 pm
content: Brilliant. Thanks, also, how would I change the output gain/volume. With a pot I suppose, but where does it go? Something to do with those 4.7K and 47K resistors?CheersMichael

author: SeanB
author_rank: Super Contributor
timestamp: July 12, 2013, 05:54:38 pm
content: Quote from: IanB on July 12, 2013, 04:34:31 pmMotor start are a lot smaller value for value than motor run units, as they do not have to survive being connected to the mains permanently. Typically I buy values of 35, 40, 45, 50, 55 and 60uF capacitors with a 440VAC rating for airconditioners. They typically will withstand 1kV on test, and if they start to break down with a 30 second test ( you hear the internal arcing) then I replace them, this is typically after 4-5 years of running. Smaller units of 1, 1.5, 2, 4, 8, 10 and 12uF are used for fan motors, as well as for power factor correction on rapid start ballasts. The PFC and fan motor units are typically rated for 275VAC though.

author: lewis
author_rank: Frequent Contributor
timestamp: July 12, 2013, 07:26:39 pm
content: Ah yes....You're absolutely right about the 4.7K and 47K resistors changing the gain of the amplifier, and if you find you don't have enough volume you can increase the 47K to 56K or 68K or something. But that's probably not the best place to put the pot. This is because when you adjust the pot, the DC gain of the power amp will vary slightly and the speaker cone will wobble in and out slowly. The 2.2uF cap and the 2200uF output cap will negate this somewhat, but you'll still get a bit of DC at the output of the TDA2030.The best place to put the pot is at pin 1 of the TDA2030. Replace the 100K resistor with a 100K pot, and connect the wiper to pin 1. I'll modify the diagram if you don't know what I mean.By the way, you will get a thump from the speaker every time you power on the unit because the 2200uF capacitor needs to charge through the speaker. There's a way round it if it bothers you.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 12, 2013, 08:39:12 pm
content: Yeah, if the "thump" could be avoided that would be great. And I think I get what you mean, is the attached schematic correct?Cheers

author: lewis
author_rank: Frequent Contributor
timestamp: July 12, 2013, 08:46:41 pm
content: Perfect! See how you get on with the thump, it might not be that bad.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 12, 2013, 08:52:22 pm
content: Okay, cheers, I'll let you know how I get on

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 22, 2013, 11:23:57 am
content: Update!Breadboarded it and bad newsI have double checked and triple checked my breadboarding, and am sure it is as per the diagram.When I apply power without the inputs connected there is a pop, as predicted.http://youtu.be/36a-JNcXPuUNow, when I connect the sound output from the decoder (but not actually producing any sound yet) and then the power the speaker makes a horrible buzzing noise. Then it sort of just faded away until it was almost unnoticeable.http://youtu.be/_nDE_FE1eUkNow, when I connect the inputs and turn the sound on, there is something that sounds a little like what its meant to, but its so distorted. This is the sound it makes:http://youtu.be/7KqO2jCMPkgas opposed to the "raw" output from the decoder which is this:http://youtu.be/rPsp1CLVEoUI am using the same speaker in each of the videos. Audio taken with my iPhone so perhaps not the best but it should give you a fair representation. The amplifier circuit at the moment is being powered by 12V, and ground is common with everything elseAny help would once again be greatly appreciated.ThanksMichael

author: lewis
author_rank: Frequent Contributor
timestamp: July 22, 2013, 04:55:14 pm
content: Quote from: MichaelKavanagh on July 22, 2013, 11:23:57 amOh no! Not to worry, sometimes things don't work first time. Try providing the circuit with a known clean audio signal such as the output from a CD player or PC (don't use the iphone, it might have a class D amplifier on the output too). Better still, if you have a signal generator, use that. Basically we want to try to feed the circuit with a clean (no class D nasties) signal to eliminate possible problems and noise caused by the class D amplifier.It's also possible the TDA2030 amplifier is clipping. Running from a 12V supply means the amp can only throw about 2-3V into the speaker which is not a lot. Try turning the volume pot down to see if the sound quality improves.Also, do you have an oscilloscope? One would be very handy right now...I was making so many assumptions about your setup which may not be accurate. We need to troubleshoot possible problems one at a time to track down the cause.

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 22, 2013, 06:26:01 pm
content: Hmm, I have a CD player with headphone output, but when I connect that (shielding + one of the signal wires) it just makes some constant unchanging low-ish freq buzzing.Upped the voltage supply to 24V, still the same behaviour but louder.I have an oscilloscope that was lent to me, how useful it is I don't know, the owner doesn't seem to care for his stuff much. Its an analogue scope, but better than nothing.

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2013, 07:06:52 pm
content: An analog scope should be fine for some basic testing. Do you have a model number or any specifications for it?The idea would be to feed a clean tone (like a sinewave) into the front of the circuit and then trace the signal through each stage looking for any obvious problems like major distortion or clipping.Possibly you can find and download some WAV files with pure tones in them, or even find some utilities that can generate those for you. Burn these to a CD and play them to get your test signal. I have not done any experimenting in this area but I speculate that if you look you will find such things.

author: lewis
author_rank: Frequent Contributor
timestamp: July 22, 2013, 07:17:40 pm
content: I'll order a TDA2030 and breadboard the circuit myself, give me a couple of days...

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 22, 2013, 07:22:21 pm
content: Quote from: IanB on July 22, 2013, 07:06:52 pmIts an "Advance Instruments OS3000"Quote from: lewis on July 22, 2013, 07:17:40 pmWow, that is very good of you! Actually I did order a TDA2050, but I didnt think that should make much of a difference after skimming the datasheet...

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2013, 07:39:33 pm
content: Quote from: MichaelKavanagh on July 22, 2013, 07:22:21 pmWow, does it really weigh 12 kilos?

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 22, 2013, 07:42:43 pm
content: Quote from: IanB on July 22, 2013, 07:39:33 pmYep, is that abnormal for an analogue scope? The friend who lent it to me has a storage scope that you'd need a van and 2 strong men to transport around

author: lewis
author_rank: Frequent Contributor
timestamp: July 22, 2013, 08:04:40 pm
content: Forgot to mention - if using a CD player or some other signal source that's isolated from the circuit's ground, you will need a 2.2K (or thereabouts) resistor on each input down to ground. Also, the ground of this circuit needs to be connected to the DCC ground when you're using the DCC as the source.TDA2050 should be fine.Nice scope!

author: MichaelKavanagh
author_rank: Contributor
timestamp: July 23, 2013, 05:29:16 pm
content: Hmm, any chance you could provide a diagram for that? What ground are you referring to? CD players output ground or amplifier circuit ground?Yep, the grounds are connected when using DCC as the source.Actually, unrelated but was just wondering, is it acceptable to have a 12V regulator, then a 5v regulator on the output of that? I read somewhere that this could damage it, but couldn't see why as long as you don't exceed current ratings.Cheers.