DELTA: Curious pgm out spike/rolloff

Archived from groups: rec.audio.pro (More info?)

Using a 1 KHz square wave, the L/R program channels of the Delta masters
show something a little odd at the active balanced outputs.

For each output channel, the circuit consists of a predriver op amp and
two line driver amps. The predriver output is Y'd and drives the + input
of one line amp and the - input of the other. The individual feedback
loops on each driver amp goes to the - and + inputs, respectively.

The square wave is clean from the unbalanced output of the pre-driver amp.

But when I look at the + output of the balanced line I see a narrow spike
at the leading edges of the square wave. The amplitude of the spike is
approximately 15% of the waveform amplitude. The width of the spike is
roughly 10 microseconds.

When I look at the - output, I see a slight "rounding off" of the leading
edges of the square wave, with same width as the spike on the + amp.

The spike/round off width does not vary at all over the hz range of
square waves that the system will will reasonably pass.

If I put the + and - output signals on a dual trace scope, invert the
scope input of one of them, then move the trace vertical positions so that
the waveforms just touch top to bottom, the spike of the + line driver
fits neatly into the leading-edge round off area of the minus side. It's
an exact fit, in fact.

I have tried the original 5532s and also the new 2134s and get exactly the
same thing. Both L and R channels show the same thing.

I have adjusted the balanced line symmetry control and only change the
amplitude of the + and - signals. The spike and rolloff do not go
completely away. (This 1K pot, with its wiper to ground and legs fed
through 1K resistors from the + and - output lines, determines the
relative levels of the + and - signals.)

I'm curious as to what might cause a spike on the + side and a rounding on
the - side, and whether it could have audible effect given that it appears
to be supersonic. Is there something inherently problematic about this
type of balanced line driver configuration? Seems like it ought to be
pretty straight-forward.

I have not yet replaced the resistor packs with discrete resistors as Jim
had suggested sometime back. (These resistor packs are used around the
line driver opamps to set gain/feedback and such.)

What am I missing?

Thanks in advance,
Frank Stearns
Mobile Audio
--
.

Archived from groups: rec.audio.pro (More info?)

Frank Stearns wrote:

> Using a 1 KHz square wave, the L/R program channels of the Delta masters
> show something a little odd at the active balanced outputs.
>
> For each output channel, the circuit consists of a predriver op amp and
> two line driver amps. The predriver output is Y'd and drives the + input
> of one line amp and the - input of the other. The individual feedback
> loops on each driver amp goes to the - and + inputs, respectively.
>
> The square wave is clean from the unbalanced output of the pre-driver amp.
>
> But when I look at the + output of the balanced line I see a narrow spike
> at the leading edges of the square wave. The amplitude of the spike is
> approximately 15% of the waveform amplitude. The width of the spike is
> roughly 10 microseconds.
>
> When I look at the - output, I see a slight "rounding off" of the leading
> edges of the square wave, with same width as the spike on the + amp.
>
> The spike/round off width does not vary at all over the hz range of
> square waves that the system will will reasonably pass.
>
> If I put the + and - output signals on a dual trace scope, invert the
> scope input of one of them, then move the trace vertical positions so that
> the waveforms just touch top to bottom, the spike of the + line driver
> fits neatly into the leading-edge round off area of the minus side. It's
> an exact fit, in fact.
>
> I have tried the original 5532s and also the new 2134s and get exactly the
> same thing. Both L and R channels show the same thing.
>
> I have adjusted the balanced line symmetry control and only change the
> amplitude of the + and - signals. The spike and rolloff do not go
> completely away. (This 1K pot, with its wiper to ground and legs fed
> through 1K resistors from the + and - output lines, determines the
> relative levels of the + and - signals.)
>
> I'm curious as to what might cause a spike on the + side and a rounding on
> the - side, and whether it could have audible effect given that it appears
> to be supersonic. Is there something inherently problematic about this
> type of balanced line driver configuration? Seems like it ought to be
> pretty straight-forward.
>
> I have not yet replaced the resistor packs with discrete resistors as Jim
> had suggested sometime back. (These resistor packs are used around the
> line driver opamps to set gain/feedback and such.)
>
> What am I missing?

It's a potential characterisitc of the 'cross-coupled' balanced outputs. Might
be parasitic capacitance somewhere. Or even slightly mismatched caps around
the feedback loop.

The important thing as you found is that the two outputs when summed are
accurate.

It shouldn't be the ICs btw. I do use 1% Rs in this configuration myself
though.

Graham

Archived from groups: rec.audio.pro (More info?)

Mike Rivers wrote:

> In article <11a3qol58kh15db@corp.supernews.com> franks.pacifier.com@pacifier.net writes:
>
> > Using a 1 KHz square wave, the L/R program channels of the Delta masters
> > show something a little odd at the active balanced outputs.
>
> > The square wave is clean from the unbalanced output of the pre-driver amp.
> >
> > But when I look at the + output of the balanced line I see a narrow spike
> > at the leading edges of the square wave. The amplitude of the spike is
> > approximately 15% of the waveform amplitude. The width of the spike is
> > roughly 10 microseconds.
> >
> > When I look at the - output, I see a slight "rounding off" of the leading
> > edges of the square wave, with same width as the spike on the + amp.
>
> > If I put the + and - output signals on a dual trace scope, invert the
> > scope input of one of them, then move the trace vertical positions so that
> > the waveforms just touch top to bottom, the spike of the + line driver
> > fits neatly into the leading-edge round off area of the minus side. It's
> > an exact fit, in fact.
>
> That's the way this cross-coupled transformer-emulating circuit works
> when everything is at unity gain (which they do to keep the headroom).
> The good news is that it appears that you output stage is working
> properly - when summed (differenced?) at a differential input, the
> nasties exactly cancel. But it's a good lesson in why you shouldn't
> use just one side to ground with this kind of output stage if you're
> feeding an unbalanced input. You should tie the low side (pin 3) to
> ground (pin 1) and take the unbalanced output between pins 1/3 and 2.
>
> Try slamming it so that the output stage runs into clipping and you
> may be really horrified at what you see. The Mackie Onyx line (except
> the 1220) use an output stage like this. If this works, you'll find a
> picture of what the two sides of the balanced output look like when my
> 1620 clips. Sum them together and you get a nice, symmetrical waveform
> that's flat on the top and bottom.
>
> http://www.imagehosting.us/imageho [...] ?id=510053

Curious. Do you mean Mackie use the cross-coupled configuration with unity gain from
unbalanced in ( usually post fader buffer ) to balanced out ?

I always add 6dB to indeed 'preserve headroom' so that the balanced out doesn't clip until
it's at +28dBu ! This means that all the mix bus etc runs 6dB lower than 'external' level
too giving a bit more mix headroom pre output level fader.

Graham

Archived from groups: rec.audio.pro (More info?)

In article <42A25AB5.3BBF0BFD@hotmail.com> rabbitsfriendsandrelations@hotmail.com writes:

> Curious. Do you mean Mackie use the cross-coupled configuration with unity gain
> from
> unbalanced in ( usually post fader buffer ) to balanced out ?

Yes.

> I always add 6dB to indeed 'preserve headroom' so that the balanced out doesn't
> clip until
> it's at +28dBu ! This means that all the mix bus etc runs 6dB lower than
> 'external' level
> too giving a bit more mix headroom pre output level fader.

Good policy. The Mackie is a bit skimpy on the headroom, and I think
the maximum output level (differential, between pins 2 and 3) is only
+22 dBu. Curiously, the Onyx 1220, the "baby" of the line, uses the
same output configuration as the earlier series, a unity gain inverter
between pin 2 and pin 3. That one will put out +26 dBu before
clipping.

Since the Onyx already has a single-ended output on a TRS jack, with
the ring tied to ground through a resistor, I questioned why they
bothered with this cross-coupled output configuration at all. I never
got a really good answer, but I suspect that their thinking was that
it would end the "if you use a balanced output unbalanced, you lose
6 dB" discussion - that is, if you ground the low pin (which you DON'T
want to do if it's driven by a straight inverter). However, I think
that it backfired, at least for anyone (like me) who bothered to look
at the individual halves of the differential output.

If you looked at the photo, you'll see that the amplitudes of the two
sides aren't equal. That wasn't a matter of scope gain not being
equal, they're really different voltages. Not only that, but I
measured a wide variation in the amount of "unbalance" between the two
legs of the output among two mixers and the eight outputs of the 800R
stand-alone preamp. A few were very close to equal in amplitude, some
were as far as 50% different, but always adding up to the right
amplitude when driving a differential input. Without a trimmer (like
the Soundcraft has - I also have a Soundcraft 600) you just can't get
resistors close enough in value, even 1% values, to have it completly
symmetrical.

So (and I think I may have said this here before) if you're using the
XLR outputs on an Onyx mixer, either connect them to differential
inputs or tie Pin 3 to Pin 1. Otherwise you can't be sure what level
you'll get out when the meter reads 0 VU.

--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo

Archived from groups: rec.audio.pro (More info?)

Mike Rivers wrote:

> In article <42A25AB5.3BBF0BFD@hotmail.com> rabbitsfriendsandrelations@hotmail.com writes:
>
> > Curious. Do you mean Mackie use the cross-coupled configuration with unity > gain from
> unbalanced in ( usually post fader buffer ) to balanced out ?
>
> Yes.

I'm astonished !


> > I always add 6dB to indeed 'preserve headroom' so that the balanced out > doesn't clip
> until it's at +28dBu ! This means that all the mix bus etc runs 6dB > lower than
> 'external' level too giving a bit more mix headroom pre output level > fader.
>
> Good policy.

Seems common sense to me. Ok - it can make the mix bus a fraction noisier overall but not by
much and the headroom increase is great esp for those who like to run a 'hot mix' - not
unusual in these days of digital.

Let's put it his way. it was good enough for Neve V series !

> The Mackie is a bit skimpy on the headroom,

Now I understand all those user comments ! Clearly the gain structure isn't optimised for
what I'd call 'real world' use.

> and I think
> the maximum output level (differential, between pins 2 and 3) is only
> +22 dBu. Curiously, the Onyx 1220, the "baby" of the line, uses the
> same output configuration as the earlier series, a unity gain inverter
> between pin 2 and pin 3. That one will put out +26 dBu before
> clipping.

Oh yuk ! That crazy configuration ! Have they not learnt ?


> Since the Onyx already has a single-ended output on a TRS jack, with
> the ring tied to ground through a resistor, I questioned why they
> bothered with this cross-coupled output configuration at all. I never
> got a really good answer, but I suspect that their thinking was that
> it would end the "if you use a balanced output unbalanced, you lose
> 6 dB" discussion - that is, if you ground the low pin (which you DON'T
> want to do if it's driven by a straight inverter). However, I think
> that it backfired, at least for anyone (like me) who bothered to look
> at the individual halves of the differential output.

They don't appear by the sounds of it to have a coherent design policy to me.


> If you looked at the photo, you'll see that the amplitudes of the two
> sides aren't equal. That wasn't a matter of scope gain not being
> equal, they're really different voltages.

I did wonder !

> Not only that, but I
> measured a wide variation in the amount of "unbalance" between the two
> legs of the output among two mixers and the eight outputs of the 800R
> stand-alone preamp. A few were very close to equal in amplitude, some
> were as far as 50% different, but always adding up to the right
> amplitude when driving a differential input. Without a trimmer (like
> the Soundcraft has - I also have a Soundcraft 600) you just can't get
> resistors close enough in value, even 1% values, to have it completly
> symmetrical.

I agree. Well.... I know it to be a fact with 1% values to be entirely accurate ! I believe
the trimmer to be a necessary evil. See my comment to Jim Williams.


> So (and I think I may have said this here before) if you're using the
> XLR outputs on an Onyx mixer, either connect them to differential
> inputs or tie Pin 3 to Pin 1. Otherwise you can't be sure what level
> you'll get out when the meter reads 0 VU.

That's bad ! Considering that I thought the Onyx to be a major improvement in Mackie's line
I'm truly amazed.

Btw - you get +28dBu out with proper balanced trimming on this little beast.

http://www.studiomaster.com/c1.html


Graham