MiniWatt N3 problems, oscillation thumping and RF/motor problems. - Page 2 - diyAudio
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Old 28th June 2010, 11:26 AM   #11
DF96 is offline DF96  England
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If the N3 has CCS bias for the output, which the advertising suggests, and negative feedback (quite likely to keep the distortion down in such a simple circuit) then it could be suffering from the low frequency instability discussed in this thread. The simplest solution would be to replace the CCS bias with a normal cathode resistor. In my opinion CCS bias is unnecessary in most circuits and it adds to LF problems when global feedback is used.
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Old 28th June 2010, 03:40 PM   #12
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What value do you think would work well? That sounds like the problem. I can also try the original cap value with the new working lm317's though and maybe it will bias right and not cause instability.
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Old 28th June 2010, 05:02 PM   #13
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Destroyer OS. View Post

<snip> 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.

<snip>MiniNightmare is that it is...
Tube gear should not be turned on, then off, and then on in short succession. You will always get thumps if you do not wait for the tube cathodes to cool down sufficiently to prevent immediate conduction from occuring the moment the power comes on. This is totally normal and in a lot of cases may actually result in blown fuses and damage to the amplifier. (This is also a very common cause of fried tube rectifiers in amps like the Dyna ST-70 and MKIII.)

In short wait at least 2 minutes before turning the amplifier back on.

You are the direct cause of at least some of the problems you are having...
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Last edited by kevinkr; 28th June 2010 at 05:05 PM.
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Old 28th June 2010, 07:14 PM   #14
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Oh well that is good to know. Perhaps the low volume is just my speakers not uber efficient? I suppose then since I corrected the popping noise I am good to go. The small change in capacitor value might be safe in the circuit? (150uf to 220uf)

It seems a little distressing that I have to crank the volume to just over half way up to get much anything. The fits 1/4 of it is about nothing. I thought the oscillation was draining the volume. My speakers were originally rated at 95db, and could be that up to 97db since the Fostex 126 updated driver are a little more efficient. I am using a 25k stepped attenuator. The original was either a 25k or 50k (not sure, have to find it again).

The capacitor is in between ground and the LM317.

Last edited by Destroyer OS.; 28th June 2010 at 07:20 PM.
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Old 28th June 2010, 08:52 PM   #15
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From the LM317 data sheet it looks like these are the output capacitors and the size does not matter. Woo! They are going in.
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Old 28th June 2010, 08:58 PM   #16
DF96 is offline DF96  England
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You will get lowish volume from a 3W amp unless your room is very small or your speakers are very efficient.

The 'oscillation' could be user error, as kevinkr suggests. Take note of his advice. It could just be RF pickup from the speaker leads, so a ferrite choke may help.

Changing capacitor values in a circuit with feedback can lead to trouble, so best left alone unless you have reason to believe the original designer got it wrong. I'm not sure what the correct cathode resistor is, to replace the CCS. The EL84 data sheet gives several values, depending partly on the screen grid voltage. Maybe other people on here can help. Personally, I would get rid of the CCS as it is a bad way to bias an output stage. It just happens to be popular at present for some reason.
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Old 28th June 2010, 09:13 PM   #17
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Well this sucks. I have to wait, again to listen to music. Plus every time I open this amplifier I shorten some transformer leads because the wires break off so easy.
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Old 28th June 2010, 09:28 PM   #18
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Oh I would replace it all with a resistor if I had a schematic or any information... to bad. I just have to wait for capacitor, this is irritating to the millionth degree.
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Old 6th July 2010, 12:14 AM   #19
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I got it all done. It still makes lots of noise when things turn on or off, or any RF comes in.

Well whatever I guess... I think it is just super sensitive to changes in current. None of my other equipment is like it is... I guess it is just a cheapy so that is why?
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