Mic pre idea

[whitelabrat]

I've got a project a brew'n in my head. I've got a good ole Shure SM-57, but I've dumped my dumpy Rad Shack mixer board so I don't have anything that I can use as a pre for recording. I was thinking of buying a cheap ART mic pre then I started thinking (uh oh).

I've got a bunch of lo-z transformers I've pulled from a Bogen PA amp. I don't know what the exact ration is, but I'd guess about 1:4? I've also got a beefy 15vdc wall wart and miscellaneous 12vdc wall warts. I've also got a salvaged transformer that could probably spit out about 45vac if I ignore the center tap.

I could copy the mic pre from the Bogen MX60a, but it only uses half a 12ax7 from what I can tell. So I'm thinking going my own way and using perhaps a small pentode of some sort or both sides of a 12at7.

The Emu sound card I'd like to feed has a 10k ohm input on it, so I'm guessing that should be tube friendly.

I'd like to use some of my neglected tubes: 6AU6's, 6KT8's , 6GH8's, 6AN5's, 6AN8's.

Any suggestions?

[whitelabrat]

I'm thinking either a 6KT8 or a 6GH8 may be the way to go even if I can only use one side of the tubes. I don't see why I couldn't use the pentode side in triode operation.

It appears the Bogen TM-200 mic transformer has an input of 200ohms. I need to figure out what the output is.

I figure a target voltage of about 80v would do. If I go over 100v then the electrolytics I've got laying around can't be used in the power supply.

Time to fire up Tube Cad.

[Geek]

Hi,

All of those tubes you mentioned can be rather microphonic, depending on brand.

The 6GH8 is a nice tube, but save it for a mu-stage follower for a line driver. Maybe on the backend of your mic pre

The 6AN8 and 6AU6 would be my choice of those for the frontend, especially if trioded.

Though I have not worked yet with the 6KT8.

Cheers!

[whitelabrat]

I thought this link has an interesting idea. Figure 4 especially.

http://www7.taosnet.com/f10/mustage.html

I'm thinking the 6KT8 would be ideal where the triode side could provide a good bit of gain. Microphonics may be a problem though, but I guess I'll have to just try it and see.

I won't need a extra gain stage will I? I'm hoping to get away with a single 6KT8 if possible.

[whitelabrat]

I've rummaged up some parts.

3 9 pin sockets
4 6KT8's
Bogen TM-200
0.47uf film cap
0.047uf film cap
4 100v 2200uf caps
35v 1000uf cap
20vct transformer
rectifier diodes
terminal strips
chassis

[whitelabrat]

I was just playing with Duncan Amps psu tool. A voltage doubler circuit with the 40vct transformer using the 2200uF 100v caps may squeeze about 90vdc out. I'll have to find something for the heaters.

I think Geek has the right idea. 6AU6 frontend followed by a 6GH8 mu-stage. I've got a mess of 6AU6's and seven 6GH8's about. I suspect things will be sub optimal at the low voltages though.

[dsavitsk]

Originally posted by whitelabrat
I suspect things will be sub optimal at the low voltages though.

Transformer in the picture looks like it is capable of plenty of current -- why not build a voltage doubler?

[whitelabrat]

why not build a voltage doubler?

Absolutely. The PT was pulled from a yucky power amp, so I'm guessing it's good for at least 1A at 40v. I just realized that it probably won't fit into the chassis I have sitting there.

I do have an unhappy Realistic MPA-95 (100W mono). It could be a donor for a chassis, XLR sockets, and Power Transformer. I'll check it out tonite.

[whitelabrat]

I'm liking the Realistic as an iron donor. The PS appears to generate about 65vac, which should be good for about 140vdc from a voltage doubler circuit. That should be fairly optimal for the tubes involved. I'll just need to use higher voltage rated caps on the smoothing end of things.

[DigitalJunkie]

I'll just need to use higher voltage rated caps on the smoothing end of things.

You could 'stack' your capacitors in series (parallel a 220k? resistor on each cap) and make a 200V 1100uf cap.

Or,do you have any old PC power supplies around,or other likely switch-mode supplies? They usually have at least one HV (200-400V) rated cap inside.