I know a mixer is required, but I just love thinking about that scenario.
What happens when you connect two audio outputs without using a mixer?
Anybody have a graph showing all three, both audio outputs and the point where they are connected? Or is that even possible since it becomes all one point? It's just weird. Fun weird IMO.
What happens depends on what sort of amplifier you have. All the modern "sane" audio amplifiers that I know of have a very low output impedance. This means that the amplifiers will fight each other, and dissipate a lot of heat doing so. If they're very powerful at all, they'll both get damaged.
Many older (and modern crazy) tube amplifiers have higher output impedance, and may survive just fine. You can't just say "tube amplifier" and walk away happy, though -- some tube amplifiers (like the "super linear" topology) also have voltage feedback in them, which makes for low- impedance outputs that can't be blithely paralleled.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
If the output stage is a complementary PNP/NPN follower, which is common, the base/emitter of your output final transistors can be reverse-biased by up to the full (dozens of volts) power supply range. That can kill the expensive parts of your two audio outputs.
All audio sources have an impedance. In most cases at signal level that act s as your mixer, and no harm is done. You just don't always get the mix rat io you wanted.
At power levels, as Whit3rd said you quickly wind up with output stages fig hting and frying. Overcurrent, overdissipation and excess Veb are all probl ems.
Valves are always high impedance unless there's a transformer in the way, o r a most unusual multiparallel tranformerless output. They can pretend to b e low impedance upto a point with nfb, but the valve itself is still a high impedance source and can thus only maintain the low impedance effect withi n narrow limits of current, outside of which its high impedance becomes evi dent.
NT
** You can connect microphones in series, or in parallel. Works best with the kind that have 3 wires, two for signal and a ground.
Only identical mics should be wired in parallel.
.... Phil
Depends on the amplifier. Some, particularly class-D, audio amplifiers are designed to be paralleled. Them that ain't tend to go up in smoke, if you do.
If each audio source is a line output with an output impedance in the range of 100-1k and the input impedance of the following device is much larger than the parallel combination, then the two sources should sum by superposition forming a crude mixer.
Assuming each of the two sources has the same impedance and same output level, if the two audio sources are uncorellated then the sum output at the junction will have approximately the same audio power as one output in isolation
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This is of course assuming that both line output levels are not so large such that current limiting/voltage headroom issues in the output stage come into play into driving the reduced load.
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Oh I like that! Connect your amps in series.. (assuming the outputs can float.)
George H.
** Bridge mode = two output stages in series, driving the same load.
.... Phil
Oh nice, In one configuration you get the sum and the other the difference. I wonder what the bleep it would sound like?
George H.
like the input signal? Mine does anyway.
NT
bridging is a common configuration used in pro audio power amps to get 4x the power out..
two outputs are connected in series to drive one load
they don't have to be floating outputs, they can be ground referenced outputs
the speaker load must be floating
the input signal is simply inverted through one of the two sides.
the floating load is connected across the two + outputs
Mark
...
The original poster, however, was talking about connecting two outputs, and did not suggest that the outputs were driving complementary signals. When complementary output signals are available, a (resistive) load between them is effectively a resistor to ground (the Thevenin or Norton equivalent of a resistor to ground), which ought not to be a problem.
With non-complementary signals, it might be a problem.
an oddly persistent myth
NT
Please explain. 2V => 4P
You don't happen to be talking about Crown's patented grounded bridge configuration by chance.
First time I opened one of them up I was like WTF ? Two power transformers and bridge rectifiers and only two main filters ? Not even center taps on the power transformers.
Nifty, but I think it is more trouble than it is worth.
Page 6 of :
has the details but is a different paper than I read before.
** Bridging merely combines the power output of both channels into a single load.
If a stereo amp is rated for min 4 ohms loads, then you must use min 8 ohms in bridge mode.
No free lunch and getting it wrong can prove expensive.
.... Phil
Some amps have one channell inverted. I think the Carver 1.5T is like that. The reason given is that only one channel is sucking down the + or - suppl y on bass, which is usually in phase. that kind of amp is alrdy easily brid gable as long as you feed it mono equal to each channel and don't go under double its minimum impedance load, as you said.
Seems ironic to those who aren't hep to this stuff that if you bridge such an amp, if you drive one channel harder, the OTHER channel will get hotter and more likely to blow first. Seems exactly opposite of how it seems it sh ould be but nope. The harder driven channel will run cooler.
h an amp, if you drive one channel harder, the OTHER channel will get hotte r and more likely to blow first. Seems exactly opposite of how it seems it should be but nope. The harder driven channel will run cooler.
** Here is an easy way to think about it.In the normal case, with matched outputs, there is a point in the middle of the load that is always at zero volts - imagine it is grounded. So each ch annel sees load of half the total impedance.
Imagine one channel is driven at half the level of the other - then the ze ro point moves towards that channel and it sees a load of 1/3 the total imp edance.
Imagine one channel is driven at 1/10th, then the load is 1/11th of the tot al.
The worst case is with no drive at all to one channel - it *still* has to c ontinuously sink current equal to what the other channel is sourcing but th e voltage across each power device is fixed at maximum level so has much hi gher heat dissipation than the driven channel.
In this last case, the current limiting circuits will often trigger and cau se gross distortion of the sound.
.... Phil
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