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Old 18th November 2012, 06:11 PM   #11
JForged is offline JForged  United States
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Originally Posted by Mooly View Post
Your bridge rectifier is wired incorrectly... maybe just a typo The two right hand diodes both need to be the other way round.

More smoothing yes The 47uf cap could usefully be a little bigger. Try at least 1000uf.

The centre tap from the tranny can be left floating. As it is you must ensure that it does not connect to the negative rail of the amp as if it does it will short out one winding of the tranny via the bridge rectifier.
Thanks. The rectifier was a stock image in visio. I am actually using a full wave bridge rectifier so I know it's wired correctly. I'll try a larger cap. Thanks!
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Old 18th November 2012, 06:17 PM   #12
Mooly is offline Mooly  United Kingdom
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I've just seen in the small print your note about the diodes

Anyhow you want the transformer wired like this with the ground connection to be the from the zero volt point.

Whether you add any power supply regulators depends on the voltage the amp needs to work and the voltage you actually measure across the reservoir cap in the PSU. A 7812 type needs around 2.5 volts or more differential so you would need 14.5 volts minimum across the cap to use one. If you have less than that then a good option is an active ripple filter using a transistor a resistor and cap. These are just guestimate values. The transistor would be a medium power high gain device (preferably) like a TIP41C
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Old 18th November 2012, 08:27 PM   #13
JForged is offline JForged  United States
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Originally Posted by Mooly View Post
I've just seen in the small print your note about the diodes

Anyhow you want the transformer wired like this with the ground connection to be the from the zero volt point.

Whether you add any power supply regulators depends on the voltage the amp needs to work and the voltage you actually measure across the reservoir cap in the PSU. A 7812 type needs around 2.5 volts or more differential so you would need 14.5 volts minimum across the cap to use one. If you have less than that then a good option is an active ripple filter using a transistor a resistor and cap. These are just guestimate values. The transistor would be a medium power high gain device (preferably) like a TIP41C
Mooly,

Thank you for your help.

Perhaps you can help my understanding with regulators. I have read that it is not always wise to use a regulator in front of an audio circuit because the demands from the music require more or less passages depending on the transients and dynamics of the music.
Do you believe this to be true?
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Old 18th November 2012, 08:30 PM   #14
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12 V transformer will give maybe 17V DC, subtract the diode drop. So, say 15 or 16

This is too much for your filaments.

Mooly, JForged, I think the voltage regulator is actually wired as a constant current regulator for the mosfet.

Last edited by Robert Kesh; 18th November 2012 at 08:35 PM.
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Old 18th November 2012, 08:46 PM   #15
JForged is offline JForged  United States
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Wodehouse,

I get 18.9 volts after rectification.\

Also which voltage regulator are you referring to?
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Old 18th November 2012, 09:07 PM   #16
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Um, the one on the schematic.

Your transformer is giving out more than it should. Is that under load? Is the primary rated the same as your mains supply?
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Old 18th November 2012, 10:04 PM   #17
JForged is offline JForged  United States
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Yup, just checked again. 18.5 volts. No load attached. Yes the primary is matched to my mains.
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Old 18th November 2012, 10:11 PM   #18
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Yup, just checked again. 18.5 volts. No load attached. Yes the primary is matched to my mains.
well, you're going to have to get another regulator for the filaments.
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Old 19th November 2012, 02:13 AM   #19
JForged is offline JForged  United States
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So to wire another regulator in a run run two lines off of the capacitor to two separate regulators?
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Old 19th November 2012, 06:38 AM   #20
Mooly is offline Mooly  United Kingdom
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Originally Posted by JForged View Post
Mooly,

Thank you for your help.

Perhaps you can help my understanding with regulators. I have read that it is not always wise to use a regulator in front of an audio circuit because the demands from the music require more or less passages depending on the transients and dynamics of the music.
Do you believe this to be true?
That's more applicable to larger power amplifiers where an unregulated supply can deliver more "power" on transients. Imagine your 18 volts or so powering an amp driving speakers. Short term 18 volts is available supplied via the reservoir caps (although this voltage will quickly fall under sustained load). So lets say 18 volts peak to peak (thats 9 volts peak or around 6 volts RMS). For an 8 ohm speaker that equates to 4.5 watts RMS (for a brief moment on a transient until the supply collapses). If the amp were on a regulated supply of 12 volts then the power under all conditions, transient and continuos would be only 2 watts RMS. Both amps would have the same "continuous" RMS power rating because the 18 volt rail collapse down to 12 volts in a few 10's of milliseconds under load. The unregulated one would have "better" transient ability.

For your headphone amp the most important thing is to ensure the filament volts is correct. Is it 12 ? or 12.6 ? Also the power involved driving headphones is of the order of a few milliwatts so transient ability doesn't even figure into it. The regulator already used in your amp is nothing to do with the power supply but is used as a constant current load for the FET output.

What you really need to confirm before all this though is what the voltage is under load i.e. running the amp properly (with a 1000uf or more reservoir cap) because that voltage determines what kind of regulation or filter can be used. If the voltage falls under load to around 14 or so then a normal 7812 type regulator isn't suitable because there isn't enough "extra" voltage to work with.

With 18 volts available you should use a proper regulator such as a 7812.
http://www.datasheetcatalog.org/data...0/44435_DS.pdf

Three pins, in, out and ground. All you need add is two caps of around 10uf close to the regulator pins to decouple input to ground and output to ground. The regulator would need a small heatsink.

Lets see what we are working with first
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