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CurtZHP

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Mar 21, 2005
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Location
Allentown, PA
I've come across a couple UTC transformers:  an LS-10x and an LS-50.  Thought they might make good input and output for a possible mic preamp.  Bodged this schematic together.

If this works, I'd like to make a few additions to it.  I'd like to possibly add some sort of basic tone control, maybe based on a tone stack from a guitar amp.  And maybe some sort of compression or limiter circuit.  FX loop?  And where would be the best place to put these in the circuit if I were to do so?

Also, on the LS-10x are multiple taps for different input impedances.  Any benefit to rigging some sort of switch to take advantage of this?


 

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That's a pretty snazzy transformer to use with a guitar amp circuit. I don't know anything about tube mic pre circuits but I would find one that is known to be quiet and more linear and generally more "high performance" to match the transformer. Guitar amp circuits are all about high-gain, band-pass filtering and power and not so much about fidelity. But who knows. Maybe it would sound cool anyway.
 
1Meg gain pots, at half-way, are 250K sources.

12AX7 grid capacitance is near 100pFd.

100pFd against 250K is 6,666Hz high-cut.

You potentially have two, for -2dB at 3,333Hz.

Your smallish 47K plate loads could easily drive lower-R pots for a higher high-cut.
 
1Meg gain pots, at half-way, are 250K sources.

12AX7 grid capacitance is near 100pFd.

100pFd against 250K is 6,666Hz high-cut.

You potentially have two, for -2dB at 3,333Hz.

Your smallish 47K plate loads could easily drive lower-R pots for a higher high-cut.


Oops!  Sorry.  Most of this schematic was cut and pasted from a similar project I did a while back.  Forgot to change a few things!
Nothing like a bad MP3-quality tube preamp!  :-[

Meant to up the plate resistors to 100K.  I was going to leave the pots at 1M, but should I drop those to maybe 50K?

 
Pots would generally be 2X to 5X the plate resistors.

50K pot on 100K Rp is going a bit far.
 
CurtZHP said:
I've come across a couple UTC transformers:  an LS-10x and an LS-50.  Thought they might make good input and output for a possible mic preamp.  Bodged this schematic together.
Do you need that much gain? Wet finger says about 85dB.


If this works, I'd like to make a few additions to it.  I'd like to possibly add some sort of basic tone control, maybe based on a tone stack from a guitar amp.
  A guitar tone stack is nowhere near flat; typically has a midrange cut of several dB.


And maybe some sort of compression or limiter circuit.  FX loop?  And where would be the best place to put these in the circuit if I were to do so?
I would not create an insert point. Just use the low impedance output at the secondary to drive outboard stuff. An insert point needs buffering.

Also, on the LS-10x are multiple taps for different input impedances.  Any benefit to rigging some sort of switch to take advantage of this?
The only case where you would want a different impedance is when you have low impedance (30-50ohm) mics.
I would just wire the 200/250 ohm primary.
 
Do you need that much gain? Wet finger says about 85dB.

I'll have to make a few revisions at repost the schematic.  If you look over my exchange with PRR, I have a few component values wrong.
 
A guitar tone stack is nowhere near flat; typically has a midrange cut of several dB.

I would not create an insert point. Just use the low impedance output at the secondary to drive outboard stuff. An insert point needs buffering.


The reason I was considering such features was that this thing is going to be a single channel unit, so I was thinking in terms of building a "channel strip" type gadget.

Then again, if I could scare up another set of transformers, it would make a decent stereo preamp.
 
CurtZHP said:
I'll have to make a few revisions at repost the schematic.  If you look over my exchange with PRR, I have a few component values wrong.
Even with some component value changes, the 3-stage topology without NFB yields too much gain for most purposes. Indeed, you can attenuate with the pots, but I reckon there are better alternatives.
 
Even with some component value changes, the 3-stage topology without NFB yields too much gain for most purposes.

Since, as you mentioned earlier, I would need a buffer stage if I wanted to add other features, could I repurpose one of the existing stages accordingly?
 
CurtZHP said:
Since, as you mentioned earlier, I would need a buffer stage if I wanted to add other features, could I repurpose one of the existing stages accordingly?
It depends on what you want to achieve in terms of maximum gain. With two stages, I think you could achieve about 50-55dB.
 
Then you may reconfigure the second stage as a voltage follower driving an unbalanced Insert Send; the unbalanced Insert Return could drive directly the SRPP output stage. Max gain at Insert Send would be about 54dB. You would have about 5-6dB gain between Insert RTN and output (using the 600 ohm secondary).
 
OK, this has me scratching my head....

According to one set of calculations I did, I came up with a gain of 30 on the first stage (and pretty much the second stage too, since it's all but identical...).  Then I found another formula that factors in the cathode bypass cap, and when that's factored in, the gain goes way up to 55!  That's just on one stage!

 
Not needed.  Doubt you need R7 either.  C1 is far too large. 

LS-10 and LS-50 are far too nice to build a fuzzbox.   
 

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