Quick question on L-pads

[PonchoGreg]

Hi all,

I'm thinking of putting together an L-pad attenuator for low-wattage amps. Nothing too drastic in terms of attenuation, just taking the edge off a bit!

I've looked at a few DIY examples and have a couple of questions for those who have done it:

- I only need one of those, right? http://www.ebay.co.uk/itm/L-Pad-Atte...709#vi-content ? Just put it between the head and speaker (via the appropriate connectors of course) and Bob's your uncle?

- How hot is the L-pad likely to get? I was thinking of putting it in a pedal enclosure, but obviously not if the whole thing is going to melt...

Thanks for your help!

[paultheoneyoulove]

Yeah, that's the one. And it depends how high the output on the amp is. Mine is a five watter (very loud for five though). The attenuator gets a bit warm, but that's it. Certainly nothing approaching hot. The 50w rated one is apparently good for the 18 watt circuits.

Also, it is linear. This means most of the attenuation is in the last quarter of a turn. You can hear the edge get taken off, but if you're planning on running the balls off your amp, be prepared for some heavier attenuation and a bit of treble loss (I like this as my amp is a bit bright).

[PonchoGreg]

Cheers mate! The worst this baby would have to attenuate would be a not-so-loud 4 watts... Should be OK!

Funny about the pot taper. Does that mean that some L-pads are logarithmic, then?

[ICBM]

Cheers mate! The worst this baby would have to attenuate would be a not-so-loud 4 watts... Should be OK!

Funny about the pot taper. Does that mean that some L-pads are logarithmic, then?

Unlikely given the way they're constructed. Linear is just what you get with a wirewound rheostat.

Personally I wouldn't like to trust the power handling at anything over about 1/5 of the rated power, for a cranked valve amp of that power - ie use a "100W" L-pad for up to a 20W amp - although if people have been running a 50W one reliably with an 18W amp I could just about believe it.

4 watts should be fine.

[PonchoGreg]

Thanks!

[PonchoGreg]

Sorry to be a pain again, but I can only seem to find 8 ohm L-pads... do they not exist in a 16 ohm configuration? Can an 8 ohm one be adapted for that purpose?

Thanks again

[ecc83]

Sorry to be a pain again, but I can only seem to find 8 ohm L-pads... do they not exist in a 16 ohm configuration? Can an 8 ohm one be adapted for that purpose?

Thanks again

Do you mean that your amp does not have an 8Ohm output setting or that you just have a 16 Ohm speaker?

If you DO have an 8R amp tap use that with the 16R speaker. The worse case load on the amp is 5.3Ohms and will not bother it diddly.....Might even improve the "law" a bit!

Dave.

[PonchoGreg]

Do you mean that your amp does not have an 8Ohm output setting or that you just have a 16 Ohm speaker?

If you DO have an 8R amp tap use that with the 16R speaker. The worse case load on the amp is 5.3Ohms and will not bother it diddly.....Might even improve the "law" a bit!

Dave.

Cheers for the advice, Dave.

That's exactly that, actually: 16 speaker, amp with both 8 and 16 ohm outs. But I thought one should avoid mismatches? Because that would essentially send an 8 ohm load to a 16 speaker... But if you reckon that's fine...

[ecc83]

Cheers for the advice, Dave.

That's exactly that, actually: 16 speaker, amp with both 8 and 16 ohm outs. But I thought one should avoid mismatches? Because that would essentially send an 8 ohm load to a 16 speaker... But if you reckon that's fine...

Yes, you should avoid a GROSS mismatch. For instance an amp set for 4Ohms with a 16Ohm speaker is asking for trouble but even then you would have to drive the amp pretty hard for it to be a problem.

You are going the "other way" putting a lower load on the amp but a higher load on the L pad I am not sure what the resistance of an 8R Lpad is (will check) but it shunts the 16 Ohm speaker, so cool.

Dave.

[ICBM]

In situations like this, as long as you can match the amp to the attenuator it's fine - the amp sees the attenuator as the load not the speaker, if you're attenuating more than about 6dB, and close even at less than that.

Mismatches are fine within 2:1 in either direction with most valve amps. Some are more sensitive to mismatching in one direction than the other, but it's rare that you would be likely to compromise the performance much, and even rarer to do any damage.