Let's Discuss Servos (Previously OPA188 Thread)

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Let's Discuss Servos (Previously OPA188 Thread)

Post by JR. »

Yup, and the phase response is not going to be very filter like, again 1- the other pass.

I suspect a usable filter can be made, but I never tried and am not inclined to. By the time we throw a servo at an audio path, we are by definition not trying to save parts, and concerned about sonic performance. One pole filters are generally the most benign sounding so why not KISS.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Let's Discuss Servos (Previously OPA188 Thread)

Post by mediatechnology »

These are still a mystery....


What's up in the right channel during the intro?
Image
Switchfoot, "We Are One Tonight."

What happened before t=0?
Image
Fiction Plane, "Two Sisters."
ricardo
Posts: 262
Joined: Sun Apr 10, 2011 7:24 am

Re: Let's Discuss Servos (Previously OPA188 Thread)

Post by ricardo »

mediatechnology wrote:
I have never tried to make a proper filter from a servo but I suspect it will behave similarly to the class of derived crossover filters, where you subtract a LPF from unity to make a HPF response. The resulting response curve for the HPF is not remotely symmetrical.
IIRC, the HPF derived is always 6 dB/oct regardless of the LPF order isn't it? (Or vice-versa if the derived is the LPF section.)
You can have symmetrical sections & high order roll-offs but only if you accept quite high peaking of the individual sections on either side of the xover point. ... from my previous nerdy loudspeaker existence. :ugeek:

None of the versions are in any way useful except to Golden Pinnae. But they do sum to unity with no phase shift.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Let's Discuss Servos (Previously OPA188 Thread)

Post by JR. »

But summing to unity is not a concern for a single HPF. I think I looked into making a variant of derived where a state variable topology only had a HP and LP output closing the NFB loops, I forget why exactly I abandoned it, it didn't look that promising to me for high orders (back in the '80s) and crossovers are always conflicted with on axis response where unity sum is useful and off axis where flat power is more important.

In that servo, we get the bad half of a simple derived filter and don't sum them back together, so sum benefit doesn't apply.

=======

Wayne I don't recall the resolution of that, apparently it wasn't resolved, looks like DC (very LF) bounce. Some shift in operating point between no signal and signal present. A very HF oscillation could do that, but no evidence of that in the scope. I would try to chase it down with sine waves, maybe tone bursts...Some kind of DC operating point shift appears to be going on. (BTW I don't remember what we're looking at so just shotgunning wild guesses.)

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Let's Discuss Servos (Previously OPA188 Thread)

Post by mediatechnology »

Those were in the original soundfiles so I have no clue. Rarely do I see these types of artifacts.

The lower example with the bowing in the right channel is so LF that it makes me think it entered the production chain near the end.
When I saw both of them I thought servos?
If the even-order harmonics shifted during that short segment and the servo integration time were too short (cut-off too high) I can see how the DC baseline would shift.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Let's Discuss Servos (Previously OPA188 Thread)

Post by JR. »

mediatechnology wrote:Those were in the original soundfiles so I have no clue. Rarely do I see these types of artifacts.

The lower example with the bowing in the right channel is so LF that it makes me think it entered the production chain near the end.
When I saw both of them I thought servos?
If the even-order harmonics shifted during that short segment and the servo integration time were too short (cut-off too high) I can see how the DC baseline would shift.
Ok if we are guessing what kind of signal chain causes that. I would be inclined to suspect a VCA/gain element in a compressor. Many gain elements can be guilty of a DC error term referenced to their input, so as the gain changes, due to signal level change, a DC component can be added to the signal.

So perhaps a DC step from a compressor kicking in and the recovery as a servo or HPF returns to nominal 0V. I would expect a similar dip and recovery when the music stops.

Such LF signal bounce is pretty common in the world of cheap gain elements. The OTAs that we used at Peavey in the power amp clip limiters were a special house number graded for input referred DC offset to manage that LF bounce.

JR
Cancel the "cancel culture", do not support mob hatred.
User avatar
mediatechnology
Posts: 5444
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Let's Discuss Servos (Previously OPA188 Thread)

Post by mediatechnology »

Visit this post "Servo with Non Inverting Differential Output using a Deboo Integrator."

viewtopic.php?f=6&t=14&start=114
Post Reply