Long Preamp to Power Amp Cable Lengths

[Loren42]

Originally Posted by palmas

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

Yes. Hum from a ground loop. The cable ground is also disconnected at the power amp.

I am looking for a long term robust solution.

[palmas]

Originally Posted by Loren42

Yes. Hum from a ground loop. The cable ground is also disconnected at the power amp.

I am looking for a long term robust solution.

I had some similar problem from the phono amp to preamp (4m in single-ended).
Had a pair of Lundahl LL1592 and some balanced cables laying around, and as my pre has balanced input...

hum and noise disapeared and sound is great. So great I'm thinking of getting some more of these trafos for balanced to SE input on the amps
Hope this helps

[Antoinel]

If the output stage of the preamp is a current source [high Zo] instead of a voltage source [Zo ~ 0], will it help resolve this problem. Is it Krell which uses this non-standard approach?

[Chris Hornbeck]

Originally Posted by Loren42

Yes. Hum from a ground loop. The cable ground is also disconnected at the power amp.

I am looking for a long term robust solution.

You have two options for the ground loop, to break the "galvanic path" of the safety grounds plus signal grounds, or to shorten the ground loop.

The first option requires transformers, which are great if properly applied, but don't like high impedances and especially along with high external capacitances.

The second option is to shorten the ground loop by bringing AC line power along with signal from the source end. A simple DIY and "code" solution exists:

Amazon.com: Datacomm 50-3323-WH-KIT Flat Panel TV Cable Organizer Kit with Power Solution - White: Electronics

All good fortune,
Chris

[Loren42]

Thanks. That is an interesting product.

[Speedskater]

I would reconnect all the safety grounds.
Connect all the components to one AC power strip (with a long extension cord to the amp).
Connect the RCA chassis jacks directly to their chassis's.
Use the above " LC-1 Blue Jeans cable "

[Loren42]

Called Jensen transformers and they recommended simply using one CI-2RR Iso-Max transformer at the power amp.

[Eli Duttman]

Loren42,

I've done some thinking about your long cable issue. Blue Jeans, IIRC, uses a simple coaxial construction, which is not suited to the task at hand. Shielded "twinax" in a differential, if not true balanced, configuration seems correct. That length of shield, when carrying signal, can all too easily pick up noise. The shield should not carry signal and only be connected to the upstream end chassis, not signal, ground.

You previously mentioned wall plates. 2X Neutrik 203Ms will do the job for you. Online Components carries the Neutrik plates.

No ifs, buts, or maybes, you will have to buffer the PAS preamp. I'm giving the buffer some thought. More about that to to follow soon. However, this much is clear. You will need trafos to break ground loops up. Jensen can be pricey. CineMag offers competetive products, at lower cost. CineMag's CMOB-2L looks like a candidate for your situation. A DC blocking cap. is needed between the buffering voltage follower and the trafo. Contact CineMag for assistance in sizing that part.

[jcx]

heavy shield coax works fine in many instances
Mogami with good Belden coax can be excellent


really cheap RCA cables may only have a dozen or so spiral wrapped strands with <50% coverage, pigtail termination - they suck

[Loren42]

Originally Posted by Eli Duttman

Loren42,

I've done some thinking about your long cable issue. Blue Jeans, IIRC, uses a simple coaxial construction, which is not suited to the task at hand. Shielded "twinax" in a differential, if not true balanced, configuration seems correct. That length of shield, when carrying signal, can all too easily pick up noise. The shield should not carry signal and only be connected to the upstream end chassis, not signal, ground.

You previously mentioned wall plates. 2X Neutrik 203Ms will do the job for you. Online Components carries the Neutrik plates.

No ifs, buts, or maybes, you will have to buffer the PAS preamp. I'm giving the buffer some thought. More about that to to follow soon. However, this much is clear. You will need trafos to break ground loops up. Jensen can be pricey. CineMag offers competetive products, at lower cost. CineMag's CMOB-2L looks like a candidate for your situation. A DC blocking cap. is needed between the buffering voltage follower and the trafo. Contact CineMag for assistance in sizing that part.

Eli,

Thank you for the information. Would a Jensen CI-2RR work as the isolation transformer if I can get it at a competitive price?

Long term I would like to roll my own preamp, but in the interim, does it make sense to upgrade the PAS 3 with one of those Tubes4HiFi PC boards?

I would like to retain the tone controls, so I was considering the TCLA here:

Dynaco PAS3 upgrades & modifications & parts

Unfortunately, I can't find a schematic nor a specification for the output impedance (uses a 12AX7), so I have no idea if it would be a solution or a waste of money. Your thoughts?

If I understand your wiring scheme, you would use a high quality shielded twin lead cable. Connect the two inner leads to the preamp out hot and ground. Do not connect the shield at the preamp end.

The other end goes to the isolation transformer where the shield is connected to the isolation transformer box, the hot to hot, and the other lead to the return side of the transformer.

The cable from the transformer box would connect that shield to the amp chassis, while the output of the transformer connects to the input of the power amp.