The BA-3 as preamp build guide

[Dneu2011]

Yes. I will need to start diagnosing. I checked the input diodes and all of them match and pass the diode test. Regulators both test fine.

I re-flowed all solders on the side giving me issues but did not have any luck.

 

[6L6]

Can you get any voltage to be even on both rails? The circuit will run beautifully from 15 to 24.

 

[Dneu2011]

Yes. I could easily get 22.75V to 23V on both rails. If that’s fine then I can even both sides at the highest possible matched voltage.

 

[6L6]

+/-23v it is!

Carry on.

 

[cubicincher]

to Dneu2011 #1543

Hello Dneu2011,


I would adjust your regulated powersupply to +/- 22.75 Volt. Without the BA-3

preamp hanging on the PSU. First try to get a nice `balanced`or symmetrical
voltage out of your powersupply.
Then hang your BA-3 Preamp on your powersupply and start the adjusting- /
biasing procedure with the pots on the board. (I think you read how it is done?)
Pot 3 into midposition. Bringing it slowly up to `life`with pot 1 and pot 2 (on each channel).Will take some time!!! Patience....

My BA-3 Preamp is running on +/- 24 V up to +/- 32 V perfectly (I made a PSU with a C-L-C - filter followed by LM317/LM337 regulators - so I can adjust the voltage in a pretty wide range).

Still one of my most preferred Preamps!


Have fun and success!
If you listen to it - you will forget the `troubles`



Greets
Dirk

 

[margol]

I am planing to build the preamp with the BA3 front end, but I also want it to act as a headphone amp. I believe that I need buffer for this. Do you have any suggestions? What buffer do you recommend?

 

[Alfetta87]

Voltage accross R10 - R11

The same - something close to +/- 24vdc.

For R10 and R11 I have 20 ohms, not 22 (couldn't find any).
the voltage drop should be higher than 1V, what would be ideal?

I have exactly +/- 24Vdc.

- dan

 

[6L6]

20 vs 22 ohms is an inconsequential difference in the circuit. Bias to 1V.

 

[Alfetta87]

20 vs 22 ohms is an inconsequential difference in the circuit. Bias to 1V.

Thanks! Done already. Changes all the time, but after some hours it becomes more consistent.

On your build guide it says I should put a DMM in VDC across R12 to check the offset. It's 0.000... and doesn't change at all.
Output V after the cap is 150mV.

Any concern, or can I leave like that?

- dan

 

[6L6]

Yes, there is no temperature compensation in the circuit, so it does change with heat in the chassis.

Not across R12, instead measure offset from the node of R12 and C3 to ground. That will show offset in the circuit before the output cap.

 

[Itsmee]

I am planing to build the preamp with the BA3 front end, but I also want it to act as a headphone amp. I believe that I need buffer for this. Do you have any suggestions? What buffer do you recommend?

Build a mini crippled F4.

 

[margol]

Build a mini crippled F4.

That's a nice idea! And what about driving the headphones from full-sized f4? Are there any cons?

Should one place a load resistor in parallel with headphones in that case?

 

[thompsontechs]

Looks like the Tubecad.com Select-2 is not in stock... anyone have an equivalent?


Thanks



JT

 

[6L6]

New AC Selector Switch

 

[Hikari1]

That's a nice idea! And what about driving the headphones from full-sized f4? Are there any cons?

Should one place a load resistor in parallel with headphones in that case?

Cons are noise and the ability to destroy your headphones and hearing.

I'd use the output stage of the "Whammy" headphone amp.

 

[thompsontechs]

New AC Selector Switch

Thanks 6, but I got that, the one they didn't have was the Select-2 not the AC switch. The signal input...


JT

 

[margol]

I'd use the output stage of the "Whammy" headphone amp.

Can it work outside of the feedback loop?

 

[Hikari1]

Can it work outside of the feedback loop?

Sure. It’s a mini f4 with a different bias scheme. I do not know if it will be dc stable though.

The ba3 cannot drive headphones because the output impedance is too high. If we make it with feedback it lowers the output impedance and it becomes a mini f5 which is the subject of quite a few headphone threads.

 

[6L6]

Thanks 6, but I got that, the one they didn't have was the Select-2 not the AC switch. The signal input...

Aha... the select-3. When you talked about -2, I though you were wanting the AC switch.

How about this— comes with an attenuator, and input switching for 3 sources. I’ve found the fine/coarse setup in John’s attenuators to be wonderfully useable. Much better then you’d think, they aren’t awkward at all. FrontPanel

 

[thompsontechs]

Aha... the select-3. When you talked about -2, I though you were wanting the AC switch.

How about this— comes with an attenuator, and input switching for 3 sources. I’ve found the fine/coarse setup in John’s attenuators to be wonderfully useable. Much better then you’d think, they aren’t awkward at all. FrontPanel

Thanks 6, I ordered it... I was trying to keep the cost down, as all this and the F5T are a Christmas present.



What chassis, currently available, would you put it in. DIY store is good, but other options welcome too. Also, input from other DIY members welcome.


JT