L-pad selection for amp's tweeter?

[mexicanyella]

I'm refining my long-titled post below into a new thread, after a thrilling and successful experiment.

I removed the tweeter from my Fender Acoustasonic Jr. and tried it as a slave, fed from my Peavey Audition 20's line out, and WOW!!! Exactly what I wanted! Using just the two 8" drivers, it reproduces everything good about the Peavey's gritty almost-clean sound, and it can get much louder.

But I may want to use it for an actual acoustic guitar or full-range portable PA again, so I think I want to install an L-pad in the back so I can adjust the tweeter level down to 0% for slave duty, or varying levels up to 100% for other duties.

Acoustasonic schematic:
http://support.fender.com/schematics/guitar_amplifiers/Acoustasonic_Jr_schematic.pdf

In the lower left of the diagram, more or less, you can see where the "HF horn" taps into the L speaker output with a 22 ohm resistor, a 47 ohm resistor and a .1 mf capacitor. I think I could drill a hole in the back of the chassis and install an L-pad and tap it into this HF horn feed...right?

If so, where in the HF horn's wiring do I insert the L-pad, in relation to the resistors, capacitor and the horn itself?

If the left side speaker is fed by a 40-watt power amp, how much of the wattage am I attenuating to the horn; in other words, what wattage rating of L-pad do I need to buy? Initial searching has shown me 8-ohm L-pads that look like big potentiometers an are rated at 15, 50 and 100 watts.

Thanks in advance.

[mexicanyella]

Nobody's taking a crack at this one, huh?

I'm asking because I don't understand the effect on output wattage the two resistors and the capacitor will have. I'm assuming they reduce the 40 watts available from the left-side power amp to something less before it hits the tweeter, along with acting as a high-pass filter, but I don't know this for sure. So I was wondering what wattage of L-pad to select.

And if anyone had a fairly short explanation of how the cap and two resistors filter out frequencies/affect the wattage, I'd be interested to learn that info.

Anyone?

[J M Fahey]

Nobody's answering because you did not ask the proper question.
1) you misread that "capacitor network" .
R120 and C108 are not related to the tweeter at all, they just go from amp out to ground, it's a stabilizing Zobel network.
You'll find a similar one in the other amp channel: R220/C208
2) your tweeter is a very simple one, looks like it's a Piezo, which does not need a crossover because it behaves like a capacitor.
R121 is in series with it simply to make sure that its impedance never falls below 47 ohms, which could happen if the amplifier oscillates (not at normal audio frequencies).
3) all LPads you'll find are meant for regular (magnetic , voice coil) Tweeters ; Piezos are too humble (read cheap) for that.
Reconnect the Piezo where it belongs and add a switch in series with it, to turn it on and off, as needed.

[mexicanyella]

A-ha, I see R220 and C208 now that you point them out. I do not know about Zobel networks or what they do, but had I noticed that the resistor and cap were present on both sides, I might've at least figured out that they weren't part of the tweeter circuit. Thank you for pointing that out.

I would still like to have the tweeter's output be variable from 0% to 100%; can an L-pad work with a piezo tweeter, even if it's "overkill?" I get that the switch would be the simple solution, but I wonder if I'd find use for the amp in certain settings with the tweeter "dimmed?" Is there another way to do this?

[J M Fahey]

Can you get, say, a 100 ohm wire wound potentiometer ?
May be up to 200 or 250 ohms.
5 to 10W dissipation?
If not, I can suggest a 3 way switch with Full/Off/Low settings, better than nothing.

[mexicanyella]

Thank you for the suggestions. I will look into that and report back when I have progress.

[phatt]

Hi Mex,
Go find some HiFi Crossover setups ,, that might give clues.

Yes Lpad is what you need and not that hard to wire up.
Depending on the Amp you need a 50~100 watt L-Pad.
If in the land of Oz we have them through a place called Jaycar.

As JMF noted you may not need a crossover *IF* it is a peizo horn.

try these for clues;
http://www.colomar.com/Shavano/lpad.html

And good pics here;
http://www.audiokarma.org/forums/showthread.php?t=328900

please NOTE these are a little bit different to normal pots,,, one centre wiper with TWO separate resistive pads.
Phil.

[mexicanyella]

How about this idea: a single-pole/three-position rotary switch, like this one:

http://search.digikey.com/us/en/products/HS13Y/360-2559-ND/1046760

installed through the Acoustasonic's back panel, with a pointer knob or something on it, and wired so one position passes signal straight to the tweeter as per stock configuration...then another position incorporates a capacitor (and resistor? not clear on how this works yet) to "dim" the tweeter output, and the remaining terminal isn't wired to anything, so there's an "off" position too?


If that would work I could have one dim position, or more than one progressively more dimmed positions...right?

[J M Fahey]

http://search.digikey.com/scripts/dksearch/dksus.dll

[mexicanyella]

JMF...are you saying "nope, keep looking?" Or were you attempting to link a specific item and the link didn't work out?

Thinking about this further, it seems that I might need a dual-pole three-position switch if I want on/dim/off; the switch I mentioned above might have been useful if I wanted to select between a couple of tweeters or something but not varying paths to the same tweeter, some attenuated, some not, etc.

[phatt]

Buy an L pad,,, otherwise you will spend forever trying different attenuation's before it sounds right.
The Lpad you just progressively dial in the amount from none to full power to tweeter.
Done.
Phil.

[J M Fahey]

The link showed 4 different answers, must have been chopped in half.
Here you have the first three.
1) the "raw" rheostat/potentiometer
http://search.digikey.com/us/en/products/026T419S101A1A1/026T419S101A1A1-ND/468347
2) same "with mounting hardware" (L bracket/small panel/knob?)
http://search.digikey.com/us/en/products/026TB32R101B1A1/CT2154-ND/203775
3) same with switch. Not bad if when set to minimum you want to fully remove it from the cabinet.

You will also need to order a 1uF bipolar capacitor to be used in series with the pot, so as stopping frequencies below 1600Hz, which the Tweeter won't reproduce anyway
http://search.digikey.com/us/en/products/ECE-A1HN010U/P1196-ND/227637

Dear Phatt: Lpads are needed in conventional crossovers so they see always the same impedance at different attenuation levels. Usually 8 ohms.
This is a Piezo Tweeter where the impedance is around 1000 ohms , so a 100 ohm pot is the right attenuator.
Plus the capacitor so the pot does not get unnecessary mids and lows.
Good luck.

PD: and a cheap SPDT switch with center OFF can and does provide 3 levels: High/Off/Low .
SWR (among others) uses that on their Bass amps.
Too busy now to draw it, will post it later.

I guess they are the same but the 2nd also comes with mounting hardware (an L bracket or something?)

[phatt]

Ding!!! Arrh I see da light,,,,,,, Yes of course Peizo,, no voice coil.
Don't mind me I'll go to bed and recharge the brain. :duh
Phil.