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Old 28th November 2012, 09:55 AM   #121
bigwill is offline bigwill  United Kingdom
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I made some measurements on my circuit today. Unfortunately, either the circuit is shockingly bad, or I'm getting half wave currents dirtying my earth. I suspect the latter due to the lash-up nature of the breadboard and supply wiring. Also due to the fact that adjusting the bias had zero effect on the measurement. An epic spray of harmonics was measured!
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Old 28th November 2012, 09:58 AM   #122
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Quote:
Originally Posted by bigwill View Post
I made some measurements on my circuit today. Unfortunately, either the circuit is shockingly bad, or I'm getting half wave currents dirtying my earth. I suspect the latter due to the lash-up nature of the breadboard and supply wiring. Also due to the fact that adjusting the bias had zero effect on the measurement. An epic spray of harmonics was measured!
Do you have a schematic for it? I wonder if there's been any recent changes that make it more susceptible to power noise?
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Old 28th November 2012, 11:09 AM   #123
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Wild guess: In this range of output power, the transformer's voltage output is so low that small noise could account for a more impressive percentage of the total power supply voltage. Power noise pollution and bridge rectifier noise are weak current sources that can cause extraneous rail variance. And, the noise itself can cause extra heat in the amplifier. Perhaps you'd want to snub the rectifier and/or use a CRC?
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Old 28th November 2012, 11:24 AM   #124
bigwill is offline bigwill  United Kingdom
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It's probably down to the fact that everything was half-assed together with long clip leads without much regard to anything

I'm going to order a 20VA transformer to power the thing. Do I have to put out 9W from both channels at the same time?

Edit: Nevermind, I uprated it to a 50VA.

Last edited by bigwill; 28th November 2012 at 11:28 AM.
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Old 28th November 2012, 03:14 PM   #125
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Quote:
Originally Posted by bigwill View Post
It's probably down to the fact that everything was half-assed together with long clip leads without much regard to anything
I'm going to order a 20VA transformer to power the thing. Do I have to put out 9W from both channels at the same time? Edit: Nevermind, I uprated it to a 50VA.
For decently low ripple performance, 18W*3=54VA. The 50va is right.
About a 2a center tap (13.5+13.5)*2a=54VA
Capacitance could be 4400u (2 paralleled 2200u) on single rail or approximately 10,000u per each rail for split rail. Those are sort of minimums for low ripple performance, but it could use something better. The capacitance looks big because the transformer is quite small and so is the amplifier, but the speakers didn't get smaller. If less capacitance is desired then a higher amperage transformer is needed. Also, bigger transformer plus good size capacitance could sound just like a bigger amp that isn't turned up quite as loud, instead of sounding only like a little amp. I think I'd use regs.
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Old 28th November 2012, 04:06 PM   #126
bigwill is offline bigwill  United Kingdom
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Quote:
Originally Posted by danielwritesbac View Post
For decently low ripple performance, 18W*3=54VA. The 50va is right.
About a 2a center tap (13.5+13.5)*2a=54VA
Capacitance could be 4400u (2 paralleled 2200u) on single rail or approximately 10,000u per each rail for split rail. Those are sort of minimums for low ripple performance, but it could use something better. The capacitance looks big because the transformer is quite small and so is the amplifier, but the speakers didn't get smaller. If less capacitance is desired then a higher amperage transformer is needed. Also, bigger transformer plus good size capacitance could sound just like a bigger amp that isn't turned up quite as loud, instead of sounding only like a little amp. I think I'd use regs.
I nearly got it right, thought I ordered a 4700uF cap for each rail. It'll do for now! I'll make space on the power supply PCB for another pair.
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Old 28th November 2012, 04:21 PM   #127
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One thing to do with long wires is to pinch the wire so it's parallel with itself and twist it. This way the magnetic fields cancel and it's no longer an inductive anttenna. It can be untwisted for longer connections.

I gave up on breadboards and now just make solder sculpture circuits. After all, get some gunk in the breadboard and one random connection goes open circuit, this can blow up components through no fault of the experimenter.
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Old 28th November 2012, 05:02 PM   #128
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Just managed to get (in simulation) 0.004% distortion at 20Khz with TIPs on the output and 18mA Iq. Big performance gains were had with buffering the VAS from the output stage (which isn't really a huge surprise).

This is with CFP outputs too. If I can get reality to reflect this I will be very happy.
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Old 29th November 2012, 12:41 AM   #129
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That's amazingly good performance. 0.004% thd with a 9W amp is sort of like 0.0004% with a 90W amp.

I was thinking about your LTP and your power supply. Naturally, these two circuits are enemies. The LTP will gladly throw the baby out with the bath water if exposed to power noise. Power noise rejection is an excellent feature we hope never to use. But, actually we hope that instead of a huge workout, the power noise rejection is only needed for spurious noise. Therefore, I think you'd like a CRC. I think you can do it by adding 2200u and a series resistor (per each rail) directly at the bridge rectifier. This CRC will have your 4700u||4700u power reservoir full up with reasonably clean DC instead of harmonic distortion. I think that it would be good to turn (dis)harmonic noise into heat inside of the power box instead of heat inside of the mint tin.

P.S.
You can also try snubbing the bridge rectifier for about 1v less noise (that's impressive if seen as a percentage) and a somewhat noticeable percentage less heat. Using a CRC decreases the reliance on snubbing the bridge rectifier and could allow using approximately 4.7n or smaller lossy/cheap polyester dip caps to snub the rectifier very easily (its either that or real RC's).
OR, with faster didoes (such as the extremely quiet MR diodes), you can snub "before and after" the bridge rectifier with RC's across the secondaries and across the rails. I actually forgot how to do this one, but there's a couple of posts by PacificBlue that looked good. If I remember correctly, it was an equal-and-opposite sort of arrangement that makes DC, instead of connecting the noise to the 0v. I do remember that the parts count was quite low and that, if using fast/soft diodes its more about snubbing the transformer, not so much about snubbing diodes.
And, For mains side filtering, it should be fairly easy to extract the MOV and mains side filters from a discarded computer power supply, for free.
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Old 29th November 2012, 11:15 AM   #130
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Almost got the board together... just got to wangle my 2 input caps into a spot and i'll be ready to plug it in and test it.... Oh yes and I gotta find a heatsink that will fit, do the job and leave space for input, output and a volume pot!

Not sure if it will fit in a tin, but i'll be happy if it works.. Maybe if i just tap the caps down a bit with a hammer and twist the chip with a screwdriver it just squeeze in.
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