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Hum problem of preamp
Hum problem of preamp
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Old 3rd May 2007, 04:40 AM   #1
B&W_arthur is offline B&W_arthur  Hong Kong
diyAudio Member
Join Date: Jun 2006
Talking Hum problem of preamp

First of all, excuse me if you find this problem is asked repeatedly in this forum.

In fact, I have searched and read related thread in this and in other forums. Yet, can't understand nor figure out where the problem lies.

Here are some common possibilities:

1. ground loop
2. inferior transformer quality
3. excessive currect draws from the transformer

Here is my situation:

I am building a line preamp, a modified version of Convergent Audio Technology CAT SL-1 preamp, using a PCB produced in China. I tested the power supply and did not find any problem. However, when I connect the High Voltage secondary (300VAC) from the transformer to the power supply. There is noisy hum. The noise is not coming from the speaker because I did not connect the preamp to any source or ampifier. The transformer seems to be vibrating.
Also, the transformer seems quite hot after switching on for a while.

I connect the earth pin of the IEC plug to the chasis. Also, the earth wire of the transformer is also connected to the chasis. These are the 2 only connected ground. All other ground of the amplifiying circuit are floating, i.e. not connected to the chasis.

The transformer provide (a) 6.3VAC for the 6DJ8 filament, (b) 18VAC to PSU and converted to 12.6VDC for 12AU7, 12AX7 x 2 filaments. (c) And 300VAC to PSU and converted to 260VDC for the High Voltage.
The transformer specification 100W: 6.3(2A), 18V(1A), 300V(100mA).

My questions are:

How can I identified the cause of the hum or vibration?
How to fix it?
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Old 3rd May 2007, 08:59 AM   #2
Karsten Sømand is offline Karsten Sømand  Denmark
diyAudio Member
Join Date: Mar 2006
Location: Outside Copenhagen
Hum problem of preamp
Default Hum from trafo

Hi B&W_arthur!

Hm, if you say it is your trafo humming it might be because of (DC)noise on your AC line. Have you tried to install an AC line filter between your AC line and your amp?

You say your trafo is very hot but not "how hot". If you bought the whole kit at the same supplier then things might be dimensioned OK. Just in case, give the supplier a call - he might be able to help you and he has probably heard about your problem before.

Good luck.

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