MiniWatt N3 problems, oscillation thumping and RF/motor problems. - diyAudio
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Old 25th June 2010, 04:03 AM   #1
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Default MiniWatt N3 problems, oscillation thumping and RF/motor problems.

I removed the POT on my Miniwatt N3, great improvement. Next up I replaced one of the sockets that had a broken leg that eventually burned itself into a large gap (scary).

Now whether it did this before or not I am not sure. I must state I live in a really old building in a studio apartment half bath (sink room rather). I have screw in fuses on one wall.

Luckily I grounded my socket I use for stereo to the box because the flex conduit running in the building is all grounded (wheeew thought I might have to run something out the window!)

Whenever my fan is turned on or off and between speeds I get audible pops. My understanding is this if strong RF, and it is low frequency that must be below 1600hz. I say that because I have a power conditioner that starts in pretty strong attenuation at 1600hz. If it was at 1600hz I would probably not notice it. My mini-fridge gives me a little noise but not as much. I am not sure if I can hear anyone else in the building doing anything since my room has it's own 60a service (two 30a breakers, newer).

My first thought was the MiniWatt is screwed up. I touched up all the soldering points, added better thermal paste on the heatsinked T220 IC, and through on some old thermal guards onto the front LM317's for fun. (my room is warm during summer). I figured since the longer it is on the more audible things seem to be. This might mean a cold solder joint that is building up resistance but that is no longer a problem. I also thought maybe some capacitors were not doing anything so I was getting more noise from the wall, due to bad solder joints. The noise from turning the fan on may be reduced slightly now, but it is far from gone. The fridge makes much less of a noise but perhaps when I get the socket for it grounded the noise from it will be eliminated if it has a good circuit. If it does not maybe I will install a soft start in it. Hell maybe I will put soft starts on my fan!

The strangest thing is when I turn that amplifier on after being on a long time, and turn it back on after that I get a strong oscillation thumping. That one baffles me.

What am I going to do about the AC motors? Not sure... I hate to do anything to the AC signal that chokes it down. My best ideas are along the ideas of adding more capacitance to my power conditioner (already at just over 100uf across the mains). However there is a point where it can be too much obviously and I start attenuating a lot of the line which is not smart obviously. That or adding soft starts to my other devices.
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Old 25th June 2010, 04:41 AM   #2
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Nope minifridge still does it.
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Old 25th June 2010, 05:16 AM   #3
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One thing that changed in the system was the removal the two resistors that were performing a log change to the POT. I wonder if this was valuable to the circuit? Hard to imagine but possible.

Look at "Changing the Law of the Pot"

Potentiometers (Beginners' Guide to Pots)

However maybe it proved a better purpose. I am going to re-insert them.
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Old 25th June 2010, 05:50 AM   #4
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Well that did not fix the problem. It actually made it more audible. I mean when the fridge turns on it is like a freaking surprise and the lights dim and everything in the room. Scary!

The Miniwatt people are sort of lazy and put in two resistors to act as the log curve for the built in POT. However these appear to also be part of the load so a DAC or whatever can drive the unit easier. That and the super low quality tube sockets are a bit of disappointment. Overall though the unit is great if I can just get these sounds to stop.
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Old 25th June 2010, 06:47 AM   #5
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...if lights are dimming etc etc whe nyour fridge turns on then your "60A" supply is weak as weasel's ****. Start there....
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Old 25th June 2010, 07:01 AM   #6
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Yes that is true. I would like to figure that out.

However the oscillation is not associated with that.

If I turn off all my lights then the mini fridge makes almost no difference on or off, as far as the stereo goes. The fan however still makes noise.
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Old 25th June 2010, 07:02 AM   #7
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Yes that is true. I would like to figure that out.

However the oscillation is not associated with that.

If I turn off all my lights then the mini fridge makes almost no difference on or off, as far as the stereo goes. The fan however still makes noise.
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Old 25th June 2010, 07:10 AM   #8
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that still makes sense. The noise you are hearing is probably the commutation of the motors. This will be worse with higher line load, eg when your lights are on. Specially so when everyone else's A/C is also going. I'm picking that if you check the line voltage, its through the floor and drops further as load increases...
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Old 25th June 2010, 07:15 AM   #9
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Yes that probably is fair to assume. I still need to figure out the oscillation problem.

It was pin 9 that was shorted to begin with. I am worried a capacitor or two went bad. Both tubes do the strong oscillation thumping to speakers that fades slowly. Specifically maybe a capacitor or two are open to ground instead of loading to ground. However once again I am far from a Tube guy. I have just been reading a little while tonight about oscillation causes.

I am going to go over the wiring in the apartment with an electrician on Sunday. I might have to check the box downstairs and see if the service to the breakers is adequate or if they screwed it up.
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Old 28th June 2010, 06:15 AM   #10
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Well I replaced the socket in the wall that my mini-fridge is on. The old wires fell off the old socket when it was pulled out .

In my MiniWatt I replaced the voltage regulators (LM317's) and that got rid of the sound I get when things in the room turn on or off.

I still have low volume and get the really low thumping sound when I turn it on, play it for a few seconds, turn it off, then turn it on again.

I did find one bad capacitor and replaced it and replaced the one on the other side with the same thing. They went from 150uf to 220uf, which could result in some sort of feedback if I am not allowing the right frequencies through but the thumping has to be super low, way way way below 60hz. I might try some 150uf replacement capacitors but still there is not a good explanation of why both speakers would do it if the problem was the circuit for the one particular voltage regulator and capacitor for the side that had the bad tube socket to start.

MiniNightmare is that it is...
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