MM cartridge body modifications for mono? - diyAudio
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Old 9th September 2007, 11:29 AM   #1
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Default MM cartridge body modifications for mono?

I've been wondering about the feasibility of modifiying a MM cartridge body permanently for mono after disassembling an old Stanton 500 body; I basically made a single coil to replace the two former ones (a lot less gain, but I expected that) however I think the audio I'm getting is still essentially similar to combining the channels since I suspect the 45/45 orientation of the magnet in the cantilever is causing all movement to be picked up.

I'm wondering; is there any substance I could try coating the upper half of the pole piece assembly that would essentially block pickup of lateral movement? Fingernail polish or something of that sort? Or is the fact that the cantliever responds to vertical movement the limiting factor as it's likely going to get picked up regardless because of proximity to the coil?

Understand I'm just experimenting here...I know there are mono cartridges that can be purchased (I have a DL-102) and such, just wondering if this is feasible or if the orientation of the cantilever in a stereo cartridge makes true mono pickup not possible.
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Old 9th September 2007, 07:33 PM   #2
oshifis is offline oshifis  Hungary
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Vertical movement (mono component) causes both L and R channels in-phase, lateral movement (stereo component) causes out-of-phase. Same principle as M-S microphone recording. If you add the L and R channels electronically, the stereo component cancels and you get pure mono.
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Old 11th September 2007, 07:39 PM   #3
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Hello vanmeter and oshifish, as far as I know mono signal causes lateral movements, and out-of -phase signal causes vertical movements (I remember I realized a simple rumble filter for a phono stage mixing in mono signals below 40 Hz: the vertical oscillation of a warped LP causes out-of-phase noise in the subsonic field that can be cancelled with this kind of mix).
Apart from this, just for fun, have you thinked about connecting the two channels in series? Probably you should change the input resistor of the preamplifier (or simply put in, if you're going to build your own...) with a double-valued one (it should be about 100 KOhm) and halve the value of the capacitor (generally around 150 - 270 pF to 68-150 pF) to reach the right load for the new coils configuration. Pay attention to use the earth connected pin (generally to the cartridge body) for the - signal.
(e.g. L- (earth, connected to the body) L+ R- R+ (out)).
I don't know how it works, but surely it should work. The other solution is to mix the signals, but it must be done actively as oshifish says, after the first stage of preamplification.
Good work!!!
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Old 12th September 2007, 12:37 AM   #4
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Right, but the goal here would be to ignore vertical conversion totally since on a mono disc there's nothing there but noise. Combining the channels will certainly reduce it, but it's there. I have tried series, using L+ and R- for the leads (since the R- is grounded) and wiring the other two pins together, but it only seems to reduce the noise by about (I'd guess) 6 or 8 db.

I mentioned at the Vinyl Asylum that this was bourne out of the realization that if I hook a stereo cartridge up for vertical movement only, it works (at least with my Stanton 500 body). In other words, both + pins are on one coil internally and both - pins are on the other, so if you connect your + and - leads to either both + pins or - pins you have a signal path internally that appears to give totally vertical movement. A noisy 78 will play virtually no music and just noise like this - not 6db lower of music, virtually no music.

I'm just trying to figure out a way to make that work for lateral. I guess I'm going to have to replace the coils with one lateral one, but I'm trying now to figure out the best way to go about that.
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Old 12th September 2007, 01:01 AM   #5
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6dB sounds about right to me for complete cancellation.

If you consider the noise to be produced by asperities on the surface which cause the stylus to deflect over them, only those asperities which caused purely vertical deflection would be completely cancelled. All other asperities would cause a combination of vertical and horizontal deflection, the horizontal component of which would be picked up.

If the noise is a random distribution of asperities the average direction of deflection would be about evenly split between the two components so you would expect about half to remain eg -6 dB.
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