Reactive Load for silent guitar recording

[bjosephs]

Just posting a build photo of a work in progress. It’s working but I still have some more metal fab then a bit of paint.

It’s based on Randall Aiken’s reactive load design with the values scaled to a nominal 8 ohms. I have some amps with no 16 ohm taps and frankly a smaller inductance was easier to source anyway. You can hear and feel a difference between reactive and resistive loads though the difference is hard to quantify. I’ll post more when I have some time if anyone is interested in making one for their own recordings.

 

[user 130681]

When recording, using a valve or indeed solid state amplifier, I find the most accurate dummy load is an 8 Ohm or 16 Ohm wire wound ceramic resistor is as good as anything otherwise one tends to be looking at snake oil.
A suitable tapping to set the desired level can easily be added, even a matching transformer (but that may cause distortion and unwanted peaks) if required.

 

[john12ax7]

When recording, using a valve or indeed solid state amplifier, I find the most accurate dummy load is an 8 Ohm or 16 Ohm wire wound ceramic resistor is as good as anything otherwise one tends to be looking at snake oil.

Strong disagree. Real world speakers are complex impedances, resistive and reactive parts. If you want the amp to respond the same you can't neglect the reactive part. It's not snake oil at all.

Good job OP. The hardest part for this tends to be sourcing a suitable inductor. I ended up just buying a commercial unit, which interestingly is just a clone of a diy design.

Edit : make sure your resistors are sufficient wattage and properly heat sinked.

 

[bjosephs]

Strong disagree. Real world speakers are complex impedances, resistive and reactive parts. If you want the amp to respond the same you can't neglect the reactive part. It's not snake oil at all.

Ya I’m not sure why Jon S is so sure about their position. A resistor is not an “accurate” load in any sense of the word and a reactive load does sound different and (IMHO) better.

Good job OP. The hardest part for this tends to be sourcing a suitable inductor. I ended up just buying a commercial unit, which interestingly is just a clone of a diy design.

Edit : make sure your resistors are sufficient wattage and properly heat sinked.

Thanks. I bought my inductor from Willy’s hifi. It’s a 33mH but it measures closer to 28mH. My resonance peak is a tad higher than I had hoped at 95Hz but I’m not rushing to tune it. I’ll order a slightly larger resonance cap when convenient.

Right now I’m planning to use this with my 20 and 50 Watt amps. The reactive parts are rated for plenty of current and the resistor is a 100W thermally bonded to the tray. Not a great heatsink but the resistor isn’t getting warm after plenty of play time.

I have a pair of 15 ohm 75W resistors that I’d originally prototyped with but I couldn’t fit them in the box. They’d be a good option if someone were to build one of these into a 1U cabinet but a more compact inductor becomes the next challenge.

 

[bjosephs]

For those who are interested here is the impedance plot. I took it with REW. I presume the little glitch at 60Hz is an artifact of power supply noise. I don't know what the little bar graph at the bottom is so if anyone does I'm all ears.

 

[john12ax7]

Sometimes I use the reactive load in parallel with a cab, gives you 50% power attenuation with little to no changes in tone.

If you really want to go down the rabbit hole you could measure the impedance of your cab and then adjust the reactive load to match. And then even do your own IRs.

 

[emrr]

Interesting to look at real time plots of an amp connected to a speaker, watching response change as the speaker moves.