Go Between with Tube Mixers - MX10, 1567a

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mcshimby
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Joined: Thu Oct 13, 2011 6:42 am

Go Between with Tube Mixers - MX10, 1567a

Post by mcshimby »

Hi so I made a 4 channel Go Between front end for using condensers with some older preamps. All four seem to work great when plugged into my Ampex 351, but I can't get a good signal when using condensers going into my Ampex MX10 and I had a similar issue with an Altec 1567a.

They do everything they're supposed to if I run a dynamic through them into the mixers, but when I plug in a condenser (AT4033 or Earthworks TC25) I get a very weak distorted signal when the 48V is engaged. The level actually seems to go up a little bit when I engage the pad. And the weirdest thing is that when I disengage the pad there are a few seconds where it actually sounds good, loud and clear and then fades away back to the quiet distorted signal.

As a side question, is the phase switch before or after the 48V? Might there be a pin 2 hot vs 3 issue that the could be resolved if I rewired the cables going to the mixer inputs? I kind of figured the phase would fix that. And to be perfectly honest, I think I was only getting signal when the phase was engaged, but can't quite remember as I'm away from the gear right now.

I'm guessing it's impedance related, but just don't know enough to say.
Any help on this would be appreciated. Thanks!

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Joe Malone
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Re: Go Between with Tube Mixers - MX10, 1567a

Post by Joe Malone »

mcshimby wrote:Hi so I made a 4 channel Go Between front end for using condensers with some older preamps. All four seem to work great when plugged into my Ampex 351, but I can't get a good signal when using condensers going into my Ampex MX10 and I had a similar issue with an Altec 1567a.
As long as the Altec 567a has the 4 plug in mic input transformers still fitted it will be the same as the MX10 input circuit wise which is a floating balanced input and will work perfectly with the GO between kits.
They do everything they're supposed to if I run a dynamic through them into the mixers, but when I plug in a condenser (AT4033 or Earthworks TC25) I get a very weak distorted signal when the 48V is engaged. The level actually seems to go up a little bit when I engage the pad. And the weirdest thing is that when I disengage the pad there are a few seconds where it actually sounds good, loud and clear and then fades away back to the quiet distorted signal.
This would suggest the 48v phantom power is not working properly.

What power supply is supplying the 48v power rail?
Is it wired to 48v and 0v on the go betweens?
Are all XLR pin 1 wired to 0v on GO betweens?
Are the 48v leds wired across the top to legs of the 48v switch?
Does the 48v led light when the 48v switch is turned on?
Are the in and output XLR wired in mirror image of each other (pin 1 ground opposite sides) ?
With phantom turned on what voltage do you get between pin 2 and pin 1 on the input XLR with no mic plugged in?
With phantom turned on what voltage do you get between pin 3 and pin 1 on the input XLR with no mic plugged in?
With phantom turned on and mic plugged in what voltage do you get between pin 2 and pin 1 on the input XLR with no mic plugged in?
With phantom turned on and mic plugged in what voltage do you get between pin 3 and pin 1 on the input XLR with no mic plugged in?
As a side question, is the phase switch before or after the 48V? Might there be a pin 2 hot vs 3 issue that the could be resolved if I rewired the cables going to the mixer inputs? I kind of figured the phase would fix that. And to be perfectly honest, I think I was only getting signal when the phase was engaged, but can't quite remember as I'm away from the gear right now.
Switches are wired int he order on the front panel. eg 48v pad phase. If switching the phase changes anything then the wiring to the go between is wrong.
I'm guessing it's impedance related, but just don't know enough to say.
No this is a wiring or 48v power problem. Answer the above questions and we should be able to get you going quickly. Otherwise some photos of your wiring always helps.
Joe :-)
JLM Audio
Capturing Audio without Injury

mcshimby
Posts: 3
Joined: Thu Oct 13, 2011 6:42 am

Re: Go Between with Tube Mixers - MX10, 1567a

Post by mcshimby »

Joe,

Thanks so much for your thorough reply and your excellent kits. There was no problem with the GoBetweens.
It turns out that someone modded the inputs of the MX10 to be unbalanced, so pin 2 was shorted to ground. Once I removed these cables everything works great!

Thanks again.
Shimby

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Joe Malone
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Re: Go Between with Tube Mixers - MX10, 1567a

Post by Joe Malone »

mcshimby wrote:Joe,

Thanks so much for your thorough reply and your excellent kits. There was no problem with the GoBetweens.
It turns out that someone modded the inputs of the MX10 to be unbalanced, so pin 2 was shorted to ground. Once I removed these cables everything works great!

Thanks again.
Shimby

Cool. Good to hear you got it all working :-)
Joe :-)
JLM Audio
Capturing Audio without Injury

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