Practice Amp buzzing by design - pls help improve it

Hi.
I bought this little practice amp, Behr* vtone GM108, and it sounds great.

Actually, it sounds so great, that I plan on recording through it, both guitars and synths. Problem is, that it was not built for recording. It has slight buzzing present all the time.

It is not factory failure, I tried others in shop and all had the same buzz. It is slight, and not heard while practising, but will be picked by close miking. Buzz is the same independently of what is plugged in, and independently of settings on buttons and independently of volume setting. At high volume white noise comes out, but that's fine. I just want to get rid of buzzing.

I opened it (see photos below).

http://84.255.203.119/amp/20081001_0001.JPG
http://84.255.203.119/amp/20081001_0003.JPG
http://84.255.203.119/amp/20081001_0004.JPG
http://84.255.203.119/amp/20081001_0005.JPG

Buzzing is visible on scope. It is not sinusoid, but spikey-triangle-something signal, with 50Hz frequency.

I checked power supply signal, and it is not nice sinusoid. It is like cca. 10% overloaded sine, sinusoid with sharply flattened top with some hi- freq dirt on it, so probably transformer core is overloaded or something.

In power supply part there are two 1000uF capacitors. Should I add in paralell some bigger ones? There seem to be no cheramic bypass caps. Should I add some 100n from both PS rails to ground? (edit: i noticed two ceramic 100n caps hidden behind pot, so there is bypassing)

Or is it maybe inducted noise from transformer, and will not be cleaned by modifying power supply?

What would you recommend?

If it's 50 Hz I wonder if it's a magnetic induction from the transformer. Maybe try to rotate the power transformer?
Power transformers are Behringers weakest spot...

Michael

[quote author="Michael Tibes"]If it's 50 Hz I wonder if it's a magnetic induction from the transformer. Maybe try to rotate the power transformer?
Power transformers are Behringers weakest spot...

Michael[/quote]

After removing screws I tried moving transformer few centimeters here and there, rotating it in all sides, and there is no audible change in buzz.

Transformer leads are short, but I was able to move it 5cm here and there. If induction was the reason, shouldn't those 5cm movements cause some audible difference in buzz?

Based on this, can I conclude that induction is not the cause?

So, would the reason then be insufficiently regulated PS voltage?

Sure that it's not exactly double that; -100Hz?

That would point to DC line ripple.

Keith

Posting an audio sample (30 s) might help.

Samuel

Here is 30s sample of buzz, recorded with sm58 3 centimeters from speaker, from behind:
http://84.255.203.119/amp/amp%20buzz%20x30dB.mp3
It was recorded with high preamp gain, and later amplified by 30dB in Cool Pro, so you can hear it clearly. I'd say it is some 40 dB below usual recording full scale level.

Looking at it with analyzer in Cool Pro, it is silent 50Hz base, but 100Hz is first prominent harmonic.

Here is photo of scope at speaker output:
http://84.255.203.119/amp/20081001_0014.JPG
Buzz is constant and independent of any settings, tone controls, volume.... It stays the same no matter what.

Here is scope photo of power supply, negative, measured directly at power amp pin. Positive looks is bassically the same, but positive.
http://84.255.203.119/amp/20081001_0016.JPG
Ripple can be seen easily even on 5V/div scale

Next are scope photos of loaded (connected) transformer output.
http://84.255.203.119/amp/20081001_0017.JPG
http://84.255.203.119/amp/20081001_0018.JPG
Probably what is seen is due to diode regulation, is it?

(edit) Power amp IC datasheet can be seen here:
http://www.datasheet4u.com/html/D/2/0/D2030_ShaoxingSilicoreTechnology.pdf.html
click at "DataSheet View"
...

Bump...

Based on above data, would you say that power supply ripple is the cause of buzz?

What do you recommend? Shall I just connect some big capacitors in parallel to existing ones, and see? Clearly, I do now want to destroy it. Something else to consider before doing that?

...

I can't derive a clear cause from the available data. Both magnetically induced hum and supply-injected hum could be the origin, perhaps even a combination. Doubling the smoothing capacitors should not cause any harm, so you could start with that if it is easily done. While this will surely reduce supply-injected hum it might unfortunately increase magnetically induced hum as the transformer peak currents will increase.

Samuel

I would dress those transformer wires away from the pcb just for general housekeeping but that is not the problem... your tests show 100Hz buzz which seems to indicate a post-rectifier problem, and I think you need to parallel those two 1000uF caps with another pair of capacitors. At least 1000uF each (2200uF is probably better), and see how that sounds.

Be sure the big caps are drained before messing with them.

regards, Jack

Oh, man, you fix it in ProTools. Don't PT have a buzz-filter, stock or add-on? Even old CoolEdit2000 let me program narrow notches at 60, 180, etc. (CE2K also had a noise reduction which would "learn" noise and then subtract that from music.)

Anyhow....

The chip claims 40dB PSRR.

1V ripple at rail means 10mV at speaker. This is about what you observe. So Working As Designed.

1V ripple at dead-idle seems high. For 2*1000uFd it suggests like 0.5 Amps idle current, and that can't be right.

I don't see the rectifier. I do see the PT leads and they don't land near the main filter caps. The PCB traces to the main caps are ample on the common ground sides, but are skinny (or hidden?) to the hot side of the caps.

A common mistake when you send a plan to PCB layout without being real fussy is to have a long length of wire (trace) in-common between rectifier current spikes and load hot lead(s). The auto-router cleverly zig-zags the trace around the jack, pads, and LED which the auto-layout (or front panel artwork) put in the way. The result is the load is better connected to rectifier spikes than to the filter cap. Top sketch.



Unless B-ringer has done something too clever around the rects, there is a fix. Take rectifier DIRECT to the caps. Bottom sketch. Rather than re-work that too-tight PCB, throw in a $2 bridge rectifier and abandon the old rects in place.

Lift all 3 PT leads off the board. Get a >50V >1.5A bridge rectifier. Butt-solder PT CT to that lovely big pad between the two caps. Butt-solder bridge rect's + and - legs directly on the filter caps' hot-leg pads (in correct polarity). Connect PT hot legs to bridge's "AC" legs.

If B-ringer has done nothing odd around their rects, this should work and may even address the buzz.

If B-ringer did something odd, or if you mis-connect, it could smoke and burn at turn-on. I suggest taking it outside, or at least use a Lamp Limiter, for initial smoke-test.

This butt-solder hanging-on bridge rectifier is liable to pull-off the PCB pads when you drive the long bumpy road from Lodi to Winnemucca Flats. Maybe do something with ShoeGoo. Or just don't use this amp for road-trips. Heck, for $60 you can buy one for the studio, one for home, and several for the road.

I am wondering where the hell is the connection to Chassis Ground?

geez, I don't see how hard it could be for B to make a decent PS.

Ok, I just finished it, closed and working. It is silent now.

It was 1000uF caps. Not enough. I paralleled them with 10000uF/35V, glued them to chasis with thermal glue, and all is fine.

There is still slight amount of buzz, but it is far from what it was before. Now it is well burried in hiss, and almost not perceivable. On recording it will be way back comparing to signal, and I can gate it out as needed.

So, I'm good. :grin:

Otherwise, PRR, thanx for great reply. A bit late, though.... :?
There are two rectifier diodes hidden way back behind.... And it is double sided PCB, so you can't see all traces on photo.

True, this amp is dirt cheap. And it sounds great, I mean, GREAT!!!! It is surprising, really, I'm not joking - it sounds really good, even if it would cost 600eur it would be worthed, but it is 60 eur..... Works nicely at low and mid levels. Sure, forget about live gig with it - it is a small thing, practice amp, and will not be loud enough for stage. And besides it is not strongly built, you would drop it once, and it would fall apart. :wink: But I don't use if for live, those days are gone.... It will sit in my studio, rarley moved, so all will be fine, I guess. When it dies, I'll just buy another one.

Guavatone, connection to chasis is through input jack.

Thnx all for the help.