Nige, global feedback does nothing for the behavior of individual stages, which could compromise transients even up to the level of indivual stage overload, and in the extreme, clipping.
I agree with Wave.
I still maintain that no one single approach could ever be ideal, it takes a balance of local and global to produce truly ideal sonic results. This also means tons of work, especially fine tuning after it's electrically operational.
I agree with Wave.
I still maintain that no one single approach could ever be ideal, it takes a balance of local and global to produce truly ideal sonic results. This also means tons of work, especially fine tuning after it's electrically operational.
Hi DVV , Hi wave .
I love that EL84 amp . I must build that as I have all the bits . It is like something I had in mind .
The idea of unity gain power amplifier is to see if it will work just as a buffer . It will struggle . My feeling is we should be able to ask it to do that . In inverting mode perhaps ?
The point about local feedback is it is a useful thing and sometimes is the key to the difference between competent and excellent amplifiers . My brother modified many commercial amplifiers using it . His amplifiers would be more musical some said .
When I use local feedback I draw that resistor or circuit in purple to say it was designed by ear . This means I found a critical valve . My 8 mA VAS often have 15R purple VAS resistor . This is not strictly speaking feedback . It is to lift the VAS up 0.12 V which helps the current mirror ( 0.65V - 0.2V = 0.45 if you are lucky then - 0.1 V balancing resistors , + 0.12 V is useful ) . I can not say for certain the sound improvement is due to mirror balance , easier to drive VAS input impedance or local feedback , or all 3 ? All I know is it works . Below 47 R all measurements remain about the same . The difference is the sound becomes softer as the resistance climbs . If listening to rock music all night to short out the 15 R would be preferable . Note I can not measure any real difference . The VAS is correct even at 0R I should add . I usually try to get 1:3 impedance ratio if I can , it might be 1:1 at 0R . The most complicated VAS would be something like a Darlington cascode . With a little care a 1 x 2N5551 will do the job . The interesting thing is the VAS is described as a trans-impedance amplifier . That is it is neither a voltage amp nor current amp . It is a form of I to V converter . The preferred option is to make it into a voltage amp especailly if using a Darlington . My instinct is to say what a very bad idea . My experience says do it . So I design to make it > 1:1 . Many amps were < 1:1 . I suspect when TID was talked about that ended ? My ears did not like it although it measured fine .
I love that EL84 amp . I must build that as I have all the bits . It is like something I had in mind .
The idea of unity gain power amplifier is to see if it will work just as a buffer . It will struggle . My feeling is we should be able to ask it to do that . In inverting mode perhaps ?
The point about local feedback is it is a useful thing and sometimes is the key to the difference between competent and excellent amplifiers . My brother modified many commercial amplifiers using it . His amplifiers would be more musical some said .
When I use local feedback I draw that resistor or circuit in purple to say it was designed by ear . This means I found a critical valve . My 8 mA VAS often have 15R purple VAS resistor . This is not strictly speaking feedback . It is to lift the VAS up 0.12 V which helps the current mirror ( 0.65V - 0.2V = 0.45 if you are lucky then - 0.1 V balancing resistors , + 0.12 V is useful ) . I can not say for certain the sound improvement is due to mirror balance , easier to drive VAS input impedance or local feedback , or all 3 ? All I know is it works . Below 47 R all measurements remain about the same . The difference is the sound becomes softer as the resistance climbs . If listening to rock music all night to short out the 15 R would be preferable . Note I can not measure any real difference . The VAS is correct even at 0R I should add . I usually try to get 1:3 impedance ratio if I can , it might be 1:1 at 0R . The most complicated VAS would be something like a Darlington cascode . With a little care a 1 x 2N5551 will do the job . The interesting thing is the VAS is described as a trans-impedance amplifier . That is it is neither a voltage amp nor current amp . It is a form of I to V converter . The preferred option is to make it into a voltage amp especailly if using a Darlington . My instinct is to say what a very bad idea . My experience says do it . So I design to make it > 1:1 . Many amps were < 1:1 . I suspect when TID was talked about that ended ? My ears did not like it although it measured fine .
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@Nigel
Well, if it's simple cuorsity, that's understandable, why not try it anyway?
But honestly, I see no further point to a unity gain POWER amplifier, unless it is in fact a simple current boost stage to whatever is amplifying the signal. On the other hand, if it's an op amp doing the signal amplification, there are far more practical ways of current boosting them.
Well, if it's simple cuorsity, that's understandable, why not try it anyway?
But honestly, I see no further point to a unity gain POWER amplifier, unless it is in fact a simple current boost stage to whatever is amplifying the signal. On the other hand, if it's an op amp doing the signal amplification, there are far more practical ways of current boosting them.
I suppose it is like having a super low gear on a car . When will it be useful ? As an engineer it is always good to know . The number of times I have been asked a question and have been able to say " it's funny you should ask that ".
As I said before my clock stopped at age 10 . When I write here I usually have some child's project on the go . This week it is playing with 555 timers . Still didn't make a crude class D that worked . I have a blown up inverter I would like to replace . I hoped to make something powerful of about 0.5% distortion .
As I said before my clock stopped at age 10 . When I write here I usually have some child's project on the go . This week it is playing with 555 timers . Still didn't make a crude class D that worked . I have a blown up inverter I would like to replace . I hoped to make something powerful of about 0.5% distortion .
I believe it has a typo in the article, it should use EL34 tubes, not EL84. For EL84 470 Ohm in cathodes is too much.
EL 34 is fine . Mine came out of Chipping Norton Studios , they are antiquated with good music . The topology was my interest . Things I have tried but not in this mix .
Thanks anyway . Like seeing a Delta-wing it's the shape I need . The bottle I would have worked out . The triode like anode to grid shunt feedback unimpeded by the impedance of the driver stage I like . I think that's right ? Nice to see transistors lending a hand .
Thanks anyway . Like seeing a Delta-wing it's the shape I need . The bottle I would have worked out . The triode like anode to grid shunt feedback unimpeded by the impedance of the driver stage I like . I think that's right ? Nice to see transistors lending a hand .
That's very nice and a little bit like what I have been building . Mine is SE . I gave up with the Alex Kitic type feedback on mine . I kept my notes so can always go back to it . The best I saw was a DC coupled 1000 V Fet driving 5881 in a similar circuit . Half grid bias and half cathode bias with a 180V zener to G2 . The distrotion spectrum a near perfect exponential . I think I might have it somewhere . Forgive any errors , this is similar . If people need more info I will find the original and it's author ( Michael Kloster is it ? ) . Forgive as it is to show an idea only .
GU50 Does Not Fit Nicely
Wavebourn
Your suggestion of "the GU50 fitting nicely" needs some qualification. Direct substitution is not an option as from the GU50 curves at Vpk = 15V, Vpk = 380V and Vg2 =250V, the GU50 draws 200mA hence is dissipating 76W, way more than its rated 40W maximum. You might want to elaborate on your definition of "fits nicely" with a more appropriate operating point.
Wavebourn
Your suggestion of "the GU50 fitting nicely" needs some qualification. Direct substitution is not an option as from the GU50 curves at Vpk = 15V, Vpk = 380V and Vg2 =250V, the GU50 draws 200mA hence is dissipating 76W, way more than its rated 40W maximum. You might want to elaborate on your definition of "fits nicely" with a more appropriate operating point.
01N100 has wide spread of parameters, as well as tubes, so anyway you need to select proper value for the resistor in drain; no way 820 Ohm will fit any random mosfet and tube combination. Select it for 100V on cathode, you will get 100 mA idle current as the result. That means 35W dissipation. For an average Gu-50 in such mode you will have about -30 V on G1 in respect to cathode. But if you set idle current 70 mA as drawn on schematic you will have about -35V on g1 in respect to cathode and even lower anode power dissipation.
Yeah, only toobes, with their sticky, syrupy, THDing and IMing, full of transformer garble sound.
Fortunately, in about an hour, I'm off to Budapest and will be back on Friday evening, when the wrath of the righteous has calmed down, as per the old Latin saying: Ira furor brevis est (Their wrath is short lived).
Don't bother gathering a posse, Wave, even if you already have a rope ready.
I shall report on the quality of the goulash and the Szerbo cake.
GU 50/5881 ?
I found the original . I did have a go to build it . Start with a cathode current sink I would say . I think some call these Frankenstein amps . That makes me want to build them . I did find using this type of shunt feedback to the power valve gives exact triode distortion spectrum with a nice gain bonus . About 30% typical in RH88 version . It is a cheap lunch . I must say when I use valves the Frankenstein comes over me . I have to see the anodes glow even though I shouldn't . The only transistor I knew to tolerate that was a BD131 . Smoke came out yet it continued to work when the fault corrected !
I found the original . I did have a go to build it . Start with a cathode current sink I would say . I think some call these Frankenstein amps . That makes me want to build them . I did find using this type of shunt feedback to the power valve gives exact triode distortion spectrum with a nice gain bonus . About 30% typical in RH88 version . It is a cheap lunch . I must say when I use valves the Frankenstein comes over me . I have to see the anodes glow even though I shouldn't . The only transistor I knew to tolerate that was a BD131 . Smoke came out yet it continued to work when the fault corrected !
DVV , you neatly bring me full circle . This is an amplifier that can have no existence without global feedback . The output transformer-less type . Philips I seem to remember used it in pentode form for TV's . If a special speaker is made it can have special advantages ( 32 ohm ) . Having heard such an amp I was very surprised , lightning fast . It might have been the distortion spectrum fooling my ears .
No way. Use resistor in cathode, because MOSFET can't keep stable voltage on the control grid.
Use 1k trimpot for R2 in source of MOSFET. Rotate it on minimum. Power on. Slowly rotate the trmpot to set desired idle current measuring voltage across cathode resistor. Listen and enjoy.
I was thinking just to get some indications . Myself , I used a high value resistor and reduced it . Something went wrong and it got a bit wild for a minute. I think I calculated >100 W dissipated . The valve was fine . I wouldn't advocate a current sink for permanent use as it is at best too complicated . Many times I wish I had used one in the initial stages of a design . I have a current setting on my bench power supply , it has save the day so many times . I must say DC coupling an amp like this seems a bridge too far .
Hi Wave I am new to valves and learning , it is about having knowledge rather than loving them . I prefer diesel to steam locomotives so there is no hope I guess ( forget electrics that's class D ) . It has to be said steam comes very close and valves too . The Brits call old radios " steam radios " !?
I found by happy accident the right way to do it and the one you recommend . I had a nasty moment when my head went into reverse gear applied the opposite bias that I intended . As you say it survived . The heat haze coming off of the cathode resistor let me know first .
I found by happy accident the right way to do it and the one you recommend . I had a nasty moment when my head went into reverse gear applied the opposite bias that I intended . As you say it survived . The heat haze coming off of the cathode resistor let me know first .
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