L Pads

olafgarten

Hi, I am thinking of forgetting a low wattage tube amp and getting a Jet City JCA2212c and then making an L-Pad to go with it, I'll probably make it Switchable and between 10 and 20 dB. 

I'm just about to come up with a circuit diagram, but just wanted to make sure my calculations are correct for the 16 ohm speaker in the combo. 

For 10db of attenuation, I would get roughly 2 watts of output with a 11 ohm 15 watt series resistor and a 7.5 ohm 5 watt Parrallel one. 

For 20db of attenuation, I would get 0.2 watts of output and would need a 14.5 ohm 18 watt resistor and a 2 ohm 2 watt resistor. 

Of course my numbers have been rounded off to the nearest 0.5 as I don't think they make fractional resistors, also I may increase the power ratings to support larger amps which I might use in the future. 

Can someone please verify these calculations. 

olafgarten

I've made a circuit, the 2 spdt switches are just to show 2 halves of the dpdt switch. I might also add some switchable brightness capacitors as well.

https://www.circuitlab.com/circuit/55ms7stk3kg4/switchable-l-pad/

olafgarten

Ok, I've just seen the variable L-Pad's available, can't find a 16 Ohm one, but they look like this: 

http://cpc.farnell.com/monacor/at-62h/high-power-l-pad-mono/dp/LS00547

olafgarten

I could probably also get this and wire in in series for the 16 ohm load.

https://willys-hifi.com/collections/l-pad-attenuators/products/l-pad-attenuator-100w-8-ohm-wirewound-volume-control-1

EDIT: Something like this: https://www.circuitlab.com/circuit/7h4xx6qm8wmj/variable-l-pad-16ohm/

normula1

I have one of the Monacor ones fitted to my Laney Lionheart and it works pretty well.

ICBM

Just bear in mind that a "100W" L-Pad isn't suitable for a 100W guitar amp, or anything even close. The rating is for when they're used in their original application, which is in a hi-fi speaker system (usually as only a treble control), with a 100W amp. But a 100W hi-fi amp won't be putting out any more than about 25W on average even with fairly undynamic programme music, and even less into the tweeter. You're probably safe with a guitar amp of up to about 15-20W, but I definitely wouldn't go any higher and even that may be pushing it.

The resistances don't need to be at all accurate if you're building a separate-element one - bear in mind that the impedance of a speaker varies by a factor of at least 4 or 5 over the audio range anyway.

olafgarten

ICBM said:

Just bear in mind that a "100W" L-Pad isn't suitable for a 100W guitar amp, or anything even close. The rating is for when they're used in their original application, which is in a hi-fi speaker system (usually as only a treble control), with a 100W amp. But a 100W hi-fi amp won't be putting out any more than about 25W on average even with fairly undynamic programme music, and even less into the tweeter. You're probably safe with a guitar amp of up to about 15-20W, but I definitely wouldn't go any higher and even that may be pushing it.

The resistances don't need to be at all accurate if you're building a separate-element one - bear in mind that the impedance of a speaker varies by a factor of at least 4 or 5 over the audio range anyway.

Thanks, do you recommend anything other than an L-Pad that would work better, I'm sure the master volume on the Jet City is pretty good, but not sure if it will be enough.

Also, I'm going to put this in a seperate box, what wire and jacks should I use? 

ICBM

olafgarten said:

Thanks, do you recommend anything other than an L-Pad that would work better, I'm sure the master volume on the Jet City is pretty good, but not sure if it will be enough.

Jettenuator . Although the price seems to have gone up a bit recently.

olafgarten said:

Also, I'm going to put this in a seperate box, what wire and jacks should I use? 

Proper Switchcraft 1/4" open-frame jacks (not copies) or Cliff/Re-An plastic 'Marshall type' jacks (probably the best). Any fairly decent gauge cable is fine - ripped-down light gauge power cable is fine if you don't want to spend money.

underdog

I made a few L pad boxes back in the day, they work well and are incredibly cheap to make (like mentioned above I just used wire from an ordinary electric cable and some decent Jack sockets) but I only ever used them on "small" amps, 5 - 15 watts and always used the 100 watt rated L pad. Use to be able to get my Epi valve junior sounding epic even at whisper volumes.