that's NOT what i was suggesting...... just using them as TO-3P case dimension examples..... it's what i happened to have at hand for comparison...... didn't mean to open a big can of worms.... please note i used a couple of bipolars as examples as well..... i was only discussing dimensional and mounting considerations, NOT electrical characteristics......
Hi, I am also busy with a small single mosfet headphone design and have a little quetsion...
Now I know its not a perfect CSS, but I have used it before on a heater grid in a tube based pproject..
I was wondering which is the better of these 2 evils...
useing a large wattage low impendance biasing resistor (8R2) which will see 5V dropped over it... so 610mA or 3W
Or useing an LM317 as a constant current source (this is what I was talking about in first paragraph), useing a 2R2 resistor from adjust to ouput tab should give me about 570mA...( a little less about 6%)
Now I know its not a perfect CSS, but I have used it before on a heater grid in a tube based pproject..
I was wondering which is the better of these 2 evils...
useing a large wattage low impendance biasing resistor (8R2) which will see 5V dropped over it... so 610mA or 3W
Or useing an LM317 as a constant current source (this is what I was talking about in first paragraph), useing a 2R2 resistor from adjust to ouput tab should give me about 570mA...( a little less about 6%)
If you are asking about the output stage DC load, I would go with the current source, though not necesairly an LM317. The reason being, a current source will attempt to source the set current into any load no matter what the voltage is across the current source - ideally. In practice, there are limits of course. With a resistor, the maximum current sourced depends on the resistor AND the load, as there is a voltage divider thing going on. In general, for a given idle current, a current source will limit your output voltage swing less than a resistor.
In reality, however, things may be different. A LM317, for instance, needs a minimum voltage across it to work correctly, a few V. So, adding that to the 1.25V required on the current set resistor gives you some 4V as the minimum across this current source, for it to still act as a current source - so there you have an output voltage swing limiting mechanism. Fortunately, LM317 is not the only way to make a current source. It would help if we could see the rest of the schematic, but then we are going off topic with this, aren't we?
In reality, however, things may be different. A LM317, for instance, needs a minimum voltage across it to work correctly, a few V. So, adding that to the 1.25V required on the current set resistor gives you some 4V as the minimum across this current source, for it to still act as a current source - so there you have an output voltage swing limiting mechanism. Fortunately, LM317 is not the only way to make a current source. It would help if we could see the rest of the schematic, but then we are going off topic with this, aren't we?
Yes, I would hate to interrupt a productive thread, but I think my matter can be resolved with one or 2 small posts, and it relates directly to biasing a mosfet...
All the other Css methods I am familiar with tend to be pretty low wattage...
we need to manage about 3W over the CSS, as far as I see... which I think lies in the LM317's SOA
Ignoreing the pots etc before the fet, the circuit is basicaly 12v rail at the top, 8R2 resistor to irf610 0R82 resistor from irf610 to Gnd.
P.S. how do I calculate the voltage over the small resistor to ground?
All the other Css methods I am familiar with tend to be pretty low wattage...
we need to manage about 3W over the CSS, as far as I see... which I think lies in the LM317's SOA
Ignoreing the pots etc before the fet, the circuit is basicaly 12v rail at the top, 8R2 resistor to irf610 0R82 resistor from irf610 to Gnd.
P.S. how do I calculate the voltage over the small resistor to ground?
An externally hosted image should be here but it was not working when we last tested it.
Attachments
Lol, noone is useing this thread, so I might as well continue...
Got confused, between a few schematics, so ended up with 2r2 resistors instead of 0r82, when i came back from the shop.
Anyhow, I decided to parallel two of those for 1R1 ....
Well it runs pretty nicely... and I made a few test points for analysis, my last one was a beast to faulttrace...
I suppose the change in the resistor is responsible for the operateing point to be diffirent... about 8.85V is the lowest I can get the point marked 7V in the schematic... below this... a humlike distortion appears...I will swap in another 2R2, rarallel to the 1r1 I have currently, but wanted to know; does this affect the gain ?
Also, I have 47k pot on input... it hums slightly when turned to 0... but is dead quiet at 100% with source puased...
Kindly share any insights, advice, optimisations or usefull formulas that would help me conceptualise better...
I get the impression the bias current, input voltage and this resistor has an intimate relationship...
Power supply is 2x12VAC transformer with secondaries in parallel, followed by bridge made from 8A diodes, diodes bypassed with 100nf paper and film caps... followed by 2200uf, lm7812 and 1uf poly... was expecting to hear alot of PSU... but it is quiet...
Next I will construct the second channel, but this time with LM317 + 2r2 current source, in place of the power resistor... should make AB comparison easier... still have to swap the earpiece from ear to ear, due to diffirent frequency sensitivity of my ears... one seems to exlude low frequencies, and the other excludes very high frequencies (frequent grommet receiver of the year here)...
All of the technical things asside, this baby once again convince me that I would be a very easy Mosfet convert...
I love the solid powerfull bass, and slightly rolled of high frequency sensation, and lack of audiable artifacts, would make a very forgiveing headphone amp for bad sources as is... No sibbilance, which tends to be my main gripe with 70% of the headphone designs I try... something in the way high frequencies and rustles dont fade away properly...irritates me quickly...
If anything the sound reminds me of an e188cc cathode follower buffer...
The plan is to liberate a set of Logitech x-530 PC speakers of their crappy amp... they are quite efficient @ 94db/W, and not awefull sounding at all...this I want to use to drive front right and left channels at my PC.
Thermal Runaway
I have worked quite a bit with IRF's in amplifier applications, and they will work fine. No thermal runaway problems to deal with, however the bias will fluctuate a bit, if the heat sink allows the temperature to swing.
I suggest adding a simple VBE multiplier of 3V, in series with a 3.9V zener. It will work dead stable.
I have always been a fan of simple designs. Not least because they are easy to make, and fun to start up.
However when trying to design an ultrasimple amplifier, you always run into big compromises, that favours throwing in a few more parts, to overcome the flaws of the components.
In case of the IRF's as output devices, they have a very high impedance at low currents. At higher currents the impedance drops off, so this gives problems with distortion. Around 0 output current the MOSFET only draws idle current, so the impedance is high, but as the signal goes up or down, the current rises.
The result is x-over distortion of 2-3%. Unless you go Class A, in which case you can easily get a good result.
Next problem is the high rail loss, that will make this design not only unltra simple, but also ultra inefficient. You can probably get twice the power out with the same heat sink, power supply and MOSFET's by optimizing rail loss.
Next again the poor bandwidth is a direct consequence of the over-simplified design.
I have worked quite a bit with IRF's in amplifier applications, and they will work fine. No thermal runaway problems to deal with, however the bias will fluctuate a bit, if the heat sink allows the temperature to swing.
I suggest adding a simple VBE multiplier of 3V, in series with a 3.9V zener. It will work dead stable.
I have always been a fan of simple designs. Not least because they are easy to make, and fun to start up.
However when trying to design an ultrasimple amplifier, you always run into big compromises, that favours throwing in a few more parts, to overcome the flaws of the components.
In case of the IRF's as output devices, they have a very high impedance at low currents. At higher currents the impedance drops off, so this gives problems with distortion. Around 0 output current the MOSFET only draws idle current, so the impedance is high, but as the signal goes up or down, the current rises.
The result is x-over distortion of 2-3%. Unless you go Class A, in which case you can easily get a good result.
Next problem is the high rail loss, that will make this design not only unltra simple, but also ultra inefficient. You can probably get twice the power out with the same heat sink, power supply and MOSFET's by optimizing rail loss.
Next again the poor bandwidth is a direct consequence of the over-simplified design.
OK, so I dropped in the 0r82 resistors as per the schematic....
... and it is not behaveing as expected..
I can get it to work quietly unless I turn the trimpot till the 8r2 resistors drop less than 2V over themselves...
It worked alot better with the 2r2 and 1r1 to ground that I tried over the weekend...
I just dont unerstand how the designer claimed to get a 5V drop over the power resistors...
Could the load affect this point?
Changeing the pot on the input from 47k to 10k was an improvement
... and it is not behaveing as expected..
I can get it to work quietly unless I turn the trimpot till the 8r2 resistors drop less than 2V over themselves...
It worked alot better with the 2r2 and 1r1 to ground that I tried over the weekend...
I just dont unerstand how the designer claimed to get a 5V drop over the power resistors...
Could the load affect this point?
Changeing the pot on the input from 47k to 10k was an improvement
Re: Thermal Runaway
And did your IRF devices have source resistors? (note the MOSFETs in the simple schematic do not...). My last two amps have IRFs and no source resistors and they DO indeed go into thermal runaway without a bias servo, even though the heatsinks are huge. of course, the runaway may not be destructive, but the bias fluctuation can get way too high and the adjustment completely unreliable - not to mention the bias current being very impractical. So, when you say the bias fluctuates a bit, how much is 'a bit'?
Hold on, I think something got lost in the translation here. What do you mean, impedance? Output impedance? If so, it is completely normal and not at all limited to IRFs. It has to do with gm rising with output current, which is one of the reasons higher bias currents are preferred with MOSFETs. With IRF devices this tends to be more obvious because the gm is so high. Higehr bias and some source resistance can aleviate the problem, but efficiency is down. lateral MOSFETs are, of course, not at all immune to the same effect.
Yes, but see my simple proposal for a modification (bootstrap 'current source' for VAS). The main reason for the rail loss is the low gm of the laterals, they need about 8V or so Vgs at mximum current, which means this is the rail loss one can expect for more difficult loads. A separate negative supply for the driver stage (positive is not needed with a slight modification of the VAS bootstrap arrangement) would take care of this but at the cost of higher distortion setting in at high power due to nonlinear Cgd effects with Vgd becoming negative. Assuming the amp is modified for IRFs with the inclusion of a Vbe or Vgs multiplier, the rail loss would be somewhat lower due to higher gm of the IRFs.
Yes, dependant on the stage currents. There is room to play around with, however. Doubling the currents would not be out of the question (low impedance sources would be preferred!), but even as it is it's usable. not optimal, but as you say, it is a very simple design.
Lars Clausen said:
And did your IRF devices have source resistors? (note the MOSFETs in the simple schematic do not...). My last two amps have IRFs and no source resistors and they DO indeed go into thermal runaway without a bias servo, even though the heatsinks are huge. of course, the runaway may not be destructive, but the bias fluctuation can get way too high and the adjustment completely unreliable - not to mention the bias current being very impractical. So, when you say the bias fluctuates a bit, how much is 'a bit'?
Hold on, I think something got lost in the translation here. What do you mean, impedance? Output impedance? If so, it is completely normal and not at all limited to IRFs. It has to do with gm rising with output current, which is one of the reasons higher bias currents are preferred with MOSFETs. With IRF devices this tends to be more obvious because the gm is so high. Higehr bias and some source resistance can aleviate the problem, but efficiency is down. lateral MOSFETs are, of course, not at all immune to the same effect.
Yes, but see my simple proposal for a modification (bootstrap 'current source' for VAS). The main reason for the rail loss is the low gm of the laterals, they need about 8V or so Vgs at mximum current, which means this is the rail loss one can expect for more difficult loads. A separate negative supply for the driver stage (positive is not needed with a slight modification of the VAS bootstrap arrangement) would take care of this but at the cost of higher distortion setting in at high power due to nonlinear Cgd effects with Vgd becoming negative. Assuming the amp is modified for IRFs with the inclusion of a Vbe or Vgs multiplier, the rail loss would be somewhat lower due to higher gm of the IRFs.
Yes, dependant on the stage currents. There is room to play around with, however. Doubling the currents would not be out of the question (low impedance sources would be preferred!), but even as it is it's usable. not optimal, but as you say, it is a very simple design.
Nordic said:
It would help if you could tell us the impedance ofyour headphones. What do you mean when you say 'it is not quiet'?
It is a bit odd that the difference between 0.82 ohms and 1.1 ohms would be so drastic. The laod itself should not affect the DC operating point, but the DC operating point to set does have an optimum, which depends on the load - in order to get maximum swing.
The improvement with the 10k pot is not surprising, the input capacitance is quite high.
BTW an LM317 current source is not suitable for that design because the voltage drop on it would be too low for the LM317 to work properly. There are other ways to build current sources, though, and some would work just fine in your schematic, but the complexity would be a lot greater.
Nordic said:
Are you sure your power supply can give you the required current (with 0.82R it can be a bit higher than with the other combinations). If it can, what you describe sounds more like oscillation. Is your 0.82 ohm resistor a wirewound type (inductive)?
For 32 ohms load the DC voltage on the drain of the MOSFET should be about 5.4V for maximum swing. This means the amp runs VERY rich class A, given the 32 ohm load (not a bad thing if you can cope with the heat). The idle current will be about 805mA, and the peak current will be about 1.33A so your power supply should be able to deliver at least that. If it were me, i would put a capacitor from pot wiper to ground, say 10u or so. In theory it is not needed, but if your power supply is marginal, you may get oscilations from it without that cap.
Ilimzm: I guess there are also other factors. I have only worked with original IR versions (from International Rectifier). Also the thermal connection to the heat sink is of importance.
I wouldn't say thermal runaway is impossible, with the proposed circuit, just that i have never encountered it. On the contrary it seemed surprisingly calm.
I wouldn't say thermal runaway is impossible, with the proposed circuit, just that i have never encountered it. On the contrary it seemed surprisingly calm.
Thanks, I am useing a 2x 12VAC 50VA transformer, with secondaries paralleled...
But it did cross my mind... think it is too small?
I will play around with a single channel (half the job for PSU).
Only have 2200uf on PSU as well... but increasing that would at best take care of the issues at peaks...
The 10uf on the pot... is that for the bias trimpot?
On the input I have 10k resistors to ground for testing purposes...
............................................
Received transistors for my Destroyer DX amp this evening, and just finished installing it... amazeingly, when the fake set self destructed, it killed nothing but the fuses.... clever design I would say...
------------------------------------
I will probably only get to play with the headphone amp again tommorrow evening, as I have to go to hospital tommorrow to swallow a camera
- unless I can't sleep tonight...
But it did cross my mind... think it is too small?
I will play around with a single channel (half the job for PSU).
Only have 2200uf on PSU as well... but increasing that would at best take care of the issues at peaks...
The 10uf on the pot... is that for the bias trimpot?
On the input I have 10k resistors to ground for testing purposes...
............................................
Received transistors for my Destroyer DX amp this evening, and just finished installing it... amazeingly, when the fake set self destructed, it killed nothing but the fuses.... clever design I would say...
------------------------------------
I will probably only get to play with the headphone amp again tommorrow evening, as I have to go to hospital tommorrow to swallow a camera
- unless I can't sleep tonight...Nordic said:
Your schematic says it is a regulated 12V supply. What do you use for regulation? It is possible that by increasing bias current you simply load the supply enough to casue a dropout of the regulator. It seems likely given your 12VAC trafo and 2200uF filters. Do you have a scope at hand? If not, keep in mind that measuring DC voltages with a DVM will show you the average or even maximum level on a ripply DC voltage, when you are actually interested in the 'valleys' in the ripples on top of the DC.
Yes. If you aheva ripple problem with the PSU, this will make a difference as it will locally filter the bias voltage for the MOSFET. If you get a lower volume buzzing compared to before, ripple in your power supply is the problem.
Yes, my supply is embarrasing.... I physicaly cut it out of the board of another doomed project, and just wired it up...
It was doing ok with the 2k2 and 1k1 setup, but the heatsink on it went pretty hot (about 50 to 60C)... I have designed "copied" a bit more robust version useing 2 tip42s and an LM7812.
I did notice the rail was sitting on 11.76V, when I tried to debug the amp...didn't think it would be too bad... so I'm wrong?
I'm sure the cap there would be good... my instincts told me one belonged there and I also think the resistor to ground shoud have a cap over it maybe.... Like on a valve's cathode resistor...
Please excuse I I sound haphazzard... aneasthetics still wearing off... I had to sneak away from hospital... as I had to drive myself
It was doing ok with the 2k2 and 1k1 setup, but the heatsink on it went pretty hot (about 50 to 60C)... I have designed "copied" a bit more robust version useing 2 tip42s and an LM7812.
I did notice the rail was sitting on 11.76V, when I tried to debug the amp...didn't think it would be too bad... so I'm wrong?
I'm sure the cap there would be good... my instincts told me one belonged there and I also think the resistor to ground shoud have a cap over it maybe.... Like on a valve's cathode resistor...
Please excuse I I sound haphazzard... aneasthetics still wearing off... I had to sneak away from hospital... as I had to drive myself
The absoulte value of the rail is not that critical, it is just important that ih has no ripple feeding through. If you measure it with a DVM set to DC you will not see this until it really becomes unbearable.
The problem is that rectifying 12VAC gives you about 17VDC with no load, with a load the DC becomes 'ripply' on top, like an inverted sawtooth. So, while the top of the sawtooth may still be at 17V, the bottom might be several V less. A 7812 needs about 3V or so between it's input and output to work right. This means that the minimum (bottom of the sawtooth) must always be at least 15V, or whenever the wawtooth falls below 15V, the output of the 7812 will also fall by that amount - in essence you get part of the sawtooth 'feeding through' because of regulator dropout. In other words, regulator dropout means the voltage across the regulator became smaller than is needed for the regulator to regulate.
Based on what you have been saying about the regulator, I am guessing this is your problem.
In theory, your trafo is big enough to provide the necessary current you need for both channels of the amp. However, the ripple you start with depends on the size of the filter caps - bigger caps, less ripple (too big is not good either because other problems start).
So, if your regulator is dropping out you have several strategies:
- Obvious, use lower load. Of course, this may not be an option.
- Increase filter caps
- Use lower dropout regulator
- Lower the supply voltage if the circuit supplied is tolerant to this, so you create more headroom for the regulator
- use low drop (schottky) diodes for rectification, so you get the highest possible rectified voltage before regulation
The problem is that rectifying 12VAC gives you about 17VDC with no load, with a load the DC becomes 'ripply' on top, like an inverted sawtooth. So, while the top of the sawtooth may still be at 17V, the bottom might be several V less. A 7812 needs about 3V or so between it's input and output to work right. This means that the minimum (bottom of the sawtooth) must always be at least 15V, or whenever the wawtooth falls below 15V, the output of the 7812 will also fall by that amount - in essence you get part of the sawtooth 'feeding through' because of regulator dropout. In other words, regulator dropout means the voltage across the regulator became smaller than is needed for the regulator to regulate.
Based on what you have been saying about the regulator, I am guessing this is your problem.
In theory, your trafo is big enough to provide the necessary current you need for both channels of the amp. However, the ripple you start with depends on the size of the filter caps - bigger caps, less ripple (too big is not good either because other problems start).
So, if your regulator is dropping out you have several strategies:
- Obvious, use lower load. Of course, this may not be an option.
- Increase filter caps
- Use lower dropout regulator
- Lower the supply voltage if the circuit supplied is tolerant to this, so you create more headroom for the regulator
- use low drop (schottky) diodes for rectification, so you get the highest possible rectified voltage before regulation
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.