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# Millett "Starving Student" hybrid amp - Page 9

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Quote:
 Originally Posted by tomb Good point - it looks to me the differential has to remain between 3V and 40V, so I think you're right: 37V limit.
No, you just have to float it and keep the differential between in and out low enough and it can be used as a high voltage reg. You can also use it as a floating CCS with a shunting resistor to ground and it will work as a shunting V-reg.

Holland's suggestion will work, too, though it wouldn't be my first choice.
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Quote:
 Originally Posted by holland No. The differential between in and out is between 3V and 40V. I may be wrong, but I didn't get the impression that there is a 37V output voltage limit from the datasheet.
This surely won't go down as the last time I read a data sheet wrong. I'd swear I found one reference that specifically called the Vout (not differential) limit 37V, although now that I think about it I see nothing that would preclude it's use provided appropriate steps are taken.

dsavitsk, is there a way in which something like a TREAD could be used while floating the LM317? I don't fully understand the floating concept.
Quote:
 Originally Posted by n_maher This surely won't go down as the last time I read a data sheet wrong. I'd swear I found one reference that specifically called the Vout (not differential) limit 37V, although now that I think about it I see nothing that would preclude it's use provided appropriate steps are taken. dsavitsk, is there a way in which something like a TREAD could be used while floating the LM317? I don't fully understand the floating concept.
Me, too. I think if it's referencing ground at zero, then it will see 48V. On the other hand, even a TREAD or STEPS only uses a differential of 5-10V (going on memory, here, so don't shoot me), whereas the supply voltage is much larger.

On a different subject - sort of - it appears that many of the Cisco PS's are still available on ebay:

eBay Stores Ã¢â‚¬â€œ Cisco PSA18U, Networking Communications, Desktop Laptop Accessories, PC Components Ã¢â‚¬â€œ Original items at low prices.

You get zip if you do a simple search on regular ebay, but if you click on "Sellers", you get that list of 23 above right now. Several of those sellers have multiples - up to 25 for the few I checked. Most seem to be less than \$20-\$25 total, with shipping, too.

One thing not mentioned is that these don't come with a power cord. I got two from that first big seller and was surprised to find the AC cords weren't included. I guess many of us have those laying around - but just in case some of you were expecting a complete product ...
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It can certainly be made to work, but you must take care using an LM317 at voltages over 37V. Consider what happens when power is off and input and output are both at zero volts. Now flip the power switch, applying 48V to the input pin almost instantaneously. Until the output cap charges up, there is 48V across the regulator.

Also consider what happens when you slip with a scope probe and accidently short the output to ground .

Those issues are why the NS app note has transistors and zeners in it...

There's really no need for good regulation. I think a power transistor/zener regulator would be plenty good.

A decently stiff unregulated supply may sound even better. Often people don't realize that the gain in a voltage regulator is very active at audio frequencies, and unless you have very large output caps, you're actually listening to the voltage regulator as much as your amplifier.

Pete
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You can do this - I use a LM317 regulated supply for the screens in my EL84 amp - in the range 270-310V.

I don't have the schematic to hand but I'll try to dig it out.

Fran
Quote:
 Originally Posted by n_maher It's possible, look and see if it's just an ROHS thing, perhaps an alternate part is already listed. I'll have a look later.
duh, yeah that's it...somehow i missed the "click here for substitute" link. But I was wondering, would some Wimas be a good upgrade? Hah, gotta love upgrades that mean spending an extra 30 cents. Would the metalized polyester film or the polypropylene be preferable? I know they'd be a bit of a hassle using them in p-to-p construction with their short leads, but nothing a little bit of perfboard & some wire can't handle

Quote:
 Originally Posted by n_maher Without looking no idea what the N means. And yes, I'm using the 1-1/2 tall version and things seem fine. I'll try and ask Pete if they should have been 2".
ahhh the ones with the N at the end of the p/n have are notched in the middle below where the transistor mates with the sink...not sure what the point of that is, but they're not worth an extra \$2 to me on a sub-\$40 amp

Quote:
 Originally Posted by n_maher I wouldn't use the chassis as the ground plane, I think it'll open up the possibility of noise. I used a terminal strip at the rear of the amp as a star ground which I then tied to the chassis in one location.
what's the difference? Isn't the ground still in electrical contact with the whole chassis?
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Quote:
 But I was wondering, would some Wimas be a good upgrade?
I think that's going to have to fall to the Y in DIY.
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 ahhh the ones with the N at the end of the p/n have are notched in the middle below where the transistor mates with the sink...not sure what the point of that is, but they're not worth an extra \$2 to me on a sub-\$40 amp
For me it made it much easier to mount the sinks with a hole grommet so they were worth it.

Quote:
 what's the difference? Isn't the ground still in electrical contact with the whole chassis?
As far as I understand it it's a bad idea because then you're using the chassis as a conductor rather than a single point. The latter setup doesn't require the chassis to flow any current and lets it act like a shield. At least that's the way it was once explained to me. And all I can tell you is that with my Menace build it made a huge difference in the noise floor of the amp.
Quote:
 Originally Posted by n_maher I think that's going to have to fall to the Y in DIY.
ya, they're actually out of stock on those ROHS compliant versions of the ones pete has listed on the BOM anyway

Quote:
 Originally Posted by n_maher As far as I understand it it's a bad idea because then you're using the chassis as a conductor rather than a single point. The latter setup doesn't require the chassis to flow any current and lets it act like a shield.
ahhh that makes sense

edit: u ever figure out what those little resistors are that you used? I remember seeing them somewhere else but I can't recall where and for some reason it's really bugging me
Quote:
 Originally Posted by pinkfloyd4ever u ever figure out what those little resistors are that you used? I remember seeing them somewhere else but I can't recall where and for some reason it's really bugging me
Those little tiny ones are Yageo Carbon Films. At Digikey, use the value+EBK to find the part, i.e., 22EBK, 1.5KEBK, etc.

Anyone e,lse do this yet?
I have my tubes and powersupply, just waiting for a few things from mouser to show up(with the rest of my parts for my mosfet max)
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