The Unsure DIY Preamp

[skipwave]

Let me preface this by saying that I am very new to DIY electronics, and have yet to graduate from modding to building from scratch. I picked up one of these for $10 on evilbay:

http://www.shure.com/pdf/discontinued/m64a.pdf

The manual convinced me this would be a good piece to hack apart and experiment with. I put a couple of Beyerdynamic TR/BV TXs in front of it, and a bipolar output buffer after, using a scavenged TL084. I also did the mod described in the manual, placing 330 Rs across R13 and R14.

With a pot inbetween the m64 and the IC, this is a cheap little mike pre, though I'm un-s(h)ure that the sound is usable for much of anything. A couple of questions:

Would it be worth replacing the transistors? The manual for the original m64 (the A generation seems nearly identical) shows the following replacement parts for these:

Q1,Q2: NPN Transistor, Silicon, T.I. TIS97
Q3,Q4: PNP Transistor, Silicon, similar to Motorola 2N5087

Also, can I remove the coupling capacitors on the output of the m64, since this now goes to the pot and the IC output stage has caps?

Thanks in advance.

 

[rodabod]

So, does it actually sound good with the current mods?


If it is not excellent, then you could use your beyer transformer to make the preamp which Samuel Groner suggested which utilises 2 NE5534 or OPA604 opamps.

I can post a link to the schematic.

I will post pictures of mine soon and maybe give some specs. It has a huge amount of gain - more than enough for even very quiet ribbons. Very low noise too.

 

[skipwave]

Hey Rodabod,

Thanks for the response.

I pulled the coupling caps off the input and output, as well as a 1k resistor on each ouput. Now its just Input TX -> transistors -> volume pot -> TL084 Buffer Out.

Sounds pretty nice, IMHO. A slightly grainy quality to it, but I'm not expecting ultra clean from this. Next I'm going to add a second gain stage after the S(h)ure, consisting of a couple LT1363s I have lying around. That'll bring the gain range up to a more respectable 60db.

I'll report back when that's done. Any suggestions are, of course, welcome.

 

[rodabod]

Sounds ok. I'll try and get a proper a/b of my preamp to show how it sounds in comparison to a bog-standard preamp.

I'll see if I can also get a sig. gen. and calculate the actual max. gain since it seems to have a hell of a lot.