Inrush thermistors on Xono phono stage - diyAudio
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Old 22nd May 2007, 03:19 PM   #1
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Default Inrush thermistors on Xono phono stage

Hi!

I have just finished up my preamp boards and am moving on to the power supply for my Xono. I have found a number of designs out there, two of which have caught my attention. My question has to do with the proper fuse and thermistor configuration. My toroids are dual 50VA Plitron toroids rated at 30 VAC x 2 at 0.83A with 13% regulation. I am using a Delta 06AK2D inlet, which contains an EMI filter, and a 6A fuse (by default). I'm not sure if I should use two inlets or just one, so I'd like to pose that question as well. The design I plan on following (closest) is located here:

http://www.r-stens.de/diy/AlephOno/Rev2/PowerSupply.pdf

http://www.r-stens.de/diy/AlephOno/Rev2/PSAnalogDC.pdf

However, I'd like to add the inrush protection. I have found another Xono schematic, which I'm attaching, that includes inrush protection that might be a good model.

Also, I have looked at the power supply for the F4 located here, which has a similar use of thermistors as the attached schematic:

http://www.passlabs.com/np/F4%20Power%20Amplifier.pdf

I guess my question is that there are a number of possible configurations, including not using any inrush protection at all. I was wondering if you guys could help me sort out what's best in this case for my Xono.

So in summary, I'm asking about fuse rating, thermistor configuration and number of IEC inlets.

Thanks!
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File Type: pdf ono_unreg. pcb_schematic.pdf (18.2 KB, 127 views)
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Old 22nd May 2007, 03:41 PM   #2
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I'm not familiar with this particular project, but why do you think you might need two mains inlets?

I don't think you need inrush limiting for 30VA. Ten times that VA then I would start thinking about it.
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Old 22nd May 2007, 06:00 PM   #3
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Default Re: Inrush thermistors on Xono phono stage

Quote:
Originally posted by luvdunhill
So in summary, I'm asking about fuse rating, thermistor configuration and number of IEC inlets.
1A slow blow, any thermistor similar to a Keystone CL60 or so,
and 1 IEC outlet.

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Old 22nd May 2007, 07:11 PM   #4
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hi! okay, that's short and sweet! What about the configuration of said thermistors? Should I do something like in the F4, where there are three, two between the IEC inlet and the toroid primaries and one between the output ground and the chassis ground (which is shared with the IEC inlet earth) *or* is there no need for something this fancy...

as to richie00boy's question, well I see a lot of products with two IEC inlets, and besides the fact that it looks cool, I could never figure out why they did it, except for a bit more isolation when using power conditioning? who knows, probably all marketing hype...
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Old 22nd May 2007, 09:54 PM   #5
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How many things have you seen with more than one power connection? With the arguable exception of a dual mono amplifier in a single chassis, I fail to see any use for such a thing.
I'm also not sure why you see the need for inrush limiting on a low level piece of equipment. The usual application is to keep from blowing fuses when charging a very large capacitor bank in an amplifier (note that the F4 is a power amplifier). In a phono stage, you're going to have to go into some really heavy capacitance to need such a thing. Any other variation would be as well served by a straight resistance, e.g. reducing the rail voltage.

Grey
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Old 22nd May 2007, 10:18 PM   #6
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Grey:

Points taken. I believe the piece of equipment I was referring to with two power cords is one of the Aesthetix phono stages, but I cannot find pictures at this point to confirm.

In any case, the Xono is indeed dual mono in both the preamplification and power supply portions of the design.

As to your point about the amount of capacitance and straight resistance, I am following this schematic here:

http://www.r-stens.de/diy/AlephOno/Rev2/PSAnalogDC.pdf

which is part of this larger design:

http://www.r-stens.de/diy/AlephOno/Rev2/PowerSupply.pdf

Finally, the last attachment is another Xono design that incorporates thermistors, hence my interest in the subject.

It may be useful to note that this is definitely a tour-de-force phono preamplifier design
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Old 22nd May 2007, 10:48 PM   #7
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Quote:
Originally posted by luvdunhill


It may be useful to note that this is definitely a tour-de-force phono preamplifier design


So I had assumed, but I see no thermistors in the schematics you linked to, only fixed resistors, and you haven't suggested anything that would necessitate their use.

Grey
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Old 22nd May 2007, 11:22 PM   #8
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well, the attachment in my first post named ono_unreg. pcb_schematic.pdf shows two thermistors labeled TH1 and TH2.

Also, the second link in my first post (http://www.r-stens.de/diy/AlephOno/Rev2/PSAnalogDC.pdf) shows 80,000uF in the power supply chassis. In addition, there will be another 37,600uF in the preamplifier chassis before regulation. Hence my concerns.
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Old 23rd May 2007, 12:48 AM   #9
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I took the pdf you're mentioning to be yours, not someone else's; so of course it shows them because you put them there. Perhaps I misunderstood.
There are numerous ways to look at power supplies. My point of view--and I do not claim that it's shared by everyone--is to reduce the impedance looking upstream into the AC line to a bare minimum. Resistors and thermistors are contraindicated. Yes, there are times when they're necessary, but I avoid them if I can. 80,000uF doesn't scare me, but if you feel that it's a problem, then use the thermistors, but bear in mind that by using a lot of capacitance fed by resistance you are engaging in a tug-of-war with yourself. One of the reasons for using a lot of capacitance is to lower the impedance seen by the circuit when it demands current. By definition, putting resistance in the line will work against that.
Your response to that will probably take one of two forms: The resistance is on the other side of the power transformer--or--I'm not storing energy, I'm making the rail quiet.
If the first, my reply is that any impedance will retard the flow of charge to the power supply caps.
If the second, my response is that caps are good, but there are more elegant ways to make a rail quiet than brute force capacitance.
Again, if you want to use a lot of capacitance, I'm all for it. If you feel that thermistors are part of your vision of how to get where you want to go, then have at it. For something small like a phono stage, even assuming an inrush current problem (which I'm still not convinced is the case), you could use something like inductors wound with heavy gauge wire--minimal impedance, except at turn-on. Or use a circuit topology that sums to DC for current draw. Or use a capacitance multiplier. Or active regulation. Or...any number of things.
Incidentally, I don't see that schematic showing 80,000uF. Are you sure we're talking about the right schematic?

Grey

P.S.: Bear in mind that since you're referencing schematics showing CRC filters, everything after the R will, by definition, not develop much inrush current because the R will show an increased voltage drop due to the current. One the surge is over, things will assume a more modest value.
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Old 13th July 2007, 02:04 PM   #10
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Grey:

First of all, thanks for the well thought out response! I realized I never thanked you.

Grey and Co.:

Well, I'm now out of 1A fuses I was really jonesing for some Mahler last night, so I stuck in a 3A fuse after my last 1A blew and listened to Solti and Mahler 1 on Decca

To clarify, I have two power supplies, each of which has a total of 24,700uF per rail before the regulator. 20,000uF of this is in a separate power supply chassis, and the other 4,700uF is in the preamp chassis right before the regulator. So, there are a total of 8 x 10,000uF in the power supply chassis. Maybe pictures would help if this is clear only to myself?

So, it seems that I am meeting your requirement of "blowing fuses when charging a very large capacitor bank". So, should I just use a larger fuse, or consider adding a thermistor, or something else?

Thanks in advance!
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