The BA-3 as preamp build guide

[JeffYoung]

What determines the size of the output decoupling capacitor?

I'm looking at building a "beefed up" BA-3 preamp with a cascode for the JFETs and current mirrors between the JFET stage and the MOSFETs.

The HPA-1 is similar and appears it uses 100uF at the output (whereas the BA-3 uses 10uF). What are the factors that feed into the choice of values here? (Disregarding cost, space, etc. I'm just trying to understand the electrical side of it.)

Thanks,
Jeff.

 

[MEPER]

It depends of the load. E.g. if it is 10kohm or 8ohm load. It defines the lower cut off frequency of the "high pass" filter that is formed by the C (output cap) and the R (load) …..assumed it is almost an "ohmic" load.


You may use this calculator to see low end cut off frequency:
RC High Pass Filter Calculator

 

[JeffYoung]

Ahh... so it's just like the output coupling capacitors on the F3. So I just need to figure out what a reasonably range of input loads of amplifiers I might build are....

 

[mbedwani]

I can only get a hold of 18-0-18 50VA or 25-0-25 50VA. I already have an 18-0-18 50VA would that be ok to run two channel as it should give 23.5v to the regs, or should I buy the 25-0-25?

I will be using this LM317-337 regulator board - LM317 / LM337 +/-1.5V~37V Adjustable Dual Voltage Regulator Power Supply PCB 699917672211 | eBay

thanks.

 

[silasmellor]

With 23.5 you are limited to probably 22v or lower. Iirc, there was another person trying to run the ba3fe off 18 or 20V, might be worth hearing their experience.

 

[6L6]

If you use the bigger transformer you can get exactly the voltage you want.

 

[mbedwani]

Finding it hard to source these Heatsinks, I know I can use alternatives, however I like the compactness of these. Does anyone have a link to purchase these? Thanks.

 

[Itsmee]

I will be using this LM317-337 regulator board

I'd recomend

Super Regulator – diyAudio Store

 

[WKCox]

Finding it hard to source these Heatsinks, I know I can use alternatives, however I like the compactness of these. Does anyone have a link to purchase these? Thanks.

Looks like Aavid 7020 series. Available from Digi-Key. PN 7020B-MTG.

 

[silasmellor]

I bought 7020s for mine and they don’t fit great, rather go for these
551002B00000G Aavid, Thermal Division of Boyd Corporation | Fans, Thermal Management | DigiKey

 

[Twst]

Built my dual mono front end/pre and it sounds really good adding a little life into the chain.
Putting it between a passive volume control/source selector will give a flexible setup.

There is only one annoyance, it's adding hiss like an old tube preamp.
Based on others here, it should be dead quiet when input shorted and going into an F6 or M2 - here it is in neither.
The hiss is annoyingly loud at 1m from my 99db speakers. Not that much at the listening seat, but enough to kill the experience. My DCG3 pre does not make a sound in the same chain.

My build is standard parts after the BOM/schematics, board from the store and Fairchild mosfets with 1K pots in P1/P2. PSU is dual mono Salas SSLV1.3 shunts giving the BA3 front end dead on +-24V. Bias and offset seems fine.
Both channels produce exactly the same hiss.
Tried paralell 330ohms to R13 and 100ohms series to R5 after reading here, thinking it would be too much gain. That solved nothing.

Anyone clever? 🙂

 

[WalterW]

I had the same dual mono BA3s and it added just a little hiss, because of having way too much gain. If I see this build and the modifications you tried, i think you do everything right.
Don´t know why there is so much hiss. Try the volume control in front of the BA3 and after the BA3?

 

[Zen Mod]

just for fun , try one channel with basic 7824/7924 reg , to isolate problem - is it in regs or in active stage

 

[Twst]

Thank you for your answer WalterW.
I haven't tried the volume control after the BA3, only in front. I will try mixing it up a little!

Im thinking supressing the noise/hiss into a potentiometer would not remove it, just mask it until you turn the knob? But it makes sense if it's just gain related. How about impedances placing a pot between?

Others is saying it's dead silent in similar setups, so that kinda had me worrying.

I hope to mainly use this gain stage in front of the BA complementary when done, so hopefully this hiss I'm chasing will not matter.
🙂

 

[Twst]

just for fun , try one channel with basic 7824/7924 reg , to isolate problem - is it in regs or in active stage

Thanks! I will give it a try!
Or maybe put up a standard CRC psu - as I have the parts without looking through every box. Should't be too noisy?

 

[Zen Mod]

Try. That's the goal

 

[MEPER]

Is it "hiss" without any hum?
Hopefully it is kind of external noise that is amplified and not internal noise in the amp.

 

[Twst]

Defently hiss and no hum. Shorting the inputs makes no difference either.
Maybe I did an error somewhere and replicated to both channels 😱

 

[Zen Mod]

did you measured gain of stage?

any DVM set on AC will do , if you keep frequency down , say in range of 500Hz to 1K

no need for crazy precision , rough ratio of in/out voltage is enough

as sine source whatever you have , PC or even android app should be good enough

 

[Twst]

Just made a mess of one of my M2 blocks(easiest access), giving the pre a psu with enough capacitance to last a couple of tracks 😛

I let it warm up a little and did proper adjustments.

But the hiss is still there.

Atleast now I know its not the inhouse power 🙂