Benchmark Mic Preamp from 1984 - Active Preamp History

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mediatechnology
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Benchmark Mic Preamp from 1984 - Active Preamp History

Post by mediatechnology » Sun May 25, 2008 11:40 pm

This thread title exists elsewhere both in the Documents section and cyberspace. It's actually morphed into a history of the active mic preamp.

In March of 1984 I visited Al Burdick of Benchmark Audio in his garage in Garland, Texas. Al gave me a hand-sketched drawing of a preamp he had been working on and was really pumped about the Rohm 2SB737. I recently found the schematic:

Image

Note the date: March 1, 1984.

I had been led to believe that Cohen "invented" this topology and looked up his AES paper. It was presented in September 1984. Cohen characterized this preamp, with the op amps wrapped around the transistor (provding both AC and DC feedback) as "new." Now if Cohen presented it as "new" in the fall of 1984, how is it that Burdick was able to sketch it in March?

Image

Hmm. Something isn't adding up.

This is the AD524 Instrumentation amplifier introduced in 1982:

Image

Note the similarity to Burdick and Cohen.

I later realized that I has seen something before either one of these. It was the Harrison PC1041 in 1978 used in the 4032C IIRC.

Image

The front-end transistors are LM394 with a HA4741 quad op amp. The title block (in the full drawing) has the date.

https://www.ka-electronics.com/images/j ... ic_Pre.JPG

Now if Harrison used this in 1978, how is it that Cohen characterized it as "new" in 1984? Could it have been new to Cohen but more widely known?
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Re: Benchmark Mic Preamp from 1984 - History

Post by mediatechnology » Wed Jul 16, 2008 10:12 pm

Yes it was known as far back as 1968.

Image

"Evolution from Operational Amplifier to Data Amplifier," Robert Demrow, Analog Devices, 1968. https://www.proaudiodesignforum.com/for ... =12&t=1052
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Re: Benchmark Mic Preamp from 1984 - History

Post by JR. » Thu Jul 17, 2008 9:30 am

Interesting early reference,,, Looks similar.

I wonder about the value of collector resistors for input pair, and a little more clarity on the internal gain stage (UNITY CM GAIN).

Putting the variable gain in second stage is not optimal for S/N. but it looks like they were more concerned with CMRR.

Indeed close enough to claim priority.

JR
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Re: Benchmark Mic Preamp from 1984 - History

Post by mediatechnology » Thu Jul 17, 2008 9:45 am

Thanks JR. Let's see if we can find one prior to 1968!

Notice the shield drive line from the derived CM. Not much of a stretch I suppose to use it to bootstrap the input bias resistors.

I find it interesting that INAs were once called "data amplifiers." We hadn't landed on the moon when this one was published. I was 11.

One thing that is unique about Cohen (at least based on what I've found so far) is the fully-balanced topo from "in" all the way out.
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Re: Benchmark Mic Preamp from 1984 - History

Post by JR. » Thu Jul 17, 2008 10:08 am

I actually did a similar derived CM shield drive in a LOFT console back in '70s. I had a mic preamp on one end of the input strip and the gain pot over a foot away. I used 2 cond shielded and drove the shield with CM to reduce cable capacitance effects on gain pot circuit nodes.

JR
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Re: Benchmark Mic Preamp from 1984 - History

Post by mediatechnology » Thu Jul 17, 2008 10:20 am

Cool. The capacitive imbalance that can develop in the Rgain lines of the typical CFB amps we're talking about here is significant. In "Analog Secrets Your Mother Never Told You" THAT shows how even 1 pF significantly degrades audio band HF CMR.

In your Loft circuit at 1 ft you were dealing with significant capacitance to ground on both conductors. I'm guessing that even with the driven shield that your Rgain stop resistor(s) were still on board. Was that one also a CFB?
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Re: Benchmark Mic Preamp from 1984 - History

Post by JR. » Thu Jul 17, 2008 10:41 am

not sure what CFB means ... (Current feedback?) IIRC that console used Transamp mic pre, so gain pot was hanging off the emitters, so I guess yes a current feedback node.

This was before I rolled my own version a few years later in the '80s, after discovering those lovely low Rbb transistors.

JR
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Re: Benchmark Mic Preamp from 1984 - History

Post by mediatechnology » Thu Jul 17, 2008 10:48 am

Yes current feedback. So the high gain limit resistor(s) were at /near the emitters?
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Re: Benchmark Mic Preamp from 1984 - History

Post by JR. » Thu Jul 17, 2008 11:07 am

yes 2 1% resistors before the cable capacitance.

JR
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Re: Benchmark Mic Preamp from 1984 - History

Post by mediatechnology » Thu Jul 17, 2008 12:49 pm

I saw the potential for that with the 1510 and mentioned it to Frank Thomas. I think they found it too and put it in Analog Secrets. I should probably post that sim.

It's here though: http://www.thatcorp.com/datashts/Analog ... df#page=17
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