Help - Show off your DOA knowlege... PLEASE

[Handcrafted Tone]

I want to learn about discrete op amps. I admit I know near nothing so far, anyone heard of a good book or resource?

anyway here's my question, I have a Sphere M1200 card (discrete preamp). I want to shrink this down into a 2520 footprint.

So what do you think, will it work? It looks pretty simple, am I missing something? Any opinions?


Edit - changed pic to only show op-amp

 

[Samuel Groner]

Should the caps on the power rails be in there?

A matter of taste. ICs don't have them (and thus no ground connection), but here you should have enough space on the board, so you can include them.

Looks fine otherwise.

Samuel

 

[Handcrafted Tone]

Thanks Samuel, I really respect your opinion!

I'll give it a try.

 

[peterc]

This cct looks almost identical to the Quad Eight AM4 opamp - have a look here for an example:

http://www.avensonaudio.com/tech/Quadeight/mm100schematicsm1.jpg

Peter

 

[Handcrafted Tone]

Yeah it sure does, if you haven't read "The History of Electrodyne, Quad Eight, and Sphere, I recommend it. Quad Eight literally stole the early electrodyne designs, that would later become Sphere.

 

[Handcrafted Tone]

I'd like to put this amp into a 312 card, I guess the bypass caps and series diodes on the 312 can do the job.

but...

How would the signal change if I left them in? I would think they are just a little bit of protection, and shouldn't effect anything?

I've already made a layout, and I can fit them in with room to spare... So, are they really not needed?

 

[rafafredd]

There´s already a layout for the quad eight am4 (from peter), so maybe you just need to make the little parts change to use this circuit with that layout, because it´s almost identical, like he said...

 

[Samuel Groner]

The diodes protect the amp from missconnect (i.e. reversed) PSU lines--you decide if you manage that. :grin:

If you have 'lytic decoupling caps on the main board you might add two 100 nF caps on the amp board, if you like.

Samuel

 

[peterc]

Yeah it sure does, if you haven't read "The History of Electrodyne, Quad Eight, and Sphere, I recommend it.

Good read! Thanks for that.

Peter

 

[Handcrafted Tone]

well.... I had time to put this circuit in pref board.

It sounds like a strange distortion effect... Is that oscillation?... Anyone know why?

The trim pot on the 312 board sounds more like a HPF, and doesn't change volume....

I tried to disconnect ground, nada..

 

[Samuel Groner]

We need to know more:
* is the amplifier output about at 0 V (befor the cap, of course)?
* are things quiet when you short the input transformer primaries?
* what happens when you remove the output transformer?

BTW, I think there is a drawing error around the gain setting pot in the schematic you posted--would at least explain part of the story.

Samuel

 

[Handcrafted Tone]

Unfortunately I decided to try press n' peel PCBs instead of photo-PCB's, and Kinkos wouldn't let me use the p n' p paper in their printers... my prototype DOA is just a radio for now, it's picking up RF like crazy. I should have known better than to pref board a DOA, it's a rats nest of wire under there.

but I did find some interesting info on the Jensen site... does this look familiar: http://www.jensen-transformers.com/an/an001.pdf

thats even closer to this circuit than the Quad 8, It's about an exact copy. (Dean Jensen was Chief engineer at Quad 8 at one point) Does that make this circuit public domain?



Samuel, you don't have to look to deep into this one, I'll make a prototype on PC board, hopefully that one will work. To answer your questions:

shorting the mic input transformer will kill the mic, but not the noise . The noise changes when primaries are shorted though.

with out the output transformer I get nothing, unless I hit the mic hard, I'll get a pop.

I updated the schematic to show only the op-amp, the rest is going in a 312 circuit that worked with a JH990 in there.

I'm getting 0V on the output.

-Rob

 

[tk@halmi]

Break it down. I would breadboard the opamp and measure the current in each stage. The input should have way less than 1ma with that 360k resistor. (that makes the 1:2 input transfomer a poor match noise wise, a 1:8 would be more like it) The VAS should have a few ma and the output should have at least ten.

 

[tk@halmi]

[quote author="peterc"]This cct looks almost identical to the Quad Eight AM4 opamp - have a look here for an example:
http://www.avensonaudio.com/tech/Quadeight/mm100schematicsm1.jpg
Peter[/quote]

I love that circuit. Tons of sound out of five transistors. Notice the capacitor bootstrapped current source in the VAS. In those days transistors were expensive compared to capacitors.

 

[Samuel Groner]

Thats even closer to this circuit than the Quad 8, It's about an exact copy. (Dean Jensen was Chief engineer at Quad 8 at one point) Does that make this circuit public domain?

This topology is so well known that it is difficult to even think about "public domain"--don't believe that it was an audio company that invented it.

Good look with the next prototype!

Samuel