recordinggeek wrote:I also did some noise measurements. With a 150 ohm terminator at the mic input, at full gain the output noise is about -37 dBu, which gives an equivalent "referred to input" noise level of just below -126 dBu... pretty good actually, considering the crazy high gain here. Switching in the input attenuator actually RAISED the noise just a hair, and cranking the record level to minimum (24 dB attenuation) gave a reading of about -41. I scope and listen to the output as well as metering it, and it sounds like the hiss actually drops more than that. RFI may be a factor here, as I live only a couple of miles from an AM transmitter site (which I worked at some years ago). When I try to look at really low level stuff, I can often see modulated RF on the scope.
One other thing I notice is a tendency toward microphonics. Tapping on any tube or either amp card "rings" a bit. A rap on the front panel will do this also (but not as much). Again, I am not certain whether I am seeing a real problem, or if it is just a case of "well yeah, crank up 90 dB of gain and you WILL get stuff like that".
I would still like to know whether my results are in line with what should be expected when you feed the second stage directly to the input of the line amp.
We haven't had any problems with RF, or microphonic tubes (other than the occasional microphonic tube).
I've never bypassed the "cal" pot. I can't tell if what you're seeing is normal. It is possible that the C-R filter that it creates somehow stabilizes the signal at the input of the inverter.
So far, I am considering 3 alternatives to drop the maximum gain to reasonable levels:
1) Bypass the second gain stage, feeding the output of the record gain pot directly to the line amp.
2) Change the plate load and cathode bias resistors of the first two stages to reduce the gain of each stage.
3) Use a lower gain tube (with changes to resistor values if needed) to reduce gain of the first two stages.
Any one of these options, though, might materially affect the sound of the preamp, which might reduce its value (I will eventually sell this thing, as I have a hard time with the idea of using up 4U of rack space for a single channel mic preamp, no matter how "cool" it is).
See if you get good results with the unit calibrated to 65 dB max gain (Record level all the way up, loading the output with 600 ohms). It has worked well for us for 12 years now. Maybe that extra stage is the magic sound.
