bwb011

Ampex MX 10 Advice
« on: June 10, 2018, 06:34:27 PM »
Hi,

I've been lurking on this site for years and it's been incredibly helpful for my newbie education! I recently bought an Ampex MX 10, and I know that in it's current state the output impedance isn't optimal with my converter (Apogee AD 16x). 

I saw on Electrical Audio's website that Greg replaced C16 and C17 (.47uF DC blocking cathode follower coupling caps), and removed the output fader and series resistor.  Unfortunately I'm new to this all and I don't want to screw anything up.   Could anyone point out to me in kindergarten terms on the schematic what exactly I'd be removing?   Is there anything else I should do to make the Ampex work with today's audio gear?

I'm also thinking of making a 1u rack with four JLM Go Between kits and possibly two of his DI's to make the Ampex a bit more functional.  I don't want to mangle the enclosure of the Ampex so could I run the XLR output from my microphone into the JLM, and XLR from the JLM into the XLR mic input of the preamp?  Or am I totally confused?

Do you have any other suggestions or thoughts?

Here's a link to the schematic:  http://www.davegroupjapan.com/mx103.gif

Thanks so much for your help! 



Re: Ampex MX 10 Advice
« Reply #1 on: June 10, 2018, 07:47:03 PM »
You didn't need to hack the mixer at all.  I used UTC LS50 type HiZ to line transformers with the mixer unmodified with spectacular results... wire the primary to a XLR plug for the mixer output and connect the secondary to the input of the converter.  The UTC A24 should also work fine.  The most you may want to do (after you connect the transformer and listen critically) is increase the coupling capacitor to 1 uF; use a good quality capacitor here.  If it ain't broke, don't fix it.

bwb011

Re: Ampex MX 10 Advice
« Reply #2 on: June 11, 2018, 12:14:39 AM »
I'm glad I haven't hacked into anything yet!  The transformer sounds like it would be way easier and I'd love to alter the unit as little as possible.

The two transformers you mentioned look amazing but a bit expensive and difficult to find.  I'll call Cinemag and some other local manufacturers tomorrow and see if they make anything that would work.  Are there any other transformers you'd recommend that are a bit easier to find?

Thanks again for your help!

Ampex MX 10
« Reply #3 on: June 11, 2018, 01:05:39 AM »
I recently bought an Ampex MX 10, and I know that in it's current state the output impedance isn't optimal with my converter (Apogee AD 16x).
a healthy MX-10 can drive 20k, though 100k is specified in the manual.
according to Apogee, the AD16x input impedance is 100k.
http://www.apogeedigital.com/knowledgebase/legacy-products/ad-16x-input-impedance/

you need no transformers unless planning on driving 600 ohm devices.
note that unmolested MX-10 outputs are pin 3+, unbalanced.
That fact will certainly agitate some folks.

bwb011

Re: Ampex MX 10 Advice
« Reply #4 on: June 11, 2018, 01:49:27 AM »
Wow I can't believe I missed that! Thanks although now I feel dumb  :o

I was aware of Pin 3 being hot.  As far as balancing would something like the product linked below be helpful if I make an xlr to rca cable?  Would you suggest another way to connect it to the converter?  Or should I simply make an xlr to account for pin 3 and run it straight into my converter?

https://www.ebay.com/itm/Whirlwind-LM2B-2-Channel-Unbalanced-to-Balanced-Line-Level-Converter/271908093730?epid=17012048371&hash=item3f4efc3f22:g:r48AAOSwu4BVhhEw

Thanks again this has really been a huge help!

Re: Ampex MX 10
« Reply #5 on: June 11, 2018, 02:10:56 AM »
should I simply make an xlr to account for pin 3 and run it straight into my converter?
yes.
shield to shield
Ampex pin 3 to Apogee +
Ampex pin 2 to Apogee -

however, I do suggest you open the unit, find the 2 "death capacitors", C19 and C20 on the AC mains wiring.
cut them out.

Re: Ampex MX 10 Advice
« Reply #6 on: June 11, 2018, 12:01:08 PM »
I'm glad I haven't hacked into anything yet!  The transformer sounds like it would be way easier and I'd love to alter the unit as little as possible.

The two transformers you mentioned look amazing but a bit expensive and difficult to find.  I'll call Cinemag and some other local manufacturers tomorrow and see if they make anything that would work.  Are there any other transformers you'd recommend that are a bit easier to find?

Thanks again for your help!

Didn't realize the converter used hi Z (RCA plug) input.  In that case, XLR female to RCA male, pin 3 (hot) to RCA pin, shield and pin 2 tie together to RCA plug ground...Keep the cable length short (1 m or less) because capacitance/meter applies to the cable used for the unbalanced connection.    My stuff is 600 ohm so I used the transformers.  Most of the professional gear is 600 ohm, usually balanced.

Jensen, Sowter, Transcendar, Lundahl, and some others build good output and matching transformers.  You can find the UTC stuff on ebay.  You get what you pay for with transformers.

You will also find out that transformers can block RF and other disturbances on audio circuits.  The MX10 has input transformers for the mic inputs which is a good thing.  The MX10 has "saved" me several times in a high RF/noise environment.

bwb011

Re: Ampex MX 10 Advice
« Reply #7 on: June 13, 2018, 03:58:07 AM »
Good to know. Thanks for your help guys! I really appreciate it!


 

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