I have a quality linear regulated power supply that outputs 50v up to 3A. I am thinking about using resistors to split the single supply to make it +/-25v and use it to power a gainclone LM3886 amp that in turn will power a pair of electrostatic headphones (not much current needed there). I have a few questions:
1) Is the 3A rating on the power supply enough for the gainclone?
2) Is it possible to use resistors to split the supply, given the current?
3) If it's possible, what would be the power rating and impedance value for the resistors? And what types are preferred? (I have wirewounds, but just saw some Cement resistors)
Thanks!
1) Is the 3A rating on the power supply enough for the gainclone?
2) Is it possible to use resistors to split the supply, given the current?
3) If it's possible, what would be the power rating and impedance value for the resistors? And what types are preferred? (I have wirewounds, but just saw some Cement resistors)
Thanks!
There is no way you could user resistors to do this. The amount of current they would have to sink to make yourself a virtual earth would probably tax the supply before you even had a chance to use it as an amp. The only way to ever make a half decent virtual earth with a single rail supply is to actually use another power op amp to dc amplify V/2 and use that as your virtual earth.
You are better of using the LM3886 in its proper single supply configuration, as shown on page six of this pdf.
http://www.national.com/ds/LM/LM3886.pdf
Its a fair amount more complicated to use in a single supply configuration, and a bit of waste really to drive ES headphones, which are hard enough to drive as it is.
Honestly, you are much better of using your regulated supply to make a class A amp. Much better at driving ES headphones, and its regulated, which pretty well solves the biggest problem most class A amps have when building them.
You are better of using the LM3886 in its proper single supply configuration, as shown on page six of this pdf.
http://www.national.com/ds/LM/LM3886.pdf
Its a fair amount more complicated to use in a single supply configuration, and a bit of waste really to drive ES headphones, which are hard enough to drive as it is.
Honestly, you are much better of using your regulated supply to make a class A amp. Much better at driving ES headphones, and its regulated, which pretty well solves the biggest problem most class A amps have when building them.
With ±25 V the LM3886 will deliver ~30 W into 8 Ohm and ~58 W into 4 Ohm. Your 150 W power supply will be sufficient for two channels.mudihan said:
You can use resistors to do that, because they will not carry a lot of current. The application in the datasheet adds a transistor to the resistors for better stability. The transistor is rated at 200 mA, so even if you don't understand the schematic, you can deduce that little current is needed.mudihan said:
I=U/R and P=U²/R
Normal metal film types with 1/4 W rating will do. 100k is a commonly used value. If you don't like transistors, you can copy the implementation from the TDA2030's datasheet. There a capacitor is used for stability.mudihan said:
Don't forget that you need DC blocking caps at the in- and output of the amplifier in a single supply application.
The single supply version does have caps between the amp and headphones.
The only way to avoid caps is to have a split power supply.
If you dont have one, this is the only decent way to make one from a single supply that is capable of sinking/sourcing a decent amount of current without wasting it as heat in large resistors.
http://www.techlib.com/craig/craigssplitsupply.htm
The only way to avoid caps is to have a split power supply.
If you dont have one, this is the only decent way to make one from a single supply that is capable of sinking/sourcing a decent amount of current without wasting it as heat in large resistors.
http://www.techlib.com/craig/craigssplitsupply.htm
I also have +/-29v 100va transformer. After rectification, it will be around +/-40+ VDC, which is very close for LM3886/3875.
Well, I have to think about the whole thing. I am very familiar with National chips and have made a LM3886 myself. They are cheap and the PCBs are easily available. Doing class-A will need new research, which is almost impossible due to my time restraint. I am also trying to use existing materials, due to my budget limit.
For now, my newly acquired ES headphones have to rest.
Well, I have to think about the whole thing. I am very familiar with National chips and have made a LM3886 myself. They are cheap and the PCBs are easily available. Doing class-A will need new research, which is almost impossible due to my time restraint. I am also trying to use existing materials, due to my budget limit.
For now, my newly acquired ES headphones have to rest.
There is little other choice for SE, but PP works from a split supply as well, thus no caps.Rainwulf said:
This amp is going to powerSpittinLLama said:
http://www.headwize.com/projects/showfile.php?file=gilmore2_prj.htm
http://headwize.com/projects/showfile.php?file=gilmore_prj.htm
http://www.national.com/an/AN/AN-1768.pdf
It is possible to avoid the output coupling capacitor in a single supply amp.
In the PSU, instead of using one smoothing cap, use two in serial of double capacitance, parallel each cap with a a few kohms resistor. So you have a middle point at Vcc/2. Wire your speaker from the amplifier output to this point. No DC will cross your speaker and your amp will work more symetrically.
In the PSU, instead of using one smoothing cap, use two in serial of double capacitance, parallel each cap with a a few kohms resistor. So you have a middle point at Vcc/2. Wire your speaker from the amplifier output to this point. No DC will cross your speaker and your amp will work more symetrically.
bobodioulasso said:
This wont work because you have to make sure the resistors can pass more current then the speaker ever could, otherwise that v/2 point will rise/fall. A few kohms wont ever be enough, the speaker will just charge/discharge the caps until they are at rail voltage, or earth.
Think about it like this, you have a 8ohm resistor, charging a cap. Then you have say a 2 Kiloohm resistor discharging it.
Which one is going to win........
It will work fine for high impedance loads, but not with low impedance lines. When you have a speaker current of several amps, where does it go?
You can use low value resistors, but will waste a lot of power as heat.
And you do realise that you are using the caps as coupling caps right? So you are still left with caps in the power amp stage.
They are just on the other side of the speaker.
http://sound.westhost.com/project43.htm
Details the issues with using resistors to supply a V/2 supply.
You just cant do it in high current situations.
You can use low value resistors, but will waste a lot of power as heat.
And you do realise that you are using the caps as coupling caps right? So you are still left with caps in the power amp stage.
They are just on the other side of the speaker.
http://sound.westhost.com/project43.htm
Details the issues with using resistors to supply a V/2 supply.
You just cant do it in high current situations.
I know exactly how an amplifer works. The current comes from or to the power rails. The current goes through the speaker, then it has to go back to the rails. Kirchoffs current law.
If there is an earth point, current goes back through the earth. This earth carries ALL the current that goes through the speaker, which can be quite a few amps. If you make a virtual earth with 1k resistors, and your rail is 30 volts, the max current that can flow is 3ma. Thats Ohms law.
If you isolate the speaker with large value resistors, then the current cant flow, and your amp wont work.
Why do you think chip amp application notes have a split rail circuit, and then a single rail circuit that is often more complicated, and uses a coupling capacitor. Because you simple just cant use two resistors and two caps between your rails to create a fake earth. It just doesnt work like that.
Do an experiment for me. Put a 1k resistor in series with your speaker in your amp. Tell me how your music sounds then.
This is not even an argument, i already pointed you to a web page that has information on making virtual earths using resistors.
I will quote it for you.
"There will be instances where the currents from each supply will be unequal. Where this is the case, the resistor divider is not sufficient, and the +ve and -ve voltages will be unequal. "
Post 4 by pacificblue also states the same. You can not just use resistors.
here is some more information.
http://tangentsoft.net/elec/vgrounds.html
For all but the lowest currents you have to use an active device to create your virtual earth, and you still wont be able to use that to earth a speaker.
You can create a virtual earth for ONLY the speaker using two capacitors, but then thats only coupling capacitors again. Which hasnt solved anything at all.
Its just moving the cap to the other side of the speaker. You cant use that virtual earth in your LM circuit, because it will basically be the output.
If there is an earth point, current goes back through the earth. This earth carries ALL the current that goes through the speaker, which can be quite a few amps. If you make a virtual earth with 1k resistors, and your rail is 30 volts, the max current that can flow is 3ma. Thats Ohms law.
If you isolate the speaker with large value resistors, then the current cant flow, and your amp wont work.
Why do you think chip amp application notes have a split rail circuit, and then a single rail circuit that is often more complicated, and uses a coupling capacitor. Because you simple just cant use two resistors and two caps between your rails to create a fake earth. It just doesnt work like that.
Do an experiment for me. Put a 1k resistor in series with your speaker in your amp. Tell me how your music sounds then.
This is not even an argument, i already pointed you to a web page that has information on making virtual earths using resistors.
I will quote it for you.
"There will be instances where the currents from each supply will be unequal. Where this is the case, the resistor divider is not sufficient, and the +ve and -ve voltages will be unequal. "
Post 4 by pacificblue also states the same. You can not just use resistors.
here is some more information.
http://tangentsoft.net/elec/vgrounds.html
For all but the lowest currents you have to use an active device to create your virtual earth, and you still wont be able to use that to earth a speaker.
You can create a virtual earth for ONLY the speaker using two capacitors, but then thats only coupling capacitors again. Which hasnt solved anything at all.
Its just moving the cap to the other side of the speaker. You cant use that virtual earth in your LM circuit, because it will basically be the output.
If I'm imagining what's being considered, he's just making an ac voltage divider in the supply with two caps, and insuring the midpoint doesn't drift using high value resistors. It will work fine as long as the musical signal averages to 0 vdc, which is usually the case. The scheme won't work for DC signals but who cares. IMHO, none of the splitting techniques are very desirable and I'd go find a suitable transformer with a CT, or build a switcher with a CT.
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