[AMRadio] Sound advice


Jay Bromley jayw5jay at gmail.com
Tue Aug 23 15:29:56 EDT 2016


HI Rob,
Couldn't you use a cathode follower if one didn't have a transformer on
hand?  I had a few of the LS jobs here and they didn't always sweep that
great compared to the modern day Jensens line of transformers.  Those could
have been bad and why they ended up in my junk box, hi

73 de w5jay/jay..


Rob wrote>>>  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.

73

Rob
K5UJ


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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