Attenuator L-Pad wattage

[rockaffe]

Hi, I'm planning to build an attenuator for my Vox AC30CC2 and I've seen that L-Pads are sold with different wattate values. Do I have to buy one that match my amp wattage?

[Chuck H]

You have to buy one that EXCEEDS your amps wattage. I'd suggest double. Your amp can actually put out a lot more than 30 watts in bursts and those L-pads are generally rated for nominal power. Unless you're talking about a plain ol rheostat which some people call L-pads.?. And...

IMHE those prefabricated L-pads made for in house sound systems don't sound good for guitar amps. I scoped the output of my amp once while using one (because it sounded 'off' somehow) and the clipped waveform had a very unusual rounded bump in the middle of the upper clip and an equivalent rounded divot in the lower clip. And believe me you could hear it. I honestly think you'd be better off building a simple resistive attenuator with a couple of big resistors and a rheostat.

[xtian]

I've had good luck using L-pads for low-wattage amps. I can fit a 15w L-pad into a Hammond 1590 footswitch enclosure, which I use for my 2-watt tube heads. I also have a 50w L-pad in a larger Hammond enclosure--sounds great with my 10-15w amps (supro, princeton reverb style builds). When compared with my Weber 100w Mass reactive attenuator, there is very little difference.

[Chuck H]

I do know they're used by a couple of members here that report good results. I only tried it once so it's possible I had a particular model that's goofy. I've been neck deep in the homemade attenuator game and have ended up with the full reactive load enchilada. But those are a little spendy to make. Though a lot more spendy to buy.

[rockaffe]

You have to buy one that EXCEEDS your amps wattage. I'd suggest double. Your amp can actually put out a lot more than 30 watts in bursts and those L-pads are generally rated for nominal power. Unless you're talking about a plain ol rheostat which some people call L-pads.?. And...

IMHE those prefabricated L-pads made for in house sound systems don't sound good for guitar amps. I scoped the output of my amp once while using one (because it sounded 'off' somehow) and the clipped waveform had a very unusual rounded bump in the middle of the upper clip and an equivalent rounded divot in the lower clip. And believe me you could hear it. I honestly think you'd be better off building a simple resistive attenuator with a couple of big resistors and a rheostat.

Thanks for the replies!
This is the one I talk about: https://www.topservicepro.it/wp-cont...049_YAT100.jpg

[xtian]

This is the one I talk about: https://www.topservicepro.it/wp-cont...049_YAT100.jpg

Yeah, looks similar to the ones I get from parts-express.com and Amazon.

[Chuck H]

I do think that's just a big ol rheostat. So, 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. Still useful though. I have a few threads here where my resistive attenuator is discussed. It's much better than just a parallel load with a rheostat because it matches the ideal load better and offers more adjustability in the louder range so you can turn down just enough to make the bar owner stop crabbin'.

[Chuck H]

Well I went looking for threads and I decided it would be easier to just post the schem and diagram yet again. The resistors represented in the diagram are those ribbed chassis mount jobs available from many manufacturers. Mount them with heat sink paste if you have it. This is good with a tube amp to 50W for an 8 ohm load. If you have different requirements I can make adjustments and repost.

[xtian]

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

[Chuck H]

Ok. Though I have to say that the skinny unit on display surely doesn't look like it has two 100W wire wound elements in there. But I'll take your word for it. In any case... It's a parallel load that is almost certainly creating a less than ideal impedance. Consider that the most popular resistive attenuators use a HIGHER than rated load as the attenuated resistance/impedance. I think this is because an actual speaker only presents the ideal load in a fairly narrow frequency band. Though most (if not all) amplifier testing happens with a purely resistive load at the ideal resistance/impedance an actual speaker is MUCH higher impedance outside of the audible midrange frequencies. So to preserve the "feel" of a speaker the most popular purely resistive attenuators use higher loads. Some are grossly high in fact. I didn't do that with mine in the interest of amplifier safety and preservation. And, as I mentioned, it does sound a little flat at high attenuation settings. Whatcha gonna do? The Trainwreck Air Brake, for example, has something like 32ohms at some lower settings. Hardly the actual 70 to 100 ohms an actual speaker would present at critical, but extreme HF and LF frequencies. But I think the designers must have considered it an "average" of the load presented to the amplifier with guitar amp use. YMMV.

The advantage to the attenuator I posted is that it only reduces the signal by shy of 2dB when turned full up. A parallel load at the speaker impedance is a clearly audible reduction in volume making it less than great for "just taking a little off" when gigging. That's the actual reason I decided to go after this issue in the first place. For my own purposes. I only hope it can help others.

[tedmich]

In my experience the "rated wattages" on commercial L-pads are grossly overestimated; they blow with almost any appreciable signal. I've had much better luck with fixed resistors and a rotary switch.

[g1]

In my experience the "rated wattages" on commercial L-pads are grossly overestimated; they blow with almost any appreciable signal. I've had much better luck with fixed resistors and a rotary switch.

I think of those L-pad ratings as similar to the 'program power' rating on speakers. Or maybe peak program power.

[Chuck H]

I think of those L-pad ratings as similar to the 'program power' rating on speakers. Or maybe peak program power.

That's a funny thing, isn't it? For years I used a 5 way speaker system with a CD/DVD/multi media player purchased at Best Buy which rated the little 4"x6" plastic cubes at 500W?!? Really? Of course I never tested that and the system actually sounded pretty good. Otherwise I wouldn't have troubled to wire it up in four corners of the room and position the bass driver in a stealth, but suitable location in THREE DIFFERENT DWELLINGS. But 500W? Not on your life. I'm pretty certain that if I plugged my 20W guitar amp into one of those speakers it would have blown it to smithereens. Good thing that wasn't what I needed it for.

[Mick Bailey]

To my ears an L-pad on its own sounds a little dull when turned down and needs a frequency compensation capacitor to sound better. I use switchable 2.2uf polyester caps - the further you attenuate the more capacitance needed - here's an article showing how to wire these in; https://guitar.com/guides/diy-worksh...wn-attenuator/

[rockaffe]

Well I went looking for threads and I decided it would be easier to just post the schem and diagram yet again. The resistors represented in the diagram are those ribbed chassis mount jobs available from many manufacturers. Mount them with heat sink paste if you have it. This is good with a tube amp to 50W for an 8 ohm load. If you have different requirements I can make adjustments and repost.

Thank you very much for your time. I think I'm gonna go for this one. 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