Changing feedback resistor in 1176 preamp section - quietly!

sircletus · Oct 21, 2013

So, in the never-ending desire to tinker, I've put "mode" switches in my DIY 1176-alikes. The mode switch changes the feedback resistor in the preamp section (R15 in a Rev. D schematic, or R21 in a Rev. K "Gyraf" schematic), raising it (and reducing NFB) for some more funk and grit. I'm using a relay switch for switching to keep paths short and obviously, with DC on it, it pops like a mofo. That, and I like having a toggle switch on the front panel to match the other toggles for power and stereo link.

Any ideas for switching quietly with a toggle/relay combo in DC land?

PRR · Oct 22, 2013

Make-before-Break, of course.

But still it is going to POP. No easy answer.

joaquins · Oct 22, 2013

Switch with fet at slow speed, a couple of ms switching will change the pop for a thumb, if you can wait 100ms to switch it shoudn't be nasty noises... look at boss pedals switching to go discrete, you will need two fets and one inverter (may be one BJT and 2 resistor) to do a SPDT switch and if you think it a moment you could use the DC that's already there as bias, then you should get a higher voltage and a lower voltage to turn on or off the fet, that will be around because you already have the PS, an as it be slow shouldn't make noise on the line.

JR may be the person you have to ask about this...

JS

Brian Roth · Oct 22, 2013

It looks like R15 is involved with DC operating point, so I'm unsure how you can switch that around without any popping.

Bri

okgb · Oct 22, 2013

unless the switch included something to mute it at the same time, but maybe that's
getting too complicated .

sircletus · Oct 22, 2013

Yes, I was looking for make-before-break relays in an attempt to minimize the issue, and I've only managed to find the Fujitsu RY-12D-K. Mouser is back-ordered until deep into 2014 (!) and I can't find them anywhere else. Not even dubious suppliers on eBay.

I had thought about going the JFET route here for a bit of delay, but it would still be "passing around" DC. Might be worth trying.

Does anyone know how Wade at Chxxxler handles the feedback controls on the Germanium stuff without DC crackling on those pots? Maybe there's a related solution...

I was also taking a look at Gyraf's G9 mic preamp. I'm seeing a lot of blocking caps and what I assume are 1M pull-down resistors. Please correct me if I'm wrong, but it appears in the "gain" positions of the switch (pos. 5-11), he's shunting AC feedback to ground through a resistive divider while C7 (10µ) holds the DC bias for the first triode steady (attached).

Would something similar work here, perhaps?