# Is it possible to design this circuit with a single potentiometer (L-pad)?

I'm designing an L-Pad for my speakers. They attenuate the signal while keeping a consistent effective impedance, which is necessary for the amplifier and crossover circuits.

Typically, these values are computed and hard soldered, giving one 'set' attenuation level.

I want to use a single potentiometer (though it could be a dual-wiper pot) to make this adjustable, and include 0 dB (completely out of the circuit).

Would this be possible to design, without the use of a microcontroller and programmable potentiometers?

It needs to maintain a constant impedance, and I'm using them on 8 Ohm speakers.

Solving for my application:

R2 = Variable from (infinity) Ohms / disconnected

R1 = 8 - (R2 * 8) / (R2 + 8)

Help!

## 1 Answer

Normally reactive components such as inductors and capacitors are used to provide attenuation within/outside of specific frequency ranges and this by definition is how cross-overs are built.

Reactive components are favorable because they do not dissipate power apart from minimal R losses.

If you want to attenuate the signal being driven into a power load such as a speaker then I believe you will need a fairly robust potentiometer. If you are delivering 40 watts into a speaker, and you want to attenuate it by -3dB, then your potentiometer will need to be 8ohms and capable of handling 20 watts. Something the size of a high school lab rheostat would be suitable, but impractical and slightly inductive. I would suggest you consider attenuation of the signal entering your amplifier rather than the signal coming out of it.

If however you're dealing with 250mW headphone speakers then the potentiometer will work, however you may want to consider a Logarithmic type as the parallel impedance equation is not linear with differences in R.

• I'll be delivering up to 135W RMS to the speaker. It will be 2x8" woofers in series (for 8 ohm) and one Ribbon tweeter (at 8 ohm), crossed over. I'm guessing that's way too much power for any potentiometer to handle. Would there be a way to independently attenuate the woofers and tweeter with reactive components WITHOUT coloring the sound? (I.E. equal attenuation at all frequencies) – Ehryk Jan 13 '14 at 19:51
• you can dissipate as much power as you like if you're comfortable with localised heating. Normally selective attenuation of woofers and tweeters is achieved through an equaliser. To achieve attenuation at high power with a flat frequency response would require high power resistors. Is an EQ on the input to your amplifer not an option? If you insist on varying the power on the output side of the amplifier then this is one option... farnell.com/datasheets/31802.pdf – Martin Jan 13 '14 at 22:13