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Loren42 16th January 2013 01:02 AM

Long Preamp to Power Amp Cable Lengths
Unfortunately, my tube preamp and tube power amp are separated by about 20 feet of cable. I can not shorten that distance.

My first thought was to use a balanced line between the two components as it would solve the cable length issue and avoid any ground loops.

I have a Jensen PC-2XR transformer (actually, I have two!) that could be placed near the power amp, but what can I use at the preamp's output?

The PC-2XR is a pro to consumer level transformer, so that may not be a good choice unless I add a gain stage to the preamp?

Is there a better way to do this or what should I do?

Eli Duttman 16th January 2013 02:28 AM

Cables of that length MUST be shielded. Long shielded cables have significant capacitance. It is required that the circuitry driving the cables have both a low O/P impedance and substantial current capability.

You don't need a balanced setup. Shielded "twinax" wire, with the shield grounded only at the "upstream" end is quite suitable. However, the driving electronics must have elephant sized cojones. What does the circuitry driving your preamp's O/Ps look like? You probably are going to need a buffer and FETs do the buffering job quite well. If the thought of "sand" in the signal path makes you break out in hives, build the buffer around 1 of the "super" twin triodes: 5687, ECC99, or 6H30П (6n30p).

DUG 16th January 2013 02:33 AM

Three issues for long signal runs (as I see it):

Ground loop.

Signal loss due to line capacitance.

Potential for EMI...pickup of unwanted signals

Ground loop can be reduced significantly with a transformer or with balanced feed.

Signal loss due to cable capacitance can be compensated with inverse roll off filtering at either end or by reducing the feed (source) driving impedance.

EMI can be reduced with shielded lines, twisted balanced feed, ferrite clamp-on sleeves or low pass filtering at the load (amp) end.

There are many other ways but it depends on the issues that actually crop up in your installation.

Chris Hornbeck 16th January 2013 03:26 AM

Another approach to the ground loop issue is to run the amplifier(s)' power cord(s) back in close proximity to the signal cords. Draw it out before dismissing the idea. Courtesy of member jneutron.

All good fortune,

Loren42 16th January 2013 11:23 AM

The driving amp is a PAS 2 tube amp.

I know that is pretty junky, but at some point I will roll my own preamp and that should allow me to address the drive issue in a more eloquent way.

The power amp is my own design and has a 10K Ohm input impedance.

Using a total run of 20 feet of LC-1 Blue Jeans cable (with a capacitance of 12.5 pf) at 10K impedance at my power amp yields an F3 of 64 kHz, which sould be fine.

The caveats are that the run needs to go through a wall, so I would need to add connectors and a wall plate. Not sure what that does to my calculations.

The beauty of the balanced line is no ground loops, which is an issue that I need to contend with.

My choices there are to either buy a set of transformers or build a balanced out drive circuit.

Using tubes to make a balanced out circuit seems excessive and just adds too many components in the chain.

I can use something like a DRV134 diff amp, but the mystique of tubes is kind of broken, but more importantly, it requires an additional power supply for those chips.

I have two pairs of Jensen PC-2XRs that I could use one at the power amp side, but they require a +14 dB higher drive than a simple 1:1 transformer.

I am thinking I should just sell those and look into buying two pairs of something else. Not sure what.

Eli, I assume that the 5687, ECC99, or 6N30P would be a cathode follower circuit to get the low impedance drive?

Chris, The preamps and all of the associated inputs run off of a circuit that is unique to the cabinet they reside in. The power amp must use a neighboring wall socket, so it would not be easy or possible to simply plug the amp in at the same outlet strip as the preamp.

Lastly, if the room layout is changed, the amp may need to be moved another 15 feet away, so the balanced line becomes even more attractive.

Loren42 16th January 2013 11:32 AM

I also see that Jensen has a PI-2RX Consumer to Pro 1:1 isolation transformer that might serve as the beast for the preamp side.

Would that work in conjunction with my Jensen PC-2XR or will I still have a 14 dB drive loss?

Shoog 16th January 2013 12:09 PM

I have used a 5687 as a Long Tail Pair driving an output transformer, for differential drive, with step down for about 2x gain overall. This involves a voltage steep down of about 9:1 which means that the ouput impedance is very low from an already low impedance tube and the current capability must be in the 100's of mA.

This was not a difficult build and only comprises 1x5687 which is transformer loaded (Push Pull) to the output. I happen to use an input phase splitting choke - but this is not essential. All in all its about the best thing I have built and is so simple its laughable.

A valve differential preamp can be very simple and relatively efficient to boot.


Tom Bavis 16th January 2013 12:34 PM

A PAS-2 won't drive 10K happily, much less 10K in parallel with 400-1000 pF of cable. The PAS-2 is designed to drive 100K or higher. It won't drive a transformer without adding a buffer stage.

Loren42 16th January 2013 01:05 PM


Originally Posted by Tom Bavis (
A PAS-2 won't drive 10K happily, much less 10K in parallel with 400-1000 pF of cable. The PAS-2 is designed to drive 100K or higher. It won't drive a transformer without adding a buffer stage.

Yes, I think I need to either add a buffer or bite the bullet and simply build my proposed preamp circuit.

palmas 16th January 2013 02:26 PM

I could not find if you tryed already the normal long SE cable solution and found a problem?

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