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Capacitance on pre-amps

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mutant-caterpillar View Drop Down
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    Posted: 01 Nov 2019 at 8:05pm
Hi everybody,
I wonder if someone can answer a query about capacitance loading on pre-amps. I'm getting thoroughly confused here!

Essentially, it boils down to this: I have a few turntables. My primary one sounds fantastic, and has a Graham Slee (obviously :) ) Reflex M connected to my Axis/Akito/Goldring 2500 combo. I modded the Reflex M to slightly lower impedance and it sounds awesome.

I'm now looking to upgrade my second turntable which is a Pink Triangle LPT, Rega RB250 and 2M Bronze. To do this I need a new amp, and this is where things get confusing.

With the 2M Bronze, it seems I need to reduce the capacitance to 100pF. I did this in my NAD PP-2 and PP-3, but I didn't do this in my Reflex M (does this have 100pF or 220pF?) for my Goldring 2500.
I noticed that Rega make cheapish pre-amps which have 100pF, but the amps get more expensive and have 220pF - and the Nait I was looking at (XS3) has 470pF! That will soundabsolutely awful with my 2M Bronze.

Am I fundamentally misunderstanding something here? It makes no sense that the more expensive the amp I look at, the more awful it will make my cartridge sound. The Bronze is supposed to work at 150-300pF, but of course by the time I've added capacitance of the cable on, it's out of spec and I get a 10KHz bump in the audio, followed by nasty roll-off.

My system upstairs where the Bronze is also has a 2M Mono deck set up, hence why I want an amp with an acceptable pre-amp, and I'm hoping to maybe connect some kind of mono-capable pre-amp from Graham Slee as well, but this is just confusing me too much.

Can anyone help with this conundrum? Why do amp and most pre-amp makers use a capacitance that's too large and make the cartridge sound wrong?

To make the comparison, I wired up a mixer and compared a digital output (MP3) in one channel with an LP known to be a digital (probably CD) transfer, in this case a Coldplay LP. I then compared the spectrograph from the LP with the MP3, and this is how I saw a large bump around 10KHz (I already knew the cartridge was sounding very harsh and trebly so it supported what I thought anyway).

Can anyone help with this apparently conundrum? Any help would be very gratefully received. 
Thanks!

-- 
Ian Gledhill
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2019 at 6:55pm
Some 75 years ago Blumlein invented the stereo MC cartridge whose magnet was so powerful it destroyed itself when used with a steel platter turntable (crunch!)

Steel platter turntable users would have to wait a little longer until Alnico magnets could be connected to the stylus and made to produce an electrical signal by the movement of the magnets inside fixed coils.

The fixed coils needed lots of turns for the signal to be large enough to drive an EQ'd valve microphone amplifier.

All coils are inductive and with the number of winding turns required, the resultant inductance approached 1 Henry.

Operating into a low noise valve stage having a 50,000 ohm grid resistor, the frequency response was lucky to reach as high as 10 kHz.

Standardisation of resistor values made 50,000 ohms into 47,000 ohms (47k).

Some time passed and then Shure came up with their V15 cartridge which set a standard others followed.

Its inductance was 0.5 Henries, which into 47k gave an upper -3dB roll-off of 15kHz.

Capacitive loading was found to get the response up, and that's where capacitive loading came in.

The tank circuit formed by the cartridge inductance, capacitive load, and load resistor, peaked at around 16kHz (just above 11 kHz rotated).

The value of load capacitance was decided at 250pF, which lowered the peak back to around 15kHz (11kHz rotated), but the peak being a bit higher gave a -3dB upper roll-off at nearly 19kHz (rotated).

The 11kHz peak was roughly 1.4dB.

By reducing the load resistance to 39k, the peak is damped to about 0.5dB, but the peak is at a lower frequency of 8kHz, and the -3dB roll-off 17kHz.

By lowering capacitance to 200pF the peak is still around 10kHz but less than 1dB and the roll-off goes to 20kHz.

All the above being based on 0.5 H (500 mH) coil inductance.

Usually the arm wires and connecting cable contribute an inescapable 100pF (+/- 25pF) on average.

A 100pF capacitance to a phono stage input will therefore give the usually satisfactory 200pF total.

A 220pf phono stage input capacitance also causes a 10kHz peak, but at +2dB and a -3dB roll-off at 17kHz.

470pF peaks by +4dB at 8kHz and is -3dB at 14kHz.

However, with only 100pF (the arm capacitance alone) there is no peak and -3dB is at 21kHz.

Having said that, the phono stage input capacitance helps guard against radio breakthrough.
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (1) Thanks(1)   Quote mutant-caterpillar Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2019 at 8:27pm
Thank you Graham! Capacitance and loading seems to be one of those black arts that nobody online ever agrees on, so it's nice to see a potted history like that that helps to explain things.

I've now set my pre-amp (not my Reflex M, but my NAD PP-2 which is being used upstairs until I can afford another Reflex) to 31pF capacitance, and 47K impedance. It seems to be good so far, but obviously I shall have to do a lot of "testing" (listening to stuff)!

You have confirmed what I thought, which is that the Naim is going to sound very poor, which is quite frankly bonkers for such an audiophile item, but there we go.

Thanks again for your help and detailed explanation!

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Ian Gledhill
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