Schematic Review SSL Type Preamp

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sr1200

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I think I've ironed out the problems with the schematic. In trying to lay out the board i found a bunch of silly errors.
 

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sr1200

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Need to look at the DI circuit

Also, pin 1 of both XLR's go to chassis ground, not to audio ground.
I had a big boo boo there. That section was overhauled in the schematic i just posted. Thanks for the heads up. When i was trying to lay out the board i was wondering why the net was telling me to dump the DI to ground... in the words of Homer J. Simpson... "DOH!".
 

abbey road d enfer

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I think I've ironed out the problems with the schematic. In trying to lay out the board i found a bunch of silly errors.
I think you should swap the values of R8 and R11, and increase R3 to about 100k.
Strictly, C6 and D1 should be returned to chassis ground. Probably not a big issue in a small preamp, though.
 

sr1200

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@abbey road d enfer appreciate the input, ill get on that. Also, i keep having this feeling that the transformer is backwards. Should i be using it 1:10 no? I think i currently have it as 10:1...

should i up all the resistors going to the LED's to 100k or is it just that one in particular for some reason?

 
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abbey road d enfer

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@abbey road d enfer appreciate the input, ill get on that. Also, i keep having this feeling that the transformer is backwards.
Yes it is backwards. You have it as a 10;1 step down ATM. Mirror it.
Should i be using it 1:10 no? I think i currently have it as 10:1...
Correct.
should i up all the resistors going to the LED's to 100k or is it just that one in particular for some reason?
Sorry, typo, I meant R30.
 

sr1200

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updated.
  • Changed the resistor values (if you could explain why, im curious about the R8/R11 swap, I assume it has to do with how U2 is being loaded)
  • reversed the input trafo (on the board layout i put a footprint for a cinemag or similar can trafo as well)
  • changed all the LED grounds to chassis as well as the C6 cap.
 

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sr1200

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Anyone else see anything that would cause catastrophic failure? Before i go and spend a bunch on protoing boards... im not entirely sure that c1 needs to be there and perhaps i should leave it in case the transformer selected requires it?
 

sr1200

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So im breadboarding this thing out and ive run into an issue. At the first gain stage i have a nice signal going in but am only getting an enormous square wave out. No matter the setting at the pot, its either a small square wave or a very large one.
Not quite sure whats off here. Thoughts?
 

sr1200

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What amplitude? The minimum gain is pretty high ( for line level input, fine for mic level).
Ive sent in mic level pre transformer, i've sent it in post transformer as well, just seems that that first gain stage is running away and maxed out no matter what.
 

abbey road d enfer

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I was under the impression R73 from the original SSL schematic was to bias the opamp to class A.
Biasing Op-Amps into Class A
The SSL "Class A arrangement" works with about 1.5mA (typically a 10k resistor tied to one of the rails), so the opamp thinks it's in class A for signals up to nominal operating level. Above operating level, cross-over distortion is masked by the signal. Here the current is about 100 times smaller, so the class A behaviour would be restricted to levels below about -40dBu.
 

sr1200

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You said it was breadboard, that sounds like the feedback connection is not made correctly, so the op-amp is running open loop.
Far as i can tell it looks right. Im wondering if i should just bite the bullet and proto some boards. I never get breadboarding right with something complex. Guitar pedals, yeah, this… not so much.
 

sr1200

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Cant quite figure out whats wrong on teh breadboard. I found a company to proto the boards for relatively cheap ($65 for 5 boards plus shipping). I have most of the parts i need on hand already in the shop, so, here's to hoping it works! If anyone sees anything glaringly obviously wrong still, please let me know, i have about 24 hours to cancel the order.
 

sr1200

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As long as the signal path is good, resistor values can be swapped out at build time.

Someone mentioned the possibility of the feed back loop being open (why the breadboard was just cranked out full). Does the pot going back to the negative input of U2 seems right since we're already feeding back though R17/R15?

Having non polarized caps at C7 and C16 seems unique, as every variation on this circuit ive looked at puts polarized caps there.

Edit: also noticed that the original SSL (later version) schematic is putting +17v into the negative input of the ne5534. What is the significance of this?
 
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