Amp earthing

pre65 · Post by **pre65** » Tue Apr 01, 2008 1:58 pm

Hi-not wishing to hi-jack the pre amp earthing thread !

My first amp (the 832A) was exhibited at Eggborough a couple of years ago,just after its birth,and although i love the sound,i need to re-assess the way it is earthed,so i am asking for advice.

The "bus bar" runs between the input phono neg and the speaker neg terminal and all the earths were soldered to the nearest part without any regard to loops,hum etc.

For some time i have been thinking of "star" earthing,and was wondering how to do it in practice .

Do i take each individual earth from one channel to a "star",then join both "stars" together and connect them to chassis earth via a resistor ?

Or could i join all the first valve earths together (12AT7) and then go to "star" ?

I would like to get it quieter for the fest.

Post by **Nick** » Tue Apr 01, 2008 2:14 pm

Can you post the circuit, and we can see what suggestions come to mind.

You need to consider the power supply in the scheme as well, the phono discussion intentionally ignored that other than the caps across each valve stage.

pre65 · Post by **pre65** » Tue Apr 01, 2008 2:18 pm

Hi-i attach a link to the circuit.

http://www.audiohobbyist.com/images/832/832sch.jpg

Cressy Snr · Post by **Cressy Snr** » Tue Apr 01, 2008 2:31 pm

Right Phillip

I was writing this as Nick was posting so it might not be what you want but I'll leave it anyway

Here's how I'd do it. I'd use a bus bar earth system using a 2.5mm bare copper wire. I'll try to give you a direction to run the bar.

It needs to start at the mains earth connection on the back of the power input socket. After that take it past the power supply capacitors. Then carry it on to run past the two 832s. Take it round to run past the two input driver valves, then direct it to towards the volume control and finally the input sockets.

After you have secured it in place, take a tee of solid copper wire off a point on the busbar just after the power supply and heater earths and before the output valves. This tee should be terminated at the speaker socket negative terminals. along this tee off is where you terminate the common terminal from the secondary of each output transformer.

Back to the main bus bar route, working away from the PSU,terminate the 832 earth end of your CCS then the grid resistor, repeat for the other 832.
Now go round to the input/drivers and terminate the cathode bypass cap, followed by the cathode resistor then the grid resistor. Repeat again for the other valve. Next terminate the volume control earth and finally the input socket earths. Job done.

If you are not satisfied with the hum levels then a 100R resistor can be placed between the beginning of the busbar and the mains earth pin. I've never needed one but YMMV.

Steve

Post by **Nick** » Tue Apr 01, 2008 3:13 pm

What Steve says would work. Another way using the same sort of star that was talked about in the other thread.

I would connect the -ve terminal to the power supply cap -ve. Make th -ve post of the cap the star point.

Start at the front of the amp, connect the ground from the vol pot to the first catjode resistor, and also connect the -ve of the 33uf cap to this point. Then run a wire back from this point to the star point. Using as short a wire as possible,

Now repeat with the second valve cathode and the 47uf cap, this point then back to the start point.

Now the output stage, connect the cathode resistot to the two 390k resistors and the 47u cap to a point, and take this point back to the star.

Then take the ground termnal of the TX secondary back to the star.

Repeat with other chanell.

Then take the star to the chassis/saftey ground, possibly via 1 100R resistor if you need to, but not between the earth on the IEC and the chassis, make sure thats a low resistance link.

pre65 · Post by **pre65** » Tue Apr 01, 2008 3:16 pm

Hi Steve-at the moment the bus bars (i have one for each channel) are thinner than you recomend.

Is there any advantage to keeping with that arrangement,with the right thickness,and both terminated at the IEC earth one end and the input phonos the other ?

I have a pseudo centre tap for the 6.3V heater wiring,where should that earth go ?

Thanks for your time.

pre65 · Post by **pre65** » Tue Apr 01, 2008 3:19 pm

Hi Nick-thanks for your take on this.

I must admit i am more inclined to go your way,but i will mull it over for a bit.

Post by **Nick** » Tue Apr 01, 2008 3:19 pm

I have a pseudo centre tap for the 6.3V heater wiring,where should that earth go ?

Either the star, or the noisy end of the bus

pre65 · Post by **pre65** » Tue Apr 01, 2008 3:22 pm

Nick wrote:
I have a pseudo centre tap for the 6.3V heater wiring,where should that earth go ?
Either the star, or the noisy end of the bus

Muchos grassyass amigos !

pre65 · Post by **pre65** » Tue Apr 01, 2008 3:45 pm

Hi-if i go the "star" earth route should the earth wires to the "star" be of any particular size ?

Is bigger better ?

Post by **Nick** » Tue Apr 01, 2008 3:56 pm

0.6mm should do fine for most things, depends on the current you expect to be passing at that point.

Cressy Snr · Post by **Cressy Snr** » Tue Apr 01, 2008 4:26 pm

pre65 wrote:Hi Steve-at the moment the bus bars (i have one for each channel) are thinner than you recomend.

Is there any advantage to keeping with that arrangement,with the right thickness,and both terminated at the IEC earth one end and the input phonos the other ?

Thanks for your time.

You might get away with that arrangement provided you have everything terminated in the right order. The thickness of the wire is quite important as the bus needs to be of very low resistance. The currents being dumped to earth, if they meet any resistance, will generate unwanted voltages. It only takes a couple of mV to be generated and you have the dreaded hum.

With your amp fully wired as it is now, it might be a royal PITA to run a thick ground bus to pick up all the earths at the correct points. This is where Nick's scheme will be better than mine as it is much easier to run individual wires than it would be to fight with a length of recalcitrant copper wire with the chassis full of other gubbins.

Steve

simon · Post by **simon** » Tue Apr 01, 2008 5:35 pm

Nick wrote:0.6mm should do fine for most things, depends on the current you expect to be passing at that point.

Going OT a little, what do people use for wiring up in general?

I tend to use single core stuff from Rapid up to 1.5A, and 6A multicore stuff from Farnell for larger heaters, earthing and longer runs. No great thought has gone into this, other than rating, and it's cheap! Not having built enough amps to have an informed opinion what do others use?

Darren · Post by **Darren** » Tue Apr 01, 2008 6:07 pm

T&E

It's cheap

Cressy Snr · Post by **Cressy Snr** » Tue Apr 01, 2008 6:15 pm

simon wrote:
Nick wrote:0.6mm should do fine for most things, depends on the current you expect to be passing at that point.
Going OT a little, what do people use for wiring up in general?

I tend to use single core stuff from Rapid up to 1.5A, and 6A multicore stuff from Farnell for larger heaters, earthing and longer runs. No great thought has gone into this, other than rating, and it's cheap! Not having built enough amps to have an informed opinion what do others use?

I tend to use 0.6mm single core hook-up wire for general connections. I use this twisted together for small valve heaters. Power valve heaters get 6A stranded cable.
For high voltage I have an old reel of 30A 2.5mm twin and earth red/black. Red for B+ and black for earths. The bare earth wire I use for fabricating busbars.

The thick solid core wire stays put when bent and is capable of supporting components without having to pass through clamps or tags too often.

Steve

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