Background

When constantly moving and playing late it’s not easy to carry a cab around and to crank it. In the previous couple of months I’ve been building a gear around idea to have either just a preamp or a whole amp with a dummy load, use my Palmer PDI-09 as a speaker sim and plug that into Boss Micro BR recorder. That setup allows me to change amps/preamps and still be able to easily record stuff, use my headphones and play over backing tracks.

The idea for this particular dummy load came from Palmer PDI-03 which is similar to my PDI-09 just with couple of extra switches and dummy load. It uses three 22 ohm resistors paralleled for ~7.3 ohms of resistive load in series with a 100uH air core inductor. Guys at SLOClone forum reverse engineered one of these and measured the inductor to be 150uH, so I was shooting for something in between. The original inductor was wound with a 1mm wire, but since it’s a 100W dummy load and I don’t have an amp over 5W I settled with #22AWG magnet wire from Radio Shack. They have a magnet wire set with three different gauges and #22 being the thickest.

Below is a plot for 100uH inductor in series with a 7.5ohm resistor (thanks FredB!). It shows that impedance increases after 1KHz or so. For purely resistive load it is constant for any signal frequency.

Construction

When playing with a 9W resistor as a dummy load, it would get very hot even with my 2W AX84 4-4-0 amp. I got some higher power resistors just to keep everything cooler. Two 15 ohm, 17W resistors in parallel yield 7.5 ohm load with total power handling of 34W which is way more than I need. I wound the inductor on the bobbin from Radio Shack that wire was originally wound on. I wound leftover wire on top of the other gauge from the set. It wasn’t as easy as I was hoping for because #22 is a bit thick and it’s not easy to keep the windings tight and close to each other. I used the Pronine inductor calculator to determine number of turns and it was just over 3 full layers of wire. Each layer of wire is coated with nail polish to fill the gaps and keep everything in place. When it was done it measured 110uH on my DMM.
Resistors are bolted to the chassis tightly and the inductor is mounted on the top. That makes the box filled very tightly on all axis. That could be a problem with heat, but I hope that 34W don’t even get warm with a cranked 2W amp.

Results

To be honest, I was expecting that inductor will make a bigger change, but just can’t tell the difference between the resistive load and this device. To test this, I recorded a clip of clean guitar, played it from my laptop through the amp and recorded on Boss Micro BR. One clip is recorded with a resistive and the other with resistive-inductive load. I ran spectrum analysis in Audacity for both clips and put them on top of each other to make them easier to compare. Contrary to my expectations, inductive load has a bit higher treble response starting from 6KHz, but again, it’s not something I can hear. Other than that, both look pretty same (see below – red area is for resistive and blue for resistive-inductive load, purple is where they overlap). Good thing about this is that bigger resistors get just few degrees hotter than room temperature, but sonic difference doesn’t seem to justify the hassle of making an inductor . The next thing I’m gonna try is to gut out a speaker and use it’s motor as dummy load. Stay tuned…

Pictorial

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