Consul
Well-known member
I've accumulated a crapload of parts over the years for projects that were never started. I've been doing an inventory recently, and I can't help but wonder if I have all the parts I need to make a pair of decent mic pres. I don't have any Jensens or 2520s, but I do have a pair of Shure 1:10 mic pre trafos (with shields), a tube full of 5532s, and some API output iron. What could possibly go wrong?
So, with that in mind, I knocked out the attached schematic (using DaveCAD). All it is is two inverting amp stages, with the first one (gain of 2) acting more or less like a buffer for the second main stage. So, a gain of 10 from the trafo, 2 from the first stage, and 1 to ~50 from the third. Here are my questions:
1) The cap shorting across the input after the trafo. I've seen this in a lot of schematics, and from what I can tell, it's there to kill any resonant peak caused by the fact that the secondary of the trafo and the input resistance and capacitance of the first gain stage basically make an LRC filter. Is that right? If so, I'm pretty sure I can figure out the value of the cap I need.
2) I'm wondering if I should make the first stage a unity gain buffer and just run the second stage with a gain from 1 to ~100. I did it this way thinking in terms of a) not wanting to use a ganged pot, and b) Gain Bandwidth Product, but that might not matter with the 5532. I haven't done that math yet.
3) The output trafos are those quadrafilar-wound API output thingies I got from a group buy (I think) quite some time ago. I'm pretty sure the 5532 can drive those, but want to make sure.
I would like to point out that I left a bunch of the usual stuff off the schematic, like the bypass caps for the power rails, phantom power, and all that. I'm also sure there are some tricks I can use to improve the circuit's stability. Any thoughts would be greatly appreciated.
This post was written under the influence of Jim Croce.
So, with that in mind, I knocked out the attached schematic (using DaveCAD). All it is is two inverting amp stages, with the first one (gain of 2) acting more or less like a buffer for the second main stage. So, a gain of 10 from the trafo, 2 from the first stage, and 1 to ~50 from the third. Here are my questions:
1) The cap shorting across the input after the trafo. I've seen this in a lot of schematics, and from what I can tell, it's there to kill any resonant peak caused by the fact that the secondary of the trafo and the input resistance and capacitance of the first gain stage basically make an LRC filter. Is that right? If so, I'm pretty sure I can figure out the value of the cap I need.
2) I'm wondering if I should make the first stage a unity gain buffer and just run the second stage with a gain from 1 to ~100. I did it this way thinking in terms of a) not wanting to use a ganged pot, and b) Gain Bandwidth Product, but that might not matter with the 5532. I haven't done that math yet.
3) The output trafos are those quadrafilar-wound API output thingies I got from a group buy (I think) quite some time ago. I'm pretty sure the 5532 can drive those, but want to make sure.
I would like to point out that I left a bunch of the usual stuff off the schematic, like the bypass caps for the power rails, phantom power, and all that. I'm also sure there are some tricks I can use to improve the circuit's stability. Any thoughts would be greatly appreciated.
This post was written under the influence of Jim Croce.