Yes it does. Should I keep the lousy PSU and all that offset? I dont want to spoil the sound I already have 🙂 🙂
Miguel
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.....
....or gallo, didden - jung - ....alw.............anders.............................dieckmann.....😉
Guys, keep it simple for now.
Miguel has some problems to solve before he starts worrying about the PSU.😉
Miguel has some problems to solve before he starts worrying about the PSU.😉
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
Miguel,
With +/-15v it should work fine...😱
Oh, I have no input or output caps on my pre and I have 0.1~0.2mv DC on the pre outs.😉
With +/-15v it should work fine...😱
Oh, I have no input or output caps on my pre and I have 0.1~0.2mv DC on the pre outs.😉
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
Once upon a time I had a problem: without cap the offset was very low, but with it the offset was real high. Explanation?
The cap was charged a little bit.
The cap was charged a little bit.
Hi Mighuel,
I use 1% 0.6w film resistors.
Nothing fancy, really.
And I have 50 ohm resistors in series with the output.
And I also have the input resistors.
Raka, it seams to me that the BUF634 doesn't like caps.😀
I use 1% 0.6w film resistors.
Nothing fancy, really.
And I have 50 ohm resistors in series with the output.
And I also have the input resistors.
Raka, it seams to me that the BUF634 doesn't like caps.😀
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
Hi Ryan
Which type BUF are you using: to220 or dil? They have different pinouts of course. Just an idea.
And of course to be sure the probs come from the buf you can always replace it with a short and see if the opamp is fine on its own.
Which type BUF are you using: to220 or dil? They have different pinouts of course. Just an idea.
And of course to be sure the probs come from the buf you can always replace it with a short and see if the opamp is fine on its own.
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.😀
Seems I know why this happened!
Hi guys and thanks for repsonses,
Seems that I used the DIL and took the pinout of the TO-220!
Should be able to sort it out now....
Hi guys and thanks for repsonses,
Seems that I used the DIL and took the pinout of the TO-220!
Should be able to sort it out now....
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?🙂
carlosfm,
This is more than evident. I was thinking about -Vs saturation at the BUF output, which resulted from wrong package (pinouts).
Back to the case: if you change both R3 and R4 from 3.3k to 4.7k no change will occur.
Pavel
This is more than evident. I was thinking about -Vs saturation at the BUF output, which resulted from wrong package (pinouts).
Back to the case: if you change both R3 and R4 from 3.3k to 4.7k no change will occur.
Pavel
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