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-   -   bosoz buzzing problem (http://www.diyaudio.com/forums/pass-labs/61362-bosoz-buzzing-problem.html)

cowanrg 25th July 2005 04:20 AM

bosoz buzzing problem
 
yes, another problem. i just started another thread because i have a feeling someone in the future might have this problem, so instead of burying it in a 4-page thread, ill just make a new one for it.

anyways, here is the problem...

i got all the fets changed out and the preamp section tests well. the two voltage reference points are pretty close to perfect (+-1% or so). and, i got music. however, after completely finishing the faceplate, hooking up the LED's for what i thought would be the last time, now i get some buzzing and humming out of it.

i would normally assume ground loop or something like that, which i am familiar with and can fix. however, this is different. the amp belts out a loud buzz sound as soon as its turned on, and it slowly fades after maybe 5 seconds, and goes away. the amp isnt exactly dead quiet afterwards (its got some hiss and a SLIGHT hum) which increases with volume to a pretty loud buzz at full volume.

if i turn the amp on, wait until the buzz thing goes away, then turn it off and back on really quick, it doesnt happen again. so, its like it only happens when its "charging". thats just an explaination of how it feels, not a techincal explaination of what i think is happening.

also, im using a light bulb in series to check for shorts (ive tried without one, it does the same thing), and when the amp turns on, the light bulb light VERY brightly and dims in sync with the buzz. and the light bulb never completely goes off. it is always on just a little bit. it glows very dimly but is never off when the preamp is on.

the funny thing is, when i was testing the amp after i changed those resistor values that were wrong, i never noticed a hum at all. it sounded fine. the buzz is quite loud and it sounds like something is VERY VERY wrong at first, and then it slowly fades and goes away. but when it first comes on, you think the speakers are at risk or something is seriously in danger.

that doesnt seem like a good sign.

SO, i will conquer this, but i will definately think twice about building a preamp again! ill link the previous thread so you can figure out schematic and stuff if you havent been following my plight.

http://www.diyaudio.com/forums/showt...threadid=60218

p.s. - this thing is drop-dead gorgeous with its final coats of lacquer though! and i didnt put anything against the backs of the LED's so they glow inside the case to let a tiny bit of blue light spill out of the top vents. yummy. now it just needs to work.

jan.didden 25th July 2005 07:23 AM

Hi,

It sounds like it is on the verge of oscillations that just come and go as you describe. Was there any change in the wiring, wire routing, screening or something that could have caused it? Changes to grounding the connector(s) to the chassis?

Jan Didden

cowanrg 25th July 2005 10:14 PM

well, more updates.

janneman, thanks for the advice.

the amp was tested in the same environment as before. i really cant think of anything that was different. it didnt have the faceplate on, which has LED's on it. but, they are wired separately from the PSU. they have their own windings on the transformer.

i did a bit more reading on here and saw that some people had a humming or buzzing because their RCA's were grounded to the chassis. so, i used external RCA's that werent touching or connected to the chassis at all. they acted pretty much the same. it still buzzes when it comes on, and buzzes for a brief second once it turns off.

its pretty loud and doesnt appear to be dependant on the volume level of the preamp. when volume is off (which, with music, is off), it is same volume. and when its full volume, its same loudness. it happens with or without a source plugged in or turned on. i even removed the whole input selector/volume control from the circuit, so you had NO input what-so-ever and it was still there.

i did further testing and it is technically operating just fine. im getting fine voltages and its getting very warm. i did notice however that the music sounds like crap though. its some of the worst music ive heard come out of a stereo. the bass is so sloppy it almost sounds like a different song and there are virtually no highs. its MUCH different than when i made my first post. it sounded decent except for it was just a bit noisy. now, it just sounds bad.

how can there be this many problems with such a simple circuit???

steenoe 25th July 2005 10:24 PM

Quote:

how can there be this many problems with such a simple circuit???
Cowanrg, you have my sympathy;) You just seems to run into all sorts of problems, with that darn BosoZ:bigeyes: Check out your solderings in the supply, very carefully. If you shorten'd up the leads on the trafo; make sure you did clean up the ends until they are pure copper. This could sound like a cold solder point:xeye: Or a thousend other things:hot:

Steen:)

cowanrg 25th July 2005 11:00 PM

the only wires that were changed recently were the ones for the LED's. everything else is pretty much the same. all the PSU wires are wire-tied to the chassis and cant be removed easily, and havent changed since the beginning.

do i need to be shorting the - and gnd for the inputs and outputs? kristijan (if i remember right) said i didnt need to for this design. i figured it really wouldnt work right at all if i had needed to...

any suggestion is a good one at this point!

edit:

here are a couple of questions i have. they are just general, but pertain to this project, but will help me out in the future...

1.) when using the light bulb thingy in series with mains, how should it act? when i turn on the preamp, should it glow full power, then dim to just a faint glow? is that normal, or should it always be off, or just be on when the preamp first kicks on?

2.) do you have to insulate RCA jacks from the chassis?

3.) without a scope, is there any way to tell if an amp is oscillating?

4.) can ground issues alone make an amp oscillate?

5.) what are the basic symptoms of an oscillating amp? can it still play music? will the music sound different? do you just get a hum/buzz, or will the music be terrible?

steenoe 25th July 2005 11:08 PM

Quote:

do i need to be shorting the - and gnd for the inputs and outputs?
If you only use singleended input, you need to short -in to gnd. Don't short the output! Easy enough to do on the PCB. Hope I understood your question right?

Steen.:)

BTW,
Quote:

2.) do you have to insulate RCA jacks from the chassis?
Yes, that would be a good idea!

cowanrg 25th July 2005 11:14 PM

Quote:

Originally posted by steenoe
If you only use singleended input, you need to short -in to gnd. Don't short the output! Easy enough to do on the PCB. Hope I understood your question right?

Steen.:)

yes, you understood perfectly.

geez, that could be one problem...

i just checked it, and the - and GND are not shorted. they are just open. ill have to fix that... but i will only short the inputs.

regarding the RCA jack isolation... is it a good idea, or a necessity... if it MUST be done, ok. but if it doesnt have to be done, and you can get away with it not being done, that would make life easier.

i already tried the amp without completely isolated RCA jacks and if anything it was much noiser. so, if its just a "good idea" then ill keep it the way it is, but if its the problem, i can fix it.

steenoe 25th July 2005 11:33 PM

Quote:

so, if its just a "good idea" then ill keep it the way it is, but if its the problem, i can fix it.
You have to isolate the jack's. It could easily cause you hum problems, if they are not isolated. You will have several returns to gnd point, and you only want one;) Spelling "starground". I am looking for JH's nice drawing of a starground, and will post it when I find it;)

Steen:)

Edit. I found it:) I just like that drawing of JH's:)
http://www.diyaudio.com/forums/showt...503#post510503

cowanrg 25th July 2005 11:45 PM

how do you go about isolating the rca jacks from the case though?

steenoe 25th July 2005 11:53 PM

2 Attachment(s)
Quote:

how do you go about isolating the rca jacks from the case though?
I am using jacks with teflon washers. You might be able to find some kind of plastic washers, that will do the job. Something like this pic:


Steen:cool:

Edit, some of my friends use plastic for the rearpanel, to get rid of that problem. Plexiglass, fiberglass or something like that. Might save your jacks;)


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