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peterb
Senior Member Joined: 02 Feb 2017 Location: Cornwall Status: Offline Points: 332 |
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Thanks everyone for their contribution.
My scope is Picoscope 2205 which has a sensitivity of 10mV/div, just about good enough. Thanks for the link, I will study it. I have tried to follow those wiring assembly guidelines, hopefully visible in the pic below. All leads twisted. Signal leads screened. Crossing leads at right angles. AC concentrated to one area (RHS), and I even tried to allow for a steel screen to surround it, but not yet fitted. |
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Peter
--------------------------------------- Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2 |
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Graham Slee
Admin Group Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16298 |
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The 78s and 79s aren't fully stable, although the manufacturers say they are. Charging current happens 100 times a second as the diodes switch each ac cycle, so there is a top-up to the electrolytic "bulk" capacitors. The instantaneous part of the ripple voltage is fast and electrolytics are inductive, so there will be some high frequency noise. The bypass capacitor doesn't solve this, it only moves the noise higher in frequency. When we tested broadcast power supplies we tended to find the output noise increased onload, which verifies the ripple noise, there being more ripple on the bulk capacitors. Generally this wasn't a problem because the circuits being driven had massive PSRRs. Not so discretes unless built like opamps.
The noise harmonics can go so high in frequency that the 78/79 are "trying" to operate outside spec, and being high gain amps in their own respect, will tend to ring on. They're also trying to pass this into a load capacitance which is also inductive. No surprise then to see ringing at the rate seen. Solution? It cannot be completely removed but judicious placement of low ohmage resistors in series with the electrolytics may help (1 to say 3.3 Ohms), and if you take a look at some Philips audio circuits, you can see where they put theirs. |
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That none should be able to buy or sell without a smartphone and the knowledge in how to use apps
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peterb
Senior Member Joined: 02 Feb 2017 Location: Cornwall Status: Offline Points: 332 |
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I have now read the Genera design thread, very interesting.
I will see how destructive it will be to add some very small resistors to the PSU output caps, they can only help. One sentence in the thread rang a loud bell, "HiFi is listening to your PSUs" (I paraphrase). I seem to remember that this is the big USP given by Chord Audio http://www.chordelectronics.co.uk/chord-amplification-technology/ When I first read it my initial reaction was, what a lot of trouble for a small improvement. But to quote someone else, much of this is making things better by marginal gains! At the end of the day the background noise it drowned, in a good way, when music is being played. It is just that I know it's there!
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Peter
--------------------------------------- Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2 |
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peterb
Senior Member Joined: 02 Feb 2017 Location: Cornwall Status: Offline Points: 332 |
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A quick follow up.
I upped the Electrolytics to 220u and fed them from 2 ohm resistors. Where the resistors were added was determined by the pcb layout, on the +ve side it fed both the 100n and 220u, on the -ve side it just fed the 220u. The difference was not great but visible on the scope and discernible with 'my Ear In The Speaker' test. However one oddity occurred. In trying to get a clipped earth connection for the scope probe I moved it from holding against the 0V terminal of the PSU to clipping it to the 0V link on the PSU pcb. The noise level shown on the scope fell to almost nothing !! The hiss also all but disappeared from the speaker. The scope was my laptop running on its battery so it wasn't getting another earth. Don't understand that! |
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Peter
--------------------------------------- Dual 505-1, Cyrus CD T, DIY 80W MosFet amp and PreAmp, 2xKEF 103.2 |
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