Project 500 EQ from Soundcraft Series One / Two

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Menno1000

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Hi masters,

I d like to start a project building the EQ of the vintage Serie One on a 500 pcb. All discrete. It is a fixed 4 ways bax like eq.
Love the sound of this eq. I'd like to start with a aml breadboard test version.

2 questions, excuse me for asking these beginner question..

I need 24 - 30v volt on the pcb.
Does any of you have a nice and simple schematic to turn the +- 16V int into 0 and 24-30 volt?


Also attached the part of the schematic. Soundcraft uses 2sc1681. Would KSC1845 be a good alternative? Or BC550?

Any tips are welcome!


 

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boji

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Does any of you have a nice and simple schematic to turn the +- 16V int into 0 and 24-30 volt?

Note that if you use the negative 16v rail of the bipolar supply as your 0v reference, this gives you +32v with respect to it from the +16v rail.

As to your second question : https://groupdiy.com/index.php?topic=60962.0

If the above OP's figures are correct, the BC550/60 looks compatible.  Of course you'd want to test it out.

How do to you plan to balance the I/O? Sorry to say, until you draft up a schematic and submit it for review, I doubt you'll receive much engagement from the 'masters' (I am certainly not one).  8)
 

Menno1000

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boji said:
Note that if you use the negative 16v rail of the bipolar supply as your 0v reference, this gives you +32v with respect to it from the +16v rail.

Hi, thanks for your reply!
The with this i thing that the minus 16v in the bipolar cannot be connected to ground? Is there a way to turn bipolar into 0V and ground connected with a positive rail?
Thanks again.
 

Menno1000

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boji said:
Pardon me if you already understand that ground is assignable in relation to some other potential.

Will your voltage source come from a lunchbox? Wall outlet? Mixer PSU?

From the lunchbox, other modules connect the o to ground, so that would create short -16v to ground i think.
 

ruffrecords

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I would be tempted to choose transistors that are comfortable with the full 32V voltage and just connect it right across them. You do not need to worry about 0V because both the inputs and outputs are capacitor coupled.

Cheers

Ian
 

Winston OBoogie

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If you wish to keep the voltage in the 30 volt or under range you could use a regulator on the negative rail and loose a few volts that way.    If the current of the whole shebang is constant, ie class A, then a simple cap multiplier would work to shave off a couple or three volts too.
I doubt you'll have a problem working with 32 volts though.  Like Ian said, just check the dissipation etc. on the transistors you use. 


Edit: take a peek at what voltage is where and just be sure you orient your electrolytic caps the correct way, it's possible you may need to reverse a couple from how they're depicted on the scheme but, without a full  schematic, who knows?
 

Menno1000

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Hi All,

Thanks for all replies and educating me!

I understand i can work with the bipolar supply. That is nice. The voltage is ok like this, no need to shave it of I think? I made a first draw. I connected R1 to 0V and added an 100K to the output. Do i need an extra cap there? R23 was already in the original schematic.

So could this be right? Any comments welcome.

Thanks

 

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ruffrecords

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That is basically it. You just need to know the voltages on TR1 base and TR2 emitter (and the same for TR3 and TR4).

TR2 emitter is almost certainly designed to sit at about half the 24V rails (and a quick calculation confirms this) i.e 12V above  the -16V so -4V relative to the 0V signal line.

TR1 base sits about 0.65V above the -16V rail so its at -15.35.

So bot point are negative with respect to the 0V signal so the negative end of the caps should go to the transistors.

Cheers

Ian
 

Menno1000

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Ricardus said:
What are the frequencies of the 4 bands?

I'd build a few of these to have in the rack.

It says 15khz, 3,5khz 300hz 50hz.


Eq's sound very very nice. I try this circuit on aml's breadboard. Maybe ad with dpdt push pull pots second freq for all bands. And next develop a 500 pcb.
 

Ricardus

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Menno1000 said:
It says 15khz, 3,5khz 300hz 50hz.


Eq's sound very very nice. I try this circuit on aml's breadboard. Maybe ad with dpdt push pull pots second freq for all bands. And next develop a 500 pcb.

I'll take 4.  ;D
 

Winston OBoogie

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It's probably just a drafting error but, flip C11 - 220uF - otherwise he's probably gonna get mad he was invited to a party in upside down land  ;)
 

Menno1000

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Winston O'Boogie said:
It's probably just a drafting error but, flip C11 - 220uF - otherwise he's probably gonna get mad he was invited to a party in upside down land  ;)

Haha, absolutely right! Flipped it and flipped c4 and c10.
 

mjrippe

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The Series One console was famously used by dub reggae pioneer Lee Scratch Perry.  I had a pair of channel strips at one point and they included Sowter input transformers and an all discrete preamp.  Sounded very nice and as you say, the eq is smooth and hard to make offensive.  I racked them and sold them to a fellow who was supposed to be working with Perry on a project.  Perhaps you can name yours the LSP 500  ;)
 

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