Dual discrete opamp PCB in DIP8

[Calimero]

very nice work Tiroth.
I wondered if you could shine your light on the performance of this design.
How does it for instance compare to an OPA627 or AD8065.
Do you have an idea how long the Wiki-page will be open for ordering these boards.

Thanks Henk

[tiroth]

Thanks Calimero! The best person to comment on the sound is moamps, as he designed and built the original circuit. I don't yet have any 2SK389 on hand to comment, but I expect it is quite special.

The wiki "preorder" will go on for about a week and a half more, and then payments for a week after that.

[moamps]

Originally posted by tiroth
.....to comment on the sound is moamps, as he designed and built the original circuit....

Hi,

IMHO, a discrete OPAMP sounds better than ordinary OPAMPS like TL, NE, OPA .... This opamp (the term itself might be misleading in a way, though) has a small open loop gain, small bandwith and relatively high THD (0.02%). But, it has only two stages and works in class A. I reckon this would suffice for this particular application.

OPAMPS with high bandwith, high open loop gain, minor distortion remind me of many 1970s amps - great specifications and crapy sound.

I haven't tried OPA 627 and AD8065.
(too many zeros in specifications, I don't need an XVGA buffer )

Regards

[Fred Dieckmann]

"The best person to comment on the sound is moamps, as he designed and built the original circuit."

Nelson Pass actually designed the topology and provided some implementation help for those who want to build such a circuit.

http://www.passdiy.com/pdf/diyopamp.pdf

While a bipolar second stage is great for repeatability for fixed bias resistor values, I wonder if the mosfet second stage might sound better for those willing to adjust the circuit for different values for the gate to source turn on voltage for the mosfet. maybe not practical for a production circuit, but not to hard for DIY small quantiities.

[MBK]

Hmm, just checked and couldn't find MPSA92 at RS - anybody know of a valid substitute?

[moamps]

Originally posted by MBK
Hmm, just checked and couldn't find MPSA92 at RS - anybody know of a valid substitute?

Hi,

good combinations also are:
ZTX450/550
BC639/640.

Regards

[Fred Dieckmann]

"good combinations also are:
ZTX450/550"

I think you will find stability a real issue. The Zetex are really more a medium power transistor rather than a small signal amplifier type. I have been burned like this before and am pretty sure the Zetex will require much different high frequency compensation. Mr. Pass uses a
2N4290 in his DIY op amp article. I think the 2SA970 and 2N2240 would be a good bet also and widely used for audio circuits.

http://www.passdiy.com/pdf/diyopamp.pdf

[tiroth]

Just crossposting some info about using these as headphone drivers:

Originally posted by tiroth
..on the drive current required. Since MPSA92 can handle 500mA, not a problem.....the biasing may differ depending on the impedance of your headphones.

Originally posted by moamps
Hi,

You are right. This opamp works just fine with output CS set on about 20mA and high impedance headphones like AKG 240.

[Elso Kwak]

Hi, the circuit is really too simple!
You cannot expect quality from that.
The 2SK389 is a large geometry FET with big capacitances. Now what are you going to do about it?
Lay it on the line!

[Fred Dieckmann]

Elso........... I assume you are funning us. With in the limitations of a reasonably low closed loop gain and driving reasonable impedances, >5K or so and reasonable cable capacitance, the circuit will work fine. One could lower the drain load resistor and increase the jfet bias current or even cascode the jfets if one wanted less input capacitance.
The only thing I might add is a small output resistor to isolate cable capacitance a bit.

My wife's Miata can't pull a large trailer and doesn't have cruise control, but it is still fun to drive. You haven't been licking the natural hallucinogenic secretions off that blue frog by any chance?

http://itotd.com/index.alt?ArticleID=34

PS what are you doing in Papua, New Guinea?