Burning Amp BA-3b (Balanced)

[Dennis Hui]

What Idss jfets are you using? Can you measure the voltages across R6 and R7?

 

[Zen Mod]

see

for practical setting, you need just two DVMs , connected for 1 and 2

when these two are set, coincidentally 3 is very close in value to 1

 

[cubicincher]

Hello Dirk1,

you have a large DC-offset at the output of your BA-3.
No wonder, if I see the different bias -values of the N-channel and the P-channel.
First confirm, that your Pot3 is set to mid-position.
The biasing - procedure is described in the build guide.
First turn Pot2 till you get a reading (voltage) over the bias resistor R10, then turn Pot1 till you get a similar (voltage) reading over the bias resistor R11.
In my opinion most important: keep your DC-offset at the output low (below 100mV) during biasing up. The target is to get this DC-offset as close as possible to 0 mV.
How to measure DC-offset shown in the following pics (I think you measured over the outputresistor - which is wrong - your red dots in your schematic).
Cheers
Dirk

 

[Dirk1]

Hello,
sorry that I am only now getting back in touch.
Thank you for your help.
I tried both tips, the result is the same.
# ZEN-MOD, worked great with two measuring devices.
I only connected the 3. meter because of all values.
Maximum values measured above R10/R11.
P1 Maximum.

P1 = 326.2mV
P2= 324.5mV
DC Offset 0.03V
P3 = center

Can it be that Q1/Q2 has a low IDS?

 

[ItsAllInMyHead]

See post #841

 

[Dirk1]

R6= 6,91
R7= 6,83

Q1= 2SK170 / Q2= 2SJ74

Q3= 2SJ313 / Q4= 2SK2013

 

[Zen Mod]

increase P1 and P2 to 1K

set to zero before starting again

confirm zero with ohmmeter, do not risk with CW and CCW fairytales

ref. to schm in post #1

 

[Dirk1]

Thanks Zen-Mod,
have ordered 1K Ohm trimmer, contact me when I have installed it.

 

[Zen Mod]

no worries, I'll call as soon I spot you got them

 

[ozorfis]

R6= 6,91
R7= 6,83

The voltage drop across R6 and R7 should read around 0,8V. This indicates that there is sufficient current running through the jfets to bias up the mosfets with a 500 Ohms pot.

 

[Dirk1]

Hello,
I swapped the pots and was able to adjust everything.
Thank you once again for your help.
Dirk

 

[Dneu2011]

Quick question when setting bias and dc offset for the BBA3. Can each board have one side of it done at a time with only 3 DMM or is it important to have 6 DMM with both sides of the board done together?

So does tying R5 together affect the opposite side when adjusting?

 

[ozorfis]

With the inputs shorted I've set each channel in single ended mode with 2 DMMs and then switched to balanced mode, because I was of the impression that tied together via R5 one channel did indeed affect the other. To make switching easy I have those small DIP-switch boards in the attachment installed. Credit for those boards goes to ItsAllinmyHead and the original designer whose name I sadly forgot. This being DIY I am sure he is happy with you having those Gerbers wherever he is.

 

[Dneu2011]

I set the bias and offset of the preamp last night. The bias and offset was not affected when adjusting one side of the channel vs the other. Haven’t been able to test yet but hope to tonight.

That dip switch is very cool, but in what scenario would it be useful? I understand the BA3 can take single ended sourced by tying pins 1 and 3 together with adapters.

 

[ItsAllInMyHead]

With the switches, you can very quickly go back and forth from a balanced / differential monoblock to a stereo amplifier.

Edited to add - for a balanced pre-amp in a single chassis with input switching etc, the number of use cases is likely more limited.

 

[Dneu2011]

Oh yes. I forgot the thread is mostly for the BA3 amp.

I built the preamp to drive two F4’s as monoblocks.

 

[ItsAllInMyHead]

In cooking terms ... a BA3b deconstructed.

That will be awesome! Can't wait to see how it all turns out.

 

[maltboy1]

I'm looking for help with the front end for a BA-3b with 32V rails. There's not enough room on the PCBs for the SILMIC 35V caps. What are my options to fix this?

 

[ozorfis]

You can squeeze them in and leave a bit longer leads on them. Have seen it quite often - looks only slightly ugly.
You could mount one on top and the other below bent horizontally or so.
You can buy fitting parts with more headroom regarding voltage rating too 1000uF50V.
I'd probably buy the fitting part, because I like a bit of headroom.

 

[6L6]

Use smaller caps.