Does anyone know of schematics for a nice tube-based microphone
preamp ? Preferably 2 channels with phantom power.
I've found this one so far :
Any comments as to the suitability of running microphones through
tubes would also be appreciated.
Thanks for the links, lots of interesting stuff to read. I've found many
solid state circuits, which look easy enough to build, but not that many
tube circuits. The Pulteq is more or less what I was looking for, a simple
tube circuit to study.
I just found the following, which looks like it would be hard to beat. A
tube mic preamp (with ss power supply) that meets all my requirements :
except that it uses European parts which might be a slight inconvenience,
my living in North America...
This should keep me busy for a while.
Go to the jensen transformer website and check those out. One thing is if you build them go as close to the specs as you can. Whatever mic pre you build use an input transformer. The Cascodes are very nice.
I was thinking that if a dynamic mic is to be used within a few feet of the preamp, perhaps the tiny transformer could be bypassed. This way you won't require one prior to the tube. Right? Now, how to deal with the balanced end
I once looked at a Fearn and discovered that it alike a JCM800 preamp but with some negative feedback and different values. Thus to simplify the issue, aside from those pesky transformers, we could use any preamp/guitar amp for a mic preamp. Err, right? Hey, what's a little impedance mismatch. But, the grid resistor could be dropped/lower to "meet the capsule fairly, say 150K.
Paul, the EH 12ay7 preamp sounds really good stock if you're in the market and has phantom power.
'Whats a little impedance mismatch?"
In the case of a typical low imp mic like a 57 or 58, it's a lot of hiss and noise. You can use the RatShack high imp adapters(transformers) and plug into any old tube PA head and be relatively ok for rock n roll.
Even with very hot output mics like a GT model1 the results are pretty cruddy. Resistors alone can't provide the necessary impedance transformation needed to feed the grid of tube.
The basic designs for guitar preamp and mic pre are as you said very similar. The mic pre output must be dealt with to get that signal into a DAW. A transformerless cathode follower output can be done but a transformer tends to do this job better.
I take it that by "pesky" you mean expensive? Relative to many vintage guitars and amps I'd say not very. 50 bucks for the input(until Edcor antes up and gets a potted version on the market) and 13 bucks for the output.
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