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?
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?
john65b said:
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.
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?
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?
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.
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 said:
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
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Aha!
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...
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...
The amp is in a steel enclosure (an old Parasound HCA-500 chassis). The motorboating effect is apparent with cover on and off...
I have a couple ferrite beads that I can put on the outputs - I can try tonight...
Record the noise?? OK, I should be able to record it tonight and post it on my file server...
I have a couple ferrite beads that I can put on the outputs - I can try tonight...
Record the noise?? OK, I should be able to record it tonight and post it on my file server...
The chassis is not grounded (2 prongs on power cord on the Parasound HCA-500)
I can try the other items mentioned...
Incidentally, I was in a conference call with my boss yesterday when he got a call on his cell...that exact same motorboating noise came through his computer speakers, so it is interference that I am dealing with...
Re: Aha!
try 100nF ceramics from the power pins to ground and one 100nF across the rails -- as close as possible to the chip. the leads should be as short as possible -- elsewise they become receptors themselves and you make a bad situation worse.
a 220 pF silver mike across the inverting and non-inverting inputs as well.
Read the prospectus carefully before you invest -- National tells you how to cure this.
john65b said:
try 100nF ceramics from the power pins to ground and one 100nF across the rails -- as close as possible to the chip. the leads should be as short as possible -- elsewise they become receptors themselves and you make a bad situation worse.
a 220 pF silver mike across the inverting and non-inverting inputs as well.
Read the prospectus carefully before you invest -- National tells you how to cure this.
Hi,
there are a lot of chipamp schematics that omit the RF filter on the input. They even omit the LF filter (DC blocking cap) to compound the problem.
Most builders assume the schematics are correct and follow the mistake religiously.
Always fit the input filters, even if it compromises the audio performance. Once you understand the complexities of the amplifier, then you can experiment with improvements with informed decisions/knowledge.
there are a lot of chipamp schematics that omit the RF filter on the input. They even omit the LF filter (DC blocking cap) to compound the problem.
Most builders assume the schematics are correct and follow the mistake religiously.
Always fit the input filters, even if it compromises the audio performance. Once you understand the complexities of the amplifier, then you can experiment with improvements with informed decisions/knowledge.
I know that cell phone noise John is talking about. As far as I can tell, it is definitely be radiated into my line level circuitry. This is easy to replicate when using my laptop as the source. I just bring my cell phone close to the laptop and when the phone tries to send or receive a signal you can hear it coming out of the speakers. My chip amps use a differential input with differential and common mode filtering as well as a cap across the inputs of the chip amp itself.
Yeah, wireless is nice but has its disadvantages. It doesn't help that my laptop uses a wireless internet connection as well, although that has never really caused problems with my audio gear. Cell phones, on the other hand, effect my home and auto audio systems. If I put my cell phone right above the head unit in the car I hear the same noises from the speakers when the phone is transmitting/receiving as I do with my home equipment.
Here's the schematic of my amps, including filtering.
http://www.diyaudio.com/forums/attachment.php?s=&postid=1157922&stamp=1173892828
Here's the schematic of my amps, including filtering.
http://www.diyaudio.com/forums/attachment.php?s=&postid=1157922&stamp=1173892828
Also, I do have 2.2uf input caps - Bennic 5% - but I will try the .01uf caps accross the rails close to the IC pins...
Like I said earlier, I tried to stay true to the original Gaincard design as best as I could - don't see any of what you all are suggesting on the Gaincard (doesn't mean that its right/wrong)
Thanks for all the suggestions...I will try later
Like I said earlier, I tried to stay true to the original Gaincard design as best as I could - don't see any of what you all are suggesting on the Gaincard (doesn't mean that its right/wrong)
Thanks for all the suggestions...I will try later
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