Single PA top rather than amp?

[Bill Fitzmaurice]

The values each of the rheostats would have at any given level of attenuation are the same as those for a fixed LPad, which you calculate with this: https://sengpielaudio.com/calculator-Lpad.htm

[warwickhunt]

Anecdotal but I transitioned between full bass rig to IEM (no backline) via using my chosen preamp pedal (Fishman Platinum Pro) into a QSC 10 mounted on a speaker stand to my side of the stage (obviously PA doing all the heavy lifting).  The sounded that I had to work with was far better than I expected and I occasionally default back to this set up when I've doubts about the quality of the engineer/PA to supply a good IEM mix... I've not reverted back to full backline, which surprises me (40+ years of relying on backline, often to carry FOH).  

[Phil Starr]

15 hours ago, Stub Mandrel said:

 

I can see for an 8R system the series rheostat should be 0-8R, presumably linear. The parallel rheostat would, ideally be 0-infinity and 8R at the point where the series one is at its mid-point. That would give three 8R points at max and min.

 

But most rheostats are wirewound and linear, so I wonder what compromise is used.

This below is a simple version of how they work, which is as you surmise. You may need to add in an extra resistor or so to add in further attenuation or limit the range over which they operate. As you can see the example in the picture isn't hard wired so you can make the circuit as you wish. Most commercially available L-pads work at 8 ohms. You've clearly spotted the flaw, there is no way in this circuit that you can maintain a constant impedance. For those who haven't worked it out: If we take the simplest example of both rheostats being 8 ohms. At one extreme the series element is 8ohms and the parallel one is shorting out the tweeter which shuts it out completely. The amp 'sees' 8ohms and is perfectly happy. At the other end of the dial the series element is now zero ohms and the parallel one 8ohms, the circuit is now 4ohms, at the mid position it is 6.67 ohms. All of these are easily driven by an amplifier but they are also part of a tuned circuit in the crossover, often a simple CR circuit so the resonant point of the high pass filter will vary and you'll get non flat frequency artefacts around the now crossover point. It's not an issue if you are simply using the L-pad to tune in a sound you like but if you want to use a modeller to best effect you now have something to notch out to get a realistic model coming out through the speaker. I'd also be suspicious of any cheaply made wirewound product in a tuned circuit having unwanted inductance.

 

[Stub Mandrel]

6 hours ago, Phil Starr said:

This below is a simple version of how they work, which is as you surmise. You may need to add in an extra resistor or so to add in further attenuation or limit the range over which they operate. As you can see the example in the picture isn't hard wired so you can make the circuit as you wish. Most commercially available L-pads work at 8 ohms. You've clearly spotted the flaw, there is no way in this circuit that you can maintain a constant impedance. For those who haven't worked it out: If we take the simplest example of both rheostats being 8 ohms. At one extreme the series element is 8ohms and the parallel one is shorting out the tweeter which shuts it out completely. The amp 'sees' 8ohms and is perfectly happy. At the other end of the dial the series element is now zero ohms and the parallel one 8ohms, the circuit is now 4ohms, at the mid position it is 6.67 ohms. All of these are easily driven by an amplifier but they are also part of a tuned circuit in the crossover, often a simple CR circuit so the resonant point of the high pass filter will vary and you'll get non flat frequency artefacts around the now crossover point. It's not an issue if you are simply using the L-pad to tune in a sound you like but if you want to use a modeller to best effect you now have something to notch out to get a realistic model coming out through the speaker. I'd also be suspicious of any cheaply made wirewound product in a tuned circuit having unwanted inductance.

 

 

Thanks Phil, that was what my back of the envelope suggested. I did wonder if there were some 'kludges' applied to approximate a non-linear effect.