The BA-3 as preamp build guide

[Jpaul]

The BA-3 front end is able to act as a wonderful preamp, with plenty of gain that is changeable with just a few resistors.

Hello Jim, could you indicate which resistors would increase/decrease gain?
TIA,
John

 

[JLOP]

Hi all

First BA3 over 2 on the way. I have a doubt
- as mosfet radiators are standalone I guess I do not need electrical insulation to the transistors, just thermal compound ?

And a question about output capacitor C3 : i have electrolytics elna Silmic, is it worth audibly to Invest in polypropylene or PIO or Téflon. If yes, Nelson suggests I could keep both capacitors, what would be the benefit ?

Thanks a lot
Jean-Louis

 

[6L6]

Hello Jim, could you indicate which resistors would increase/decrease gain?

Certainly!

...from the article written by Nelson, bold is mine "The voltage appearing at the Gate of Q3 is amplified by something less than the ratio of R13 divided by R10"

So adjust those as necessary.

And a question about output capacitor C3 : i have electrolytics elna Silmic, is it worth audibly to Invest in polypropylene or PIO or Téflon. If yes, Nelson suggests I could keep both capacitors, what would be the benefit ?

Keep the Silmic and use a small value film cap for the bypass. (0.1uF or so) it's significantly cheaper than buying a 10uF exotic cap, and gives much of the same benefit.

 

[Jpaul]

Certainly!

...from the article written by Nelson, bold is mine "The voltage appearing at the Gate of Q3 is amplified by something less than the ratio of R13 divided by R10"

So adjust those as necessary.

Thanks Jim.

 

[JLOP]

+1

 

[starbender]

It seems a silly question, but I don't want to ruin anything because of my stupidity,
so I have to ask ;

I soldered all trimpots like this;

An externally hosted image should be here but it was not working when we last tested it.

Which way should I turn trimpots to make them zero? CW? CCW?

Thanks 😉

 

[Marra]

Put your DMM across the pads for C1/2 and turn whichever way ressults in zero ohm's reading.

 

[starbender]

Thanks 😉

 

[Marra]

I'm probably stating the obvious but don't forget to have P3 at it's mid position.

Good luck with your build.

 

[AndrewT]

It seems a silly question, but I don't want to ruin anything because of my stupidity,
so I have to ask ;

I soldered all trimpots like this;

Which way should I turn trimpots to make them zero? CW? CCW?

Thanks 😉

Starbender,
I like the "output" to increase when I turn an adjuster clockwise.
Suppose we are monitoring the output bias current.
An increase in that bias current using "my rule" would be a clockwise turn.

There is a difficult way to do this. Install the VR and measure the change in output for a change in rotation.
If it goes the wrong way, then uninstall the VR and solder back in after you have repaired the PCB.

Far easier to analyse the VR operation and predict which way you want the VR effect to work. Then install the VR in the orientation that gives you "YOUR RULE".

If you find that output bias current increases with an "anticlockwise" turn adjustment, then write that on the PCB for future reference.

This potential problem is down to the layout technician not thinking about operation and VR orientation !

 

[starbender]

Andrew, thanks for your excellent explanation! Appreciated.

 

[anilva]

The board works flawlessly. Biases to 45mA. No problem. Did not hear yet. Will need to remove my B1 and mount this in the preamp cabinet.

Cheers.

Finally connected up to hear the sound. Firstly it starts off with a huge hum on both the channels. I turned down the volume (20k ALPS at the input of BA3) and meanwhile fire up my PC connected to an external ODAC to play source music. BA3 bias stabilizes and when I turn up the volume there is no hum and starts playing music.

The reproduction chain is BA3 -> Aleph J -> Fostex FE206 full ranger in a back loaded horn cabinet.

The sound from BA3 (earlier I used to use B1) is incredible. Absolutely powerful, dynamic, ultra high resolution, clean highs, great mids and incredible bass. The imaging, instrument separation, depth every element is simply of a different league. To simply put this is the BEST PREAMP I have heard so far.

Now I need to figure out why the preamp starts off with a hum. While I adjust for zero offset before the output capacitor, strangely there is nearly 50mV added after the cap when I measure the DC at the output.

Also there is some element of dirty stray noise in the system, which I will eliminate by rewiring and re routing the system.

The gain of the preamp is a tad higher with the volume control playing loud music at nearly 1/4th way compared to 1/2 way that I am used to. I will change to a 50k alps and see.

I have work to do, but this preamp is worth the trouble. Highly recommended.

Cheers.

 

[Zen Mod]

search how to decrease gain of BA3 (resistor from mosfet drains to gnd, not how to change value of pot

it will give nothing , regarding gain issues , and you'll still waste precious signal

 

[anilva]

search how to decrease gain of BA3 (resistor from mosfet drains to gnd, not how to change value of pot

it will give nothing , regarding gain issues , and you'll still waste precious signal

Thanks. I have seen in one of the posts as well as NP's article on how to reduce the gain. Will do.

What could be the reason for hum in the beginning and then for it to disappear.

Cheers.

 

[Zen Mod]

did you had closed volume pot during first humming ?

 

[anilva]

No it was not closed first time I heard humming.

I will check again after I come back from work related travel and report. I want to be sure of what I have observed.

Thanks.

 

[AndrewT]

..........................strangely there is nearly 50mV added after the cap when I measure the DC at the output. ..............

This could be input offset of the receiver.

Normally we put the DC blocking capacitor in the Receiver and set up the Receiver to have no output offset.

Doing this Receiver set up this way, then allows one to connect any Source and it will not affect the operation of the Receiver.

 

[ArthurDK]

I have not been building for at while, because I really like my F5 with a BA-3 preamp.

I have F-4 parts lying around, and have just been ordering SJ109BL´s for some BA-2 frontends - from Ebay from China 🙂.

I am undecided if I should make my F4 monoblocks with balanced inputs. Speaker cable ruins the sound much more than balanced signal cable in my opinion.

Did anyone compare the sound of BA-2 and the BA-3 front ends? And with what result?

If I decide for balanced monoblocks I need to convert my BA-3 preamp to balanced output. It could be done with an output transformer. I suppose it has to be a 10K 1:1 gapped transformer, and that I can get rid of the output caps ?

Best regards
Arthur.

 

[AndrewT]

................. have just been ordering SJ109BL´s for some BA-2 frontends - from Ebay from China

When they turn ot to be fakes. Please report them via Ebay for FRAUD.

................need to convert my BA-3 preamp to balanced output. It could be done with an output transformer.................

A transformer will not pass DC to the next stage.
But the transformer will badly affected by any DC you pass into it.
Small signal transformers do not tolerate much DC.

 

[JLOP]

Hi all

I have successfully launched my BA3 to feed a F4, with a little help from Light Bulb limiter (;-) and great help from the build thread. Thanks again SL6. Everything work fine, no noise or hum, very good sound from first listening, maybe female voices are a little bit thin, I have to investigate if this is related to R13 Elna Silmic, not bypassed yet.

I have one issues though:
As I was expecting too much gain, I have set a 25kO potentiometer upfront to find the proper gain. And surprise, BA3+F4 can play loud on my 93dB HPD315 Tannoy, but not very loud and on some records it is a bit too limited even for domestic listening.
I found out that for +- 0.9 vAC I get +-9vAC so a x10 factor. Is this correct ? I have a Najda DAC and digital XO upfront and I am in 6Vrms output level, which is not small. I am using FQP3F30 Fairchild mosfets.

Thanks
Jean-Louis