L-pad question

[daz]

I found a 100w 8 ohm L-pad and thought i'd give it a try with my amp. It's only for use at low home volumes to make the low end of the master less touchy, so no worries there. But having used set L-pads i've made in the past i know they are tone killers. Looking at the diagram below, can anyone recommend any kind of cap tricks or such i could do to it to make it retain my tone to a better degree? Thanks kids.

[Steve Conner]

There are two fundamental problems with the L-pad.

Firstly, if you tweaked your amp to have a high output impedance, the L-pad will lower it. This lessens the effect of any presence or resonance controls you might have.

Secondly, it isn't compensated for the Fletcher-Munson curves.

These issues both conspire to make it sound a bit flat and dull when turned down.

To mitigate this you might like to try connecting a "treble bleed cap" between pins 2 and 3. The theory is exactly the same as the bright cap used on guitar and amp volume controls, just the value is a lot bigger. Maybe 1uF or thereabouts.

If you're feeling adventurous you might also like to try a large capacitor in series with pin 1 to give a bass boost at low volumes. It would have to be a few hundred uF, either a non-polar electrolytic or two regular electrolytics connected back to back in series.

[daz]

Thanks. I was thinking of trying a treble bleed cap but i wasn't sure whether it would be a good idea in this context and didn't know whether the value would be much different. But i'll try it and try a 1uf to start. Thats where it fails is in the high end. It loses that cut and sparkle even when just dropping a few DB.

There are two fundamental problems with the L-pad.

Firstly, if you tweaked your amp to have a high output impedance, the L-pad will lower it. This lessens the effect of any presence or resonance controls you might have.

Secondly, it isn't compensated for the Fletcher-Munson curves.

These issues both conspire to make it sound a bit flat and dull when turned down.

To mitigate this you might like to try connecting a "treble bleed cap" between pins 2 and 3. The theory is exactly the same as the bright cap used on guitar and amp volume controls, just the value is a lot bigger. Maybe 1uF or thereabouts.

If you're feeling adventurous you might also like to try a large capacitor in series with pin 1 to give a bass boost at low volumes. It would have to be a few hundred uF, either a non-polar electrolytic or two regular electrolytics connected back to back in series.

[daz]

Well, tried a 2000pf, a 1uf, and have nothing bigger other than electrolytics so i tried a 10uf. Nothing seems to do much. Also, wouldn't this affect impedance?

[bluesky6]

I've tried 4.7uF non-polarized with some success. I was using a WGS G12C/S which was naturally unbright. The LPAD made it even darker. The cap restored some of the brightness.

[Leo_Gnardo]

I've noticed THD uses a brightening cap on the pad (not an L-pad, just a heavy duty WW pot) they install in some of their smaller-wattage amps, UniValve for instance. They might be in THD speaker attenuators too but never had a chance to look inside. I've used from 2 to 6 uF film caps. If it's a small amp you can get away with 100V rating but if you have something big better go for 200 or 250V. Daytons from Parts Express - lots of choices & very reasonable prices. Solen 250V available from Antique/CE also.

FWIW I'd avoid so called non-polarized electrolytics. Seen too many of 'em fail in speaker crossovers. (edit) R.G. just explained on another thread: "If you put two electro caps head to head, then no matter which way the voltage points, one is low resistance and one is high resistance. The one that blocks DC flow also blocks the currents that would eat away at the oxide film of the one that's being polarized backwards. And you only paid a 2:1 penalty in bigger volume for the privilege." So the "nonpolarized" cap is a not very good compromise, especially for speaker use.

[tedmich]

IME typical crappy Lpads are VERY liberally spec'd for power handling; I've blown them with amps that put out less than 1/2 of the Lpads rated power

[bluesky6]

Daytons from Parts Express - lots of choices & very reasonable prices. Solen 250V available from Antique/CE also.
...
FWIW I'd avoid so called non-polarized electrolytics. Seen too many of 'em fail in speaker crossovers....

I'm swapping out the 100W L-PAD I'm using for a 50W one (amp is only 15W tube). Ordered a Dayton 4.7uF along with it per the advice above.

For folks wanting to order from Parts Express, do it from the Amazon website as shipping is combined there. Ordering directly from Parts Express results in "cummulative" shipping costs e.g. $5 for every item you add to your cart.

[dai h.]

IME typical crappy Lpads are VERY liberally spec'd for power handling; I've blown them with amps that put out less than 1/2 of the Lpads rated power

IIRC this has been discussed (somewhere in the archives) before. L-Pads seem to carry a "music power" rating, which is more of a peak, short term rating (as I understood), so a "100W" L-Pad won't handle 100W (also I think they are typically used with tweeters, not "guitar speaker" woofers). On another forum it was suggested to derate them significantly (I forget the specific suggestion on how much to de-rate), so if anyone is planning on using one for attenuation experimentation, be forewarned. Also, since it's an electro-mechanical component (that can wear or possibly experience malfunction from small debris getting inside (if there is an opening)), it might be wise to have a fixed load before the L-Pad (or at least have some minimal open load protection such as higher value extra R across amp output) so in case something happens to it, the amp doesn't see an open load (which I understand is esp. important for no feedback pentodes since with open secondary the primary Z goes super high).

[olddawg]

I use the Weber Mini Mass all of the time. Can't beat it for $100.

[Mick Bailey]

There's no real rule for correction caps - it depends on each circumstance and subjective listening, The last one I installed had a total of 3uf made up of three 1uf poly caps and it sounds good.

I work on 30% of rated output as a maximum dissipation level.