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Topic: Is this right? (Audiosource AMP100 manual) (Read 4666 times) previous topic - next topic
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Is this right? (Audiosource AMP100 manual)

I'm almost certain this is not correct:

Quote
BRIDGED MODE- MONO APPLICATION
For bridged mode, playing Right and Left together as mono output, use a
Y cable adapter to connect Right and Left RCA signal to the Right (red) input
at Line 2 IN.


From: http://developer.audiosource.net/wp-conten...100-Current.pdf

I'm getting this amp for a friend's small restaurant, and I need it to output a mono signal. I've done this before, but with a small inexpensive mixer. I was wondering if the amp itself would mix the signal when using stereo input, but bridged mode, but from what I can tell it can't.

Is this right? (Audiosource AMP100 manual)

Reply #1 –
No, that's definitely not right.

You can use a Y-cable to split a signal, but never to sum it together. You will get all kinds of nastiness if you try to mix two outputs together like that.

E: http://www.rane.com/note109.html

Is this right? (Audiosource AMP100 manual)

Reply #2 –
Quote from: KozmoNaut on
No, that's definitely not right.

You can use a Y-cable to split a signal, but never to sum it together. You will get all kinds of phase errors and other nastiness.


If it is "consumer" equipment, on the input side, with RCA connectors, a "Y" cable will sum the left and right signals to mono in most cases just fine.  Typically, the stereo outputs of consumer audio equipment are restively built out to several thousand ohms, and the build out resistors will work well as a summing network. 

Do not parallel the left and right outputs of a power amplifier (intended to connect to loudspeakers) These have a very low source impedance and will essentially short each other out.  At best, you will get a lot of distortion if the source audio has a lot of separation.  At worst, you'll burn out the output stages of the amplifier.   

For "professional" balanced line level (again not power amplifier outputs) sources, in most cases you can connect the high side of the left channel source to the high side of input to the mono amplifier(s), and the low side of the right channel source to the low side of the input to the mono amplifier(s).  This depends on both the high and low sides of the stereo sources being driven.  It will not work with some "prosumer" items like the "balanced" phone jack outputs of Mackie (and some other makes) mixers, where only the high side of the balanced output is driven, the low side just has a resistor to ground.

Is this right? (Audiosource AMP100 manual)

Reply #3 –
Quote from: KozmoNaut on
No, that's definitely not right.

You can use a Y-cable to split a signal, but never to sum it together. You will get all kinds of nastiness if you try to mix two outputs together like that.

E: http://www.rane.com/note109.html


Not an absolute truth. The build out resistors on the line drive amps will function as summing resistors. Many units will do OK with the mono mix via Y cord.
If you want to be certain it's OK add a resistor into each leg to do the summing function. 1K to 10K should be fine for solid state units. Toobs prefer
larger values. Whatever values should be reasonably matched. Unselected 1% would be fine.

In the broadcast world they are called 'mults' and are not unusual for mono mixing. Cheesy but effective.

G²

Is this right? (Audiosource AMP100 manual)

Reply #4 –
Well, I stand corrected, then.

I still think it's bad practice, though.

I sum my subs to mono in the active crossover, I wonder if it's actually causing some amount of phase cancellation or whether that would happen acoustically anyway if I ran them in stereo instead. I guess it would only be a problem with badly mastered tracks where the L/R channels are out of phase with each other.

Is this right? (Audiosource AMP100 manual)

Reply #5 –
'KozmoNaut' I'll still go with your original post and the Rane "wye" note.  Just because it doesn't sound too bad with some equipment, doesn't make it a good idea.

From the Rane Note:
Here is the rule: Outputs are low impedance and must only be connected to high impedance inputs -- never, never tie two outputs directly together -- never. If you do, then each output tries to drive the very low impedance of the other, forcing both outputs into current-limit and possible damage. As a minimum, severe signal loss results.

Although I doubt the 'possible damage' part.
Kevin Graf :: aka Speedskater

Is this right? (Audiosource AMP100 manual)

Reply #6 –
I have to agree with with the general rule, "Never connect two outputs together".  (There could be exceptions if you've got the schematic and you know what you are doing.)

If you know how to solder it's cheap & easy to make a little fixed-passive mixer with resistors, but I don't know where you can buy something like that ready-made.

As far as I know it's rare to find series resistors on an output.  It adds (a small) cost and it can create "impedance problems".   

It's the "nature" of solid state electronics to have low source impedance.  Usually much lower than the designed load impedance and if you "match" the impedance, bad things can happen.  When you connect two outputs together the outputs are back-driving each other and they both see an impedance lower than they were designed for. 

As a worst-case example, power amplifiers (designed to drive 4 or 8 Ohms) usually have an output impedance of less than 1 Ohm.*  If you connect another amp-output instead of a speaker (or in addition to a speaker), you'll get excess current and bad things will happen  (exactly what bad thing happens depends on the amplifier's design).   

When I was younger, I tried connecting connecting left & right line-outputs together and I got a "reverse vocal-removal" effect where the centered sound came-through OK, but the left-only and right-only sounds were highly attenuated and distorted.  It was an "interesting effect", but at some point I realized what was happening and I figured-out that I was over-stressing the electronics.





*  An amplifier with a damping factor of 100 into 8 Ohms has an effective source impedance of 0.08 Ohms.



Is this right? (Audiosource AMP100 manual)

Reply #7 –
Years ago when I was into audio electronics, it seemed to me that it was pretty standard to put a 100K resistor in parallel with an input and a 1K resistor in series with an output, but maybe things have changed.

Is this right? (Audiosource AMP100 manual)

Reply #8 –
Consumer line-outs/ins are not uncommon to have 100/10k ohm.

Even if there is no damage, performance could be degraded quite severely.
"I hear it when I see it."

Is this right? (Audiosource AMP100 manual)

Reply #9 –
The source will be a Chromecast Audio mainly, but they may connect something else to it. I already ordered the mixer, I guess that's the safest way plus lets me control the levels better for something like a Chromecast.

Is this right? (Audiosource AMP100 manual)

Reply #10 –
Quote from: DVDdoug on
I have to agree with with the general rule, "Never connect two outputs together".  (There could be exceptions if you've got the schematic and you know what you are doing.)

If you know how to solder it's cheap & easy to make a little fixed-passive mixer with resistors, but I don't know where you can buy something like that ready-made.

As far as I know it's rare to find series resistors on an output.  It adds (a small) cost and it can create "impedance problems".   

<snip>



"As far as I know it's rare to find series resistors on an output." This is true for power amplifiers but definitely not true for line level outputs. The build out resistors are needed to decouple the amplifier from the capacitance of the cable which can cause severe instability. Adding the resistor forms a single pole low pass but the corner is usually far above the audio band. Forcing 'mixing' using the build outs as the summing resistors might load the opamps to the point of current limiting - a form of clipping which is always bad. But, much  commercial 'stereo' is constructed from multichannel mono and pan pots and phase shifts. If you display this in X-Y mode on a scope it tends to look like an oval as there is some channel to channel phase difference which reduces the bad effects of the mono 'mix'. True stereo with a 2 widely separated mics  that is matrixed in the room will be all over the place phase wise. That would place much more stress on the opamps which could give distortions. Making a mono mix Y cable would simply add the previously mentioned resistors in series with the 2 inputs. I could build those inside the connectors easily but I don't like mono and have never spent the time on it.

Paralleling most solid state power amps without resistors WILL blow up one or both channels. NEVER DO THAT.

G²