I see cheapie's pre-amp in for sale forum. While not 100% sure , I think the hum come from the power supply. He is using RC 濾波, a method that is quite easy to hum. To remedy this , you may need to buy a choke from PO shan, 10H 150ma and replace the first resistor. Second thing you can do is to replace the heater supply from AC to DC. This will reduce the hum significantly. Basically, i think the builder know all these problems so the hum did exist in all machine.
You should check whether he had gound the earth to the chasis, but that is too obvious.
Secondly, check whether the input wire are shield. Just remember to shield the input side of the earth wire and do not sheild both side.
If you cannot manage to fix it, then I think the pre-amp needs some major rework.
[quote:54585682bd="cheapie"]Many thanks to all experienced DIYer for your suggestions. I'm still out of town at the moment but will try the methods from the simplest first when I'm back to HK during the May 1 holidays. I'm not a DIYer and am very grateful for your help.
I suppose it's a case of buyers beware. But this guy has several listings, all preamps ranging from $500 to $2000, in the for sale market under the same name. He trades at MTR stations, no testings allowed / arranged. Wonder how many got burned because of him.
Really hard, but I think there is a few area of which you can consider
1. whether the ground had connected to the Chasis.
2. Input wire need to be shielded on the input side.
3. Personally, i do not like those 8u 350V oil caps, if it is used on the first order rectification, it is quite easy have hum. change to some 47U350-450 E-caps will improve the problem.
4. Check whether one side of HT had connected to earth. To reduce hum, it is better to use 2x 50ohms resistor and connect earth.
Hope this will help.
[quote:fab9c6b690="alfcat"]Dear Accphoto,
These are the photos of my colleague's Pre-Amp. Please take a look and see if you can spot the source of the problem.
1. Try to connect 水線 , electronic gound ( negative) to the chasis too. Make sure that all chasis are connected as I see there is some wood in between. ALso try to sand the connector point to make sure that the connection is good.
2. Yes, any e-cap with right voltage will do.
3. Heater means 燈絲，燈絲( i.e . 12AU7 4 和 5 腳) 加兩支 50ohms 1W 電阻，不用好的，中心落地，這樣的 hum 聲最少。如不用電阻，請將燈絲一腳接地。
Yes, one end connect to HT and the other end to earth. Do not connect any HT to earth in this case, the value of the resistor is does not matter , it can be between 50-100 ohms but need to be one watt at least.
