TDA2003 grounding

[autir]

Just built a TDA2003 amp on stripboard, works like charm :-) I have read the sticky post suggesting grounding techniques at the Amplifier Discussion subsection of this forum, and I have placed the chip in the middle of the board, with power supply & speaker connections and caps and the zobel network on one side of the stripboard, and all other connections to the other. I cannot imagine a better way to do this without pcb :-S The chip's quiescent current at 12 Volts with 1.6Ω load (two 3.2Ω speakers in parallel) is 40mA, meaning I have no oscillation issues.

My problem is that I have this audible hum which is reduced only when I 1) touch the heatsink, or 2) touch the shield of the shielded input wire, or 3) connect the circuit to my guitar and touch the strings. With the guitar connected and none if its metal parts touched, the hum gets really annoying. If that helps you, the hum is exactly the one I get from my Marshall MS-2 battery-powered amp. To make matters worse, I have not installed an input pot yet, and with its 40db gain the TDA2003 is unforgiving :-)

My question is: What can I do to reduce the hum?

I have not installed it in a case yet, and supply it by my bench PSU. I was thinking of connecting the safety ground cable to the screw holding the chip to the heatsink, seems to me like a good spot. What do you think? In case of a simple 2-wire connection (no safety ground cable) what should I do?

P.s.: If it is of any relevance, I have changed the values of some parts: C4 (output coupling to load) from 1000 to 2200 μF, C2 (ripple rejection) from 470 to 2200 μF, Rx from 39 to 47 Ω, Cx from 39 to 47 nF.

[phatt]

Hi autir,
               It's called a *Case* or grounded Metal chassis.
Phil.

[autir]

Hello

When using a metal chassis the safety ground must connect to it. What about connection from the safety ground to the circuit?

What if there is no safety ground available? Just the (+) and (-) terminals from the rectifier of the transformer?

What if I use a plastic box, either with safety ground or just with the two terminals?

The last two questions are mostly because of another amp I am building. Low power one, inside a plastic box.

[J M Fahey]

In a single-supply amp as your TDA2003, the (-) terminal *IS* the ground and must be connected to the metal chassis .
If you build it on a plastic box, glue some kitchen-type aluminum foil to its bottom and if possible sides and roof, and connect that to PSU ground/negative.
Avoid shorting ypur board bottom side to it when mounting.
Use spacers, standoffs, foam or cardboard at least.
Also shield your guitat likewise.
Use a 3 pin power cord and ground the green wire.
Always touch some grounded part of your guitar.

[phatt]

Hello autir,
               Teemu's book may help to broarden your understanding of these issues.
If I remember he covers the problems you mention.
Well worth the read,, heck it's free mate.
(You can download it here on this site)
Phil.