donderdag 22 september 2011

Urei MOD-1 OpAmp

Here's another little opamp i've made a self etch layout for. It's the DOA used in the Urei MOD-1 broadcast desk. I'm fascinated with how simple some audio equipment can be. And since this is as simple as it gets for a DOA, i thought i'd give it a whirl. 

First of let me link the thread on GDIY where i first saw this opamp.
 http://www.prodigy-pro.com/diy/index.php?topic=16313.0
Since the schematic, layout and PCB that Peter Purpose (of lazpro.com Check his stuff out!) made for this OpAmp were missing. I thought it might be nice to make my own just to try it out.

So here's the schematic of the original opamp:

and you can download this schematic in HUGE format here.


The original schematic can be found in this .pdf on page 61, along with a host of schematics to build an entire mixing board using this little building block.:
http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-MOD1.pdf

this opamp was used by Peter Purpose as the differential amp on the input on his version of the API 525 compressor. As can be seen here:
http://www.prodigy-pro.com/diy/index.php?topic=22699.msg280210#msg280210
and this is the schematic of that version:

as you can see he substituted the original transistors with commonly found ones and the output pair was replaced with bc550/bc560 instead of the (medium) power transistor types. He used bc550/bc560 on the output because he did not need the load driving capabilities of the original transistors and because he did not have enough room on his pcb.

Since i do not have the same constraints a he had on his 525 pcb. I decided to make my pcb fit the usual 990/2520 pinout/layout and since i DO want the full driving capability i decided on the common BD139/BD140 transistors as the output pair.

So here's my schematic, layout and PCB:

Schematic
Layout
PCB
note: Layout has been tested in the FiveFish Studio Opamp Test Jig found HERE and is confirmed working.

Some notes on this opamp. This is about as basic an opamp can get, so the performance is of course not stellar. It's not gonna win any prices in fidelity and noise but when used wisely it can probably be a great little performer. And maybe another flavour in you arsenal.
Gain is limited to about 27 dB max so when using this in a mic-pre you might wanna use a step-up transformer and an extra opamp for some extra gain. Also there's probably some DC offset on the output so using a DC blocking capacitor is very much needed. This circuit has been verified working as a diffential amp so i believe it should perform just as wel in the 553 layout posted here earlier.

ok i think that's it for now, next up a more elaborate version of this opamp used in Quad Eight equipment.

Greetings,

Thomas

1 opmerking:

  1. hi!
    i just found this page, which looks really interesting, but all the links are dead, and i can't seem to find the meaning of "SPFD" about that C1 capacitor.
    is it maybe a 5 and not an S, thus a small 5pF? or is it something else?
    thanks, cheers!
    Frances

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