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Info on old STC-9000 pre-amps

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Rairun:
Hello!

I'd been using an old Church Audio pre-amp for about 8 years to good results. A few months ago, as I was about to record a gig, I learned the left channel was dead. I quickly bypassed the pre-amp by plugging my CA-11 cards straight into a Zoom H1, and because it wasn't a very loud performance, the recording turned out good.

I couldn't figure out what was wrong, so I went to the Yard Sale forum to try and get myself a new one. @unidentified very kindly gave me a spare STC-9000 he had got in a trade and never used. Interestingly, both units seem to be non-standard.

As far as I'm aware, STC-9000s generally had a three-step switch for gain (0, +10 and +30) and an attenuator knob that turned the unit off at 0. My first STC-9000 had a two-step switch (low and high gain, no indication of how much) and the same attenuator knob. The second STC-9000 (the one @unidentified gave me) has no knob and two three-step switches that actually work as two-step ones: the first one is an off/off/on switch, and the second one is a low/low/high gain switch. There are handwritten numbers for the gain (both 0/+10/+30 and -20/-20/20), but neither set of numbers looks right).



I had an old test recording I'd made for my first pre-amp where I recorded a hair drier from a set distance. I reproduced this test for the new pre-amp, and I got these results:

At low gain, the second STC-9000 is approximately 1.8dB louder than the first one.

First STC-9000: the difference between low and high gain was 20.5dB
Second STC-9000: the difference between low and high gain is 19.5dB

I made these recordings running CA-11s (cards) into the STC-9000 into the Zoom H1 (level 37).

Then I ran the CA-11s straight into the Zoom H1 (still at level 37) and the recording is between 15 and 17dB quieter than the others at LOW gain!

Have I really been using a +15/+35dB pre-amp all along? And could my new STC-9000s really be +17/+36dB?

The Zoom H1 supplies plug-in power, I think about 3V. Could the voltage difference really account for such a huge difference? Could it maybe account for 5dB (making the pre-amps about +10/+30dB)?

Basically, I'm just trying to figure out the specs. Any idea what is going on here?

jefflester:
The plug-in power should have no effect on the preamp. There most assuredly are blocking capacitors at the preamp output. You are probably just seeing different adjustments in his design or use of slightly different components based on either part availability or upgrade design. Nothing to be too concerned about.

Rairun:
Oh, I think I was unclear! I mean that when I run the CA-11 mics straight into the recorder with PiP, I get a recording that is at least 15dB quieter than if I ran the mics through the pre-amp set at low gain. This would mean that these two pre-amps have a minimum gain of +15dB, which sounds a bit too high to me?

I was asking whether the plug-in power being a lower voltage than the mics actually require could account for this difference in sensitivity, or if these pre-amps really are +15dB/+35dB ones with no option to run them at +0db like a battery box.

Gutbucket:
Sensitivity may decrease slightly when provided with less than optimal powering voltage (along with the audible distortion point and max SPL before clipping), but not by 15dB.

Looks like a gain of +15/+35dB through the pre-amp.  The one with an attenuation pot should let you reduce throughput gain if necessary, if at the cost of some increase in noise.. if that matters, it won't if something else is noisier, or if the noise isn't perceivable.  Technically, best noise performance will be with the pot fully open, but set it as needed to get good gain staging along with the recorder.

Rairun:
Cool, yeah, good to know. I think I might get a battery box for when I don't need the gain, since the one with the attenuation pot is giving me trouble.

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