[ruffrecords]

Open loop gain that tracks with closed loop gain is a well explored characteristic of the "cohen" topology (also used inside modern mic preamp ICs).

You gotta spill the beans on which modern mic preamp ICs use this technology.

I mentioned this in passing in my 1980 console performance article (when describing Transamp used in a summing amp).

JR

Yes, the Transamp I think was the first 'op amp' to employ this technique but I was not aware it was now embedded in ICs.

Cheers

Ian

[abbey road d enfer]

You gotta spill the beans on which modern mic preamp ICs use this technology.

the SSM2015 was launched in 1983!

[JohnRoberts]

You gotta spill the beans on which modern mic preamp ICs use this technology.Yes, the Transamp I think was the first 'op amp' to employ this technique but I was not aware it was now embedded in ICs.

Cheers

Ian

The transamp is not an op amp but hybrid combination of discreet bipolar transistors and IIRC a TL072 IC op amp, all inside a potted module.

I don't feel like making a list but pretty much any high performance IC mic preamp uses that topology (Cohen), for the obvious benefit.

Not a secret...

JR 

[clintrubber]

Here is a link to the three transistor Neve input stage.

...

Here is a link to the Helios three transistor mic pre scehmatic:

...

Cheers

Ian

Hi Ian,

Thanks for those links, hadn't realized before there's indeed a pretty elegant 'tandem action' going on in those topologies.

If this is indeed the main reason for things 'sounding better' then that'd be a nice (re-)discovery, begging to implement it 'for integrated' as well (saw the Cohen example mentioned, need to dig it up & have a look again).

How do DOAs fit in here? They'd be suffering from the non-tracking open loop gain consequences as well, although I expect DOAs don't reach the same open loop gain levels as integrated opamps do.

Bye

[abbey road d enfer]

If this is indeed the main reason for things 'sounding better' then that'd be a nice (re-)discovery, begging to implement it 'for integrated' as well (saw the Cohen example mentioned, need to dig it up & have a look again).

This topology is in use since ages (1980's) in most british mixers (Soundcraft, Amek...) and many others, where it makes a difference, i.e. in stages with a high noise gain, that is, typically, mic preamps and summing amps. Do they "sound better"?

How do DOAs fit in here? They'd be suffering from the non-tracking open loop gain consequences as well, although I expect DOAs don't reach the same open loop gain levels as integrated opamps do.

DOA's have their own raison d'être, but in this respect tend to perform less well than recent opamps with GHz response. They can be used in a transamp topology, though.

[abbey road d enfer]

Not exactly the same, but offering the same advantages, Current Feedback Amplifiers.
Not easily applicable for audio, since they require low Z design (<100r).

[volker]

Not exactly the same, but offering the same advantages, Current Feedback Amplifiers.
Not easily applicable for audio, since they require low Z design (<100r).

Not necessarily, look at the LT1364 for example, page 11. The negative input is buffered high-impedance as well. I've designed a DOA using this same topology already couple years ago, but alas where is all the time to build stuff...

[abbey road d enfer]

Not necessarily, look at the LT1364 for example, page 11. The negative input is buffered high-impedance as well. I've designed a DOA using this same topology already couple years ago, but alas where is all the time to build stuff...

Well, teh big advantage of CFA's is the possibility to achieve very low noise and high gain margin. It seems this one does not provide the utmost in terms of noise. 9nV/sqrtHz is 6dB more than a 5532.

[ruffrecords]

Hi Ian,

Thanks for those links, hadn't realized before there's indeed a pretty elegant 'tandem action' going on in those topologies.

If this is indeed the main reason for things 'sounding better' then that'd be a nice (re-)discovery, begging to implement it 'for integrated' as well (saw the Cohen example mentioned, need to dig it up & have a look again).

I am suggesting it may be an important factor compared to regular op amps with dominant poles around a few Hz. The other factor is that all the three transistor versions were 100% class A whereas all op amps have class B output stages.

Other discreet designs like the Calrec B series amps used more transistors and had a push pull output stage but it was still run in class A. (schematic attached)

Cheers

Ian

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