SIT1 Choke Input Power Supply? - diyAudio
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Old 4th January 2017, 11:50 PM   #1
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Default SIT1 Choke Input Power Supply?

I must be determined to hear if this is worth the trouble. I know Mr. Pass has done his best to dissuade me from doing this.

I made a few observations - the SIT1 uses a 72 volts AC power transformer and a single bridge rectifier which is then split into what I measure to be +- 40 volts rails. My AC line is a little high so that number is slightly higher than it should be.

My first idea was to simply use a 100 volts power transformer into the rectifier and then to a LUNDAHL LL2733 choke configured for 100 mH and .875 R.

Using the DUNCAN power supply modeler this says I will be running much more current through the choke than it can handle. I wonder if this was why I was told this was not a good idea since if the LUNDAHLs cannot be used. There is nothing of a similar physical size available.

So I wonder if I use the two 50 volts windings (instead of using them in series) and using two bridge rectifiers and two chokes I can get the current down to just within the choke's limits.

First question is: are both supplies required to deliver the same amount of current? With the extra caps on the plus side I wonder if one side is being asked to deliver more than the negative side - if this is the case that makes the chokes unusable.

If not, and the two supplies are delivering approx equal current it can work. I will have to mount the capacitor board vertically to get the room required.

My question with the sincere hope I will get an answer from the only one who REALLY knows - is am I on the right track? I know I can get this to work if this is all there is to it. If there is a pitfall I cannot see please let me know.

Initially it seemed it would be very simple and now I see it will require much more time than I thought but it is getting cold outside and one must keep themselves busy indoors.

I have this belief (and that is the only way to describe it) that this will make these wonderful amplifiers even more lovely to listen to - not night and day, of course, but when one is straining for nirvana is there any route that should not be taken? And there is the crux - is there more potential for destroying a precious output device than there is a chance of hearing a slightly better amplifier?

Unless there is something I am not aware of I feel confident I can do this without destroying anything but there is so much of which I am unaware.

I will never change these amps so concerns about resale value are not a factor; someone always has to throw that in.

Guidance is needed and greatly appreciated.

Thanks and take care,
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Old 5th January 2017, 06:00 PM   #2
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There is only 1 supply rail at about 90 volts. The -V you see in the
simplified schematic is about -5V for biasing the Gate of the SIT.

If you want to run the SIT into a choke I suggest a quiet regulated supply
at about +15V and a bias current of 1.4A.
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Old 5th January 2017, 10:07 PM   #3
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Thanks for your time.

Shows how good my analysis is. I found those two points marked D & G and did not bother to look at the polarity of the voltage.

I would like trying a choke but if that is all on one rail no chance of finding a choke that would fit int he thing. Unless you have a suggestion ...

So are you saying all of that "extra" voltage is being bled away by those resistors?

And that the SIT 2 is using a much lower voltage power supply?

I am much more comfortable (and safer) with passive components. A regulator for the amp is well beyond my capabilities. AS if I had to even write that.

THANKS again for your time and trouble.
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Old 5th January 2017, 10:09 PM   #4
eduard is offline eduard  Netherlands
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Hello,
A few days agomy Vietnamese friend Tho got his pair of lundahl ll2733. He will use them in a single ended transistor design by Nelson Pass.
Already did type to much about choke input power supplies for devices with higher currents so now it is up to the others.
I am not a lundahl shareholder and have not interest in convincing other people to try choke input.
I did travel through Asia with 2 chokes for Thomas in my luggage so they better be good lol.
Greetings, Eduard
Pso important is the use a bleeder that will take of the minimumcurrent necessary to make it work as choke input. See the graphic
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Old 5th January 2017, 10:20 PM   #5
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In this application a bleeder would only put more stress on the choke!

LUNDAHL says 5.7 A is the saturation point and the DUNCAN modeler predicts 9 A going through the choke!

Wish LUNDAHL made a BIG one!

I see nothing wrong with being enthusiastic about LUNDAHL products.
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Old 5th January 2017, 10:42 PM   #6
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Quote:
Originally Posted by rickmcinnis View Post
So are you saying all of that "extra" voltage is being bled away by those resistors?

And that the SIT 2 is using a much lower voltage power supply?
Yes and Yes.

The SIT-1 is a demonstration of the performance of a single device
surrounded by passive components doing the filtering and current sourcing.

The SIT-2 uses an active current source. As currently shipped, it is a CCS.
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Old 5th January 2017, 10:47 PM   #7
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Thanks for the confirmation.

I like the passive approach much better than the active one.

So without a choke possibility I will return to being perfectly happy with the amps as they are.

Thanks for your guidance and Happy New Year! Can't wait to find out what you come up with this year!!! Take care,
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Old 5th January 2017, 11:04 PM   #8
eduard is offline eduard  Netherlands
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Hello,
All serious publications like the ones in the technical library from Philips company did mention the use of a bleeder in a choke input power supply.if you don't want to use one then just don't. Maybe one day you will hear a strange hissing sound coming from your device. Hope you will have timeenough to switch it off before the safety vent of the power capacitor will start working.
Better be safe than sorry.
Greetings, eduard
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Old 6th January 2017, 11:32 AM   #9
Zen Mod is offline Zen Mod  Serbia
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still having troubles with Class B Flue , even if I'm feeling sensations of proper A Class one .... so , missing few days of online/reply time

as subject of thread is sayin' - you want choke input PSU , not choke loaded gain stage ....... so it seems Papa misunderstood what you really want

looking at LL2733 datasheet (Lundahl site) , series connection is ditto good for feeding one channel of your SIT

that means one bridge , then LL2733 , then cap bank , all that feeding one SIT-1 channel

you can use one common xformer for both channels , and proper AC Voltage is easiest to determine ( as you did) with Duncan's PSUD

again , fastest info one can get about choke input filtering is to search for NEM AI 50, here and elsewhere on net , you'll find nice graphs of xformer and rectifier loading

lookie : The NEM AI-50

Click the image to open in full size.

edit:

so , as Pa said - you need to shoot for 90Vdc at cap bank , and in your sim set load as 1A4 CCS
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Last edited by Zen Mod; 6th January 2017 at 11:38 AM.
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Old 6th January 2017, 03:09 PM   #10
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My friend from Holland gave to me a pair of Lundahl LL2733 made for Thomas Mayer, I have a plan to build choke input filter for my SIT Nemesis amp and I think I still apply zen V3 active supply regulation in this amp

Click the image to open in full size.
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