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Old 6th June 2006, 07:26 PM   #1
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Question Bulletproof Buzz

Hi again, fellow music-lovers!

I recently built a pretty nice amp (details in my intro: <A New Arrival> ) but I have a problem with it...

For the life of me I can't isolate the source of a slight buzz (not a hum; it comes through all speaker drivers, but it needs to be nighttime or 6 inches away to hear it). The following have been examined so far:

-volume pot from full attenuation to unity - no change.
-connect / disconnect signal sources - no change.
-input ["clean"] ground directly wired to the star, with resistive anti-ground-loop standoff to circuit ground - from 0 - 1k on "standoff" resistance - no change.
-input signal wire placement and shielding - no change.
-noisy (?) rectifier (GWPC3504A) shunt ceramic caps (0.047uF each diode) - no change.

THE QUESTION:
Should the global feedback reference the "clean" ground instead of circuit ground? I have heavy rail filtering on-board (220uF +0.1uF per rail at output, refiltered through 27-ohm -> 220uF + 0.1uF for DA and VA stages), and perhaps these are dumping any HF noise that gets through the tanks (suggested by rising ESR of tanks (37,000uF) at frequencies above 120Hz and why would be a buzz and not a hum) into the feedback reference at the N-inv input due to non-superconductor ground connection from circuit ground to the star? Shouldn't matter if it's the inv or N-inv input right? An upside-down buzz is still a buzz?

Problem is I have seen schematics both ways, and talk is usually pretty vague on input conditioning and on whether the feedback ground reference should be the same as the input ground reference. Just thought I would ask before hacking my board traces and jumpering...

If anyone can help shed some light on this I would be grateful!
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Old 6th June 2006, 11:48 PM   #2
anatech is offline anatech  Canada
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Hi acoustixman,
Feedback ground should reference the input ground. You want the difference between what is coming out and what went in.

The high frequency aspect sounds like some supply bypass currents are getting into your clean ground.

Didn't catch your schematic in the intro, I may have missed it.

-Chris
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Old 7th June 2006, 12:26 AM   #3
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anatech:

Thanks for the reply;

The more I thought about it the more it makes sense that this connection should be dedicated (feedback reference to input ground). I believe the huge caps are letting the "corners" of the ripple in due to upper harmonic content at diltered DC waveform vertex (occurring 120Hz). This remaining ripple energy is riding my rails to the on-board refilter caps and there being diverted to ground.

My new realization, especially in light of your confirmation is that this filter ground is really nothing but a garbage chute and should obviously not be expected to provide a clean ground reference for any signal.

I'm going to try severing the filter ground -> feedback reference connection and jumper the reference to input ground tonight...

On the schematic, should I worry about releasing my design if I intend to eventually produce a saleable product with it? That's the only reason I haven't actually posted the picture up here yet...
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Old 7th June 2006, 12:38 AM   #4
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Hi acoustixman,
Glad you figured it out. Let us know how the change helps things.

About iP and commercial products. There are only so many ways to make an amplifier. In the end, the circuit layout on the board and components used make a bigger difference. How well you match components and the like.

There are always people who will "steal" a design. If you want it to remain a secret, never sell one, never show anyone. The easiest way to copy an amp is to buy one. That tells you absolutely everything you need to know.

To get an idea, check out MikeB's Symasym thread. Quasi has one too as do others that they have been kind enough to share with everyone. I honestly don't know that they are worth "ripping off", but it wouldn't surprise me to see someone selling a kit either. Low lifes are everywhere. Not to say the projects aren't good, because they really are. It's just that they use parts better than the standard kit or copied design. There are always exceptions to that too.

There are designers that have shared more knowledge than I can absorb. Nelson Pass and others are great examples. I can't list them all, but they have shared more than I can come up with.

-Chris
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Old 7th June 2006, 03:22 PM   #5
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anatech:

Well, I severed the trace from feedback ground to [PCB] filter ground and jumpered it to the clean signal ground. What were we talking about again? Oh yeah, that BUZZING that's ALL GONE NOW!!! Duh!!

TO ALL AMP BUILDERS: Keep input and feedback references connected to a dedicated ray of the star ground point!!!! Spare yourself my lesson.

My amplifier is consequently capable of replaying SILENCE now, and what a pleasure. I would never have claimed to hear the buzz over even the softest music, but now the amp falls back into the blackest void during silent passages. My diff amps have 2.25 mA per leg and that seems to be low enough to really hide Johnson noise and the like (the "fffffffffffffff" sound). It's hard to believe it's turned ON, despite the LED on the front, until it springs to action for some tunes.

I have a DC cooling fan (120mm Muffin XL, 24VDC from separate toroid and regulator also used for DC speaker disconnect monitor and relays), voltage controlled for lowest acoustic noise via heatsink temperature (max air at about 60C in core), and at idle it can [now] be heard as the faintest (10cm from tweeter at night) rapping sound at about 40-50 cycles. Noise from an AC fan would probably be more rejectable with its more sinusoidal current waveform compared to the pulse-train from commutation in a DC motor, but [sonically and electrically] quiet AC throttling requires a high-end VFD (probably digital, not that I hate digital, just not in a power amp, thank you).

Good points as well on design publication. Nothing ventured nothing gained... I spoze I'd never sell to people who'd rather build'em themselves anyway (although there are kit-builders out there I'm sure...), and they might as well build mine. It would just be nice if they mention that later.

I think I'll post a couple schematics. Maybe in their own thread?

Thanks a lot!
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Old 7th June 2006, 03:29 PM   #6
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Hi acoustixman,
A good result. Excellent!

You may benefit from posting and discussion. There are many talented people around here. A new project generates interest.

And yes, don't lose any sleep over what others may or may not do with your amp design. You stand an excellent chance of "meeting" some really helpful people and improving your amplifier even further. Be prepared to be questioned on your design choices.

By all means, start a thread for your amp design.

-Chris
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Old 7th June 2006, 05:07 PM   #7
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I have the schematic updated for the latest changes, but I was actually just fixing to go to a nearby electronics shop to have my amp analyzed for THD+N, frequency response, and performance power rating. I'll start the thread with that info. That's why I had become desperate about the buzzing. I didn't want that "+N" part making my amp look cruddy on paper just cuz I couldn't shake a little buzz... More soon. Thanks again.
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Old 7th June 2006, 05:12 PM   #8
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Hi acoustixman,
Sounds like fun. I wish you good numbers, but better sound.

-Chris
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