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...There are several (DIY or commercial) copies & kits including the SSL9K but none of them even approach its performance.
On the same website, is Great Guru Baxandall's last thoughts on LN design in Microphone Design Handbook.
Good read, the lot of it.In the cohen pdf there are pictures of the circuit before potting it in epoxy. Were opamps and transistors available "open" like that, so you had to solder the bond wires and could actually see the chip internals? Or is that some kind of a special brew?
Yup, that got discussed back a few years ago.
When I first started participating here, I was confused when people mentioned the "Cohen" topology, since I had seen it around and was using it in consoles well before his AES paper publication, so from my perspective he was a later adopter. FWIW he doesn't claim to have invented it.
Anyway, thanks John. Do you happen to know of any particular early examples prior to the "so-called" Cohen that you could point to online? Just for idle academic curiosity's sake. MJK & Kingston: Yes it's all a ripping good read J.C.
When LEDs are used like this as a fixed voltage source, is there any special property of RED leds that makes them more desirable to use than other color LEDs or is it just a matter of looking for the right voltage drop across the LED and red just happens to be a convenient 2V(ish)?
I've posted my old phono preamps (there were more than one of them)
Yes it is good, and well known to those skilled in low noise design. Of you look inside most modern mic preamp ICs you will see variants on that topology. While it was exotic 30-35 years ago, today not so much thanks to these popular mic pre ICs.
In my book, a "proper" Cohen isn't about the topology.
It's about sub nV/rtHz noise performance.
it's noise performance also deteriorates less at lesser gains.
The topology dates back to Demrow in the 60's IIRC but Cohen's implementation is the one that really breaks the 1nV/rtHz barrier for a real life P48V mike preamp.
Can someone explain the reason for the 2 x LED's in his circuit?
We're always interested in seeing something new and better.
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.
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).
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.
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).
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.
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.
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.
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