Next Project - Type M2 Attenuator

My next project is an 8 ohm 50W attenuator to use with my tube amps. I came across a reactive circuit by JohnH that has become very popular on the Marshall Amp Forum over the past few years.  Many users have reported positive results in volume reduction while maintaining the integrity of the sound. The M2 design attempts to approximate the speaker impedance curve of a 4x12 cabinet with Celestion Greenbacks with minimal number of components. This reactive first stage of the design is made up of three large resistors and an inductor to achieve 7dB of attenuation. Subsequent attenuation stages are purely resistive networks to bring levels further down.

The attenuation  range of the M2 attenuator in the example build is -7dB to -31.5dB, in 3.5dB steps. My plan is to adapt the concept to achieve a range of -14dB to -38.5dB for my needs. This will be done by using a two fixed 7dB attenuation stages, and three switchable steps of 3.5, 7, and 14dB.

Layout and wiring mockup in AutoCAD.

M2 Attenuator wiring modified for my use.

I am using a Hammond 1444-22 chassis to mount all the passive components on the same plane.

Preparing the chassis to be drilled out.

All resistors are from Ohmite/Arcol (100W, 25W) and Vishay Dale (50W). The inductor is a Dayton brand 0.9mH 18AWG air core inductor from Parts Express.

Edit - 1/1/22 - I had to ask users on the Marshall Amp Forum for advice on applying thermal compound on the heat sink resistors. I am hopeful that I’ve applied enough compound for sufficient heat transfer from the resistors onto the chassis.

Resistors populated and (-) wiring soldered.

Edit - 1/2/22 - And it works! The wiring came together fairly quickly. I fired it up with my Jet City JCA22H head on the Overdrive channel with the master volume at about 10:00. At full attenuation, the amp is at low conversation volumes. My 50W Plexi-circuit amp is much louder through the attenuator, at moderately loud TV volumes.

Completed wiring of attenuator.

I put up a Shure SM57 on my Jet City 24SVe 2x12 with a Greenback. I set the attenuator on the maximum -38.5dB setting, and cranked the gain on my Stam SA73 preamp. I was able to record a quick clip sitting 5 feet away from the cabinet with IEMs for monitoring. I need to work on mic placement, but it is an encouraging first step in actually recording the Plexi circuit rather than rely on IRs.

Outmoded Electronics · Plexi 1987 Circuit - Type M2 Attenuator Build -38.5dB

I later played the same riff, but recorded it in 3 different scenarios:

1. Attenuated heavily and mic’d with a Beyerdynamic M201.
2. A post-power amp line out. The Suhr Reactive Load acted as a simple line out with the attenuated speaker load connected to the Thru jack.
3. Through the Suhr Reactive Load, as a separate take.

I made an impulse response of the speaker and mic to apply to the last two setups. I am satisfied with any of the methods to tame the 50W Plexi circuit. It is more satisfying to put up a mic myself and hearing the speaker in the room as I record. Having the option of applying IRs after the fact is a flexible way to work. I originally thought about installing a line out into the attenuator, or buying a line out box from David Bray Amps. Then I remembered that the Suhr Reactive Load already offered this functionality in a transformer isolated flavor. 

Outmoded Electronics · Very Informal Tube Amp Reactive Load Test

Edit - 2/27/22 - I hastily recorded some guitar tracks using the attenuator between my Winfield Typhoon head and my Eminence 1x12 loaded with a Weber Blue Dog. 14dB of attenuation got the 5W amp down to comfortable low volumes for recording.

To be continued…