Schematic Review SSL Type Preamp

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sr1200

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Attached is a schematic i've been trying to put together that kind of glues a few things together from various projects into a single circuit. This is pretty much the first time im doing this, and would appreciate if anyone can put a set of eyes on this to see if there are errors or anything that doesnt make sense.

Since most of this was taken from blocks of other project, some of the parts dont really make sense to me and im not sure if they're even needed. (for instance the voltage divider before the gain pot leading into R21... why? Or, the purpose of C7 and C9 one right after the other... seems redundant but the polarities are reversed, again, why?)

Sorry for the relay designations... kicad brought in some weird stuff for things that weren't in the native library.
 

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ccaudle

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What is C3 doing in the feedback loop? Is that supposed to be just an additional DC only feedback path in parallel with the gain pot?

And it looks really odd to be feeding 9V into the output loop of the op-amp in a design with bipolar supplies. Are you sure that wasn't pulled from a design made to run on single supply?
 

sr1200

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What is C3 doing in the feedback loop? Is that supposed to be just an additional DC only feedback path in parallel with the gain pot?

And it looks really odd to be feeding 9V into the output loop of the op-amp in a design with bipolar supplies. Are you sure that wasn't pulled from a design made to run on single supply?
The C3 is in the original SSL schematic as well (its C4 on the original). And I agree about the 9v thing, didn't make sense to me either, and which is why im asking someone that knows a lot more than me to check this out. The original schematic has +Ve feed going into that 1M resistor (doesn't mention what +Ve is though).

The schematic I based it off of was pulled from something I had found a while ago and never got around to. I thought someone else had made boards based off the same source, but they don't seem to work properly and I found a number of strange errors on the boards, so im trying to re-do the board myself. :(

I also realized im missing a cap in the Phantom section. which should go before the ground and after where the 2 legs branch off to go to the XLR in.

This was the original I took from
 

ccaudle

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I think I see what is going on, the gain pot has a cap in series, so there still needs to be a DC feedback path. Having that low pass filter in the resistor feedback connection allows separating the DC feedback from the higher frequency feedback that is used to set audio frequency gain.
Injecting voltage at the junction of C54 and C55 (original schematic designations) is an attempt to keep the polarized caps biased so that the insulation does not degrade over time.
I'm still not sure why R72 is included. There is no equivalent at T2, which is the line input buffer.
 

sr1200

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But is it just the +voltage (18) or something else? The only other voltages ive seen on the schematics are the 11v for the LED's and 17V (which if i remember correctly has to do with the logic on the modules).

Also, should i be putting caps on all the voltage inputs (.1u or something off to ground)?
 

abbey road d enfer

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Attached is a schematic i've been trying to put together that kind of glues a few things together from various projects into a single circuit. This is pretty much the first time im doing this, and would appreciate if anyone can put a set of eyes on this to see if there are errors or anything that doesnt make sense.

Since most of this was taken from blocks of other project, some of the parts dont really make sense to me and im not sure if they're even needed. (for instance the voltage divider before the gain pot leading into R21... why?
Totally opvercomplicated. Replace all that with a 100uF Non Polar cap.
It seems the two half-secondaries are wired out-of-phase. No bueno.
R25 should not be there. The 1646 chip must be driven with the lowest possible impedance possible. 100 ohms ruins CMRR to less than 40dB.
D2 and D3 are in the wrong direction.
Junction of D5 & D6 should be grounded.
BTW, sharing ground between relay/LED's and audio is not a good idea. You should run a separate ground for these.
Junction of R2, R4 & R6 is grounded, so phantom is shorted. Is it really what you want? I suspect not. Put a 100uF 63V cap there to ground.
I just don't understand the phantom switch.
I'm not sure using the 5534 as the DI input stage is adequate. Could be quite noisy with Hi-Z sources.
Or, the purpose of C7 and C9 one right after the other... seems redundant but the polarities are reversed, again, why?)
This a remnant of the time SSL was concerned with capacitor distortion. I don't think it's appropriate for modern capacitors. As I said replace all this s..t with one good NP cap.
Sorry for the relay designations... kicad brought in some weird stuff for things that weren't in the native library.
I often see that with Kicad. Schematics are not easily legible.
 

hildy

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there might have been some transation errors between the ka-electroncis link you gave and the kicad schematic. For example, the primary input transformer looks like its halves are wired backwards. Polarity on C7 and C9 is backwards. Polarity on K1 is backwards. The power to K3 isn't in a circuit. Is the ring terminal on the DI input feeding the base of the relay switch?
 

sr1200

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First, thanks all for the feedback on this. I'm still trying to learn here and there were a bunch of silly mistakes. I had a feeling that voltage divider section was kinda BS.
So I placed the 100u non-pol cap in there, should there be a voltage source feeding into that or no?

For the transformer, i followed the datasheet from lundahl
(https://www.lundahltransformers.com/wp-content/uploads/datasheets/1935.pdf)
Its strange as the first drawing they number the pins for the transformer differently than they did for subsequent 2 drawings. Should i then connect 2/4 together and ground 1/3?

I knew about the missing cap in the phantom section, i've updated the schematic for that and the D2/D3 diodes.

As far as the impedance going into the 1646, what do you suggest? Also, would it make more sense to put a non polarized cap at what C16 was (following that 100ohm resistor)?

@abbey road d enfer is spot on with the DI input, i thought is was a pretty cool way to not have to deal with another switch on the front panel.

As for the Phantom switch. Basically turning a light on and feeding phantom back to the balanced input. Why the 1.69K resistor is there when disengaged... not sure but it kind of looks like it creates a kind of big H pad there. Does it need to be there? If not I'll happily save the $0.13 and the extra drill holes and paths on the board lol.

Going to update the schematic now.
 

abbey road d enfer

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First, thanks all for the feedback on this. I'm still trying to learn here and there were a bunch of silly mistakes. I had a feeling that voltage divider section was kinda BS.
So I placed the 100u non-pol cap in there, should there be a voltage source feeding into that or no?
No.
For the transformer, i followed the datasheet from lundahl
(https://www.lundahltransformers.com/wp-content/uploads/datasheets/1935.pdf)
OK, I must admit the pin numbering is rather disconcerting.
As far as the impedance going into the 1646, what do you suggest? Also, would it make more sense to put a non polarized cap at what C16 was (following that 100ohm resistor)?
Just make C16 a NP and delete R25.
As for the Phantom switch. Basically turning a light on and feeding phantom back to the balanced input. Why the 1.69K resistor is there when disengaged... not sure but it kind of looks like it creates a kind of big H pad there. Does it need to be there? If not I'll happily save the $0.13 and the extra drill holes and paths on the board lol.
OK, I get it. This allows the LED to light up when phantom is applied to the input from an external source.
 

sr1200

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So I've made a bunch of changes based on the suggestions posted. I added a buffered active DI to feed into the 5534 (Perhaps that will help?). If anyone sees anything else out of sorts (assuming that transformer section is right... i cant really tell based on lundahls drawings) please let me know. Otherwise im going to start to tie up the board layout. Once thats done, ill make it available on OSHPark if anyone is interested in building one of these things.

Edit: figured id ask again, but should i put the 100n caps at the voltage inputs on the 5534's? Also, not sure how to make the 0v so i denoted it as GND2. KiCad is kind of annoying...
 

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