STC portable microphone mixers (1940's)

[barrymagrec]

Unless you go to the esoteric levels of multiple parallel FETs there probably isn`t a lot of difference in noise level between well designed later valve and discrete transistor preamps, I doubt however that the noise performance of the valves used in these mixers would be that good, there was no requirement at the time, most OB mixers were going to be feeding AM broadcasts with limited bandwidth and relatively high noise levels.

[Goldjazz]

Spec says sigal to noise ratio is "66db at 72sb gain and reffering to peak output signal of +18db"

Is that bad?
Interestingly for noise probably a good thing that it can be run on batteries

[Herald1360]

That's not too bad at all. It's comparable with typical MC front ends on the face of it, though I wonder if it would still match up by the time RIAA correction has been applied since this adds a drooping gain with frequency which could mean worse results.

[Goldjazz]

Right as in RIAA attenuates the highs so noise in the lows with be at a worse ratio to signal in the highs?

[Herald1360]

Yes. I'm speculating that since typical MC specs seem to involve about 70dB gain and about 65dB s/n at 1kHz, the mic preamp with only a flat gain of 72dB will have insufficient gain at high frequencies when the RIAA correction is imposed. In simplistic terms, I think you need about 20dB more initial gain.

This could be nonsense of course without any actual information as to what input signal level the mic preamp was designed for.

[Goldjazz]

Interesting. Well the maximum gain the unit is capable of is 92db. The s/n rating is at 72db but its capable of 92db presumably the s/n is worse at max output.

[Goldjazz]

4Just wanted to also share this attenuator that I got with the mixers. Presumably also 40's. It has the most impressive pot attenuator inside. Housed in a wooden box lined with copper sheeting inside. Still works great too, dead silent.

[Radio Wrangler]

Yes, that's a stud fader. Can be wired with simple potentiometer networks with stud tappings or more usually with multiple tracks of studs with proper Pi or T section attenuators with the studs switching both input and output to the chosen section.

I rewired one as a potentiometer with a special law to work as a feedback element of an active volume control in my homebrew preamp. The snag is getting stereo paths to step at the same time without having to fit mechanical detents. otherwise my balance wanders a little side-to-side.

David

[jamesperrett]

Spec says sigal to noise ratio is "66db at 72sb gain and reffering to peak output signal of +18db"

Nowadays microphone amplifier noise is often specified as a level relative to 0dBu (with the input terminated by a specified resistor and bandwidth specified). By my calculation (assuming the output figure is in dBm) this mixer would give an equivalent input noise of -120dBu which is comparable to modern budget microphone inputs (my Focusrite Saffire Pro26i/o interface spec says <-120dB while my budget Zoom U44 spec says -119dB). Better mic inputs (like my Audient Preamps and Allen and Heath mixing desk) seem to have a spec of around -128dBu while the best that I've seen was -131dBu (which is the theoretical limit of a 150ohm termination). All of these figures are with a 150 or 200 ohm termination and 20kHz bandwidth.