Passive Loop, Impedance, and Tone Questions

[pdf64]

'Pulling the 10k raises the output impedance to approximately 120k'
Won't the output impedance without R29 depend a lot on the setting of the reverb control, and also whether the reverb is switched on/off?
The way I see it, it might go as low as ~50k with the reverb up full but switched off (obviously still way too high for a loop send though).

How about making R32 into a 470k pot and put the cap between the slider and hot end, then you can adjust how much of a bright effect it has?

[wizard333]

Could do that except the amount I want from that cap is none, I want it to go away!

What I'm pondering now is, is there any way to keep the output impedance of that loop low and isolate it from the rest of the circuit more or less so its not such a tone sucker. Make that thing a parallel loop instead of series.

A MV could be placed at R34, but that amp is already sporting extra knobs for 1) midrange control on clean channel, 2) separate mids on the gain channel when the clean boost is engaged, 3) volume control for the clean boost I added. Not sure I can fit another knob on there and have it be usable.

[wizard333]

Against the chassis is best because it suppresses the electric field around the heater wires. Hum often arises from improper wiring of the sockets though, creating hum loops. And the twisting must be good right up tot the socket.

Yeah I twisted the heck out of mine and ran them against the chassis to boot. Seems like Greg Germino does the same thing on his builds, if I remember correctly.

What sort of improper socket wiring are you talking about here? These amps have a high amount of hum to them, anything that brings it down would help.

[Steve Conner]

I hate to be Mr. Stating The Obvious here but...

Why don't you just try a larger resistor than 10k, and/or a smaller bright cap?

Dull and brittle aren't two independent coefficients, they're just the two ends of the treble boost stick. If you don't boost treble enough, it sounds dull, if you boost it too much it sounds brittle. So you need to boost it just the right amount and then it won't be dull or brittle. Hence I argue that you don't want that bright cap to go away altogether, you just want it smaller than it is, or with a resistor in series to tame it a bit.

Or balance it with another cap across the 10k: maybe that's what the bright cap is there for anyway, to drive the cable capacitance of the effects loop send, and so the loop sounds too "brittle" when you're not using it and there's no cable capacitance. That would be a compromise that the amp designer couldn't avoid. Heck, try patching a 10ft guitar lead between the send and return and see if you like what it does.

If some poor amp ever ends up sounding dull AND brittle at the same time, it's because it boosted treble in the early stages of a dirty channel and then lost it again further on down the signal chain.

[wizard333]

Fair question, the answer would be that I'm worried about the loading of having a high output impedance loop send going into a potentially low impedance input on an effect.

The more I'm thinking about this, the more I think making it a parallel return and isolating the send from the rest of the circuit may be the answer. To make room for that mix knob though I guess I'd have to make the send jack into a TRS send/return and use the hole where the return is for the mix knob.

[Merlinb]

Fair question, the answer would be that I'm worried about the loading of having a high output impedance loop send going into a potentially low impedance input on an effect.

Whether you use 10k or some larger value, the freq response will be the same without the cap, so +1 on what Steve said; tweak the cap.