• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

tube mic preamp kit??

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hardclip

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Joined 2006
#1
Hi. I am pretty new at this stuff. I've got the idea in my head that it would be fun to build one of these preamp kits. They've been talked about in previous threads.

http://eshop.diyclub.biz/index.php?cPath=149_106

The schematics are available at kitsparts.com.

Any recommendation on which would be the most suitable for a good two-channel microphone preamp for a home recording setup? I would need 48v phantom power for condenser microphones. I'm sure this is kind of a dumb question, but I would like to learn a lot building and I'm scared that without some advice I'm going to buy and build something that turns out not to be useful.

Thanks.
 
#3
I don't see where a valve/tube microphone preamp is that difficult a project. After all it's a transformer, a low noise stage, a variable gain stage and an output driver, all linear, flat frequency response. The most complicated is the output stage; if you want a low impedance balanced line driver it's basically a tiny power amp, with an output transformer which is not easy to find; but that would be true with any tube gear you want to interface with professional studio material. Basically, if the circuit is reasonable, the sound of the preamp is the sound of the input transformer, and can be very good indeed.
Certainly, any components (particularly switches) in the low level signal path have to be highest available quality and reliability (but this is a general condition, the only really special case being if you want an attenuator switch before the input transformer).
Even if one adds a highpass filter, a variable input impedance and a hi z DI input, the project should be well within a neophyte's range; far easier than a good equaliser or a varimu compressor. And if someone else has done the metalwork… (Qmaxing out B9a holes reasonably in line was always a problem of mine, as was labelling controls with lettraset)
All right, few more slight problems; it doesn't appear they do metalwork, and stabilising the phantom power from your tube ht is an interesting extra project, plus I don't think they plan for DC heaters (not essential, but so easy to do nowadays) but still…
 
#4
For a condenser microphone amplifying conversational speech, you normally need to set the gain of the microphone channel to about 55dB, for a ribbon, 75dB. That's to bring the level up so that peaks reach +8dB above 0dBu (0.775VRMS). We'd like a S/N ratio of at least 60dB referred to peak level, and hum needs to be at least 15dB better than that, so hum needs to be below -67dBu at the output, or -122dBu at the input, which is 0.6uV. Achieving that level of hum is possible, but it requires careful attention to earthing, layout, screening, and DC heaters. Not really a good first project. If you look at the number of threads on this forum about hum in power amplifiers, you'll see what I mean.

And then there's the provision for variable gain to allow for microphones being rammed down saxophones, the quiet phantom power you mentioned, etc, etc.
 
#5
first project idea

Well, the more I look into things, the more I realize I don't know. I've got a pretty good background in basic physics, simple dc circuits, and some experience with the soldering iron, but practically no experience with analyzing and modifying these rather intimidating a/c designs.

I think my best bet is probably to learn some more basics and start out building and testing a really simple amplifier design or two before I try to make anything useful.

I'm reading through some of the resources posted on the "learning for newbies" thread. I would really appreciate any advice on a good first project and resources to help me get a good grasp of what is going on inside of the circuit.
 
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