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BA-3 Amplifier illustrated build guide
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Old 26th November 2020, 08:29 PM   #611
ItsAllInMyHead is offline ItsAllInMyHead  United States
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BA-3 Amplifier illustrated build guide
Bonsai - thank you for posting this! Pasted into my files for future reference and to ensure I read your article. Brilliant.

If you want a friend, feed any animal - Perry Farrell
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Old 26th November 2020, 08:49 PM   #612
andynor is online now andynor  Norway
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It is a post anyone with these issues can enjoy, for years to come!

I encourage all builders who are unfamiliar with it to read, store, reread and learn from Bonsai’s article, so I link it yet again: http://hifisonix.com/wordpress/wp-co...ound-Loops.pdf

Last edited by andynor; 26th November 2020 at 08:57 PM.
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Old 26th November 2020, 11:11 PM   #613
Bonsai is offline Bonsai  Europe
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Thank you. Please also read ilimzn’s excellent posts which I gathered into one document here:-


Getting a class A amplifier quiet is much tougher than a class AB amplifier.

Last edited by Bonsai; 26th November 2020 at 11:13 PM.
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Old 26th November 2020, 11:38 PM   #614
Dennis Hui is offline Dennis Hui  Canada
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BA-3 Amplifier illustrated build guide
Thank you Andrew!
We are fearless amplifier builders. A little heat doesn't scare us at all.
- NP 2002
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Old Today, 03:56 AM   #615
DronEvil is offline DronEvil
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Originally Posted by Zen Mod View Post
first thing - what kind of speaker protection circuit you are using - it must be made for balanced amp (sense side ground-free)

second - with so much wires flying everywhere, my mind would go motorboating, let alone amp itself

third - post exact schematic you're using, just so we are all on the same page

I was using the diyaudio store speaker protection circuit with the 24v relays. I have now removed it from the chain until I can do more reading about how to re-work it for balanced operation.

Should the DC power wires not be twisted?

Schematic uploaded!

Sorry for the delay I got busy with the holiday!
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File Type: jpg full-schm-bal-psu.jpg (649.1 KB, 15 views)
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Old Today, 04:38 AM   #616
DronEvil is offline DronEvil
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sorry,12v* relays on the protection boards
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Old Today, 10:53 AM   #617
andynor is online now andynor  Norway
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Originally Posted by Bonsai View Post
Hello Andreas,
I'm not an expert in these things, but let’s try to improve it. You will need to do some tests to determine what kind of problem it is.

1. Unplug the amp from the mains and remove any input cables and disconnect from the speakers.

2. Disconnect the thermistor (TH1 in post 605 circuit diagram). Measure from the amplifier 0V and input socket signal ground to the chassis to confirm it is open circuit (use high resistance mode, not diode mode). You should be measuring 100's of k Ohms or higher. If you measure a short or very low Ohms, you will have an earth loop problem in your amp and need to resolve it. Reconnect the thermistor (for safety) after you have fixed it.

3. The next test is for common impedance noise. Make sure the input connector signal grounds are NOT joined together inside the amp. Short each input, connect the speakers and re-apply power. If you hear hum, there is a good chance you have a common impedance coupling problem and this means you should look carefully at how the grounds are connected at the PSU side - I assume since this is Nelson Pass design amp board, there are no problems on the amplifier module side. If you hear no noise (or very low noise), then there is a good chance you do not have a common impedance coupling issue, or that it is not your biggest problem at this stage.

4. If there is no noise in step 3, the next test is for a cross-channel ground loop. In a typical cross channel ground loop, the amplifier is quiet with only one input channel connected, or no inputs connected, but when you connect BOTH input channels, then you get hum. To test for this, take a standard 1 metre phono cable and connect it from one input to the other so it forms a loop outside the amplifier. You should hear no noise on a good amp. If you get hum (but none with the cable disconnected both ends), its likely a cross channel ground loop and it is arising inside your amp due to the transformer mag field intersecting a loop formed by the signal ground connection from the one input, through the module, through the 0V power connection to the PSU, out to the other module via the same route and around the external cable. To overcome/reduce this issue, check you using hum breaking resistors (HBR) - see pages 30-41 in the Ground Loop presentation (link below in my signature) and then check point 5 below.

5. Have you routed the input connector to amplifier module cables correctly? The shortest connection is not always the quietest (See pages 61-64 in the Ground Loop presentation). I usually mount these next to each other not separate on either side of the rear panel (although I did this on my earlier amps). You do this to minimize the loop area between the channels. If you join the grounds together where the connectors come into the amplifier, you trap cross channel ground loops inside the amplifier. If your input connectors are on either side of the rear panel and you are running the input cables directly to each amplifier module, you will be creating a very large cross channel ground loop area and you will not help the situation by joining the grounds together on the input sockets in that case. Only bond the input grounds together if the connectors are located right next to each other.

6. This is a class A amplifier with a heavy constant current draw. The transformer will be radiating big magnetic fields compared to a class AB amplifier even with no input signal. If you are still getting hum and all the above is correct, I would lift the transformer out of the chassis a few feet away and run twisted cables to the rectifiers to see if it solves it. If it does, it’s a general mag field problem and you then need to consider a better transformer - hopefully you can avoid that though. For class A amplifier, I always recommend an oversized toroid, with a GOSS band and a pri-sec screen for the reasons I mentioned above.

7. Ground lifter: I usually use a 35A chassis mount rectifier - see page 25 in the 'Ground Loop' presentation. I have not tried the thermistor method, but suspect it works more like an HBR than a true ground lifter. Note on my commercial products I do not use a ground lifter. The amplifier 0V goes straight to the chassis and is bonded to the incoming power safety earth (I do this for safety reasons).
Greetings, Bonsai!

I finally had some time to perform the tests. I believe I have a preliminary conclusion, which in any case will require some guidance.

Results/comments in same order as you described:

NB: all tests done with transformer out of chassis. There was no change in hum or noise levels with or without shorted inputs taking the transformer out, and twisting huge gauge interconnects between rectifier and PSU.

1: Done

2: Only unreadably high resistances measured between 0v and chassis, and signal return and chassis with NTC decoupled. Same goes for speaker returns. Conclusion: No ground loop there.

3: shorted inputs, and still the same hum. Only difference with shorted inputs, is less hf background noise. Conclusion (preliminary): I probably have a common impedance issue.

4: I performed this test even though test 3 was positive. Test 4, inputs connected together with one RCA cable, actually [B]increased hum. Also, there was a little bit increased hf noise.

5: this chassis back plate have pre-drilled holes for Neutriks, and the inputs are therefore on opposite sides. So I probably have some cross channel looping, but doubt that is the main problem, since it persists completely regardless of transformer being in or out of chassis.

6: Taking the transformer out and moving it several feet away, provided no change what so ever.

7: Ground lifter against AC loops will not solve my main issue anyways, so I’ll wait.

Conclusions (preliminary):

1: I have a probable common impedance loop. It is possibly exacerbated by a mild cross channel loop.

What in my amp can be causing this? I have some hypotheses, but not sure how good they are:

1: Long interconnects from rectifiers, and using quick connects instead of on board soldering, increases impedance and exacerbates the problem.

2: I have three (3!!) junctions between neg and pos cap bank 0volts at the star point (visible in pic on post 585). In yours and others articles, it is specifically said there should be only one such junction connection.

3: my NTC is connected at this point, but my other ground connections are an inch or two away, on the neg 0v side of the PCB. Plus one of the speaker returns on the other.

4: The lengths of the GND return wires from the gain stage, are of minutely different lengths.

5: Input wires are not shielded.

What do you think is / are my problem(s)? Is it my star or T arrangement messing it up? Is it a combo? Is the tranny still a possible culprit (cap coupling pri/sec), or is that now completely ruled out?

So I plan to do the following, but need to know if the plan is good:

1: Move rectifiers close to the PSU pcbs, and solder connections.

2: Solder trannys to rectifiers

3: Remove all but one junction between cap bank 0v’s. But is one small 16-20 AWG wire thick enough for that connection?

4: Redo general grounding scheme at the star. But it is quite the same as 6L6s in post 1, except for him having 3 junction connections at the rectifier input side of the PCB, and only one at the star side.

5: cut gain stage returns to equal lengths. Also, decrease wire gauge to gain stage from 16awg to half. It does not require much power, and cables are now so thick they are a hassle to twist and work with (solder and so on).

6: Redo/perfect the rest of the wiring, twisting it all properly. Wiring right now is preliminary as I ame testing things to get rid of this hum. But even when wires were twisted and mor properly done, the hum was there nonetheless. But might help other more hf issues though.

7: Move gain stage/FE to back plate, to as far as possible do this the same way 6L6 did. Not expecting change in hum

8: flip heatsinks. Not expecting change in jum

I want to repeat one important thing: The only test that had provided change in hum levels, is moving FE returns to different grounding points.

This was a lot of text, and I am sorry for that. I have attached a photo of the testing sequence.

Attached Images
File Type: jpg AC7CEB47-8D9B-4137-AC10-822856651359.jpg (1.02 MB, 5 views)

Last edited by andynor; Today at 11:04 AM.
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