Tweaking begins; expert guidance (or shoving in the right direction)
appreciated in advance.

I've got a bunch of observations, but will keep this brief and on just one
section for now.

I have IC-socketed a few of my Delta input modules, and left the others
alone (factory soldered TLO72s). In the pan buffer section one op amp
section of a dual buffer feeds the left mix bus, while the other amp
buffers the right.

The pan pot is a 10K dual; the input signal is "Y"d so that it appears
across both pot sections. Wipers direct feed the + inputs of the amps.
Rotate the pot one way and one wiper is at ground while the other wiper is
at the "top" of the pot.

Here's what I've seen in this section:

Buffer TLO72 OPA275 OP2134

Output noise: 2 mV 2 mV 2 mV (approx)

- input 11V 0.5 MHz 22V 1 MHz 25V 1 MHz
amplitude varies from zero to level noted. Peak is at
exactly mid-rotation of pan pot.

+ Input 0-4 mV, varies with pan pot position

Output noise spectra shows a strong component of the oscillation seen at
the invert input.

Note that *all* configurations do this (soldered, socketed, all three chip
types), though the factory soldered TLO72 is at a lower freq and amplitude.

This oscillation on the invert input would then appear to be "normal" (as
there's no apparent oscillation at the output other than in the noise).

Other conditions: bus and pan switches OFF so that nothing is going out of
the module.

2.2K feeds the output to the invert input. Invert input also goes through
10K to ground.

It seems as though I could bring the noise down a bit and perhaps reduce
the oscillation level at the invert input by putting a small value cap
around the 2.2K feedback resistor, and/or a small value cap across the 10K
resistor to ground.

I did try this, and got the ouput to start oscillating -- could have been
leads too long or wrong kind of cap (470 nF/100V ceramic is what I had on
hand -- should I try something else -- polystyrene perhaps?)

It does appear that most of the op amp feedback loops in the Delta do not
have BW limiting caps in the feedback network. I could be happy with a BW
of, oh 100-200KHz, assuming cumulative BW-limited stages don't start
impacting in-band performance. I also assume there's a simple calculation
to set feedback cap value based on desired BW and invert input Z (in this
case it appears to be 10K).

Thanks in advance for suggestions; this is a bit of education and fun; I
hope the postings are of value to others.

Frank Stearns
Mobile Audio
--

"Frank Stearns" wrote in message
...

Here's what I've seen in this section:

Buffer TLO72 OPA275 OP2134

Output noise: 2 mV 2 mV 2 mV (approx)

- input 11V 0.5 MHz 22V 1 MHz 25V 1 MHz
amplitude varies from zero to level noted. Peak is at
exactly mid-rotation of pan pot.

+ Input 0-4 mV, varies with pan pot position

Output noise spectra shows a strong component of the oscillation seen at
the invert input.

Let's see if I understand what's happening: when you stick your probe on the
output of the IC, you get the oscillation component, even with the stock IC
in place? Is it that way on the modules you haven't modified? Are you using
a /10 probe? If not, try using one and see if you still see the oscillation
at the IC output. It may be that the probe's capacitance is destabilizing
the IC.

It seems as though I could bring the noise down a bit and perhaps reduce
the oscillation level at the invert input by putting a small value cap
around the 2.2K feedback resistor, and/or a small value cap across the 10K
resistor to ground.

Definitely don't try the latter. Definitely do try the former, You won't
reduce the audible noise, but you may get rid of the oscillation, if it
really exists without the scope probe present. (Are you *sure* the 10k is
from - in to ground and the 2.2k is from - in to out?)

I did try this, and got the ouput to start oscillating -- could have been
leads too long or wrong kind of cap (470 nF/100V ceramic is what I had on
hand -- should I try something else -- polystyrene perhaps?)

If you hung a cap on the input resistor, well, that's a good way to build an
oscillator. Well, not a good way, but it'll do it.

It does appear that most of the op amp feedback loops in the Delta do not
have BW limiting caps in the feedback network. I could be happy with a BW
of, oh 100-200KHz, assuming cumulative BW-limited stages don't start
impacting in-band performance. I also assume there's a simple calculation
to set feedback cap value based on desired BW and invert input Z (in this
case it appears to be 10K).

No, you want to calculate based on the feedback resistor (output to -
input). The formula is:

C = 1 / (2 * pi * R * f)

where C is the capacitor in farads, R is the value of the feedback resistor
in ohms, and f is in Hz, f being your desired rolloff frequency. Multiply
your answer by 10^12 to get picofarads.

Peace,
Paul

Frank Stearns wrote:

Tweaking begins; expert guidance (or shoving in the right direction)
appreciated in advance.

There should be no oscillation anywhere.

Maybe you'd like to start your explanation at the beginning ?

I have no idea at all why you would want to substitute different op-amps at the
'pan buffer' section now what it's meant to illustrate.

Graham

That circuit is the patented Soundcraft "active pan pot". It includes
both negative and positive feedback to obtain a near perfect pan "law".
Shove any scope probe into the inverting input of an opamp and the
stray capacitance will cause phase margin to degrade.

The important part is that the outputs are clean. Many opamps can be
used in this circuit as long as some housecleaning is done.

Place a 47 pf NPO mono ceramic cap across R54 and R 57, the 2.2 k
feedback resistors.
Place a .1 uf mono ceramic cap from pin 8 to ground, close to the pin
of the opamp. Place another .1 mono cap from pin 4 to ground. Try to
keep the lead lengths short, especially at the opamp pins. Install a
machined 8 pin socket.

For wide band opamps like the LM6172, connect a 100 ohm resistor to a
100 pf NPO cap. place this network across pins 2 and 3. Make another
and place it to pins 5 and 6. This is a phase lead function that pushes
the high freq. gain up out of the unstable region of unity gain. You
can just about put any opamp in there after that. This pan mod applies
to all Soundcrafts with this circuit like Venue, 6000, 3200, Sapphire,
etc.

Jim Williams
Audio Upgrades

Jim Williams
Audio Upgrades

writes:

That circuit is the patented Soundcraft "active pan pot". It includes
both negative and positive feedback to obtain a near perfect pan "law".
Shove any scope probe into the inverting input of an opamp and the
stray capacitance will cause phase margin to degrade.

Ah ha! Thanks for the explanation.

The important part is that the outputs are clean. Many opamps can be
used in this circuit as long as some housecleaning is done.

Okay. The outputs do appear reasonably clean with the oscillation way out
of band and the amplitude down in the noise.

Place a 47 pf NPO mono ceramic cap across R54 and R 57, the 2.2 k

Thanks once again, Jim, for your willingness to post such detailed
information.

Also, thanks to all who have posted replies.

Frank Stearns
Mobile Audio

--