Attenuators, do they work or merely reduce SPL

Rocker

In this months Guitar and Bass magazine, there is a feature on building an attenuator to go between the amp and speaker. This has been discussed many times before (sorry), but is a resistor based attenuator a good solution to reducing sound level without seriously affecting the tone of the amp? It looks fairly easy to build...

ICBM

Yes, providing you don't expect miracles.

You won't turn a cranked 100W Marshall into a bedroom practice amp with one and retain anything like the tone and dynamics.

But you can use them for moderate level reduction with good results.

Just make sure that you use appropriately-sized resistors (with the correct heatsinking, for anything over about 10W) and that you're really careful to check all your solder joints. If an attenuator fails it will usually leave the amp with an open circuit, which is really not a good idea...

ecc83

I shall buy that magazine Rocker, interested to see their design.

As ICBM says, you must make sure the load cannot go OC and one way is to use two load Rs in parallel. So for 8R you would (in theory) use two 16s but IC and others reckon load Rs should be about 20% or so higher than the nominal speaker Z for best tone?

The most reliable resistors are the aluminium clad, chassis mount jobs (CPC .com) and even the 100W ones are still very cheap when you weigh up the cost of a decent case, switches, jacks etc, so even for a 30W amp you could use two 100W Rs in pllel safe in the knowledge that you are never likely to blow it. In fact it would be safe with just about any valve amp you can lift! (unless you are Arnie!)

Will pop into Sainsbury soon.

Dave.

ecc83

Got the mag, trying to get the full technical SP on the L pad used.

In any case I have a few reservations about the design but won't comment until I have had a good read.

Dave.

normula1

I built one of those LPADs into the back of my Laney and even though it's rated for 100 watts it gets quite warm even a just 5 watts so I wouldn't want to put anything bigger than 20 watts or so through it.

It does lose a bit of treble but nothing that can't be compensated for with a little re-eqing.

ICBM

normula1 said:

I built one of those LPADs into the back of my Laney and even though it's rated for 100 watts it gets quite warm even a just 5 watts so I wouldn't want to put anything bigger than 20 watts or so through it.

Is that one of those commercial "100W" L-Pads? If so I agree, 20W is a more sensible rating.

They're designed for use as a treble attenuator in a hi-fi system with an overall rating of 100W, which is where that number comes from - obviously that's a totally different thing from an overdriven 100W guitar amp.

EricTheWeary

It surprised me as a DIY project in G and B. Not that I understand the technical aspects but if it went/ was made wrong presumably it could damage someone's amp? A DIY project for, say, a fuzz Pedal that craps out would be pretty harmless.

samzadgan

i've been wondering this too...not to build one, but to buy one...

my question is how much stress does it put on the output transformer if any??

ecc83

I have had a reply from the L pad company and they tell me the posted specification is all they have. They cannot give me any information such as the continuous dissipation or  one and ten seconds overload capacity.  This information is available for the Allyclad type resistors.

The article is well written IMHO with clear schematics and photos. My criticisms would be,..

As ICBM says, the L pad is NOT up to the 100W + you would get from a 100W valve amp (tho it DOES say in the text that 50W is the biggest amp that is deemed safe. I am with IC, 20-25W max) The L pad is the only, s or bust load on the amp. I would prefer a stonking heatsunk ally job as the primary load and then an L pad to pick off a much reduced power level, 10W say, to give infinite attenuation.

The tin is too small! All been there! We use whatever is to hand for a project and end up cramming it all in. Needs to be a good 30% bigger*.

I assume the bent tin style jacks common the sleeve i.e. earth, through the case? Not good practice.

Not THAT happy about the bypass switch? The social consequences of flipping it to bypass after few bevvies at 2am are too worrying to contemplate!

*This feeds into another beef, no line out or headphone output. I would suggest the former at least would be very sought after as home recording is now such a big market. But! There is barely room for the present components leave alone another  jack! (and maybe an OEP 1:1 traff? )

Of course, many of my complaints would add to the cost of the project but if the device WERE made bombproof and incorporated at least a line out and a bigger case it could still be done well under £100 I think and would be a very much more worthwhile project.

As it stands I agree with Eric, a potential buggerer of amplifiers!

(I built a test load, 200W well sinked and used a 100V line speaker transformer to get a low level monitoring output. Could just as easily be a "balanced" line out.)

Dave.