Fader question. Substitute 10k with 600 ohm?

Hi.

Is it possible the exchange a 10k fader with a 600 ohm one??
I am currently rebuilding my old Cadac desk which i think is specified for 10k faders.
But recently found a bunch of cool, old Painton faders that would look great (at least!).
But they are 600 ohm.
Would it be possible??
What would happen if i just did a straight swap??

Regards,
Dennis

What drives the fader? An opamp, simple buffer or discrete amp?
Probs will most likely be with the source stage as 600ohm might be too hard to drive. 600ohms should be lower noise than 10k and the load stage would most likely be ok as I suspect there will be some kind of post fader amp or buffer before feeding pans etc.

You may need to add some electronics to drive the 600ohm faders and also bear in mind that depending upon the topology, it might need to drive the 600ohm in parallel with a few other loads such as pre fade auxes etc.

You got a schemo?

Cheers Tom

Here are the schemos!
Let me know if you gets a clearer picture!

Regards,
Dennis

Hmmmm no schemos...

sorry!

http://www.groupdiy.com/index.php?topic=16164&highlight=cadac

OK cool - so it seems amp stage C364 (just after the EQ) drives the fader, the solo bus and two aux sends with fader values of 2kohm as well a buffered meter feed (the impedance of which sould be sufficiently high enough to ignore but check that...)

So for rough approximation, amplifier C364 must be able to drive 600//2k//2k//buffer(assume 100k)//solo bus ...

The impedance will be dominated by the 600ohm fader....but actually low even with a 10k if auxes are pre fade.

The 10k might have presented a load of approx: 900ohms assuming impedances of meter buffer and solo bus are 100k and 10k respectively.

A 600ohm fader should present a load of roughly 400ohms....pretty low.

What amp is C364? Is it an IC? Looking at the line out schem, thats a discrete amp with a push/pull output, so maybe C364 is discrete or an IC with P/P output stage? It will likely need to be and also biased pretty hard to achieve good output into a 400ohm load.

Forgive me I may be making heinous calculation errors....but it looks roughly in the right ballpark.

There may be more insertion loss with the older 600ohm fader...so booster amp C347 might need a trim too.

You might get away with it depending upon C364. If you use the auxes set to post fader then the load won't be as low....maybe nearer 560 ohms.
Also the solo bus value will affect it only in solo mode....do you have bus schems?

Actually I just spotted the peak meter might not be fitted (special order only) so if you don't have it that might make a very small difference but as I said, if the buffer input Z is high >100k then its a very minor change.

Someone correct me if I messed up.
HTH
Tom

Actually I looked at the EQ schems and now I can't make out whether the EQ passes signal passively when in bypass....the channel schem shows amp C364 is active after the EQ (whether in bypass or not), so I assumed that is the driving amp.

There must be an amp somewhere thats active between the inputs and the fader with the EQ in bypass otherwise the line input transformer would drive the fader directly, which may be a huge issue.

I would guess that one of those amps drawn in the EQ schems is the driving stage....someone with more discrete knowledge than me will have to tell you whether that amp can drive the 4-500ohm load...

Also the insert (or what looks like the insert) is placed after the EQ and anything inserted would have to drive the low load as well...so thats another consideration.

I'm really an idiot so hopefully somone else will give you the definitive answer.

Tom

Those figures look okay to me, Tom! It would appear that C.364 is pictured in "eq_2.jpg", and as is the same as the output amp. Note the bootstrapped o/p pair, TO-5 cans with heatsinks, drawing roughly 5mA. Nearly class B. 0.775V/400ohms = ~2mA. Granted, I don't think the 600ohm fader will be a problem, but the deifinitive answer will only present itself when you take one channel and try it out.

Just out of interest....
From the Cyril Jones thread..
"Prefer 600ohm fader - due to lack of noise but P&G will only supply non 10Kohm faders unless you buy thousands "

From the Sound Techniques web site
"The initial desks used either Elcom, EMT or Painton, quadrant, 600 ohm constant impedance, stud faders. Later we used Penny and Giles carbon track (flat) faders. The faders used to make up about 50% of the component cost in any desk. Faders are actually potentiometers, but are associated with the master gain control of an input or output channel. They are also linear in action (though log in characteristic) and bigger than an knob!
"

Nice one Family Hoof.

I agree the best way to try it is slap one in there, should be easy as the faders are on either a jack or edge connector...

The output stage as biased is barely sufficient for decent signal level through the console IMO....it may induce some extra distortion. perhaps you could try re-biasing the stage to handle a little more? Maybe 10mA. But that depends upon the transistors (which are heatsinked) and also how much current your power supply can provide....

an extra 5mA on 12 channels might not be an issue....60mA in total.

By the way - are the 600ohm faders the reverse cockpit 'thrust' style stepped ones? This console is going to look badass if you get them installed!

-Tom

Unless I'm totally misunderstanding this discussion (which is possible because I can't give it my undivided attention at the moment):

"Nearly class B. 0.775V/400ohms = ~2mA."

Click to expand...

That's fine as long as your signal never exceeds 0.775V peak.

Use the peak value, not RMS. For a maximum level of 0dBU, it's 0.775 * 1.414 = 1Volt. And then there's the matter of headroom: figure on 20dB above nominal level if you wanna be on the safe side. That results in fourteen times the current (and 200 times the power) of what you're figuring, assuming a 0dBU nominal level. But what IS the nominal internal operating level of this console, anyway? I don't have the time right now to study all those JPGs...

[quote author="NewYorkDave"]Use the peak value, not RMS. For a maximum level of 0dBU, it's 0.775 * 1.414 = 1Volt. And then there's the matter of headroom: figure on 20dB above nominal level if you wanna be on the safe side.[/quote]
Right you are, Dave. The diagram is labeled 0dBv before the fader, which then keeps 10 dB in hand. I chose to overlook the issue of headroom for discussion's sake. You could always rebias the o/p pair to run hotter. Best thing to do now is try it on one channel and see.