Looking for Info About Early Transistor Mixers and Mic Pres

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cpsmusic

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Dec 3, 2013
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Hi Folks,

The subject says it all - I’m interested in the period when transistors were first introduced to the world of pro recording. Any particular articles or books that cover this period. So far, I’ve been going through the many “hobbyist” transistor guides. A lot of these have a couple of mic pre circuits. I’m yet to find any mixers but have only been digging into this for a short time.

BTW, the thing that got me interested in this was the following:

https://www.youtube.com/watch?v=-K3juTGeT4M

Suggestions welcome.

Cheers,

Chris
 
That's a nice little PYE mixer  :)

I don't have any articles off the top of my head.  What I can say is that the first use of transistors in pro audio recording desks that I've seen were in the form of plug in, self contained, but separate gain block modules that were replacements for the earlier valve type equivalents.  This was before the idea of housing a complete integrated channel amp (with eq and echo sends etc. on board) had caught on and become the norm.

Plenty of examples of these amplifiers were made by various folks, but in my opinion,  two that were designed by John Hall stand out as being a little ahead of the others -  The Altec 9475A and the Langevin AM16 which, if memory serves, are from around 1964/65.

Over in the UK, EMI did make some transistor modules in house quite early on that were intended to directly replace the V72 amplifiers in the older REDD37 desk in studio 1.  But these were deemed as not satisfactory when compared to a V72 or the REDD47 in the studios 2 & 3 desks.  The circuit was a basic germanium design with 3 or 4  transistors (I forget and don't have the print on hand) and, if you compare something like that to the Altec or Langevin, it's a no brainer really.

Anyway, EmRR Doug will chime in I'm sure, he knows this stuff.

 
cpsmusic said:
The subject says it all - I’m interested in the period when transistors were first introduced to the world of pro recording. Any particular articles or books that cover this period.
RCA had professional transistor equipment by 1961.
take a look at the RCA "Broadcast News" dated June 1961.  It is available at AmericanRadioHistory.
There is a detailed description of the audio circuitry in the RT-21a tape machine  including details of input stage biasing through the reproduce head coil.
Ampex employed the technique several years later.
Photos of RCA transistor gear exhibited at the 1961 NAB convention including what appears to be BA-31a.

 
Hi Cpsmusic,

The BBC was at the forefront of transistor mic amp design - you can search their archives and also the articles in 'Wireless World'.

Also, EMI had a division called BREDD that made what was probably the first encapsulated amplifier - type 849 (IIRC).  It used Germanium transistors in a screened can and ran off  a -24volt rail.  This was used in various broadcast consoles including those made by Pye.  It was probably the forerunner of the TG series of in-house built modular consoles.

I have attached a couple of photos for you.

Cheers

Mike
 

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  • pre-amp chassis.jpg
    pre-amp chassis.jpg
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Thanks for the BRED pics Mike. 
I've never seen a print of what's inside those amp cans, but it's possible they're similar to the REDD designed early germanium amps used in, say, the small REDD.43 desks.   
 
Winston O'Boogie said:
Plenty of examples of these amplifiers were made by various folks, but in my opinion,  two that were designed by John Hall stand out as being a little ahead of the others -  The Altec 9475A and the Langevin AM16 which, if memory serves, are from around 1964/65.

I agree with Winston, you should check out the Altec 9475A and the Langevin AM16, those are great American mic preamplifier circuits.
Also check the Spectrasonics 101 and 110A, and the UA 1108.
All if these are great amplifiers with the use of Silicon transistors.

These are not mixers or books,
but on the Germanium end you can check the early Helios amplifier that used Germanium transistors,
and also the early Neve modules that were also Germanium based, you have a lot of info on these Neve modules in this thread:

https://groupdiy.com/index.php?topic=18356.0
 
The design of early transistor circuit was very much influenced by designers that were more familiar with tubes. Transistors were costly, so they tried to use as few as possible. The notion that transistors had low input impedance and high output impedance, combined with said familiarity with tubes often lead them to use coupling transformers.
The notion of applying heavy NFB for linearizing circuits was not so ingrained, so there were many one-transistor stages (as in the video) with local NFB, because that's how it was done with tubes.
You want to read about biasing transistors, and how loading the collector with an inductor quadruples the avaialble output.
 
Look at 1960's RCA circuits.  Early UA transistor circuits.  Early GE.  That Pye looks a lot like some GE stuff. 
 
Lots of good references so far. RCA BA-3x (the 31 and 33 being mic pre and program amp respectively IIRC) are among the best examples of germanium transistors units. Take a look at the Collins 212z remote mixer, which also used germaniums. If you’re interested in early FET designs, the UA1108 and McCurdy AT-284 are the earliest I know of.

But like Abby said, the really early transistor stuff suffered from general unfamiliarity with the requirements for transistor design by the engineers who simply applied what they knew from tube design, so there are far more bad examples from that era than good ones.
 
abbey road d enfer said:
The design of early transistor circuit was very much influenced by designers that were more familiar with tubes. Transistors were costly, so they tried to use as few as possible. The notion that transistors had low input impedance and high output impedance, combined with said familiarity with tubes often lead them to use coupling transformers.
The notion of applying heavy NFB for linearizing circuits was not so ingrained, so there were many one-transistor stages (as in the video) with local NFB, because that's how it was done with tubes.
You want to read about biasing transistors, and how loading the collector with an inductor quadruples the avaialble output.
+1.. I was not very interested in mic preamps until later (like the 70s). I recall seeing early solid state power amplifiers in the 60s that were as Abbey suggested tube designs with transistors dropped in place of the tubes, so lots of interstage transformers used inside power amplifiers. Back in the day transformers were relatively cheap compared to active stages (tubes) so they were more commonly used.  Now transformers are relatively expensive compared to active stages using transistors so much less common.

JR

PS: for a little side note, Hartley hired Jack Sondermeyer away from RCA where he was the (a?) top solid state applications engineer. Hartley spent so much time on the phone trying to keep his early solid state Peavey power amps from blowing up that he hired Jack and brought him in house to save on his phone bill.  ;D  BTW Jack was no slouch regarding vacuum tube design.
 
abbey road d enfer said:
The design of early transistor circuit was very much influenced by designers that were more familiar with tubes. Transistors were costly, so they tried to use as few as possible. 

+2.
I 100% agree with what Abbey wrote in regard to early transistor designs being, more or less, based on earlier valve designs. .  But I still stick to my initial references to the Altec and the Langevin as being a little ahead of the curve . These  two were directly coupled, differential, and transformer/inductive loaded on the output devices. 
Interesting subject none-the-less. 
 
squarewave said:
Can you explain this more?

+1.
Abbey knows his stuff so I don't doubt his statement.  But I only know of inductor loading a collector giving*double* the output -  Neve 283 output being a good example of that.
 
Winston O'Boogie said:
+1.
Abbey knows his stuff so I don't doubt his statement.  But I only know of inductor loading a collector giving*double* the output -  Neve 283 output being a good example of that.
Ah, I see what you're saying. So when the current switches directions the stored flux in a transformer primary provides a double kick whereas a resistor cannot store energy?
 
squarewave said:
Ah, I see what you're saying. So when the current switches directions the stored flux in a transformer primary provides a double kick whereas a resistor cannot store energy?

Yep, that's what I undetrstand  to be correct  but, as I said,  I know Abbey is more than clue'ed into the details of this stuff so, let's wait and see what  he has to say. 
 
It is because with an inductor load, the collector (or plate for that matter) can swing up as high as twice  the supply voltage. That is twice the range of a resistor loaded stage hence four times the power because power is proportional to V squared.

Cheers

Ian
 

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