OK, I was going to go out on a limb and say 15 min - because of the electrolytics.olafmatt wrote:At least 30 minutes, but that's not because of the preamp, it is because of the mic and all the electrolytic caps in there.
In some of the tests I did a long time ago, with only the input coupling caps, it didn't even begin to get quiet until 5 minutes.
15 minutes was where I felt comfortable saying it was quiet.
One thing I've noticed with the input-capacitorless topology is that phantom turn-on doesn't pack the expected sonic wallop. That certainly doesn't apply to a capacitor output mic which to my knowledge I don't have.
The pain is over more quickly.
Gaskell's measurements do seem to suggest that PP and PET are workable based on THD. But they seem to undermine themselves by concluding with:
I don't really know what conclusions to draw from Gaskell but its interesting.Listeners seemed to perceive a shift in the overall character of the sound due to the presence of capacitor distortions. Although the experiment was not designed to quantify preference or categorize the differences heard, it is clear that certain capacitors can produce a sonic signature. Further listening tests would be necessary to properly explore the relationship between capacitor distortion and perceived sound quality. It is generally felt that THD alone is not a good indicator of whether listeners will be able to perceive distortion [8, 9, 10]. This is particularly true in the case of capacitors’ frequency dependent non-linear behavior.
I'm pretty sure some of the big 47µF/250V Dayton PP caps used in speaker crossovers (inexpensive and available) would measure and perform as well as an input-capacitorless direct base connection. I'm not sure about PET caps. Never knowingly tried them.
I bought some of the Dayton's to test. They're huge: About 30% larger than a D cell battery. Lovely and impressive to look at.
My thought was input-capacitorless until the freak microphone came along (triggering a servo red light) where a switch could be flipped to insert the big monster PP caps.
The big PP would be off-board (since they're big) and we might eventually find out they're so infrequently needed as to not be worthwhile.
My goal is a two channel 1 or 2U gizmo and not a large multi-channel device so there would be room for battery-sized input C's.
So let me recap and see if I basically have this right:
You built at least one of these beginning around 2008 that was DC-coupled end-to-end and recorded with it.
You haven't found a mic in the wild that doesn't work with it.
The current modulation at the input, from the servo, hasn't been found to be a problem in the field.
Its quiet and doesn't sound bad.
Do you still use it?
Would you do it again? (Possibly using an updated/improved circuit.)