Re: Whose building already
Lack of funds meant I had to sit this one out, but I'm cheering you all on from the sidelines.
Oh, and if we ever get to share a bottle of Jack Daniels, I'll explain where my nickname came from. 🙂
Jozua said:
Lack of funds meant I had to sit this one out, but I'm cheering you all on from the sidelines.
Oh, and if we ever get to share a bottle of Jack Daniels, I'll explain where my nickname came from. 🙂
Slow Progress
Well, I got my resistors stuffed into the first board last weekend (rain and cold weather in Denver). When I moved on to transistors, I got stopped up on Q5 & Q6. Took a while to determine the proper orientation using the T092 devices (the type that are not round with one flat back - but sort of trapezoidal). Ended up tracking the traces to the SOT223 pads on the back to validate source, gate and drain orientation. Of course, I did that after I had placed them incorrectly. I think my slow progress has also been associated with the fact that I don't have a solution yet for heat sinks or transformers. I keep hoping that somebody will post up something that is available in the US (so I don't incur outrageous shipping charges) and will meet power and passive cooling requirements. I'm sure I'll get more diligent about my search once I have the boards completed and need power and cooling in order to test them. We're anticipating 80 degrees F this weekend and the rivers are rising, so it'll be tough to find time for indoor activities when the kayak is calling...
Well, I got my resistors stuffed into the first board last weekend (rain and cold weather in Denver). When I moved on to transistors, I got stopped up on Q5 & Q6. Took a while to determine the proper orientation using the T092 devices (the type that are not round with one flat back - but sort of trapezoidal). Ended up tracking the traces to the SOT223 pads on the back to validate source, gate and drain orientation. Of course, I did that after I had placed them incorrectly. I think my slow progress has also been associated with the fact that I don't have a solution yet for heat sinks or transformers. I keep hoping that somebody will post up something that is available in the US (so I don't incur outrageous shipping charges) and will meet power and passive cooling requirements. I'm sure I'll get more diligent about my search once I have the boards completed and need power and cooling in order to test them. We're anticipating 80 degrees F this weekend and the rivers are rising, so it'll be tough to find time for indoor activities when the kayak is calling...
Yip adding support for all those different transistors and mosfets sure can be confusing to get the orientations correct! However since the boards can work fine (and measure extremely well) without the output stage connected, it's still a good idea to finish and test the boards while waiting for a PSU & heatsink solution.
this is fascinating to read, ppl all over the world are contributing to this project.
good luck to it and look forward to a great happy ending
good luck to it and look forward to a great happy ending
I want to do a scaled down version of the KSA100-mkII, but can't decide what would be the best configuration. I want to keep the original rails voltage to keep the maximum output power, and half the total dissipation.
What would be the best:
a) keep everything the same, only reduce bias current
b) half the numer of output devices and the double pair of drivers (and keep bias per output device the same)
c) half the number of output devices and also use only one pair of drivers (again keep bias per output device the same)
Or is it perhaps something in between? Is it possible to parallel the drivers together and use 6 output devices?
Any ideas?
What would be the best:
a) keep everything the same, only reduce bias current
b) half the numer of output devices and the double pair of drivers (and keep bias per output device the same)
c) half the number of output devices and also use only one pair of drivers (again keep bias per output device the same)
Or is it perhaps something in between? Is it possible to parallel the drivers together and use 6 output devices?
Any ideas?
Hi DrS,
any of your options will work.
I would be tempted to keep the 4pair output stage and keep the two pair of drivers.
Reducing the bias will make this a high bias ClassAB amp, so what. How often will this domestic amp run at near full power?
100W into 8ohms is a superlative spec for a ClassA amp.
But step back a moment.
Optimum ClassAB requires upto 25mV across each emitter resistor. If you decide to run ClassAB and reduce the emitter resistors to 0r1 then each pair of output devices will run at 250mA.
Four pair will need 1000mA, just to run at the top end of optimum ClassAB. What was the bias you wanted for cooler running? Pretty damn close I'd say.
That Iq=1A will give 2Apk into your 8ohm load. That is about 16W, which is only 8db below maximum output. I'll quess that the ClassAB version running 90db speakers will only go into ClassB for about 0.1% of the time you are listening. And further, I suspect you'll never notice it.
How does that "sound"?
You got me wondering, I was planning a 6pair output stage.
If I reduced the Re from 0r33 to 0r1 I get upto Iq=1.5A just for optimum ClassAB. I've almost convinced myself that reducing Re from 1ohm to 0r33 isn't going far enough, maybe I should go even lower.
any of your options will work.
I would be tempted to keep the 4pair output stage and keep the two pair of drivers.
Reducing the bias will make this a high bias ClassAB amp, so what. How often will this domestic amp run at near full power?
100W into 8ohms is a superlative spec for a ClassA amp.
But step back a moment.
Optimum ClassAB requires upto 25mV across each emitter resistor. If you decide to run ClassAB and reduce the emitter resistors to 0r1 then each pair of output devices will run at 250mA.
Four pair will need 1000mA, just to run at the top end of optimum ClassAB. What was the bias you wanted for cooler running? Pretty damn close I'd say.
That Iq=1A will give 2Apk into your 8ohm load. That is about 16W, which is only 8db below maximum output. I'll quess that the ClassAB version running 90db speakers will only go into ClassB for about 0.1% of the time you are listening. And further, I suspect you'll never notice it.
How does that "sound"?
You got me wondering, I was planning a 6pair output stage.
If I reduced the Re from 0r33 to 0r1 I get upto Iq=1.5A just for optimum ClassAB. I've almost convinced myself that reducing Re from 1ohm to 0r33 isn't going far enough, maybe I should go even lower.
Hi Andrew,
Thanks for your reply. And clarifying things.
I am aiming for half the dissipation of a normally biased Krell. So I'm glad to know all options would work.
I have not calculated the values myself, simply because I'm not sure I'm doing the right calculations.
Why is 25mV across each emitter resistor needed for optimum classAB.
What setup were you planning to use with 6 pairs of output devices?
Thanks for your reply. And clarifying things.
I am aiming for half the dissipation of a normally biased Krell. So I'm glad to know all options would work.
I have not calculated the values myself, simply because I'm not sure I'm doing the right calculations.
Why is 25mV across each emitter resistor needed for optimum classAB.
What setup were you planning to use with 6 pairs of output devices?
Hi DrS,
the first time I came across bias quoted as a voltage was reading D.Self.
Since then it has become apparent that many are using Vre to define output bias and have abandoned measuring/specifying current as the output bias specification.
There is a thread running that looked at this very topic. They agreed that ClassAB EF output stage should use Vre between 15mV and 25mV (on EACH Re irrespective of numbers of output devices used). The higher figure is VERY CLOSE to D.Self's recommendation. Some other designer/builders often quote current bias that results in Vre falling inside this range.
A CFP output stage requires Vre about 10% of this value, but it varies a bit more with changing values of Re.
Does anyone want to comment on reducing Re from 1r0 to something lower?
How much lower?
How will it affect thermal stability?
Will it affect sound quality?
Will it affect surviveability on shorted output line?
I plan to use 2pair 2sb649/d669 driving 6pair MJL4281/4302.
Pd of 4pr15003/4 =6prMJL4281/4302 when Tc~=95degC.
the first time I came across bias quoted as a voltage was reading D.Self.
Since then it has become apparent that many are using Vre to define output bias and have abandoned measuring/specifying current as the output bias specification.
There is a thread running that looked at this very topic. They agreed that ClassAB EF output stage should use Vre between 15mV and 25mV (on EACH Re irrespective of numbers of output devices used). The higher figure is VERY CLOSE to D.Self's recommendation. Some other designer/builders often quote current bias that results in Vre falling inside this range.
A CFP output stage requires Vre about 10% of this value, but it varies a bit more with changing values of Re.
Does anyone want to comment on reducing Re from 1r0 to something lower?
How much lower?
How will it affect thermal stability?
Will it affect sound quality?
Will it affect surviveability on shorted output line?
I plan to use 2pair 2sb649/d669 driving 6pair MJL4281/4302.
Pd of 4pr15003/4 =6prMJL4281/4302 when Tc~=95degC.
dr.strangelove3 said:
The only way to achieve these targets + reduce the number of output devices is:
- active dissipation limiter/protection
- through sliding bias and output stage rail regulation.
For the second one you'll need something like the regulator Mr Ryan pictured to improve an Adcom 555
(ps: the Ryan regulator is a direct copy of the one used in a really exotic Japanese power amp of the 80s)
Hi Jacco,
Sliding bias. Isn't that for keeping the maximum power at class A?
I'm fine with reduced power output in class-A.
Sliding bias. Isn't that for keeping the maximum power at class A?
I'm fine with reduced power output in class-A.
small Re
my experience with the ksa50 was that a higher value of Re made the tolerances in everything less important...
To make a WAG: the amp will become more sensitive to thermal tracking and transistor differences in inverse proportion to the value of these resistors...ie using 0r3 will double imbalances between transistors.
I'd be careful going down to 0r1, at this point a 10mv difference in the Vbe of the outputs will show up as 100ma difference in the current split (with 0r68, the difference would be 14ma).
Since most people are talking about matching outputs perhaps it doesn't matter...though if matching is necessary you'd better match drivers, their Re, outputs, their Re...life's too short, and/or I'm too lazy...
If you make the outputs run in AB and they are essentially cold under normal operating conditions you can probably use half as many anyway. Someone should do the math, but (AFAICT) the only reason to use 4 TO3 pairs in the KSA100 is 'cos they are hot all the time...
A single pair of mj15003/4 are more than adequate for a 50w class AB amp...
HTH
Stuart
my experience with the ksa50 was that a higher value of Re made the tolerances in everything less important...
To make a WAG: the amp will become more sensitive to thermal tracking and transistor differences in inverse proportion to the value of these resistors...ie using 0r3 will double imbalances between transistors.
I'd be careful going down to 0r1, at this point a 10mv difference in the Vbe of the outputs will show up as 100ma difference in the current split (with 0r68, the difference would be 14ma).
Since most people are talking about matching outputs perhaps it doesn't matter...though if matching is necessary you'd better match drivers, their Re, outputs, their Re...life's too short, and/or I'm too lazy...
If you make the outputs run in AB and they are essentially cold under normal operating conditions you can probably use half as many anyway. Someone should do the math, but (AFAICT) the only reason to use 4 TO3 pairs in the KSA100 is 'cos they are hot all the time...
A single pair of mj15003/4 are more than adequate for a 50w class AB amp...
HTH
Stuart
Hi Stuart,
how did you measure those imbalanced currents for mismatched Vbe?
Since my Klone is still in assembly stage I cannot measure.
I reckon as follows, (this is based on reading the Ic vs Vbe graph).
standard Iq=400mA and Vbe650mV and substituting a transistor with 640mV @ the same 400mA Ic.
Re= 1r0, gives 409mA. Vbias=1050mV, Vbe=641mV
Re=0r33, gives 422mA. Vbias=782mV, Vbe=643mV
Re=0r1, gives 450mA. Vbias=690mV, Vbe=645mV
note that Vbias across the transistor + emitter resistor is fixed.
The Vre varies as bias current changes. This is the self correcting effect.
I had reckoned that going from 400 to 409 for the 1r0 case and becoming 400 to 422 for the 0r33 case gave a reasonable match between case temperatures. Your numbers give the impression that the case temperature mismatch is much worse.
Do you or anyone care to explain.
how did you measure those imbalanced currents for mismatched Vbe?
Since my Klone is still in assembly stage I cannot measure.
I reckon as follows, (this is based on reading the Ic vs Vbe graph).
standard Iq=400mA and Vbe650mV and substituting a transistor with 640mV @ the same 400mA Ic.
Re= 1r0, gives 409mA. Vbias=1050mV, Vbe=641mV
Re=0r33, gives 422mA. Vbias=782mV, Vbe=643mV
Re=0r1, gives 450mA. Vbias=690mV, Vbe=645mV
note that Vbias across the transistor + emitter resistor is fixed.
The Vre varies as bias current changes. This is the self correcting effect.
I had reckoned that going from 400 to 409 for the 1r0 case and becoming 400 to 422 for the 0r33 case gave a reasonable match between case temperatures. Your numbers give the impression that the case temperature mismatch is much worse.
Do you or anyone care to explain.
measurements
Hi,
My reasoning is as follows: If the Vbe difference between a pair of output transistors is 5mv, the current through Vre is where the imbalance has to be equalized, so you would see a difference of 5mv across a pair of 0r1 resistors, hence 50ma difference between them.
All the figures you show are perfectly valid, but two (Vre, Vbe) of the three variables are modified with each line, I think if Vbe had been held constant, you'd have differences that match what I'm calculating. You have adjusted the Vbe of the mismatched transistor for the current flowing through it, but AFAICT you have not done this for the control. Hence by the last measurement you only 'see' a 5mv difference in Vbe and a corresponding 50ma difference.
As for the measurements, I used a wheatstone bridge to find a set of 10 of the 0r68 resistors that were identical, then I measure them in series and divide the result by 10 to get an accurate value of Re, then I simply measured the voltage across it in circuit. Maybe +/-1% accurate.
Clearly the 50ma (or 100ma) difference is only important if one is trying to reduce the overall output stage current to minimal levels.
I measured temperature using a 'gun' (no-contact) thermometer, AFAIK not super accurate, but it seemed consistent. Short term I got differences of ~10C from the hottest to the coolest device. IIRC Vbe changes at 2mv/C...20mv is a 200ma difference if Re is 0r1 or 29ma if Re is 0r68...
Stuart
Hi,
My reasoning is as follows: If the Vbe difference between a pair of output transistors is 5mv, the current through Vre is where the imbalance has to be equalized, so you would see a difference of 5mv across a pair of 0r1 resistors, hence 50ma difference between them.
All the figures you show are perfectly valid, but two (Vre, Vbe) of the three variables are modified with each line, I think if Vbe had been held constant, you'd have differences that match what I'm calculating. You have adjusted the Vbe of the mismatched transistor for the current flowing through it, but AFAICT you have not done this for the control. Hence by the last measurement you only 'see' a 5mv difference in Vbe and a corresponding 50ma difference.
As for the measurements, I used a wheatstone bridge to find a set of 10 of the 0r68 resistors that were identical, then I measure them in series and divide the result by 10 to get an accurate value of Re, then I simply measured the voltage across it in circuit. Maybe +/-1% accurate.
Clearly the 50ma (or 100ma) difference is only important if one is trying to reduce the overall output stage current to minimal levels.
I measured temperature using a 'gun' (no-contact) thermometer, AFAIK not super accurate, but it seemed consistent. Short term I got differences of ~10C from the hottest to the coolest device. IIRC Vbe changes at 2mv/C...20mv is a 200ma difference if Re is 0r1 or 29ma if Re is 0r68...
Stuart
Hi Stuart,
But when the output stage is set up with sets of identical transistors each feeding identical emitter resistors then when the bias pot is adjusted all the transistors+ their respective emitter resistors are fed identical bias voltage. If the transistors are identical then for each transistor Vbias=Vbe1+Vre1=Vbe2+Vre2 ...=Vben+Vren and Vbe1=Vbe2=.... =Vben and Vre1=Vre2= etc.
Let's assume for a 6pair output stage that each 1r0 emitter resistor is carrying 400mA, Vbe=650mV, Vre=400mV, Vbias=1050mV and Iq=2.4A.
Now replace one transistor with an unmatched version. Let's choose one that during the selection process gave Vbe=640mV when Ic was set to 400mA. i.e. 10mV low compared to it's partners.
The low Vbe will allow an extra 10mV across Re since by construction Vbias must be the same for all the parallel pairs.
The extra 10mV now leaves 410mV across the emitter resistor and first calculation indicates 410mV is passing. But @ 410mV the Vbe will no longer be 640mV. The transistor must have a higher Vbe than it's selection figure of 640mV to allow it to pass 410mA.
If you now add Vre=410mV to the raised value of Vbe of about 641mV (from the Ic vs Vbe graph) then Vbias =1051mV. But that is not available so the current through the rogue device MUST be lower than first guess of 410mA. I estimate it is approximately 409mA.
This confirms that for 10mV spread in Vbe @ Ic=400mA then using Re=1r0 gives a spread of Ib of about 400 to 409mA.
Redoing this calculation for alternative emitter resistors gives the results I posted earlier.
It seems to show that if Vbias is held fixed for all of the parallel pairs then the unbalanced Ib is reduced from the 100mA figure you quoted to about 50mA due to the feedback action of a 0r1 emitter resistor.
Our different results are purely down to whether Vbias or Vbe is fixed.
Which applies to the operational circuit?
It would appear that the rogue (10mV low Vbe) transistor will dissipate about 15% more quiescent power than it's partners.
Assuming that the heatsink is the same temperature under each transistor then Tj will be raised by about 4.7Cdeg [34.7-30 from 0.45A*50V*(1+0.54C/W)-0.39A*50V*1.54C/W]
For those considering matching transistors that hFE has not entered any of the calculation above. I believe that Vbe is the major selection criteria.
I agree.
But when the output stage is set up with sets of identical transistors each feeding identical emitter resistors then when the bias pot is adjusted all the transistors+ their respective emitter resistors are fed identical bias voltage. If the transistors are identical then for each transistor Vbias=Vbe1+Vre1=Vbe2+Vre2 ...=Vben+Vren and Vbe1=Vbe2=.... =Vben and Vre1=Vre2= etc.
Let's assume for a 6pair output stage that each 1r0 emitter resistor is carrying 400mA, Vbe=650mV, Vre=400mV, Vbias=1050mV and Iq=2.4A.
Now replace one transistor with an unmatched version. Let's choose one that during the selection process gave Vbe=640mV when Ic was set to 400mA. i.e. 10mV low compared to it's partners.
The low Vbe will allow an extra 10mV across Re since by construction Vbias must be the same for all the parallel pairs.
The extra 10mV now leaves 410mV across the emitter resistor and first calculation indicates 410mV is passing. But @ 410mV the Vbe will no longer be 640mV. The transistor must have a higher Vbe than it's selection figure of 640mV to allow it to pass 410mA.
If you now add Vre=410mV to the raised value of Vbe of about 641mV (from the Ic vs Vbe graph) then Vbias =1051mV. But that is not available so the current through the rogue device MUST be lower than first guess of 410mA. I estimate it is approximately 409mA.
This confirms that for 10mV spread in Vbe @ Ic=400mA then using Re=1r0 gives a spread of Ib of about 400 to 409mA.
Redoing this calculation for alternative emitter resistors gives the results I posted earlier.
It seems to show that if Vbias is held fixed for all of the parallel pairs then the unbalanced Ib is reduced from the 100mA figure you quoted to about 50mA due to the feedback action of a 0r1 emitter resistor.
Our different results are purely down to whether Vbias or Vbe is fixed.
Which applies to the operational circuit?
It would appear that the rogue (10mV low Vbe) transistor will dissipate about 15% more quiescent power than it's partners.
Assuming that the heatsink is the same temperature under each transistor then Tj will be raised by about 4.7Cdeg [34.7-30 from 0.45A*50V*(1+0.54C/W)-0.39A*50V*1.54C/W]
For those considering matching transistors that hFE has not entered any of the calculation above. I believe that Vbe is the major selection criteria.
absolutely...
...the Vbe of the output transistors is, IMHO, the critical parameter, of course it can't hurt to match the other parameters as well...
I think we are in violent agreement here, you have chosen a slightly different interpretation for the Vbe mismatch, if you had chosen transistors that mismatch by 5mv in situ under the required load, the difference would, by definition be 50ma with Re=0r1 and 7.3ma with Re=0r68.
My point is still: A higher value of Re mitigates against the cumulative errors (thermal, Vbe, Re, hfe...) becoming a problem, in fact the total 'current sharing' errors are reduced in the inverse ratio of possible Re...0r1 is 6.8 times worse than 0r68, and 10 times worse than 1r0. I like a simple solution that reduces circuit errors and imbalances by a factor of 10.
I'm going to offer another WAG and postulate that Dan would have chosen 0r1 if he had felt comfortable with the tolerances and stability etc, after all the amp would have produced more power for the same components, not a small thing in the retail world. Not having to hand match all the output components is probably a nice saving for a manufacturer too.
Stuart
...the Vbe of the output transistors is, IMHO, the critical parameter, of course it can't hurt to match the other parameters as well...
I think we are in violent agreement here, you have chosen a slightly different interpretation for the Vbe mismatch, if you had chosen transistors that mismatch by 5mv in situ under the required load, the difference would, by definition be 50ma with Re=0r1 and 7.3ma with Re=0r68.
My point is still: A higher value of Re mitigates against the cumulative errors (thermal, Vbe, Re, hfe...) becoming a problem, in fact the total 'current sharing' errors are reduced in the inverse ratio of possible Re...0r1 is 6.8 times worse than 0r68, and 10 times worse than 1r0. I like a simple solution that reduces circuit errors and imbalances by a factor of 10.
I'm going to offer another WAG and postulate that Dan would have chosen 0r1 if he had felt comfortable with the tolerances and stability etc, after all the amp would have produced more power for the same components, not a small thing in the retail world. Not having to hand match all the output components is probably a nice saving for a manufacturer too.
Stuart
Hi Stuart,
agreed.
Now back to my questions.
agreed.
Now back to my questions.
Would thermal stability be degraded if lower value emitter resistors were fitted. eg when ambient temperature varies. Would the location of the Vbe multiplier become more critical if Re were lower?
I have 2 untouched spare KSA 100 blue/gold boards for sale, $35 plus S&H . Contact me if you're interested.
Do a search for the comments of Hugh Dean on the ETI466, it contains a few lines on the value of Re.
Lower Re values improves sound quality, but even pro designers such as Mr Curl need to raise the Re values sometimes, see comments on the Halo JC-1.
Lower Re values improves sound quality, but even pro designers such as Mr Curl need to raise the Re values sometimes, see comments on the Halo JC-1.
Houston, we have a go on the Krell space project but Weird Jaccowitz is wearing teflon socks again. 😴
Hit it, boys
Hit it, boys
Attachments
Clip..........Cut!
HAHAHAHA really funny! 😀 😀 😀
And you think it sounds better with Teflon sleeving? How can we take you serious after this clip?
HAHAHAHA really funny! 😀 😀 😀
And you think it sounds better with Teflon sleeving? How can we take you serious after this clip?