The BA-3 as preamp build guide

[Attila I]

@mbedwani,
in Walters picture and also in one of the first pics from Nelson you see a "PWA Claritycap"
PWA audio capacitor from ClarityCap

good quality/reasonable price
and they fit in the provided space.
have fun
A.

 

[Dirk1]

Hello,
I run my BA3 on a test basis in an old case with 50 KOhm volume control and a L'Amp 193V as power amplifier.
Are the 50 KOhms of the volume control ok, or is another value better?
I would like to build myself a new housing with an optimal volume control.

Greetings Dirk

 

[PiotrL]

Hi Guys, one more, Looks like it’s hard to find good 10µF MKP caps for “C3”, can someone please point me to a link for some not too ridiculously expensive caps, Mouser and Digi-key etc. don’t seem to stock any axial ones. Also what is their use case in this circuit, to me is seems they are being used as a HighPass filer i.e. filtering say above 20khz, am I right?

Mark.

Here is Mundorf : Mundorf MCAP 400V Capacitor. 10μF - Audiophonics

 

[WalterW]

Hi Walter,

Thanks for clarifying

A few questions regarding your build, (Very nice by the way). Why did you use two SuperRegs, I assume you could have just used one?

Also what is that Bridge Board you used, I can't make out the name and are they still available?

Mark.

Hi Mark, i used two SuperRegs to have 'dual mono', ultimate quality Most of my power amps are also 'dual mono', that's why I used two.
But one is indeed also possible.
The bridge/capacitor board is from member Salas. You can ask him if he has some PCB's left over.

 

[mbedwani]

Thank you everyone for your replies

 

[kpsthakur]

Dear All, I was just about planning to integrate BA-3 pre with my upcoming F6 project, so I ran the bias and DC offset on the board. I am taking DC offset readings between R12-R13 and GND which stabilises at around 10mV after an hour or so. BUT, i just randomly tried checking the DC Offset after the Caps i.e. point D and GND, which should ideally be 0mV (considering Caps filter DC), the reading is a bit more around 30mV DC....any thoughts? This is consistent on both channels.

 

[6L6]

How are you measuring?

 

[ggetzoff]

Dear All, I was just about planning to integrate BA-3 pre with my upcoming F6 project, so I ran the bias and DC offset on the board. I am taking DC offset readings between R12-R13 and GND which stabilises at around 10mV after an hour or so. BUT, i just randomly tried checking the DC Offset after the Caps i.e. point D and GND, which should ideally be 0mV (considering Caps filter DC), the reading is a bit more around 30mV DC....any thoughts? This is consistent on both channels.

What process are you using for bias and offset adj.? Are following ZMs process on page one?

 

[kpsthakur]

How are you measuring?

For DC offset Multimeter on mV DC, +ve lead on R13 the 3W resistor the outer most edge, -ve lead to GND.

Now since this is before the Caps, I tried to confirm if they are filtering DC away and the DC reading between the output pin listed as D and GND is 0mV, which is not the case

 

[kpsthakur]

What process are you using for bias and offset adj.? Are following ZMs process on page one?

Yup, the bias setting is very good almost stable at 0.955V and DC offset before Caps now swings between 4mV - 6mV DC.. The Pre sings very well...

 

[silasmellor]

ZM’s advice for this was to add a resistor between the signal and gnd positions while setting bias.

See https://www.diyaudio.com/forums/pass-labs/258022-ba-3-preamp-build-guide-88.html#post5359886

 

[Hook]

Can the WHAMMY (headphone amp) power supply be used for a BA-3 Preamp? It is 15VA or 25VA 15+15 to 22+22. It is to the best of my limited knowledge a regulated power supply.


I'm looking at building a preamp and am considering either BA-3 or WHAMMY. Having a power supply that is already designed, has parts list would help my level of comfort with going with the BA-3. This will be used to power an Amp Camp Amp, and hopefully later an F4, or other first watt type amp.



Thanks, Jim

 

[silasmellor]

Yes, you can also use other LM78/79 refs to reach 24V. E.g LM78/7922 with the LED ground reference will give you near 24v. You may want to increase the amount of filtering as well. You need something like 20V primaries at least to have high enough DC that the refs can drop some.

 

[kpsthakur]

ZM’s advice for this was to add a resistor between the signal and gnd positions while setting bias.

See https://www.diyaudio.com/forums/pass-labs/258022-ba-3-preamp-build-guide-88.html#post5359886

Humm, what do we mean by bleeder resistor...should I short the output and GND using 100K resistor and measure across...and does this has to be kept permanent or is it for setting DC offset purposes only. My BA-3 is driving output so not sure if this required...but the offset still exists

 

[Zen Mod]

if you have output stage connected , observe strictly OS output offset , not output node of FE

setting proper voltages on FE still applies

 

[Hook]

Yes, you can also use other LM78/79 refs to reach 24V. E.g LM78/7922 with the LED ground reference will give you near 24v. You may want to increase the amount of filtering as well. You need something like 20V primaries at least to have high enough DC that the refs can drop some.

Thanks, for your advice. I take it you are referring to the U2 and U3 regulators in the WHAMMY PSU which are 78xx and 79xx devices. Am I understanding you correctly, are you suggesting that by changing to different regulators this power supply can be increased from 22v to 24v, which would be more optimal for the BA-3.



Also, is the filtering you are referring to the filter capacitors in the PSU? Are you suggesting by changing the values of these the amount of filtering can be increased? Sorry, this is at the edge or beyond my understanding.

 

[silasmellor]

Hook, yes and yes others might have more qualified advice for how much but 6x4700 is probably loads. Of course this becomes a bit more involved if you’re new to this, but as I understand it, the whammy psu is very good for the purpose.

 

[Hook]

So increasing the 6 large capacitors from 3300uf/35v to 4700uf. (C6, C11, C14, C15, C18, C19).



Still at 35volts?


Thanks!

 

[kpsthakur]

if you have output stage connected , observe strictly OS output offset , not output node of FE

setting proper voltages on FE still applies

Hi Zen, what is OS output offset. I am just using the BA3 FE, to be coupled with F6. I want to tame DC offset for BA3 before it feeds into F6. Do you mean to set offsetting after Caps of before it?

 

[Hikari1]

Hi Zen, what is OS output offset. I am just using the BA3 FE, to be coupled with F6. I want to tame DC offset for BA3 before it feeds into F6. Do you mean to set offsetting after Caps of before it?

Zen means "Output Stage". You are not using the output stage if you are building as a preamp.

Adjust for zero offset before cap on preamp (or as close to zero as you can get, it will drift). Make sure there is no DC after cap on preamp (test at RCA jacks). Plug into F6.