True Mono from a Stereo cartridge - switchable !

[Ldg]

Prompted by thinking about the coil arrangement in 's carts on another thread, I came up with a general method for obtaining true mono from a stereo cartridge. There seems a significant (4-6dB) noise advantage versus stereo replay of a mono recording. Also described is a simple way to make it switchable between true mono and true stereo. Measurements and listening tests show a significant noise reduction, c 4-6dB, versus mono replay on a stereo cartridge.

I don't know if it's already known, but it's new to me. It has excellent results for noise improvement and sound, so I thought I'd post it anyway.

The idea is to connect L & R coils in series, and the series output is then true mono. Mono groove modulation is purely lateral movement, of course, and this method significantly attenuates noise output from vertical stylus movement. Each coil produces antiphase signals for pure vertical motion, in phase for pure lateral motion.

Although similar effects can be produced electronically by combining L+R signals later in the signal path, the method described here avoids the need for a seperate summing stage, and avoids phase and amplitude errors often arising through other methods.


This sketch shows an implementation using a DPDT switch to toggle between true stereo and true mono. I conveniently built this into the base of the arm housing under the TT I used for testing, so can switch between stereo and mono easily.

Here's a screenshot of a mono audio sample (tone), showing the effect of flipping the switch :

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Signal amplitude increases by 6dB when the mono switch is set. Note noise peaks remain same amplitude.

Here's a screenshot of a silent groove, showing the effect on noise floor of flipping the switch:

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Groove noise remains approx similar amplitude. But signal amplitude increased by 6dB (above), hence S/N improves by c 6dB when mono switch set.

Here's a spectrum plot of the silent groove samples, normalised for 6dB offset in signal amplitude:

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The benefit is clear across the spectrum. 4-6dB

Here's a plot of a 150Hz mono sinusoid tone, to check harmonic distortion.

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Harmonic distortion is more or less unaltered. There is something strange going on in the 20-50Hz range though. It remains when the stylus is lifted off, so seems hum or preamp related - this is not a very good preamp BTW.

Here's a plot that shows Vertical Modulation rejection at 1kHz is c 16dB, and across the spectrum is up to 20dB. That is how much attenuation vertical stylus noise gets.

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I tested this on an Ortofon OM5E in a project 9C arm. The S/N ratio improvement for mono records is obvious, toggling the switch.

Sound is as good as the cartridge. Voicing changes significantly due to cartridge loading on toggling the switch. I found an interim load of 68k per channel voices well enough for both stereo and mono, but I expect much more can be done to obtain optimal loading and voicing.

In the fullness of time I expect to leave a TT set up like this, because the S/N advantage when replaying mono recordings seems significant.

Neat, eh ? Appols if this is old hat, but it's not to me !

[pivot]

Cool! Thanks for figuring that out.

Betcha that wiring is how some companies do SOME of their mono cartridges. (The first company that sprang to mind I will not mention other then to note the name starts with "G")

[Ldg]

You're welcome, pivot. Yes, the big 'G' coil arrangement appears to readily lend itself well to this, or by swapping a few N-S poles and wiring differently internally, perhaps.

Should add the obvious though, that the method doesn't require internal mods, and should work with any cart in principle.

[flavio81]

EXCELLENT Thread!!!!!!!!

Note: This will also cancel vertical rumble components as well.

[Ldg]

Thanks flavio81 !. Yes, it should also take care of the vertical component of rumble.

Pure mono might be a bit of a specialist interest sport. But for those of us with a reasonable collection of mono pressings, especially classical music, this seems really useful.

I've been listening to some of my favourites, and really enjoying the sound.

[Ldg]

Thanks, Dave. Yes, by reversing one of the coils one would get pure vertical output, ideal for vertical cut 78s perhaps. Or indeed for setting azimuth...........

[1200y3]

Azimuth tests are usually recorded in antiphase on the record.

If reversing doesn't work on some cartridges, some preamps have a Null switch, it is done in the preamp electrically for isolation. When in the Null position tracking noise we otherwise can't hear will come clear, isolating the distortions so one can hear them in real time on a mono record, and balance tests can be done. It can also be heard through the centre channel (speaker hooked to each channel +.)

For "hill & dale" records, the the polarity of one channel is reversed before mono summing, or hooking in parallel.

The diagram shown is very similar to a canting control, where the positives are blended through two potentiometers, but it is done after the preamp, to line level signals.

[beatcomber]

Keep in mind that I'm a DIY newbie, but I find this very interesting.

A few months ago I built a mono/stereo project box that just sums the channels. (See illustration.) Could I modify it to actually cancel horizontal movement?

[Ldg]

Hi beatcomber. Yes, summing the channels sort of works. But each channel effectively loads the other, and generally with a load that is way below what is intended, except for the pure mono signal. So mono response and noise cancellation can depend on a whole set of undefines, also stereo summed to mono seems inevitably wrong. The series approach is better in that respect.

Unfortunately, it can't be done without rewiring at the TT armbase, though.

@1200y3. Yes, if it's done electrically it inevitably seems to require another stage, and even then some of the hazards associated with beatcombers' summing approach apply unless done carefully.

Series approach seems an elegant method, and sounds pretty good to me. Stereo summed to mono works really well too.

[beatcomber]

Thanks for the response!

[Ldg]

You're welcome beatcomber. I'm convinced the series approach has advantages though.

Listening to some familiar stereo pressings in mono tonight. Seems pure mono playback has lower distortion, IMO. Perhaps the vertical channel is the one to focus on for general stereo playback improvement, methinks?

[josephazannieri]

Yo series wirers:

First, to answer beatcomber's question, you can modify your box to cancel lateral info, but to do it you will have to lift the ground side of one channel off the chassis and then reverse the hot lead and the ground lead on the channel with the lifted ground side. This will get you the phase reversal that will bring out the vertical motion, but you can't do it if both the left and the right ground are connected to the metal chassis of the box. It's a pain, but it is possible if you are wiling to cobble the box. You can shift the polarity with a DPDT switch. You will have to sum thew channels out of phase. The series wiring method can also be used to do this, but you connect the negative side of one channel to the negative side of the other channel, instead of the way that it is wired in the diagram shown. (- side to + side of other channel)

Second, to ask some questions of my own, I have seen that series method of wiring the cartridge, but it raises some questions when you are hooking it to amp. What's the ideal load? Is it twice the ideal load for the individual channels? (I. E. 94,000 ohms?) Also, what effect does cable capacitance have? Seems like with that high an impedance, a little capacitance would have a big effect. Also, how do you get a cart that is wired in this fashon into a stereo amp? Do you put reverse Y adapter on it? Or are you stuck with a mono amp only? And what happens when you stick 2 parallelled 47,000 ohm inputs, or 23,500 ohms, across the 94,000 ohm output of that series wired cart, like you would have to do to plug it into a stereo amp?

Would be interested in the details of this hookup, as requested, with profound gratitude for the help from those with greater knowledge than I.

And good luck from that inquisitive, and appreciative old guy,

Joe Z.

[bauzace50]

Great!
Thanks!

b50

[Ldg]

Yo series wirers:

Second, to ask some questions of my own, I have seen that series method of wiring the cartridge, but it raises some questions when you are hooking it to amp. What's the ideal load? Is it twice the ideal load for the individual channels? (I. E. 94,000 ohms?) Also, what effect does cable capacitance have? Seems like with that high an impedance, a little capacitance would have a big effect. Also, how do you get a cart that is wired in this fashon into a stereo amp? Do you put reverse Y adapter on it? Or are you stuck with a mono amp only? And what happens when you stick 2 parallelled 47,000 ohm inputs, or 23,500 ohms, across the 94,000 ohm output of that series wired cart, like you would have to do to plug it into a stereo amp?

Would be interested in the details of this hookup, as requested, with profound gratitude for the help from those with greater knowledge than I.

And good luck from that inquisitive, and appreciative old guy,

Joe Z.

Yes, that's the gist of it, JoeZ. Raising phono preamp loading R and keeping C very low are pretty much mandatory with a normal MM/MI cartridge. As you say, remembering that whatever is fitted to the phono preamp by way of R & C on each channel will be in parallel under this scheme in mono.

For R loading, I use a total of 75K, that's 150k on each channel total. That's with an OM cartridge body. For C loading I use a bare minimum (22pf total, 10pf each channel) inside the preamp. For phono cable, I use a very short run, about 0.2m of low C cable with the preamp very near the bottom of the armboard. If it is wired as pure mono, and not swithable, one can save half the cable C by wiring at the cartridge end and only using one drop cable, then a splitter at the phono preamp end.

Anyways, that's what I use to good effect IME. It's a permanent setup for me. I'd recommend making it switchable and flexible, so the difference is audible. It's impressive, and worth the effort IMO.

There are various convolutions of ways to reduce C that might have application too, but in this case I kept it simple and just did what was possible with common sense. I put the switch under the plinth at the base of the armboard near the pivot so it's accessible, BTW.