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Old 23rd January 2015, 12:49 PM   #1
Ggizzy is offline Ggizzy  United States
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Default Leak monoblock hum

Good morning,

Long time reader, first time poster.

I recently purchased a Leak TL/25 plus power amplifier and two preamplifiers to use with it- Leak RC/PA and RC/PA/U. All three were purchased through a reputable UK dealer and were recapped and serviced prior to shipping.

I am having a reoccuring and troublesome problem, however. With the power amplifier plugged in with nothing attached, no hum. However, when I plug in the octal umbilical, there is a hum. With the umbilical and preamp attached, still a hum. The hum becomes more pronounced when touching or grasping the umbilical or the preamp chassis. With V1 on the power amp removed, no hum.

I thought this may be a grounding issue with the cable. I checked all pins and reflowed. I then checked to ensure the shield was grounded appropriately to both sides and not coming into contact with anything else. I also checked pin-pin continuity to ensure nothing was grounding inside the chassis. No dice.

I even went through and changed some resistors in the power amp which were out of tolerance. I also added R1, which was absent in my unit, in hopes this may help to alleviate the issue. It did not.

The only other thing I can think of is the umbilical socket on the power amp. The wiring to this had been modified, but I restored it to the schematic and it still produces the same problem.

Any insight would be much appreciated. Here is the schematic I'm using.

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Old 23rd January 2015, 05:53 PM   #2
pointy is offline pointy  United Kingdom
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Try...........yahoo groups / h.j. leak & co. ltd.
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Old 23rd January 2015, 10:23 PM   #3
Ggizzy is offline Ggizzy  United States
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Thanks for the reply. Unfortunately, I don't believe that group is active anymore and I don't see any archived information either.
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Old 23rd January 2015, 10:50 PM   #4
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Sounds like an earth loop.
Disconnect the earth on pre amp to the zero volt line and see if that improves matters.
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Old 23rd January 2015, 11:04 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by Ggizzy View Post
However, when I plug in the octal umbilical, there is a hum. With the umbilical and preamp attached, still a hum.
The only other thing I can think of is the umbilical socket on the power amp.
The audio input signal wiring is combined with the heater wiring in the cable. With such a high amplifier input impedance,
there could be hum coupling. Install RCA sockets in the amp and preamp, and connect the inputs and outputs
to those, instead of the octal sockets. This way the audio signal will be carried between the preamp and amp by
shielded cables, not in a bundle with the heater wiring.

Last edited by rayma; 23rd January 2015 at 11:15 PM.
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Old 23rd January 2015, 11:08 PM   #6
Ggizzy is offline Ggizzy  United States
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The preamp doesn't have an earth per say as it isn't hooked directly to mains. There is a chassis ground, which is a mounting screw for the input screw-terminal block.

The octal interconnect connecting power and pre amps contains signal, ground, shield ground, heater, and HT wires. This powers the singular valve in the preamp. I can post a schematic for that if needed.
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Old 23rd January 2015, 11:24 PM   #7
Ggizzy is offline Ggizzy  United States
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I had thought something like that might be the issue, but I was hoping to knock everything else out before I began disassembling cables. A few more questions.

Because of it's close proximity in the cable bundle, even with a shield, the heater wires are probably inducing some sort of hum into the signal path?

Another troubling thing I noticed today is with the voltages. I measured voltages at all the points indicated on the schematic, minus the transformers themselves. Basically. I took voltage to ground, without preamp connected, measuring the voltage at the anodes and plates of the valves. All the voltages were low, by more than 20v. I should also mention I've tube swapped, including the rectifier.

I appreciate any further assistance anyone may be able to provide.
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