The BA-3 as preamp build guide

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#2,761
Giovanni1968 said:
Of course matter of laying it all out …

View attachment 1095555
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Or, mount the toroids vertically…
I believe you then would have space for even a metallic shield between PSU and signalstuff…

[ironic mode]concerning feet, why not a bit of baroque?[/ironic mode]
 

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#2,762
Dennis Hui said:
Chip,

To measure the bias, you measure the voltage across the resistors R11 or R10 individually and not from R10 to R11 (as you seem to have indicated in your photo).
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Dennis,

First, thanks.

Second, duh!
Senior moment?
Sudden drop in IQ?
Young man in a hurry?
I could go on.

Because I had already reset the pots, I couldn't determine if the weak sound from the right channel was due to a bias change.

I've rebiased both channels and its working like a champ.

If the symptom returns, I will save myself some time, next time......

Thanks again,

Chip
 
#2,763
Chiptech said:
Dennis,

First, thanks.

Second, duh!
Senior moment?
Sudden drop in IQ?
Young man in a hurry?
I could go on.

Because I had already reset the pots, I couldn't determine if the weak sound from the right channel was due to a bias change.

I've rebiased both channels and its working like a champ.

If the symptom returns, I will save myself some time, next time......

Thanks again,

Chip
Click to expand...
Well, that was premature.

I tried playing thru a M2X amp instead of my ANK EL34 integrated amp, which has a preamp in it, and I clearly hear the weak right channel. Also hear it when I reverse the speaker connections.

My bias setting is holding., that doesn't appear to be the cause.

Something else on the amp board?

I'm wondering if my Khozmo attenuator is the source?

Issue with the Khozmo 5V PSU?

Issue on the Khozmo CB? Doesn't seem very likely.

I have a 2nd amp board built and could swap it in, but I'd appreciate all ideas, in case I'm missing something.

Thanks,
 

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#2,764
Can you flip the Khozmo channels to the ba-3? If the weak channel flips, then the Khozmos channel is weak.

Also when testing with the amplifier, switch preamp to amplifier, not flip speakers. Flipping speakers does not tell anything about the preamp. If you swap preamp interconnects to amplifier, if the weak channel follows the switch then the preamp channel is weak. If the weak channel stays, then the amplifier channel is weak.
 
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#2,766
I just need to flip the connections from
Chiptech said:
Ben,

I will try that out.

Thanks. Also, I wasn't clear, I meant to say I flipped the preamp to amp interconnects.
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I switched the outputs from the Khozmo board so left out went to right in on the amp board, and right out went to left in..

I still hear a dominate left channel.

Guess it's not the Khozmo board.

What part(s) would fail and cause this issue?

Time to swap out the amp board for a new one?
 

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#2,767
I'm not absolutely sure what else you have done. But here are all the tests I would do:
  1. Swap the outputs from the preamp to the amp. So pre L goes to amp R. Leave everything else alone. If the problem swaps, then the problem is in the preamp. If not, then put that back the way it was and...
  2. Swap the speaker outputs from the amp. If that changes, then the problem is in the amp.
The idea is to isolate each component successively.
 
#2,768
And if switching chans doesn’t help, review all your resistors - correct value in correct spot. Also, center your P3’s, and please confirm the JFETs are genuine and matched as they should.

You are not the first with this issue on a BA-3 board, though the solution escapes me right now.
 
#2,769
Also check the interconnect between the preamp and amp. For example, if originally IC1 connects preamp left-out to amp left-in and IC2 connects preamp right-out to amp right-in then switch the interconnects so that IC1 connects preamp right-out to amp right-in and IC2 connects preamp left-out to amp left-in.
 
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#2,770
Dennis Hui said:
Also check the interconnect between the preamp and amp. For example, if originally IC1 connects preamp left-out to amp left-in and IC2 connects preamp right-out to amp right-in then switch the interconnects so that IC1 connects preamp right-out to amp right-in and IC2 connects preamp left-out to amp left-in.
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I did not change the interconnects from the pre to the amp earlier today. Duh!

I just did, and now the weaker side is on the left, not the right.

That would now suggest a problem with the Khozmo board?

Any other possibility before I reach out to Arek at Khozmo?

And thanks for all the help.
 
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#2,773
I don't think it's the interconnects. I have 2 BA-3 preamps. With my 2nd one out of commission, I use my first one and everything is normal. No issues in balance.

The only difference between the 2 BA's is the 2nd one with issues uses the Khozmo with remote as shown above.

The 1st BA uses a Khozmo attenuator volume control. No remote.
 
#2,774
How about doing some measurements to isolate the component where the issue occurs? Do this just on the preamp.

Get a (stereo) sine wave, say 60Hz, close to full range if you can so the signal is fairly strong, play it through both channels. Now
measure and compare the AC voltages for both channels at every 'connection' point, starting from RCA in, volume board in,
volume board out, BA3 in, BA3 out, RCA out . Write them down and that should help you pinpoint where things go funny...
 
#2,775
Always up for a new challenge. I've been searching for sine wave files that I can play on my digital music player and feed into the RCA inputs on the BA.

So far, no go. I found Audacity, but that doesn't appear to be a quick on ramp for where I want to go.

I'm looking for a download of a 60Hz stereo sine wave full range in FLAC or Mp3 format.
 
#2,777
Chiptech said:
Always up for a new challenge. I've been searching for sine wave files that I can play on my digital music player and feed into the RCA inputs on the BA.

So far, no go. I found Audacity, but that doesn't appear to be a quick on ramp for where I want to go.

I'm looking for a download of a 60Hz stereo sine wave full range in FLAC or Mp3 format.
Click to expand...
I appear to have solved my channel imbalance issue.

Turns out it was a poorly installed RCA socket.

In the midst of all my surgery attempts to sort it out, I somehow intuited that was the problem.

Often it is the simplest of possibilities….My mind usually goes to the worst-case situation.

Anyway, happy ending.

Thanks for the support along the way.
 
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#2,778
6L6 said:
Aha! Yep, you're not setting it correctly.

(And looking at my post on setting things, I think I see the confusing bit... Sorry...)

This circuit is a teeter-totter, and it's not as simple as P1= bias and P2= offset. They work together to get the job done, and are both responsible for bias and offset.

But, before that, let's state the goals of adjusting the pots -

1)You want to measure 1.0VDC across both R10 and R11

2)You want to set that in conjunction with 0VDC (or as close as you can get) measured from the "top" of the output capacitor to ground. (I.E., the input side of the cap, as the cap should always block the DC on the output.)

It's best to do this with 3 meters, one across R10, one across R1, one from the cap input to gnd.

Stating with the P1 and P2 to zero (And P3 set to it's middle and then ignored for this entire procedure)

Increase each pot an even number of turns until you see some voltage across the resistors.

Decrease or increase one of the pots (doesn't matter which) to zero the offset. -- this will take a little of the bias voltage away; that's normal.

Increase the pots again so you show more voltage (go in 0.2V steps or so) and then zero the offset again. It will feel like two steps forward, one step back, but again, that's normal.

Continue until you have 1.0V across R10 and R11 with zero offset.

Check your PM
Click to expand...
Thanks for this...

Pete
 
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