Quad ESL57 O.T.A. EHT board?

[neskor]

I have Quad ESL refurbished by One Thing Audio.
On my EHT boards there are resistors on bass and treble connection.
On original Quad schematics they are not there.
Anybody knows why?
Original Quad schematic:

And EHT board with resistors:

An externally hosted image should be here but it was not working when we last tested it.

same here:

An externally hosted image should be here but it was not working when we last tested it.

 

[lcsaszar]

The schematics is perhaps older than the production units. The resistors are there for safety compliance, I suppose. At least they are there on my original EHT block.

 

[arend-jan]

The resistors on the Quad original EHT blocks are the ones on the input side (2.2M and 330K as shown in the circuit diagram) that make up a voltage divider. The resistors shown on the OTA boards are on the output .

One Thing Audio uses graphite/carbon based coating on their rebuild panels which has a very low surface resistance. This causes increased distortion due to constant voltage vs constant charge operation of the diaphragm. This can be mitigated somewhat by adding a large value resistor on the output of the EHT, but this resistor can do nothing to prevent charge migration on the diaphragm itself.

Measurements that I did years ago show a significant increase in distortion with high conductive coatings like graphite.

Quad used a much much much higher surface resistance coating on the original panels. So it does not need a resistor on the output nor does it make any sense.

I prefer to use the high resistance coating because the resistor on the output of the EHT can not prevent charge migration on the diaphragm. The result is a better sounding speaker.

 

[lcsaszar]

Indeed, my mistake. Nevertheless, the low value of capacitors in the voltage multiplier results in very high internal resistance in itself, in the order of several 10s Mohms. So an external resistor is not necessary.