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In Reply to: RE: Ugly, it's a good thing you asked ahead of time posted by Caucasian Blackplate on October 10, 2014 at 16:48:26
I pulled one apart and measured it while running tonight. At the transformer input, ~18VAC 0-p. Nearly 26V at the regulator inputs. Doubler? Looks like that's it."What is leading you to believe you need more capacitance?"
Oh nothing. Recall my original post was a request for power supply recommendations. This is just a tangent. I was just speculating about what might be done to solve the noise issues I'm seeing if I were to try and patch the existing power supply. "More capacitance" would likely still be my answer after poking around tonight with my scope a bit. Though the circuits obviously not what I first thought.
Using the AC coupling mode on my scope so I could zoom in on the ac component of a power supply rail noise w/ the scopes 5mV/div scale. I verified the noise riding on the regulator output is essentially identical to the noise I'm seeing at the audio output. Possibly it's some kind of summed version of the ripples on the positive and negative rails. Coincidentally it is also seemingly a scaled down version of the ripple riding on the regulator inputs. The regulators had some decent rejection of this noise, like you say.
The picture is of the output noise in MM mode with 1k terminations right on the inputs. The scope is at 5mV/div and 5mS/div.
Edits: 10/10/14Follow Ups:
That's NOT a doubly or a doubler! 18 VAC RMS, actually gives you sq. root 2 * RMS VAC = 26 P-P voltage, which is fine. An actual doubler would give you 2 * P-P or 52 volts! That is great if you need plus and minus rails, +18-0--18, which is a 36 volt difference.
You should measure and report on the scope trace at the output rails AFTER the regulators, because it is unclear whether the preamp runs on balanced rails OR a single rail. You have two regulators in the picture, so I suspect balanced rails, i.e., +9-0--9 and not +18-0. You should trace the diagrams out by careful examination. Here are ways voltage doublers actually work, but I don't think that is what you have:
Yes, in general, they are NOT a good idea. However, I had a tube preamp run off a voltage doubler. With suitable capacitors, it sounded OK. It's usually an issue of which transformer is readily available.
"That's NOT a doubly or a doubler!"It's something like that. I haven't bothered to reverse engineer the thing. I am not planning to reuse any of it.
"18 VAC RMS"
Reread what I wrote. Not RMS, I said zero to peak.
Here is a picture I took last night of the transformer output into the preamp when it was running. 5V/div, 2mS/div.
"You should measure and report on the scope trace at the output rails AFTER the regulators"
Did you read the post you are responding to? I described it in a way I considered to be pretty clear at the time. What exactly don't you get about my description? The picture didn't seem all that interesting to me since it looks almost identical to the one I posted last night.
Here it is. Ac coupled view of noise on +18V rail if i recall correctly. 5mV/div, 5mS/div.
"it is unclear whether the preamp runs on balanced rails OR a single rail."
Hmm. Only unclear if someone hasn't actualy been reading the thread. I mentioned before the audio sections have plus and minus 18V rails.
"I suspect balanced rails, i.e., +9-0--9 and not +18-0"Yes it's plus and minus 18V rails.
"You should trace the diagrams out by careful examination. Here are ways voltage doublers actually work, but I don't think that is what you have"
Not doing it. The chances of me reworking the existing supply are very slim. Doesn't seem worth it at all. My preferred method is off the shelf but if I diy I'm scrapping everything and starting over.
"Yes, in general, they are NOT a good idea"
I get why they would have gone that route but there would be no reason for me to use that type of circuit if were trying to upgrade other than if I were trying to reuse some of the existing circuitry for some reason.
Edits: 10/11/14
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