Series wired cartridge for mono

[Clock_House]

Hi all, first post.

'ld' wrote something in July 2010 about series wiring a cartridge for mono reproduction.
I recently came across the piece and tried it out and can say that it's like listening to my mono records
for the first time. The difference is subtle compared to paralleling the channels - background noise has reduced significantly and the music is smoother.

However, I'm unsure about load impedance and would appreciate advice. I am using an SME 3009
with detachable headshell(s) and have simply taken l+ and r- to one set of pins (r+ and l- joined on cartridge body) and strapped to the other set, thus retaining the existing cabling. High end seems OK
but should I be compensating somehow? As the regular impedance is 47K per channel, I reckon I'd need to put 470k in parallel at the preamp inputs? Or is it worth it?

[Rothwellaudio]

By putting the coils in series you're efectively doubling the source impedance. Then you're feeding that source impedance to two channels instead of one, effectively halving the load impedance. So, if 47k was the optimum load for a single channel (wired conventionally), then 188k (4 x 47k) should be optimum for the series wiring.
However, there's the capacitance to consider as well. To get the same HF response as before, the capacitance should be lower. Reduce it by a factor of four. However, part of the capacitive load will be due to the cable, which will be very hard to reduce.
But if it sounds good, don't worry too much.

[Clock_House]

Thanks Rothwellaudio Due to the technicalities of my preamp 'lash up' (excellent by the way) it would be difficult to reduce the capacitance, but would it be feasible to put a resistor in the headshell and if so, what value?

[Rothwellaudio]

I fail to see how putting a resistor in the headshell would raise the pre-amp's input impedance.

[Coffee Phil]

Hi Rothwellaudio,

I am close to being in agreement with you except the quadrupling of the load impedance. In this example we are putting to very similar sources (each channel of the cartridge) in series so the impedance's should pretty much add algebraically. The load should be 94 k Ohm in shunt with 1/2 the recommended cap load for the cartridge.

The 4 times impedance ratio would apply to a 2:1 transformer.

Phil

PS: Whoops! I just re-read the OPs post. Apparently he is feeding both preamp inputs with the series cartridge channels. With that in mind, yes each input needs to be 188 k-Ohms yielding a 94 k-Ohm load on the series connected cartridge channels.

By putting the coils in series you're efectively doubling the source impedance. Then you're feeding that source impedance to two channels instead of one, effectively halving the load impedance. So, if 47k was the optimum load for a single channel (wired conventionally), then 188k (4 x 47k) should be optimum for the series wiring.
However, there's the capacitance to consider as well. To get the same HF response as before, the capacitance should be lower. Reduce it by a factor of four. However, part of the capacitive load will be due to the cable, which will be very hard to reduce.
But if it sounds good, don't worry too much.

[Ldg]

Yes, that's the gist of it. It's easy to forget that both preamp channels end up loading the mono cartridge. It's a good idea to minimise cable C, and if possible, preamp C. And changing preamp internal R up a bit seems a good move. Some experimentation is in order to optimise. But, if you like the way it sounds with loading as standard, that's good too, enjoy it !

I settled on 75k/10pf preamp loading in each channel with an Ortofon OM body, FWIW, but I could readily understand higher.

This setup is part of my permanent rig, for some time now, and I wouldn't be without it for mono playback. It just seems to voice correctly, and has low noise as the OP says. I have a fair mono collection, FWIW.