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Old 5th June 2002, 03:48 PM   #1
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Default AC power connection schematic explanation needed

That's the schematic for AC connection I used in my A75 amps. The circuit around 2 bridges is taken from ML23 amp and supposed to remove DC component from the power line and prevents transformer saturation. Can somebody explain how it works and is it worth to install it?
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Old 6th June 2002, 12:38 AM   #2
paulb is offline paulb  Canada
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I haven't taken a close look at this filter, but check out this thread for some insight:
dc filter
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Old 6th June 2002, 01:08 AM   #3
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I used the same exact circuit topology for my stereo Leach SuperAmp except that it used a single bridge rectifier. The triac is actually used as the main switch. The R and the C around it are used to snub the switching triac's transients. The physical switch puts the AC line on the gate of the triac which forces it to work within the first and third quadrants. Everything else should be recognizable. I would not encourage its use even though I used it. The problem with this type of circuit is that the brief switching transients of the triac often appear out of the noise in the audio. So it leaves a tiny bit of noise. And you can see it on the scope too it you look at it. Little tiny spikes only a few millivolts rising out of the noise positive and negative 120 times per second. However, it is an easy way to turn on the primary of the transformer without using a huge ugly industrial size power switch. I used a small signal switch mounted on the front to trigger a relay to trigger the gate. Thats my experience: advantages and disadvantages.

BeanZ
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Old 6th June 2002, 01:48 AM   #4
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BeanZ,
I know how the triac switch works, what I don't know is how the circuit with the bridge works.
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Old 6th June 2002, 02:47 AM   #5
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Harry:
Could it be that their using the resistors and capacitors to create a phase difference in the ac and using the bridges as a sort of trap because of that difference. The center of the bridge looks like a dead short perhaps unless the small amount of ripple is out of phase, in which case would some cancellation occur? I really don't know, I'm just kicking some ideas around. Must have been invented when Reagan was in office. A kind of VooDoo Elecotronics Good luck in your quest, it's over my head
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Old 6th June 2002, 03:55 AM   #6
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It looks a bit strange but the fundamental idea appears to be the same as those older type amplifiers that use a single supply rail and an output coupling capacitor to remove the half-rail dc offset before it gets to the loudspeaker. Instead of the speaker being driven zero to full rail, the capacitor causes it to be driven +half rail to - half rail.

Even if the waveform fed to the cap is assymetrical, the cap ensures the waveform fed to the speaker has the same area above zero as below zero. So the average dc output is zero i.e. just what you and the speaker want. Unless of course you are also trying to reproduce the barometric pressure on the day of the concert.

This circuit does the same thing. It guarantees the transformer is driven equal volt/seconds (i.e. area under the half cycle) positive and negative.

The bridges are arranged like a pair of back-to-back zeners across each cap, limiting the cap voltage to about + - 1.5 volts either polarity so you could use a high value low voltage cap. Looks like TH at the top is a thermistor for inrush reduction. I can't see why the lower caps & bridge are needed, it is a series circuit so one set would be enough.

As for why you might need one of these, the transformer out in the street won't feed you dc as such, that's impossible, but someone else might be using some sort of load that distorts the ac by pulling unequal pos and neg current therefore the line voltage may not swing equally pos and neg. The difference appears as dc and flows straight through your transformer primary. Your tranny usually won't have an airgap in the core so it will very easily saturate, maybe even burn out at part load only.

GP.
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Old 21st September 2002, 11:20 AM   #7
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Peter I like to see your progress. Maybe I am also going to use your AC power connection within my AlephX. After I studied the schematic I have a few questions. If I am correct four fuses are drawn in the schematic. When I look at your photo I see only two fuses.

1) Did you skip the fuses in the primaries?

2) What triac did you use?

Marco Ensing
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Old 21st September 2002, 01:30 PM   #8
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Since there is an interest I post bettter pic of it. The schematic is based on A75 AC connection and ML23.5 DC removal circuit. I used 2 fuses closer to AC socket. The circuit worked very well for me over the years. Triac is DigiKey part# Q6040J7. Most of the values are given on schematic. The small circuit below is used for connection between amp's ground and chassis and is also placed on a board.
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File Type: jpg ac2.jpg (56.5 KB, 2608 views)
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Old 21st September 2002, 01:38 PM   #9
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Here's the artwork for a board. The correct positioning on copper side is when you read "O.K." properly. The size of board is 5"x 4.15".
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Old 21st September 2002, 01:43 PM   #10
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And this is component placement.
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