Benchmark Mic Preamp from 1984 - Active Preamp History

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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by Ian DuRieu » Sat Aug 02, 2008 7:49 am

JR. wrote:I guess "E" stands for earth (?)
Yes, "E" is the earth/common/ground point.
JR. wrote:I don't see much market for a stand alone mic preamp.
What about all those studios who need to get a mic or two into their HDD recorder?
I used one just the other week to run an AKG condensor mic into a laptop.

It is true that if the OM1556 hybrid was used in a traditional mixing desk front end application, its -ve output may be redundant. It would be a good place to put the phase switch however.

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mediatechnology
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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by mediatechnology » Sat Aug 02, 2008 8:04 am

It is true that if the OM1556 hybrid was used in a traditional mixing desk front end application, its -ve output may be redundant. It would be a good place to put the phase switch however.
Also perfect for driving an insert send.
I don't see much market for a stand alone mic preamp.
Can't say for sure but I think JR may have meant for him. There sure are a lot of players in that space so there is certainly a market.
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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by JR. » Sat Aug 02, 2008 9:26 am

Yup..

Another point that is mentioned and perhaps worth discussing is the nominal impedance of the feedback network. I've looked at this a little in the context of very low noise front ends for A/D and you can get into current drive issues with standard opamps if you work at very low impedances. AFAIK the 5534 per it's specification won't drive 300 ohms rail to rail. Unless the short circuit current limit is folded back from some higher level.

Of course when dealing with microphone source impedances >100 ohms we have some room to play with there... perhaps for ribbon mics there would be merit in silly low values and higher current drive capability in the first stage.

JR
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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by JR. » Sat Aug 02, 2008 10:41 am

I don't see much market for a stand alone mic preamp.
What about all those studios who need to get a mic or two into their HDD recorder?
I used one just the other week to run an AKG condensor mic into a laptop.

It is true that if the OM1556 hybrid was used in a traditional mixing desk front end application, its -ve output may be redundant. It would be a good place to put the phase switch however.
I always put the polarity switch on actual mic lines, with pad and sundry other front end fiddles. I guess if it's there for free, it's arguably better to eliminate a switch contact from low level path. I guess it's ok to just pull from just one output, since they're both differentialed at that point.
------
Yes, I meant no market for me... I have been in and around these markets for decades and modern audio gear is getting sold in consumer box goods stores. In the US "Best Buy" a big consumer chain is getting into MI (musical instrument) gear.

The stand alone mic preamps are either $25 USB output jobbies, or tweaky gold plated high end stuff... I abandoned the high end market niche decades ago when I perceived a disconnect between actual sound performance, and perceived value. I don't have the right personality to sell to people who believe in magic.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by Ian DuRieu » Sat Aug 02, 2008 10:48 am

JR. wrote:AFAIK the 5534 per it's specification won't drive 300 ohms rail to rail.
According to Graeme, the input op amps only swing half rail making the use of 300R resistors possible.
These op amps are also running class A in so far as their outputs are always sinking.
I asked Graeme some time ago about using a buffer to allow values much less than 300R. This would allow a gain set resistor of less than 1 ohm. There isn't much to be gained by going less than 1 ohm due to the series rbb and re of the input transistors which start to dominate.

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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by mediatechnology » Sat Aug 02, 2008 11:23 am

BTW JR thanks for posting the schematic. I was afraid we had fully lost you to DSP.
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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by JR. » Sat Aug 02, 2008 2:12 pm

Ian DuRieu wrote:
JR. wrote:AFAIK the 5534 per it's specification won't drive 300 ohms rail to rail.
According to Graeme, the input op amps only swing half rail making the use of 300R resistors possible.
These op amps are also running class A in so far as their outputs are always sinking.
I asked Graeme some time ago about using a buffer to allow values much less than 300R. This would allow a gain set resistor of less than 1 ohm. There isn't much to be gained by going less than 1 ohm due to the series rbb and re of the input transistors which start to dominate.

Ian D
Yup, with the diff amps summing two signals at unity gain, they will saturate while front end is still 6dB below FS.

The class A current is from the opamps supplying the operating current to bias the input transistors. I did that on my MC phono preamp too. You do burn a little signal swing in theory, but the effective gain in the following diff stage makes that inconsequential.

The range of class A operation depends on the gain set resistor. IMO it's more like class AB with the zero drive current transition point skewed away from 0V. For even moderate closed loop gains the opamps will be sinking and sourcing current.

I'll take your word for it on Rbb, the national spec sheet I found doesn't include that. I liked the LM394 more as a differential input for discrete opamps, back when such things were useful. Now we can buy opamps off the shelf that do better. I liked some really low noise Japanese transistors I learned about in the late '70s for mic pre and head amps.. But it's hard to quibble about johnson noise of a few ohms in the context of microphone self noise and any real world room. It could matter for MC phono cartridges, bet they seem pretty rare these days

JR

PS: Wayne... I'm not coding DSP today, but indeed coding up my little meter project on a different cheapo 16 bit platform... lots o' blinky lights...The more time I spend messing with digital stuff, makes analog circuitry seem a little limiting. I like using the right technology for the task... for meters,,, a cheapo micro's does the job very nicely. For a quick FFT, gimme a DSP chip.
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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by mediatechnology » Tue Mar 19, 2013 3:42 pm

A new entry into the mic preamp museum.

The VCA Associates "IMP" circa 1986.

Image
VCA Associates IMP Transformerless Active Mic Preamp Circa 1986.

VCA Associates documents may be found here: https://www.proaudiodesignforum.com/for ... f=12&t=574
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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by JR. » Wed Mar 20, 2013 1:15 pm

That topology looks a little like the innards of the early Mic pre ICs. I ASSume that is discrete (or hybrid) due to all the caps and numbered opamps.

JR
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Re: Benchmark Mic Preamp from 1984 - Active Preamp History

Post by mediatechnology » Wed Mar 20, 2013 1:58 pm

It was a hybrid: viewtopic.php?f=12&t=574
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