api312 bass response problem

After fixing the problem with my api 312 (bauman pcbs) channels that some of you may have read about, theres still one problem im having difficulty fixing.

It seems that one of my channels has about 3-4dB higher frequency responce below 63hz. At all other frequencys they seem to match pretty much.

They both sound fine, just different from another. Perhaps the one with more bass sounds a bit muddier, and less open. But not by much.

Any ideas on why this could be? And how i can fix it?

Btw, i havent done any scientific testing with noise etc. Only listening tests+software analysing in my daw.

/Jonas

all the parts are the same?
you could try bridgu=ing the in and out caps with a cap of the same value,
then you could try swapping transformers (ugh!) to see if that is the culprit. if both of these fail to turn up anything, it might be a wiring error, feedback/gain difference, input/output pad or something amiss with the opamp itself.

Sounds like the output cap, the lows are the first to go.
Could be a bad op amp, The first thing I check even with new 2520 op amps is the current they pull. Seen as low as 3 mills, 10 to 15 mills is more like it.
Not as consistant as a 990C for example, 25 mills no matter what voltage they are run at.

sorry, but which cap is the output cap?

On the opamp, or the actual mainboard? (btw, im using fabios boards)

thanks!
/Jonas

It would be on the PCB, the op amp may have the pin out on the top, O would be what it is connected to, if that helps.

Am i getting closer... is it a 100n cap?
/jonas

no. look for microfarads.

My mistake the 312 is DC coupled. So I would suspect a bad 2520.
Its possible it could be the 250uf cap in the gain circuit, in series with the gain pot. I would change that with a 470uf B.P. cap.

if dc coupled, maybe dc ids wrecking bass. check offset voltage on output of opamp, or measure dccurrent with no signal and then with signal.

ok, this is way over my head. Im only good with a soldering iron, my electronic skills arent up to par. So you have to be overly explainative for me to understand.

What is dc coupling?

Is it those 220uf caps that are on the power rails?

Btw, i tried replacing the electrolytic connected with the gain pot.

im using a 220uf/16v right now.

What would be the advantage using a Bipolar in this position Mike?

Perhaps it would be easier helping me if i link to the schematic im using:
http://www.thediypill.phx.com.br/forumfiles/R312sch_rev3.pdf

Im also using a diy 1731 melcor opamp.

Btw, another question. If i have to replace the electrolytics on my api:s,

Which cap positions are most important to use low impedance types in?

/Jonas

OK, DC coupled just means NO cap in the signal path.
Is the unit old?
Checking for a DC voltage on the output would be a good idea , if you have a VOM.
Is the Op Amp in a socket?
I would get a new Op Amp and just in case, problem is if it OK then ...
If its soldered in unsoldering can destroy a op amp.

Its soldered, and i built the opamp myself. It has been soldered and unsoldered about 20-30 times... so that could definately be the problem. Ive gotten ridiculously good at desoldering though :wink:

I got the wrong millmax pins, otherwise i would have used sockets+pins.

I have a DMM,

So basically i just measure the output from the opamp and check for DC?

before the ouput transformer that is?


Would be very happy if i wouldnt have to build a new opamp though.

Any ideas which components i could try changing?

an easier approach might be to simply install a 100 uf 50 volt, or there abouts, from the opamp output to the transformer primary lead. put positive towards the opamp.

if this improves the bass, then the dc offset is probably your problem.

I'd definitely recommend socketing the opamp. I made a couple of 312s using Fabio's boards and I change the opamps between the Melcor and API 2520 depending on my needs. I can get the part numbers for the pins and matching sockets if your interested... I could easily dig them up for you.

i might have some problems buying them from here (sweden). Or it may become to expensive.

I actually bought some pins and sockets from a guy who ordered them especially for baumans pcbs, but the sockets dont fit the holes in the pcb. Im a bit worried i might screw up if i start drilling bigger holes in the pcb.

Anyone that has any spare socket and pins they could sell me and send to sweden that accept paypal maybe?

/jonas

Small quantitys I get them from John Hardy, Gold plated.