BA-3 As Preamp

[madskaman]

I adjusted the pots from one end to the other. I found the 2nd to be higher than the 3rd at one end, and the opposite on the other end. I did a sweep a couple times and found the results similar. I'm using an app on my phone to make the measurements. While I'm sure it's not the best way to do so, it still clearly showed changed in the harmonics.

I'll recreate the test tonight and eliminate as many variables as I can. I'll use the same speaker, in the same location, same cable to same channel on my amp, ect.

 

[Hikari1]

Are you using the mic on your phone or some sort of interface?

Typically a speaker is not involved. We are typically measuring the line out signal from the preamp or amp output (which will need a load resistor and probably will damage your phone).

 

[madskaman]

Are you using the mic on your phone or some sort of interface?

Typically a speaker is not involved. We are typically measuring the line out signal from the preamp or amp output (which will need a load resistor and probably will damage your phone).

Unfortunately, I do not have that equipment. I'm using the MIC on my phone.

 

[Hikari1]

Well, if it gets you to a sound you like better than go for it.

But you are measuring all sorts of distortion and variables like the speaker, the room, etc.

But I guess it's better than by ear, I'll give you that.

One of these would allow you to measure just the preamp, you would also need a computer and some freeware.


Behringer U-CONTROL UCA202 USB-Audio Interface | Guitar Center

 

[Dennis Hui]

I recall reading that you can do a similar measurement even with just
a computer/laptop with a soundcard. You'll need to build something that
attenuates the input.

I'll see if I can find the link.

Dennis

 

[Hikari1]

Theoretically you don't even need an attenuator, just do this if you are on awindows machine.

https://m.youtube.com/watch?v=F_zgd4GqdFQ

P.S. Iam not responsible for damage to your soundcard. Don't crank the volume on the preamp. You are looking for 2v out max.

 

[madskaman]

So I tried it again last night with a different microphone and had similar results. I tested one side of the amp with one speaker. Basically, it was the same. Not much of a surprise here considering my crude testing techniques.

This morning I tried again using the method in the video in the link above. I ran the preamp straight into my computer and used the same program to test it. I was much more clearly able to see the harmonics. Adjustmenting this way made it easier to see what I was adjusting. The difference from where I had it to where I ended up wasn't drastic, but now I feel I am in a better place and I know the channels are similar.

Thank you all again for the suggestions and help.

 

[Attila I]

bigger common heatsink or not???

hi all,
as for routine i keep adjusting my ba3fe preamp every few month
and always go crazy about that jumpy behaviour of Iq & DCout.
its like trying to mount a pin upright on a knifedge.
so would it be an improvment to mount p & n output-transistors on one common heatsink in order to add some sloth to its behaviour??

has anybody done that before?

advice appreciated!

 

[6L6]

Having a common heatsink would help, yes.

However, don't worry about it, the circuit was designed knowing that there would be a bit of offset, and that it changes as it warms up -- hence why there's a big capacitor on the output.

 

[Marra]

I mounted my output transistors on a common sink but have to say that I haven't checked the settings in a while. Still making great music

 

[delecoy]

I just checked mine a few weeks ago ( after a year ). No drift on the ba-3 fe. The bias on one channel of my F4 had moved up 15mv.

 

[Attila I]

bigger common heatsink or not???

thakx for the response! i'll try bigger sinks (right now i have fischer's VPR32/38,1)

i would not argue about 15mV DC before the cap but recently i measured 35mV DC after the cap. so the real offset was in the hundreds of mV!!
the output fets are irf530/9530 maybe i should change that too???!!!

kind regards,

attila i

 

[Zen Mod]

there can't be offset after the cap , except in case of bad one

however , if you don't have any bleeder resistor there (output side of cap) , due to huge DMM impedance , you'll have false DC reading

so - when measuring positions like that , put 100K resistor parallel to probes

however - why at all measuring side of cap , flapping in da breeze ?



just chill , and enjoy in muzak

 

[Attila I]

what the heck is "flapping in da breeze" ????
sounds like an idiom for diarrhoea.

sorry i'am not a native english speaker.

there are 100R in series before the cap with 330R to ground as is in the official
circuit.
i came to measure the output offset while measuring the output noise.

besides-> muzak is licensefree audio rubbish as is heard in elevators eg.

cheers

attila i

 

[6L6]

What ZenMod means is that there needs to be a resistor after the capacitor, 100K or so, in order to read the actual DC offset after the cap.

Do you have 25-turn potentiometers in your bias circuit? That helps.

Also, when these circuits get very, very touchy to adjust, I would suspect a cold solder joint somewhere.

 

[Attila I]

@6l6
so i shall put 100k between output and ground as a bleeder just for measuring; ok done!

and i'll do some reflow on the pcb.

thanks to all!
attila i

 

[Zen Mod]

 

[Attila I]

Soooo....
reflow done, there were indeed a few suspect solderpoints
adjustment done, a bit easier than before.
dc out pre cap circa -15 to +15mVdc
noise below 0.5mVac

again, thanks for your kind assistance.

right now listening to "Seks Bomba" from Boston
recommended!!!

attila i.

 

[Attila I]

for those who care

 

[Zen Mod]

neat

what's that in silver box ?