More home made attenuator questions

[Regis]

I've been building attenuators again, and have some questions.

Some of you may remember my Ampwell House I built last year with a variable L-Pad from Parts Express. I've now built a similar one with a dual ganged 100 watt 8 ohm L-Pads wired in parallel to get 4 ohms. The parallel wiring changes the taper a little but that's ok. The attenuator sounds pretty good, and because of the dual pots it will handle more power. Not sure how much tho.

I also built a static attenuator with sand cast power resistors from Ted Weber. Each section has two 50 watt resistors and it's a 4 ohm impedance. I did a thread over at TGP about it, you might get a chuckle regarding the enclosure I used.

First question: The power handling capability is hard to determine. I drove the variable L-Pad with a dimed Bandmaster for about twenty minutes, and the pots were hot, meaning, put your finger on it, and two or three seconds pull it away. A buddy brought his Vicky 30 watt tweed Bandmaster over and we drove it hard with that, and it got real warm but not too bad.
The static attenuator did the same thing with a dimed Pro Reverb. With a fan on each of them they ran a bit cooler.

So, how hot is too hot? How do I tell? Do the Calvin and Hobbes thing and keep driving them with bigger and bigger amps until something burns up? I know it's about heat dissipation and if I go by the wattage ratings the one should be rated at 200 watts and the other at 100 watts, but that don't seem likely in real world testing.

Second question: I've seen commercial attenuators that have fans powered by the speaker voltage coming from the amp. How do they do that? Use AC fans or rectifiy it to DC?

Thanks all,
regis

[kg]

I've been building attenuators again, and have some questions.

Some of you may remember my Ampwell House I built last year with a variable L-Pad from Parts Express. I've now built a similar one with a dual ganged 100 watt 8 ohm L-Pads wired in parallel to get 4 ohms. The parallel wiring changes the taper a little but that's ok. The attenuator sounds pretty good, and because of the dual pots it will handle more power. Not sure how much tho.

I also built a static attenuator with sand cast power resistors from Ted Weber. Each section has two 50 watt resistors and it's a 4 ohm impedance. I did a thread over at TGP about it, you might get a chuckle regarding the enclosure I used.

First question: The power handling capability is hard to determine. I drove the variable L-Pad with a dimed Bandmaster for about twenty minutes, and the pots were hot, meaning, put your finger on it, and two or three seconds pull it away. A buddy brought his Vicky 30 watt tweed Bandmaster over and we drove it hard with that, and it got real warm but not too bad.
The static attenuator did the same thing with a dimed Pro Reverb. With a fan on each of them they ran a bit cooler.

So, how hot is too hot? How do I tell? Do the Calvin and Hobbes thing and keep driving them with bigger and bigger amps until something burns up? I know it's about heat dissipation and if I go by the wattage ratings the one should be rated at 200 watts and the other at 100 watts, but that don't seem likely in real world testing.

Second question: I've seen commercial attenuators that have fans powered by the speaker voltage coming from the amp. How do they do that? Use AC fans or rectifiy it to DC?

Thanks all,
regis

well as you know, putting them into enclosures severely reduces max power handling.

generally when you start getting over 140*f things become more serious. wire insulation can start breaking down. 60/40 melts at ~370*f. the resistors themselves can probably handle more than that, being sandcast wirewound type.

obviously you could torture test the devices to find their breaking point. you might end up doing that since there are so many variables involved.

[dai h.]

hey Regis, here's some info I found for a thread on another forum websearching for info on L-Pads:

-----------------------------------------------------------
thanks! Tried searching for a bit and found this:

http://colomar.com/Shavano/lpad.html

scratching my head for a bit it sort of looks like (for an 8ohm L-Pad) the first VR would be 8ohms and the second (must be the one on the inside in the pic above) would be higher (since it ends up in parallel with the output, i.e. spk--haven't figured out what value it would be though)?

this is the highest rated one I could find (200W) :

http://www.madisound.com/catalog/produc ... cts_id=263

price is not listed but fwiw here(japan) it's priced around USD$35.

Also, (since it's two internally ganged) rheostats) I think it'd be possible to get higher rated ones if you had a ganged rheostat custom made. (The limitation on wwnd. rheostats as I understand is the current rating with generally higher ohm rheostats having a lower current rating--the higher the resistance the thinner the wire on the substrate(?)--the ceramic "body" portion).

dai h.

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Re: Weber MASS Attenuators

Postby dai h. on Wed Apr 30, 2008 9:38 pm
found this which shows a value for the 2nd VR:

http://www.bcae1.com/lpad.htm

and 120 and 8(using DC) in parallel is 7.5ohms, so seems not quite perfect. Looking at the diagram, one can see that you could also use a rotary sw. and indiv. resistors for fixed attenuation steps.

couple more L-Pad related links I found:

http://www.prodigy-pro.com/forum/viewtopic.php?p=268166

http://www.texatone.com/pages/Attenuators/Designs.html
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a guy on the prodigy pro forum (in the thread above) suggests de-rating those commonly avail. L-Pads to 1/10 or less which seems to make sense from their size. I think they are generally meant for use in line with tweeters, and for woofers it seems fixed power resistors are typically used (in crossover L-Pads). The highest rated variable one I found was a 200W Fostex (sells for around 35bucks here in Japan), so using the 1/10 figure, you would treat it as about 20W. You could also use a fixed L-Pad to knock some of the power down before the variable L-Pad to help ensure you don't overdissipate the var. L-Pad. I think the dBs for the L-Pads basically add, so a -6dB followed by a -12dB equals -18dB down, so (I think) simple summing(adding) works to figure the attenuation.

as for a fan circuit you might take a look at the Marshall Power Brake schematic (rectifies spk. signal then sends to DC fan which varies along with the signal).

[Bruce / Mission Amps]

...A buddy brought his Vicky 30 watt tweed Bandmaster over and we drove it hard with that, and it got real warm but not too bad.
...
regis

The last + half dozen Victoria tweed 2x6L6 amps I've worked on over the last 18 months, (not Bassman amps) never made more then 19 to 22 watts (cleanish) output into a 4 or 8 ohm dummy load, as viewed with my scope and about a 100mv input signal.
Not related much to your heat question but I'm curious if you actually measured the Bandmaster power output.

[Regis]

Guys, thanks for the replies.
KG, I suppose I could put a thermometer in there somewhere and experiment.

Dai, I've seen that Shavino site, he has good stuff there. The Madisound L-Pads are essentially the same thing I am using.
I know these pads should be conservatively rated but I think 1/10 is a bit much.
That guy at the texatone site has essentially done the same thing I have, interesting site, he and I have a lot of the same opinions.

Bruce, I don't have the equipment to measure output power, I wish I did. We just went on what Vicky people told him about how much power the amp had.

Thanks,
regis

[dai h.]

well, I'm guesstimating, but for example the "200watt" Fostex one looks waaay too small to handle anything close to that much power. Check this out for comparison

(a 200watt 25ohm rheostat in the weber vst attenuator kit: )

https://taweber.powweb.com/store/200atten.jpg

Fostex R82B 200watt ("music power" rated--whatever that is) L-Pad

http://www.geocities.jp/moasmaster/system8-network.jpg

[dai h.]

(sorry for digging up ye olde thread , but)

fwiw, I found a bit more info in regards to the power rating. According to the Fostex datasheet for their attenuators (L-Pads), the ones listed apparently have a "music power" rating,

http://www.fostexinternational.com/d...mp/pdf/att.pdf

which according to the page below is basically a bogus rating designed to help make something look bigger than it really is.


http://users.chariot.net.au/~gmarts/amppower.htm

PMPO (Peak Music Power Output). There's no technical definition I can find for this, but it appears to be marketing speak for: "Let's find any excuse we can to advertise more watts"! So we use the peak power rating mentioned above (a fictional double of the RMS power), usually a stereo system, so double again, then sprinkle the fairy dust. There are suggestions that because some music played on these systems is highly percussive (such as techno), that the system can deliver much more power to handle these peaks for very short times. This mutiplies the figure yet again by some arbitrary figure, but more
importantly, it's very relevant to guitar amplifiers.

Also, this is what it says on wikipedia:

http://en.wikipedia.org/wiki/Audio_power

'Music power' — the real issues

The term "Music Power" has been used in relation to both amplifiers and loudspeakers with some validity. When live music is recorded without amplitude compression or limiting, the resulting signal contains brief peaks of very much higher amplitude (20 dB or more) than the mean, and since power is proportional to the square of signal voltage their reproduction would require an amplifier capable of providing brief peaks of power around a hundred times greater than the average level. Thus the ideal 100-watt audio system would need to be capable of handling brief peaks of 10,000 watts in order to avoid clipping (see Programme levels). Most loudspeakers are in fact capable of withstanding peaks of several times their continuous rating (though not a hundred times), since thermal inertia prevents the voice coils from burning out on short bursts. It is therefore acceptable, and desirable, to drive a loudspeaker from a power amplifier with a higher continuous rating several times the steady power that the speaker can withstand, but only if care is taken not to overheat it; this is difficult, especially on modern recordings which tend to be heavily compressed and so can be played at high levels without the obvious distortion that would result from an uncompressed recording when the amplifier started clipping.

An amplifier can be designed with an audio output circuitry capable of generating a certain power level, but with a power supply unable to supply sufficient power for more than a very short time, and with heat sinking that will overheat dangerously if full output power is maintained for long. This makes good technical and commercial sense, as the amplifier can handle music with a relatively low mean power, but with brief peaks; a high 'music power' output can be advertised (and delivered), and money saved on the power supply and heat sink. Program sources that are significantly compressed are more likely to cause trouble, as the mean power can be much higher for the same peak power. Circuitry which protects the amplifier and power supply can prevent equipment damage in the case of sustained high power operation.

More sophisticated equipment usually used in a professional context has advanced circuitry which can handle high peak power levels without delivering more average power to the speakers than they and the amplifier can handle safely.


So in total, looking at the size of the L-Pads and the illusory "music power" rating, the advice to massively derate them does look logical and sound.

[Regis]

Funny that this thread should be resurrected.

Just last night I got my Pro Reverb out. I haven't played through it it a while and I have a blues gig next week that would suit it. It was late and I was going to hook up my commercial power soak but thought to try one of my homemade ones I built. This one is the same design as my Havanatone.

I dimed the amp and also drove it with a tube screamer. I dialed the attenuator down pretty low, and played for about 45 minutes. This attenuator has a fan in it and I think that helped a lot, plus the L-pads were wired it parallel to give a 4 ohm load so the power is distributed over two L-pad sections.

Every few minutes I would feel both gangs of the L-pad and the sections got very warm but not too hot to touch. The fan definitely made a difference in keeping it cool. I'm going to play it some more but I think it would work fine for a gig to shave off some of the horsepower.

I state on my Havanatone web page not to use a Pro Reverb with this thing but never really tried it until now. I just didn't want to be responsible if someone built one of these and blew up his amp.

The Pro is rated at what, 40 watts rms?

[Enzo]

Peak music power. Imagine your foot is stuck under a 900 pound piano. You jerk upwards with all your might and it moves enough you can slip your foot out from under it and go hopping around in pain. You certainly can;t lift 900 pounds, but you do have 900 pounds of peak lifting force at your command.

It is a useless spec that is based vaguely on how much powr you could measure instantaneously from an amp.

[tubeswell]

Peak music power. Imagine your foot is stuck under a 900 pound piano. You jerk upwards with all your might and it moves enough you can slip your foot out from under it and go hopping around in pain. You certainly can;t lift 900 pounds, but you do have 900 pounds of peak lifting force at your command.

If that was my foot I wonder if I would yell at rms power or peak power too :-)

[dai h.]

(this isn't a really unique idea or anything, but) you could always knock the power down a bit before it hits the L-Pad using a fixed L-Pad with adequately rated power resistors (so the power resistors take the bulk of the power). There's a real elaborate series of attenuators made by Sequis (UK), and from what I gather (descriptions, chassis pic) it looks like that is what they are doing. The volume range will be more limited of course but I suppose if you only need the volume in a certain low range, it can make sense. A pro might be not needing to buy a special high power L-Pad and being able to use a common off the shelf L-Pad.

Going back to the power rating, another guy on a different forum related (when he was experimenting with variable L-Pads) how his started smoking (when he was apparently overpowering the unit). So when using L-Pads, don't let the smoke out!

[Regis]

you could always knock the power down a bit before it hits the L-Pad using a fixed L-Pad with adequately rated power resistors (so the power resistors take the bulk of the power).

Yes I've been toying with this idea. Even if it was only a 3db reduction those L-pads I've been using would be able to be used with a bigger amp.

[dai h.]

I think that would be safer also, just in case the variable L-Pad after it went open (the fixed L-Pad would still load the output).

re: Fostex's power rating, I think it'd be a little unfair to portray the rating as malevolently deceptive since it does seem clear they are intending for the L-Pad to be used with hi-fi speakers in line with the tweeter and not guitar amps( and the "woofers" used as speakers--although the "music power" rating still seems kind of bogus). If it was intended for use with guitar amp outputs, I'd imagine they would give it a more conventional rating (otherwise, I'd expect a LOT of people would be quite unhappy with them after the L-Pads were overpowered and blew up).

Also, FWIW I found some pictures of the Fostex transformer type attenuator R100T if anyone is interested in looking at their innards:

http://roberto.blog.drecom.jp/archive/731

http://www.hifido.co.jp/KW/G0210/J/0...2855-11356-00/