Member
Joined 2003
Member
Joined 2003
kmj said:
Just about any single opamp with the same pin out as the opa627/637 will work. You should not need to change the resistor values unless you need more or less gain.
As always check the opamp datasheet, but the PCb is very flexible as far as opamp choices go.
Some I have used:
LT1115
OP227/228
AD8610(with brown dog adapter)
RC4560
All work fine.
Cheers!
Russ
Russ White said:
Just about any single opamp with the same pin out as the opa627/637 will work. You should not need to change the resistor values unless you need more or less gain.
As always check the opamp datasheet, but the PCb is very flexible as far as opamp choices go.
Some I have used:
LT1115
OP227/228
AD8610(with brown dog adapter)
RC4560
All work fine.
Cheers!
Russ
Russ, how much of a difference in sound did you find between the OPA627AP and the OPA227AP? The difference in their price is about 4 times here. If there is not too much, I may go with the OPA227AP initially.
Shehzad
Russ White said:
There is not as much difference as you might think.
Cheers!
Russ
Russ, thanks for your reply. Does a gain of 5 mean a very low gain? Sorry, I am new at this. Another question would be that of using an ALPS small rotary pot be unwise as compared to the more reknowned ALPS Blue Beauty and the Black Beauty? If there is something else that you could recommend, it would be most appreciated.
BTW, a Merry Christmas and Happy New Year to all....
Cheers!!!
Shehzad
I usually setup my preamps for a gain of 3 or so, but it is really depndant on the end target, for example if you are driving something with a gain of 20-25 or so like most chipamps 3 is a good number for the preamp.
If you are driving headphones you can actually go down to unity gain in some cases but I usually stay in the 2-3 x gain even for headphones.
A gain of 5 is certainly reasonable, but a little more gain than some would use. I would start with the opa228 at a gain of 5-6 to keep it stable and see what you think. If you build the with the default values you will have a voltage gain of 5.7 which should work just fine for the OPA228. The formula to set voltage gain is G = 1+(RFB1/RFB2). If you detect any instability(oscillation, really bad sound, ringing, and/or a hot opamp) add a 10-20pf cap at CC otherwise leave that space empty. CC is only needed if you have an instable opamp, it is used to compensate the opamp externally which makes it more stable, but lowers bandwidth. This circuit is well bypassed so it is pretty forgiving at low gains where some opamps are sensitive.
Cheers!
Russ
If you are driving headphones you can actually go down to unity gain in some cases but I usually stay in the 2-3 x gain even for headphones.
A gain of 5 is certainly reasonable, but a little more gain than some would use. I would start with the opa228 at a gain of 5-6 to keep it stable and see what you think. If you build the with the default values you will have a voltage gain of 5.7 which should work just fine for the OPA228. The formula to set voltage gain is G = 1+(RFB1/RFB2). If you detect any instability(oscillation, really bad sound, ringing, and/or a hot opamp) add a 10-20pf cap at CC otherwise leave that space empty. CC is only needed if you have an instable opamp, it is used to compensate the opamp externally which makes it more stable, but lowers bandwidth. This circuit is well bypassed so it is pretty forgiving at low gains where some opamps are sensitive.
Cheers!
Russ
safetyman said:Hi Russ, how does the power supply side look like? In an earlier post you had mentioned a 15V supply input. this is 15V prior to rectification? Sorry, i know this is a basic question but my friend asked me and I did not know how to reply him.
The supply voltage will be dictated by the opamps you choose. But for OPA627 and a host of other common opamps 15VDC per rail is just fine. If you wanted to build a power supply that would be safe for a wider range of opamps I would go +/- 12VDC.
To get 12-15VDC I would use a regulator (LM78xx or any other suitable VREG). It just depends on how comfortable you are with various techniques,
A dual 9V secondary (or 18VCT) trafo ungregulated dual rail supply wll get you about 12.7V which should be fine.
A dual 12V (or 24VCT) should be regulated down to 12-15V.
Cheers!
Russ
Russ, thanks for the explanation. If it was implemented with a 9V-0-9V transformer, with both the 9V connections to a bridge rectifier and the +/- DC from the bridge rectifier to the board, and the centre connection of the board direct to the 0 tap at the transformer secondary, would this be right?
Thanks for the help and looking forward to your reply.
Regards.
Thanks for the help and looking forward to your reply.
Regards.
safetyman said:
with 9-0-9 (18VCT) trafo you would run the center tap directly to GND. and both 9V wires to the bridge, with the positive side of the bridge to the positive rail, and the negative to the negative rail.
One simple PS (and certainly good enough) approach would use a 1000uf or so cap on each rail to GND directly after the bridge to help the transient response and smooth out the DC.
Cheers!
Russ
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