unknown tube mic pre (w/schem)

[12afael]

this preamp is based on a design from a big L.A. area estudio back in the 90s.
someone recognize the design?

http://www.mediamax.com/plexilandia/Hosted/redpre.gif

Rafael

 

[Marik]

On my screen the cathode resistor of the first tube looks like 20K. Are you sure about that?

Also, it seems without NFB the circuit will be overloading with some hot mics, but I guess it is not your question.

 

[12afael]

the 20k catode resistor can perfectly be my mistake, a 2k R have more sense there.

the guy who build this pre hate NFB and the pre is compressing the signal.
I would like to see the original schematic.

 

[blandman74]

Looks interesting, with lots of gain. I don't understand the pad switch. Seems like it would present a very low load to a microphone, if using 150:15k input transformer.

 

[pstamler]

The pad switch is screwy; as drawn, either the tube's grid is not connected to the transformer (muted), or it's connected with a 2.5k resistor in parallel with the secondary (which will give 25 ohms at the primary, way too low), or it shorts out the 25k resistor but doesn't connect to the transformer (muted again).

If the swinger of the switch was attached to the grid, one of the other terminals to the transformer and one end of the 25k resistor, the other terminal to the junction between the 25k and the 2.5k, then it's work right -- although the input impedance would still be 275 ohms, still quite low for dynamic mics and way low for condensers and ribbons.

The rest of the circuit looks like boilerplate, done by somebody who doesn't know much about good tube design. The input section has way too much gain and will be overloading most of the time (especially with only 200V plate supply); the output section uses a 12AX7 triode as another voltage amplifier, driving a pair of 12AX7 sections in parallel as cathode followers, which drive the transformer. With a plate supply voltage of 250V, this setup won't have much headroom, given the stepdown of the transformer.

When I've done similar designs (different tubes) with the plate supply at 330V, they clipped at about +24dBu into, say, 20k. I'd expect the clipping point to be more like +22dBu with a 260V supply; stepping that down 12dB through the transformer, it'll clip around +10dBu out, or 6dB of headroom over standard +4dBu operating level. Pretty pathetic.

And that's ignoring the 12AX7 cathode follower trying to drive a 10k load. It won't do it without generating lots of distortion. A 6SN7 would at least have enough current, although not enough voltage.

My guess is that somebody at the studio read a couple of application notes and threw this thing together -- "Hey, wow, look at all the distortion! I guess that's what tubes are about, huh?"

Peace,
Paul

 

[Peter Simonsen]

[quote author="12afael"]this preamp is based on a design from a big L.A. area estudio back in the 90s.
someone recognize the design?

http://www.mediamax.com/plexilandia/Hosted/redpre.gif

Rafael[/quote]

Hi rafael,

Naargh not really?...But how does the thing sound?..Could be sounding darn good ..

Kind regards

Peter

 

[NewYorkDave]

I'm with Paul; this circuit doesn't look particularly clever or good.

 

[12afael]

I`m using it to record some vocals with an ADK 51s, It have a lot of gain too much maybe, I use the gain at 3 and with the pad. If I measure it right it is loosing high frequences and is a bit sensible to mic cables.

the studio was A&M

 

[pstamler]

If the input impedances are what was given, not only will it lose high frequencies due to transformer overdamping (in the mic and in the preamp), but it'll probably make the microphone's head amplifier distort.

Peace,
Paul

 

[skipwave]

Looks like they wanted an altec 1566.

 

[ruffrecords]

Seems rather reminiscent of the old Altec 1556 design. Look here:

http://www.tangible-technology.com/tubes/1566/1566.html

Ian

 

[skipwave]

Like I said...



:razz:

 

[12afael]

I had a short talk with the guy how built this preamp, he just built it as in the schematic.

I agree, the altec 1566 is very similar.