explain this electronically balanced mic pre, can I replace with transformer?

[Vyvyan]

can someone explain the input of this amp? and could i possibly replace the resistor network on the input with a mic in transformer with 250ohm primary and 7.2k secondary followed by a gain pot?

 

[Newmarket]

Is this a schematic of a piece of kit that you have ? If so what is it ?

 

[RuudNL]

Input impedance looks too low to me for a 250/7.2 K. transformer.
(Pretty old design, 741 OpAmps...)

 

[Vyvyan]

yes, 1975 allen & heath sd12-2, also have a 10 channel "quasi" they have similar two transistor mic pres and im thinking of mixing and matching parts to build a colorful tacking\summing box. taking the sowter input transformers from the quasi and adding output transformers to all direct outs and the 2 buss, I have a pretty large collection of vintage mic and line transformers and have more than enough 600:600 from old yamaha consoles, and find a way to replace the ic with a discrete stage, maybe replace some coupling caps with line transformers


https://www.allen-heath.com/ahproducts/sd12-2/https://www.allen-heath.com/ahproducts/quasi/

 

[Vyvyan]

Input impedance looks too low to me for a 250/7.2 K. transformer.
(Pretty old design, 741 OpAmps...)

yea i figured that too. thinking of just swapping the mic pres from the quasi now, everything pre eq. schematics on the link in previous post dont show the sowter transformers but they are there and apparently 250:7k2

 

[gyraf]

Are you really sure you want to spend money, time and energy on this system? The type of eq circuit sported here tends to not bring out anything charming or vintage'ish..?

/Jakob E.

 

[Vyvyan]

Are you really sure you want to spend money, time and energy on this system? The type of eq circuit sported here tends to not bring out anything charming or vintage'ish..?

/Jakob E.

Not planning on spending money just time, I have the parts. The eq is fine for me, but I will probably end up messing around with customizing it. I've got the simple baxandall eqs from the quasi and have some inductors from an API build, and some old random crossovers I can source parts from to play around with. This is what I do for fun lol, but would also be great to end up with something useful, unique and colorful. A character build, I'm not interested in anything "transparent" anyone with a decent modern interface with mic/line in's has transparency right there. I love coloring sounds

 

[abbey road d enfer]

can someone explain the input of this amp?

It'(s basically a one-transistor inverter followed by a voltage-follower. Gain combines two actions. Moving to the left shorts the input, after the two 100r resistors, and moving to the right shorts the 5.6k emitter degeneration resistor.

and could i possibly replace the resistor network on the input with a mic in transformer with 250ohm primary and 7.2k secondary followed by a gain pot?

That would necessit a complete rework of the topology.

 

[ruffrecords]

The quasi mic pre is quite different and has a relatively high input impedance so the transformer will work. 250:7K is just over 1:5 so it will give close to 14dB of gain. 250 is probably the expected source impedance which will be reflected to the secondary as 7K2 so you probably want to load this with 10 time should s this or more. The 220K pot before the MA1 should be fine.

Cheers

ian

 

[Vyvyan]

It'(s basically a one-transistor inverter followed by a voltage-follower. Gain combines two actions. Moving to the left shorts the input, after the two 100r resistors, and moving to the right shorts the 5.6k emitter degeneration resistor.


That would necessit a complete rework of the topology.

That's why I'm thinking of replacing the mic pre stage with quasi mic pre.

 

[FIX]

A good lesson on unbalanced designs... The Line in also serves as the insert send and return... " I be I can design a console strip with 9 parts"

 

[JohnRoberts]

I see little redeeming qualities about that design.

You could consider redesigning the input, the middle , and output.

JR

 

[abbey road d enfer]

That's why I'm thinking of replacing the mic pre stage with quasi mic pre.

That's the venerable two-transistor preamp. Millions of them are still in operation, although the more serious pro audio designers tend to add one or two to improve the outout capability.
It's a lot of work to only figure out in the end that the rest doesn't follow.
RE A&H. There's a stark contrast between the crap they made in the 80's and what they build today.

 

[Vyvyan]

That's the venerable two-transistor preamp. Millions of them are still in operation, although the more serious pro audio designers tend to add one or two to improve the outout capability.
It's a lot of work to only figure out in the end that the rest doesn't follow.
RE A&H. There's a stark contrast between the crap they made in the 80's and what they build today.

As mentioned above, I'm not interested in clean or hifi or transparency, I'm using a antelope audio discrete 8 and ferrofish pulse 16 so I've got plenty of clean "pro audio" I/O the simple quasi strips only have 4 transistors per board and that's with the preamp, passive eq and fader. I've wired them for post fade direct outs and theyve sounded wonderful to me plenty of out and very little noise (post recap). I'm just interested in color and character not "pro audio character" I like dirt and I like the idea of the post fader direct out amps on the sd12-2 which look like they'd drive a 600 ohm tranny better than the quasi and extra mid frequency on the eq is a nice addition. Seems good for color and would probably work well with my tascam tsr-8 for some all analog sessions

 

[Vyvyan]

I see little redeeming qualities about that design.

You could consider redesigning the input, the middle , and output.

JR

Honestly I am open to redesign ideas. As stated I'm not worried about creating the perfect channel strip or anything like that, not expecting never or API quality (already have those colors in other gear. I've built tube and solid state gear based on schematics but have very minimal circuit knowledge and am trying to learn what I can because I enjoy working on old tube/transistor gear. So I'm open to any ideas of improvement even if seemingly miniscule. As stated previously I have plenty of parts at my disposal (caps, resistors as well as tubes transistors transformer and other parts from old alter, Yamaha and Sony channels) and I mean plenty. Transformers For example I have the 10 sowters from the quasi 12 mic xfrmers from a Yamaha m512, 16 from an m916, 4 from a pm1000, 2 from a pm2000 and an array of vintage tube mic xfrmers and probably 25+ 600:600 outputs from the Yamahas as well as some altec 15k:15k and 10k:10k, 10k:600, gates, UTC, ECT...


This is what I do for fun and I like having lots of character pieces in the studio. Everyone has access to perfection and the old standards thru plugins and modern interfaces, I like having all the strange homebrew sounds you cant get with.plugins or the old standards, the perceived quality of the circuit is all relative. But I'm definitely open to thoughts on redesigns because I'm interested in soaking up any knowledge I can and before anyone suggests, I've already got many of the books suggested on this forum on order and plan on tearing through them when I can.

 

[Vyvyan]

Can someone help explain the purpouse of the 741 here? is it configured as an inverting op amp for post eq makeup gain? ive only worked with baxandall eqs fallowed by a single transistor buffer (no makeup) driving a 10k fader followed by another post fade buffer into the final routing section.

i guess i just dont really understand active eq circuits? is makeup gain necessary when using the channels post fade direct out amp? or is the 741 only necessary to drive the 2k2 fader into the master summing section?

can the eq section alone (excluding the 741 and 2k2 fader) followed by a simple buffer drive a 10k fader into the post fade direct out?