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Old 17th November 2005, 05:03 PM   #1
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Default Another Buzzing GC Query

Hey, All!

I finished my first amp a few weeks ago, and I'm now ready to think about tackling it's only problem -- a bit of a buzz.

I didn't try to fix it yet, as I'm *really* enjoying this amplifier. A LOT.

A brief description of the problem
- Buzzes like MAD with no inputs
- Absolutely DEAD quiet with the inputs shorted at the end of a 6' RCA cable
- Buzzes quietly when hooked up to any source in my rack. Buzz is audible at about 6-8" (15-20cm), but no further. (Thank God my listening position is at 1m).

Before I installed the amp in my rack, I tested it on the kitchen table with an old pair of speakers and a battery-powered CD player. No audible buzz.

I then racked it up, and put a set of more effecient speakers on it. It now buzzes with any source. The "source" I use most is a Carver C11 preamp (two-prong). Oh! It was buzzing a fair bit (audible at 6 feet/2m) when I first racked it up. Since I left my channel grounds "available" and capped with marrette connectors, I was able to reach inside the amp and connect the two channel grounds together; this got the buzz down to the level it is at today.

A bit about the amp:
- Dual Monoblock design with a single toroidal transformer, built around BrianGT LM3886 kits
- Earth ground is connected to chassis and no where else
- Chassis is ~18awg stainless steel
- All grounds for each channel meet for the first time on the amp PCB (so, basically, a signal/power star on 1 square inch of copper). They are not connected anywhere else
- It's a power amp -- no tone controls, no volume knob
- Inputs leads are quite short, Belden shielded twisted pair. Shield drain is connected to chassis at one end.
- All wires are way thicker than they need to be (had to pare them back to fit in the PCB holes)
- Only "quick connects" are on the AC side; everything else is soldered
- Inputs and outputs are also run in parallel with (currently unused) 1/4" TS jacks (switchcraft or neutrik, I forget which). Twists in input wire were preserved between jacks.
- 22k Phoenix SRS resistors mounted in parallel with the inputs
- Plate where the jacks are mounted is fiberglass PCB material (copper etched away)
- I tried to keep input and output wires apart and orthogonal

Any thoughts would be appreciated. I really love this amp, but once I get the face plate made up, it will only have ONE flaw IMHO -- the buzz. And even though I can't hear it from my listening position, just knowing that it is there will make me mad!

Oh -- If I had to guess, I'd say that the buzz is in the 150-200Hz neighbourhood, but I haven't thrown a frequency counter at it or anything.

Tools at my disposal -- 'scope, audio freq generator, cheap multimeter, brains.

Here are some photos of the amp which will show the construction:
http://flickr.com/photos/shellyandwes/sets/1291427/

BTW, that paint job was hard as hell and chips too easily. And CarlosFM, thanks for the mouse pad idea!

Wes
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Old 18th November 2005, 02:18 AM   #2
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Hi Wes,

The thing I'd suggest is try removing the connection to chassis of the input wiring shield drain (I'm assuming you are running it from the socket end, and that it is also connected to the PCB star ground), and instead run a wire from the Star point to the chassis or from the center of the PS caps to chassis, whichever seems to work best... hopefully might make a difference.

one question too. are any of the components connected to the pre earthed via the mains, if so try disconecting them and see if it goes away.

Tony.
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Old 18th November 2005, 01:04 PM   #3
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Tony;

The drain wire connects to the chassis near the PCB, not near the inputs. It doesn't actually hit the star ground anywhere (as the chassis ground is not part of the star).

The preamp, likewise, is not connected to earth at all.. unless it's some how getting connected through it's rack ears (but I don't think this is possible due to the chassis design -- I should have another look, though!).

The pre *does* have a ground terminal IIRC, but I haven't connected that anywhere. I can't imagine that I should need to, as the amp is set up so that it should be able to float without difficulties.

Puzzling, eh? You think you do everything good enough, and the damn thing still buzzes.

I'm still pleased, though, I can't believe these simple-to-build amplifiers sound SO good!

Wes
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Old 18th November 2005, 01:42 PM   #4
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Probably not the case there, but after fuming at the humming Rev. C amp I built, I noticed that I let the mains lead run over the input leads, while quicly setting it up to test.... lol I picked up the mains cable, and carried away the noise...
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Old 18th November 2005, 02:20 PM   #5
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Hi Wes,

I know how you feel, my 100W mosfet amp has a buzz, which I've never been able to eliminate... it's only audible within about 20cm of the speakers, and is only that bad on one channel. In RMAA it gives an excellent rating for S/N ratio, but it bugs me!!

I've had a look at brians PCB's I'm assuming you have got the signal sheild connected at the SG pcb point, and the connection to your chassis ground connected at the CHG pcb point. Looking at the layout of the PCB and how the tracks are connected that seems the logical way.

Have you checked whether the preamp earth connection has continuity to the gnd on the rca's?? maybe it is isolated, but connected to the case?? if it is isolated, you could try running an earth wire back to the amps chassis.

Oh and one other thing... the wires in the first pic (on the standoff), I assume that they are all mounted on a nylon bolt with nylon spacers??? Looks like a clever way to get high density without the problems of a terminal block

Tony.
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Old 23rd November 2005, 03:59 PM   #6
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Hey, Tony!

Good thinking about the possibilty of a problem with the preamp's grounding. I sort-of eliminated that by plugging my keyboard directly into the amplifier; the buzz really doesn't change in character, and the keyboard is also a two-prong jobbie. I also know that internally, the keyboard's chassis is connected to the system DC ground.

The wires on the standoff are indeed mounted on a nylon bolt, but using nuts rather than spacers between then. Makes assembly easier.

I used a #6 nylon bolt and #6 connectors for 16awg wire. I chose #6 because it was the smallest size for those connectors, and the the way they are made, they vary the hole size rather than the tab size (so they just change the punch). That means more surface area between the connectors. The connectors were crimped and soldered, then assembled on the bolt three at a time, the nuts tightened, then the bolt treaded into a pre-tapped plastic standoff. The whole unit functions as a 4-wire Y-splitter.

I'm glad you think it's clever, it took me a long time to solve the problem I was having -- basically, oh crap, all this stuff is ready to go here, but I don't have an elegant way to hook the wires up, and the terminal block doesn't fit!

The signal shield -- not connected at the SG point, it's connected to the chassis, and hence earthed. My signal wires are shielded twisted pair, so the signal ground (TS and RCA rings) is connected at the SG point.

Hmm, I wonder if signal foil is picking up earth ground noise and inducing it in the inputs? You wouldn't think it would a big deal even if it was, at that would be common-mode anyhow. Hmm. Hmm. I used the shielded wire to try and make my amp cellphone-noise-proof, I wonder if I should build a high-pass filter for the signal shield.

Like. Maybe. The world's most complicated filter, a ceramic cap. That should let though just high frequencies (radio) and kill all the low freq (audio, power) stuff? Or is it the other way around? Crap, I can never remember. I wish they had taught AC theory in school.

Wes
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Old 24th November 2005, 01:31 AM   #7
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Hi Wes,

OK sheilded twisted pair I get it now I doubt that the shield is causing problems, should be fine as is. One thing though, did you install the optional zobel components on the boards?? the 2.7 Ohm 2W and 100nF cap??? If you didn't then you may have a slight oscillation which can also cause a buzz in the output.

yeah you got the cap right it will pass the high freqencies only (lower the value the higher the freq).

Tony.
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Old 24th November 2005, 01:38 AM   #8
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Zobel.. zobel.. zobel..

I don't know if I put it on.

The boards are fully stuffed, do you know if Brian's boards are drilled for the zobel network?

I recall add an optional 100uF cap. I don't recall any 2W resistors on the amp boards, though.

Have you built Brian's kit?

Incidentally, would slight oscillation explain the LOUD hum when nothing is connected?

Wes
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Old 24th November 2005, 02:10 AM   #9
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Hi Wes,

I used Brians LM3886 schematic, but I did p2p no pcb's.... yes the board has the provision for the zobel components... Not sure about the osciallation vs loudness, I've just been advised when querying about my 100W mosfet amp, that oscillation can cause buzz...

One thing I did notice with my GC though was that I had a loud buzz with no input connected when I didn't have a connection from the PS star ground to the chassis (which is earthed), the odd thing was when I connected it to my the amp to my PC (which is earthed) the noise level dropped dramatically....

After connecting the PS star point to the chassis, the amp was dead quiet (could only just hear a very minimal hum and hiss with ear right against the speaker). It still has a bit more noise than that when connected to the pc, but I think that is to be expected

actually now that I think about it, from memory you don't have anything but the signal wire shield connected to the chassis right (I was thinking it was a normal shield and signal two wire setup)?? try getting an aligator clip lead and connecting from the CHG point on the amp PCB to the chassis ground point, and see whether your hum goes away

Tony.
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Old 25th November 2005, 02:59 AM   #10
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Well, Tony -- you've provided some very interesting input.

I was not able to get the hum to away with equipment plugged in, BUT if I connect CHG to chassis ground, the LOUD hum goes away when there are no wires connected!

This suggests to me that the amp is on the very of oscillation, and having no input throws it over the edge. Grounding the amp probably "pulls" things a little, and keeps it out of oscillation. Does that make sense?

Now, the other finding I made tonight. It turns out that it doesn't matter what equipment is plugged in. If I have two wires wire dangling, the quiet hum is there. If I plug them into the pre, the hum is there. If I connect the pre's ground lug to my channel ground (which now has the chassis ground on in), the hum is there. It's there if I use the quarter-inch inputs, it's there I use the RCA inputs.

But, with nothing at all connected, the hum is gone. And previously, it was gone if I shorted the ends of an RCA cable plugged into the inputs. (Didn't test it today, I'm sure it would be fine).

Odd, eh?

Here's a very bad picture of my wiring scheme. It's accurate in terms of connections made, but not in terms of how the wires are actually run. It also doesn't show the shield on the input wires.

Wes
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