DIY Stereo Micpre Sowter 7847 input trans( What is this circuit based on? )

[bluesblake]

Picked up this Stereo Mic pre.  Has  48V, Pad,Phase .  Wondering what circuit it is based on.
Here is a page I put up with a bunch of pictures.  https://www.icloud.com/sharedalbum/#B0H5qXGF1uhU4w
Cannot find info anywhere on the Sowter 7847 input transformer.

Any info  would be greatly appreciated.

 

[ruffrecords]

The part with the 2N3055 etc is a power supply. The actual mic pre looks to be a three transistor design of some sort. I suggest you trace the circuit out from the PCB and post it here.

Cheers

Ian

 

[bluesblake]

Circuit tracing is above my musician pay grade I think but I will give it a go.   Thanks for the info.

 

[Evertide]

Sowter 7847 ; Blue / Red Primary , Green / Yellow Secondary no info on winding numbers ...

Clear Picktures form front and back of preamp-print could help us to help you ... ( transistor type , resistors , connectons )

 

[ruffrecords]

Sowter 7847 ; Blue / Red Primary , Green / Yellow Secondary no info on winding numbers ...

Clear Picktures form front and back of preamp-print could help us to help you ... ( transistor type , resistors , connectons )

I could not find any info on the transformer on the Sowter site. I agree that good pics of the top and bottom of the 3 transistor mic pre PCB would be helpful.

Cheers

Ian

 

[bluesblake]

Got this info from Sowter.

Hello Blake

Unfortunately we do not have much information on this old design but it was built for nominal impedance of 1.2kohm Primary and 32Kohm Secondary, Turns ratio is 1:5.16 step up. Maximum input is 2.75Vrms at 30Hz. Frequency response +/- 1dB 20Hz – 20kHz.
It has a note on the original design that says ‘similar to Trident/Unitel’ but does not name the actual circuit.
Primary colours are Red & blue and secondary colours are Green & Yellow.

Hope this is of help

Best regards,
Ruth

Mrs Ruth Loveday

E.A.Sowter Ltd.
The Boatyard
Cullingham Road
Ipswich
IP1 2EG
England.

 

[bluesblake]

Handmade Preamp          Pictures        https://www.icloud.com/sharedalbum/#B0H5qXGF1uhU4w


Parts.

Input + Output Volume ( stepped) ,  Phase switch, Pad


            PARTS
Front of Circuit board 

BC416C transistor

Caps -    682K    Brown    Ceramic disc
            2.2 35V    Tant Cap



Resistor

RN55  ( glass precision ? )

8K25  Metal film resistor
150K
100K
121K
150    ( clear see through)  can’t see it very well


220uf  50V  Filter Cap

Back of Circuit Board


( 2)  BC413B Transistor

Caps -   
  (2)          22u 35V    Tant Cap        ( one i couldn’t read well)


Black metal film resistors

33K2
6K81
221R
68K1
18K2

120K  5% (  brown , red, yellow, Gold )

 

[bluesblake]

back of circuit board ,    I also added more pictures to the  site.      https://www.icloud.com/sharedalbum/#B0H5qXGF1uhU4w

 

[mjrippe]

back of circuit board ,    I also added more pictures to the  site.        https://youtu.be/AfsRmITHtpQ

"This video is private".  And if it is more of those moving zoom photos, I don't want to see it anyhow.  Attachments are much better.

 

[bluesblake]

Sorry.

Maybe try this  :  https://www.icloud.com/sharedalbum/#B0H5qXGF1uhU4w

I had to right click on the link and open in new tab.  It just shows the pictures,  I hope.

 

[analogguru]

Picked up this Stereo Mic pre.  Has  48V, Pad,Phase .  Wondering what circuit it is based on.
.....

It's a clone of the Trident A-Range Mic Preamp.  The same circuit was used by Revox in their A77 (open reel tape recorder).

 

[bluesblake]

Sweet,  I haven't even plugged it in yet but am interested in how it sounds.  Wasn't the Revox a tube pre though?

 

[RuudNL]

Wasn't the Revox a tube pre though?

No, the Revox A77 is all transistor.

 

[ruffrecords]

No, the Revox A77 is all transistor.

And definitely pre-dated the Trident.

Cheers

Ian

 

[pucho812]

trident, neve and others all read the same wireless world article that showed a three transistor gain block and discussed it.  They took the info and ran with it in different directions.

 

[ruffrecords]

trident, neve and others all read the same wireless world article that showed a three transistor gain block and discussed it.  They took the info and ran with it in different directions.

A link to this supposed article would be nice.

Cheers

Ian

 

[mhelin]

This WW circuit doesn't differ much, on Neve the first and last transistor though share the common emitter resistor:

http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1972/Low-noise%20Audio%20Amplifiers%20-%20H%20P%20Walker.pdf

 

[PRR]

> wireless world article that showed a three transistor gain block and discussed it. 
http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1972/Low-noise%20Audio%20Amplifiers%20-%20H%20P%20Walker.pdf

Yes, that was a classic.

Note, bottom page 3, reference to 204401/2N4403. And do we know Mr Curl?

There was a parallel but less formal article in Audio Amateur a few years later, coming to about the same noise parameters with a mildly modified topology.

While these are shown as Phono preamps, the same works for transformer input mike preamps. I recall a Langevin mixer which came to (or adopted) this approach.

 

[ruffrecords]

This WW circuit doesn't differ much, on Neve the first and last transistor though share the common emitter resistor:

http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1972/Low-noise%20Audio%20Amplifiers%20-%20H%20P%20Walker.pdf

Except the Neve design pre dates it by a few years. The basic dc coupled pair was well known long before that - it was in one of the textbooks I used at university back in 1969. The Goldring Lenco RIAA preamp I had before that used the same circuit.

Cheers

Ian

 

[PRR]

> The basic dc coupled pair was well known long before

From an unpublished historical survey--