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Old 11th October 2006, 11:55 AM   #1
kmj is offline kmj  Sweden
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Default Buzz in gainclone, looking for source.

Hi, you all know that kind of background buzz that is present on almost all amps. The low level noice that you can hear if you put your ear to the tweeter, not a ground humm but a buzz like a static sound.
You know what I mean?

I recently put a gainclone with LM3875 to a friend and I get this sound but at a higher level and I would like to know where to look for the solution to the problem.

It is configured as dual mono to the socket and the grounding is as follows:
for each channel there is a signal starground on the groundlug of the RCA-input and a power starground right between the 1500uF capacitors. These are connected with a thin wire (0,4mm). The speakerreturn is connected to the power-star.

The two powerstars are then connected to eachother with a solid wire and fron the center of that is the safetyearth connected.
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Old 11th October 2006, 12:03 PM   #2
kmj is offline kmj  Sweden
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Old 11th October 2006, 12:39 PM   #3
AndrewT is online now AndrewT  Scotland
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Hi,
I used to have the same problem, but this Forum taught me how to do it properly.

I have just built up 4 monoblocks (two completely different amplifier types) and all of them are completely hum and buzz and noise free.

There is no need to tolerate any buzz from normal sensitivity speakers (mine are 89db/2.83V).

Read some of my grounding posts for extensive advice.
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Old 11th October 2006, 02:57 PM   #4
kmj is offline kmj  Sweden
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Ah, so it is the grounding then. I accually thought it was something else since it was more a buzz than a hum. But I'll split the connection chassiground-powerground inte two wires, one for each channel and then lift the signalground with a resistor and see what happens.

Btw, your posts are really informative, you just have to know what to search for
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Old 12th October 2006, 08:42 AM   #5
fallow is offline fallow  Finland
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Hi,

Does the buzz appear when the source of gainclone is shorted, i.e. the input of gainclone is connected together?

If there is then buzzin sound the grounding would effect there, but if there is no buzz in output the noisefloor of source is making the buzz.
If the gainclone itself causes the problem then it might be the chassisground connection to chipground.
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Old 12th October 2006, 12:39 PM   #6
kmj is offline kmj  Sweden
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Hi Fallow, there is no noice at all when I short the input to ground.
(I just put a solid copperwire between the input and signalground)

I have used two sources as of today. A laptop and a portable CD since I don't have a pre-amp ready yet.

Edit:
Theres a buzz on my laptop, stationary computer and portable CD. On my MP3-player there is no buzz.
So I guess that the grounding should be correct then, however, can I get rid of the noice in another way then?

Edit2:
I went through my attic and connected the optical output on my stationary computer to a externad D/A-converter and the noice was back.
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Old 12th October 2006, 01:11 PM   #7
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by kmj
no noise at all when I short the input to ground.

Theres a buzz on my laptop, stationary computer and portable CD.
Sounds like you need a better sound card in the computers.
When the portable CD is battery powered, does the buzz disappear?

If all your mains powered sources are correctly grounded and the power amp is incorrectly grounded, there should be no hum.

However, if one source and the power amp are incorrectly grounded, then you can get a ground loop which will hum.

Attaching only one incorrectly grounded source to a correctly grounded power amp should have no hum.

The result is that a correctly grounded amp can hide upto one incorrectly grounded source.
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Old 12th October 2006, 01:41 PM   #8
kmj is offline kmj  Sweden
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Quote:
When the portable CD is battery powered, does the buzz disappear?
No, I only use batteries.

Quote:
If all your mains powered sources are correctly grounded and the power amp is incorrectly grounded, there should be no hum.
I connected the poweramp to a grounded socket in the kitchen and connected the portable cd and the hum was there. I tried the simple thing to turn the powerplug and still there was buzz.

I tried to add a 10ohm resistor between the singal and powerground, it increased the noicelevel so I removed it again.

I fiddled with some of the internal wiring and nothing happend.

I went back to the livingroom and connected the portable and the noice was lower and acceptable, I guess. Audiable at 1m in an otherwise quiet room.

I connected the computers again and the louder noice was back.

So I guess it's a matter of source whick is bad since a computer is my friends primary use.
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Old 13th October 2006, 05:18 AM   #9
fallow is offline fallow  Finland
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Hi,
Try to connect signal ground and mains ground with resistor and capasitor parallel, this should be better than single resistor.

Or try to put thinner or longer wire(to offer weaker path from power ground to signal ground) connecting between signal ground and power ground, because there should not be any current flowing from power ground to signal ground.

Have you tried to put 220nF capasitors parallel of the 1500uF capacitors? Or maybe move the starground connection away from capacitors, because those big caps tend to produxe noise when those are used. (Just some ideas which might work, dunno)


I'm fighting with same problem with my own preamp, there is buzzing sound at the vlme controller of my source, when I connect my gainclone to commercial source, i.e. The EMU 0404 or CD-player, the buzzing sound dissappears or it is significantly lowered. There is one trick which might help to lower the buzzing noise when the machine is on, but nothing going in (this trick does not help on silent part's of the music). You can put a resistor division to input of your amplifier, this lower's the volume, but at the same time it lowers the noisefloor, when the noise of resistor's is not greater than the noisefloor.

then back to the original problem:

Do you have a way to measure the output inpedance of your source and input impedance of your gainclone, then put similar load to your source and measure it's noisefloor.

And check the impedance matching of your source and gainclone,
this might also be the problem for generating the high frequency noise to your system.

Good luck of hunting the source of noise.
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Old 13th October 2006, 04:37 PM   #10
traw is offline traw  United States
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yet another consideration...

on one channel for trial, put 2 bigger caps, say >= 4700 . then use 2 x 100uf instead of the 1500uf... in some of my own experiments, notice use of bigger, > 470, near chip amp can be touchy and sensitive to grounding issues(not always). it would seem the all or significant power supply filtering near the chip amp can be problematic but i haven't confirmed across all builds to make that an absolute. i had one board layout that was lucky and immune to ground noise pickup breaking that rule. i had caps located together in opposing layout (not having stripes same direction).

or if resources limited, borrow the other 2 1500's, put those 4 further away from chip and find some 33-100's to place on the rails closer to chip. should be a quick little experiment (curious myself if theory, errr, observation holds any water)
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