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Old 29th July 2005, 10:16 AM   #1
Keeso is offline Keeso  United States
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Default Attenuation Box for Guitar Amp

I'd like some advice on this ... I know this is pretty basic stuff, but working on even the most basic schematics and DIY projects makes me have to think really hard!

A long while ago, I made a very simple load box to be used with a guitar amp (8-ohm load). Four 20W 8-ohm resistors wired in series-parallel ... a basic dummy load. Never used it, though. Then I decided to re-work it, to add a 50W mono (8-ohm) L-pad to be able to have variable attentuation for my 35W guitar amp (and with knob turned all the way, it would be a dummy load so I could use my slave out, which has a pot -- plus I have pads on a speaker sim box -- as a direct out for silent recording ... and I know the EQ for that might still have to fairly tweaked to sound OK) ... but my main concern was to make sure the circuitry was sound: no fry-mo, please, for the amp!

So then I saw the Bruce Collins schematic (I think the file was volreduc.gif) and modified that for this new idea (very, very basic, I know). I thought it might be nice to have a line out, but it's optional, really. But I hemmed and hawed about how to frequency tweak for best tone ... to minimize the need for EQ and cab sim, although I know that even using certain values of caps or inductors, there might still be a need to EQ-ing the post-attenuation signal; depending, of course, on the amount of attentuation.

So, anyway, here it is, attached: a very basic schematic (I decided to remove one of the 20W resistors due to space limitations) that I hope isn't complete hoo-hah. I think it would work but would like some advice before I actually start soldering. Is this new worked-up idea a "sound" schematic? (not really a pun, per se, just a question regarding whether I have totally missed the mark or not ... please bear with a newbie!)
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Old 29th July 2005, 10:31 AM   #2
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You really don't need the three 20 watt 8 ohm resistors.
The L pad will provide the load to your amplifier.
Everything else looks good.
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Old 29th July 2005, 11:53 AM   #3
Keeso is offline Keeso  United States
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Frank:

Thanks very much for your reply! Yeah, the 20W resistors are rather redundant, although I thought perhaps I could keep heat down by increasing the total wattage/load capacity to 110W; but then, I already bought a small cooling fan (to run with a 12VDC adapter) so I suppose this is overkill!

I could always just put the 50W L-pad in a small separate project box ... and use just that for variable attentuation. When I want just a dummy load, I could use the existing box with (4) 20W 8-ohm resistors.

OR .... I could do this (see new attachment) and lend it out to friends with much beefier amps.
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Old 2nd August 2005, 04:02 PM   #4
igge is offline igge  Sweden
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Is the L-pad you are planning on using the one from Monacor? And is your (and your friends beefier amps) by any chance tube amps? In that case you will burn the carbon resistor inside the L -pad, because it can not handle the high current (1 - 2.5 A) of a tube amp greater than 15 watts. I know, because I fried one 50 watt Monacor L-pad with my 50 watt JCM 800.

Not even the 100 watt L-pads from Monacor will do in the long run as load - dumps. They cannot handle the current either. They are for solidstate hifi amps.
The watt-ratings Monacor uses are not the "proper" RMS-watts.

If you want tips on load - dumps, check out this website:

Randall Aiken

click "tech info" in the menu, then "advanced" , then "dummy loads".

You are also welcome to check out these two schematics I have on my website:

mass

and:

attenuator

Good luck.
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Old 3rd August 2005, 06:36 AM   #5
Keeso is offline Keeso  United States
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igge:

I used the Speco model AT-50 for the L-pad. Currently I am constructing it based on the second schematic I attached (above), minus the inductors. With the (4) 20W 8-ohm resistors in addition to the Speco AT-50 I was hoping I could prevent a "toasty" experience. Would this (possible "toastiness) be dependent on how hard I push the amp into this attentuator?

My Mesa-Boogie Maverick is a pretty loud and peppy 35 watts so this is obviously a concern.

But in the meantime (and since I was rather burnt-out on looking for the inductors) I've been mulling over capacitor values for treble roll-off (very, very simple tone compensation). I thought of using a much lesser value for the main output (to speaker) since obviously my own Celestion V-30s will add their own coloration.

For the line-out circuit, I may want to use a higher value to cut even more treble since I may use that for (at least) headphone output (I realize it may not sound all that great for direct recording). The Maverick has a slave output with a pot and I purchased a Behringer G-100 cab sim (it has pads, so I'm fairly covered there) ... I could use that slave output with G-100 for recording if the DIY line-out in my box sounds crappy -- assuming of course that the whole thing doesn't turn to toast. So far, so good -- (checking continuity and resistances with my new multimeter). The last thing I need is a short to make the Maverick transformer go blammo. (Compulsive Soldering-Quality-Control Man) ...
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Old 3rd August 2005, 12:08 PM   #6
igge is offline igge  Sweden
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Interesting. Checked out the Speco site, appear to be slightly different from Monacor. Odd that they don't list how many volts their 50 watt attenuator can handle when they do it for the others I actually did a calculation for their 35watter & based on their 25 V rating it's good for 1.4 A. Which wouldn't do me any good since my AC-4/November amp pushes 1.5 A...

Anyway, as for tone quality these L-pads really eat up a lot of the mid/high frequency bite we all love with our guitar amps. So it does need some sort of treble boost (in my opinion anyway). Trebel rolloff? You feel your Boogie is too trebly, then?


Quote:
Would this (possible "toastiness) be dependent on how hard I push the amp into this attentuator?
Yep, it would. Yours is a push-pull AB amp right? I'm pretty safe now with my 15 watt AC-4/November (push-pull A) coupled with a 100 W L-pad, but it still does get very hot to the touch so I do not turn the L-pad lower (or higher, lol) than -16/-18 dB while keeping the mastervolume on 4 or 5.

Anyhow, good luck with getting your little homemade attenuator up and running.
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Old 3rd August 2005, 09:52 PM   #7
Keeso is offline Keeso  United States
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Wow, thanks for telling me that: I wasn't aware these L-pads eat up that much of the mid/high frequency "bite" -- I love bite! In fact, I just finished doing the "Sniper Mod" on my Boss GE-7 EQ pedal so I can do higher treble boosting (and level) much more cleanly. Yay! (it's very cool).

I was working up very basic low-pass circuits (figuring cap values for the 200-ohm output and the 8-ohm main-out to speaker ... thought I'd have two separate switchable selections on each output, for various amounts of treble cut). I realize this is something more important for the line-out (speakers having their own coloration/treble roll-off), but what the hey ... it's insurance against the "thin and fizzies" (see below).

Here's the site I was "playing with options." Cool little Java-driven calculator:

http://www.st-andrews.ac.uk/~www_pa/...wpass/lpf.html

But now ... I have to re-think the whole schmeer ... because, as I said, I love bite! My previous ideas were based on all this stuff I keep reading about high attenuation levels causing the "thin-and-fizzies." So before I run out to Radio Shack to buy up some switches and caps to start wiring up treble-cut/low-pass circuits, I may have to just finish the basics and see how it sounds (maybe I just wanted some extra soldering practice). I would have preferred to use .33 mH 150W inductors instead of this other method (as shown in the Bruce Collins schematic), but I'm impatient and thought it would take a while to get those in hand.

And as for the Speco, I think I'm OK ... I think it won't turn into "toast." We'll see!
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Old 3rd August 2005, 10:39 PM   #8
igge is offline igge  Sweden
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Glad to be of assistance
As for loosing the bite or attack, lots of people I know experience the same with the commercialy available products from THD, Marshall and the rest. The Weber Mass is probably the least criticised, as it's design is frequency compensated & uses a speaker as load.

Just do a listening test with the L-pad & amp at different levels
to see where you need to boost/cut frequencies.
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