Eliminated horrible hum of guitar just with a 10nF cap!

Thread Starter

Willen

Joined Nov 13, 2015
318
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?
 

vu2nan

Joined Sep 11, 2014
188
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.
 

LowQCab

Joined Nov 6, 2012
1,183
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.
.
.
.
 

Ian0

Joined Aug 7, 2020
3,463
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
 

Hymie

Joined Mar 30, 2018
998
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.
 

MisterBill2

Joined Jan 23, 2018
9,467
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.
 

Thread Starter

Willen

Joined Nov 13, 2015
318
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.
 

MisterBill2

Joined Jan 23, 2018
9,467
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.
 

MisterBill2

Joined Jan 23, 2018
9,467
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..
 

vu2nan

Joined Sep 11, 2014
188
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.
 

MisterBill2

Joined Jan 23, 2018
9,467
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.
 
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