High impedance balanced output: how long the cable?

[Alfetta87]

Tube preamplifiers have often a typically 600 ohms output impedance. How long (max.) should I keep the cables (in balanced mode) to the active speakers, which are distanced 3.5 metres (11.5ft) from each other?

Thanks,
-dan

[piano3]

With an output impedance as low as that you should be able to drive quite long cables assuming that the output stage has a high enough standing current to drive a capacitive load. I suggest you read SY's "a heretical line stage" and/or from page 572 onwards in "Valve amplifiers" by Morgan Jones if you have not already done so.

[janneman]

Originally Posted by Alfetta87

Tube preamplifiers have often a typically 600 ohms output impedance. How long (max.) should I keep the cables (in balanced mode) to the active speakers, which are distanced 3.5 metres (11.5ft) from each other?

Thanks,
-dan

An easy, rule of thumb is to calculate the frequency where Zout = cable (and amp input) capacitance, because that's the -3dB freq of the low pass filter formed by Zout and cable cap. Alternatively, if you have a 'target' -3dB, say 30kHz, you can calculate the max cable cap you can afford. Then when knowing the cable cap per foot you know the max length you can afford.

Example: you want the -3dB at 30kHz. Zout = 600 ohms.

The impedance of the capacitance Zcap = 1/(2*pi*f*C); Zout = 600 ohms so:

600=1/(2*pi*35*10^3*C) so you rework that and you get C = 1/(600*2*pi*35*10^3) and that comes out to about 750pF. With a reasonable cable of 150pF/feet you can afford about 5 feet.

jd

[jcx]

I hope you meant pF/m there jann - I'm sure you could find cable C that high but I would put the typical range at ~ 1/3 that # for twisted pair and < 15 pF/ft "low capacitance" cabling is available

also single pole roll off is slow, to keep 1 dB 20KHz flatness you would want >40 KHz fc, 0.1 dB requires over 120 KHz

[Alfetta87]

Thanks a lot guys, didn't expect that much help in such short time. A knowledge of mine (not a friend, my friends don't have that money) is thinking to buy an Audio Research REF5 preamp, which is 600 ohms. The active speakers are ATC SCM150 SL AT (active), but they have 10k ohms input impedance. AR says that it should have min. 20k input impedance and 2000pF capacity. The cable is one I made myself, Furutech SA22 in 3 metres length per side. The guy is thinking to switch to Kimber twisted, because of its low capacitance. I'm worried this might not work...

[janneman]

Originally Posted by Alfetta87

Thanks a lot guys, didn't expect that much help in such short time. A knowledge of mine (not a friend, my friends don't have that money) is thinking to buy an Audio Research REF5 preamp, which is 600 ohms. The active speakers are ATC SCM150 SL AT (active), but they have 10k ohms input impedance. AR says that it should have min. 20k input impedance and 2000pF capacity. The cable is one I made myself, Furutech SA22 in 3 metres length per side. The guy is thinking to switch to Kimber twisted, because of its low capacitance. I'm worried this might not work...

2000pF capacitive load sounds a bit high for this preamp.

jd

[janneman]

Originally Posted by jcx

I hope you meant pF/m there jann - I'm sure you could find cable C that high but I would put the typical range at ~ 1/3 that # for twisted pair and < 15 pF/ft "low capacitance" cabling is available

also single pole roll off is slow, to keep 1 dB 20KHz flatness you would want >40 KHz fc, 0.1 dB requires over 120 KHz

Agreed. I was just using some numbers to illustrate the process.
I also think that with a balanced 600 ohms Zout the value to use is probably 1200 ohms, no? Meaning even less capacitance should be allowed.

jd

[Alfetta87]

It say that the maximum capacity shouldn't exceed 2000pF, but preferable less:
Audio Research REFERENCE 5 Vacuum Tube Line-Stage Preamplifier

[Alfetta87]

Jann, where Germany, Belgium and The Netherlands meet, sounds like Vaals, no? I'm from Eupen, and spend some youth in a disco called I think Spuugh or something like that.
Was a lot of fun...

[janneman]

Originally Posted by Alfetta87

Jann, where Germany, Belgium and The Netherlands meet, sounds like Vaals, no? I'm from Eupen, and spend some youth in a disco called I think Spuugh or something like that.
Was a lot of fun...

Heerlen. 15 miles from Vaals.
The only disco I know from way back is the Queens Club in Vroenhoven, your country. Only one that stayed open till 04:00 AM.

jd