Need Help With Audio Amp Circuit - Page 1
Forum section: electronics / beginners

author: prwiley
author_rank: Contributor
timestamp: February 07, 2018, 12:37:56 pm
content: It would be charitable to call me a noobie. I am trying to connect an audio amplifier (this one:https://www.canakit.com/20w-bridged-audio-amplifier-kit-ck193-uk193.html) with a balanced input circuit (this one:https://www.diyrecordingequipment.com/pages/balanced-input-output-assembly-guidesee schematic below).I have the two circuits connected to the same 12volt power supply (this one:https://www.trcelectronics.com/View/Mean-Well/RS-35-12.shtml) pins 4 and 7 of U1 are connected to the 12V+ and 12V- of the power supply. Pin 1 of U1 is connected to the ground of the power supply. U1 is this chip:http://www.thatcorp.com/datashts/THAT_1240-Series_Datasheet.pdfI hope someone experienced will be able to suggest what's going on from my description of what's happening.I find that when I connect the OUTGND and the OUT+ of the balanced input circuit to the +/GND inputs on the amp, I get a buzz out of the speaker and the audio signal is very distorted and is only just audible. When I disconnect the OUTGROUND from the GND input of the amp so that only the OUT+ is connected from the balanced input, I get fairly clear audio out (there is some AC hum, but I'll leave that issue aside for the moment).My question is: why does the audio sound better when only the OUT+ is connected?My presumption is that is has something to do with the how the power supply is connected to both circuits, but I really don't understand how/why and would really like to know. Can someone please explain what they think might be going on. I hope is that it's obvious to someone with more experience. The amp circuit itself does not seem to be an issue because I get the same behavior if I swap in a completely different amp.Many thanks is advance for solving/explaing this puzzle.

author: Benta
author_rank: Super Contributor
timestamp: February 07, 2018, 01:46:17 pm
content: The problem is that you're using a single output power supply, that is: +12 V and 0 V.You need a dual: +12 V, 0 V, -12 V.You could use two of of the power supplies you link to with the outputs connected in series.Supply 1: +V to +12 V, -V to 0 V.Supply 2: +V to 0 V, -V to -12 V.It's not a very good solution though, as -V is internally connected to Ground, meaning you need to make sure the two enclosures don't touch each other.

author: prwiley
author_rank: Contributor
timestamp: February 07, 2018, 02:02:27 pm
content: @Benta, thanks very much for your reply.I am a little confused because the power supply, per the spec sheet (https://www.trcelectronics.com/ecomm/pdf/rs35.pdf) does have +12 and -12 outputs. I thought it was a dual supply.

author: Benta
author_rank: Super Contributor
timestamp: February 07, 2018, 02:07:52 pm
content: Yes, the data sheet is extremely misleading. If you got to the product page instead (your first link), it specifically says "single output".See the schematic on data sheet p.2, you'll see that there are only two outputs, 0 V is missing.

author: paulca
author_rank: Super Contributor
timestamp: February 07, 2018, 02:15:44 pm
content: Doesn't an audio "balanced" signal require one normal signaled and one inverted signal? The idea being you add the two together at the other end to get your noise, the subtract the noise from the normal signal? Thinking of XLR style balanced mics.How does that amp circuit handle that?

author: prwiley
author_rank: Contributor
timestamp: February 07, 2018, 02:39:45 pm
content: @paulca you are right, the signals need to be combined and that's what the THAT1240 chip is doing, but that's not related to the power supply issue.@benta. Thanks again, got it. Searching around, I can see that the term "dual power supply" is ambiguous on supplier's descriptions of power supplies. It's this kind of thing I need?http://electronics-diy.com/12v-dual-power-supply.phpit's ground being the 0v, yes?

author: Benta
author_rank: Super Contributor
timestamp: February 07, 2018, 03:20:40 pm
content: Yes, that one will work.

author: rstofer
author_rank: Super Contributor
timestamp: February 07, 2018, 07:58:35 pm
content: Quote from: prwiley on February 07, 2018, 02:39:45 pmBe careful! That supply, as shown at the link, has 230V input ONLY. I don't pretend to know anything about the PS or your project but the parts lists shows the transformer with a 230V primary.

author: Benta
author_rank: Super Contributor
timestamp: February 07, 2018, 08:03:16 pm
content: Quote from: rstofer on February 07, 2018, 07:58:35 pmYes, well, finding a transformer with 120 V primary is possible. And the other way round is worse.

author: rstofer
author_rank: Super Contributor
timestamp: February 07, 2018, 08:22:05 pm
content: Quote from: Benta on February 07, 2018, 08:03:16 pmI don't know anything about that PS. Is it a kit? If so, is there a transformer option? Is it built from parts? If so, a 120V transformer might be pretty easy to find. But for a noobie? Does it have to fit a specific PCB pattern?

author: Benta
author_rank: Super Contributor
timestamp: February 07, 2018, 08:55:55 pm
content: The question from the OP was "is this the kind of thing that I need?" Yes, it is. It has +12 V, 0, -12 V and is meant as an example.I'm certain that he will find a ready-made product that fits.

author: Audioguru
author_rank: Super Contributor
timestamp: February 08, 2018, 01:45:04 am
content: The Canakit audio amplifier uses obsolete TDA20003 ICs that might be impossible to replace if one fails.

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 08, 2018, 02:29:33 am
content: Quote from: paulca on February 07, 2018, 02:15:44 pmNo. A balanced signal requires exactly equal source impedances on the positive and negative signal lines. These are then fed to a differential amp at the destination, whicha) nulls out any common mode signal picked up along the wayb) doesn't need to assume that 0V at the source is the same as 0V at the destinationOne common way to do this is, as you say, to double up the output circuits to give one normal and one inverted copy of the signal. This will give a slight increase in common mode rejection, and also a 6dB signal boost at the receiving end which may or may not be desirable. Which is why commercial line receiver ICs like the ones from THAT Corporation offer -6dB as well as 0dB versions.Quote from: paulca on February 07, 2018, 02:15:44 pmYou subtract, not add, the two signals. That is why the noise tends to cancel out.

author: prwiley
author_rank: Contributor
timestamp: February 08, 2018, 01:22:34 pm
content: I was not proposing to use the supply I referenced, I just wanted to know if the design was the type I might need.Since posting my question and reading answers here, I've a crash course on power supplies, and I can see that for my application a switching power supply of the kind I was trying to use may well be problematic because of noise issues. A bipolar linear supply would seem to be a better choice for an audio application, something like this, perhaps:https://www.jameco.com/z/JE215-Dual-Output-Adjustable-Linear-Regulated-Power-Supply-Kit_20626.html

author: Svgeesus
author_rank: Regular Contributor
timestamp: February 08, 2018, 09:14:00 pm
content: A linear supply may give better performance (also more heat) than a switcher. The supply you linked to is bipolar (the circuit diagram in the assembly instructions shows this clearly, with a positive and a negative voltage regulator), so it can produce +12, 0V, -12 (or +15, 0V, -15). There is no figure quoted for ripple, so they make no promises there.However, look at the power rating (remembering that power is voltage times current, so the 750mA rating will be for the ±5V option):Quoteand compare with what your power amp needs:QuoteThe power supply is likely fine for the preamp/ balanced input section, but is very underpowered for the power amp.Then again you already have a unipolar power supply which works with the power amp, right?

author: prwiley
author_rank: Contributor
timestamp: February 12, 2018, 02:22:22 pm
content: @Svgeesus I do understand that that power supply is under specs for the 20W amplifier.What I have now are two inexpensive Mean-Well 12v 3A switching supplies that I have connected separately to the balanced input stage and the amplifier. This arrangement allows the GNDOUT of the balanced input circuit to the GND of the amp with no distortion of the audio. Cost wise, the two supplies are not any more expensive than a single linear supply would be, maybe even a little cheaper.I have found this kit:https://www.vellemanstore.com/en/velleman-k8042-symmetric-1a-power-supply-kitto which adds a transformer appropriate to one's needs, but this only gets one up to 1A and a good transformer appears not to be inexpensive.I am surprised that it's as hard as it is to locate linear supply kits that would work. Perhaps I am looking in the wrong places or have the wrong search terms.

author: BrianHG
author_rank: Super Contributor
timestamp: February 12, 2018, 03:31:58 pm
content: For generic amp boards, I like these little class-D amps. No extra heat-sink needed and better performance:https://www.aliexpress.com/item/TPA3116D2-2-50W-Digital-Power-Amplifier-Board-Class-D-Two-channel-Stereo-New/32815406508.html?spm=2114.search0104.3.2.375f4eead9jfbJ&ws_ab_test=searchweb0_0,searchweb201602_4_10152_10151_10065_10344_10068_10130_10342_10547_10325_10343_10546_10340_10548_10341_10084_10083_10618_10139_10307_5711211_5722315_10313_10059_10534_100031_10103_10626_10625_10624_10623_10622_10621_10620_10142,searchweb201603_38,ppcSwitch_5&algo_expid=e2d541e0-b111-49ad-95eb-804d7a7e285a-0&algo_pvid=e2d541e0-b111-49ad-95eb-804d7a7e285a&priceBeautifyAB=0or:https://www.aliexpress.com/item/TDA7498-2x100W-Digital-Power-Amplifier-Board-Audio-Amplifier-Class-D-Dual-Audio-Stereo-DC-14-34V/32807680649.html?spm=2114.search0104.3.16.375f4eead9jfbJ&ws_ab_test=searchweb0_0,searchweb201602_4_10152_10151_10065_10344_10068_10130_10342_10547_10325_10343_10546_10340_10548_10341_10084_10083_10618_10139_10307_5711211_5722315_10313_10059_10534_100031_10103_10626_10625_10624_10623_10622_10621_10620_10142,searchweb201603_38,ppcSwitch_5&algo_expid=e2d541e0-b111-49ad-95eb-804d7a7e285a-2&algo_pvid=e2d541e0-b111-49ad-95eb-804d7a7e285a&priceBeautifyAB=0