TT PoS X

Tube amps don't really sound like tubes until they are opened up wide; unfortunately this can result in a deafeningly loud volume. In order to reduce the output to a reasonable level, the energy generated must be dissipated; which is normally achieved through a series of resistors.

However, it isn't enough to simply string a series of resistors somewhere between the output of the amplifier and the loudspeaker; several factors must be taken into account:

a) Impedance

b) Power output

The schematic and layout diagrams below are for a simple and economical solution for an attenuator. The attenuator can dissipate up to 50 watts without additional cooling when the attenuator is assembled in a metal chassis.

A series of individual power resistors is used rather than a high-power potentiometer to keep the costs down. S2 switches these resistors in various combinations to set the amount of attenuation.

Impedance matching is accomplished by the resistors R8, R9, and R10, which also do the real work and, of course, get quite warm. S3 sets the desired impedance. The circuit is quite simple and self-explanatory.

S1 is a hard-bypass switch which removes the entire attenuator from the signal path, in which case the power at the output is equal to that at the input. Be aware that caution is required when switching at a high attenuation and volume levels.

Of particular interest is the tone stack, which is activated with switch S4. The purpose of this tone stack is to compensate for a possible high frequency loss through the resistance network. The high frequencies bypass the resistance network and are inserted in the output signal.

A simple line out is also included, although it is not necessary for attenuation. The line-out signal can be sent to other amplifiers, or even into an additional input of the same amp, if it has multiple inputs.

The photos below show one possible layout. In this case, the attenuator is built in an aluminum Hammond 1444-16 chassis. This chassis is big enough, and there is enough room to easily position and fasten the components.

Silicon wire is used for the hookup. The power resistors themselves are mounted on standoffs. They could just as well be mounted directly to the chassis, but the standoffs improve ventilation.

The view from the front and back:

The schematic

This is just one example of how the attenuator could be laid out. The
circuit is not sensitive to interference and can be easily laid out as
suits your purpose and available space.

Bill of Materials