Attenuation Question

[agreatheight]

So I have a bit of an odd question. Let's say I want to reduce the volume output of my amp. I have a 50 watt head and it's loud when dimed out. So I look into standard attenuators, but read that they can mess with the sound. Is there a way to run a device to lower the wattage / volume that is NOT in-line with your speaker? Shouldn't there be a way to send out 4 ohms from the head, but split it so that half goes to the cab I have at 8 ohms and half goes to a dummy load also at 8? That would drop the wattage to the cab to 25, with the remaining 25 going to the dummy device, yes? Am I insane? Missing something obvious? Would this even help?

Thanks in advance!

[John Phillips]

Insane - no. Missing something obvious - yes, you'll kick yourself when you hear this :

Get a standard attenuator, set it to maximum attenuation or the 'load' setting if it has one, and plug it into the other speaker output. Set the amp to half the impedance of whichever (attenuator or speaker) is the lower, and that's it . (Don't worry about not having a speaker connected if the attenuator doesn't have a 'load' setting - at the quietest setting, only around 1% of the output would be going to it anyway, so it makes no practical difference.)

If the attenuator is the same impedance as the speaker, half the power goes to each, giving a 3dB reduction. If it's not, the power distribution will be off a bit, but you'll still get close to the same volume reduction - somewhere between 2 and 5dB usually.

But this still won't not affect the tone, at least a little - the attenuator won't have exactly the same frequency-dependent impedance as the real speaker, so the power balance will be uneven to some extent depending on frequency. But generally it does give less alteration than using an attenuator 'normally'.


There are also a few other things you can use as dummy loads if you know what you're doing, which you probably wouldn't believe if I told you .

[agreatheight]

Thanks John, that all made sense! I appreciate it greatly!

[justonwo]

They don't really have much affect on tone even when they're inline as long as you don't crank the attenuation. I use a Hot Plate on 8 dB with minimal effect on my Super Lead's tone. I would buy one used and try it. If you like it great. If not, ditch it in the classifieds.

[stratopastor]

Originally Posted by John Phillips

There are also a few other things you can use as dummy loads if you know what you're doing, which you probably wouldn't believe if I told you .

Oh do tell. Do any of them involve hamster wheels?

[Steve Foley]

Hmm....this is an interesting concept.
Let me be sure I understand how you're describing this, John Phillips:
Let's say I have an 8-0hm Hotplate, and and 8-ohm speaker. I would plug the speaker into the 4-ohm output on the amp head, and the Hotplate "amp" jack into the 8-ohm speaker jack on the head, with no speaker connected to it? And the Hotplate would be set to the 16dB setting, cranked down to a low setting....and this would cause the head to "see" two speaker "cabs", and the volume output would be split between the actual cab and the 8-ohm speaker, effectively reducing the volume in "half" (nominally, of course) between the actual speaker, and the Hotplate.
I've just never considered this concept, before, and of course will have to go home and try it.... I just want to be sure I understand what you're describing before I blow something up.
Does it sound like I'm describing it similarly to what you are? I've re-read the post a few times, but I'm not sure we're talking about the same setup.
El Steve-0

[John Phillips]

Originally Posted by Steve Foley

Hmm....this is an interesting concept.
Let me be sure I understand how you're describing this, John Phillips:
Let's say I have an 8-0hm Hotplate, and and 8-ohm speaker. I would plug the speaker into the 4-ohm output on the amp head, and the Hotplate "amp" jack into the 8-ohm speaker jack on the head, with no speaker connected to it?

No - both the speaker and the Hotplate need to be connected to the 4-ohm jacks. If the amp has only one 4-ohm jack, you need a splitter cable.

And the Hotplate would be set to the 16dB setting, cranked down to a low setting.

The Hotplate does have a Load setting, which you should use - not all attenuators do though, in which case use the lowest (quietest) setting.

...and this would cause the head to "see" two speaker "cabs", and the volume output would be split between the actual cab and the 8-ohm speaker, effectively reducing the volume in "half" (nominally, of course) between the actual speaker, and the Hotplate.

Exactly. Basically you have two 'cabs' connected, but one of them is a dummy and absorbs half the power of the amp without making any noise.

Originally Posted by stratopastor

Oh do tell. Do any of them involve hamster wheels?

Not quite...

Any domestic heating appliance of the right resistance will work... like a fan heater, or a kettle. You just need to pick the correct power rating relative to the supply voltage to be roughly equivalent to a nominal speaker load.

For example, if you're in the UK with a 240V supply and you have a 2KW appliance, its resistance is 29 ohms (resistance = voltage-squared divided by power). This is a good substitute for a 16-ohm speaker, since the average impedance of a speaker over the full audio range is usually around double the nominal value.

Or, if you're in the US the same 2KW appliance would have a resistance of about 7 ohms, which is perfect to simulate a 4-ohm speaker.

(Heaters with elements which are designed to run red-hot may have a slightly lower 'cold' resistance, since the resistance of wire increases as it heats up, but it should still be in the right ballpark.)

By a nice coincidence the typical resistances of American appliances - because of the lower supply voltage - are close to those required for typical American (Fender etc) amps, and those of typical British appliances are closer to those needed for British amps (Marshall etc)...

All you need to do is find an old thing like this (junk sales are a good bet, you don't want anything sophisticated and modern with electronic controls or variable power levels etc - just plain old electrical), make sure it works and its resistance is correct, cut off the power plug and fit a 1/4" plug instead. It might be a good idea to bypass or tape down any switches to make sure it doesn't accidentally get turned off while you're using it, but if you're only using it as a parallel load (rather than a sole dummy load) it doesn't matter too much.

Don't expect it to heat the room or make the coffee while you're using it though - it probably won't even get very hot. You shouldn't even need to put water in a kettle if you're using one, the elements are robust enough to stand being run in the air at that sort of power input.

This may sound like a joke, or unbelievable, but to an amp impedance is impedance, and it doesn't matter whether it's a speaker, a 'proper' dummy load or anything else of about the right electrical value.

[jdzialak]

Originally Posted by John Phillips

No - both the speaker and the Hotplate need to be connected to the 4-ohm jacks. If the amp has only one 4-ohm jack, you need a splitter cable.

The Hotplate does have a Load setting, which you should use - not all attenuators do though, in which case use the lowest (quietest) setting.

What about an amp (Marshall) with 2 jacks and an ohm selector?
16 ohm cab and 16 ohm HP with the impedence selector on 8?
I also have a UA that acts as a load when turned all the way down and no speaker is connected.

Cool concept that I'd heard of but never understood until I read your detailed description, John.
Thanks,

Joe

[Melodic Dreamer]

I have a THD Flexi 50/2x12 cab/Hotplate and I have know clue of what your talking about. lol
I tried reading along, but don't understand a thing.
Would like to though.

[Steve Foley]

John Phillips:
You never fail to amaze me, with your breadth of knowledge! Is there ANYTHING you don't already have a handle on?!?!?! You 'splain "complex" things thoroughly and clearly, in a way that even a guitar player can understand, and that's really saying something.

Thanks!

El Steve-0

[JimH]

John - Great stuff, love the kettle thing. Hilarious.

...What if the amp's not got an impedance switch?
My 20W plexi head is always 8 ohms. The way I see it I have to use a 16ohm cab and set my power brake to 16 ohms to get the correct 8 ohm match - correct?

[Hand of Doom]

Originally Posted by JimH

John - Great stuff, love the kettle thing. Hilarious.

...What if the amp's not got an impedance switch?
My 20W plexi head is always 8 ohms. The way I see it I have to use a 16ohm cab and set my power brake to 16 ohms to get the correct 8 ohm match - correct?

I am in the same boat, my amp is fixed at 16 ohms.

Am I getting this right? I ran one side to an Ultimate Attenuator and the other to the cab. The cab is 16 ohms I should wire it to 8 I know but I just wanted to try this. If I did it right, I like it much better than the UA. I think the tone was better using this method over the UA. The over all vol didn’t decrease as much as I had anticipated, but it did allow me to hit the amp harder than I normally would.

[John Phillips]

Originally Posted by JimH

John - Great stuff, love the kettle thing. Hilarious.

...What if the amp's not got an impedance switch?
My 20W plexi head is always 8 ohms. The way I see it I have to use a 16ohm cab and set my power brake to 16 ohms to get the correct 8 ohm match - correct?

Yes.

The kettle thing was a rumor I first heard back in the mid 80s, it may have come from an old pic showing someone (I think it was Jeff Beck, but I could be wrong) in the studio with a kettle on top of his amp! I have no idea whether it was being used for attenuation or just making the tea , but that's the way rumors like this often start... some of which are really wrong. But with this one, I thought about it and did a quick calculation, and realised it would work perfectly .

Originally Posted by sangre azul

I am in the same boat, my amp is fixed at 16 ohms.

Am I getting this right? I ran one side to an Ultimate Attenuator and the other to the cab. The cab is 16 ohms I should wire it to 8 I know but I just wanted to try this.

It should be OK. You're getting a mismatch by doing that but within the normal safe range - especially since by being a fixed resistance (roughly equivalent to a 16-ohm speaker), the UA will smooth out the frequency/impedance curve quite a lot. You won't get much volume reduction, probably only about 4dB (roughly 2-3dB from splitting the power not quite evenly, and another 1-2dB from the mismatch). Don't rewire the cab, you'll make it worse - both the mismatch and the amount of useful volume reduction.

[jlh]

The Ultimate Attenuator is the deal, get one, a double volume rig, youl love it