Help requested to cure hum problem of preamp

Bought a $500 Twins Audio Serenade pre-amp from a DIY seller / member through this site. It has mixed, non-labelled 'GE' tubes (3 Nos 5654 + 1 No 6AK5) , loosely-mounted volume control and serious hum heard throughout all output levels except when the volume is turned fully clockwise.

My appeal to other DIY experts - please advise how the hum problem can be reduced or removed. The seller refused to look at it except to sell me more expensive products. Thanks to all those who are willing to assist.

cheapie  :cry:

I guess posting some photos maybe helpful

CAT

Hi Cat

Thanks for your response and suggestion.  I'll be away from HK until May 1.  Will find the time and post some photos upon my return then.  

I'd like to correct myself.  The noise is more like high-voltage electrical buzzing D-noise (similar to that described in another thread regarding a Mac preamp) than hum.  It becomes loudest mid-way through the volume control and dies down at full volume.

Thanks again.
cheapie

Hi Cheapie,

How u do the grounding? kindly follow accphoto's way of ground... besides, check your volume pot's shaft's grounding whether your sfat n the chasis is touching, normally it should touch...

Thanks windwss for the suggestion.  I'd look into the box when I return to HK on Apr 30.

Dear Cheapie,

One colleague got another Pre-amp from this person. He got the "grounding" problem.

You can check whether you got this problem as well.

1. Turn both Pre-amp and Power Amp ON. Volume can set to low level.
2. By using your finger, knock gently on the metal chasis / body.
3. Check to see if the "knock" will come out through the speakers.

TEST TWO

1. Turn on BOTH Pre and Power Amps
2. Don't connect any source yet.
3. Touch the volume. If the knob is plastic, remove the knob to show the metal part.
4. Touch the volume knob, you will probably hear "sss...sss" noise when you contact the metal knob.

CAT

Hi, how can he sell this pre-amp???? It is not even debug.......

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.

In this case  i will sugget to do something on the grounding to reduce the hum 1st,even using RC filter the hum sound also can be reduce by proper grounding,then change to choke for further improvement.

1) ensure the contact point of the volume port to the chasis is connected, may need to remove the paint on the chasis where the volume pot shaft is srew on the chasis. (if the volume pot & chasis not connected, while at minimum volume setting it will has a lot of noise)

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.

cheapie

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.

Thank you.

CAT

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.

cheapie[/quote]

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.

Thank you.

CAT[/quote]

Dear Accphoto,

Thanks for your quick reply.

1. The Ground has not connected to the Chasis. I checked it out for him.
    Even after I connect the ground at the plug to the chasis, the problem
    is still there. I tap on the chasis, the "dong" sound came out from the
    speakers.

2. I can work on the Input wire and the e-caps for him. Will ask him
    whether he would like to spend the money first. No need for really
    expensive caps, right?

3. What is "HT"? You said one side of the HT should be connected to ground
    Please advise.

CAT

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。

Dear Accphoto,

I see. HT = Heater. Thank you.

But I still got one point not clear.

"燈絲( i.e . 12AU7 4 和 5 腳) 加兩支 50ohms 1W 電阻，不用好的，中心落地"

That means ONE end of EACH resistor connect to pin 4 and pin 5 respectively and the other end twisted together and ground it?

Sorry for my lacking in knowledge.

CAT

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.

Millions of thanks to the expert DIYers who have offered advice, particularly accphoto.  I spent an hour doing the simplest to my el cheapo preamp - connecting a ground wire from the 'micro' volume control to the power in ground and then to chassis - by a telephone cable core stripped appropriately and twisted to wrap around the contacts, no soldering.  

Bingo! It worked.  Please see attached photos.

cheapie :wink:

A little bit danagerous for not soldering. ^_^

Yeah, I got this same feeling with Accphoto. I would rather have it soldered.

CAT

Thanks.  I intended to solder the connections ultimately.  But on May 1 morning I had to get packed for another trip and only returned late last night.  Then it's a full day's work in the office, pack and leave at 0800 for China again tomorrow.  I'll miss enjoying my equipment for the coming few months.

Hence, just a make-do and minimal work for the time being.  

cheapie