LPF placement

[Greg Robinson]

I also agree that in the amp isn't the right place to do this, but anode bypass cap doesn't need to go from plate to B+. It can go from plate to cathode or plate to ground. Then no concerns about B+ on the switch, just put the anti-pop reference resistor and switch on the ground side.

[The Dude]

The same would apply to bass, IMO. I wouldn't mess with the preamp for fear he wouldn't like the amp tone. In fact, it may make the situation worse. If the stage amp is not as responsive to high frequencies, the player is going to want to boost highs with the preamp to get the sound he wants on stage causing even more HF to arrive at the mixer for monitors. I would still change only the direct out. FYI: Here's a screenshot of the very simple Rockmaster direct out. Of course, your component values may be different depending on transfomer impedance, etc.

[Helmholtz]

The purpose/advantage of the transformer is providing a balanced output. So with a balanced interconnect cable there is no signal current through the shield and it only needs to be grounded at one end. This avoids a ground loop when connecting to other class 1 equipment.

[hylaphone]

The same would apply to bass, IMO. I wouldn't mess with the preamp for fear he wouldn't like the amp tone. In fact, it may make the situation worse. If the stage amp is not as responsive to high frequencies, the player is going to want to boost highs with the preamp to get the sound he wants on stage causing even more HF to arrive at the mixer for monitors. I would still change only the direct out. FYI: Here's a screenshot of the very simple Rockmaster direct out. Of course, your component values may be different depending on transfomer impedance, etc.

Cool, thanks. So, R44 and R45 knock down the signal, and R44 C34 low pass at 150Hz? How does the impedance ratio figure in?

[Helmholtz]

Cool, thanks. So, R44 and R45 knock down the signal, and R44 C34 low pass at 150Hz? How does the impedance ratio figure in?

Yes, R44/R45 form a signal voltage divider.

The relevant R for the LPF is not R44 but the effective source resistance that feeds C34.
It is essentially given by R45.
The corner frequency will be between 1.5kHz and 1.7kHz.

The impedance of the transformer should be high enough to not influence the LPF.

Connecting to other safety grounded (class 1) equipment via unbalanced interconnect cable might cause ground loop hum.
In that case a DI box can help.

[hylaphone]

The relevant R for the LPF is not R44 but the effective source resistance that feeds C34.
It is essentially given by R45.
The corner frequency will be between 1.5kHz and 1.7kHz.

I don't understand, how does parallel R45 change the source resistance? Not sure what is happening earlier in the Rockman, but mine will be driven by Mosfet follower without Zout around 150 ohms

[Helmholtz]

Circuit source resistance is measured from output to ground. Maybe look up Thevenin's rules. (https://www.electricaltechnology.org...s-theorem.html)
Textbook formulas for filters typically assume zero source resistance and infinite load resistance, so have to be modified for real circuits.

With a low source resistance before R44 total source resistance for the filter is essentially given by R44 in parallel with R45 and corner frequency will be around 1.7kHz.

[hylaphone]

Circuit source resistance is measured from output to ground. Maybe look up Thevenin's rules. (https://www.electricaltechnology.org...s-theorem.html)
Textbook formulas for filters typically assume zero source resistance and infinite load resistance, so have to be modified for real circuits.

With a low source resistance before R44 total source resistance for the filter is essentially given by R44 in parallel with R45 and corner frequency will be around 1.7kHz.

Great, I'll chew on that for a while. Thanks again!