I have a wonderful SD45 that I love, and I have a question about using an attenuator with it. I often use a THD Hot Plate (8 ohms) between

I have a wonderful SD45 that I love, and I have a question about using an attenuator with it. I often use a THD Hot Plate (8 ohms) between the amp and my Budda 2x12 cab, which works fine but gets a little too compressed at times. Recently I heard somewhere that you can use your attenuator in parallel (rather than in series) with your cab, and I was wondering how to safely do that with the SD45. Would I plug the cab into the speaker 1 output, and plug the hot plate into the speaker 2 output, and then set the impedance switch to 4 ohms? Have you ever tried this, and can it be done safely?

Thanks a ton and keep making killer amps!


Hi Matt,
First let me pass on an official “thanks from the Budda guys” for playing a Superdrive 45 and 2x12 cab. That is certainly some monster tone, so I can understand your need to tame those KT66s once in a while. The THD Hot Plate is one of my favorite pieces for this task, but since your question is dealing with an alternative use of a THD Hot Plate, I called my friend Andy Marshall, the man behind THD (thdelectronics.com), to get the “official” company policy on such use. Here’s basically what he had to say.

Yes, the Hot Plate can be used as a parallel attenuation device, but there is a recommended way to do this so that the constant impedance virtue of the device remains intact. Matt, it turns out that your hook-up assumption above was correct. Your speaker cabinet should be plugged into speaker output 1 of the amp, and the “input from amp” on the rear of the Hot Plate should be plugged into speaker output 2 of the amp. Since you’re using an 8-ohm cabinet and an 8-ohm Hot Plate (the purple version, the perfect Budda compliment), the impedance selector on the amp should be set to 4 ohms.

Here is where you’ll see that this type of setup is theoretically very limiting. The only setting on the Hot Plate that will, in conjunction with the speaker cabinet, present the amp with a constant 4-ohm impedance in this configuration is the “load” position. This will give you a total attenuation of 3db, which is half-power, but definitely not half-volume. If you normally use the Hot Plate for attenuations greater than its 4db setting, this setup is theoretically not for you due to its lack of versatility.

I also personally A/B’d this 3db parallel configuration against the standard usage 4db attenuation setting on the Hot Plate. The result? After a lengthy session culminating in volume-induced vomiting, heart palpitations and loss of balance (Okay, it really wasn’t that loud!), I determined that there was only a negligible difference in tone and feel between the two, the result of which I determined to be the extra speaker cable length between the Hot Plate and the speaker cabinet. Bottom line? I really found no major benefits in using the Hot Plate as a parallel 3db attenuation device.

I did however, against official company policy I might add, try the other attenuation settings on the Hot Plate. My favorite and most transparent was the 4db setting, which in this parallel configuration produced only about 1.5db of attenuation. With that little attenuation, it’s easy to see why it sounded so transparent. The other settings did produce greater than 3db worth of attenuation, but none came close to their stated degree of attenuation when the device is used properly. Keep in mind that these settings also do not maintain a constant and proper impedance on the amp, but no setting will take you below 4 ohms. Since Budda amps don’t seem to be particularly impedance sensitive with this small degree of mismatch, you can set the amp to 4 ohms, give the other settings a try and judge for yourself.

In conclusion, 3db of attenuation works pretty well no matter which way it is incorporated, but the one point that I think everyone needs to realize is that if you’re trying to throttle down a high-powered amp to bedroom levels, no attenuation device is going to sound “just like the amp.” That’s the reason we make the Superdrive 18!

Now go forth and assault the masses. That’s what rock n’ roll is all about!

Jeff Bober
©2007 Jeff Bober

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