Blue Jeans LC-1 is quoted at about 12pF / foot. Very good shielding. A little heavy and not flexible though. I think it is basically a high grade 75 ohm coax.
Grado carts sound nice into 12AX7, but I found I had a steady hum, which was cured by using a blocking capacitor. As a bonus that filtered out low frequency noise/warp on records. Maybe grid leak current affects the Grado? My Ortofon did not suffer from hum.
FWIW my turntable interconnect (hardwired to the turntable) measures 26 pF using a cheap multimeter connected to the rca without the cart installed...
I don't have the math to calculate "miller" on the input of the phono, but the input tube is 6N1P with another 6N1P as the active load. 47k input, 180R grid stopper.
I don't have the math to calculate "miller" on the input of the phono, but the input tube is 6N1P with another 6N1P as the active load. 47k input, 180R grid stopper.
Yeah, not likely that low, unless its made out of some mammoth 75-ohm solid-core coax cable or something.
I measured my super low C tonearm cable (75pF by itself) connected to the tonearm and it measures 122pF. So that means the tonearm cabling (before the DIN connector) has about 50pF capacitance.
I have an old Monster Cable digital interconnect (75-ohm coax) with stranded center conductor that measures 130pF for 6 feet (almost 2 meters), so about 22pF/foot.
As jgf mentioned earlier, Blue Jeans Cable makes a quite reasonably priced DIN-5 to 2x RCA-M tonearm cable using their LC-1 cable, which they say is a 75-ohm coax cable with flexible jacket, with 12.2pF/foot capacitance. That's about 40pF/meter, which is nice and low.
But... How much capacitance is added by the DIN-5 and RCA connectors? I don't know.
Maybe it's impossible to make a 1m tonearm-RCA cable that has less than about 75pF capacitance.
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I measured my super low C tonearm cable (75pF by itself) connected to the tonearm and it measures 122pF. So that means the tonearm cabling (before the DIN connector) has about 50pF capacitance.
I have an old Monster Cable digital interconnect (75-ohm coax) with stranded center conductor that measures 130pF for 6 feet (almost 2 meters), so about 22pF/foot.
As jgf mentioned earlier, Blue Jeans Cable makes a quite reasonably priced DIN-5 to 2x RCA-M tonearm cable using their LC-1 cable, which they say is a 75-ohm coax cable with flexible jacket, with 12.2pF/foot capacitance. That's about 40pF/meter, which is nice and low.
But... How much capacitance is added by the DIN-5 and RCA connectors? I don't know.
Maybe it's impossible to make a 1m tonearm-RCA cable that has less than about 75pF capacitance.
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What is Miller Capacitance?
The formula for determining the total input capacitance of a triode stage is as follows:
Cin = Cgk + Cgp*(A+1)
where: Cin = input capacitance
Cgk = grid-to-cathode capacitance, composed of the internal tube capacitance plus the stray capacitance
Cgp = grid-to-plate capacitance, composed of the internal tube capacitance plus the stray capacitance
A = stage gain
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For a 12AX7, this comes out to at least 150pF and often as high as 200pF, for the triode alone.
I assume the worst, so 12AX7 Cin = 200pF for me.
50pF for the tonearm wiring,
100pF for the tonearm to preamp cable,
200pF 12AX7 Cin
20pF for preamp input wiring
total Cin for a 12AX7 phono preamp is 370pF at minimum
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What I meant is the response of the LRC circuit formed by the cartridge impedance and the input impedance of the amplifier, from the EMF induced in the coil to the voltage across the amplifier's input terminals. With mechanical response I meant the response from record cutter velocity to magnet or coil velocity.
When the combination of those responses is more or less flat, the record is cut properly with an accurate reverse RIAA correction and the phono amplifier has an accurate RIAA correction, then the whole record-playback process has a more or less flat response.
Today's experiment...
I added a measured 465pF for C4, in parallel with the measured 3340pF cap in the C5 position (as calculated by the TCJ RIAA app). That brings the total shunt capacitance to 3805pF, which is a lot more than the calculated value. Simulation said adding that 465pF should reduce output from 10kHz to 20kHz by -1dB.
Sure enough, the sound is not quite so bright. It actually sounds more 'correct' to me now. I had brought the cartridge load down to 32k ohms before this change, but now I like the way it sounds with 47k.
I generally like a 'warm' (dark?) tonal balance, so this is working for me. The preamp also sounds like it plays with less background noise now. Ticks and pops are handled more gracefully.
I've only tried it with the Shure M35X/Jico elliptical so far.
More to come...
I added a measured 465pF for C4, in parallel with the measured 3340pF cap in the C5 position (as calculated by the TCJ RIAA app). That brings the total shunt capacitance to 3805pF, which is a lot more than the calculated value. Simulation said adding that 465pF should reduce output from 10kHz to 20kHz by -1dB.
Sure enough, the sound is not quite so bright. It actually sounds more 'correct' to me now. I had brought the cartridge load down to 32k ohms before this change, but now I like the way it sounds with 47k.
I generally like a 'warm' (dark?) tonal balance, so this is working for me. The preamp also sounds like it plays with less background noise now. Ticks and pops are handled more gracefully.
I've only tried it with the Shure M35X/Jico elliptical so far.
More to come...
You could use two RG179 cables tied together between turntable and anplifier, but not in the actual tonearm, as it is too stiff for that.
I found the Belden website where they have good listings of their various types of 75 ohm coax cables, including the pF/ft (or pF/m) and the materials used.
I figure the following are the features you'd want in a DIN-5 to 2x RCA-Male cable:
- Copper conductors (silver-plated copper is available, so perhaps 'best' choice?)
- Lowest possible capacitance (it seems 16.5pF/foot is about average)
- Braided shield (not foil)
- Stranded inner conductor (more flexible)
- Flexible jacket
- Small diameter jacket so two can fit into DIN-5 connector
- Air or gas-injected foam insulator for center conductor (flexible, lowest capacitance)
Anything else to look for?
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I figure the following are the features you'd want in a DIN-5 to 2x RCA-Male cable:
- Copper conductors (silver-plated copper is available, so perhaps 'best' choice?)
- Lowest possible capacitance (it seems 16.5pF/foot is about average)
- Braided shield (not foil)
- Stranded inner conductor (more flexible)
- Flexible jacket
- Small diameter jacket so two can fit into DIN-5 connector
- Air or gas-injected foam insulator for center conductor (flexible, lowest capacitance)
Anything else to look for?
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The one I referred to in post #210, of which I couldn't remember what it was called, is Belden 179DT 
It's a 75 ohm cable with a foam dielectric and a capacitance of nominally 57.0894 pF/m. It is more flexible than normal RG179, but I still think it's too stiff for tonearms. It should do fine between turntable and amplifier. It has a foil and a braid, by the way.
There are only two things a manufacturer can do to reduce the capacitance per unit length of a coaxial cable: use a dielectric with a small dielectric constant and use a large ratio of the inner diameter of the shield to the outer diameter of the centre conductor. Both increase the RF characteristic impedance (which is the square root of the ratio of the inductance to the capacitance). Polyethylene foam is about as good as it gets regarding dielectric constant, so the foam-insulated cable with the highest characteristic impedance will have the lowest capacitance.
It's a 75 ohm cable with a foam dielectric and a capacitance of nominally 57.0894 pF/m. It is more flexible than normal RG179, but I still think it's too stiff for tonearms. It should do fine between turntable and amplifier. It has a foil and a braid, by the way.There are only two things a manufacturer can do to reduce the capacitance per unit length of a coaxial cable: use a dielectric with a small dielectric constant and use a large ratio of the inner diameter of the shield to the outer diameter of the centre conductor. Both increase the RF characteristic impedance (which is the square root of the ratio of the inductance to the capacitance). Polyethylene foam is about as good as it gets regarding dielectric constant, so the foam-insulated cable with the highest characteristic impedance will have the lowest capacitance.
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I think Blue Jeans Cable has this figured out pretty well. According to their site, they had Belden make them a flexible 75 ohm cable with a 25 ga. copper center conductor, polyethylene foam (with air bubbles) dielectric, copper braid, flexible jacket. Its capacitance is 12.2pF per foot, so about 40pF/meter. They'll make you a 3 foot long 5-pin DIN-female to 2x RCA-male cable with a ground wire for $44.35 USD + tax and shipping. Since the 5-pin DIN socket alone costs at least $25, that looks like an awfully good deal, and convenient too.
By "too stiff for tonearms" did you mean for the cable connecting the DIN socket to the preamp? Or is that what you meant by "between turntable and amplifier"? I'm confused...
In other news, I just had a chance to try the Audio Technica AT-VM95E in this new less-bright version of the phono preamp. While the AT still is a bright sounding cartridge (AT carts in general have that reputation), it sounds pretty nice now if I load it with 15k ohms (47k//22k). It has really good clarity. Different instruments in the mix stay more 'seperated' than they do when the Shure M35X/Jico elliptical is playing the same record. Also, dynamics are fun played by the AT-VM95. Snare drum thwacks have nice weight to them. I really liked this cart playing through the Hagerman Bugle, so I was surprised when it sounded so harsh through the first iteration of the 12AX7 preamp.
At some point I'll take this over to a friend's house who has an inverse RIAA box already made up, and we'll check the frequency response. It will be interesting to see how far from the target it is in real life. A good test of SPICE vs. Reality.
Now I need to hook up the Denon DL110 and give that a listen with this less-bright version of the preamp.
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By "too stiff for tonearms" did you mean for the cable connecting the DIN socket to the preamp? Or is that what you meant by "between turntable and amplifier"? I'm confused...
In other news, I just had a chance to try the Audio Technica AT-VM95E in this new less-bright version of the phono preamp. While the AT still is a bright sounding cartridge (AT carts in general have that reputation), it sounds pretty nice now if I load it with 15k ohms (47k//22k). It has really good clarity. Different instruments in the mix stay more 'seperated' than they do when the Shure M35X/Jico elliptical is playing the same record. Also, dynamics are fun played by the AT-VM95. Snare drum thwacks have nice weight to them. I really liked this cart playing through the Hagerman Bugle, so I was surprised when it sounded so harsh through the first iteration of the 12AX7 preamp.
At some point I'll take this over to a friend's house who has an inverse RIAA box already made up, and we'll check the frequency response. It will be interesting to see how far from the target it is in real life. A good test of SPICE vs. Reality.
Now I need to hook up the Denon DL110 and give that a listen with this less-bright version of the preamp.
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Thanks MarceldvG and Salas.
I'll look into fabricating my own super-low-C DIN-RCA cable. Belden 179DT looks good, but its capacitance is a little high at a quoted 17.4pF/foot. However, the cable is nice and skinny. Belden makes so many cable types, I'll have to do some serious research...
Salas, thanks for the tip. Yes, he has a signal generator and 'scope.
What should I look for when viewing the square waves? I'd imagine that a 10kHz should look flat on top with no ringing or overshoot on the rising (left) edge, and it should be flat across the screen, not tilted. Should I expect a little bit of a tilt to it? A little bit of a rounded leading edge due to -1dB falling response above 10kHz?
What should I look for with the 1kHz square wave? The same? Should I expect a flawless square wave at 1kHz?
Thanks.
I'll look into fabricating my own super-low-C DIN-RCA cable. Belden 179DT looks good, but its capacitance is a little high at a quoted 17.4pF/foot. However, the cable is nice and skinny. Belden makes so many cable types, I'll have to do some serious research...
Salas, thanks for the tip. Yes, he has a signal generator and 'scope.
What should I look for when viewing the square waves? I'd imagine that a 10kHz should look flat on top with no ringing or overshoot on the rising (left) edge, and it should be flat across the screen, not tilted. Should I expect a little bit of a tilt to it? A little bit of a rounded leading edge due to -1dB falling response above 10kHz?
What should I look for with the 1kHz square wave? The same? Should I expect a flawless square wave at 1kHz?
Thanks.
1kHz should have no undershoot or overshoot else the audio band response is flawed. 10kHz shape quality depends on extended bandwidth and conformity. The nicer it looks the better. Anti-Riaa circuit should be high frequency compatible. If it includes "lost constant" and the phono network doesn't, the result will be skewed anyway.
Great Salas, thank you.
I'm pretty sure my friend's reverse RIAA is the Hagerman one.
iRIAA2 - Inverse RIAA Filter – Hagerman Audio Labs
"...the iRIAA Filter includes a correctly placed upper 3.18us (50kHz) corner in its transfer function." I expect that is the "lost constant" you mentioned, yes?
I'm pretty sure my friend's reverse RIAA is the Hagerman one.
iRIAA2 - Inverse RIAA Filter – Hagerman Audio Labs
"...the iRIAA Filter includes a correctly placed upper 3.18us (50kHz) corner in its transfer function." I expect that is the "lost constant" you mentioned, yes?