Hi all, The hum while not touching the guitar metal body or string, was so high and irritating! The cheap amp has two wire (no earth) mains line and has linear Xformer. I watched dozens of videos about noise gate, ground loop problem, shielding guitar's internal chambers etc. All of them looked complicated or time consuming. Today what I did is: replaced two pin supply cable into 3 pin (with earth) and connected guitar input 'ground' to earth through 10nF 1kv capacitor. Amazing! I eliminated the hum! Is this a good way to do? Do it need to connect the earth directly to 0V DC too, for further improvement? or throught the another 10nF cap?

 

Was the guitar connected to the amplifier with a shielded audio cable? Did the amplifier produce hum when the guitar's cable was disconnected?

 

Hi Willen,

No hum, while touching the guitar metal body or its strings, confirms the correctness of its wiring.

The apt cum safe solution would be to use a 3-core mains cord, with a 3-pin plug, for the amplifier. The earth wire from the 3rd pin is to be connected to the chassis / common of the amplifier.

There would be no need for the capacitors.

Nandu.

 

Be very careful,
many of these Amps have the chassis connected to the Mains-Voltage,
or worse, the Plate-Voltage,
which can easily kill You if You don't watch what you're doing.
.
.
.

 

Be very careful,
many of these Amps have the chassis connected to the Mains-Voltage,
or worse, the Plate-Voltage,
which can easily kill You if You don't watch what you're doing.
.
.
.

If so, the barrel of the input jack would be live

 

Old Musical-Instrument-Tube-Amplifiers ARE DANGEROUS.
Telling someone to ignore that FACT is irresponsible.
.
.
.

 

Les Harvey (of Stone the Crows), younger brother of Alex Harvey (he of The Sensational Alex Harvey Band) would probably agree if he was alive today.

 

Be very careful,
many of these Amps have the chassis connected to the Mains-Voltage,
or worse, the Plate-Voltage,
which can easily kill You if You don't watch what you're doing.
.
.
.

There was not one word about the amplifier type except cheap. Tube, transistor, IC, or??? And so it is probably not a tube type of amp. The very first question would certainly be about the input cable and connections. BUT it may be that the TS has no clue.

 

Was the guitar connected to the amplifier with a shielded audio cable? Did the amplifier produce hum when the guitar's cable was disconnected?

Yes the guitar is connected to the amp with a shield wire audio cable. I tested the wiring but the shield wire of the cable is not connected to 0V (ground) of the 12V DC supply. Amplifier do not produce hum while not connected cable or guitar. The Amp has a TO-220 amp chip driven by an audio OpAmp.

Hi Willen,
No hum, while touching the guitar metal body or its strings, confirms the correctness of its wiring.
The apt cum safe solution would be to use a 3-core mains cord, with a 3-pin plug, for the amplifier. The earth wire from the 3rd pin is to be connected to the chassis / common of the amplifier.
There would be no need for the capacitors.
Nandu.

In case if the another device in the same house leaking huge AC current to the earth cable and the earth rod of the house is poor and not pulling down all the AC leakage to the earth, then the earth wire has AC voltage. To avoid that direct leaked AC accident, my mind ordered me to use a 10nF 1KV ceramic cap. What about it?

Be very careful,
many of these Amps have the chassis connected to the Mains-Voltage,
or worse, the Plate-Voltage,
which can easily kill You if You don't watch what you're doing.

It looks pretty safe because the mains directly goes to the 12V linear transformer. Chassis is connected to 0V filtered DC.

 

It may be that for some reason the shield side of the input jack is not tied to the amplifier common. In that case the capacitor to common is the correct choice. But probably more investigation is in order.

 

Of course the shield of the input cable must be connected to the amplifier 0V. Try it.

 

If the input amplifier IC is not set up to handle a normal input signal that is referenced to the power common, which most circuits do, there may be a problem. I have seen other versions that do not use power common as the input reference. CERTAINLY they are very poor designs, but consider that there are also some very poor designers around..

 

In case if the another device in the same house leaking huge AC current to the earth cable and the earth rod of the house is poor and not pulling down all the AC leakage to the earth, then the earth wire has AC voltage. To avoid that direct leaked AC accident, my mind ordered me to use a 10nF 1KV ceramic cap. What about it?

Hi Willen,

Connecting a capacitor from 'common' to chassis is a throwback to the days of AC/DC sets e.g. the 'All American Five' radio. Those sets did not have an isolation transformer and there existed the possibility of the chassis being live, in the event of inversion of the 2-pin plug. Hence the capacitor from 'common' to chassis, instead of a direct connection, for the chassis to provide a convenient RF ground.

With present isolated power supplies, it is standard practice to directly ground the common/chassis. It goes without saying that an effective earth connection must be ensured.

Nandu.

 

A whole lotof consumer electronics uses an external wall ot tablettop supply and so it is not grounded. And we are not discussing tube type radios or such. So please give us a break from the grounding religion. The discussion is about a cheap guitar amplifier that is powered from a 12 volt AC output transformer.