Xs-PreAmp

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buzzforb

buzzforb

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Joined 2010
:tongue:

With this, I tihnk the old school Laterals used by Patrick in his pre might be real nice. Dont underestimate a balanced version of the LSk preamp. Both are bruisers and if the SE version of the folded is any indication, it is a very musical topology. Neither are lacking in my experience.
 
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H

hm4nine

Member
Joined 2005
I want to make my personal contribution to common confusion….

What do we see on picture?:
Rather close neighborhood of optocouplers and servo loop double OPV (OPA2604?) let me assume they work together.
On board they are located on negative supply side, and below SL-opamp we see 2 resistors (R5&R6) ea. 453E. These resistors are probably collector resistors of lower cascode.
Now we know from UGS-modules that absolute offset is adjusted on neg. supply side; normally there is a single trimpot between joining collector resistors (of lower cascode) and neg. supply.
Could it not be that - instead of this trimpot - Wayne has incorporated both opto couplers as SL-controlled variable resistors?
On positive supply side we see a trimpot (50E) and close to this 2 resistors (R15&R17) ea. 562E. I assume that these resistors are collector resistors of upper cascode and this trimpot is still responsible for adjustment of differential offset. May be this trimpot is still needed because regulation headroom of SL should be kept (or is) small.
And we also see that many resistor values have been decreased, e.g. only 75E as source resistors for TO220-mosfets, 475E as their gate stoppers, 22E as source resistors for input-jfets, cascodes are referenced with 10k/4k75, and collector resistors of casodes are now 562E & 50E-trimpot or 453E & optocoupler.

Interestingly also that all resistors seem to be European and not brown colored US types.
 
CeeVee

CeeVee

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Joined 2006
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buzzforb said:
Interesting thoughts. Not positive those are opto's, but very possible. Makes my head hurt to think about. Servo is probably both simple and another point o manipulation. Unity gain FE could setup interesting output. Transformerish. If your hallucinating, I'm the walking dead.:)
Click to expand...

Yes those are Opto's : L 1205 4N35...and the OA is OPA2604AP....have a hires photo.:cool:
 
buzzforb

buzzforb

Member
Joined 2010
It is a high bandwidth design, I fI had to guess, and is probably susceptible to instablilty if not laid out correctly. I would imagine the caps on the input are to limit bandwidth, but they could be there to eliminate DC interaction with the FE. Seems unlikely with the servos, but could be wrong.
 
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