L-pad, potentiometer, volume control - Whats the difference?

[piece-it pete]

I don't want to monopolise this board but I'm learning that I've got a lot to learn :).

Thanks to many helpful people here and at AA (Alchoholics, not Audio hehe) I'm starting to "get it".

However I'm still not clear on what I need for a level control before an amp. 100k, 50k, pot, L-pad? Shorting out inputs?? Will a normal volume control work? Arrrrrgh!

Does anyone have a link or some basic instruction?

Thanks!

Pete

[Dual-500]

100 ohm should work fine - 100 ohm load on the output stage of your crossover should work. If not, 1000 ohm will work. 100 ohm will provide for better coupling with your amp - which likely has a 10k ohm input impedance.

Best to ask Marchand this question or look at the schematics of some of their crossovers with output gain controls in them.

I would probably opt for a "Trim Pot" for this application - mounted inside your crossover case and adustable with a small screwdriver.

[highend64]

The difference between is very simple. L-pads are use to control the output level of a speaker while mantaining the same impedance. Pots, short term for potentiometer, is use to resist the flow of current for two paths. A balance control is a pot. If you turn the knob, it will decrease current flow on one side whlie increase current on the other.
Volume control is a rheostat. Rheostat is similar to the pot except the rheostat has only path for current to flow. So when you turn up the volume, your decrasing the resistance. Hope this helps you.

[piece-it pete]

OK, I have a confused picture of these things :).

So, to control the line-level signal I should use a L-pad? Trimmer pot?

"Somewhat mystified" Pete

[Dual-500]

OK, I have a confused picture of these things :).

So, to control the line-level signal I should use a L-pad? Trimmer pot?

"Somewhat mystified" Pete

Use a Trim Pot if you are going to "Set and Forget" the crossover output.

Use a standard Potentiometer if you want a knob to adjust settings.

You will probably also want "Audio Taper" pots, and not "Linear Taper" for this application.

The nature of your question warrants a proper answer and honestly it would be bit wordy for this media - maybe read up on a few "How to" electronics books would help.

"L-pads" and "T-pads" are used for gain adjustments on speakers. Commonly used in mid/high frequency gain controls and also as gain controls for ceiling speakers. L/T pads are used for Low Z applications only and are NOT suited for what you are doing.

[skeptic]

If reading the other replies left you confused, don't feel too badly, they left me confused too. At least they would have if I hadn't known before.

L-pads are variable volume controls usually used to control the level of a loudspeaker and is placed between the amplifier or receiver output and the speaker. It allows a more or less constant impedence load on the amplifier while allowing the loudness of the speaker to be varied. This is important for tube amplifiers but not particularly important for solid state amplifiers because if the tube amplifier sees a very high impedence such as when a normal volume control is turned down, its output stage could be damaged. This type of control was once commonly used for tweeter and midrange level controls and for remote speaker level controls but NEVER for controlling the woofer level or the overall level of a high fidelity loudspeaker because they introduce a series resistance and therefore reduce amplifier damping factor. They are rated according to impedence such as 4 or 8 ohms, and wattage, 25, 50 etc. Sometimes they come ganged together for stereo.

A potentiometer is a variable resistor used for controlling the level of an electrical signal. It has 3 terminals. Two terminals connect across the entire resistance and these are connected to the source of voltage. One of these is designated as the "common" terminal. The other terminal is connected to a movable conductor which slides across the fixed resistor and the output or load is connected between this element and the common. As you adjust the position of the movable conductor, the ratio of a resistive voltage divider bridge changes. The motion of these can be rotary (commonly used for audio), linear (a slider pot) or even a multiturn screw potentiometer where great precision of adjustment is needed. The rate of change of resistance can also be proportional (equal change in position equals equal change in resistance) or logarithmic (change in position results in a lograthmic change in resistance.) For audio volume controls, we always use logarithmic pots because our hearing sensitivity is logarithmic so a change in a rotary pot from say 9 o'clock to 10 o'clock will give about the same number of decibels change as from 1 o'clock to 2 o'clock. Typically for audio amplifiers and preamplifiers they are 50Kohms to 100K ohms.

At the input of an amplifier, these controls can be used to compensate for differences in loudness between a phonograph cartridge and a tuner so that you don't get a sudden change in volume when you switch from one input to another. Usually very expensive preamplifiers of yesteryear like McIntosh incorporated these types of controls.

Shorting plugs are used for preventing a popping noise when you switch inputs on a preamp with unused high gain inputs such as tape head or microphone. They simply assure a zero volts across these inputs. It's just an RCA jack with a short piece of wire between the hot and ground. You can make them yourself for a few cents each or just take an old unused interconnect (cheapest you can find) cut it in half, strip the insulation back and short the conductors.

There are other methods of adjusting the level of an electrical signal such as a variable transformer (Powerstat or Variostat) which are not used in audio circuits.

[piece-it pete]

I'm slowly but surely working this out!!

So, I'm thinking I should get a potentiometer to control the sub-line out from my electronic crossover.

Would a shorting plug stop the massive "whump" from my subs when I turn the unit on?

Pete

[Dual-500]

I'm slowly but surely working this out!!

So, I'm thinking I should get a potentiometer to control the sub-line out from my electronic crossover.

Would a shorting plug stop the massive "whump" from my subs when I turn the unit on?

Pete

It's evident from following the thread that you either haven't contacted Marchand for advice with their product or they haven't answered your query.

Not a big deal - just pointing that out.

To take care of startup transients you need to drop the gain on the input of the amp, or change your power up sequencing (recommended whatever you do anyway), and/or add an output relay to the crossover.

These are things that Marchand should be able and willing to assist you with as a customer.

Power up signal processing first, then amps last. Power down amps first and signal processing gear last.

:)

[piece-it pete]

D'oh!!

Thanks for pointing that out. I must be working too hard - and not listening to the music enough!!

Thanks for all the help.

Pete