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Old 22nd April 2002, 07:36 PM   #1
Ralph is offline Ralph  Netherlands
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Question Just finished my first tubes-preamp

Hi all,

I'm new to this forum and to the DIY-amp world and I just built my first pre-amp with the PSU/PRE-II kit from World Audio Design. When I hooked it all up last week and plugged in ... It hums awfull, sometimes.

The PRE is very sensitive about placement close to my Linn poweramp. But mostly it hums when I switch on the cdplayer or the TV (very much). It also hums when the PRE is turned off, so it must pick up some sort of interference from other equipment. In fact, I had to use all of the length of my interconnects to get rid of the loud hum. Now it's acceptable, except when the TV is turned on.

The PRE
http://www.worldaudiodesign.co.uk/pdf/precdia.pdf

The PSU
http://www.worldaudiodesign.co.uk/pdf/psu.pdf

I also think the gain (5x) is a little high.

Anyway I really enjoyed building the kit and experimenting with the tube pre-amp. I'm looking for a new preamp project (not a kit) that:

- sound very good :-)
- absolutly doesn't hum! (I mean it)
- works with my ss Linn poweramp (output imp 10k, input 650 mV)
- I don't need a phone stage

What are the options you recomment?

Thanks in advance,

Ralph
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Old 22nd April 2002, 08:14 PM   #2
stigla is offline stigla  Norway
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Some tips;

-650mV input sensitivity is pretty high. A normal CD player puts out a good 2V. This explains the "high" gain of the kit preamp. A pre-amp is not really needed...

Nice topology of that preamp! transformer coupling, I like it. Only thing with Tranformers is that they have a tendency of picking up hum. I don't know if the ones in your amp is potted or screened in any way, but that is the obvious "soft-spot".
Humm can be sourced in the strangest of places, go through all your solderings and connections, try putting a screen over the tube. Is the chassis grounded? Try to change the direction of your signal cables... is there more hum in either of the channels than the other? etcetc...
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Old 23rd April 2002, 08:58 AM   #3
Ralph is offline Ralph  Netherlands
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The output transformers are the obvious weak part, they pick up the hum indeed. Is there a possibility to used shielded transformers, are they available? How do I know what 'value' to look for?

And looking at the schema a second time I've noticed that in my design there is a LR network parallel to the output sockets. R1.5k and C8.2uF. Does anyone have a clue what it is meant for?

Ralph
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Old 23rd April 2002, 09:49 AM   #4
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Default Bas Horneman

If the preamp is humming when it is off. The humm is clearly not the preamps problem.

Most propably the problem is a ground loop.
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Old 23rd April 2002, 11:13 AM   #5
Ralph is offline Ralph  Netherlands
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This is the current situation:

1) CDP, PRE and poweamp on: almost no hum if I place the PRE 1m away from the CDP and poweramp. More hum if the distance is less.

2) Situation 1 + TV on: loud(er) hum, audible from the listening chair. Hum varies with distance PRE to TV (normally 50 cm)

3) Situation 2 with PRE switched off: hum is real loud.

What can couse this strange behavour?
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Old 25th April 2002, 12:15 PM   #6
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Default Step Down Trannie

Hey Ralph

I've been looking for a low output impedance, ultra simple pre-amp design. Do you happen to know the value of the step down trannie there (ie turns ratio and anode load?)

Thanks
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Old 25th April 2002, 02:53 PM   #7
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Quote:
I've noticed that in my design there is a LR network parallel to the output sockets. R1.5k and C8.2uF. Does anyone have a clue what it is meant for?
It's probably there to control a high frequency resonance in the transformer.
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Old 26th April 2002, 09:26 AM   #8
Ralph is offline Ralph  Netherlands
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Hi Gumboots,

I'm not so technical, but from my manual I read: Primary winding HT Voltage 100 Vdc and Anode 99vDc; secondary winding to output sockets. That's all I know.

Ralph
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