L-pad Attenuator Build

[jjasilli]

Finally built an attenuator in connection with this thread:  http://www.el34world.com/Forum/index.php?topic=15868.msg153909#msg153909
It works fine as an ordinary attenuator.  But it has two output jacks: bypass & attenuated.  It's main purpose is to work with my vintage PA amp to reduce the vol to a monitor speaker, while feeding full vol to the main speaker.  (These are house speakers at a venue I play at.  They don't seem to have vol controls and are out of reach.) It's rated for 100W 8ohms.  The amp will see 4 Ohms with 2 speakers plugged in.

Only a minor issue:  the L-pad seems to have a slight effect on the bypass circuit.  Don't kn ow why.

The circuit is based on the diy Ampwell circuit, but no switches.  It's built into a mini computer power supply box which I salvaged.  For higher power amps it has a mini cooling fan wired to work off a 9VDC wall wart. 

Schematics & photo's below.

[jojokeo]

Only a minor issue:  the L-pad seems to have a slight effect on the bypass circuit.  Don't kn ow why.

The circuit is based on the diy Ampwell circuit, but no switches.

It appears that the bypass circuit is not isolated from the attenuator circuit as it would be with the switch, therefore it is in parallel with it. Test the "bypass" output jack's resistance through the range of the l-pad with a multimeter and see if it changes, this will confirm it.

[jjasilli]

Yes, my connectivity test after the build did show interaction on the meter.  But it works well enough and in a small light package. 

Some ideas for a possible next project:
* try a 1 or 2 Ohm resistor of correct wattage to isolate the L-pad input from the bypass circuit.
* use a 1:1 tranny with dual secondaries.  This would be bulky.
* run a 2nd lo wattage power amp for the monitor speaker.  Also bulky.

* Use an inductor to ground rather than a bypass cap to preserver hi frequencies at lo L-pad volume settings.  This "ampwell" circuit does not wire the L-pad "correctly".  It should be wired as a variable resistor, not a voltage divider.  The latter allows the familiar bypass cap to be used, but does not properly preserve impedance matching.  Wired properly as a variable resistor, I'm thinking that a shunt inductor, under the L-pad, would lift hi frequencies from ground.

[stratele52]

jjasilli , are you shure these L Pads are safe for an amps ? 

[jjasilli]

It would seem L-pads are as safe as a dummy load (except to the extent that a pot or rheostat is inherently less dependable than a fixed resistor).  This L-pad is a rheostat rated to dissipate 100W.  A 50W amp distorted puts out, say, 100W.  Full attenuated, that's too close for comfort.  But that's why there's a fan.  A car engine would fail if not for fan cooling.

A 20W amp puts out, say, 40W distorted -- well within the comfort zone of the 100W L-pad; fan not needed.  Almost fully attenuated, the L-pad didn't even get warm with all guitar and amp controls dimed using a PP el-84 amp @ about 15W, overdriven = 30W.  Sounded great too.  No loss of hi's; maybe a bit bright. (Perhaps the 4uF bypass cap could be a larger value - but good enough for a first try.)

My PA amp is 20W and used only clean.  Running 2 speaker cabs, if the L-pad fails in that setting, there's still a load on the amp through the bypass circuit within a 2:1 ratio.

Note this is a low cost project, especially with a salvaged enclosure.  The L-pad cost $14, plus a few bucks for the fan's 9VDC power jack.  Everything else was on hand.