building a mic preamp... transformers?

[imaradiostar]

Then build something akin to a neve input stage- that's where a lot of that color will come from.

jamie

[Steve Conner]

All the old preamps people love seem to color your sound and use transformers.

It doesn't necessarily follow from that that the transformers are responsible for the coloration. Those old preamps also used discrete circuitry that overloaded in a musical way.

If you just have to have a transformer, that Edcor MXL5 looks fine for the input, and the PC10K/10K should work as the output transformer.

[TimWaldvogel]

Ok perfect. Would that edcor input transformer work with the Jensen Mic preamp design Jamie posted above? Or would it need some minor modification case the op amp is seeing a different input impedance?

[imaradiostar]

The edcor will work fine but don't expect it to sound any different from an op-amp based preamp without a transformer.

If you want a super simple, decent sounding but slightly colored preamp that'll work with an MXL5 at the input and your 10k:10k at the output you should find the tape-op article on the "hamptone" jfet preamp. It's Jeff Hampton's design IIRC and it's simple and effective. It will achieve coloration similar to a tube circuit when pushed without hitting an immediate "brick wall" like an opamp based pre.

Steve and I have previously discussed building common source jfet based preamps for our own use- I'd probably use something akin to the "fetzer valve" from runoff groove for it's triode like distortion characteristics. If you've never built anything like that before the hamptone circuit is a great intro to it and should give a reliable outcome with little to no tweaking.

jamie

[TimWaldvogel]

I have bad luck with transistors. I have built 5 or 6 guitar pedals and I am lightly active in the diystompboxes forum. I just have no knowledge of how a mic preamp works. I am learn though. I was looking at that design. One of my questions is are there any other transistor substitutes for that circuit?
I am cursed with transistor circuits haha

The goal I have is to order the transformer and avoid ordering any other parts lol

And since I have some dual op amps laying around I was considering designing it around 2 stages that push each other

[imaradiostar]

I understand what you're getting at. Biasing transistors can be frustrating. Biasing jfets is much simpler than bipolars in my opinion- they bias up like tubes! Connect the gate to ground with a 1 meg resistor. You can pick a smallish source to ground resistor- say 470 ohms- and use a trimpot for the drain to positive. Vary the value of the pot till the drain is at 1/2 of b+ voltage. Signal output is taken using a film cap connected at the drain. It should amplify nicely and use less components than an op amp circuit. Add an electrolytic cap from source to ground for a gain boost, skip the cap from source to ground if you want "tubier" distortion.

There is nothing wrong with an op-amp based pre but you'll get similar results by putting a 600:600 transformer in front of any commercial op-amp based preamp.

Do you have a breadboard? That's where I'd start. Try some things and see how they sound. They'll likely be a touch noisier on the breadboard but it'll give you a feel for how they work.

jamie

[TimWaldvogel]

Well I was thinking about taking the Jensen schematic for a low noise opamp, lowerin the gain a little, and adding bypassable passive 3 knob eq after the initial gain stage. after the eq I would add another adjustable gain stage after the fact to have loads of gain with plenty of options for color and a gainy overdrive

[TimWaldvogel]

Change of plan

Let's say I want to use an op amp to bring in the signal balanced instead of a transformer and then do my edcor pc10k/10k to rebalance te signal after my amplification.
Can someone refer me to a page to learn how to bias op amps and maybe a circuit with a low noise op amp mic pre schematic like the ne5532/34

[imaradiostar]

I'll say it one more time then I'll let it go. You probably won't like an opamp mic pre that is distorting. It doesn't generally sound good. If you'd like I can refer you to an AES paper that describes the details- if I can find it.

Classic transformer+op-amp mic pres such as an API mic pre have a good rep because they're excellent microphone preamps- not because they distort in some perfect way. There are some op-amps that distort in a more pleasing way than others but in general they're never going to sound like a discrete class a mic preamp. In a similar manner- classic Trident A-range and Neve preamps are desirable because of a very subtle but unique sonic coloration, not because they're some sort of panacea for audio. There is no golden signal path.

Jamie