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JamMan 1st November 2004 01:02 PM

Power supplies in mic tube preamps
 
Hello to you all,

I want to build a mic tube preamp, as simple as possible, with a good 70's rock sound.

There are lots of schematics out there. Can recommend one particularly?

My main question concerns the power supplies. For example, in the Gyraf audio G9, there are more components in the PS than in a whole tube amp I made for guitar. Are PS:s as extensive as this really needed?

http://www.gyraf.dk/gy_pd/g9/g9pd.htm

I would really like to make my preamp PTP, on an eyelet board or on a turret board, and mount tube sockets in the chassis. But then again, can I make my PS "good" enough with this style?

What would be wrong if I used good quality power transformers (not toroid), good quality components for filtering, rectification etc. ?

Thanks for any replies,
JT

Brett 1st November 2004 05:48 PM

Re: Power supplies in mic tube preamps
 
Quote:

Originally posted by JamMan
Hello to you all,

I want to build a mic tube preamp, as simple as possible, with a good 70's rock sound.

There are lots of schematics out there. Can recommend one particularly?

To my mind, a mic pre should not have a sound at all. Get any variation you want in post-processing, not in the initial record, 'cos then you're stuck with that sound.

As for a circuit, this is what we use. Ditch the RIAA components and volume control, and add transformers if you want them. But you'll need to include +48V phantom power, which you can take from the Gyraf with minimal mods.

Quote:

My main question concerns the power supplies. For example, in the Gyraf audio G9, there are more components in the PS than in a whole tube amp I made for guitar. Are PS:s as extensive as this really needed?
IME, yes, but I wouldn't build anything like the Gyraf cct. If you think that PS is big, you should see the one powering mine. Think of the signal levels you're dealing with here, and that this is the primary component in the entire recording signal chain.

Quote:

I would really like to make my preamp PTP, on an eyelet board or on a turret board, and mount tube sockets in the chassis. But then again, can I make my PS "good" enough with this style?
If you don't make any mistakes wiring it up, it's likely to be better p2p then pcb.

Quote:

What would be wrong if I used good quality power transformers (not toroid), good quality components for filtering, rectification etc. ?
As I said, I don't like the Gyraf cct, based on my own experience. I'd prefer EI core mains trans (with ES sheild), seperate filament trans, damper diode rectification and C(small)LCLC filtering. Shunt regs are good too.

JamMan 2nd November 2004 08:48 AM

Thanks Brett,

Quote:

Originally posted by Brett
To my mind, a mic pre should not have a sound at all. Get any variation you want in post-processing, not in the initial record, 'cos then you're stuck with that sound.
Well, you're right. But I don't have enough good recording gear for that to matter. If it colours the sound, I don't mind as long as it sounds good!

Quote:


As for a circuit, this is what we use...

That looks pro...and a bit too complicated for me...

Quote:


If you don't make any mistakes wiring it up, it's likely to be better p2p then pcb.

OK.

Do you or someone else think that all the voltages used must come from a regulator? I'm not sure I can find a regulator for 250 VDC or similar...actually I could have troubles finding a regulator for the phantom power...

Quote:


As I said, I don't like the Gyraf cct, based on my own experience. I'd prefer EI core mains trans (with ES sheild), seperate filament trans, damper diode rectification and C(small)LCLC filtering. Shunt regs are good too.

I would like to use the same components one uses for guitar tube amps as much as possible. I'm not sure how the power transformers in those would perform...

EC8010 2nd November 2004 01:28 PM

The signal leaving your microphone is fragile. A microphone amplifier that amplifies that signal cleanly must have clean power supplies - that's why the power supply is the most important part of the circuit after the input transformer. Microphone amplifiers are very like RIAA stages, but with one important difference: if you play a record and it doesn't sound nice, you simply go away, build a better RIAA stage and play it again. You don't have this luxury with a microphone amplifier. You have one chance to capture the sound and that's it - you're stuck with it.

As Brett says, point to point construction will be fine. One other thing worth considering is placing the microphone amplifier in the studio rather than the control room, thus lessening the length of cable that the microphone level signal has to travel down and the amount of noise it can pick up.

Incidentally, it's better to apply the +48V phantom power to the centre tap of the input transformer via a pair of parallelled 6k8 resistors (thus relying on transformer balance) than an individual 6k8 resistor to each leg (relying on resistor balance).

Brett 2nd November 2004 01:39 PM

Quote:

Originally posted by JamMan
Well, you're right. But I don't have enough good recording gear for that to matter. If it colours the sound, I don't mind as long as it sounds good!
If you're going to go to all the effort of building it, do the very best you can, and it can stay with you through workstation and other upgrades.
Quote:

That looks pro...and a bit too complicated for me...
It's not really that difficult. And it sounds superb.
Quote:

Do you or someone else think that all the voltages used must come from a regulator? I'm not sure I can find a regulator for 250 VDC or similar...actually I could have troubles finding a regulator for the phantom power...
Most regulators, especially series regs sound worse to my ears than a well filtered passive supply.
Quote:

I would like to use the same components one uses for guitar tube amps as much as possible. I'm not sure how the power transformers in those would perform...
Why? Most of the components in guitar amps are crap for use in high-fidelity. They're fine if they do the job reliably in an MI amp and create the sound you want, but still usually low performance rubbish. FTR, I've repaired a lot of MI amps and build some customs now.

PS: +1 for everything in EC8010's post, especially moving the preamp into the studio when in use.

mcs 2nd November 2004 02:48 PM

Re: Power supplies in mic tube preamps
 
Quote:

Originally posted by JamMan
[B]There are lots of schematics out there. Can recommend one particularly?
I would recommend the G9. Many people I trust like it a lot. I haven't completed my own yet.

Quote:

My main question concerns the power supplies. For example, in the Gyraf audio G9, there are more components in the PS than in a whole tube amp I made for guitar. Are PS:s as extensive as this really needed?
You could also make a more "normal" powersupply with a tube rectifier and more than one L-C filtering stage - but it will be a lot bigger and more expensive than the G9 supply. You will also have to place the transformer and chokes far away from the signal transformers and tubes. If you use a toroid (or R-core transformer), you can place the power transformer closer to the amp.

Quote:

I would really like to make my preamp PTP, on an eyelet board or on a turret board, and mount tube sockets in the chassis. But then again, can I make my PS "good" enough with this style?
Yes, you can build everything PTP, but why bother? If you use the G9 PCB (which you can buy ready-made BTW), you will get a stable working preamp very easily.

Best regards,

Mikkel C. SImonsen


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