Electronics > Beginners
12v DC to 12V AC
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Ian.M:

--- Quote from: Hero999 on August 19, 2018, 09:45:09 pm ---
--- Quote from: Ian.M on August 19, 2018, 08:46:54 pm ---It will probably *work* off a squarewave but you are likely to get excessive noise breakthrough of the switching frequency and its odd harmonics as most valve amps don't have great PSRR.

--- End quote ---
Yes that's a possibility. It can be mitigated somewhat by keeping the rise/fall times reasonably slow, at the cost of efficiency, but I agree, it's a risk.

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If one's going down the squarewave route, it's probably worth replacing the rectifier diodes in the preamp on-board PSU circuit with fast enough ones to work efficiently at 50KHz to get the switching noise and any of its intermodulation products with the audio signal, well above the audio frequency range.   It will probably also need ceramics added across the reservoir caps, if they are electrolytic, to get the impedance @50KHz low enough.


--- Quote from: james_s on August 20, 2018, 03:27:19 am ---Why do you have all these separate power supplies? You only need one, just a single 12VAC power supply and then a rectifier and regulator for each DC output you want. There is no advantage to having multiple separate supplies, that's just pointless.

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Yes, though it may be worth using a boost-buck non-isolated DC-DC converter rather than a linear regulator, with *just* enough bulk capacitance on its input to keep it running through the zero crossings at full load.  That will provide secondary side power factor correction and make *far* better use of the transformer's VA rating.
 
james_s:
If someone is using a tube amp I would guess they're far enough down the audiophile rabbit hole that a switching regulator is probably off the table. As long as the power level is modest there's nothing wrong with a linear regulator, the wasted energy is probably less than that consumed by the tube heaters.
Brumby:
Looking at the silk screen of the underside of the unpopulated PCB, you can see three pairs of pads labelled -20V, +20V and 12.6V.

Seems to fit what people have been saying...
ArthurDent:
My personal opinion is that all these tube-type preamps benefit the seller much more than the end user. If you check Ebay (US) you will find over 500 listings for this type of low-end preamp. Some designs I’ve seen just have the filaments of the tubes powered so they glow but have a simple transistor circuit to do the actual work, a lot require 12VAC because that is required for the cheap voltage multiplier circuit to get about 28VDC they use for the plate supply and that multiplier won’t work on DC. There are a few that use a DC-DC converter to get about 100VDC for the plates of the tubes and I’d consider these as better, but not good designs. I doubt that you’ll find any of these cheapy designs that have the equalization required for different input devices. The reason most do not include a power supply is that shifts the cost to you and makes for a much lighter package to ship from China.

Most of these will kinda work and you can convince yourself that they sound great but if they are checked using audio test equipment I think they will be found lacking. I believe that new MOSFET amps can be made to duplicate the previous tube circuits for softer sounds without the ‘sharp edges’ of the bipolar junction transistor designs and I’d be looking for those designs. As to these cheap amps, you are probably getting what you pay for and a glowing vacuum tube (valve) looks great.  8)
james_s:
Oh I agree, but that's beside the point.

Tubes are pretty neat, I mean all else aside they look pretty cool. Just because something has a tube in it doesn't mean it's any good though.
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