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Old 5th June 2008, 04:19 PM   #1
PRNDL is offline PRNDL  United States
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Default tube microphone preamp design ideas

A friend asked if I could build him a nice tube microphone preamp, which would sound as good as some of the high end retail ones.

I've searched and came up with a few examples, with these two as good candidates:
http://www.frontiernet.net/~jff/pult...onePreamp.html
http://boozhoundlabs.com/

Transformers appear to be the most crucial selection with the usual trade-off of price vs. quality. Several recommend Edcor, although some didn't like their input transformers.

Tube choice is also important. The Pultec MB-1 used a 12AX7, which some don't like for mic pres. The BHL designs use 6SL7 and 6V6 with about 100V B+. The BHL Aikido uses 6SN7/6SL7 with 300 B+ and Altec/Peerless 15095 input and output transformers.

At this point, the BHL Aikido looks like a great idea.

I thought it might be wise to ask for some advice before proceeding.
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Old 5th June 2008, 04:43 PM   #2
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Hi!

I would suggest you designs that are known for good sound and well documented. IMO, the most important thing for diy mic pre is documentation; there are many schematics with mistakes, some are designed by people who don't really know what mic pre is, other are not tested etc.
I think Pultec MB1 is very nice if you need one channel of simple tube preamp so your friend can try real tubes. Documentation is also well done, so i don't think you would have problems building it. And it's very cheap.
You could also build G9 preamp from Gyraf:
http://www.gyraf.dk/gy_pd/g9/g9pd.htm
Many people built it, there is plenty of informations about possible problems and tweaks, it sounds very good etc. To keep the cost low you can use OEP transformer for the input and Cinemag for the output, check for options. If your friend need only one channel you can build it p2p. This one is a bit more expensive and complicated than the Pultec if you build it on pcb...
I suggest you to check Prodigy Pro forum. That's the place for this kind of thing. If you want something even simpler than the Pultec check for NYD One Bottle. It's tube preamp based on 12AV7 tube, very very simple and cheap. Many people built it and i think there is also layout fot it.
Ok, enough I would build Pultec with Cinemag in and Edcor out...

Good luck!

Miha
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Old 5th June 2008, 07:30 PM   #3
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The Ampex 351 recorder electronics has long been revered as a great mike preamp. Just look at the price they command on fee-bay. I once built a nice multi channel mixer using several of the first two record input stages in parallel. (top section of the 1st link below) Yes, it's a 12AX7, but so what. This recorder, and the 300 with a similar input, was used to make those Mercury Living Presence and RCA Living Stereo recordings that people swoon over.

I used a UTC HA-100X input trans (very expensive) but an A-10 should be just as good as should a Lundal equivalent or the smaller Ampex plug-in types. The Ampex 601 also had a nice mike input as well. The first stage plate load resistor is a critical low noise part. And you should also shock mount the tube if at all possible.

http://ftp.ampex.com/ampex/manuals/a...0960.schem.pdf

http://ftp.ampex.com/ampex/manuals/a...n/601schem.gif

http://ampexdata.com/html/legacy_manuals.html

Victor
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Old 6th June 2008, 01:00 AM   #4
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Steve Eddy is fond of CineMag mike trafos. The CM stuff costs less than Jensen (Cal.) and, according to SE, sounds fine.
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Old 6th June 2008, 04:42 AM   #5
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Do you need design ideas, or advices about which known brand to copy?
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Old 6th June 2008, 06:27 AM   #6
PRNDL is offline PRNDL  United States
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Quote:
Originally posted by Wavebourn
Do you need design ideas, or advices about which known brand to copy?
It would be easier to copy an existing design, but I have found only those two that have build info and use modern components, which is surprising.

I'm interested in the BHL design that has 6SL7 and 6V6 tubes. It's similar to a Fender Champ (6SL7 is a better choice than a 12AX7).

From that perspective, you could make a high quality direct out for a Fender Champ by adding a balanced output transformer and an 8 ohm 30W resistor connected to a switched output jack.

To add a mic pre, you'd add a balanced input transformer and a mic connector (phantom power would be more complicated).

At this point, I'm still researching.
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Old 6th June 2008, 06:47 AM   #7
Gordy is offline Gordy  United Kingdom
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Half way down this page:

http://www.prodigy-pro.com/forum/viewtopic.php?t=16909

More than you can shake a stick at.

: )
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Old 6th June 2008, 07:34 AM   #8
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Quote:
Originally posted by PRNDL


It would be easier to copy an existing design, but I have found only those two that have build info and use modern components, which is surprising.

I'm interested in the BHL design that has 6SL7 and 6V6 tubes. It's similar to a Fender Champ (6SL7 is a better choice than a 12AX7).

From that perspective, you could make a high quality direct out for a Fender Champ by adding a balanced output transformer and an 8 ohm 30W resistor connected to a switched output jack.

To add a mic pre, you'd add a balanced input transformer and a mic connector (phantom power would be more complicated).

At this point, I'm still researching.
It sounds like you are going to build one more guitar amp with microphone transformer on input.
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Old 6th June 2008, 09:52 PM   #9
EC8010 is offline EC8010  United Kingdom
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A microphone channel is as challenging as an RIAA stage. Perhaps even more so. The problems are very similar in that you need low noise, wide dynamic range and high overload. Added to that, you need good CMRR at the input, and that's down to your input transformer choice (yes, you want a transformer). Try looking at known good RIAA stages for ideas. Remember that a ribbon microphone might need 70dB of gain to bring conversation to line level whereas a condenser microphone in the throat of a saxophone might need attenuation!
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Old 7th June 2008, 05:43 PM   #10
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Quote:
Originally posted by PRNDL


It would be easier to copy an existing design, but I have found only those two that have build info and use modern components, which is surprising.

I'm interested in the BHL design that has 6SL7 and 6V6 tubes. It's similar to a Fender Champ (6SL7 is a better choice than a 12AX7).

Do you want guitar amp or a mic pre? For a good mic pre, i would choose dedicated design, not moded git amp.
There are more documented designs if you search the internet. For example; SlowBlow, old RCAs, Redd47, V72, Tube-Tech MP1A, NYD One and Two Bottle and so on. If you can't build it from schematic, you better buy a kit or make something ss instead.
I don't think you will ever need 70dB of gain. If mic pre goes into console or recorder, you can amplify there. That's what people do with V72s and similar preamps which were built as a line amps...

Miha
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