It's been a while, can't remember. I must've used a 5V or so relay, powered off the heater supply. I remember mounting the current sink resistor directly to the switch, then ran leads to 0V and H+ in a hokey way.
M49c tag-board layout (Oliver Archut circuit with 5718)
- Thread starter OneRoomStudio
- Start date
Help Support GroupDIY Audio Forum:
One thing to look out for if searching for relais in this very high impedance area is the insulation resistance and capacitance.
The Littlefuse relay "only" has an insulation resistance of 10^10 Ohm, while the Coto relais have 10^12 Ohm.
That's 10 Gigaohm for the Littlefuse and 1000 Gigaohm for the Coto.
10 Gigaohm is still really high and probably enough but there are also small form factor 5V reed relais from Coto with 10^12 ohm (https://cotorelay.com/product/9011-9012-9117-miniature-sip-relays/). Especially the 9011 series is even smaller than the Littlefuse. So I would chose these just to be on the safe side.
Capacitance is also important as only small capacitance directly at the capsule can weaken the signal. I did not find any information in the Littlefuse docs but the Coto is in the range of only 1pF.
The Littlefuse relay "only" has an insulation resistance of 10^10 Ohm, while the Coto relais have 10^12 Ohm.
That's 10 Gigaohm for the Littlefuse and 1000 Gigaohm for the Coto.
10 Gigaohm is still really high and probably enough but there are also small form factor 5V reed relais from Coto with 10^12 ohm (https://cotorelay.com/product/9011-9012-9117-miniature-sip-relays/). Especially the 9011 series is even smaller than the Littlefuse. So I would chose these just to be on the safe side.
Capacitance is also important as only small capacitance directly at the capsule can weaken the signal. I did not find any information in the Littlefuse docs but the Coto is in the range of only 1pF.
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A word of caution:
I was having issues with the polar patterns on this mic and couldn't figure out what was going on. Every time I took the mic body off and measured the voltages, everything was correct. Then I'd put the mic back together and have issues again. Finally, I found the culprit:
One of the body-tube mounting screws had been pressing into this capacitor (C2 on the AMI schematic), shorting the backplate to ground. Sneaky! I switched out the polystyrene cap for a slimmer polyester cap, and all is good now.
I was having issues with the polar patterns on this mic and couldn't figure out what was going on. Every time I took the mic body off and measured the voltages, everything was correct. Then I'd put the mic back together and have issues again. Finally, I found the culprit:
One of the body-tube mounting screws had been pressing into this capacitor (C2 on the AMI schematic), shorting the backplate to ground. Sneaky! I switched out the polystyrene cap for a slimmer polyester cap, and all is good now.
