A Stereo Instrument Preamp/Headphone Amp (formerly "a JISBOS-with-gain Headamp")
Oct 19, 2009 at 6:07 AM Post #16 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
Much cleaner, no?
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Much!
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By the way, the mode switch is for swapping left and right channels, correct?

se

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Oct 19, 2009 at 10:17 PM Post #17 of 109

Zaubertuba

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
Much!
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By the way, the mode switch is for swapping left and right channels, correct?



Well, in a way.

It swaps L- and R+ so that L+ and R+ goes to one fader and L- and R- goes to the other, giving two discretely controlable unbalanced stereo outputs, when necessary.

Unswitched, L+/L- and R+/R- each get their own fader, so balance can be controlled between the two ganged faders.

Of course, this is contingent on me getting the ground path right.
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Oct 19, 2009 at 10:22 PM Post #18 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
Well, in a way.

It swaps L- and R+ so that L+ and R+ goes to one fader and L- and R- goes to the other, giving two discretely controlable unbalanced stereo outputs, when necessary.



But one would output an inverting signal and the other a non-inverting signal.

Is that actually what you want? Why would you want two unbalanced outputs with different signal polarities?

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Oct 19, 2009 at 10:44 PM Post #19 of 109

Zaubertuba

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
But one would output an inverting signal and the other a non-inverting signal.

Is that actually what you want? Why would you want two unbalanced outputs with different signal polarities?



If they're driving two separate headphones, the signal polarity shouldn't make any difference, should it, as long as L and R are the same for each headphone?

Now, as far as the line outs, if you needed to use both (like one signal for L & R speakers and one sent to a subwoofer), you'd either need to invert one signal before the output or know you're going to be able to invert it at the SW (my subwoofer happens to have a phase-inverting switch on it).

...or, if any of this creates any "issues" I haven't foreseen, there's always that transformer output stage you suggested.
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Oct 19, 2009 at 11:19 PM Post #20 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
If they're driving two separate headphones, the signal polarity shouldn't make any difference, should it, as long as L and R are the same for each headphone?


Well, there are those who would argue that signal polarity should be preserved. But if it's not an issue for you, then it's not an issue.
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Quote:

Now, as far as the line outs, if you needed to use both (like one signal for L & R speakers and one sent to a subwoofer), you'd either need to invert one signal before the output or know you're going to be able to invert it at the SW (my subwoofer happens to have a phase-inverting switch on it).

...or, if any of this creates any "issues" I haven't foreseen, there's always that transformer output stage you suggested.
wink.gif


Yes. And that would also give you balanced output even for "unbalanced" phones. Though you'd still have the common contact resistance in the "ground" of the TRS plug/jack combo.

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Oct 20, 2009 at 4:25 AM Post #21 of 109

Zaubertuba

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Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
Well, there are those who would argue that signal polarity should be preserved. But if it's not an issue for you, then it's not an issue.
atsmile.gif



Well, I'm not so much a technician or physiologist that I could argue about the merits of soundwaves being created in- or out-of phase or whether the human ear could actually tell the difference.
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Quote:

Yes. And that would also give you balanced output even for "unbalanced" phones. Though you'd still have the common contact resistance in the "ground" of the TRS plug/jack combo.


Wait...so are you saying that using transformers I could basically sum R-/L- and send those outputs to "ground" on a standard headphone? Interesting. Are there amps out there that already do this?

Of course the catch is, then I wouldn't have discreet control of two headphone stages. I suppose this is why there's 6-board builds (yes...I did spend a little time with the b22 active ground thread
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)?

...has anyone tried summing L-/R- on a third board (not actually "active ground," but actually sending a signal to the board)? I suppose that could get messy real fast.
 
Oct 20, 2009 at 4:30 AM Post #22 of 109

amb

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
...has anyone tried summing L-/R- on a third board (not actually "active ground," but actually sending a signal to the board)? I suppose that could get messy real fast.


You would be creating a "ground" that's swinging an out-of-phase version of the MONO signal. Think about the implications -- I don't think you'd really want to do this.
 
Oct 20, 2009 at 4:52 AM Post #23 of 109

Steve Eddy

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Quote:

Originally Posted by Zaubertuba /img/forum/go_quote.gif
Well, I'm not so much a technician or physiologist that I could argue about the merits of soundwaves being created in- or out-of phase or whether the human ear could actually tell the difference.
rolleyes.gif



Well, like I said, if it's not of any concern for you, then it's not a concern.
atsmile.gif


Quote:

Wait...so are you saying that using transformers I could basically sum R-/L- and send those outputs to "ground" on a standard headphone?


Yes.

Quote:

Are there amps out there that already do this?


I believe Donald North's amps do this.

Quote:

Of course the catch is, then I wouldn't have discreet control of two headphone stages.


Nope.

Quote:

I suppose this is why there's 6-board builds (yes...I did spend a little time with the b22 active ground thread
tongue_smile.gif
)?


If I never hear "6-board builds" again it'll be too soon.
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Quote:

...has anyone tried summing L-/R- on a third board (not actually "active ground," but actually sending a signal to the board)? I suppose that could get messy real fast.


You can't do that with bridged outputs. They would effectively be driving a short circuit, i.e. L- would be driving R-'s output and vice versa. The output impedance of a B22 is below 1 ohm, and the parallel combination of the regular channel's output impedance and the output impedance of the "ground channel" board, it would be half that again.

Bad juju.

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Oct 20, 2009 at 5:57 AM Post #24 of 109

Zaubertuba

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Quote:

Originally Posted by amb /img/forum/go_quote.gif
You would be creating a "ground" that's swinging an out-of-phase version of the MONO signal. Think about the implications -- I don't think you'd really want to do this.


LOL, O.K., I can sort of see that. But then how is this different then summing L- and R- via a transformer?
 
Oct 20, 2009 at 8:37 AM Post #25 of 109

amb

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Whether summing with a transformer or summing via a summing amplifier (or some other means), the effect would be the same. The sum of two signals is what it is.

You cannot simply connect the L- and R- outputs together, though. The two amplifier's outputs would "see" each other and fight. One or both of them would most likely be damaged as a result.
 
Oct 20, 2009 at 5:27 PM Post #26 of 109

Steve Eddy

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Quote:

Originally Posted by amb /img/forum/go_quote.gif
Whether summing with a transformer or summing via a summing amplifier (or some other means), the effect would be the same. The sum of two signals is what it is.


No, it's not the same.

If you common the L- and R- outputs of a pair of bridged amplifier channels as would be the case with a four board B22 or similarly configured amplifier, the two channels see each others' outputs as an effective short circuit and output current from one channel will flow through the output of the other and vice versa.

If you common the bottom secondary windings of a pair of output transformers, neither secondary sees the other and there is no current flowing from one channel's secondary into the other channel's secondary.

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Oct 20, 2009 at 7:37 PM Post #29 of 109

Zaubertuba

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Quote:

Originally Posted by bada bing /img/forum/go_quote.gif
lol


Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif
Care to elaborate? Or is that the extent of your vocabulary?


O.K., Let's try and keep it on the up-and-up, guys. Koyaan's been exceedingly patient with me and has provided a lot of useful information on this thread. I am kind of expecting a *productive* debate betwixt Him and AMB about this whole "summing L- & R-" thing, from which I expect I'll learn a whole lot more.
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Provided, we keep it to sensible, scientific debate, and keep the tempers down.

It hasn't really degraded that bad at all, I just wanted to head it off before it gets there. I would really dislike asking the mods to close a thread that I (and I hope others) have been getting a lot of useful information on.

Cheers!
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