Custom Mixer Build - Mic Preamp PCB resource(s)

[boriscrispin]

Hi all,

I've been a lurker for many years, I have no technical skills of which to speak but being a music recording person, end up filling my head with all sorts of 'information' on transformers and old mixing desks.

I am soon to ask my tech to undertake a custom small mixer build (something portable with preamps and summing bus) and would like to know if there is an open source style place I could be looking for some discrete mic preamps and the like.

I have (amongst quite a large resource of gathered line in/out transformers for the rest) some good quality Sowter 1:5 mic transformers (see attached pic from some 70's Audio Developments channels* which they are on currently) that I would ideally like to use in the preamp - so anything relatively off the shelf that will save my tech time (and me money!) seems like a wise place to start?

From Sowter:
"It is a 1:5 200/25K mic transformer with a geometrically balanced primary with CT for phantom powering.. The black wire is an internal electrostatic screen."

*It crossed my mind to ask my tech to copy the existing preamp perhaps but have no real knowledge of its quality/sound as I've never experienced them functioning, so its hard to know if they're old and good or old and 'bad'

Any advice would be great and perhaps I can string you along for the rest of the build if that is interesting at all!

 

[L´Andratté]

Soundcraft Series2
Ricardo´s Dark Side MicPre

 

[boriscrispin]

Thanks for those.
They look like they result in quite colourful preamps perhaps? As long they live up to modern noise reality then I'm sure they'd be interesting.
Do you have personal audible experience with either?

 

[iampoor1]

Decide what type of preamp you want first. Trying to save money on the pcb designs on a custom mixer is an exercise in false economy. Retrofitting the design to whatever ever metalwork you make and the overall system design may not seem like a big deal but that will easily outweighs any savings.

I think building some 500 series buckets into a small mixer format would be your best bet if you want to use off the shelf modules and designs.

 

[abbey road d enfer]

From Sowter:
"It is a 1:5 200/25K mic transformer

This is contradictory. 1:5 implies a ratio of impedance of 1:25, so for a 200r input, it results in 5k secondary, which is good, because it matches well with most discrete and monolithic opamps, such as the venerable 5534 (monolithic - think SSL4000) and 2520 (DOA - think API), and also many full discrete mic preamps such as Neve and Helios. So you have a large palette for selection.

*It crossed my mind to ask my tech to copy the existing preamp perhaps but have no real knowledge of its quality/sound as I've never experienced them functioning, so its hard to know if they're old and good or old and 'bad'

AD mixers have a different approach than most music recording mixers. They are designed for the rigours of location recording, and long battery life. It doesn't mean they sound bad at all (I made some pretty good recordings with an AD075 in jurassic times).

 

[boriscrispin]

Decide what type of preamp you want first. Trying to save money on the pcb designs on a custom mixer is an exercise in false economy. Retrofitting the design to whatever ever metalwork you make and the overall system design may not seem like a big deal but that will easily outweighs any savings.

I think building some 500 series buckets into a small mixer format would be your best bet if you want to use off the shelf modules and designs.

I have definitely considered trying to utilise the 500 series concept - I'm not looking truly to use off the shelf complete items so rather wondered if there was anything of an archive of browsable designs one could pillage, including 500 circuits.

I'll be working with a panel/metalworker person and have found an old chassis which is suitable size and "vibe" wise, so will be working within it's constraints. I am in line with your comment though, I am fully aware of the kind of rabbit hole one can go down, so I think my first line of thinking was 'what circuit can I utilise'

My requirements for the mic pre stage are boringly common I'm sure, transformer balanced, hi-fidelity but not 100% transparent type thing. I don't have any pointless bias against using op-amps but I know for a fact that the pre-amps I have used that have been more sonically rich have been discrete designs - which is perhaps why I'd like to hear any users of the previously link 'ricardo dark side' preamp concept

 

[boriscrispin]

This is contradictory. 1:5 implies a ratio of impedance of 1:25, so for a 200r input, it results in 5k secondary, which is good, because it matches well with most discrete and monolithic opamps, such as the venerable 5534 (monolithic - think SSL4000) and 2520 (DOA - think API), and also many full discrete mic preamps such as Neve and Helios. So you have a large palette for selection.

Interesting, perhaps a typo on Brian's part, do you feel confident that is a mistake?

Now this 'large palette for selection'....has anyone put that up in an archive of circuit designs anywhere?

 

[abbey road d enfer]

, hi-fidelity but not 100% transparent

Which, for an electronics designer, is somewhat moronic...

 

[boriscrispin]

Which, for an electronics designer, is somewhat moronic...

Ha! I know you're quite right, it never feels good writing about sonic desires.

I'm certainly keen not to inspire annoyance with my hopes. Some useful circuits to pass on to my friend would be a great outcome

 

[sahib]

This is contradictory. 1:5 implies a ratio of impedance of 1:25, so for a 200r input, it results in 5k secondary......

Abbey, I am glad you picked up on this but (as boriscrispin has also pointed out) Sowter's data sheet is indeed full of similar given specifications. PRO AUDIO TRANSFORMERS ARCHIVE

Could it be that, as these were custom designs they were driving into a specific input impedance? Which still does not make clear sense. I am not sure if Brian is still with the company but perhaps you (or Mr CMRR) can bring some clarification

 

[abbey road d enfer]

Abbey, I am glad you picked up on this but (as boriscrispin has also pointed out) Sowter's data sheet is indeed full of similar given specifications. PRO AUDIO TRANSFORMERS ARCHIVE

Could it be that, as these were custom designs they were driving into a specific input impedance? Which still does not make clear sense. I am not sure if Brian is still with the company but perhaps you (or Mr CMRR) can bring some clarification

It seems that some of the specs are legit, and others are a statement of the nominal expected loads, and a few being either figments of imagination or plain typos.
I tend to think that the turn ratios are correct, the impedances much less so.

 

[boriscrispin]

I'm not sure if this helps to shed any light on the possible circuit which follows the transformers but the only two schematics I have gathered which are close to my channels in date, I have attached. Perhaps if anyone would find it interesting to determine if the photos "match" with either of those?

 

[abbey road d enfer]

the only two schematics I have gathered which are close to my channels in date, I have attached. P

These are typical of the design created by Peter Levesley when he was head of design at AD. It uses a rather unusual combination of voltage and current NFB. The actual gain depends on the impedance of the mic that is connected to it. When there is no source connected, the gain is reduced, so a mic that comes disconnected (it happens in location recording) the noise is also reduced. It doesn't sound bad, but it doesn't shine on mics that are sensitive to load, i.e. most dynamics and ribbons.

 

[boriscrispin]

These are typical of the design created by Peter Levesley when he was head of design at AD. It uses a rather unusual combination of voltage and current NFB. The actual gain depends on the impedance of the mic that is connected to it. When there is no source connected, the gain is reduced, so a mic that comes disconnected (it happens in location recording) the noise is also reduced. It doesn't sound bad, but it doesn't shine on mics that are sensitive to load, i.e. most dynamics and ribbons.

Very interesting to hear, thanks

 

[L´Andratté]

Hi, Boris, yes I built these "Dark Side Preamps", I still have some flying around here (s.below) and like them a lot for their simplicity, they are Low noise and clean up to a point. If you want a circuit explanation, see "Art Of Electronics", 2nd edition, page 238...
On a highly subjective basisd I perceive it as the nice sound of classA (and transformers of course). But as Ricardo writes, with this circuit, the performance is dependent on the load impedance, which you can define (thus the missing output buffer).
My proposal would be with circuits as simple as these, just build and listen. Take the SC series2 circuit (eq, filter, aux, faderamp) I posted and replace the overly complicated preamp with the dark side snippet (that´s what I did, only with SC series1, which is similar to series2, just with even more clunky preamp config.). For that you have to run the channel on +30V, which is not a problem for the SC,which usually runs on +40V.

 

[boriscrispin]

Thanks for your suggestions, appreciate your input here - my tech is tied up renovating his house so I can never really get him to easily test anything out!

May I ask with regards to the Sowter transformers mentioned in the thread, the 1:5 (200:25k) spec may perhaps be erroneous, but if that turned 25k out to be correct, would they still be of use in the Dark-Side preamp? - What transformer did you use for yours?

 

[L´Andratté]

25k cannot be correct.
1:5 turns/voltage ratio transformer is correct for nominal microphon output impedance of 150/200R,
which (as impedance gets transformed as the square of turns/voltage ratio=25 here) gets transformed to 3k75/5k which is close to optimum source impedance aka best noise performance from the input transistor.
200/25k transformer doesn´t make sense that would be a 1:125 voltage ratio/over +40 dB, and not optimum source impedance.
Otherwise it would be a 1k:25k but that would not make sense too...

The Sowters I use are from a SCseries 1 mixer, that was beyond repair. parts #6145, for which strangely 2 different data sheets exist, iirc??? Mine are 1:7 measured with my DMM.

Edit: I think this preamp came up recently on the io micbuilder group, by Ricardo/R.Lee