BA-3 As Preamp

[ammel68]

Essentially yes. The Jfets self-bias, as based on their Idss, and the Mosfets bias via the pots.

You an change the bias of the Jfet by changing the source resistance, but it's easier to just use them in the proper Idss range - anyting from 7.0-11.0 will work with no issues.

So if you have Jfets with a Idss of say 6.5mA for example, does that mean you will automatically read 65mV across R3 and R4?

If 8-10mA is recommended for passing through Q1 & Q2, shouldn't you be reading 80-100mV across R3 and R4?

Is it even possible to bias a Jfet with a Idss of 6.5mA to 8mA by lowering the source resistors below 10 ohms?

 

[Zen Mod]

whenever you introduce resistor in Jfet's source , while prior to that - gate and source were on same voltage potential , result is less current then Idss of part

just take Jfet as small pentode ....... or read this , for instance Junction Field Effect Transistor or JFET Tutorial

 

[impuls]

I just finished my BA-3. I'm using 220 ohms for R13 and still have almost too much gain. You may want to try 180 ohms.

Look at post 495

 

[ammel68]

Look at post 495

OK, I did read post 495. I'm not sure what you mean by "out to ground 170 ohm".
Are you referring to R12?

The resistors are all numbered in the schematic, why not just state which resistors you changed like R10 & R11 to 65 ohms, R13 to 200 ohms, etc.?

I assume your combination resulted in much lower gain?

 

[impuls]

R10, R11 65 ohms, R13 170 ohms, R6, R7 200 ohms.

 

[ammel68]

Thank you for clarifying which resistors you changed.

 

[6L6]

Check this out -- Very nice writeup of a whole club who made the BA-3 FE their reference preamp.

https://sites.google.com/site/audiosocietyofminnesota/Home/diy-projects/pass-ba-3-preamplifier

 

[vdi_nenna]

Check it out! Wow! That's so cool!

 

[Zen Mod]

sissies

 

[6L6]

Vince - I also placed the 47K resistor in the R2 position and it got rid of the stepped attenuator clicks & pops. Thanks for mentioning it!

 

[vdi_nenna]

Vince - I also placed the 47K resistor in the R2 position and it got rid of the stepped attenuator clicks & pops. Thanks for mentioning it!

I didn't didn't that. I don't have it in and have no pops or clicks.

 

[Hikari1]

I mentioned it. I thought i was going to have to get a new ladder pot, the thing was so noisy. But adding the resistors solved the problem.

 

[6L6]

Oh, sorry... I knew it was one of you guys...

It's a heck of a nice preamp stage. I would love to try it with a shunt regulator - that seems like the next logical step, but it really is fine as-is.

 

[ammel68]

What's so special about a shunt regulator vs. a regulator like the LM317/337?

Thanks...

 

[Hikari1]

What's so special about a shunt regulator vs. a regulator like the LM317/337?

Thanks...

Sounds like I am going to let 6L6 find out before I try the shunt reg. C'mon 6L6, we all got our BA-3s built now we need to tear them apart again!

Seriously, I have been looking at the Salas shunt:

http://www.diyaudio.com/forums/power-supplies/153484-using-hypnotize-general-shunt-reg-pcb.html

And the kubota:

Kubota Low Noise Regulator Full Blown Version Kit | eBay

And this shunt reg:

Low Impedance 500mA Class A Shunt Regulator Kit | eBay

But I just bought a Rega P5 so I'll probably play with that for a while till I get some motivation to work on the BA3 again.

 

[Zen Mod]

What's so special about a shunt regulator vs. a regulator like the LM317/337?

Thanks...

you can take shunt reg as A class amplifier , while (any ) series reg is B classs amp

one of important things is that shunt reg is bidirectional in nature - it can sink current , same as giving it

take a look at Cook Book ; if not much about general characteristics of shunt regs , something is written about operation of one

 

[brianco]

ZM

Think there would be less room for misunderstanding to say:

Shunt is first quality amp and series reg is second second (or lower) quality amp (in this application)…….


or have I misunderstood you?

 

[Zen Mod]

it depends of what you prefer

I know I prefer A class amps

so , taking that in account ..... I chose speakers easy to drive , so no need for drekload of watts

I could drive mine with beefed ....... or tube WOT preamp

 

[brianco]

I was able to drive mine quite well with an ARS preamp…so there!

 

[6L6]

Class-A amps have all the power running all the time and release it to the speakers. Class-AB have to modulate the power from the PSU.

It's the same for Shunt regs vs. series regs.

(Yes, the descriptions of operation are paraphrase, but you get the idea...)

But I just bought a Rega P5 so I'll probably play with that for a while till I get some motivation to work on the BA3 again.

Sounds like you need to build a Pearl.