M-1/312 hybrid preamp built question

[Dyonight]

First of all, this is my first attempt in DIY mic preamp so if I ask too much newbe questions in this thread just tell me and I can send PM instead.

From the pics of a 4 channels build on your site, I see that 5 horizontal channels would fit. So I'll design the front panel to accept 20 horizontal modules in a 4U.

I'd like to make a hybrid version (since it's possible), with a M-1 front end and a 312 back end.

I'd like to put the Lundahl 1538 at the input, the Hardy 990C+ and the Ed Anderson 2503 output transformer.

A quick question about the Lundahl 1538 vs 1538XL:

1. Both SPL and Focusrite use the XL version in their design, is there a benefit in using the XL instead?

ok next

I'd like the stepped input gain to drive the Lundahl/990 and the variable trim pot to feed the 2503.

So:

2.  Would it work? Is the TRIM pot before the output transformer? Sorry if the answer is obvious...

And:

3.  Does the trim pot act as an attenuation only or does it act like a "fader" to push the out level +6db or so?

Last:

4. To implement those parts (Lundahl 1538 (XL?), Hardy 990C+, EA 2503), could I built the 312 and simply swap the input xformer and opamp or the circuit parts must adapted to accept those two "non-312" parts?

On a side note, the DIY JE-990 provided by Hairball audio look like a very nice alternative to the Hardy's own 990C+ for half the price.

Any experience with this one?

This price difference will truly matter when multiplied 20 times...

 

[Dyonight]

ah yeah, I copied/pasted my questions from another forum to help others (and let others help me ) but the question is refering to the hybrid pcb offered by mr Bob Schwenkler here

 

[bobschwenkler]

1. Both SPL and Focusrite use the XL version in their design, is there a benefit in using the XL instead?

2.  Would it work? Is the TRIM pot before the output transformer? Sorry if the answer is obvious...

3.  Does the trim pot act as an attenuation only or does it act like a "fader" to push the out level +6db or so?

4. To implement those parts (Lundahl 1538 (XL?), Hardy 990C+, EA 2503), could I built the 312 and simply swap the input xformer and opamp or the circuit parts must adapted to accept those two "non-312" parts?

Thank for your help!

1. It can accept higher levels before distortion. You'd have to ask others to be sure but the normal version should be more than adequate for any normal situation.

2. The stepped AND trim controls are within the negative feedback for the op amp. They'll only affect the level going out to the 2503.

3. It's arbitrary, depending on your reference point. It changes the NFB of the op amp.

4. Sort of. Your gain will drop a fair bit because of the ratio difference between the two types of transformers. You'd ideally want to recalculate the negative feedback network to get the gain back up to a more useable place. I provide an Excel sheet you can use to do this easily. You'll also want to lower the value of the resistor that sets the input impedance, R4.

I've done my own DIY 990 amps and I would totally recommend the same to any other confident builder.

 

[Dyonight]

ok so it looks like I'm asking for troubles...  like I said I'm new to all this so I'll keep everything as simple as possible.

I'll probably just go for 10x 312 and 10x M-1.

I noticed on the LL1538 BOM that the ouput stage is solid state, what kind of transformer could replace this in a "put it there and it work" fashion?

 

[bobschwenkler]

A 1:1 would be ideal to keep gains the same, but on the technical side I don't have much experience here. Anyone else have thoughts? You might email Cinemag, or any of the other xfrmr companies to ask their thoughts on an output for the 990.

 

[Dyonight]

very nice I'll just do that and post the results of my findings!

 

[mitsos]

I would recommend a nickel output trafo, 1:1 for the M1 circuit.  You can get these from Cinemag (CMOB-2H or CMOB-2L) and Altran (don't remember the part number offhand, but we did a group buy years ago).  Contact member Nielsk, he bought tons back then and may have some left over. In fact, ask him if he has any of the other group buy trafos too (we did 312 and 990 in/out trafo group buys, for a 312/990 PCB, similar to Bob's).