LDR 6 input attenuator in a Quad 33 case

[Light Dependant Resistor]

Some details about my rebuild of my quad 33 preamp to be a 6 input LDR passive attenuator,

First thing was to take out the vertical and other internal circuit boards, and remove the power supply. Next was to convert the rear panel
from 5 pin dins to proper 5 pin locking sockets.

The power supply is external which is 12v AC from a 20VA toroidal , so a bit more space is afforded for the 6 LDR inputs
More in the next post

[Light Dependant Resistor]

With the boards now mounted in, I am planning to finish the wiring today.
I wired the series anode from main board to the pole of the switch and shunt anode wire to the LDR board last night.

Will begin with wiring from the 6 contacts to the LDR anodes - the switching unlike other preamps is contact-less
The LDR exhibits extremely high resistance when the LDR anode is off so works as a switch when off and as a
variable resistor when on. So contact-less volume and switching - perfect.

Image shows the rear panel 5 pin locking sockets, and wiring to the external 12v AC supply
27589

[Barry]

What are the sockets - 5-pin 'Domino' DINs or are they Tuchel sockets? How are the LDRs selected - do you use the push buttons of the Quad 33?

[Light Dependant Resistor]

Hi Barry
Connectors are manufactured by Maojwei http://www.maojwei.com/maojwei/products/details/n/2/pid/39.html
and are 5 pin types, chassis female and male connector. I arranged earth of each to the centre pin and the 6th input
uses all the 5 pins. Image shows wiring complete, I will post a final image showing it back in its wooden sleeve with the FM3

The build took a few days but was pretty straightforward , as the LDR boards fit so well

Yes you can use the 33 push buttons, but takes quite a bit of probing with a multimeter to discern the pole and contacts of each.
I have done one conversion that way, but took the easier path this second time with a rotary switch. The switch selects one series pair of
LDR's - its anode at a time. arranged via the first dial. Volume pot is 50 k located between the shunt and series cathodes, so all contact less volume
and input switching. https://www.stereo.net.au/forums/topic/301377-2020-stereocoffee-ldr-passive-preamp-redesign/

Cheers / Chris

[Barry]

Thanks for the information Chris,

Neat work by the way. :)

[Light Dependant Resistor]

Thanks for the information Chris,

Neat work by the way. :)

Thanks Barry, final image shows the 33 back in its wooden case with FM3