I intend on using a "hybrid" PSU for my RevC's, as follows:
An SMPS (with a hefty tranny, ~6 cu.in) (link's on my dad's comp, i'll attach it ASAP)
Two two-stage LM338s, first stage being a conventional reg, and the second being a "super-reg" (as in http://www.neilmcbride.co.uk/preamp.html), followed by the snubber that CarlosFM recommended.
I just received my 5-sample order of LM3886s from NatSemicon, so i hope to start work ASAP
I hope that this "frankenstein" of a PSU will sound near perfection (considering the good reviews that SMPS's). Has anyone else tried anything similar???
An SMPS (with a hefty tranny, ~6 cu.in) (link's on my dad's comp, i'll attach it ASAP)
Two two-stage LM338s, first stage being a conventional reg, and the second being a "super-reg" (as in http://www.neilmcbride.co.uk/preamp.html), followed by the snubber that CarlosFM recommended.
I just received my 5-sample order of LM3886s from NatSemicon, so i hope to start work ASAP
I hope that this "frankenstein" of a PSU will sound near perfection (considering the good reviews that SMPS's). Has anyone else tried anything similar???
Yes, I have tried several variations on various themes of PSUs for "My Ref" RevC, but never a SMPS. It should be an interesting experiment. Just out of curiosity, have you tried it as designed? The PSU is actually very purpose designed for the interaction with the opamps configured as they are (think PSSR). You may not feel the need to do anything else, but curiosity says "hey it would be fun to try". Certianly it is fun to fiddle. So go for it.
Cheers!
Russ
Cheers!
Russ
By "have you tried it as designed" do you mean the PSU on the RevC PCB? If you do, then i haven't tried that yet.
It's just that my dad had the great idea of modding some PC PSUs that we had lying around. After we redid the windings on the power tranny, results looked promising, so we're gonna build an SMPS according to the schematic/PCB from a site i'll have to ask dad about.
I think a first reason would be costs, then weight.
A several hundred watt tranny / toroid is, to say the least, huge, not to mention sort of expensive compared to what i'm willing to spend
It's just that my dad had the great idea of modding some PC PSUs that we had lying around. After we redid the windings on the power tranny, results looked promising, so we're gonna build an SMPS according to the schematic/PCB from a site i'll have to ask dad about.
I think a first reason would be costs, then weight.
A several hundred watt tranny / toroid is, to say the least, huge, not to mention sort of expensive compared to what i'm willing to spend
The link to the SMPS, only mine will have two separate 35Vdc secondaries.
http://www.qrp4u.de/docs/en/smps_new/
http://www.qrp4u.de/docs/en/smps_new/
PC Supply for LM3886
Khron,
If you're considering using an AT or ATX supply, don't even think of doing so until it has been modified to put out a (+/-) bipolar, symmetrical supply, first. The +12V and -12V lines out are not up to the task of powering the LM3886 for the following two reasons:
1) The +12V output voltage is tied to loading of the +5V line, and will actually read only about +10.5V if the +5V line is unloaded. Not a good thing to have happen;
2) The -12V line is good for only about 100mA, and connecting it to the negative supply pin of the LM3886 will surely drag it down. Also, this output's unloaded voltage is tied to loading of the +5V line in the same way the +12V line is.
At the very least, you will have to rewind the transformer's secondaires to put out the +/-12V to +/-42V (min & max) needed to properly bias the chip.
There are numerous threads in the Power Supplies Forum covering this subject. Many of the heavies on this subject, EVA, Jackinnj, and others have weighed in on this. Having said that, I cannot emphasize the next thing enough:
CAUTION!
If you attempt to modify an AT or ATX supply, you will be working with AC and DC voltages that are LETHAL!! 
PLEASE PLEASE PLEASE don't even try to attempt this unless you are very experienced, and I mean very experienced, at working with high voltages.
As a minimum, you should take the following cautionary measures:
1) Since these supplies work directly off the AC line, and hence, are NOT isolated from it, serious injury and DEATH can occur if you fail to isolate the supply from the AC line. A 220-220V (or 120-120V if you live in the 'States) isolation transformer, with sufficient capacity to handle your loads will accomplish this;
2) You wear clothing and rubber shoes that are insulative;
3) You never have more than one hand in a potentially hot spot at any time. Anything over 35VAC or 60VDC across the human chest could cause heart defibrulation, as it only takes a milliamp or two to do this;
4) you measure all voltages with a multimeter, not your body (See #3)
, and;
5) Just use common sense when working like this.
If you are sufficiently experienced and have observed, at a minimum, all of the cautions mentioned here, you should be OK. I know I might be sounding like an old granny saying "it cannot be done" or "oh, you can't do that", or just otherwise raining on your parade, I just want to emphasize how important safety here REALLY is.
Hope this gives you some insight on how to best proceed.
Regards,
Steve
Khron,
If you're considering using an AT or ATX supply, don't even think of doing so until it has been modified to put out a (+/-) bipolar, symmetrical supply, first. The +12V and -12V lines out are not up to the task of powering the LM3886 for the following two reasons:
1) The +12V output voltage is tied to loading of the +5V line, and will actually read only about +10.5V if the +5V line is unloaded. Not a good thing to have happen;
2) The -12V line is good for only about 100mA, and connecting it to the negative supply pin of the LM3886 will surely drag it down. Also, this output's unloaded voltage is tied to loading of the +5V line in the same way the +12V line is.
At the very least, you will have to rewind the transformer's secondaires to put out the +/-12V to +/-42V (min & max) needed to properly bias the chip.
There are numerous threads in the Power Supplies Forum covering this subject. Many of the heavies on this subject, EVA, Jackinnj, and others have weighed in on this. Having said that, I cannot emphasize the next thing enough:
CAUTION!
If you attempt to modify an AT or ATX supply, you will be working with AC and DC voltages that are LETHAL!! PLEASE PLEASE PLEASE don't even try to attempt this unless you are very experienced, and I mean very experienced, at working with high voltages.As a minimum, you should take the following cautionary measures:
1) Since these supplies work directly off the AC line, and hence, are NOT isolated from it, serious injury and DEATH can occur if you fail to isolate the supply from the AC line. A 220-220V (or 120-120V if you live in the 'States) isolation transformer, with sufficient capacity to handle your loads will accomplish this;
2) You wear clothing and rubber shoes that are insulative;
3) You never have more than one hand in a potentially hot spot at any time. Anything over 35VAC or 60VDC across the human chest could cause heart defibrulation, as it only takes a milliamp or two to do this;
4) you measure all voltages with a multimeter, not your body (See #3)
, and;5) Just use common sense when working like this.
If you are sufficiently experienced and have observed, at a minimum, all of the cautions mentioned here, you should be OK. I know I might be sounding like an old granny saying "it cannot be done" or "oh, you can't do that", or just otherwise raining on your parade, I just want to emphasize how important safety here REALLY is.
Hope this gives you some insight on how to best proceed.
Regards,
Steve
N-Channel
Thanks a lot on the safety warnings; i know many others would take it as being talked down to, but you can never be too safe
Now, on to the bulk of your post...
The part with the PC PSU modding was just as a "feasability test", with the power trafo re-winded for 2x ~28-29V. My dad did most (in fact all) of the work. Nothing blew up, fortunately, which means we have enough confidence to move on to re-winding a trafo with a substantially larger core (about twice as big & bulky as a "normal" power trafo you'd find in a PC PSU).
We intend to slightly modify the schematic and PCB found here: http://www.qrp4u.de/docs/en/smps_new/
The mods include re-working the PCB to accomodate symmetrical voltages (i.e. 2x ~30Vdc), and maybe even the "super-regulator" part i described in my initial post.
Anyway, especially because it was actually my dad's idea, i would never have started work on something like this all by myself. At least for now, i've only done a couple of "conventional" linear supplies (trafo - bridge - el. caps).
Thanks again on the input, even though the 1st part is a bit... "redundant", so to speak (no offense), and for the (never too many) safety tips.
I hope to have some actual results ASAP, but it all depends on my dad's spare time, and my spare time in the weekends (i'm at college, and when i'm home, i do have some other "obligations", if ya catch my drift..)
Thanks a lot on the safety warnings; i know many others would take it as being talked down to, but you can never be too safe
Now, on to the bulk of your post...
The part with the PC PSU modding was just as a "feasability test", with the power trafo re-winded for 2x ~28-29V. My dad did most (in fact all) of the work. Nothing blew up, fortunately, which means we have enough confidence to move on to re-winding a trafo with a substantially larger core (about twice as big & bulky as a "normal" power trafo you'd find in a PC PSU).
We intend to slightly modify the schematic and PCB found here: http://www.qrp4u.de/docs/en/smps_new/
The mods include re-working the PCB to accomodate symmetrical voltages (i.e. 2x ~30Vdc), and maybe even the "super-regulator" part i described in my initial post.
Anyway, especially because it was actually my dad's idea, i would never have started work on something like this all by myself. At least for now, i've only done a couple of "conventional" linear supplies (trafo - bridge - el. caps).
Thanks again on the input, even though the 1st part is a bit... "redundant", so to speak (no offense), and for the (never too many) safety tips.
I hope to have some actual results ASAP, but it all depends on my dad's spare time, and my spare time in the weekends (i'm at college, and when i'm home, i do have some other "obligations", if ya catch my drift..)
That site shows that the rectification is done with a couple of FETs on each rail (i'm gonna have two separate secondaries). That should be modable into symmetric rectifiying, with a pair of double Schottkys, right?
Plus, i'm gonna slap on a pair of regs on each rail, like the one i gave the link for in my original post (it's a bit lower than the middle of the html). I'm hoping that'll be enough.
Plus, i'm gonna slap on a pair of regs on each rail, like the one i gave the link for in my original post (it's a bit lower than the middle of the html). I'm hoping that'll be enough.
A pair of double-schottkys will do just fine, or, you could use a pair of double diodes like they use in car amps, if you only want to wind one centertapped secondary, which you might want to consider, especially if you're running out of room on the transformer bobbin. OnSemi has two I have recco'd a few times, and worked with in the past with great success. They're not Schottkys, but their forward voltage drop is not too bad. They are:
MUR1620CT
MUR1620CTR
As the part number implies, the 'CTR is the double diode used to recover the negative voltages. At 7A, the forward voltage drop is only 0.85V for either polarity. See the following datasheet.
http://www.onsemi.com/pub/Collateral/MUR1620CTR-D.PDF
MUR1620CT
MUR1620CTR
As the part number implies, the 'CTR is the double diode used to recover the negative voltages. At 7A, the forward voltage drop is only 0.85V for either polarity. See the following datasheet.
http://www.onsemi.com/pub/Collateral/MUR1620CTR-D.PDF
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