ISA 110 Mic Preamp Circuit

Hello, is it possible someone has a semplified version of the ISA 110 schematic with pot instead of switch?
Attached the circuit I have found on the net

Use two pots like 50k shunt. and I think a 5k feedback. but these are usual rev log tapers. I would rather use a switch 6 pos for shunt and 5 k reverse  log for feedback side.  Breadboard and play around the logic of that switch is pretty obvious

To me it's the LL1538 transformer. That gives them their unique sound.  The gain switch is calibrated and so quickly repeatable. 

Hello Fazer, do you have a schematic with the pot instead the switch?

I don't have a schematic but the switch is 2 sections.  One shunts the output of the transformer to  input side of the opamp.  The other is a resistor string on the feedback side of the 5534 opamp.  You can breadboard the circuit on a pin board like from radioshack and then see how you like using it as a 2 stage gain control.  The switch logic is a string of resistors chosen to create exact gain steps.  So add up the string and thats the value of the pot you sub in.  I think the shunt side is about 40k.  Good luck with that value so substitute a 50k pot for the string and now the shunt is variable.  The feedback string is about  5 k so use a 5 k pot for it.  Just think of the switch as  steps and the wiper of the pot makes contact where the switch positions are.  Realize that the 2 pole switch as 2 resistors / pots that take over where the shunt string stops having positions that matter which is where the resistors on the switch contacts are connected together with a jumper from pole to pole.

The shunt side of the switch is on the left side and the feedback resistor string is on the right side of the schematic.  One more thing Focusrite also uses an output amp with a transformer that might be hard to find.  It is the other side of the equation that can have an effect on the sound.  Its not in your diagram.  The output of that preamp is going to be down at  -10dbV or so.  The output amp lifts that up to +4 dbu.    Depending on what your driving this might be low level for a workstation input.  Depends on what your line level is for your interface. 

Get an opamp cookbook and study how  a simple mic preamp is put together.  I would breadboard this circuit and play around with it.  Also the Green preamp circuit is here but is transformer less.  You could add an LL1538 to it and I would think get a very similar sounding preamp.  The output amp and transformer schematic can be found in the focus rite  Red 6 in the Tech section on this site.

Thank you Fazer, but unfortunately I am not too expert to realize what you have written, so if someone can redraw the circuit with the pot  may be a great help 

> pot instead of switch?

No.

The switch is doing two different things-- amplifier gain and input attenuation. You could not afford such a custom dual pot.

A 2P12T switch is not common but not THAT expensive.

I think the idea was to replace the switch with two pots?

Strawtles said:

Hello, is it possible someone has a simplified version of the ISA 110 schematic with pot instead of switch?

Click to expand...

how about a simplified version with a one pole switch?
ditch the presence sucking attenuator between the transformer secondary and op-amp.
the concept isn't "modern" but will surely transfer the most sonic detail from that fine Lundahl input transformer.
if you need more input gain options, consider a primary winding series/parallel switch (-6 dB)
as well as the traditional -20 dB pad switch.

What's so awful about switches? They work predictably, they are extremely clean, and you can pull off the complicated attenuation and gain control needed for that circuit very simply. It's hard to say that about pots. I found some dual deck 11 position Grayhill rotary switches at Mouser that were less that $20, but were sealed, with gold over silver contacts - essentially ideal for decades of use. I bet other places have less fancy switches for less money.

How cheap does it have to be?

My only problem with the switch is that I don't know how to wire it ... the pot is simpler 
I have considered this preamp because I need some good but simple preamps with unbalanced output for external recordings

Well, using a switch, you already have a circuit that actually works. Nobody here knows how to make that circuit using pots, and the designers certainly thought not to use pots, so that seems a few steps back, possibly an infinite number of steps back. ;-)

Learn how to read schematics and you'll be much happier in the end! It's not terribly difficult, but you do have to  get used to the symbols used and the traditions that people used when drawing schematics. It's sort of like reading music - it's not obvious immediately, but it's a consistent, finite language with a few variations here and there.