Mono with a stereo cartridge

Posted by: JeremyB on 13 March 2003

Just got my copy of "The Audiophile Voice" - usually a sad reminder that The Listener is no more.

However, this month there was an article about playing 78s by George Graves.

He made an interesting assertion that you can turn a stereo cartridge into a true mono one that only reads horizontal modulation by connecting the cartridge coils so that the phase of one channel is inverted and "combining them" (series, parallel? - doesn't say).

Anyone tried this? It reminds me of a review I read about the mono Helikon design which appears to be based on the same principle. Reported benefits are drastically improved S/N and dynamic range IIRC.

It would be great to do this without shelling out on a new cartridge! (as well as the problems of swapping them over).

Maybe this type of connection is a potential feature for the future stageline 2 (we can hope...) complete with operation enabled from the Flash remote control perhaps?
Posted on: 14 March 2003 by syd
Surely ypu would need a specialist cartridge anyway for playing 78rpm shellac discs?.

Yours in Music

Syd
Posted on: 14 March 2003 by Keith Mattox
I should see it soon - same reason. Frown

Any cart set up for 78 rpm should specifically be a mono cart - I have never heard of the existence of a single stereo 78 rpm recording.

This will differ though, for cartridges that support swapping normal and 78 styli, such as the cheaper Grados. I think in that case the cartridge body's pole structure is built for stereo, so the article applies, in a sense....

Ah, mebbe I'll just read the darned article. Smile

Cheers

Keith.
Posted on: 14 March 2003 by Paul Ranson
I think a stereo record groove can be considered in two ways. Each groove wall, when considered on an axis at 45 degrees to the record surface carries an independent channel. But if we consider the lateral and vertical motions then we have the sum of the two channels in the horizontal plane and the difference in the vertical. This happily ensures mono compatibility, although to play stereo records a mono cartridge needs to be 'compatible' to the extent of allowing vertical modulation.

I think that if you invert one coil and add it to the other you will get the difference signal, and if you just join the two coils you will get the sum, or the mono.

This shouldn't be too surprising given it is what the mono switch on an amp does.

The question is whether series or parallel. Your mono button does parallel, so intuitively, if one is going to be messing with cartridge wiring, series seems sensible. But in phase.

Paul
Posted on: 15 March 2003 by JeremyB
Sorry, I see I didn't explain myself properly and I thought everyone would have read the whole article by now.
The point was not playing 78s with an LP stylus! The author used a Shure cartridge that had both LP and 78 removable stylus options. He maintained that the body should be stereo etc etc.
This prompted the thought and my thus my comment about rewiring a stereo cartridge to play mono LPs with the same principle.

Paul, here's my understanding so far, I don't know if it is correct but I can't see a problem with the theory. If you connect the coils in phase as you suggest (ie press the mono button), you do not get full horizontal modulation. You get a reduced half modulation in each channel which gets summed with the other half modulation, presumable with a resulting distortion component (like crossover distortion). An unhappy byproduct is that you also get double the vertical modulation which unlike stereo has no music - only noise.

Now if you connect the two coils OUT of phase (I can't see any difference between serial and parallel, parallel will sum the two currents, serial will sum the voltages) you get a true full horizontal modulation since the +ve and -ve cycles will generate equal and opposite currents in the two coils. The clever bit I think is that the vertical component will now be of opposite phase in the two coils, hence they will cancel out! This is theoretically similar to NOT pressing the mono button (ie listening in stereo), except that the noise (vertical modulation) will not be cancelled out.

I wondered if anyone has tried it, it seems an intresting way of playing mono LPs.
Posted on: 15 March 2003 by Paul Ranson
If you consider a perfect mono signal from the pov of each stereo generator then they must generate the same output, which implies that internally they are wired 'out of phase'. When the stylus moves 'left' one channel sees the cantilever moving in, the other out, but both must generate a positive signal. When the cantilever moves up both see it moving 'in' and vice versa, but these must generate out of phase signals.

So I contend that wiring a cartridge for mono involves joining the coils together in phase, the vertical motions will then produce a +v in one channel and a compensating -v in the other, which will cancel.

Simply paralleling the coils is probably imperfect since the difference voltages will interfere with each other and won't cancel properly, you need separate loading resistors or something. So join them together in series. This will also double the output voltage, which may be a bad thing. So perhaps a series resistor equivalent to the load presented by the head amp?

This must be covered in some ancient book from the dawn of stereo. But not in any of mine, unfortunately.

Paul