Speaker L-pads

[Steenamaroo]

Hey guys,
Can anyone tell me a little about low wattage speaker attenuation?

I want to have a final output volume control on a 1watt amplifier.
The problem is I can find suitable lpads, or values for stepped attenuators, but they are always specific to 8ohm speakers or 4ohm speakers.

My amp's output transformer is made for 3 ohm but I'll probably use it with 4 and 8 ohm speakers at different times. (different cabinets).

I think I might be set on manual resistor values on a 2p3t rotary giving volumes of full-quiet-quieter.
So...anyone know the best way to choose the values?

Many thanks in advance.

 

[ecc83]

This...TE CONNECTIVITY / CITEC - 404802692043 - Rotary Potentiometer, Cermet, MCU Series, 10 ohm, Linear, 1, 2 W, Panel | CPC UK

Would suit I am sure. The only advantage of an "L"pad is that it keeps the load on the amp constant (sort of!) but in practice this hardly matters for am amp attenuator.

If you used that 10R pot and a (worse case) 4 R speaker, the lowest the load could be is 2.85 Ohms, near enough t 3 foe jazz.
When wound back the load increases but never gets higher than 10R so the amp will be quite happy.

A warning about L pads? They are often rated at the SYSTEM power and that is no where near the juice a guitar amp will dump into them. Worse case, a pot rated at 100W (system) might in practice only handle 25W or so.

As a rule of thumb attenuators should be rated to at least 2 times the amplifier rating. Goes for speakers as well!

And (silly old fool forgot!) If contemplating BIG Mother attenuators investigate PA 100 volt transformers (70.7 V in US for some strange reason!). Can give a big voltage reduction but dissipate next to no power.
Dave.

 

[Steenamaroo]

This...TE CONNECTIVITY / CITEC - 404802692043 - Rotary Potentiometer, Cermet, MCU Series, 10 ohm, Linear, 1, 2 W, Panel | CPC UK

Would suit I am sure. The only advantage of an "L"pad is that it keeps the load on the amp constant (sort of!) but in practice this hardly matters for am amp attenuator.

If you used that 10R pot and a (worse case) 4 R speaker, the lowest the load could be is 2.85 Ohms, near enough t 3 foe jazz.
When wound back the load increases but never gets higher than 10R so the amp will be quite happy.

A warning about L pads? They are often rated at the SYSTEM power and that is no where near the juice a guitar amp will dump into them. Worse case, a pot rated at 100W (system) might in practice only handle 25W or so.

As a rule of thumb attenuators should be rated to at least 2 times the amplifier rating. Goes for speakers as well!

And (silly old fool forgot!) If contemplating BIG Mother attenuators investigate PA 100 volt transformers (70.7 V in US for some strange reason!). Can give a big voltage reduction but dissipate next to no power.
Dave.

Hi Dave,
Thanks for the info. So, this is kinda why I'm asking.

You'd use that pot as wiper= output, other two lugs = input and ground, right?
So that means the minimum additional resistance I have across the speaker (since one side is grounded) would be 10 ohms. Won't that reduce my max volume quite a bit?

Of course I can just pull out a bag of resistors and do some trial and error, but I'm curious how these values are calculated.

Thanks again.

 

[ecc83]

Hi mate! Power soaks, as I prefer to call them, are not that easy to design.

The main problem is getting sufficient attenuation in smooth steps, ideally with continuous control but NOT burning the pot out! Best bet is to sit down with p&p and a calculator and work down the chain of resistors and resultant voltages then turn those into watts . For One Watter you can use a simple pot but even at 5W track burn out can happen.

Your 1watter will lose practically nothing subjectively with a 10 Ohm load and 4 Ohms in parallel. Yes, just under 1/2 the power is going into the 10R but that's less than 3dB, barely noticeable. Even then you COULD fit a bypass swich?

For practical, non chav awakening levels you probably need 100mW or less. (0,63V into 4R, 0.9V into 8R, 1.3V into 16R and that's ALL the Ohm's Law I am doing for you!) so the general idea could be to use fixed, switched resistors to get down to a couple of volts (from full chat*) then use a suitably rated pot.

Don't worry too much about the load on the amp* it will be quite happy if it is within 50% or nominal. I have done very little work with soaks ("they" do not approve!) but the general wisdom is the amps sound better loaded a bit higher rather than lower then nominal.

*And herein lies the rub! The best tone from an amp is unlikely to be at its ball breaking max! Then the load will affect the tone.......And we haven;t even STARTED on tone bending capacitors!

Dave.