Passive mixer questions

[Schmilsson]

I'd like to use some old 600 ohm faders, is that a bad idea in a passive mixer?
Usually you see something like a 10K. (the mixer will btw also include an aux send and a pan pot)
Also, can you also one as a master fader?

Example
Channel 1 -> 600 ohm fader -> Aux send -> Pan -> Sum -> 600 ohm master fader -> make up gain.

 

[PRR]

> Usually you see something like a 10K.

Only in sillycon-state gear.

Can all your sources drive 600 ohms?

Don't fade, sum, fade again. Fade, sum, make-up, then master-fade with probable final amp.

 

[Schmilsson]

I decided to skip the master fader, so no worries there.
As for if the source can drive 600 ohms, I don't know, perhaps not.
The source will be an 8-track of some sort. Not specifies as of yet, but something along the line of tascam 238, 38, teac 80-8.

 

[PRR]

I want to say that most TASCAMs are scaled to 10K -10dBv interface; I may not have met all those models.

Sell the 600 faders and get some nice 10K pots.

 

[Schmilsson]

Here's the 238 specs:

 

[Speedskater]

With a 100 Ohm Output Impedance and a 0.6V normal output, I don't think that your 238 will be happy. Back in olden times when equipment was built to operate in true 600 Ohm systems, the output stage was a small power amplifier.

 

[emrr]

Every sort of true 600 ohm mixing system I've seen starts with a set of amps that can drive 600 ohms, has 600 ohm in/out amps for summing gain, master fade, then another set of 600 ohm amps for output.  Many broadcast consoles have a few input channels using 600 ohm repeat coils, which will still require an external source capable of appropriate drive.  The Altec 250SU and 250T3 consoles used the 15K:600 15095 octal transformers for bridging inputs, but you get about -14 dB loss that way.   

If you want to stay passive up to the makeup amps, you probably need bridging input transformers of some sort.  Otherwise, consider various buffer amp inputs that can present higher input Z and likewise drive the mixing stage. 

My long term restoration pile ha a number of vintage 600 ohm mixers on it, one being a large custom Altec that used the 9470A amps at input, channel output, and master output.  I don't expect to ever use the input stages with mics, so I don't need 45 dB input gain.  I will likely add some sort of unity gain buffer amp to the inputs, as the channels themselves already have considerable loss, and there are a lot of them.  The thought of 28 channels of bridging transformers hurts, as does the additional losses. 

 

[Schmilsson]

Yeah, I suppose those 600 ohm faders won't work, too bad since they're so pretty.

 

[emrr]

All depends on what you want to do. 

What kind do you have, and do I want them?  ;D

If on the fence, you might want to wire up a test situation and check them all, see if they are clean, dirty, and cleanable.  That might make your decision for you. 

 

[Schmilsson]

I actually haven't even got em yet.
But I was thinking of these:
http://schmitronicshop.de/index.php?main_page=product_info&products_id=434

 

[emrr]

Those are very pretty.  Don't know anything about them at all. 

 

[PRR]

> Here's the 238 specs:

As with most commercial specs: not the whole story.

However the -10dBV reference tells me this is the TASCAM interface. The output amps were originally simple transistor amps with 16V-24v supply and 4K7-10K collector loads. With ~~40dB NFB they could present 100 ohms small-signal output impedance, but sure would not swing much into 600 ohms.

Your model probably has opamps, but possibly 2K minimum-load types.

> I was thinking of these

Those have so much visual mojo that I would consider building 600 ohm buffers.

But where does it say 600 ohms?

 

[Schmilsson]

The almost are cool enough to go through the hassle!

I'm not 100% sure that they are 600 ohms, but I've seen EAB w66a (which I assume are similar enough) that have been labeled 600 ohms. I'll send them a mail.