Hi there,
I have decided to go for a class-A design DIY power amp.
I was lucky to get hold of some good 500VA+ toroids at a good price.
The most powerful one has 2x55V secondaries, which will give roughly 75V DC under load.
However most Pass designs and many other DIY amp designs are specified to a rail voltage of around 30-40V.
Are there any Passlabs designs suitable for such high rails, or is it possible to make simple tweaks to accomodate it ?
Are there any other well proven designs for higher rails that you would reccomend ? (FET, Bipolar, or whatever)
I appreciate all comments and hints please
Regards,
Bjørn
I have decided to go for a class-A design DIY power amp.
I was lucky to get hold of some good 500VA+ toroids at a good price.
The most powerful one has 2x55V secondaries, which will give roughly 75V DC under load.
However most Pass designs and many other DIY amp designs are specified to a rail voltage of around 30-40V.
Are there any Passlabs designs suitable for such high rails, or is it possible to make simple tweaks to accomodate it ?
Are there any other well proven designs for higher rails that you would reccomend ? (FET, Bipolar, or whatever)
I appreciate all comments and hints please
Regards,
Bjørn
What will you do with a 500 VA transformer and such high rails?
Figure that a 500 VA will be good for about 100 watts/ch stereo,
what amplifier do you want to make that swings +/-75 volts and
only delivers 100 watts?
Perhaps you could tap the primary for 240V and use it at 120
for +/-35 volts or so, or perhaps you could use it to make a high
voltage front end for an output stage having a lower voltage supply,
or perhaps you could build an amp which will drive the crap out of
a 16 ohm speaker like a Lowther or Feastrex
Figure that a 500 VA will be good for about 100 watts/ch stereo,
what amplifier do you want to make that swings +/-75 volts and
only delivers 100 watts?
Perhaps you could tap the primary for 240V and use it at 120
for +/-35 volts or so, or perhaps you could use it to make a high
voltage front end for an output stage having a lower voltage supply,
or perhaps you could build an amp which will drive the crap out of
a 16 ohm speaker like a Lowther or Feastrex
Hi Bjørn
As Nelson writes - 55 volts rails is way to high for a 500VA trannie if it is to be used for a class A amp. 18-22 volt would be just nice for an Aleph 3, 30, Zen, F, .......
It is however a nice trannie for some of the high efficiency switching designs out there, UCD, IcePower, ZAP and what their names are - mayby you could have a look in the Class D forum on this site?
Kind regards
Hans
As Nelson writes - 55 volts rails is way to high for a 500VA trannie if it is to be used for a class A amp. 18-22 volt would be just nice for an Aleph 3, 30, Zen, F, .......
It is however a nice trannie for some of the high efficiency switching designs out there, UCD, IcePower, ZAP and what their names are - mayby you could have a look in the Class D forum on this site?
Kind regards
Hans
Thanks to Nelson and Hans for kind and useful replies !
It confirms my suspicion, but I had to check it out before writing that nice powerful toroid off. I only paid a little more than domestic shipping for 6 transformers so never mind.
However I have noticed that some of the commercial amplifiers like the Denon POAs and such have 72V PA rails, but of course these are 200W amps and up.
Some of the Class-D modules are utilising higher voltage rails, so it might come to use some day
Some of the smaller toroids (250-400VA) have 2x24V and 2x31V and will probably be more suitable for a low power, good sounding amp design. Probably too small for a class-A, but well suited for a simple chipamp design (LM3886, etc)
I think I have found my new favourite community !
Regards,
Bjørn
It confirms my suspicion, but I had to check it out before writing that nice powerful toroid off. I only paid a little more than domestic shipping for 6 transformers so never mind.
However I have noticed that some of the commercial amplifiers like the Denon POAs and such have 72V PA rails, but of course these are 200W amps and up.
Some of the Class-D modules are utilising higher voltage rails, so it might come to use some day
Some of the smaller toroids (250-400VA) have 2x24V and 2x31V and will probably be more suitable for a low power, good sounding amp design. Probably too small for a class-A, but well suited for a simple chipamp design (LM3886, etc)
I think I have found my new favourite community !
Regards,
Bjørn
Magura said:Using it with an LC filter would be an option. That would give you like 35V rails.
Magura
Hi Maguram, thanks for yopur reply !
Could you please explain how ? From my understanding I would then have to drop the remaining 40V over the L of the filter, and hence dissipate more then a 100W across it. Not very practical.
I have considered dropping a smaller portion of the voltage across a power resistor in a RC filter, but either way, the power potential of the big toroid will be wasted.
I studied a Pioneer SX-1250 schematic (the old monster receiver) and found that the power stage (BJT) had 70V rails. It is approx 150W per channel, and runs class A for lower listening volumes.
I think that the transformer was around 600VA.
Based on this example it should be possible to make something out of the 2x55V transformer.
Regards,
Bjørn
A LC filter VS. a CLC filter, will give you an inductive load, and hence 0.7 x input voltage, instead of the 1.4 x voltage.
http://en.wikipedia.org/wiki/LC_circuit
Magura
http://en.wikipedia.org/wiki/LC_circuit
Magura
Magura said:A LC filter VS. a CLC filter, will give you an inductive load, and hence 0.7 x input voltage, instead of the 1.4 x voltage.
http://en.wikipedia.org/wiki/LC_circuit
Magura
You mean that an L in series and a C i parallel after the diode bridge will transform the 55V AC into 35V DC with little loss ?
This is new to me, but nevertheless great news !
1) What will be an appropriate value of the L in such a circuit ? I know it has to handle high current.
2) Do I need ekstra filtering capacitors after the C, provided it is big (>10mF) ?
Regards,
Bjørn
You can answer those questions yourself, by putting the numbers into this:
http://www.duncanamps.com/psud2/index.html
Magura
http://www.duncanamps.com/psud2/index.html
Magura
flg said:I think an L input filter will give you .9 Vrms and 1.41 Irms
Wiki Who?
Please explain.
I've used this trick before.....I have even had this working against me once, I had forgotten about a removed cap.
Magura
EDIT: I have not read that particular Wiki page, so if it's dodgy, don't look at me
Thanks flg !
These figures explains the topologies well.
It seems to me that all the choke topologies that gives a substantial reduction in Vdc utilises the centre tap of the transformer.
My trannie is 55-0-55 (0 on the centre tap).
My quick conclusion is that only the full bridge choke topologies (0,9xVac) will be of any use. Does it sound correct ?
Regards,
Bjørn
These figures explains the topologies well.
It seems to me that all the choke topologies that gives a substantial reduction in Vdc utilises the centre tap of the transformer.
My trannie is 55-0-55 (0 on the centre tap).
My quick conclusion is that only the full bridge choke topologies (0,9xVac) will be of any use. Does it sound correct ?
Regards,
Bjørn
I have successfully utilized a choke input on a tube amp. I am considering it on a SS amp for the exact same reasons as the poster.
Putting a small cap (.01uf to 3uf) in front of the choke will vary to output voltage from .9vac to 1.4vac. I know this is not what you are seeking, but its cool to be able to "dial in" a required voltage with a cap on a choke input filter. Use PSUdesigner to check your design.
Is there a way to physically check your output voltage rails on a choke input without having the amp board connected? I know the Lcrit current is key here.
A load resistor of value X and wattage Y would be what I am requesting to test a desired +/-35VDC rails @ 6A Choke input power supply??
Putting a small cap (.01uf to 3uf) in front of the choke will vary to output voltage from .9vac to 1.4vac. I know this is not what you are seeking, but its cool to be able to "dial in" a required voltage with a cap on a choke input filter. Use PSUdesigner to check your design.
Is there a way to physically check your output voltage rails on a choke input without having the amp board connected? I know the Lcrit current is key here.
A load resistor of value X and wattage Y would be what I am requesting to test a desired +/-35VDC rails @ 6A Choke input power supply??
Hi John65b !
Isnt it a matter just applying ohms law ?
R=V/I=35V/6A=5,83ohm so you should look for a 5,6 or safer 6,8 ohm resistor from each rail to ground.
of wattage: P=V x I=35VX6A= 210W i.e. 250W
This will be a maxiumum load test of the supply, and for prolonged testing a more relaxed (higher) value of R must be chosen.
Please take reservation for my possible misunderstanding of your specs.
NB! Dont forget to attach your dummy load resistors to a big piece of aluminium, if testing for a long duration.
Isnt it a matter just applying ohms law ?
R=V/I=35V/6A=5,83ohm so you should look for a 5,6 or safer 6,8 ohm resistor from each rail to ground.
of wattage: P=V x I=35VX6A= 210W i.e. 250W
This will be a maxiumum load test of the supply, and for prolonged testing a more relaxed (higher) value of R must be chosen.
Please take reservation for my possible misunderstanding of your specs.
NB! Dont forget to attach your dummy load resistors to a big piece of aluminium, if testing for a long duration.
Bonna,
Maybe I left out a little more info in my post -
I was wanting to ask if that test load resistor needs to be designed with respect to Critical Inductance, and not the 6A. The 6A was just the amp rating of the tranny. The amp draw of the eventual amp should be quite a bit less than the 6A...maybe half? I really don't know which amp it will be going into yet (but will be high current Class A)
From what I understand, the LC PS needs a minimum current that the at all times, or all hell supposedly breaks loose - really don't know what exactly happens - the choke starts to smoke, or the PS sees the choke as a short and your back to a C filter and the 1.414*Vac outputs.
Anyway, the bleeder resistor is supposed to flow this minimal current, and drain the cap at shut down.
So my question should have been, what minimum load resistor is required be to test a LC input PS, as I don't have a 250W resistor anywhere...?
BTW -
Lcrit = Vdc / Ima
where Lcrit is the minimum (critical) choke inductance, Vdc is the supply DC output voltage (35VDC), and Ima is the load current in milliamperes (guessing 3A).
So in operation:
Lcrit=35/3000=.0166mh, and I will be doing at least a 2mh choke, so for 3A draw, Lcrit is not an issue.
Maybe I left out a little more info in my post -
I was wanting to ask if that test load resistor needs to be designed with respect to Critical Inductance, and not the 6A. The 6A was just the amp rating of the tranny. The amp draw of the eventual amp should be quite a bit less than the 6A...maybe half? I really don't know which amp it will be going into yet (but will be high current Class A)
From what I understand, the LC PS needs a minimum current that the at all times, or all hell supposedly breaks loose - really don't know what exactly happens - the choke starts to smoke, or the PS sees the choke as a short and your back to a C filter and the 1.414*Vac outputs.
Anyway, the bleeder resistor is supposed to flow this minimal current, and drain the cap at shut down.
So my question should have been, what minimum load resistor is required be to test a LC input PS, as I don't have a 250W resistor anywhere...?
BTW -
Lcrit = Vdc / Ima
where Lcrit is the minimum (critical) choke inductance, Vdc is the supply DC output voltage (35VDC), and Ima is the load current in milliamperes (guessing 3A).
So in operation:
Lcrit=35/3000=.0166mh, and I will be doing at least a 2mh choke, so for 3A draw, Lcrit is not an issue.
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