Stereo Amp into one PA speaker? - AVS Forum
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post #1 of 9 Old 05-26-2013, 09:50 AM - Thread Starter
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Let me start by saying: Yes, I have read a couple other posts similar to my question on this board, but I still have questions.


I have a small stereo amplifier (2x120 watts) that does not have a mono switch. I also have a 15" PA speaker (8ohm) that can handle 400 watts max. Is there any way to put the full power of the amp (240 watts max) into my one speaker? I have read running two pairs of wires into the same speaker terminal can damage the amplifier by overheating. I know the ohms have something to do with this and I thought since my speaker is an 8ohm speaker it might not be stressful to the amp. Correct?

I want to do this for two reasons.
1. I need to run both the left and right channels audio (stereo recordings) from only 1 speaker.
2. The speaker will sound a bit 'better' and louder with more wattage going into it. Right now the bass is severely lacking.

or

Am I just screwed and I need to buy a bigger/better amp?

Thanks in Advance!

I don't have a "home theater"... I have a "home cinema". Nobody is singing and dancing in my house.
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post #2 of 9 Old 05-26-2013, 11:53 AM
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get a Y cable to take the stereo output into a single channel of the amp. Run mono.

Connecting both amps to one speaker is not significantly from simply wiring the two amps into each other with no speaker at all. Bad smells and dead amps ensue.

The extra power, if you used it, would add 3 dB (about one notch louder) so it's not that big a deal.

The speaker's power handling is, like most, probably a bit optimistic. Moreover, those are usually thermal (will it melt) numbers and the speaker is distorting and compressing (stops actually getting louder) somewhere below half of its thermal capability.

The speaker doesn't need the power. You understand more power means louder, it appears. That HAS TO MEAN that if the speaker is not ALWAYS AS LOUD AS IT CAN GET, sometimes it's receiving less than full power. In fact, even outdoors, a couple of watts sounds real loud if you're close to the speaker. If you've never destroyed a speaker by turning down the volume, you have about all the proof you need that the pspeaker doesn't need full power all the time.

If the one speaker isn't loud enough, you'd get a lot mlre kick (actual SPL increase) by adding another speaker than by adding power . . .
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post #3 of 9 Old 05-26-2013, 02:07 PM
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Quote:
Originally Posted by JHAz View Post

get a Y cable to take the stereo output into a single channel of the amp. Run mono.

. .
That is NOT a good idea. Yes it will "work" for a little while.

The problem with using a simple Y on the outputs is 2 fold. But them come from the same problem.

The output of each channel "sees" the impedance of the other channel. Modern electronics is designed to operate with at least a 10:1 ratio-because it is voltage that we are transferring-not power (like the old days).

So when one signal sees a really low impedance-distortion is usually the end result.

The other problem is because the output devices of each channel are driving impedances well below what they are "looking for", this can result in damaged outputs and a trip to the shop.

YES in some cases you "may" be fine-due tot he design of the particular device-but unless you KNOW that they can drive those low impedances-it is best not ot.

The solution is simple however. Just insert a 1KΩ resistor in series with each output BEFORE you parallel them together.

Yes you will lose a little bit of signal level-but the sound will be better and the outputs will be nice and happy.

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post #4 of 9 Old 05-27-2013, 07:37 AM
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Hmmm...

  1. The theoretical output from bridging the amp would be 480 W, not 240 W, as the voltage swing is doubled and thus the power is quadrupled (power goes as the square of voltage). It is, however, unlikely your amp's power supply and output stages will support the current level (amperes) needed.
  2. Bridging an amplifier requires polarity inversion of one channel. Amps built for this usually have a mono switch. You could build the circuit but I would respectively suggest refraining if you do not have the expertise to both build the circuit and to measure the amp's output to ensure it is still stable.
  3. The effective load a bridged amp sees is halved compared to a single amplifier channel. That is, an 8-ohm speaker "looks like" 4 ohms to a bridged amp. If the speaker dips lower in impedance, a bridged amp may have trouble driving them or even be unstable.
  4. I strongly suspect whatever bass problem you have is not the amp. twice the power is only 3 dB, hardly noticeable, and 4x power is still only 6 dB. It takes about 10 dB to sound twice as load, and that takes 10x the power. Many factors other than amplifier power affect frequency response.
  5. You could set your source to mono and use one output with a Y into the input of the amp to drive two speakers. Or use a passive summing circuit as suggested above. I would probably use 10k ohm resistors in the passive summer instead of 1k as that is a more typical load for a preamp output. 1k seems kind of low to me. YMMV.

Personally I would add a second speaker, and work on figuring out why the bass is lacking. It is probably the room or lack of LF response from the speaker you have. A PA speaker designed tor vocal reinforcement might not have much LF response (by design). Or if you must have an amp pick up an inexpensive pro amp, either mono or with a switch to bridge it.

FWIWFM - Don

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post #5 of 9 Old 05-28-2013, 07:22 AM
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Quote:
The output of each channel "sees" the impedance of the other channel. Modern electronics is designed to operate with at least a 10:1 ratio-because it is voltage that we are transferring-not power (like the old days).

Input impedance is typically 50K, output impedance under 100 Ohms...there is no problem here.
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post #6 of 9 Old 05-28-2013, 07:27 AM
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Just use one channel of the amplifier, and do not connect the other.

The chances are that you will have more power than you need.
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post #7 of 9 Old 05-28-2013, 11:05 AM
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Use only one channel output of the amp but it is best to use an in-series isolation resistor in BOTH channels of the output device (CD player etc) as the paral;lel impedance of the line outs will dampen the dynamic range of them both. Typically, a 10k resistor is used in series with the signal conductor of BOTH channels in a passive mixer for this purpose.
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post #8 of 9 Old 05-28-2013, 11:15 AM
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I assume the amp is a HiFi solid state type and therefore the second channel can just be ignored, that is left completely unconnected. But if this is a tube amp or an old SS amp with output transformers, you must have a load on the unused output even if the input is unconnected. Otherwise the unused channel could be destroyed. Doesn't have to be a full 8 ohm load, a small 1 watt 100ohm resistor is fine.

It has to do with the near infinite primary impedance of a transformer with an unloaded secondary. In a tube amp the AC plate voltage could rise to several kilovolts if any signal is applied to the amp including noise and crosstalk from the other channel.
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post #9 of 9 Old 06-01-2013, 07:59 PM
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Quote:
Originally Posted by SAM64 View Post

Input impedance is typically 50K, output impedance under 100 Ohms...there is no problem here.
That is EXACTLY the problem. The output impedance is "looking for a high impedance.

But when you short them together the impedance "seen" by each channel is the OUTPUT impedance of that channel. NOT the high input impedance. When you parallel 50K with 100 ohms-the 50K just doesn't matter any more. The dominating impedance is the 100 ohms (give or take.)

And when you drive a output into a very low impedance-you get distortion-and possible output device damage-over time.

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