Cable length conundrums

[Dr. Toobz]

A new walnut console didn't end up having quite enough space for all of my components (including the turntable), leaving me to run some interconnects across the room to available shelving. Due to the room dimensions and odd layout, approximately 5m of cable are needed to go between the shelf and console cabinet.

Right now, this has meant keeping the Stereomour, a headphone amp (either the Crack or Asgard, depending on what headphones I am listening to that month), and a DAC (Bifrost Uber) inside of the cabinet, and the turntable and Seduction on the aforementioned shelving. I'm trying to determine what the least damaging solution (sonically) would be here. My options (some of which have been tried):

1) I've used a cheap ADC (48kHz) to digitize the output of the Seduction, whose benefit is the ability to run an optical cable all the way to the DAC, picking up no noise in the process (and thereby avoiding any analog signal issues). Problem is, this sounds like feces - shrill, thin, and defeating the purpose of vinyl. A better ADC might help, perhaps.....

2) Run 5m of cable from the Seduction to the Stereomour. The problem here would be cable capacitance. If I went with something like the 12pF/ft stuff from Blue Jeans Cable, 5m would mean the Seduction would only have to drive 180pF of capacitance. Is that tolerable, especially if I went to a larger output capacitor? The usual limit I see with the .47uF output capacitors is about 1m, but typical cables are also like 50-75pF/ft, not 12.

3) A third idea would be to slip an op-amp buffer under the Seduction for a low impedance output, thus avoiding the cable capacitance issue. Downside here is the insertion of silicon in an otherwise all-tube signal chain.

4) A fourth idea would be to move the Seduction to the cabinet (which has just enough room) and run a line for the cartridge. I think this would be the worst option, due to the very low voltage (4mV for my cart) and aforementioned capacitance issue.

5) A final idea would be to insert the Quickie in the chain, wired as a cathode follower. The 3S4 tubes are pretty low current, however, and I seem to remember that the output impedance of a cathode follower is simply the inverse of its transconductance. I can't find that number for the tube run as a triode, however. Any guesses as to what the output impedance would be as a cathode follower? I'm guessing that this would not yield much improvement in terms of driving cable over the Seduction with larger capacitors and really low capacitance cable.

Any thoughts?

[2wo]

I think you need a good line stage/preamp next to the Seduction, to drive the cable run to the rest of your gear...John

[Paul Joppa]

I don't see a frequency response problem; I get -3dB above 200kHz.

Conventional wisdom comes from experience with passive preamps which can have a high output impedance, and it is usually said that they sound poor with longer (2 meter) cables unless the output impedance is less than 2500 ohms. All out phono preamps have 4K nominal output impedance, so we recommend short and/or low capacitance cables.

The main calculable effect is frequency response, which has a corner frequency of 1/(2*pi*R*C). At 180pF that is 212kHz.

I have always presumed there are other more subtle effects, less obviously calculable, that affect the sound so I have not chosen to question the conventional wisdom.  Such knowledge is based on extremely widespread information, but is also subject to whim, assumptions, fashion, and rumor. Makes it as hard to discount as it is hard to rely on!

[Dr. Toobz]

I concur regarding the "subtle effects;" a test using 18' of really cheap interconnects from Best Buy or the like (whose total capacitance is probably closer to 1nF!) resulted in somewhat rolled-off bass and treble, and a shouty quality to the midrange as a result. Seems like my best bet is to spring for the low-capacitance cable mentioned earlier (which is claimed to be 12.2pF/ft). Or, pair the Quickie with said cable, using the standard resistor plate load, for a slightly better output impedance of 2000 ohms. That would buy some more gain, too - though now we've added yet another component into the signal chain, which I tend to avoid doing.

Besides the risk of picking up noise, could you explain what electrical properties the cartridge is "up against" when running to the inputs of a phono preamp? It seems like that's one place where nobody recommends extending interconnects, even with good cable.

For what it's worth, this is probably a temporary problem: the original plan was to house the amplification inside of this mid-century styled console cabinet and place the turntable on the top. (The other half of the console is drawers). However, I'm not planning to stay very long in my current (noisy!) apartment, so rather than mounting an LED TV on the wall above the console, it's just sitting on top of it for the time being.

[Paul Joppa]

Bass should not be affected by cable length. Not saying it isn't, just can't think of a technical/theoretical reason.

There are two reasons phono cartridges are sensitive to long cables - capacitance and noise pickup. Capacitance is critical for the usual MM cartridges, which have high inductance and resonate it against cable and phono preamp capacitance to extend the highs. Noise pickup is always a worry at very small signal levels; for coax the quality and coverage of the shield become of fist importance.

Now an MC cartridge has impedances and signal levels similar to a dynamic microphone, and balanced-line technology for long mic cables is well established and can be quite sophisticated. I'm frankly surprised at the rarity of balanced lines in this specific application...

Incidentally, headphone amps can make dandy drivers of long lines. Some of them even sport balanced outputs.