Audio amplifier - Making a better design - Page 1
Forum section: electronics / projects

author: NaokiS
author_rank: Newbie
timestamp: November 02, 2023, 07:36:51 pm
content: Hi all!I've been working on a large project to make a scale arcade cabinet and have been making all the electronics to make it a fully working model. Part of that was designing a custom amplifier to drive the speakers in the cabinet however I have been running into trouble with this, mainly in that audio design is not my forté and so I figured it was probably best to turn to people who know what they are doing.My project is using two Dayton Audio ND15-A drivers at 4ohm, about 15w max a piece, coupled with two 28mm "tweeter" speakers. The amplifier is a class AB design using a TDA7377 main amplifier for the main drivers and two LM4871 3w amplifiers for the tweeters. I went with class AB because of my level of skill in audio design, it seemed to need less bulky parts and given everything else, it would give a cleaner signal for less hassle. I've attached a screenshot of the PCB 3D design to hopefully give an idea of how much space I have to work with, maybe to justify not doing a class-D designThe basic design was to have a three knob amp. Master volume, bass and treble where bass and treble pots in the middle would mean no EQ was added.I've tried to design what I can and done at least three iterations but the current PCBs have multiple problems and I cannot troubleshoot them due to lack of experience and familiarity. So I asked someone more familiar in this field and they made some suggestions which I tried to incorporate, but I wanted to run this design past some people and ask if there's and suggestions or fixes that they would make? I've attached a PDF of the schematic of my current design, though I appreciate that the schematic might not be the best laid out so I apologise in advanced for that.Thank you

author: moffy
author_rank: Super Contributor
timestamp: November 02, 2023, 09:22:17 pm
content: Just a quick look at the schematic and one problem, U2A&B have 100k resistors R17, R18, R32, R20 that are connected directly to the +/- inputs of the opamps, the ones on the -ve input should not be there. The ones on the -ve input, feedback, can cause instabilities and at best case both will cause some offset issues and certainly noise.

author: NaokiS
author_rank: Newbie
timestamp: November 02, 2023, 10:13:43 pm
content: Ah, I will take a look into it. I was copying from another design but I suppose because I'm running on a single supply won't help.Thanks!

author: oPossum
author_rank: Super Contributor
timestamp: November 02, 2023, 11:02:19 pm
content: You have to provide a negative supply for the op-amp V- or convert everything to a virtual ground. Connecting V- to Gnd will not work.

author: jonpaul
author_rank: Super Contributor
timestamp: November 03, 2023, 05:55:48 am
content: Suggest that you Look uo some books on audio amplifiers you will learn something.But No custom amp needed.Parts Express /Dayton has many choices, including speaker plate amps with everything built in incl controls.Easy to build speakers and powered speakers nowadays.j

author: coromonadalix
author_rank: Super Contributor
timestamp: November 03, 2023, 06:31:22 am
content: https://www.ebay.ca/itm/203479081622?with bthttps://www.ebay.ca/itm/296022205649?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 03, 2023, 07:24:07 am
content: Quote from: NaokiS on November 02, 2023, 07:36:51 pmWhy not just select a module from eBay /AliExpress that does what you need, at low cost and already proven ?You will need good equipment and some iterations to build audio amplifiers. Not worth it for one-off.

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2023, 08:09:09 am
content: Yes you can buy off-the-shelf amplifiers, but given your PCB artwork I assume you're having some fun with this.Quote from: oPossum on November 02, 2023, 11:02:19 pmThis ^Your opamp is powered by 9V and 0v, so it can't output any signals outside this range. If your audio signal is centred at 0V then it's going to struggle.In reality it's worse than this, as the NE5532 cannot output within a few volts of its power rails. It might only be able to reliably output 2V to 7V."Virtual ground" means you move the signals up a bit, eg to be centred at 4.5V. Then you modify the opamp circuits to use this 4.5V virtual ground instead of the 0V ground -- there is nuance to this, you will probably want to test on a breadboard before committing to ordering PCBs.Adding -9V is simpler and lazier, so I approve fully

author: NaokiS
author_rank: Newbie
timestamp: November 03, 2023, 05:11:36 pm
content: Quote from: coromonadalix on November 03, 2023, 06:31:22 amQuote from: PCB.Wiz on November 03, 2023, 07:24:07 amMainly because those are too big to fit in the given space and also they generally sounded awful, had bad noise interference from the BT audio circuit. I could make them work but then I would have made something I'm not happy with and learnt nothing from it either. Iwantto learn why my design sucks, not what cheap hack boards I can use to avoid the issue.Quote from: Whales on November 03, 2023, 08:09:09 amExactly! This was a project where I wanted to learn a lot of things. I could have done a lot of things with off the shelf parts but again, I would have made something I wouldn't be as proud of and I've already learnt a ton of things from getting this far.Quote from: Whales on November 03, 2023, 08:09:09 amI'm starting to see why. For some reason I forgot that your PSU doesn't need to supply all the voltages, I can use onboard regulation for -12/-9v. I guess I was just assuming that the negative voltages were going to be more of a problem to learn about, especially as single rail designs are proving to be more difficult. Is it better to change the opamp to something better or would it be fine to keep this one?Thanks

author: Whales
author_rank: Super Contributor
timestamp: November 03, 2023, 08:58:20 pm
content: Changing the opamp wouldn't help here. You need to either add a negative voltage rail or redesign your opamp circuits to use a virtual ground.QuoteI'm not sure what you mean by this.N.B. All opamps can be used with negative voltage supplies OR 0V and a positive voltage. There isn't a special type of opamp for this, your NE5532 is fine. If you see the words "single supply" in the datasheet then it's just marketing. Sometimes the datasheet will specify the power supply in terms of 0V to +xV, other times it will specify it in terms of -yV to +xV, but they're both equivalent if the total voltage difference is the same. Where and what 0V is depends on how you design your circuit.All opamps have different limits. Check the NE5532 datasheet, but I believe it can do max 44V, so you would be OK with -12V +9 or -9 +9 or similar.Another sidenote: there are such things as "rail-to-rail" opamps that can output to within millivolts of their supply rails. These are typically used in lower voltage circuits (eg if you only have 3.3V or 5V) to make the most of what you have. Many are built on "modern" silicon processes that can't handle more than 5 or 6V anyway. Their performance will have difference compromises because of a different design of output stage.

author: temperance
author_rank: Super Contributor
timestamp: November 03, 2023, 11:39:01 pm
content: This will help you a lot:https://www.ti.com/lit/an/sloa030a/sloa030a.pdf