Attenuator L-Pad wattage

[Helmholtz]

- here's an article showing how to wire these in; https://guitar.com/guides/diy-worksh...wn-attenuator/

FWIW, the circuit from the link is missing a (4R) series resistor (at least if the "L-pad" is a simple pot).

[Helmholtz]

Does the 0.5R resistor really make a difference?

[Helmholtz]

Chuck, an L-pad is not just a rheostat--it has two variable resistances that present a constant load to the amp:

This type of L-pad requires two wipers.

Many devices sold as L-pads are just power potentiometers, i.e. simple voltage dividers.

Literature doesn't seem to agree on the L-pad term, e.g. http://www.sengpielaudio.com/calculator-Lpad.htm

[Chuck H]

Does the 0.5R resistor really make a difference?

No one ever asks me about it. So I'm glad you did. To answer simply, no. It has nothing to do with the load I wanted to present to the amp. But what it DOES do is mitigate the glitchy actuation at the lowest setting. Without the resistor you can theoretically turn the attenuation down to no signal. But in actual use there is a little signal intermittently and sort of glitchy and inconsistent as the rheostat is turned up from 'off'. With that resistor in place you can't turn all the way off but the glitchy performance doesn't happen. And that's the only reason it's there.

I tried doing it without the extra .5r resistor several times and the weird behavior at 'off' happened consistently with any rheostat. So I just include it as part of the build now.

[Chuck H]

This type of L-pad requires two wipers.

Many devices sold as L-pads are just power potentiometers, i.e. simple voltage dividers.

Literature doesn't seem to agree on the L-pad term, e.g. http://www.sengpielaudio.com/calculator-Lpad.htm

Yes. This is my experience also. Some "L-pad"s seem to be plain ol rheostats. Others have inductors and resistors included as a "circuit" working with a variable power resistor and, though I haven't seen it, still others seem to be the sort of dual ganged rheostat shown in xitan's post. Which, if you analyze the circuit, is nothing more than a potentiometer (as wired) unless the two wire wound elements are of different values.

[Helmholtz]

The performance of an attenuator is likely to change with the amp used.

E.g. the Vox AC 30 has a high output (source) impedance of 70 Ohm at the 8 Ohm output and 140 Ohm at the 16 Ohm output.
The high source impedance (meaning very low damping) emphasises the bass resonance as well as the treble response of the speakers.

If with the attenuator the speakers look into a low impedance/resistance, the sound will be different and maybe disappointing.

But with an amp using heavy NFB, the difference might be very small.

[Chuck H]

The performance of an attenuator is likely to change with the amp used.

E.g. the Vox AC 30 has a high output (source) impedance of 70 Ohm at the 8 Ohm output and 140 Ohm at the 16 Ohm output.
The high source impedance (meaning very low damping) emphasises the bass resonance as well as the treble response of the speakers.

If with the attenuator the speakers look into a low impedance/resistance, the sound will be different and maybe disappointing.

But with an amp using heavy NFB, the difference might be very small.

Great information. And yes. As I said in post #7

.., purely resistive. Just fine for taking a little off the volume. If you actually want to turn down low a purely resistive load seems to flatten out the eq and the feel.

I notice this with the amps I build that do use a typical amount of NFB for a guitar amp. So it seems this would be even more pronounced with an AC30. Yucko.

I mentioned that my own attenuator uses a reactive load in place of the resistive load in the attenuator I posted. I could post the reactive load version (yet again) if rockaffe is interested. As I also mentioned, this is a more expensive project. Requiring bulky inductors and non polar electrolytic caps, etc. There's also some handiwork involved to make the "inductor sandwich" that I've discussed on other threads here.

[g1]

As I said I have a Vox Ac30cc2 that should be 30w but looking at the back of the amp, I've found this, written on a plate: 220-230v 50hz 165w

The 165W is power consumption from the wall outlet. Amplifiers are not terribly efficient, especially tube amps. So the amp is consuming a lot more power from the wall than it can deliver into the speaker. Most of the losses become heat.

[Mick Bailey]

FWIW, the circuit from the link is missing a (4R) series resistor (at least if the "L-pad" is a simple pot).

The part number given in the article is for a proper Lpad https://cpc.farnell.com/monacor/at-6...ono/dp/LS00547 The picture with the cover removed shows the 2nd inner wiper (obscuring the second resistive element).