Power attenuators are very useful tools for tube amps. As you know, tube amps sound best when cranked all way up. The only problem with that is that most of them are PAINFULLY LOUD. So it would be the best if we could crack amp and then somehow reduce output volume to level that will not make our heads explode. L-Pad is the simplest available power attenuator. It consists only two resistors (one in parallel with driver and one in series) which means it’s purely resistive.

Simple resistive attenuator

This could be a problem because at higher attenuation levels (lower output levels) there’s more treble loss. There’s also variable L-Pad available and it looks like huge potentiometer, but you can build fixed L-Pad very easily (and even cheaper). However, you must use resistors that will match your amp power and output impedance. You can further improve this design by using non-polarized treble bleed cap across R1 to compensate treble loss (for variable L-Pad it should go between input and output pads).


You can use this calculator to determine resistor values for desired attenuation level.


R1 =

R2 =

Variable L-pad

For more flexibility you can get variable L-Pad. As you can see below, it looks like rather large potentiometer. Treble bypass cap is installed between lugs 1 and 2. In my Firefly build I used 2.2uF Solen Fast cap. These L-Pad attenuators are linear which means that if you use it with 10W amp you get 5W output with L-Pad set to 50%. Since humans hear is logarithmic there will be only little volume drop with half power. To get half volume (-3db) power needs to be reduced ten times.

Here’s interesting and easy to do implementation of L-Pad. Using three way two pole switch (either slider or rotary) you can get nice versatile output. One position would give unchanged output level and other two positions can decrease level in two stages, for instance 3db and 6db. Of course, one could use switch with more positions and more options, this is just the idea.

Also, treble bleed caps could be added across R1a and R1b to reduce treble loss caused by attenuation. Below are listed resistor values for 3db and 6db attenuation with 8 ohms impedance. Cap values are usually 1uF-5uF, try different values to see what suits your needs.

2.3 ohms
4 ohms
19.4 ohms
8 ohms

Simpler solution would be using only SPDT switch for one attenuation step. Below is perfboard layout that shows both parallel and series resistors and treble bleed cap Cs that can be omitted.

12 Responses to “L-pad”
  1. elkulon says:

    Hi. Greetings from Argentina!

    I was looking for something like this, as simple as possible, because I’m newbie in this hobby.
    Asking to some friends of mine, who know a lot more than me, told me than “often, attenuators bring the tone lost, which can not be recovered again”. Looking at this page, I see that you have looked after that kind-of-matter. So, a couple of questions came to my mind…
    Is it just one cap for treble teed, in case of Variable LPad??? Wouldn’t be more effective one cap for each R1-R2 combos??

    Another couple of questions, about resistors… My design is intended to be 5W, no more. But for home use, I’d like to attenuate the output power to 1-1,5W, no more than that… What kind of resistors should I use???? The should be in the same range of wattage????

  2. JD says:

    Hmm, doesn’t the bypass cap change the impedance?

  3. Miguel says:


    Alright, this is a very good idea to attenuate an amp signal. I will try it with the switch in a Pro Junior (15w), I have ohms values for resistors with the calculator, but what about watts for these resistors?

    • Bancika says:

      For a 15W amp I’d get at least 25w resistors….maybe slightly more, you want a lot of headroom here

      • Miguel says:

        What kind of resistors are these, ceramic?

        • Bancika says:

          I’d get ones with the heatsink

          • Miguel says:

            This is a good one, I’m going to look for one of these here (Spain). I´m thinking about several resistors in paralel, sharing the watts out, could be another possibility.


  4. Kevin says:

    Very cool. Very helpful. A few questions:

    Your example for a 10w amp is 6db for half (5w) power, does 3db attenuation yield 2.5 W? Half volume?

    If I have a 5w amp, would half power still be a 6db attenuation?


    • Troy says:

      Not quite. For a 10W amp power is 40dBm half power is 5W (37dBm) and half volume is 1W (30dBm). For your 5W amp, half power would be 2.5W or 34dBm.

  5. Martin says:

    I don’t think so.
    we are talking about currents of a couple of amperes in 50-100 watt amp

  6. Glen Ritchey says:

    Could a ldr be somehow used as the switch to implement a simple type of tremolo effect?

    • Jesse says:

      Mmm. That is an interesting idea.
      You could switch the circuit in and out with a simple 555 timer and a relay, this would handle the
      power. My only concern is that it might introduce audio clicking as the relay brings the L-pad in and out of the signal path. Try it.

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