GME-20 Melcor EQ

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RSRecords

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Jun 8, 2009
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Snagged a couple of Melcor GME-20s eq's. I am thinking of adding Input and output transformers for a balancing and "substance".

I was planning to use a couple API 2623's (or similar) on the output. I'm not sure what to use on the input.
Any 10k 1:1 input tranny or should I be looking for a step down 2:1 transformer like I read somewhere? Any recommendations are welcome!

Thanks!
 
2623 wired 1:2 on the output.  bypass the 10 ohm series output resistor (R34?),
check for any stability issues.
increase the output capacitor (C23) from 100 uF to 470 uf.
a resistor in parallel with C23 might be necessary to prevent a LF bump,  47 ohms could work.
a 2:1 on the input would be complementary.  maybe  a 31267 wired 9600:2400 ?
disable the bypass function.
 
gridcurrent said:
2623 wired 1:2 on the output.  bypass the 10 ohm series output resistor (R34?),
check for any stability issues.
increase the output capacitor (C23) from 100 uF to 470 uf.
a resistor in parallel with C23 might be necessary to prevent a LF bump,  47 ohms could work.
a 2:1 on the input would be complementary.  maybe  a 31267 wired 9600:2400 ?
disable the bypass function.

Awesome, thanks for the info. The 31267 is a marinair right?

I've got a couple UTC A-21s I was thinking I might try but maybe they'd be better suited for something else? Might look at some Edcors but I don't have experience with them.
 
RSRecords said:
The 31267 is a marinair right?
yes.
there is a Carnhill "equivalent".
A-21's are a bit down on rated level but do try them.
I've had excellent results with the 1/2 watt Edcors fed from zero source.
 
I think I'm going to try the UTCs and look into the edcors. I need to place an order from them anyway.

Why do you think removing the bypass switch from the circuit necessary? Noisy?

I was also toying with the idea of adding an attenuation stage. I have a couple vintage Altec variable attenuators 600:600. They've been looking for a good home. I don't know if it's worth the effort to have a level control though I could see the use. Any thoughts on adding something like that?
 
After some thinking, and some coffee, scratch the attenuators. I'll save that for a tube pre or limiter of some ilk. Keep it simple for once...
 
Found some specs on the GME-20 from MDI. Posting for posterity!

The most interesting things are the impedances. Signal source impedance is 600ohms while input impedance is 6kohms. What do they mean by signal source impedance. The output of the device before it? Seems high right?

as always, please pardon my ignorance.
 

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600 ohms is the classic output impedance. 6K in is a "bridging" load, gives little loss of voltage. Also means you could run 10 of these boxes on one 600r output with only small loss of signal.

This is all classic old-school interfacing.
 
PRR said:
600 ohms is the classic output impedance. 6K in is a "bridging" load, gives little loss of voltage. Also means you could run 10 of these boxes on one 600r output with only small loss of signal.

This is all classic old-school interfacing.

Gotcha, so "signal source impedance" is just what it says. So If I'm looking for a input transformer, one with a 600r secondary would be in accordance to specification? And if I'm looking for a 2:1 whats that 2.4k input impedance?

Maybe this in 2.4k:600: http://edcorusa.com/wsm_series
or this one:  http://edcorusa.com/xs1100
 
I would not  assume the Modular Products GME-20 is the same as the Melcor.
While MAP appears to have carried on some of the Melcor line, some details may differ.
The Melcor GME-20's output impedance includes a 10 ohm resistor in series with the nearly zero source Z of the 1731,  not 600 ohms.
The Melcor GME-20 has a 3.6k resistor (R21) in series with the "hi" input circuitry,
thus the input impedance is substantially greater than 600 ohms.
 
gridcurrent said:
I would not  assume the Modular Products GME-20 is the same as the Melcor.
While MAP appears to have carried on some of the Melcor line, some details may differ.
The Melcor GME-20's output impedance includes a 10 ohm resistor in series with the nearly zero source Z of the 1731,  not 600 ohms.
The Melcor GME-20 has a 3.6k resistor (R21) in series with the "hi" input circuitry,
thus the input impedance is substantially greater than 600 ohms.

Oh yea, that's a good point.  I was assuming they were the same since one eq has  MDI branded 1731s.

 
Sweet! "musical  'A'"

It's officially. All front panels will hence forth be referred to as "escutcheon".

that's good to know.  So any opinion on the edcor options above? I picked up a couple 2623 for the output.

I was also thinking of doing a diff. input for unbalancing. There's even a spot in the module for another opamp. I assume that was for the AE-20 with a mic preamp but it seems like it could be re configured by cutting a couple traces. Not sure if it's a good Idea to do that to a vintage pcb though.

 
Yea, that's what I thought. My idea was to install an amp without the transformer and try and wire it as a differential line input. This would be inplace of using a line input transformer to "un-balance" the signal.  It be kinda cool to be self contained but I haven't studied the schematic to see how exactly it could or could not work. I could forego creating any new holes in the escutcheon.
Maybe somebody more familiar with these and AE-20 might have some insight.
 
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