Problem: Older 8 x 2 SSI mixer, unbalanced line input ground hum

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Trench Recordings

Well-known member
Joined
Oct 9, 2011
Messages
159
Location
Toronto, Canada
I have this SSI mixer from the late 60s / early 70s. Sounds great, uses melcor 1731 DOAs. I love the thing and use it every day. But I does have its problems.

Hoping some of you guys might have some input or suggestions on how to improve it.

The line inputs are unbalanced. Some gear works great with it. Some gear wont pass signal at all with it like my Studer A80. A80 Output to SSI mixer line in does not work ?

Other gear introduces ground hum like the output of my converters, Apogee AD8000. The ground hum stacks up for each connection made. I would love to use this as a summing mixer from my converters but the ground hum !

I think the problem maybe that there is no line input transformers in the design ? so I was thinking of installing some Lundal transformers (maybe LL1540) right after the line input connection.

Would that help the ground hum issue ?
 

Attachments

  • IMG_0394lr.jpg
    IMG_0394lr.jpg
    459.4 KB · Views: 111
check and see, it might be pin 3 hot. That might explain why some pieces of gear won't work with it. Too bad you don't have a schematic or can reverse engineer it. I imagine its very simple.

I bet fixing the ground issue is simple also, though probably very frustrating! Quick and dirty fix would be to check how the grounds of the XLRs are connected. If each ground fro mthe XLR runs back to the audio PCB then convert each XLR pin 1 directly to chassis then on only one pin1 also connect that to the audio PCB. But that could be how it is done already, or it might use a different scheme.

Oh yea, it looks really killer!!

Also are you sure its unbalanced?

edit: i guess that close up is from the outputs...also inserting input transformers would solve the hum issue, well assuming the power supply and everything else is noise free.
 
The unit I have has been modified. The insert jacks are now just unbalanced line in. There is a blue ribbon connector at the back for the mic pre direct outs. The mic pre outs do not go through the rest of the circuit/mixer unless I patch them into the line ins. Done with my patchbay or I have a blue ribbon jumper connector that connect them all.

The line in distorts if I connect a trs balanced source. The trick was to cut the cold "ring" and leave it floating. This works for 90 % of my studio gear. But some gear will not pass signal this way. Also ground hum can be an issue with some gear.

So the idea now with the help of another forum member... is to have another 1U rack to house line input transformers. I'm thinking of using carnhill VTB 9046 and wiring it in a 10k:600, or perhaps a 2:1 or 1:1. Depending on how much line in gain I'm comfortable losing.

http://www.audiomaintenance.com/downloads/carnhill_design_guide.pdf

Any other transformers you would recommend ?


The other idea I had was to install stepped gain pots for the mic pre direct outs on the front or the 1U case. They come out screaming hot and no real gain control. Ok for some quiet sources but besides that not really usable. Would the stepped pot kits from classic api work for this ?

http://classicapi.com/catalog/product_info.php?cPath=51_133&products_id=237

Thanks,
Dave
 
Very cool looking mixer.
As an aside, SSI's founder (and I believe the designer of this mixer), Neil Muncy, passed away a few days ago.
(More information here: http://www.thewombforums.com/showthread.php?p=360885 )
He was an expert on grounding (see: http://www.aes.org/historical/oral/?ID=9 and http://www.rane.com/pdf/ranenotes/Pin_1_Revisited.pdf ) and an all-around affable fellow who was very generous in sharing his quite vast knowledge.
RIP

 
Trench Recordings said:
I was thinking of installing some Lundal transformers (maybe LL1540) right after the line input connection.
This is the simplest way to solve ALL your problems.

Some gear wont pass signal at all with it like my Studer A80. A80 Output to SSI mixer line in does not work?
Probably one or the other uses the old Yank pin3 is +ve standard instead of the current AES pin2 is +ve standard.  Sensible to convert all your gear to AES.

The line in distorts if I connect a trs balanced source.  The trick was to cut the cold "ring" and leave it floating. This works for 90 % of my studio gear.
This gear has horrible 'balanced' outputs where the +ve signal is simply inverted by an OPA to feed the -ve pin.

But some gear will not pass signal this way.
In fact this is the good gear.  eg proper transformer output gear.
 
Depending on what you would like to spend, go for transformers at the input.  All of your gear listed has bridging outputs that can handle a 600 ohm load, so go for a 600:600 and keep your gain.
You have Carnhill, Lundhal, Jensen on the high end and Edcor, OEP, Radio Shack on the low.  If you want to keep with the topology, I think a purist would use a Cinemag.
If I had this project I would start with a comparison with an Edcor and a Jensen on two inputs and go from there. 
Mike
 
Ok, sorry I haven't replied to this in a while. I've been busy at work and building 1176's.

I will open her up and try to identify the transformers in there one day soon.

In the meantime, I designed and ordered the front panel for the 1ru chassis. I'm gonna order the go between and variable pad kits from JLM audio. That should make the mic pre direct out usable with some pads and attenuation.
 

Attachments

  • Untitled-1.jpg
    Untitled-1.jpg
    88.6 KB · Views: 29
Sorry I'm still learning this stuff...

So whats the difference between a 10k:10k transformer and 600:600 ? there are both 1:1

what would happen if I used a 10k:10k or 2.4k:2.4k here  ?
 
Attempt to resurrect this thread... I recently got a SSI mixer,.. It's the "remix module" basically it's 8 transformer ins to a passive stereo summing network and a stereo Melcor 1731 makeup gain amplifier.

I'm curious if anyone knows anything about this module, specifically the transformers... They are UTC o37 (which are liste as transistor to line) inside UTC o17 (listed as 25db shields ).

Curious about what the 15db sheild does and what it's for.. As well as the transformers..

Dunno.. Anyone ever come across this module before? I'd like to modify it with another 8 ins, but curious how that would affect the summing bus, as its using  1k buss resistors now.. And getting similar input transformers etc..

Anyways.

Thanks
 
Fuccimain said:
Curious about what the 15db sheild does and what it's for..

Well I know the answer to that one. Thats 15db of attenuation from outside interference. Transformers are just coils or wire around a metal core, right? Great at transferring energy but also great at pulling energy out of the air, like from the power transformer or power lines or anything. Hence the shielding. Shielding is most important on the input side of things, and also MOST important on mic input transformers. You generally don't see output transformers shielded because of the low impedance and usually outputting a large enough signal were any induced interference is way down in the dirt.
 
guess I had a type and meant 25db shield, but the answer is the same.. thanks for the info.. I'm having some other issues with the mixer, but I'll start a new thread for it

thanks
 
Back
Top