SSlmixed.com Forum

Tech discussion => 4K,6K,8K Series => Topic started by: MarconeMusic on April 09, 2022, 01:20:08 AM

Title: 291 preamp / line schematic.
Post by: MarconeMusic on April 09, 2022, 01:20:08 AM

Does anyone have a high res schematic for the 291 available quickly ish.

Mines missing and can’t find my digital copy.

Pls and thanks!

M

Email to me if you need.
Marconemusic@gmail.com

Title: Re: 291 preamp / line schematic.
Post by: Chris Simon on April 11, 2022, 03:23:12 PM

See attached.

Title: Re: 291 preamp / line schematic.
Post by: MarconeMusic on October 11, 2022, 10:54:18 AM

Did I forget to say thank you??
Sorry! Thank you!!

Title: Re: 291 preamp / line schematic.
Post by: pascal.verdet on October 12, 2022, 09:09:40 AM

Hi,
I tried to replicate the mic preamp. Lots of worries, until Andy came to my rescue.
The wires between the preamp and the gain switch must be exactly the same length, otherwise there will be oscillations...
Certainly a capacity problem.

Cheers,
Pascal

Title: Re: 291 preamp / line schematic.
Post by: amillar on October 13, 2022, 12:17:01 PM

Absolutely...it's actually the cable capacitance between the screen and core that stabilises the circuit.  You'll see that on the '291 card there are not fitted components R16/C7 and R17/C8, those were there to stabilise the circuit, but by absolutely pure luck we found that the cable capacitance did it just as well! By using the neat trick of driving the cable screen from IC1 and IC2.

If you built this as a stand alone circuit with short leads to the gain switch then R16 and R17 would be 649R (replacing R26 and R27) and C7 and C8 would be...quite small? 10pF perhaps?
 
We were really lucky that the cable didn't put in too much capacitance, if you use leads that are too long you will start losing a bit of the high end.

Cheers,

Andy

Title: Re: 291 preamp / line schematic.
Post by: Chris Simon on October 15, 2022, 12:38:34 PM

You're very welcome.