Tonestack switching confusion

[Zenabi]

Hi all,

I came across this in a schematic and can't wrap my head around it.

Switching only the output and not the input of multiple tonestacks, wouldn't this make the controls interactive with each other? Or would the interaction between the tonestacks be too small to be of any concern?

Note: The pic only shows 2 of the 4 tonestacks being switched.

Thanks.

[PRR]

Suppose the tonestacks were lightbulbs. If the source were a small battery, yes, it might have trouble driving all the lightbulbs. If the source were a monster utility power plant, it could power all the lights in town and hardly notice.

Suppose V3b was output of a 100 Watt amplifier. It would pay no attention to how many puny tonestacks loaded it.

V3b is a cathode follower of maybe 700 Ohms output impedance. This is FAR less than any part in the tonestacks. Loading is tiny.

[Zenabi]

Thanks, PRR.

Am I correct to assume that this switching scheme would be more problematic if it was switching plate driven tonestacks of a typical fender?

I am still quite confused, as I keep thinking that since everything is in parallel, one set of controls must have an effect on other sets of controls.

[sluckey]

I am still quite confused, as I keep thinking that since everything is in parallel, one set of controls must have an effect on other sets of controls.

See those 680K mixing/isolation resistors? They minimize the interaction between tone stacks.

[pdf64]

The DCCF output is low impedance, but pretty limited current output, no higher than that available from the anode.
At low signal levels the affect of adding further tonestacks should be minimal.
But the point of using a DCCF in later / final stage of an overdrive preamp is that it’ll be running under large signal conditions.
Hence adding tonestacks in parallel will tend to reduce the output voltage swing available.
And as the tonestack input impedance is to some extent variable / frequency dependent, the large signal waveshape (as well as its magnitude) at the cathode will be affected by parallelling up tonestacks.

[Zenabi]

See those 680K mixing/isolation resistors? They minimize the interaction between tone stacks.

Seriously? Despite it being after the switches?

Honestly I don't even know why there are 680ks there. There seems to be quite a bit of weird stuff going on with this schematic. It's a Diezel VH4 for those who's wondering. And, no, I'm not trying to build one.

[sluckey]

See those 680K mixing/isolation resistors? They minimize the interaction between tone stacks.

Seriously? Despite it being after the switches?

The resistor is in series with the switch. Makes no difference which comes first.

This partial schematic is flawed.

[Zenabi]

The resistor is in series with the switch. Makes no difference which comes first.

This partial schematic is flawed.

When the switch is open, wouldn't that make the 680k redundant?

At any time, only 1 out of 4 tonestacks has its switch closed, I fail to see how the 680ks comes into play.

[sluckey]

At any time, only 1 out of 4 tonestacks has its switch closed, I fail to see how the 680ks comes into play.

If you had revealed that up front I would have just kept my mouth shut!

[Zenabi]

Apologies, sluckey, I thought I was being clear in the OP.

The 680ks do look like mixing resistors. Which is strange because there's no mixing happening here.

I came across this schematic and saw that it used 4 switches to switch between 4 tonestacks. I usually just use dpdt relays. But if this works without any interaction between tonestacks, I'd use this instead of the relays.

[Zenabi]

The DCCF output is low impedance, but pretty limited....

I guess switching both inputs and outputs is unavoidable.

I'm just trying to figure out a way to avoid using dpdt relays and at the same time not replacing said relay with 4 switches.

[dwinstonwood]

When I look at the schematic snippet I see two identical parallel tone stacks fed by a single cathode follower.

I can't get my head around "why ."

I could maybe understand someone wanting to switch between two different tone stacks, but not switching between two identical ones.

What am I missing?

[pdf64]

… What am I missing?

Perhaps it’s seen to be beneficial because it allows the user to remotely footswitch between different settings?

[Zenabi]

What am I missing?

It's a 4 channel amp with independant tone controls.

[Zenabi]

I just came across another amp that does something similar. The marshall jvm410.

The difference is that the switch, instead of being series to the circuit, shunts to ground. In this case, the mixing resistors make sense.

Perhaps that's what the Diezel is doing as well, but the schematic is wrong.