oh, when i said ebay I refered to ebaystores like westfloridacomponents and such. But perhaps these also are samples.
Member
Joined 2003
I got mine from USACaps on ebay but it looks like they don't have any for sale at the moment.kmj said:
Member
Joined 2003
OPA604 = 
I would suspect that the OPA132 or OPA134 would work perfectly fine in this circuit, but I think that Russ should verify that first as I am just guessing.
I would suspect that the OPA132 or OPA134 would work perfectly fine in this circuit, but I think that Russ should verify that first as I am just guessing.
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:
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
safetyman said:
There is not as much difference as you might think. 🙂 The OPA627 is certianly a fine chip, but the OPA227/228 is no slacker. If you can go with a gain of 5 or more use the OPA228, the high slew rate it beneficial.
Cheers!
Russ
Russ White said:
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
Thanks Russ.... You're the man. 😀 I am going to try and etch the boards and give it a go. Did think of hard wiring it too but 😀
PS
Could someone post a PS schematics for the Freebird preamp? 🙂
Or even better for me, a pdf with copper and component side..
Or even point me in a direction where i can find a suitible one
That would be great! 😀
Thanx alot! 🙂
Could someone post a PS schematics for the Freebird preamp? 🙂
Or even better for me, a pdf with copper and component side..
Or even point me in a direction where i can find a suitible one
That would be great! 😀
Thanx alot! 🙂
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. 😀
safetyman said:
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
Hi, I did put a freebird (monoboards) together for a friend but when I plugged it in there was a high pitch screeching comming out of the speakers, no music played.
What do I need to look for when troubleshooting? (opa604 used)
What do I need to look for when troubleshooting? (opa604 used)
oh, it seems like the negative regulator has died. The psu tested ok so I connected it to the amps and then the sound was there, now I disconnected it and tested with a 100k load and only the positive regultor regulates.
So I guess that the problem is solved, or atleast the source is found.
So I guess that the problem is solved, or atleast the source is found.
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