Low-cut when vari-mu comp is engaged?

Hi,

A while back I posted a schematic for a conversion I was doing on a pair of CBS vari-mu limiter / amplifiers. At the time both units were working fine with the original memory unit.

Just after posting the question I got kicked out of my workshop and developed a bit of a problem with regards to a suitable venue for tweakery. At this point I made all the attck/rel hardware, but did not have space to open the units right up and reconfigure the units for the adaptor (I can make small stuff in the studio, but don?t have facility to take apart the units as it requires a large bench to open up all the sprawling P-P wiring). So I took both units back to the tech / designer who designed the mod in light of the fact that only a small job was required.

Said tech charged a nominal fee, and stated that the units were ready to use except that I might want to experiment with resistor values for a time-constant selection to my taste.

I?ve just powered up both units, but when I engage the compressor everything below around 300hz drops out.

A source of comfort I can think of is that both units have exactly the same problem? What could it be? I tried substituting valves - 6386, 6AL5, OA2 and 12AU7, but no difference.

I?ve left a message with the tech, but he is usually booked-up for weeks so it would be much appreciated if anyone here had a theory to offer, I will be moving into a new workshop in the next few days.

edit: I should add that both units "seemed" to be compressing ok, with the needle jumping, and time-constant changing when adaptor was adjusted.

Thanks,

Justin

Schematic:

http://www.gyraf.dk/schematics/CBS_Audimax_440.gif

(Thanks again to Jakob for hosting!)

Hi Justin,

Be sure to drive these units from a true low-impedance source. Trying to drive those input transformers from a medium- or high-impedance source will cut lowend significantly.

Make sure that input is strapped (on TB1) to 600 Ohms, output is not important if it's 150 or 600 - but it NEEDS to be strapped to one or the other.

Just a guess..

Jakob E.

Thanks Jakob.

The only issue I could imagine that would cause the problem would be a transformer loading issue, but I was driving the i/p with the master out from a pa mixing console, one would like to think an ic-based console could drive it!

I had a long day yesterday, and was trying the comps quite late, so the obvious might have eluded me. I'm going to power them up again today, but with another source driving them, such as a 24v preamp.

Cheers,
J

Again, make sure the in/out transformers are strapped properly..

.. and as these units are transformerbalanced, you'll need to debalance correctly if running unbalanced. Connect minus-phase to gnd..

If problem persists, check for DC balance at the output transformer T2 - pins 7&10. You should have approx. the same DC voltage on both, to keep the trafo happy. If one half of V4 (ECC82) is bad, the unbalanced DC in the output transformer could reduce low-end.

..and further - use a scope to try tracing the signal to the point where low end disappears..

Jakob E.

Thanks again Jakob.

I checked the transformers were strapped correctly, for 600Ohm operation in and out.

My current theory is that the impedance of the pa mixer was not low enough. To be honest I had a long day, and wasn't thinking as clearly as I should have been. I will post back here if there is still a problem, but I reckon the problem was really down to my brain at 11pm last night...

J