Can someone explain L-pads to me?

[itsGiusto]

In particular, I'm interested in L-pad attenuators with continuously variable controls, not L-pads just made out of two resistors.

For example, look at this image:

In this particular image, I don't understand why it matters that there is a ganged potentiometer. Couldn't the same effective schematic be accomplished with one single non-ganged potentiometer? How does ganging these and hooking them up in this way create a constant load on the amplifier (as is the intent)? Maybe the two potentiometer have different resistances, and in that way, the load ratio works out better in some way?

 

[tubeswell]

Google is your friend.

“It consists of a parallel and a series rheostat connected in an "L" configuration. As one increases in resistance, the other decreases, thus maintaining a constant impedance, at least in one direction.”

https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html

 

[itsGiusto]

Google is your friend.

No need to be condescending. I have googled it and not found the information I'm looking for, which is why I asked here.

“It consists of a parallel and a series rheostat connected in an "L" configuration. As one increases in resistance, the other decreases, thus maintaining a constant impedance, at least in one direction.”

https://www.electronics-tutorials.ws/attenuators/l-pad-attenuator.html

That quote is not in the site you linked. I think it's from here: https://www.wikiwand.com/en/L_pad

Not to mention, that quote could easily apply to one single pot, not just a ganged pot. As you sweep across a single pot hooked up as a voltage divider, the series resistance increases while the parallel resistance decreases, and yet I don't think we'd say that constant load is achieved in that scenario.

The site you linked (which I've also seen before) only talks about L-pads made of two resistors, which I specifically said is not the source of my question. I understand how those work.

Either way, neither the site I linked above or the one you linked answered my question: why do you need a ganged potentiometer? Electrically, you could achieve the same thing with a single potentiometer, that is, unless the two ganged potentiometers have different resistances. In which case, I'd like to understand why that would be beneficial, and see a mathematical explanation of why that helps.

 

[radiocaster]

I don't think it's ganged like a dual potentiometer. However, the construction is different, the materials are different and meant to endure much more current.

Also the accuracy is different and the tolerance is maybe lower. A 500k ohm pot can often be 470k or something like that when turned all the way.

Despite that, the values do fluctuate some when you turn the l-pad, which isn't so much the case with a regular pot that has constant equal variation as far as I know. I noticed this when I built my attenuator. As in the result wasn't always 8 ohms and can vary some as you turn the pot. There is especially a very weird area in the extreme counterclockwise part.

 

[bebopbrain]

Couldn't the same effective schematic be accomplished with one single non-ganged potentiometer?

I recommend you take an interest in L-pads made out of two resistors. Design an L-pad that presents 8 ohms and dissipates 75% of the input power.

Then design an L-pad that presents 8 ohms and dissipates 80% of the input power. Then 90%.

You will have answered your own questions.

 

[itsGiusto]

I recommend you take an interest in L-pads made out of two resistors. Design an L-pad that presents 8 ohms and dissipates 75% of the input power.

Then design an L-pad that presents 8 ohms and dissipates 80% of the input power. Then 90%.

You will have answered your own questions.

If what you're getting at is that that's how a continuous L-pad potentiometer would be made, essentially the sum of each of the power dissipation division steps, then I think you'd end up with two rheostats with tapers that don't really exist, as far as I'm aware. In order to present the exact same load to the amp, the resistance increase/decrease in each resistor in the L-pad would not be constant from say 70% to 80% as it would be from 80% to 90%.

 

[tubeswell]

No need to be condescending. I have googled it and not found the information I'm looking for, which is why I asked here.


That quote is not in the site you linked. I think it's from here: https://www.wikiwand.com/en/L_pad

Not to mention, that quote could easily apply to one single pot, not just a ganged pot. As you sweep across a single pot hooked up as a voltage divider, the series resistance increases while the parallel resistance decreases, and yet I don't think we'd say that constant load is achieved in that scenario.

The site you linked (which I've also seen before) only talks about L-pads made of two resistors, which I specifically said is not the source of my question. I understand how those work.

Either way, neither the site I linked above or the one you linked answered my question: why do you need a ganged potentiometer? Electrically, you could achieve the same thing with a single potentiometer, that is, unless the two ganged potentiometers have different resistances. In which case, I'd like to understand why that would be beneficial, and see a mathematical explanation of why that helps.

In this case wikiwand had a concise and accurate description. The electronics tutorial explains it simply, because an L-pad is essentially just a voltage divider (albeit with a variable output). My help is free. Google saves both of us time.

A ganged rheostat works because it’s linear (not tapered). Rheostats are more suitable than pots because pots aren’t available in appropriate power ratings, and a linear taper is needed for a ganged arrangement to present a constant* impedance (as seen by the source).

*Source impedance is constant across each rheostat (end to end).

 

[Swirling Snow]

The resistive elements in an L pad are different values. Does that help?

 

[Len058]

An L pad is like a ganged pot but uses ohm's law to keep the total resistance the same. It consists of one resistor in series and one in parallel.

When you increase the resistance that is in series with the speaker, you have to change the resistance in parallel with the signal. The resistor in series eats the signal, the other one is to keep the right resistance for the amp.

16-16 in series is 32 ohm in paralel its 8 ohm 16-8 in series is 24 ohm in parallel its 5.3

Eg speaker is 8 ohm. Add 8 in series, to lower the signal. You need 16 ohm in parallel to get back to the 8ohms, your OT wants to see/

 

[dan40]

@itsGiusto ...if you are interested in building yourself an attenuator, check out the design by "JohnH" over on the Marshall Forum. He started the design back in 2017 and the thread is now 139 pages long. Many folks, including myself, have built his reactive attenuator circuit over the past five years and they have all been very happy with the sound and performance. For just a few dollars in parts, you can end up with a great little unit.

Simple Attenuators - Design And Testing

Background Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve...

www.marshallforum.com

The circuit has evolved throughout the years but I believe the version shown on page 1 is his latest design.