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Author Topic: can i swap a resistor for a speaker in a cabinet?  (Read 16391 times)

rjd2

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can i swap a resistor for a speaker in a cabinet?
« on: October 19, 2006, 09:43:53 pm »

i was considering bypassing one speaker in an AC30 for the purpose of making it less loud. amp runs at 16 ohms. 2 8 ohm speakers. can i "swap" one speaker for a resistor of the proper value? or is this a generally bad idea? thanks.
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rj krohn

Sin x/x

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #1 on: October 19, 2006, 10:13:28 pm »

It should work, but you will get just 6dB of reduction.

I use a bridged -T attenuator.

It does change the tone of the amp, but not much.

I've heard great things about Weber, but have no experience with them.


Here's a link:http://amps.zugster.net/articles/attenuation
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Tom C

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #2 on: October 20, 2006, 08:36:00 am »

rjd2 wrote on Fri, 20 October 2006 03:43

i was considering bypassing one speaker in an AC30 for the purpose of making it less loud. amp runs at 16 ohms. 2 8 ohm speakers. can i "swap" one speaker for a resistor of the proper value? or is this a generally bad idea? thanks.



Why don't you use a (high power) potentiometer to control the
loudness of both?
You'd have much more control this way.

[Edit because of speeeliing]
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Tom

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Andy Peters

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #3 on: October 20, 2006, 04:36:32 pm »

rjd2 wrote on Thu, 19 October 2006 18:43

i was considering bypassing one speaker in an AC30 for the purpose of making it less loud. amp runs at 16 ohms. 2 8 ohm speakers. can i "swap" one speaker for a resistor of the proper value? or is this a generally bad idea? thanks.



You'd be better off getting an AC15.

-a
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danlavry

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #4 on: October 23, 2006, 04:15:14 pm »

rjd2 wrote on Fri, 20 October 2006 02:43

i was considering bypassing one speaker in an AC30 for the purpose of making it less loud. amp runs at 16 ohms. 2 8 ohm speakers. can i "swap" one speaker for a resistor of the proper value? or is this a generally bad idea? thanks.


I would not rush to assuming that it would work. Speakers are NOT resistors, and the design of a speaker (a very complex device) is not optimized for such a practice. Typically, a driver is low impedance source, and most often there is some series resistance paralleled with an inductor in series, for a good reason. That "network" is near short at low frequencies.

But if you insist doing is with a series resistor, it will have to be a very high power device, rather large, and it will run hot when the signal is high. So be careful not to get burned.

And given that many high power resistors are "wire wound", many such resistors may have too much inductance at the higher audio frequencies, which may further muck with the response...
Will it? At say 10KHz, it only takes 16uH inductance to have 1 Ohm or reactance. I do not know of the top of my head how much inductance to expect, but I would be carfull to be sure that inductance is not a problem...

Some of us pay a lot of attention to matching source and load (amp and speaker in this case). Others view all speakers as being "about the same load". But replacing 2 speakers with a resistor and a speaker? I am not sure it is such a great idea....

Regards
http://www.lavryengineering.com
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Transcending Music

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #5 on: October 26, 2006, 12:43:52 pm »

rjd2,

did you consider a power soak or "hot plate" or even the attentuator suggested by sin x/x ? they work fairly well and keep the loads safe.
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danlavry

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #6 on: October 26, 2006, 12:58:45 pm »

Transcending Music wrote on Thu, 26 October 2006 17:43

rjd2,

did you consider a power soak or "hot plate" or even the attentuator suggested by sin x/x ? they work fairly well and keep the loads safe.


That would answer only one aspect of the problem, the heat. It still falls short in the area of impeadance matching, and potential inductance issues when using wire wound power resistor.

Regards
Dan Lavry
http://www.lavryengineering.com
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Teddy G.

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #7 on: October 27, 2006, 12:22:11 am »

I must stumble in, here - these kinds of posts and replies hurt my head. I just can't NOT look at them - sorry.

I know I'm not getting it, but, I guess that's what these forums are for - to learn.

Why don't you just turn the thing down? Tone not the same?

Have you tried the little foam widgets you stick in your ears?

Are you using it mic'ed in the studio and can't get the mic far enough away? Can't turn the pre down far enough?

So loud at good tone level it interferes with other instruments in the room? Neighbors can't hear their own lawnmowers running?

Seems like a bit of a pricey gadget to tear into to mess with, anyway, even if it would "work"?

If a non-inductive resistor is the largest stumbling block(I know, it's not, but in the interest of more goofy thoughts, maybe useable in the future...), no reason to look for a single, perfect, resistor is there? One should, with a bit of head scratching and paper and pencil be able to figure out a non-inductive resistor network(Several/many resistors correctly wired together) even if all of the 2 watt variety(1/4 watt if you enjoy soldering.). Hams do this quite often with success, for power levels of hundreds of watts - even drowning the poor little resistors in cans of oil, to absorb the heat. Messy, but....?

Sometimes our questions are a bit goofy in the first place(That's fine, I ask LOTS of goofy ones!) -- and while it is interesting to see everyone jumping in with highly technical talk of the if's and's and but's, sometimes the simple answer is "best", I think.

I must go with "-a" --- go to the store and get the smaller amp.

Move on......


TG

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Transcending Music

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #8 on: October 27, 2006, 05:56:18 pm »

Hi Dan, correct me if I'm mistaken...

The THD hot plate for example dissipates the heat but are also manufactured to match impedance.
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danlavry

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #9 on: October 27, 2006, 07:47:07 pm »

Transcending Music wrote on Fri, 27 October 2006 22:56

Hi Dan, correct me if I'm mistaken...

The THD hot plate for example dissipates the heat but are also manufactured to match impedance.



Hi,

I do not know that much about it. But looking at their site it says:

"How do I use it?
The THD Hot Plate
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maxdimario

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #10 on: October 28, 2006, 07:15:28 am »

leave it as it is, it won't get much quieter and you won't have the phase cancellation and distortion which comes from running two speakers instead of one (side-by-side).

both acoustically and electronically.
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GoobAudio

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #11 on: October 30, 2006, 05:43:49 pm »

[quote title=danlavry wrote on Fri, 27 October 2006 21:47]
Transcending Music wrote on Fri, 27 October 2006 22:56



Built-in Noise Reduction
The THD Hot Plate
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Spock

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #12 on: October 30, 2006, 10:25:50 pm »

It's been a very long time, but I know it can be done without added too much inductance.  A 100 to 200 watt resistive dummy load for an RF transmitter for 30Mhz or lower would be put in a paint can with mineral oil to help with the heat transfer.
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danlavry

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #13 on: October 31, 2006, 01:00:12 pm »

[quote title=GoobAudio wrote on Mon, 30 October 2006 22:43]
danlavry wrote on Fri, 27 October 2006 21:47

Transcending Music wrote on Fri, 27 October 2006 22:56



Built-in Noise Reduction
The THD Hot Plate
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danlavry

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Re: can i swap a resistor for a speaker in a cabinet?
« Reply #14 on: November 01, 2006, 03:07:20 pm »

More about wire wound power resistors:

The type of wire itself is much more resistive then say copper, and that is how one gets to build resistance. Say one wants to have 8 Ohms, and the resistive wire used is the type of 4 Ohm per foot. That would call for 2 feet length for 8 Ohms.

But, given that we would not wish to just have 2 feet of wire, it is often being wound around a cylinder made of an insulating material. The standard way to wing is one layer (single row), and that works great for some applications such as DC power, or 60Hz AC line and more.  

The inductance of wire wound resistors in a single row depends on the a "form factor" (it has to do with ratio of length to diameter), the inductance is proportional to the diameter of the windings and most of all on the number of turns (proportional to number of turns square).

Say we have 1 inch diameter, and 5 inch winding length.  
That would yield form factor F=.003.
Say we have 20 turns (windings), then the inductance will be around .003*1*30^2 = 2.7uH

That would yield .339 Ohms of reactance at 20KHz. Such reactance against say 8 Ohms resistive load resistance means a loss of nearly .01dB. So the example above shows a resistance construction that has little effect on audio.

But say we have 2 inch diameter and 5 inch length. That would yield a form factor of around .006.
Say we have 40 turns (windings), then the inductance will be around .006*2*40^2 = 19.2uH
 
That would yield 2.4 Ohms of reactance at 20KHz. Such reactance against say 8 Ohms resistive load resistance means a loss of nearly .38dB. The loss at 10KHz is around .1dB. That may be  audible.

As a rule, the large single winding power resistor can work, but for lowering inductance, one needs to watch for a few factors:

1. Stay with single winding. Multiple winding may increase inductance exponentially.

2. Look for a wire wound resistor with fewer winding.

3. Go for a long resistor, with as small diameter as you can find

Also, consider paralleling wire wound resistors. Say you parallel 2 resistors of 16 Ohms each to get 8 Ohms. The inductance will be also cut by half. The purist may wish to mount the resistors in such a way that the winding directions of the 2 resistors is opposing each other. It should not matter here, but it is a good thing to keep in mind for higher frequency signal applications.

I am still not convinced that a series resistor to a speaker is a good idea. This post is a follow up of the inductance comment of wire wound resistor inductance.

Regards
Dan Lavry
http://www.lavryenginering.com
 
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