L-Pad query

knuckleberryfinn

I've been using a 100W L-Pad with my 5E3 for a while and was used to it being there.  However, after removing it recently to lend it to someone, and then a couple of weeks later reconnecting it, I noticed that there is a significant loss of volume and high frequencies even when it is not supposed to be attenuating at all.  Measuring across the jack terminals in and out, I'm getting 0.8ohms (no attenuation) through to about 36ohms (the highest measurement before the last turn of the L-pad when it reaches infinite resistance).  I thought L-Pads did not affect volume or tone when they weren't attenuating at all.  Am I wrong?

goldtop

For sure, if you attach a speaker to the output, and then measure the DC resistance across the input, while you sweep the L-pad's knob, you should see numbers closer to the nominal L-pad's impedance. (NOTE: the whole kit and caboodle must be unplugged from the amp to do this.)

I know I did this once, but I can't quite recall the actual numbers I got. It did go up and down a bit, but nowhere near the 0-infinity that you have and not enough to worry.

If you post a photo of how it's wired up, we'll be able to double-check that it's connected right. I'd post a pic for the hook-up of a typical L-pad (the Monacor that CPC sells), but it seems that attachments aren't allowed here.

ICBM

knuckleberryfinn said:

I've been using a 100W L-Pad with my 5E3 for a while and was used to it being there.  However, after removing it recently to lend it to someone, and then a couple of weeks later reconnecting it, I noticed that there is a significant loss of volume and high frequencies even when it is not supposed to be attenuating at all.  Measuring across the jack terminals in and out, I'm getting 0.8ohms (no attenuation) through to about 36ohms (the highest measurement before the last turn of the L-pad when it reaches infinite resistance).  I thought L-Pads did not affect volume or tone when they weren't attenuating at all.  Am I wrong?

The L-Pad will be loading the input even when it's set to full up.

Measure the resistance between the input tip and ground, with the level up full and nothing connected to the output.

knuckleberryfinn

ICBM said: knuckleberryfinn said: I've been using a 100W L-Pad with my 5E3 for a while and was used to it being there.  However, after removing it recently to lend it to someone, and then a couple of weeks later reconnecting it, I noticed that there is a significant loss of volume and high frequencies even when it is not supposed to be attenuating at all.  Measuring across the jack terminals in and out, I'm getting 0.8ohms (no attenuation) through to about 36ohms (the highest measurement before the last turn of the L-pad when it reaches infinite resistance).  I thought L-Pads did not affect volume or tone when they weren't attenuating at all.  Am I wrong? The L-Pad will be loading the input even when it's set to full up.

Measure the resistance between the input tip and ground, with the level up full and nothing connected to the output.
Resistance is 9.2ohms.  Why isn't it zero?!  I don't understand why it is loading the input when it is supposedly bypassed.

goldtop said: For sure, if you attach a speaker to the output, and then measure the DC resistance across the input, while you sweep the L-pad's knob, you should see numbers closer to the nominal L-pad's impedance. (NOTE: the whole kit and caboodle must be unplugged from the amp to do this.)

I know I did this once, but I can't quite recall the actual numbers I got. It did go up and down a bit, but nowhere near the 0-infinity that you have and not enough to worry.

If you post a photo of how it's wired up, we'll be able to double-check that it's connected right. I'd post a pic for the hook-up of a typical L-pad (the Monacor that CPC sells), but it seems that attachments aren't allowed here.
There was no speaker attached - I was trying to measure minimum resistance, but obviously was using the wrong method.  Surely an attenuator should have a sweep of zero to infinity resistance?

knuckleberryfinn

ICBM said: knuckleberryfinn said: I've been using a 100W L-Pad with my 5E3 for a while and was used to it being there.  However, after removing it recently to lend it to someone, and then a couple of weeks later reconnecting it, I noticed that there is a significant loss of volume and high frequencies even when it is not supposed to be attenuating at all.  Measuring across the jack terminals in and out, I'm getting 0.8ohms (no attenuation) through to about 36ohms (the highest measurement before the last turn of the L-pad when it reaches infinite resistance).  I thought L-Pads did not affect volume or tone when they weren't attenuating at all.  Am I wrong? The L-Pad will be loading the input even when it's set to full up.

Measure the resistance between the input tip and ground, with the level up full and nothing connected to the output.
Resistance is 9.2ohms.  Why isn't it zero?!  I don't understand why it is loading the input when it is supposedly bypassed.

goldtop said: For sure, if you attach a speaker to the output, and then measure the DC resistance across the input, while you sweep the L-pad's knob, you should see numbers closer to the nominal L-pad's impedance. (NOTE: the whole kit and caboodle must be unplugged from the amp to do this.)

I know I did this once, but I can't quite recall the actual numbers I got. It did go up and down a bit, but nowhere near the 0-infinity that you have and not enough to worry.

If you post a photo of how it's wired up, we'll be able to double-check that it's connected right. I'd post a pic for the hook-up of a typical L-pad (the Monacor that CPC sells), but it seems that attachments aren't allowed here.
There was no speaker attached - I was trying to measure minimum resistance, but obviously was using the wrong method.  Surely an attenuator should have a sweep of zero to infinity resistance?

goldtop

knuckleberryfinn said:

There was no speaker attached - I was trying to measure minimum resistance, but obviously was using the wrong method.  Surely an attenuator should have a sweep of zero to infinity resistance

Well, with the speaker attached, the L-pad is meant to provide a near-constant load to the amplifier. This diagram...



...shows how there are two resistive elements that are swept at the same time to achieve this (near) constant load.

So, the speaker sees anywhere from a near-zero to near-full part of the amp's output depending on teh position of the dial.

ICBM

knuckleberryfinn said:

Resistance is 9.2ohms.  Why isn't it zero?!  I don't understand why it is loading the input when it is supposedly bypassed.

It's not bypassed, it's turned up full - which is not the same thing.

If you look at goldtop's diagram you can see that the second resistive element is still connected in parallel with the speaker. This will reduce the volume by about 3dB and take a bit of top-end off due to it being a straight resistance compared to the speaker's rising impedance - ie at high frequencies the reduction will be more.

knuckleberryfinn said:

 Surely an attenuator should have a sweep of zero to infinity resistance?

A proper attenuator does, yes. Or rather, its not as simple as that but it should be fully bypassed - direct connection between input and output, no connection to ground - if it says it has a bypass setting. Not all do.

A L-Pad is not the same thing as an attenuator, it's a much simpler device.

martinw

The shunt resistor of the L-pad is always in circuit, in parallel to the speaker, and thus will always have an effect on the sound, even when the L-pad is set to zero.

knuckleberryfinn

Right, I get it now - thanks you three.  I'll add a bypass switch.  

I might also add a couple of bypass caps I can switch between to let more top end get through when its attenuating to different degrees.  Would this work?  If so, could you suggest some values?

ICBM

knuckleberryfinn said:

I'll add a bypass switch.

I wouldn't, it introduces unreliability.

Just unplug the unit and plug the speaker directly into the amp - that way it's absolutely definitely bypassed and there is no potential for trouble.

knuckleberryfinn said:

I might also add a couple of bypass caps I can switch between to let more top end get through when its attenuating to different degrees.  Would this work?  If so, could you suggest some values?

Yes, it will work - I'm not sure of the values though.