How connect stereo cartridge to mono riaa ? - diyAudio
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Old 30th November 2013, 07:57 PM   #1
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Default How connect stereo cartridge to mono riaa ?

I want to do listening tests on a mono amp with built-in riaa stage. The amp's input is just the grid of a tube plus 47kOhm.
The cartridge of the turntable however is stereo.
If I just connected both channels in parallel to the riaa, one channel of the cartridge would load the other; doesn't sound right ...
connecting in series is also problematic because again the signal from one channel would flow thru the other and because of the external RCA plugs.
So ... how could this be done correctly ?
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Old 30th November 2013, 08:13 PM   #2
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Put a 1k resistor in series with each output.
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Old 3rd December 2013, 01:48 PM   #3
DF96 is offline DF96  England
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Quote:
If I just connected both channels in parallel to the riaa, one channel of the cartridge would load the other; doesn't sound right ...
It is not as bad as you fear. The signal in both channels will be largely similar, so it is only stereo information which will be affected and you are throwing that away anyway!

The real issue is that a mono 47k input impedance shared between two MM channels will look to the cartridge like 94k. If you are worried about sound quality then add a 47k resistor in parallel.

Last edited by DF96; 3rd December 2013 at 01:48 PM. Reason: correct resistor value
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Old 3rd December 2013, 06:11 PM   #4
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The signal in both channels might be largely >dis-similar< and I don't want to throw them away ...

94kOhm ? really ? ...
the cartridge's left channel sees the 47k of the mono input plus the impedance of the cartridge's right channel in parallel - which is unknown;
the same holds true for the right channel's cartridge.
I can't believe that connecting the real ohms of the right cartridge and it's inductance and it's capacitance in parallel with the 47k of the amp >increases< the load impedance as seen by the left cartridge - it should be lower ... shouldn't it ?
Depending on the magnitude of the source impedance of a cartridge, part (or the better part maybe) of the signal produced by one cartridge will be fed to the other one instead of to the amp and what is worse, the combined load impedance will be frequency dependent.
And just as I write this: wouldn't the signal produced by the left actually >drive< the right cartridge into mechanical motion ? even with poor efficency this might cause any sorts of trouble.
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Old 3rd December 2013, 06:20 PM   #5
DF96 is offline DF96  England
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You have two near-enough identical signal sources in parallel, so each sees half the conductance of the load.

Your thinking would be correct if the two paralleled sources were independent, but they are highly correlated.
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Old 3rd December 2013, 06:42 PM   #6
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The only information that will be thrown away is the out-of-phase (L-R) information.
You always lose this information when listening to a stereo recording in mono.
I'd follow DF96's advice. Just combine the two cartridge channels and (if necessary) add another 47k loading resistor in parallel with the existing resistor.
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Old 3rd December 2013, 07:08 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

A stereo cartridge is most likely to work best wired in parallel
into a standard 47K whatever pF input. Series will work too,
with higher output, but loading is more complicate, 100K, and
source inductance is twice normal, needing low capacitance.

For parallel its much easier to add capacitance if needed.
Parallel with ideally 22K or 25K loading is the easiest.

rgds, sreten.
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Old 4th December 2013, 08:28 PM   #8
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there is a pic of how to do it in this manual.

dont know if it will be the same with your cart.
Attached Files
File Type: pdf ve_stanton_881s.pdf (426.2 KB, 34 views)
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Old 4th December 2013, 11:25 PM   #9
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Default What I did

This is what I did in same app as yours. I didn't have the values shown and used 680R instead of 470 and 22K instead of 20K.
Instead of using a stereo and mono 1/4" jack as shown in the diagram, I used two female RCA connectors and one male connector to make the wye. It worked well .
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File Type: png Summing Stereo to Mono.png (18.2 KB, 71 views)
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Old 5th December 2013, 06:17 PM   #10
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Thank you for your answers, but for some reason it does not seem to work for me.
First I thought it's the mono amp under test which is not sounding good, but after some experimenting I suspect it's the paralleling of the cartridge.
So I compared a commercial ss pre-amp with 2-ch input but set to mono out ... good.
Same cartridge paralleled, same pre-amp ... sounds worse.

Finally I built a simple cathode follower, dual triode ECCxx, 47k grid leak each, connected stereo to the cartridge, common 1k cathode resistor to do the summing and 10u cap to link to the amp under test. (Finally an application for those computer dual triodes with common cathode )

I think this is a sound solution and it also sounds much better.
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