The BA-3 as preamp build guide

[codyt]

Intermodulation product? Spurious signal is alternating upper and lower sideband from the harmonic. Sure the 1KHz generator is clean?

That’s a good point - I’ve been using the REW generator. I’ll try a different source.

 

[paulmarinis]

hello , i have finished building the ba3 preamp , and i am at the setting the bias process.
what is an acceptable value for dc offset?also mine is changing by 0.1 to 0.2 volts is that normal?
also what is the advisable value for the potentiometer?I see that the original poster 6l6 used 50K but from i undestand alot of people are using 10k.

 

[6L6]

Offset measured before the output capacitor will drift, particularly if there is no load on the output. This is normal and the reason the capacitor is part of the circuit.

10K pot is fine.

 

[ColinA123]

From my experience, (and from what I have read others found the same.)
The BA3 pre is very sensitive to temp when biasing.

You need to close the case and let it warm up for some time, they get in quickly to set bias. Do one side at a time, letting it stabilize between sessions.
It will be worth it.

 

[paulmarinis]

Hello, thanks for your repplies.
I noticed this, i spent half a day doing just that for one side,closing the lit, then trying to get to 0, then closing the lit again, but the best i could do is somewhere between 0.1 to 0.2 v, is that ok?it does sound nice though

 

[Russellc]

Hi!

I think I'll settle down on the BA-3 for nursing my F4...
It's clear that this is a great combo, but I am confused about the difference between the BA-3 as line-stage or as pre-amp...

as a line-stage, it hasn't no attenuator/selector, is that it?

The other question: What would be a good (good as in as good as reasonably feasible) PSU for it?

Thanks!
david

Same thing. It's the front end of a BA3 amplifier, which alone makes a great linestage. As near as I know it is same as a "preamp" as well, but many units labeled preamp also have a phono stage.

BTW, it is a great preamp, linestage, control amp whatever. Along with output stage it makes a great amplifier.

Russellc

 

[myleftear]

Same thing. it is a great preamp, linestage, control amp whatever. Along with output stage it makes a great amplifier.

Russellc

[emoji1303]

 

[Russellc]

From my experience, (and from what I have read others found the same.)
The BA3 pre is very sensitive to temp when biasing.

You need to close the case and let it warm up for some time, they get in quickly to set bias. Do one side at a time, letting it stabilize between sessions.
It will be worth it.

Yes, and any breeze, breath etc. We will not discuss my attempt to adjust the first one I built while sitting under a ceiling fan.....

Russellc

 

[matchstick20]

Hi there - I have a question regarding C3's orientation while biasing. The Audience XO caps (10uf 400v) I purchased were much larger in size than I anticipated, and while I was able to mount them to the board, they currently sit vertically vs horizontally and are ziptied to C1 and C2 for stability.

Now I don't think that should be an issue - however, with this unorthodox position, I'm left with no space to attach leads to the input of the cap or resistor. I am though able to get at the resistor if I cut the zip-ties and let them "hang".

My question remains - if i go about biasing with them in the hanging position, and then i proceed to re-zip tie them upright, will that cause any changes to the biasing?

 

[McQuaide]

I don't see how this could affect biasing.

 

[matchstick20]

Hey all - so I just finished assembling my BA-3 (first diy project) and power supply is running with 24+/- on all rails (amb sigma 22). However I’m running into a bit of an issue with biasing. I have 2 meters across R11 and R12 set to DC V (third is arriving tomorrow but I wanted to just see what the whole process is like). I power it up and I begin to turn the pots counter clockwise (they were set to 0 ohm previously), and I am not getting any rise in voltage. I then went to check the output rca and I’m seeing about 120mv... I tried this again on the other side and yet again, no gain of voltage... so I’m not entirely sure what is happening. Can’t post a picture right now, but will tomorrow.

 

[matchstick20]

To follow up, I’ve read that if there’s dc after the output cap, a bleeder resistor should work. However, that doesn’t solve why I’m not getting any rise in voltage across R11 and R12 on either channel.

 

[Russellc]

On all three of my BA3 builds, I started with the pots in the middle, as stated in build guide. Maybe you haven't turned them enough? P3 should definitely be set to the middle of its range to start. Mine were all at middle setting and I remember turning them quite a bit before "effect."

Also, the build guide is a little confusing for some as to measuring the offset. It is measured from the top of the cap and ground. Well, it confused me anyway maybe I'm easy to confuse.

I have no idea about "if there is DC after the cap, a bleeder resistor might work."?

Oh, don't adjust this under a ceiling fan like I did. Or breath on it while adjusting.

Russellc

 

[matchstick20]

False alarm on my part. I just didn’t twist the Bourns far enough and I finally got a reading.

Right now I’m letting it cook with the lid on. My right channel is sitting at:
-1.005v across R10
-1.009v across R11
- -0.0244 across R12 to ground

I think I need to back off just a tad, but man these pots are sure very sensitive.

 

[matchstick20]

My build is finally completed! Can't thank all of you enough, especially those who have posted before me with very thoughtful questions and responses. I've hooked it up to my pair of active monitors and it sounds fantastic, soundstage is super wide, and resolution is very clear as well.

However, I do have some hum and hiss taking place when I turn the volume knob (50k) all the way up. It isn't noticeable from listening position but if put my ear right up next to the speaker within a few inches, it is definitely there - considering that it supposed to be dead quiet, I know i can do some tweaking. So here are my questions:

1. I've read that this may be due to a gain issue. I have yet to do a test to measure voltage in vs voltage out, but if that is the case, I understand that a resistor from MOSFET drain to ground would solve the problem. How does one calculate the required resistance to implement? I do plan on building an F4 next so I will need SOME gain, so don't want to go overboard with the resistor

2. If it turns out it isn't a gain issue, what other faults may be causing this issue? I've attached a pic below to see if any of you can find any issues with my build.

 

[matchstick20]

Sorry here's the pic

 

[MEPER]

You could try different experiments like moving the transformer far outside the chassis to check if hum is from the magnetic field from transformer or disconnect one channel completely (remove power) to check if it is ground loop issue?
I assume the green wire from PSU to NTC is signal Gnd?

 

[matchstick20]

Yes Green wire from PSU to NTC is signal ground. I'll definitely give those experiments a shot.

Also I just did a little test and I noticed that the hiss is present even when the volume knob is turned all the way down, while the hum only comes on when the volume is turned up. Not sure if that helps diagnose in anyway...

 

[matchstick20]

Ok i was able to remove one of the channels from power to check for ground loop, hum and hiss remained.

I then checked my gain - i used my phone to play a 1k sine test tone using the 3.5mm output into RCA, then measured the output of the ba-3:

In: 0.847 Vac
Out: 6.22 Vac

I also had an output reading of -0.25 Vdc, would that have any effect?

Would appreciate all the help I can get!

 

[McQuaide]

Hum and hiss with the volume maxed that you can't hear from the listening position could be your source or power amps or a lot of things. Personally I wouldn't worry too much about it.