The Leach Amp

[AndrewT]

Hi,
yes, we are both referring to the same resistor.

IT DOES NOT CONNECT CHASSIS TO SIGNAL GROUND.

Read my post again.

[acenovelty]

Howdy folks,

This review from May 2000 may give some insight to pollypocket's observations.

http://www.hometheaterhifi.com/volum...-i-5-2000.html

In particular:
"I found that the Aerius i's to be a very picky speaker where they are placed. Although proper calibration is a necessity for good sound with any loudspeaker, the MLs require three times the effort just to accomplish bearable sound. Because of the dipole nature of the electrostatic panel, the environment and positioning must be well prepared, for them to perform optimally. To this effect, Martin Logan produces a 23-page owner's manual with about half the content devoted to speaker positioning and environment. The manual includes recommendations for floor and wall types, acoustical wall treatment, distance from side and rear walls, distance from each other, distance from the listening position, and on and on."

Electrostatic panel with cone driver for low frequencies
ESL Panel Size: 40" H x 9"W
Cone Size: 8"
Crossover: 450 Hz
Lateral Dispersion: 300
Nominal Impedance: 4 Ohms - Min 1.7 Ohms at 20 kHz
Sensitivity: 89 dB/2.83 volts/m

A"veiled and undetailed" sound description give many of us flights of fancy bordering on the whimsical.
Since it sounds so bad driving those window panes, you really should unload this piece of junk for the price of shipping to member djk. He will give it a loving home.

Regards

[Bob2]

Gold_xyz

This is what Leach states about R51 under the power supply section

The circuit board has two ground leads, both of which connect to the central power supply ground. One lead grounds the signal reference points for the diff amp input stage. The other grounds the power supply decoupling capacitors and provides a ground reference for the protection circuit. R51 connects the two ground leads together on the circuit board. This resistor is small enough to look like a signal short circuit between the two grounds but large enough to force the currents in the two grounds to flow to central ground through the separate wires. This helps to prevent hum induced by power supply ripple currents in the ground system.

Bob

[AndrewT]

Hi,
thanks Bob for confirming Leach's own words.
Note that chassis NEVER gets a mention in the grounding description.

I use my own names for the references to help avoid ambiguity.
Audio ground and safety earth. Keep them separate.

[Bob2]

Hi Andrew

Leach states the following under the construction section

The central ground point can be made with a #4 or #6 machine screw through the bottom panel with several #4 or #6 solder lugs and a nut over it inside the box. The solder lugs must make good electrical contact to the bottom panel.

I'm still learning this stuff myself, but that sounds like the central ground is the chassis and the safety earth connects to the central ground.

Bob

[AndrewT]

Originally posted by AndrewT
There is a mistake in the PSU schematic where he shows the same symbol for the safety earth and the PSU 0volt. This implies these should be connected. Don't.

Keep the safety earth separate from the audio ground.

Run a wire from PSU 0v to audio ground.

Run a wire from safety earth to a disconnecting network and then a wire from the other side of the disconnecting network to audio ground.

Note what I said in post8.

If you want to avoid hum loops and buzzing coming from the PSU or from the mains then I very strongly suggest you do not connect chassis to audio ground.

[Bob2]

Andrew

Could you explain what a disconnecting network is. Like I said I'm still learning this stuff.

Bob

[AndrewT]

Hi Bob,
the exposed conductive parts of mains operated equipment MUST be permanently connected to safety earth.
That rule NEVER varies.
It's there to keep you and other users alive!!!!

Except in approved, double insulated equipment which are designed to be safe without the third safety earth wire.
As always there has to be an exception.

With that safety message in the back of your mind, but not forgotten, because we must come back to it, the audio ground should be a clean reference for all the audio side of the amplifier circuitry.
But there is a risk of calamity if the audio side is completely separate from the safety earth. The failure senario is as follows:-
The input live connection breaks free and touches some other internal metal component (eg. PCB track) and makes it live!
Now pull out the input RCA and inadvertently touch the RCA socket barrel (the nice gold bit sticking out and just waiting for a toddler to touch). Explain that one to the fatal accident inquiry.

So we need to isolate the audio ground from chassis (and mains) to keep all the horrible noise and spikes and hum and all else we don't want sounding in our speakers while still keeping the metal box safe to touch when something goes wrong.

That's the purpose of the disconnecting network. To keep you alive. Not to make the amp work and not to make the amp quiet.

The various components used in the disconnecting network must be robust enough to blow the mains input fuse when the mains fault develops. These fault currents while the fuse is melting and before the arc extinguishes can run to many kA. yes thousands of amps.
The network and it's connecting cables must survive long enough to take the fault current to earth and keep the exposed parts at a voltage that is low enough to prevent electrocution. I believe this voltage is around 50Vac (when in non hazardous areas).

The disconnecting network can consist of a parallel set of some or all of the following:-
Power bridge diode connected in inverse parallel,
Power resistor,
Power Thermistor,
High frequency capacitor.

Some commercial equipment also add a switch to bypass all these components. I would only ever close the switch as a debugging tool to try to isolate/identify a miswired accessory unit. These switches are often referred to as earth (or ground) lift switch. They MUST leave the permanent connection between chassis and safety earth wire intact whichever position the switch is in. They must NOT break the safety earth connection. I will never fit the switch.

[Bob2]

Andrew

Thanks for the explaination, very informative. Now as Leach stated, would R51 then qualify as the disconnecting network, seperating the two grounds, power from signal.

Bob

[AndrewT]

Hi bob,
I don't think so.

It is certainly not the safety connection, but if one bolts the audio ground to the chassis that obviates the need for the safety disconnecting network.

However, there is still a connection from signal ground to chassis and power ground to chassis. I think this may give rise to buzzing and mains borne interference at the amp output.

But worse, as soon as one connects a mis-wired ancilliary with the safety earth connected to the audio ground you are guaranteed to close an earth loop and that in turn guarantees hum (a lot of it) at the amp output. The creation of an earth loop requires two pieces of mis-wired equipment. Don't go half way there by mis-wiring your power amp when it is clearly avoidable.

Leach explains why he has put the signal to power ground resistor on board

is small enough to look like a signal short circuit between the two grounds

and this demands it is physically short to minimise inductance.
It is not doing the same job and was never needed to do the same job in his version of mis-wiring the chassis to audio ground.