Soundcraft Series One * Legacy Console * Any Info?

[L´Andratté]

I didn't write it to diss you, but for warning you that you're facing a very steep learning curve. 

No offence, just thanks for the help.   And you were right, not yet appropiate for me, but that´s exactly what I want to learn from this project, how do these parameters interact with transistors. All over Horovitz/Hill, Ebers/Moll etc. by now (when I find the time : ).

Outline of the mixer I´m imaginig (rough sketch, the devil in the detail will interfere; and sorry for rambling):

-Metal work: each channel will be two modules, one for the jacks and additional switches pre transformer and one for everything after the transformer. I will order the aluminium and bend, cut and drill in a local community metal shop that is free for all.

-Power supply: see next post

-Motherboard: like in the original mixer, but +V and GND will have individual paths to PS. I have this, but it´s just a cut off piece, I will make a better one.

-6 channels mic/line in, as per original schematic with these modifications: switchable +48V phantom power, padded line in through transformer (like Steve Dove did), switchable phase reverse, switchable eq bypass, additional post fader buffer (on a little sandwich pcb), direct out. I will check how noisy they are and modify the mic preamps when necessary, but as little intrusive as possible.

-2 channels unbalanced mono line input (or R/L for one stereo channel of course) : these are the additional channels I got without transformers, the mic amps will be modified into lower gain amps (12dB ?),no additional post fader buffers, and no direct out.
Edit: canceled, will use the extra channels for parts (transistors)

-Main R/L and foldback sum and output: as per schematic, but with much reduced gain (6 dB?) in the output amps (s. above, re. nominal operating level). I have one of these (say the foldback) and will need to make pcbs for the other two. Output transformers. Need to find some (aka need to find some money  ), preferably 600:600 sowters, for kicks. Edit: Sowter Type 3292. The Lo/Hi EQ on these will be left out

-PFL sum, stereo headphone drivers and meter drivers: I have these already, same gain reduction in the hp-amps as in the main outputs Edit: not necessary. I have a nice stereo VU-meter reserved for this

That´s it. At the moment I am recapping everything and thinking about the power supply.

 

[L´Andratté]

Power supply (please correct me if I´m wrong )
I have not as much going on as the original mixer design, but it´s good practice to overrate the power supply so I will try to get as close as possible to the original design.
In the bad copy servive manual-pdf linked in the first post are pencil scribbles all over, probably from a non-soundcraft service tech, in my personal folder that I got from the seller is even a later pencil scibble above that. The supply voltage is supposedly around +40VDC.
15/CT/15 transformers are readily available. CT can be ignored so I have +30VAC. VDC=VAC/0.71=+42V (*)
There is a voltage drop after the RC.filter, so that´s in the ballpark, and will vary with mains voltage anyway.

Beside the rectifier diagram in the service folder resides a bizarr little arabesque, which, with digital enlargening, can be imagined to read "1SB05". And there it is (second row): http://www.datasheetarchive.com/dlmain/4b4d2746be05d6c6bfef7cf0be3cca4c90028e/M/1SB05
I think this is the only hint at the nominal current draw of the circuit. Idc=1A
IAC=IDC*1.61 (*again). So the transformer should have at least 1.61A*30V=48.3VA. I´m looking at 100VA, 30V 3,3A transformer.
(*) http://www.sowter.co.uk/rectifier-transformer-calculation.php

Fuse rating should be 1.5*Ipri.
Ipri/Isec=Vsec/Vpri so Ipri=(Vsec/Vpri)*Isec =0.45A
So Ifuse=0.45A*1.5=0.675A.

Edit: Will use overrated bridge rectifier, like mentioned below. Will use 7*1000uF/63V electrolytics in parallel with 100n/63V film to replace each of the three 6800uF capacitors. Will use 15R/7W resistor like original circuit. Will figure phantom supply when I got this up and running...

 

[abbey road d enfer]

I think this is the only hint at the nominal current draw of the circuit. Idc=1A

It seems consistent with the general design. However, you know that, today, bridge rectifiers rated at 5 A cost close to nothing, contrary to when this mixer was designed. $1.31 gets you a 5A 200V bridge at Mouser.

IAC=IDC*1.61 (*again).

Every now and then, I see one of these coefficients coming out of the blue, without context. Since the AC current is considerably distorted, is it Iavg, Irms, Inom or Ipk, and which one does matter? And what is the limiting factor, Joule losses, core saturation,  sag?

So the transformer should have at least 1.61A*30V=48.3VA. I´m looking at 100VA, 30V 3,3A transformer.

You should be rather good with that. You may have even more than 42V, since the transformer would be underloaded. Not a problem at all, just make sure the smoothing capacitors are rated for this elevated voltage. 63V should be perfect.

(*) http://www.sowter.co.uk/rectifier-transformer-calculation.php

I have a lot of respect for Sowter, but here, I believe there's a combination of typo errors and plain misconceptions. "Pac = Pdc x 1.41", wow! 

Fuse rating should be 1.5*Ipri.
Ipri/Isec=Vsec/Vpri so Ipri=(Vsec/Vpri)*Isec =0.45A
So Ifuse=0.45A*1.5=0.675A.

I would agree if the load was purely resistive, but here, the current surge due to the capacitors charging makes all calculations only approximative. Make sure you use slo-blo fuses and experiment.

 

[mjrippe]

Another thought about powering: if the board is supposed to run at "around +40v" it may run just fine a bit higher.  When I have racked Yamaha PM1000 modules which are supposed to run at +45v I have found that I can set the (regulated) power supply to +48v and use it for phantom power as well.  Just make sure any power filtering caps are rated at 63v for safety.  Of course phantom power specification is actually +44 to +52v so you have some leeway.  I've just heard some folks complain if their p48 is not exactly 48v!

 

[L´Andratté]

$1.31 gets you a 5A 200V bridge at Mouser.

So be it!

Every now and then, I see one of these coefficients coming out of the blue, without context. Since the AC current is considerably distorted, is it Iavg, Irms, Inom or Ipk, and which one does matter? And what is the limiting factor, Joule losses, core saturation,  sag?

I wouldn´t aspire to build this supply, if I hadn´t read up on all that beforehand. But since there is no precision needed for this PS, maybe the wrong calculation has given the right answer (although I never before came across the formula and it looked suspiciously simple to me, I wasn´t prepared to dispute Mr. Sowter´s transformer math)? Note to self: question authorities!

You should be rather good with that. You may have even more than 42V, since the transformer would be underloaded. Not a problem at all, just make sure the smoothing capacitors are rated for this elevated voltage. 63V should be perfect.

Ordered already. Higher voltage is welcome (s.below about P48), the transistors can take 60V. I have given up on pondering BC550 as equivalents/substitutes anyway.

I would agree if the load was purely resistive, but here, the current surge due to the capacitors charging makes all calculations only approximative. Make sure you use slo-blo fuses and experiment.

Understood.

...their p48 is not exactly 48v!

Weird world!  ;D

I also thought about that phantom power solution and I like it´s simplicity, I´m not so much concerned with the voltage not being exactly 48V, but with the phantom line to be too noisy (only one way to find out). There´s always the possibilty to use the voltage doubler/regulator way of ssltech´s 9kpreamp, since I´m using a center tapped power transformer...(that wouldn´t be in the spirit of the mixer, but what use is a noisy 48V?)

 

[PRR]

> IAC=IDC*1.61
> coefficients coming out of the blue

1.61 is correct for heat.

If sag is the limiting fact, the number is nearer 1.8.

Both are explained in Grossinger Grossner and Lee.

In DIY work, use "2".

> I believe there's a combination of typo errors and plain misconceptions. "Pac = Pdc x 1.41", wow!

That is essentially correct. The DC Watts Out is significantly less than the VA that the same transformer could deliver as AC. Cap-input rectification is transformer abuse.

 

[abbey road d enfer]

> IAC=IDC*1.61
> coefficients coming out of the blue

1.61 is correct for heat.

If sag is the limiting fact, the number is nearer 1.8.

Both are explained in Grossinger and Lee.

In DIY work, use "2".

> I believe there's a combination of typo errors and plain misconceptions. "Pac = Pdc x 1.41", wow!

That is essentially correct. The DC Watts Out is significantly less than the VA that the same transformer could deliver as AC. Cap-input rectification is transformer abuse.

It would be correct if Pac, Pdc, Iac and the rest were clearly defined. There's a big difference between rated power, apparent power and real power. As you mention, there are other limiting factors than R.I², so what's the point in defining a factor with 1% precision when several other constraints advocate using a much higher value.

 

[PRR]

Plitron gives the factor as 1.8.

https://www.plitron.com/news/technical-notes/

The main point is that you will not get DC Amps equal to AC Amps, not even close. This disappoints many builders.

 

[abbey road d enfer]

Plitron gives the factor as 1.8.

https://www.plitron.com/news/technical-notes/

These figures seem to come out of thin air or wet finger, without any mathematical demonstration. Plitron says 1.8, Sowter says 1.61; that is not science, it's haggling.

The main point is that you will not get DC Amps equal to AC Amps, not even close.

That is absolutely clear. And the explanation is so simple I fail to understand why some have deemed necessary to cloud the subject with unexplained (and often erroneous) formulae.

 

[PRR]

Now you are just being pissy.

> thin air or wet finger, without any mathematical demonstration

I gave the specific reasons for "two numbers" and also references for anybody who wishes to see the math. (Yes, I got one name wrong but have corrected my regrettable mistake.)

 

[abbey road d enfer]

Now you are just being pissy.

I'm cool. There's no cause for anyone to lose their temper.

I gave the specific reasons for "two numbers" and also references for anybody who wishes to see the math.

I've googled Grossner, Lee and "Grossner and Lee", to no avail. But I know that any formula that accounts 2V for the voltage drop of two diodes and neglects the RI drop is an approximation.