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Soundcraft 200B project form a designers point of view.
Old 25th January 2019 | Show parent
  #91
Chips! I forgot to include the types of the small signal transistors.

I used the 2SC2389 and the 2SA1038. Both 50 m.a transistors.
They aren't in production anymore.
I think the KSA 992 and the KSC1845 would be perfect too.

For the headers I used MOLEX 3 pin with spacing 2/10 inch.
I use soldering for connection: better than the female part that goes with the male part on the PCB.

For the drivers T7&T8 it would be better to use the 2SC2705(Y) and the 2SA1145(Y) for dissipation reasons. Unfortunately both not in production anymore. If you want to look yourself for replacement of these "elongated TO92" transistors look for a low Cob and low current capability.

It is really a shame that many outstanding transistors like the last mentioned are not produced anymore.

Of course I have many hundreds lying around here, but I would't like to add an commercial aspect to this. (perhaps not allowed here also)
Old 25th January 2019
  #92
Some more about the regulator: The focal point was to get a good PSRR and a regulator that is "at ease" with the load at it's output.
I didn't want capacitors or elcaps have an influence on the sound, so safe for the 33pF it doesn't uses any elcaps in the regulator.

About the raw power supply for lets say the 1.2A maximum: first elcaps pair 10,000 mu would be more than enough, 4.700 could be used too I guess.(the latter only when using chokes) As for the transformer I would recommend an old fashioned E I core lamination transformer, preferably with two winding chambers.
Use fast soft recovery diodes. Do NOT use decoupling caps over the elcaps.
Using a choke the second pair of elcaps could be in the same ball park as mentioned values.
Old 25th January 2019 | Show parent
  #93
Lives for gear
 

Interesting. So 1.2 A @ +/- 17.5V is the maximum for this supply?
Old 25th January 2019
  #94
The power supply in this shape and form yes.

Provided enough cooling for the serial devices and lowering the 0.5R to about 0.25R the power supply could deliver 2A.
Dissipation in the 0.25R will call for a 2W device.

Higher voltages would also be possible changing resistor values.
Old 25th January 2019
  #95
Try to upload a better picture of the schematics
Attached Thumbnails
Soundcraft 200B project form a designers point of view.-sr1-new1.jpg  
Old 25th January 2019
  #96
Raising (or lowering) output voltage:

Let's say output voltage increases with 20%, you raise R2/R16/R28 and R1/R20/R27 with about the same amount of percentage.

Output current is solely depended on the R6 and R7 "current sensing" resistors. Lower the same amount as the % of higher wanted maximum current output.

T7 and T8 dissipation will be higher with higher current and voltage, so 300 mW will be often not enough, use 800 mW or so in these cases. and of course you will have to pay attention to the dissipation of the series TO220 device (transistor), including watching the max. current specs of this transistor.
Old 27th January 2019
  #97
As I haven't looked into this regulator for a couple of years, I decided to have a closer look again doing some computer simulation in MicroCap 9 pro.
+3dB point of the PSRR of an LM317 regulator is about 2.2 KHz, from this regulator somewhat above 100KHz...
Peak in phase shift between input and output are 15 KHz and 1.2 MHz respectively.

Furthermore: the base current of the serial transistor subtracts of course from the current of the current source providing the bias for the driver transistor. So when using larger output currents R3 and R4 may have to been lowered to provide the right bias current for the driver transistor.

If the power supply isn't loaded all the current from the bias current source (D2, T1, R4 for the positive side) flows through T8. This will lead to roughly 160 mW in dissipation. So, after all it would be possible to use 300 mW transistors for T8 and T7, even if we lower R3 and R4 to remain enough bias at larger output currents than 1A.
Old 23rd May 2019 | Show parent
  #98
Here for the gear
 

Hi there,
I've been following this post from long time ago and I have to say that it helps me a lot with the mods I'm doing in my SC 200B, thank you so much for sharing so detailed info and experience. The sound of this nice desk is getting pretty good, at least for my needs

I have a question about the Dale resistors, I have ordered the RN60 1/4 W for the Master section, it is alright or they should be 1W? does it make any difference.

Thank you so much in advance.
Agus
Old 3rd June 2019 | Show parent
  #99
Quote:
Originally Posted by agustindavid View Post
Hi there,
I've been following this post from long time ago and I have to say that it helps me a lot with the mods I'm doing in my SC 200B, thank you so much for sharing so detailed info and experience. The sound of this nice desk is getting pretty good, at least for my needs

I have a question about the Dale resistors, I have ordered the RN60 1/4 W for the Master section, it is alright or they should be 1W? does it make any difference.

Thank you so much in advance.
Agus
Hi there, Thank you very much, it is great to know you are benefiting from what I wrote.

1/4 W is fine! the RN60 is the mil spec version of CMF60, which if I recall it right is specified at even 1/2 W, so no sweat.
Old 5th June 2019 | Show parent
  #100
Here for the gear
 

Quote:
Originally Posted by Panoramix View Post
Hi there, Thank you very much, it is great to know you are benefiting from what I wrote.

1/4 W is fine! the RN60 is the mil spec version of CMF60, which if I recall it right is specified at even 1/2 W, so no sweat.
Thank you so much! I really appreciate your help and asure you that your post about the SC 200B has been, and still is, my main source of tech information for my own 200B. Thanks to you now it starts to sound in another level!

Big thanks, man!
Best,
Agus
Old 2nd September 2020 | Show parent
  #101
Gear Maniac
 
YashN's Avatar
As the owner of a Soundcraft Delta mixer, this is a great thread. Well done on the analysis for enhancements, @ Panoramix .
Old 2nd September 2020 | Show parent
  #102
Thank you YashN!
Old 16th September 2020 | Show parent
  #103
Gear Maniac
 
YashN's Avatar
I have pretty much the same experience as you concerning the structure of PSUs pre-Reg. Built a few of them like that, including a dual PSU for Lab work, and one for my external DAC. More work needed on them to really optimise them though.

I do notice in my sims (so far) quite a high Voltage peak before it settles back. Do you get this too?

It's pretty amazing that most commercial PSUs aren't that well designed, no?

This year, the initial stages of lock-down gave me some free time and I have done some good breakthroughs for audio at the Reg section too. More work needed here as well for further optimisation.

I still have the CPS-150 for my Soundcraft Delta to re-work.

I had a little Soundcraft Compact 4 before that one. Still have it and opened it up for cleaning with a view of selling it.

I found that a resistor near a Reg in the Compact 4 had a burnt appearance. Looks like this design flaw in this particular spot in common to some SC mixers of the time as I pretty much found a similar thing in the CPS-150!
Old 18th September 2020
  #104
Lives for gear
 
ruffrecords's Avatar
Fascinating thread, full of mystery, myth, magic and a little engineering common sense.

Cheers

Ian
Old 18th September 2020 | Show parent
  #105
Gear Maniac
 
YashN's Avatar
And that specialised return for the bypass caps, that's a big can of worms too!

That's a whole level of re-work necessary... on almost all existing PCBs.
Old 2nd October 2020 | Show parent
  #106
Gear Maniac
 
YashN's Avatar
Just posted this about El. Caps leaking and damaging traces (that's for my Lexicon LXP-15 multi-effects rack) because IIRC someone mentioned this here (actually, I was wrong - it was mentioned in another thread of interest - let me know and I can delete it). It's not just old batteries leaking that can do bad damage.

I was lucky, most of the damage went onto the top cover, but there was still some PCB damage I had to fix.

Last edited by YashN; 20th October 2020 at 07:10 AM..
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