Bringing dummy load output down to stompbox level

[vigilante397]

I've been working on an idea (not super innovative, but neat nonetheless) of having a dummy load in a box with a cab sim, so I can plug my amp in and run straight into a PA system, recording interface, etc. I've tested the dummy load and it works great, successfully ran my amp head into a PA without damaging anything. The cab sim also works great, I ran my guitar straight into the PA without an amp and it sounds fantastic.

The problem I'm having is that when I take my "line level" signal off the dummy load into the cab sim (working right now with Tonepad's Speaker Simulator) it overdrives the crap out of it and my clean guitar tone gets distorted in a nasty way. I've tried filtering (high and low pass), knocking down the gain of both stages into fractions (meaning it should actually decrease the signal level), series resistors, resistor dividers, and everything else I could think of. I've de-soldered and re-soldered the board so many times pads are starting to lift.

What am I missing and how to I fix it?

[Rob Strand]

The problem I'm having is that when I take my "line level" signal off the dummy load into the cab sim (working right now with Tonepad's Speaker Simulator) it overdrives the crap out of it and my clean guitar tone gets distorted in a nasty way. I've tried filtering (high and low pass), knocking down the gain of both stages into fractions (meaning it should actually decrease the signal level), series resistors, resistor dividers, and everything else I could think of. I've de-soldered and re-soldered the board so many times pads are starting to lift.

You need to put a resistive divider between the dummy load and the input of the cab sim.  The signal at the dummy load is *way* higher than line level.

Another issue is getting rid of the heat from the dummy load.

[vigilante397]

You need to put a resistive divider between the dummy load and the input of the cab sim.  The signal at the dummy load is *way* higher than line level.

Any thoughts on how much I need to dump? Or should I put in a pot, adjust to the right level, measure it, then stick in the corresponding resistor values?

Another issue is getting rid of the heat from the dummy load.

So far that's been the easy part

[iainpunk]

You need to put a resistive divider between the dummy load and the input of the cab sim.  The signal at the dummy load is *way* higher than line level.

Any thoughts on how much I need to dump? Or should I put in a pot, adjust to the right level, measure it, then stick in the corresponding resistor values?

I have build a dummy load to line level box before. I used a lockable 10 turn 100k potentiometer parallel to the 8 ohm resistor. But a simple trim pot would suffice.

I also buffered the output and used an audio  transformer (1:1 ratio) to decouple the ground from the output side of the amp, since some of my amps have a BTL output. But you don't have to do that if it's not necessary.

Best of luck, Iain

[vigilante397]

I used a lockable 10 turn 100k potentiometer parallel to the 8 ohm resistor. But a simple trim pot would suffice.

That's what I was thinking, I'll give that a try tonight (assuming the pads on the PCB still hold up). Thanks!

[Rob Strand]

That's what I was thinking, I'll give that a try tonight (assuming the pads on the PCB still hold up). Thanks!

Yeah, best to tweak it to suit how hard you normally run your amp.

[mth5044]

A sprinkling of math never hurt anyone!

Power = resistance x voltage^2

Or

Voltage^2 = power/resistance

OR

Voltage = sqrt(power/resistance)

You didn't say what the wattage of your amp was or the load you were running it into, but I'll make that up and you can plug n play with your real numbers.

Say your amps runnin 5W into an 8ohm load when you turn it all the way up.

V = sqrt (5W/8R) = 0.8V RMS, or * 1.414 = 1.1V peak. That's definitely line level, as you say.

Is your cab sim built as a guitar effect? Is it meant to take pickup or effect level signals? Perhaps try and knock it down with a 10:1 divider to get it down. I've also seen 150k resistance in series feeding a 1M pot. I think this was on an amp made to be run into another amp.

[vigilante397]

A sprinkling of math never hurt anyone!

The tricky thing there is that I'm trying to find something that will be useful for any setting on any amp (up to 50W for now, end goal is 100W), and it's a variable load, switchable between 4, 8, and 16 ohms. The goal was to make it as flexible as possible in case I end up selling a couple of them to people that aren't me

I haven't finished testing all of my amps, but so far the most useful thing I've found was an 82k resistor in series with a 10k log pot before the cab sim. I started with a 100k trimpot but even with the amp turned down it only got to a usable level at the last 10% or so, and the resistor/pot combo gives me an excellent range. Using a low power amp or a clean amp with the levels low? Crank the pot all the way and it doesn't break up. Running a more powerful amp with the level turned up a bit? Fine, just turn down the pot until it stops clipping the input. I tested 5, 15, and 20W last night, I just need to run my 45W into it and if that works I can write it off as a success

Is it meant to take pickup or effect level signals?

Short answer is yes Also a good reason to leave an external pot. If I set the divider level for worst-case scenario, like a cranked JTM45 with an overdrive in front of it, then my little 5W practice amp will come out sounding weak and thin. So for my 5W amp 1:10 ratio works very well, but for my 15-20W amps the ratio is closer to 1:20. We'll see how the 45W does tonight