miguel2 said:
Yes it does. Should I keep the lousy PSU and all that offset? I dont want to spoil the sound I already have
Miguel
I would cange the regulators to 7815/7915 if I was to keep the transformer.
And you have to get a solution for that problem.
Do you have DC at the pre out, after the cap?
You could try to put the 22uf cap on the NIGC.
Like this:
Attachments
double 317/337 is a deal... its not because of the current it is useful to use a bigger trafo... increasing trafo power gives you a lower source impedance, making your psu a closer match to a perfect voltage source for your given required current...
I use 2 x 50VA with 317/337 for my single opamp OPA627BP pre.... another possible step in the psu is impedance correction for the LM... but i dont think its worth the fuss..... you can still use a schulzer/borbely one for reg.....
I use 2 x 50VA with 317/337 for my single opamp OPA627BP pre.... another possible step in the psu is impedance correction for the LM... but i dont think its worth the fuss..... you can still use a schulzer/borbely one for reg.....
I am!!
This weekend I decided to sit back and start to check everything.
First replaced the 7818/7919 to 7815/7915. Sound become really crappy and a faint hiss was audible, like a distant radio station...
Then I added the input resistor (oops, I forgot it) R1, 330R, metal oxide 2W. And I added output resistor R10 (forgotten too), 49.9R, metal oxide. I am not using R12 (output to ground) and I believe that is the reason why my DIP package 8like the OPA627) BUF634 does not get hot (just warm).
Replaced feedback resistor from 3.3k carbon to 4.7 metal film.
Added ceramic 221pF cap between output and +in of the opa627. (Should I use 2x100pF?)
I did not put a cap around R2, as Greg recommended.
I checked it out after this and I had no problem with voltage on speakers output of the GC. But still had around 210 mV offset. So I took the caps off between the pre and the GC, so I have them now DC connected. The offset of the GC is now 10mV on R channel and -10mV on the other!!! Good.
The flu is attacking my head head so I have not listened to it in a critical way, but I think the first version was better than this one, in sonic terms.
I think that the psu voltage is one of the main factors. Also, I guess that carbon composition resistors may also have an effect.
Miguel
It works fine with +/-15VDC. When I swap the regs I should have done something wrong in other area.
So now the sound is great, BUT I think that in the begining was better. Carlos, which type of resistors do you use? What about the output resistor, is it necessary? I think I am going to try it without this one but I guess the input resistor is a must. I am also suspecting of the feedback one...
Miguel
So now the sound is great, BUT I think that in the begining was better. Carlos, which type of resistors do you use? What about the output resistor, is it necessary? I think I am going to try it without this one but I guess the input resistor is a must. I am also suspecting of the feedback one...
Miguel
Problems with Macura buffer
Hello all,
I have built the pre-amp described by Carlosfm at the start of this thread and have an unusual problem:
When applying the +-15V to the board, without an input signal, I measure 0V at the input pin of the OPA627 (pin 3), which seems correct, but I measure negative 14.3V at the output of the BUF634 (pin 4). This seems very wrong! I know this is a stab in the dark, but anyone know why, in this simple circuit, this would be the case?
My set-up is as shown by Carlosfm in his diagram, except:
R11= wire, i.e. BUF634 set for wideband operation
R4 = 3.3k, not 4.7k as Carlosfm used
R12 is not used, as suggested by Carlosfm
Power supply decoupling takes the form of 100uF for + and -15V lines for now.
Thanks.
Ryan
Hello all,
I have built the pre-amp described by Carlosfm at the start of this thread and have an unusual problem:
When applying the +-15V to the board, without an input signal, I measure 0V at the input pin of the OPA627 (pin 3), which seems correct, but I measure negative 14.3V at the output of the BUF634 (pin 4). This seems very wrong! I know this is a stab in the dark, but anyone know why, in this simple circuit, this would be the case?
My set-up is as shown by Carlosfm in his diagram, except:
R11= wire, i.e. BUF634 set for wideband operation
R4 = 3.3k, not 4.7k as Carlosfm used
R12 is not used, as suggested by Carlosfm
Power supply decoupling takes the form of 100uF for + and -15V lines for now.
Thanks.
Ryan
Dr. H,
there must be a fatal mistake in your circuit. The buffer normally does not exhibit any problem and as far as I know more than hundred of them was built. Please check all the connections. 4.7k resistor instead of 3.3k makes no difference.
The pin numbers at the circuit described are for BUF634T (TO220-5).
Pavel
there must be a fatal mistake in your circuit. The buffer normally does not exhibit any problem and as far as I know more than hundred of them was built. Please check all the connections. 4.7k resistor instead of 3.3k makes no difference.
The pin numbers at the circuit described are for BUF634T (TO220-5).
Pavel
i might have suggested this earlier or in another thread but here's why one of my pre's had an 8V dc offset. 
Because it was a quick'n dirty thingie i used some flux stuff (s39 or s65, i can't really remember). As you solder this stuff it boils and splatters a little bit. Unfortunately the flux agent is also highly conductive ...
Cleaned it up and... 0.9mv offset. Good enough !
which is why even in test things i don't use that kind of stuff anymore.
Because it was a quick'n dirty thingie i used some flux stuff (s39 or s65, i can't really remember). As you solder this stuff it boils and splatters a little bit. Unfortunately the flux agent is also highly conductive ...
Cleaned it up and... 0.9mv offset. Good enough !
which is why even in test things i don't use that kind of stuff anymore.
PMA said:
It makes a difference in gain.
4.7 / 3.3 + 1 = 2.4242424242424242424242...
Ryan, why did you use 3.3k for R4?
You're making a pre with a gain of 2, a little low.
Later, I changed R3 to 1.9k, that gives me a gain of 3.47368...
The little extra gain is welcome, it's more standard.
I'm glad to see some people doing this pre.
I've completely forgot my pre.
It's always on my rack and in service.
No tweaking needed.
I've made several GCs, experimented, changed components... and the pre is always the same.
Migueeeeeeeeeeeeeeeeeeeeeeeel
How's your pre, did you finish it?
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