30W Crafter DSP1 Acoustic Guitar Amp - constant level hum underlies output

[robdean]

Phatt,

I think I read that myself regarding the DSP introducing noise! Thank you for thinking of it.

In this case I'm not so sure: there is no hum in the send of the send/return loop, which I believe is post-fx. There is hum before the signal-path relay is triggered at power on. The power amp still hums when the coupling capacitor of its signal input has a leg lifted (C46 on the schematic).

What is interesting is that there does seem to be unrectified AC tapped into the FX, relay and FET2/3 subcircuits, so maybe something there could be leaking. Just for my education, can anyone explain why there is a power connection from both unrectified (pre-D6) and post-rectified power (after R75)? All 3 subcircuits seem to be part of that power loop. As I mentioned earlier, there seems to be lots of ripple within the relay circuit power, the FETS seem to get clean DC via D3. I'll measure the fx circuit power in situ when I get the chance.

So the anomalies are hum in the power amp and a nasty ripple on the +ive power rails: if the hum has a cause in one of the above subcircuits it seems it is being induced in the power amp via the power rails rather than noise in the input signal.

I've retraced the rectifier and smoothing capacitor circuit today and they appear to be in agreement with the schematic. The key components seem to meter as one would expect.

[g1]

 Instead of lifting C46, ground it's end that connects to Fet3.  Any effect on the hum?
The unrectified AC that is feeding the relay and Fets is for turn on muting.
When you say there is large or nasty hum on supply rails, please provide numbers you have measured.

[robdean]

Thank you! Grounding that end of C46 increases hum. Short recording attached.

As yet I only have the measurements I posted earlier in the thread which I have been warned by JMF are untrustworthy since I have a cheap DMM which probably cannot measure ripple. I'm puzzled that attempting to use a capacitor in series with the probe simply yields 0v on all scales and measurements. I hope I haven't invented a whole new kind of stupid.

I'll order a better DMM today if i can find a worthwhile one I can afford which will offer better ripple measurements.

Would anyone encourage me to buy, or warn against, a UNI-T UT39C http://www.ebay.co.uk/itm/291301645135 or a Mastech MS8268 http://www.amazon.co.uk/dp/B000JQ4O2U ? I could spend a bit more if need be, but can't go 'pro'!

I sincerely appreciate your help with this amp...

[phatt]

Yep a pretty bad hum/buzz.

I wonder if you could tempoarily bypass the relay and mute?   That seems to be part of the issue.

I'm thinking,, lift output side of R27 (Preamp output) while also lifting Input side of C46 (poweramp input), then bridge the two with a wire.

Of course you still may need to kill power to both relay and mute to be sure they are not introducing hum. Of course making sure you keep power at IC7 to run FX circuit.
All that likely easier said than done ay? 
Phil.

[phatt]

Had time so I dug into the picture and schematic 4U.

My suggestions would look something like this. I've marked with a red cross each part to lift.
Just stand R27 up on end and a wire across to C46. (I think it's C46) A couple of parts are hard to read so double check.
Phil.

[robdean]

Thank you! I'll take a run at it tomorrow...

[phatt]

Opps, forgot about the Q of meters.

This one is better;  Mastech MS8268 (Higher CAT rating :tu:)

The UT39C has the lower Cat rating and is fine if you never need to work on mains voltage, obviously Co$t goes up with the better units.
Phil

[robdean]

1. The mod to take the relay out of circuit left the hum unaffected. I have left it in place in case of any suggested checks/tests. Thank you so much Phatt for marking it out for me.

2. Using an iPad app and a USB audio dongle I grabbed a trace of the hum waveform via the headphone output - image attached

3. I bought a more sophisticated DMM: UNI-T UT60E http://uni-trend.com/UT60E.html - true RMS AC
This meter does not report AC on the + rails VCC+, [A] and [C], although both my basic DMM and Analog meter do - none of the meters show AC on the -Rails

RESULTS:

RMS DMM:
-VCC: -32v DC, under 0.1v AC
B   : -25.7v DC, under 0.1v AC
[D]   : -24.7v DC, under 0.1v AC
+VCC: +32v DC, under 0.1v AC
[A]   : +11.5 vDC, under 0.1v AC
[C]   : +10.8v DC, under 0.1v AC

Simple DMM:
-VCC: -32v DC, negligible AC (due to diode bias?)
B   :  -25.8v, negligible AC (due to diode bias?)
[D]   : -24.8v DC, negligible AC (due to diode bias?)
+VCC: +32v DC, 71.1v AC
[A]   : +11.6 vDC, 25.5 AC
[C]   : +10.8v DC, 23.6 AC

Analog Meter:
-VCC: -32v DC, negligible AC (due to diode bias?)
B   :  -25.5v, negligible AC (due to diode bias?)
[D]   : -24.5v DC, negligible AC (due to diode bias?)
+VCC: +32v DC, 90v AC
[A]   : +11.5 vDC, 26v AC
[C]   : +10.6v DC, 24v AC

Incidentally the Hz range on the meter reads 50Hz against speaker/headphone output.

[g1]

  That waveform does look like something a bad cap could cause.  Do you see that kind of waveform on any of the supply rails?

[robdean]

I don't have an oscilloscope, only an app that can plot an audio waveform
:-/

I hear tell of kludges using voltage dividers, I don't know if anyone here would recommend it... ?

I've changed the two 4000uf, and two 470uf caps plus C60 and C49. Any other suggestions?

Thank you for the thought y'all have been putting into this, I really appreciate (and need!) the help.

[phatt]

Now that you have Fets and Relay out try lifting R59 this will kill power to FX but the amp should still work.  If you still have hum then maybe a dead diode in the bridge Rectifier.
Phil.

[robdean]

Thank you again! Just checking in to say my lovely, much used top-of-the-range butane soldering iron has stopped working and is going back to the (excellent) manufacturer (Antex) in the hope they can revive it. I'm asking them to send me a cheaper model as a spare, but I'm out of action for a couple of days :-(