Chip Amp - Motorboating putt putt noise?

[john65b]

Hey all, got another one fer you all...just finished a point to point LM3886 al la gaincard...same component values as the famous one we all know...zobel network, 680/22k FB resistors, etc

Anyway, I got some motorboating or something going on. I can waive my hand over the amp chips (without touching) and it affects the motorboating sound - it can slow down, speed up, and almost dissappear altogether...some kind of antenna effect pehaps? I have longer wires from the chip to the binding posts (12 inches to reach the posts) and thought of shortening them, but the speaker wires are over 6 feet long, so there goes that idea...

Anyway, I don't have anything special on the PS, but I have +\-39V on the rails, and 6800uf/rail. I started out with +/-20V /rail and 1000uf at the pins...after not getting anywhere on the motorboating, I changed PS to the 39V with the additional 6800uF smoothing caps (from a an old Parasound HCA-500) - no change...

I have been thinking of .01uf bypass caps at the 1000uf, but don't think it will do anything....seems to be running hot too (rails at +/-20V and +/-39V) - oscillations pehaps?

[BWRX]

Originally posted by john65b
oscillations pehaps?

Sure sounds like it. Do you have any other appliances turned on that are connected to the same mains outlet? Like a fluorescent/halogen light, computer, air conditioner, refrigerator, etc.

What are you using for a source?

Double check your wiring and your solder joints. Twist your supply and input wiring and reflow suspect solder joints if necessary.

[john65b]

This is my test rig - Source is a Technics SH-AC500D Preamp with a Sony CD player...all are on a power strip with a lamp - moved the lamp to another outlet ...

The motorboating comes and goes, so I will check again to see if it is the lamp...and recheck all the connections too.

Think it is odd about the antenna thingy I mentioned above?

[BWRX]

It certainly is odd that you can change the frequency of the sound just by moving. Although our bodies can make good antennas. Have you tried the same experiment while you are grounded (i.e. touching earth ground)? Problems like these are often a result of improper grounding. Not grounding of the listener but of the circuit

Also, how close are the chips to the power supply transformer, rectifiers, and supply wiring? If they're close, move them further away if possible and see if that helps.

[john65b]

Trannie and rectifier/smoothing caps are on side of the amp chassis separated by metal shield...

I will look into the better solder connections later...I have moved the lamp and isolated the plug for just the amp/source and it has not yet happened...need to give it a bit more time...

[gootee]

Originally posted by john65b
Hey all, got another one fer you all...just finished a point to point LM3886 al la gaincard...same component values as the famous one we all know...zobel network, 680/22k FB resistors, etc

Anyway, I got some motorboating or something going on. I can waive my hand over the amp chips (without touching) and it affects the motorboating sound - it can slow down, speed up, and almost dissappear altogether...some kind of antenna effect pehaps? I have longer wires from the chip to the binding posts (12 inches to reach the posts) and thought of shortening them, but the speaker wires are over 6 feet long, so there goes that idea...

Anyway, I don't have anything special on the PS, but I have +\-39V on the rails, and 6800uf/rail. I started out with +/-20V /rail and 1000uf at the pins...after not getting anywhere on the motorboating, I changed PS to the 39V with the additional 6800uF smoothing caps (from a an old Parasound HCA-500) - no change...

I have been thinking of .01uf bypass caps at the 1000uf, but don't think it will do anything....seems to be running hot too (rails at +/-20V and +/-39V) - oscillations pehaps?

It sounds like it could be a (non-)grounding problem. How/where did you connect the signal and power and zobel grounds?

Check your layout. Also make sure that your power and signal wiring is well separated, especially AC mains and input signal.

But...

If you suspect oscillation, why not simply hold a small (probably 10pF to 50pF, to start with) cap across the OUT and IN- pins of the chip, and see if it stops it?

I would certainly also want 0.1uF (or 0.01uF) caps directly from each power pin to (power's) ground, in parallel with your larger caps. (It's so easy to try, anyway.)

If none of that works, you could try adding a simple RF low-pass RC filter, on the signal input; just a series resistor (close to the input pin) with a cap downstream (i.e. between resistor and input pin) from the resistor to ground. If it's for the negative input pin, you'd need to use two resistors, with the cap going from in between them to ground. Maybe something like 1.5 kOhms and 220 pF would work (i.e. an F3dB of approx 480 kHz). If that works, look for a layout problem to fix so you can remove the filter (since resistors genrerate their own noise).

If RF _is_ getting into one or both of the chip's inputs (or output, or power pins), it can show up as rectified DC at the output. So a changing-in-amplitude (e.g. AM) RF signal might result in a low-frequency signal being added into the output, which might be able to sound like motorboating (or even (gasp!) distortion).

Classical "motorboating" is usually caused by low-frequency instability, in the mathematical "feedback control systems" sense. So maybe check your circuit components and wiring for errors, bad parts, and questionable layout practices, too. FWIW, the original Gaincard was not the greatest design, in a technical sense (in my opinion); too minimalistic, allowing too much potential for weird stability (and other) problems to rear their ugly heads.

Good luck.

- Tom Gootee

http://www.fullnet.com/~tomg/index.html

-

[john65b]

Well I was listening in close proximity to the amp last night and I got a call on my cell - the Putt Putt sound returned. I waived the cellphone over the amp and sure enough, the sound frequency and pitch changed...I hung up the call and the noise disappeared altogether. Looked around the room and my simple jeweler's lamp has one of those curly cue florescent bulbs that last forever (don't know where the hell it came from either)

So apparently, the noise can be traced back to interference with external emitters...so there may not be any major problem with the amp oscillating, correct? Or is there another more serious grounding problem that still needs to be addressed from this interference?

What can I do to the amp to rid its interference issue with cell phones , lights, computers, etc? It does not seem to be a problem with any of my other amps... I simply cannot dedicate the rooms power circuitry to the amp alone...

[gmphadte]

Have u put it in a box, preferably metallic...

Gajanan phadte

[vikas456]

Record the noise comming from you AMP, convert it to MP3 or WAV format, and attached it to your post for others to analyse.

[Minion]

If it is RF ir EMI Interferance that is causeing this Problem then maybe adding a Ferrite bead to the Input/Output would help??

Just a thought....