I have a recapped Scott 344C that seems to be working well except for a significant thump when turning off or on. it clearly move the coils on the speakers so i turn them off beforehand. reading related posts in AK seems to often mention the protection relay which this unit doesn't have, and also an "AC snubber cap" but not sure which cap this might be?
thump turning on/off Scott 344c
- Thread starter trkyle
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This is a cap coupled amp, its fairly normal for these to have some amount of thump on power up at least. You're hearing the 1000uf cap charging up to 1/2 the supply voltage. Leaky caps make this worse but its sort of a "feature" that you can't do much about.
There does not appear to be a suppressor cap across the power switch.
There does not appear to be a suppressor cap across the power switch.
thanks. i'm looking at other comments (including yours) in similar threads and trying to get a bit more educated. while this unit was working before i recapped it it also had a significant amount of hum that turned out to be due to failing transistor Q401 just before the power supply (i believe it is called a voltage regulating transistor?). The thump seemed to increase after this was done but my recall may be off. i'll check the voltages again and then try to look at replacing those coupling caps. in the meantime i liked some of the workarounds to avoid this "feature" of this nice sounding unit. thanks for the education and direction.
It may be worth fitting a small protection circuit inside the amp. Although not strictly 'original' they do have their uses.
It will run from a suitable power supply within the amp and will add a pair of relays to the amplifier outputs.
Feed the amp speaker outputs into it and feed the speaker terminals (or speaker selector switches) out of it.
It will provide the following features:
1. Speaker connection delay on power up - No thump.
2. Speaker quick disconnection on power down - No thump.
3. Output stage high DC voltage detection - Speaker protection from shorted output stage transistors.
4. Speaker short circuit protection - Output stage protection from shorted speaker cables.
They are available as small self contained circuit boards that can be mounted unobtrusively inside the amplifier enclosure.
It will run from a suitable power supply within the amp and will add a pair of relays to the amplifier outputs.
Feed the amp speaker outputs into it and feed the speaker terminals (or speaker selector switches) out of it.
It will provide the following features:
1. Speaker connection delay on power up - No thump.
2. Speaker quick disconnection on power down - No thump.
3. Output stage high DC voltage detection - Speaker protection from shorted output stage transistors.
4. Speaker short circuit protection - Output stage protection from shorted speaker cables.
They are available as small self contained circuit boards that can be mounted unobtrusively inside the amplifier enclosure.
3 isn't really a concern. You can short an output transistor in one of these and the cap blocks it.
also from experience I can tell you that powering a cap-coupled amp up with the speakers disconnected, then re-connecting them tends to make a popping noise anyway. The cap has no bleed resistor on the speaker side and the internal leakage will put some DC at the speaker jacks. Once the speakers are switched on it discharges through the voice coils. Its a different noise, but its still a noise. It will shut up power-down noise.
bad output coupling caps and bad caps elsewhere in the unit can make all of this worse than it should be though.
also from experience I can tell you that powering a cap-coupled amp up with the speakers disconnected, then re-connecting them tends to make a popping noise anyway. The cap has no bleed resistor on the speaker side and the internal leakage will put some DC at the speaker jacks. Once the speakers are switched on it discharges through the voice coils. Its a different noise, but its still a noise. It will shut up power-down noise.
bad output coupling caps and bad caps elsewhere in the unit can make all of this worse than it should be though.
goldfish
New Member
Hello, sorry to revive an old thread, but I'm taking a look at a friend's 344C and it has ridiculously high DC offset in one channel (18V!). Obviously it also has quite the thump on power-up (though once I measured the DC voltage I immediately unhooked her speakers). Would this suggest at least one of the output transistors is bad plus the 1000uf output cap?
Doesn't really indicate anything about the output transistors, but maybe not a good sign for the output cap. If its completely unloaded though the voltage is sort of meaningless. May want to put a resistor across the output and work out the leakage current to really see whats happening.
With stuff like this, if one of the output transistors has died the input side of the cap will either be at zero volts or rail voltage, but the cap itself should block whatever is on that side of it.
You may be able to get a guess about whats going on by measuring voltage on the amp side of the cap though. Typically its around 1/2 of the supply voltage, often a bit lower but there is normally an adjustment for it. If its supply voltage or zero and the fuse didn't blow, fair guess one transistor is open. A short should pop the fuse. Also there will be no or extremely weak output if one of the output transistors quit. If you get basically normal volume, its not blown.
if all thats OK, the method I use to set the center voltage is to run the amp up to where it just starts to clip into a normal 8 ohm load, then tweak the center voltage pot until it clips symmetrically. That will maximize clean power. Too far either direction and it clips top or bottom of the wave and limits clean power output.
With stuff like this, if one of the output transistors has died the input side of the cap will either be at zero volts or rail voltage, but the cap itself should block whatever is on that side of it.
You may be able to get a guess about whats going on by measuring voltage on the amp side of the cap though. Typically its around 1/2 of the supply voltage, often a bit lower but there is normally an adjustment for it. If its supply voltage or zero and the fuse didn't blow, fair guess one transistor is open. A short should pop the fuse. Also there will be no or extremely weak output if one of the output transistors quit. If you get basically normal volume, its not blown.
if all thats OK, the method I use to set the center voltage is to run the amp up to where it just starts to clip into a normal 8 ohm load, then tweak the center voltage pot until it clips symmetrically. That will maximize clean power. Too far either direction and it clips top or bottom of the wave and limits clean power output.