[AMRadio] Sound advice

Rob Atkinson ranchorobbo at gmail.com
Tue Aug 23 06:04:09 EDT 2016

Equalizers accept line level audio, not mic level.  You have to have a
mic preamp to get it to line level ahead of the equalizer.

I went through a lot of this last summer.   Look for a vintage tube
amplifier usually called a speech amp, mixer, or preamp.  Besides
12AX7 you can use 6SC7s if you want a larger octal size to deal with.

The two triodes in one of those tubes will get you up to line level.
Put a pot between the first triode and the second to vary gain.  As
Jim wrote, look at circuits in tube manuals to get an idea.

Here's the good and bad news:  The good news is that you can have as
many dual triodes as you want, each with its own input and pot going
out to the equalizer, and on each one's input you can put a grid
resistor that matches the nominal mic impedance.  Crystal would take
10 megohm, dynamic maybe 50K depending on when it was made--the new
ones are low impedance but you wrote "old mics" so I'm guessing maybe
50 to 100K for the dynamics.  Easy.

The bad news is that the equalizer may be one of these boxes that
takes a 600 ohm balanced input.  If so, you have to find a way to step
 down the high Z of the tube plate (via a dc blocking cap) to the
input of the driven circuit and get from unbalanced to balanced audio.

I took a UTC LS27 (or maybe it was a 26) that was around 5K to 60K and
flipped it to step the tube down to lower impedance.  The ends of the
former primary now secondary became the + and - balanced leads into
the stage being driven with ground to the transformer case.

This does several things.  It gets your impedance down to a level that
is close enough to the next item being driven, transfers from
unbalanced to balanced audio, and isolates the two so RF and ground
loop problems don't occur.  If you don't do some kind of impedance
step down your audio will sound tinny with no low frequency at all.

Another gotcha to avoid--don't directly ground the cathodes in the
preamp.  It will work great until you transmit RF then everything will
fall apart.   Went through all of this last summer, hi.



On Mon, Aug 22, 2016 at 9:20 PM, CL in NC via AMRadio
<amradio at mailman.qth.net> wrote:
> I have been wanting to experiment with some of my old mics that basically sound pretty bad, either too tinny or too bass, (not the fish) and run them through a graphic equalizer.  The motivation for this is that I found a really nice single channel, 32 filter unit for a song, but it does not have enough sensitivity to handle crystal, dynamic, or controlled magnetic mics, giving only minimal indication on the output bar graph at high voice input, so I need some ooomph on the head end.  A homebrew 12AX7 amp to fit between my various basket case mics and the equalizer is required.  Does anybody have experience in this area with some pointers?  Ideas on a solid state preamp instead of a tube?  I checked several designs online and the ones in the 'Encyclopedia of Electronic Circuits', but maybe somebody here has a bullet proof schematic. The DBK2031 equalizer performs as it spec sheet advertises, handling inputs and outputs to 40K loads.
> Charlie, W4MEC in NC

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