Discrete opamps in the instrumentation config.

[TomWaterman]

Hey rafa!

You can get it from the AES website for $5 if you're a member...I think it may be $10 for non-members.

I'm not a member but I know a few guys who are and will try to get it from them when I'm back at school.

If and when I get it I can email you a copy if you want??

So do you guys think this preamp would be any good? It uses all of the ideas from Freds website.........the balanced output etc.

Cheers Tom

 

[rafafredd]

I think it would be on the very transparent side...

I wanna try it with the discreet jfet opamps and maybe with other opamp designs also...

 

[volki]

[quote author="AP"]The purpose of that 'instrumentation' amp configuration is to:
- provide (usually) a high input impedance on both input legs
- provide best in class CMRR
- provide the necessary gain
- provide low noise and distortion.
[/quote]

along with the main purposes, a short sidenote/question about a possible drawback: in klaus heyne's mic lab at the PSW forums, there's a statement that instrumentation amp's are supposed to exhibit 6db less headroom if used with a xformerless mic in the impedance-balanced configuration (only one lead of the mic output driven actively):

http://recforums.prosoundweb.com/in...225/3055/?SQ=1f4c2ff7af352dd8ecca58932a61855b

can anyone comment? why exactly does this happen?

 

[PRR]

> instrumentation amps... exhibit 6db less headroom if used with a xformerless mic in the impedance-balanced configuration

I think he is wrong except in the specific case he mentions, or a carelessly designed instrumentation amp and VERY hot mikes.

 

[NewYorkDave]

CJenrick, what you're thinking of is a "decade amp", which is used in laboratories. Since a decade amp is a piece of instrumentation, I guess I can see your point. The instrumentation amp as a circuit topology seems to have been developed for transducers such as strain gauges, or as a null detector in bridges, etc.

 

[SSLtech]

[quote author="NewYorkDave"]CJenrick, what you're thinking of is a "decade amp"[/quote]
I had one o' dem, about ten years ago... or it could have been twenty...

Keef