| tonyptony |
I put this post in PS Design, but I'm thinking what I'm also looking for are recommendations for the individual components. So I'm posting here as well.
I am attaching a linear design I intend to put together. I wanted to get input from the gurus out here, in particular with regard to the sizes and types of caps. Even though I don't have the bridge rectifier marked, it will actually be made up of four Fairchild TO220 "soft recovery" type diodes, with appropriate heat sinks. I don't have the transformer yet, but I'm looking at varieties in the 10 VAC secondary, 3.8 amp out category. Input is standard 120VAC, 60Hz. The output of the circuit is meant to be 5VDC, capable of driving up to 3A. In reality it will be supplying about 1.2A on average but should be able to peak up to just under 3.
The other thing to note going in is that I intend to build this so that the transformer, the PCB containing the bridge + C1 + C2, and the remainder of the circuit will be physically separated from each other but in one enclosure. I will put shielding around each section to reduce electrical and magnetic junk from either the tranny or the regulator diodes from getting into each section 2. So there will be leads from the tranny to section 1, and from section 1 to section 2 (between C2 and C3).
Here's what I've thought about using for the various capacitors:
C1 - either a Jensen 4 pole 15000uf, 25V or (4) 4700uF Panasonic FC types
C2 - a 10uF bypass, but I don't know if I should use something like an Auricap polyprop, or a tantalum. And should I use another even smaller value (1 or .1) on top of the 10?
C3 - either the same type Jensen 4 pole 15000uF, or their 4 pole 47000 uF. I'm doing this because, again, this section will be separated from the other, but I'm not sure if I need another bypass here.
C4, C5 - LT recommends on the order of 20-25uF for decent ripple rejection at 120 Hz, but then goes on to say that a .22uF would provide rejection at the higher frequency range (10KHz). I see I also made a mistake here. These caps should be electrolytic, but here's where I could use the greatest help with both value(s) and type.
C6 - Jensen 4 pole, 47000uF
C7 - Sonicap or Auricap bypass, but I'm not sure if I need more than one (10uF + .1uF) or just go with 1 (1uF).
R1 and R2 will be selected to generate the required output voltage. Probably use Dale low noise types or Caddocks.
It should be obvious to those with greater knowledge that I have just enough understanding to get myself in trouble. So I turn to you all to help me get the best I can out of this basic design. Thanks in advance. |
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| TerryC |
I recently constructed an outboard power supply for an MSB Link DAC III. It was my first DIY project in nearly twenty years, so you can take my advice with a grain of salt.
Here are guidelines I used for selecting capacitors:
1) Use the highest quality film capacitors that the budget will allow for all 1uf and smaller, and the highest quality electrolytic capacitors that the budget will allow for all values greater than 1uf. The price point for quality film capacitors seems to be around 1uf, and then gets insanely expensive above that.
2) Where tantalum capacitors are specified by a chip manufacturer, use high quality electrolytics and increase the value by 10-20 times. Tantalums are recommended by the manufacturer based on cost and life expectancy, with no regard for the performance demanded by high-end audio applications.
3) The capacitor voltage specification should be at least twice the expected voltage that the capacitor will ever see. This is particularly important for electrolytics in order to maximize their life expectancy.
Some other obervations about your schematic:
1) According to the LT specification, the diodes D2 and D3 are not necessary because the 1085 already provides device protection.
2) Ideally you would want a bank of capacitors following the rectifier that would range from very high to very low values. For example 1000uf, 100uf, 10 uf, 1uf, ,1uf, 0.1uf, 0.01 uf, 0.001uf. The purpose being that each capacitor would filter out noise at a higher frequency, assuming that noise is even present at those frequencies. I couldn't justify this for my project, so I settled with 4700uf, 1uf, and .1uf.
3) The bypass capacitor at the Vin terminal of the regulator is there to keep the circuit stable, and should be installed as close to the device as possible. Use the manufacturer's specified value unless you find that the circuit is unstable.
Have fun! |
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| tonyptony |
Terry, thanks for your feedback. Some of what you've indicated goes back to good, solid "audiophile" electronics. What's funny is that some of it - like the use of multiple caps for filtering - seems to not be fully agreed to anymore. I did a search here to find that some, on doing listening tests, have come to the conclusion that using multiple caps detracts from what is being described as a natural musical presentation. And on top of that, there are also those who are no longer in the camp of big caps. The reporting here is that, while it produces a more extended low end, it also provides the impression of "slowing" the musical sense down. I'm going by memory here, but if you look around you can find these threads pretty easily.
I thought one of the reasons the tantalum was recommended in those cases was because of a much lower ESR compared to a larger value 'lytic. |
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| Curmudgeon |
(Solid) tantalums measurably add distortion. Too bad, as they are good for ESR, inductance and life. See the original article on Walt Jung's website.
Personally, I have a LOT of difficulty with the concept of slowed music, I just don't hear that way. A lot of very high end equipment has, over the years, paralleled many fast electrolytics (or even films) in the power supply. One of the major differences between the Bryston ST and SST generations is a considerable increase in power supply capacitance. I've always used the various fast Panasonic electrolytics, but I can't say that I did comparisons.
In crossover applications, I've had good luck with the Rel-cap multi-cap polyprop film/foil caps, and also with the North Creek in the large metallized polyprops. If you want to bypass with polyprops, those might be a good place to begin investigating. |
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