We're always interested in seeing something new and better.
I was impressed myself with the performance of the circuit I mentioned above although I won't say it's better. It was just a different approach
to solving the already solved "problem"
of amplifying a microphone signal quietly. Nothing about it is rocket science and beyond the capability of any number of folks on this forum . It was very quiet but certainly no quieter than the best we have out there already. I was impressed though with how well it worked wrt CMMR, even at low gain settings but... maybe I just don't get out often enough?
The basic topology, as I said, isn't mine so I wouldn't feel comfortable posting a schem, without asking permission first.
Better preamps have been in the 1-2dB NF range for decades, so no amount of liquid nitrogen, or design magic will make more than a 1 dB improvement. A 1 dB difference in noise floor is not likely to be a game changer.
I would agree completely with you.
while some mixer/console companies, who shall remain nameless, have invested millions of dollars advertising how their preamps are "so much quieter" (cough BS cough).
Damn, now I'll be lying awake tonight wondering who you mean! Care to drop a few discrete clues?
I miss the old low Rbb parts (2sb737 with typical 2 ohms base spreading resistance), but these were better than needed for 150-200 ohm sources. Some tweaks prefer the newer noisier but faster parts, I don't generally hear things like that.
Thanks for that! Yes, that's the transistor I was referring to - I mistakenly wrote 2sD
and I knew there was a 7 in it somewhere but... ?
I don't think I could hear the difference either (ignoring noise for a minute) with using a faster part but...
Becoming overly fixated on nV may not give the optimal result. For example the 737, that I happen to have the data sheet open for, delivers around .4nV rt Hz at 10 mA which is a nice low voltage, but the noise current increases as you would expect, so NF to a 200 ohm source (at 10 Hz) is 5dB. For 200 ohms the 737 delivers better NF at 1 mA even though the noise voltage rises to the horrendous .6 nV rt Hz (yes I'm being sarcastic).
I don't remember exactly off hand but, for one particular ribbon mic pre I optimised, I was somewhere around 3mA per.
Back when we all played back our music from vinyl sources, I don't think I used anything other than a 737 (or sometimes a couple-or-three in //) on the front end. Standing current being matched to the cartridge du jour's impedance but also generally ending up in the round about 3mA region.
Note: Ricardo made a comment in passing that GC has lower noise than other approaches at low gain... While this is a rather broad and sweeping claim. Surely some approaches are worse than others.
Those're my thoughts too. Although in a lot of circumstances with high level sources from, say, guitar cabinets, requiring low pre amp gain, you hope/expect that the sn ratio issue will be mostly masked and irrelevant.
Ignoring topology and discrete device specs for a moment, the resistors in the feedback network also contribute noise, so just like we want low Rbb in a transistor, we need to use low value resistors in the feedback network to keep their contribution low. While this is no accident some of the newer uber-opamps have very healthy drive capability, this allows us to drop down the resistor values in the feedback network to lower their self-noise contribution.
Yep. Ignoring for now the loading from the stuff downstream, I feel quite comfortable these days with a good Op-Amp being able to drive quite low FB impedances. I've had good luck with the LM4562 myself as a not too exotic (read uber expensive) lower noise, pretty much a straight drop-in replacement to "old faithful" - the 5532/34. It (4562) seems quite unfazed with loads down to around 500 ohms.
At the end of the day, the active electronics inside the mic will probably dominate the path performance. I know this is an area of Ricardo's interest and expertise, not mine.
I have zero experience with designing mic head amp electronics of any kind. Other than altering the topology a smidgen on a few valve types that needed valve replacements but that we couldn't warrant spending the £1000 going rate for the "correct" valve replacement. I've always deferred to people such as Ricardo for the more up to date in depth stuff.
One thing: I only just realised that the first pair of Op-Amps in the "Cohen" operate in current mode with a resistive current source, hence are operating class "A" DOH!!! This for the folks that want/need/insist on such things.