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Resistance reducing device


arthurhenry
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Since many of us use only one speaker cab for portability, but amps put out more power if connected to two, surely there exists some kind of device into which one could plug a second speaker cable and "trick" the amp into "thinking" that it's connected to another speaker cab. A stompbox sized unit with some circuitry inside is what I imagine. I've never heard of such a thing, have you? Perhaps there's a good reason why it wouldn't work.

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7 minutes ago, Tim1 said:

I think the problem with this idea is that the device would have to be some sort of (large)  resistor which would then absorb its share of any extra wattage you gained, hence making no significant difference to actual volume.

 

2 minutes ago, barkin said:

My amp does 500W into a 4 Ohm load. 350W for an 8 Ohm load. Adding an 8 Ohm dummy load would mean 250W into an 8 Ohm speaker (<350W) and 250W into the dummy load, serving no useful purpose. Or something.

My simplistic way of thinking is that if the amp was plugged into an 8ohm speaker and into a magic 8ohm box, it would think it was going into 4ohms and be louder. 

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2 hours ago, arthurhenry said:

My simplistic way of thinking is that if the amp was plugged into an 8ohm speaker and into a magic 8ohm box, it would think it was going into 4ohms and be louder. 

It would be going into 4 ohms. Half the power would drive the speaker, half would drive the resistor, and when you factor in that an amp doesn't develop twice its maximum power into a halved impedance load the result is that it won't be as loud.

There is an impedance reducing device that will give you more output: a second speaker.

Edited by Bill Fitzmaurice
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The short answer is an emphatic "no".

Slightly longer answer:

The device you're imagining does exist. It's generally referred to as a "dummy load", and is essentially a big chunky resistor. Attached in parallel with the existing speaker, it will reduce the impedance seen by the amplifier's output, and thereby cause the amp to produce its full rated output power, as you suggest.

However, this won't produce your desired result (more volume from the speaker), as the additional output from the amp will simply be dissipated (as heat) by the dummy load itself.

To get more volume from the existing speaker, you would need to reduce the impedance of the driver itself, which isn't feasible in any practical way.

Edited by paulbuzz
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6 minutes ago, paulbuzz said:

To get more volume from the existing speaker, you would need to reduce the impedance of the driver itself, which isn't feasible in any practical way.

That probably won't have any effect either. What limits a speaker's output is the cone excursion limit, xmax. Most drivers will reach xmax with less than half their rated power, so unless your speaker is rated for at least twice what your amp is chances are a lower impedance version would go no louder.

What would happen if you connected a extra speaker but with no cone attached I wonder?

The same as with a resistor.

Edited by Bill Fitzmaurice
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2 minutes ago, stingrayPete1977 said:

What would happen if you connected a extra speaker but with no cone attached I wonder?

Same as answered above. Half the power would drive the speaker, half would drive the speaker without a cone. Remove the speaker cone and it is just a voice coil or resistor.

 

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3 minutes ago, Bill Fitzmaurice said:

That probably won't have any effect either. What limits a speaker's output is the cone excursion limit, xmax. Most drivers will reach xmax with less than half their rated power, so unless your speaker is rated for at least twice what your amp is chances are a lower impedance version would go no louder.

Fair point! :) I was just trying to envisage a setup that came as close as possible to fulfilling the OP's original scenario. Not that I'm proposing to create a driver with switchable alternative voice coils!

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I'm just thinking without the cone the motor would move freely and produce no sound but would have the variable characteristics of a speaker rather than a fixed value resistor. 

 

I've never had any amp flat out with a single cab anyway, either I had enough volume or the single cab farted out before, I'd imagine that is the case for most of us? 

Edited by stingrayPete1977
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32 minutes ago, stingrayPete1977 said:

What would happen if you connected a extra speaker but with no cone attached I wonder?

These exist (as an alternative to resistor-based dummy loads) :

https://www.tedweber.com/gadgets/attenuators/speaker-motor-models

As Bill and Bass Bunny said though, this doesn't come any closer to acheiving what the OP was asking.

Edited by paulbuzz
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What the OP wants is the Holy Grail of Bassplayers, a small speaker that goes low and loud. The only obstacle to reaching that goal is Hoffman's Iron Law. If you're a bassplayer and don't know what that is look it up, because you should. There are some ways to get around it to some extent, like extra long xmax drivers, but you can only go so small with a cabinet no matter what the driver. Bottom line, if you want a small easily portable rig that still covers a large room without PA support...take up guitar.  :dash1:

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10 hours ago, paulbuzz said:

The short answer is an emphatic "no".

Slightly longer answer:

The device you're imagining does exist. It's generally referred to as a "dummy load", and is essentially a big chunky resistor. Attached in parallel with the existing speaker, it will reduce the impedance seen by the amplifier's output, and thereby cause the amp to produce its full rated output power, as you suggest.

However, this won't produce your desired result (more volume from the speaker), as the additional output from the amp will simply be dissipated (as heat) by the dummy load itself.

To get more volume from the existing speaker, you would need to reduce the impedance of the driver itself, which isn't feasible in any practical way.

Excellent explanation, thanks.

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There is such a device, in theory. It's called a transformer and there is one in every valve amp to match the impedance of the speaker to the amp. An audio transformer that handles 500W would be very heavy and would cost more than a second speaker especially as it's not a sensible solution and you'd have to get it custom wound.

The other issue is that most of the 'extra oomf' you get from a second speaker is down to the extra cone area and raising the speaker closer to your ears. Just pushing one little cone harder won't give you a lot extra even if it is a long excursion driver and can handle that power

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14 hours ago, arthurhenry said:

Since many of us use only one speaker cab for portability, but amps put out more power if connected to two, surely there exists some kind of device into which one could plug a second speaker cable and "trick" the amp into "thinking" that it's connected to another speaker cab. A stompbox sized unit with some circuitry inside is what I imagine. I've never heard of such a thing, have you? Perhaps there's a good reason why it wouldn't work.

Even if your dummy load did work as you wanted, there is more to consider...

1. Nearly all of them are designed to be used with guitar amps and are rated accordingly at 100W maximum. It's not going to very happy for long attached to a 300W + bass amp.

2. They are neither small or light. I own a Marshall PowerBreak for use with my 50W guitar amp. It's a chunky metal box 220 x 120 x 170 mm plus handles and feet and weighs in at just under 5kg! (A Barefaced One 10 is only 7kg). It also has a substantial fan in it to dissipate the heat generated by those resistors whose intake and output vents need to be free from obstructions, and which comes on as soon as you start driving it reasonably hard with a guitar amp. It's not the sort of thing that would fit on your pedal board - it's actually quite a bit bigger than most class D bass amps!

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You could add the dummy load box, the amp would then give you even less output (as with one 8Ohm speaker it will put out a bit more than half the power, with one speaker and a dummy load it would split the ouptput in two, given the same resistance) but there is a plus with doing this, you could have the dummy load designed as a base plate so you could stand on it during gigs and have your feet heated by the heat dissipation. Great for those outdoor winter gigs ;) 

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