Altec 9470A / 9475A rack-up

[emrr]

Layout planning:

Metalwork:

Front in progress:

Front panel controls mounted:

Back in progress:

Labels on:

Four response plots of Altec 9470A and 9475A preamplifiers, gain adjusted for comparison:

Gain differences unadjusted:

There's some blow by blow info over here:

http://www.groupdiy.com/index.php?topic=638&highlight=altec

 

[3nity]

Doug.
Did you change the DI Jack? i mean that looks like a Neutrik jack. not at all like the ones JLM included with the kit??

Jorge.

 

[3nity]

Sorry thats the new FET DI? looks cool!

 

[emrr]

Yeah, it's the FET DI jack, which is vastly superior to the opamp DI jack. I've had several of the opamp type strip after glancing at them, and don't recommend them; the jacks can't be sourced in the US and you have to get replacements from JLM. Total PITA. They have no trim washers to prevent panel scuffing, and are extremely flimsy. The FET DI jack is a huge step up, and also uses relay switching versus jack switching.

 

[Piotr]

Hi !

I'm Piotr from France. I have two 9475As that I plan on racking these days.

I've already built the JLM part of the project and I am trying to decide what option would be the best for gain control. Assuming I use a JLM input module which incorporates a -20dB input pad, should I refine this with a stepped input attenuator (from -3dB to -18dB let's say...) or should I rather design an H balanced output attenuator (resistor network and double pole switch) following the 9475A ?

Which solution is more transparent, best sounding, simple ???
All inputs welcome !!!! Thanks, Piotr.

 

[MartyMart]

Great work Doug looks like a beauty !!

MM.

 

[emrr]

Multi position stepped output T attenuator, maybe 3 steps of 4 db each might be most useful. 1K audio pot works fine too. You won't ever be doing more than 10-12 dB of output attenuation with most systems (max output of pre compared to max input of next device). JLM has already taken care of what the input can do correctly for microphones. You get max input of -24 or -4 dBm with JLM 20 dB pad option.

 

[griffmeister]

....... I'm a bit late on this one but;
Great work Doug and cheers for all the background and info!
Just curious, where did you wire the pots toon the o/p? I was thinking about going down the same route JLM i/p boards and was thinking stepped o/p attenuation (before i saw it could be "pot"-able...!)
Also, does anyone know the id/ din no. of the connectors plz? Unfortunately i'm without the trays...
Thanks, lee

 

[emrr]

Pot resistance across the output trans, 1 side directly to xlr pin, pot sweeper to other xlr pin. 

 

[griffmeister]

Many thanks for the swift response Doug!

 

[mrclunk]

Hey Doug, where do u get those custom stickers made?

 

[emrr]

Scanner, photoshop, printer, glue, clearcoat. 

 

[stitch-o]

Long time fan, first time caller:

What are those hefty power supplies you use?


very nice box!!

 

[emrr]

International Power / Power One.    Whichever they are called these days.  Mouser.  Been in the catalog for years. 

 

[griffmeister]

Sorry for the "newb-ish" question guys but.... :-\
Please could someone tell me if the 1k attenuator would "get along" with a switchable 600R terminating resistor? ..... Just before i drill so many holes that my case starts to look like swiss cheese.... ;D
Cheers!

 

[emrr]

The 1K is the terminating resistor.  Besides, these amps don't require terminating resistors on the output. 

 

[griffmeister]

Many thanks Doug, once again!

 

[Piotr]

Hi !

I'm finally finding the time to rack up 4 Altecs 9475A's, I've already posted a couple questions here, and a new one arose. I'd like to use the input transformer's dual impedance ability. My little idea is to use a DPDT switch (check both images below) and be able to switch between 150 ohms and 600 ohms input impedance. But I'm not quite sure this DPDT solution is correct. Let me know what you think of this.

So this is the input transformer wiring, I've used the primary's wire point numbers on the DPDT diagram.

And here's my idea. The signal path would then be: input XLR>JLM Go-Between>Input transformer with input points 2-8 and 4-6 connected to the switch. If I'm correct, in one position 2-8 are connected to nothing on the switch while 4-6 are jumpered. In the other position the 4-6 jumpered connection is broken and the connections 2-4 and 6-8 are established...

Am I right ??? Thanks a lot, cheers, Piotr.

 

[lassoharp]

This looks correct - just make sure and reverse the post position of either 8/2 or 4/6.

As it's drawn, for most standard DPDTs, you'll get 4-8 and 2-6 in the 2nd position instead of 2-4 and 6-8

 

[Piotr]

Hi !

thanks you're right I've drawn it too quickly !!! Here's the correct impedance switch... Thanks, Piotr.