Low Distortion audio power amplifier - Page 1
Forum section: electronics / projects

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 12:16:00 am
content: Hey guys, fairly new here but I have a project at school and I wanted to ask a few questions. My professor told me we can use PCB design or use breadboard. I am trying to simulate this in PSpice and its not going well. Also any tips would be appreciated. I have attached the schematic and I don't know how to proceed with this.I circled the parts that confused me, any help is greatly appreciatedCheers

author: GerryBags
author_rank: Frequent Contributor
timestamp: February 25, 2018, 01:38:03 am
content: Only a noob myself, but I think the big circle on the left is the chassis ground, or the PCB ground plane, connection. All the other grounds are connecting back to this. The variable resistor leading to the base of the BC184 is the pre-settable type, I think, with the flat line on the wiper instead of the usual arrow. The other two that aren't wires (?) are a switch and an air-core inductor with 17 turns which is (again - I think) acting as a choke filtering out high frequency AC.

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 01:59:29 am
content: Heres is the preliminary pspice simulation, idk what the output is, i ran a transient analysis.

author: Audioguru
author_rank: Super Contributor
timestamp: February 25, 2018, 02:34:03 am
content: It is obvious.

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 03:00:54 am
content: Quote from: Audioguru on February 25, 2018, 02:34:03 amHey man thanks for the input, I fixed it and I think i got a stable output. My prof said I need to able to output to some kinda speakers so im thinking something small, what kinda speaker should i go for and what would be the R load I should put in for the schematicAlso the top of the output is kinda getting clipped, how can I change that? I want a smooth wave.Thanks for all your helpCheers

author: Audioguru
author_rank: Super Contributor
timestamp: February 25, 2018, 04:49:30 am
content: R10 has a resistance that is 1000 times too high but has no effect with no load.Your amplifier clips symmetrically at 3V peak when it should make at least 20V peak (50W peak into 8 ohms which needs a pretty big speaker, not a small one). Most home speakers are 8 ohms.Maybe the sim program knows that the PNP transistors were destroyed when they were backwards?I think 10mH for the output inductor is way too high, maybe it should be 0.1mH.

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 04:59:38 am
content: Quote from: Audioguru on February 25, 2018, 04:49:30 amI changed r10 to 1 ohm, and I deleted the old transistors and replaced it with new ones so i dont think PSpice lite is that smart. Also Im still only getting a gain of 15 V/V. you are saying I need a gain of 50 for it to be used with a normal speaker? (didnt really understand what u meant)also I changed inductor, doesnt really make a difference. How can I smooth out the output? I keep changing values but its not helping, Idk y its clipped

author: Audioguru
author_rank: Super Contributor
timestamp: February 25, 2018, 05:00:09 am
content: Hey! The amplifier voltage gain is 100k/6.8k= 14.7 times and the input signal is only 200mV peak. Then of course its output should be 200mV x 14.7= 2.94V peak which it is.But the clipping looks fishy. The top clips into a triangle peak then the bottom clips with a left slant. Then the top clips with a right slant then the bottom clips with a triangle peak. I think the signal generator does that.

author: Audioguru
author_rank: Super Contributor
timestamp: February 25, 2018, 05:04:26 am
content: Since the voltage gain is only 100k/6.8k= 14.7 times then for it to produce its maximum output of 50W into 8 ohms (20V peak) then its input must be 20v/14.7= 1.36V peak.

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 05:08:35 am
content: Quote from: Audioguru on February 25, 2018, 05:00:09 amShould I try changing the source? I mainly use the V/sin source that I used here in class and in lab. idk what else to doBtw thanks for all your help, really appreciate it EDIT: i added a new input vs output with 1.36V as input source but its not reaching that 20V out, its getting clippedAlso should the frequency be 1k? or should I go with normal 60hz since I need to implement thisCheers

author: Audioguru
author_rank: Super Contributor
timestamp: February 25, 2018, 05:39:43 am
content: Now it clips with an 18V positive peak so reduce the input a little so it reaches +17.8V without clipping. I simply guessed that it should reach a peak of 20V but more than 17V is fine.

author: Audioguru
author_rank: Super Contributor
timestamp: February 25, 2018, 05:42:12 am
content: You asked if the 1kHz frequency should be 60Hz. Don't you know the frequencies of normal hearing?

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 05:52:18 am
content: Quote from: Audioguru on February 25, 2018, 05:39:43 amAwesome, Thanks so much for your help. for the output would something like this work?https://www.amazon.com/uxcell-Metal-Internal-Magent-Speaker/dp/B0177ABRQ6/ref=pd_sbs_147_1?_encoding=UTF8&pd_rd_i=B0177ABRQ6&pd_rd_r=RQZYH2E3JS0JDP1ACJ5P&pd_rd_w=kHSI0&pd_rd_wg=1ieVC&psc=1&refRID=RQZYH2E3JS0JDP1ACJ5PAlso how should i feed the input?Sorry if Im bothering you with all these noob questions, EDIT: about the frequency, I thought the source should be same frequency as the wall but it was a stupid question.your help is very much appreciated. Trying to get this project done professor is strict lol

author: ArdWar
author_rank: Frequent Contributor
timestamp: February 25, 2018, 06:31:27 am
content: Quote from: Audioguru on February 25, 2018, 05:00:09 amI suspect that's actually caused by the simulation timestep being too coarse

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 06:35:58 am
content: Quotehow can i fix that? I set the transient simulation for 3ms and started saving data after 0 sec. I didnt change any other options.I can report back asap if you have any suggestions,Thanks for your reply, really appreciate it

author: Audioguru
author_rank: Super Contributor
timestamp: February 25, 2018, 02:25:17 pm
content: The cheap tiny 1.1" diameter speaker you found has a maximum rated power of only 2W. It would explode if you feed a peak of 20W into it. A 20W peak rating is 10W continuous rating if the manufacturer does not lie about it. A speaker rated for 10W continuously is probably at least 6.5" or 8" in diameter and must be in a proper speaker enclosure.If you change the timebase of your oscilloscope to 2ms per division then it should show six sinewaves without the odd distortion.

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 06:20:53 pm
content: Quote from: Audioguru on February 25, 2018, 02:25:17 pmHey how did you get the 20W value? Vout is 18V so using P=V^2/R ( R would be the speakers so 8 ohms)i get around 40Wis my math wrong?Also can u give me a link to a compatible speakers? I cant find any, the ones amazon has are all USB powered.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 25, 2018, 07:29:13 pm
content: Quote from: Audioguru on February 25, 2018, 05:04:26 amAnd that's easily achieved from a circuit that runs from a 3.3V supply or higher. Back when that sort of amplifier design was common, even 3.3V wasn't very common in consumer electronics.

author: Adgj533
author_rank: Contributor
timestamp: February 25, 2018, 08:04:53 pm
content: And that's easily achieved from a circuit that runs from a 3.3V supply or higher. Back when that sort of amplifier design was common, even 3.3V wasn't very common in consumer electronics.In my case (schematic posted above) what would u do for input and output, I have to show prof my amp works and have to play music thru it, I cant figure out if i should do like a 3.5mm from my phone as input and speaker as output, but I cant find 20W speakers, they are all USB. can i somehow cut the USB and turn it into breadboard friendly?Thanks for looking at my posthighly appreciatedCheers

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 26, 2018, 12:14:44 am
content: Just get some cheap stereo speakers from Goodwill and use any audio device as a source.

author: Audioguru
author_rank: Super Contributor
timestamp: February 26, 2018, 01:25:13 am
content: Quote from: Adgj533 on February 25, 2018, 06:20:53 pmSorry, I said 20V peak awhile back. A problem with this website is that I cannot scroll back to see the schematic like on all the other amplifiers on all the other websites.18V peak into 8 ohms is 40.5W peak which is 20.25W continuously just before clipping. Is this the amplifier powered from plus and minus 22V?QuoteUSB does not drive a speaker, it charges the battery in a low power portable speaker that already has a power amplifier in it. You need an 8 ohm speaker that survives 20W continuously.Madisoundhas expensive speakers andParts Expresshas a variety of speakers. Maybe you need a 20W 8 ohm 8" diameter guitar speaker. It must be mounted in an enclosure that matches its detailed spec's.

author: Adgj533
author_rank: Contributor
timestamp: February 26, 2018, 02:03:15 am
content: QuoteYes but if its too much Watts, i can put a few diodes in to cancel the negative part out?Thanks for all your help btw, I learned a lot during these past 2 days ur amazingCheers

author: Audioguru
author_rank: Super Contributor
timestamp: February 26, 2018, 02:41:20 am
content: Quote from: Adgj533 on February 26, 2018, 02:03:15 amNo. This amplifier is designed to have the same negative voltage as its positive voltage.Nobody makes a +22V and -22V power supply. You must design it and make it. If you wanrt less maximum power then use a 20VAC center tapped transformer which will produce plus and minus 13V. Then the output power will be about 7W continuously into 8 ohms.

author: Adgj533
author_rank: Contributor
timestamp: February 26, 2018, 02:50:28 am
content: Quote from: Audioguru on February 26, 2018, 02:41:20 amI misread your question earlier, I do have a +22, -22 (max being 30v/5A i think)bench DC power supply, and the school has one too. so ill use thatAlso, Im trying to put this on PCB using eagle software, any tips?

author: Audioguru
author_rank: Super Contributor
timestamp: February 26, 2018, 03:57:30 am
content: Quote from: Adgj533 on February 26, 2018, 02:50:28 amIf you use two power supplies for the positive voltage and the negative voltage then adjust their voltages to be the same.I have never used Eagle software.

author: Adgj533
author_rank: Contributor
timestamp: February 26, 2018, 05:12:34 am
content: Quote from: Audioguru on February 26, 2018, 03:57:30 amI want to buy the transistors and other parts tmrw, what should i look for when buying it?Also I saw Daves how to NOT blow up ur scope video, I was kind of confused by it. Is it safer if you have a separate ground for your scope? and connect circuit grounds and PSU grounds together?

author: Zero999
author_rank: Super Contributor
timestamp: February 26, 2018, 10:22:02 am
content: Don't connect the amplifier's 0V rail to mains earth and it should be fine.

author: Adgj533
author_rank: Contributor
timestamp: February 27, 2018, 02:58:55 am
content: Hey im trying to put this on a breadboard and giving it input from my phone using a 3.5mm jack to 2 wires (trying to play a song and hopefully it will be amplified by speakers). I found an old 60W 6 ohm speaker at home. But wouldnt that just be DC voltage since its coming from a phone with a battery or do I need function generator hooked up too?Also I need a higher voltage to go in, how can i get that?

author: Audioguru
author_rank: Super Contributor
timestamp: February 27, 2018, 03:54:20 am
content: That audio output from your phone is not DC, it is audio. Audio is AC.If the output level from your phone is not high enough for this amplifier to produce its full output power then you need to change the resistor in the amplifier that sets its sensitivity, or add a preamp to the input of the amplifier.You can feed the amplifier with a function generator if you want.A solderless breadboard cannot pass the high currents this amplifier uses. An 18V peak into 8 ohms produces a current of 2250mA. The contacts on a solderless breadboard might be able to pass 100mA.

author: Adgj533
author_rank: Contributor
timestamp: February 27, 2018, 04:37:29 am
content: 2 questions:1) can i reduce the input voltage to like 50-100mv so that the output wont be high and still be manageable to do in breadboard2) how do I find the correct parts to build this thing, i was searching digikey and there are a lot of transistors with diff wattage etc, idk which one to chooseaudioguru ur a life saver man, thanks for all your help these past few daysreally appreciate it

author: Audioguru
author_rank: Super Contributor
timestamp: February 27, 2018, 02:18:31 pm
content: Quote from: Adgj533 on February 27, 2018, 04:37:29 amThe power supply voltage(s) of an amplifier and the speaker impedance determine the maximum output current and power from an audio amplifier. All amplifiers have a volume control to adjust the input level.2) how do I find the correct parts to build this thing, i was searching digikey and there are a lot of transistors with diff wattage etc, idk which one to chooseYou simply calculate the maximum current and heating in each transistor. Since I am posting now I cannot see the schematic to see if capacitor and resistor values were already chosen.Are you supposed to design the amplifier of just put it together?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 27, 2018, 02:27:25 pm
content: One could change the gain to a higher value to make it work with an 100 mV input level. However, this will reduce the feedback and would incease distortion. The inverting type of circuit looks good on paper, but it need the stage before the amplifier to deliver some current and this stage will add to the distortion, especially at higher gain. So the proper way would be to add a stage before this amplifier to bring the 100 mV level to some 1-3 V level first.The output stages with 3 transistors in a NPN/PNP/NPN (and complementary) combination with local feedback can be tricky. It can depend on the speed of the 3 transistors to have a suitable combination. So it is not only wattage to look at, but also speed. Finding suitable sets of transistors is where you might need the simulation for. Depending on the transistors used it might take some extra pF range caps to stabilize this. This is likely the really tricky part with this type of amplifier. The nasty part is that also parasitic properties of the layout may come into play here. The shown circuit is one of those that can give pretty good performance if done right, but could as well end up as a MHz range power oscillator if a different brand of caps / transistors are used.

author: Audioguru
author_rank: Super Contributor
timestamp: February 27, 2018, 02:42:26 pm
content: The schematic on the first page of this thread shows that all parts are already chosen. Only the two Oriental transistors are not supplied at Digikey but a quick look at their datasheets shows that they are huge high voltage transistors. Huge high voltage transistors are not needed since the amplifier's collector and emitter resistors for them shows that their current is very low so that anylittle44V or more Western transistors will work. A few of resistors must have their current and amount of heating (therefore their size) calculated.

author: Adgj533
author_rank: Contributor
timestamp: February 27, 2018, 06:26:25 pm
content: yeah i was given the schematic by professor, so I just have to put it together, but he didnt mention what type of transistors we should buy etc,Im gonna use 2N3904 and 3906. Hopefully they workthanks for your suggestions

author: Adgj533
author_rank: Contributor
timestamp: March 04, 2018, 06:06:15 am
content: hey man, i put it on a breadboard, and the output looks like this, similar to a capacitor charging, any idea why?Also the gain is only 6, supposed to be around 18 according to my calculations

author: Audioguru
author_rank: Super Contributor
timestamp: March 04, 2018, 03:19:49 pm
content: You cannot measure the gain when the output has such severe distortion.If the wiring on your breadboard is as messy as the wiring on your schematic then it will not work.The contacts on a solderless breadboard cannot pass the high currents of this amplifier anyway.

author: Adgj533
author_rank: Contributor
timestamp: March 08, 2018, 01:17:38 pm
content: Hey AudioGuru, Thanks for your input but I was able to build it in a breadboard by limiting the current on the power supply, and there were 2 other kids in the class that did it using the full current limit, one them were using a bigger resistors and a heat sink.Btw, now I have to use PCB to implement it, and I was wondering if I can put a pot after 100k resistor (pls look at original clean schematic) can I control the volume?Also one of the groups did use a PCB and they only used a small speaker and the song played fine. No distortion etcHe used a knob to control volume but idk where I would place that. any ideas?

author: Audioguru
author_rank: Super Contributor
timestamp: March 08, 2018, 04:50:51 pm
content: Quote from: Adgj533 on March 08, 2018, 01:17:38 pmThe ratio of the 100k negative feedback resistor and the 6.8k input resistor sets the gain of the amplifier to be too high with noise, distortion and poor high frequency response or low gain with oscillation. A volume control for an audio amplifier goes at its input as a signal voltage divider without affecting the gain. But since this amplifier has an inverting input then it has a low input resistance. I do not know if your teacher will allow you to change its input to be non-inverting with a high input resistance which is more suitable for an input volume control.