Flip polarity of inductor EQ?

[hageir]

I haven't opened this up yet, it's a small, rare Sony MX-20 mixer (it was the flagship of its time; transformer balanced ins and outs).

The EQ curves are all subtractive, I got the idea of adding a switch to reverse the poles? Making them boost instead of cutting..

Am I being ignorant here or is it a simple mod?
(I'm still reading upon LCR circuits, mostly passive stuff) and am learning more..
I usually repair synthesizers and some audio gear..

-Geir

 

[JohnRoberts]

Not simple...

JR

 

[ruffrecords]

If the EQ switch is anything to go by the left two positions are HPFs and the first right is a peaking high boost and the second is a combined high peak and HPF. and after a first look, the schematic confirms this.

Cheers

Ian

 

[gyraf]

the eq stuff is in the cathode feedback path, not subtractive and not all that easy to change imo

 

[hageir]

Not simple...

JR

Hey JR, you helped me with a Ashly SC-66 EQ many moons ago
I respect you and I will take that seriously, I have way too many projects anyways.
thanks for the quick answer!

If the EQ switch is anything to go by the left two positions are HPFs and the first right is a peaking high boost and the second is a combined high peak and HPF. and after a first look, the schematic confirms this.

Cheers

Ian

Oh snap! So it's actually in reverse?
the coil is responsible for the accentuated highs?

 

[hageir]

the eq stuff is in the cathode feedback path, not subtractive and not all that easy to change imo

Good stuff.
Thank you.
I'll leave it as is, it's fine (unorthodox pinouts on the XLRs; before the standardization, like my Interface Electronics console).

I think this Sony will be ideal for mixing down to a four track, might use it like that someday

have a good evening guys.
Love this forum.

 

[ruffrecords]

Hey JR, you helped me with a Ashly SC-66 EQ many moons ago
I respect you and I will take that seriously, I have way too many projects anyways.
thanks for the quick answer!



Oh snap! So it's actually in reverse?
the coil is responsible for the accentuated highs?

Correct. It reduces NFB and hence increase gain.

Cheers

Ian

 

[NOON]

I own one of these mixers as well, need to get around to doing some restoration work on it. Unusual preamp, transformer then FET. I made a 'translation loom' to reverse the XLRs without rewiring the unit.

 

[abbey road d enfer]

I haven't opened this up yet, it's a small, rare Sony MX-20 mixer (it was the flagship of its time; transformer balanced ins and outs).

The EQ curves are all subtractive, I got the idea of adding a switch to reverse the poles? Making them boost instead of cutting..

It's not how it works. Inductors have no polarity, there's no plus or minus.

Am I being ignorant here or is it a simple mod?

Impossible mod!

 

[gyraf]

..note that they actually use electrolytics as frequency-determining for high-pass filters. Not only does this introduce large'ish amounts of unnecessary distortion around cut-off point (because of the significant voltage drop across it) - it also moves this point unpredictably over time as the 'lytics dries out.. Unless, off course, it's tantalums..

/Jakob E.

 

[tinn]

I think there is a connection missing in the schematic
(green dot)

 

[ruffrecords]

I agree.

Cheers

IAn

 

[FIX]

Sometimes a green dot is all it takes to keep the boat afloat...

For those who are new at designing, one rule, never attach four wires to the same spot on a schematic. The reason is that you then have to do a bump over line if you cross wires in the schematic. You can see that the last op-amp input has four connections at the input, but has a crossover by the output. The very green dot is exactly why. If you forget a dot, a more complicated schematic might go un-noticed.

 

[JohnRoberts]

For people new at reading old hand drawn schematics don't ASSume that the schematics are 100% accurate. Human generated schematics often contain errors or omissions. Updates and fixes don't always get reflected in old schematics. This is part of the challenge in troubleshooting old designs.

Modern computer (CAD) generated schematics are generally more accurate.

JR