Stupid question. Is the low bias, mulitple output pair solution that Nelson utilizes in his Pass amps chosen for reliablity and durability? In theory, a clone could reduce output number and increase output bias, corrrect. I know hiis soluton would provide greater DF, but at some point its moot, as it is already incredibly high.
ahahaha...I don't think so. Nelson said that Bias for each device is around 2A
Nope, he gave the maximum conditions they would see, just like the max voltage of 70.
200W continuous class A in 8 ohm requires a peak output current of ~7.1A
SE without a dynamic bias on demand á la Aleph would have a static bias level of at least that value : 7.1A
200W continuous power in balanced mode requires rails of 28.3Vdc minimum.
To function properly, voltage at the drain of the MOSFETs has to be ~5 points higher than the max output at the source, aka Vgs plus slack.
So minimum rails of 35Vdc.
7.1A times 35V rails times 4 rails is ~994W dissipation.
Reality is that crossover distortion will raise it's head before output current reaches the 7.1A level.
For REAL 200W SE class A, the bias current level would need to be a little higher than that.
That little extra is set for each output device, so the more power devices in parallel, the more added in total on top of the 7.1 amps.
Extras added, dissipation will go beyond the 1kW mark per channel.
Theoretical efficiency of Push-Pull is 50%, without losses.
Theoretical efficiency of Single-Ended is 25%, not including losses.
A classic Single-Ended (static bias level) output stage, with MOSFET devices, in balanced mode, is the worst variety (as in the example, efficiency less than 20%)
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Same deal as during the Threshold days, sales rep story's the same too.
Why change a winning team ?
I fully agree with your calculation which leads to about 1KW dissipation for a 200W Class A amp.
However the calculation you implemented is no different, or in other words, is not "topolgy-dependand" which means that you get to the same number whether this is a push-pull or a SE or a SE balanced version.
In other words, if the calculation above is correct that should work for the XA as well which is a Class A push-pull topology just like the XA.5 but with much more Bias.
Would you please mind explaining a bit further how would you count in power losses into your calculation?
If we are just looking at the specs for the XA200.5, it says to leave Class A at a peak of about 400W and IDLE bias is set to 700W.
Here are some of my calculation:
max class A 400W, rated @8ohm is 200W.
i.e. V_PK = 56.56V and for a balance version V_RAIL=28.3V (I am not considering the extra margin of 5V).
I_PK@8ohm = 7.1A
However the amp doubles with 4ohm so I_PK_@4ohm =14.2A <- this is the bias per quadrant.
If there are a total of 80 devices, we can conclude
I_Bias_device = 0.787A -> P_device = 19.8W
Idle dissipation would be
P_IDLE = 14.2 * 28.3 * 4 = 1.6KW!!!!
This number is more than twice as much what declared by NP thus there must be some flaws on my calculation!!!
What is it?
However the calculation you implemented is no different, or in other words, is not "topolgy-dependand" which means that you get to the same number whether this is a push-pull or a SE or a SE balanced version.
In other words, if the calculation above is correct that should work for the XA as well which is a Class A push-pull topology just like the XA.5 but with much more Bias.
Would you please mind explaining a bit further how would you count in power losses into your calculation?
If we are just looking at the specs for the XA200.5, it says to leave Class A at a peak of about 400W and IDLE bias is set to 700W.
Here are some of my calculation:
max class A 400W, rated @8ohm is 200W.
i.e. V_PK = 56.56V and for a balance version V_RAIL=28.3V (I am not considering the extra margin of 5V).
I_PK@8ohm = 7.1A
However the amp doubles with 4ohm so I_PK_@4ohm =14.2A <- this is the bias per quadrant.
If there are a total of 80 devices, we can conclude
I_Bias_device = 0.787A -> P_device = 19.8W
Idle dissipation would be
P_IDLE = 14.2 * 28.3 * 4 = 1.6KW!!!!
This number is more than twice as much what declared by NP thus there must be some flaws on my calculation!!!
What is it?
You could do what I suggested earlier => http://www.diyaudio.com/forums/pass-labs/73543-ax100-100w-aleph-x-monoblocks-3.html#post1216044
(your first post there is 1 day later, after Paps')
You did hear that Papa is the emperor of bias on demand, with pink slippers ?
(your first post there is 1 day later, after Paps')
You did hear that Papa is the emperor of bias on demand, with pink slippers ?
ahaha I am completely thrown off now...sorry....but thanks for linking to that post.
I don't have with me now the Aleph 0 SM I will look for tonight, but if I don't recall wrong that is a pure SE design.
If I am not understanding wrong, the first generation XA was like a SE design like the Aleph 0, right? If I still understand correctly on the .5 he went to th Push pull topology.
But now if the current bias does up to 40A that would completely thrown off the calculation I have made above.
Would you like to point out how can you come to an Idle power of 700W and yet having a bias of 40A for a balanced PP topology of 200W @ 8ohm?
I think we can see why Class A amps with high output powers are not common I like class A amps, but the massive heat dissipation is a bit of a dampener when it comes to practicality.
I think the recommendations made here to actually measure the voltage peaks at your loudspeakers under normal listening, then calculate the worse case current necessary to deliver that peak into the hardest phase angle/impedance is a very good idea.
The general concept of building a massive class A amp is commendable, but simply diving into a huge design without actually knowing how much you really use is a bit premature. You might find out that you actually need more power then you've currently got, which would mean a class A amp for all occasions is really out of the question, or you might find that for 95% of your listening you only actually need say less then 50 watts give or take and most of the time might only need say 25 watts. If you measure and see that you only need actually a low amount of power for a lot of your listening, then it would make far more sense to build a 30 watt class A amp and see if it sounds how you want. If it does what you want then you could go for something more extravagant.
That is one of the issues with the B&Ws and almost all expensive loudspeakers, the manufactures don't seem too concerned with making them all 4 ohm loads or tougher with horrible phase angles. This is alright if your idea of the perfect power amp is some Krell monster, but if you're a fan of class A amps you're on a hiding to nothing. Pure proper class A into a nominal 8 ohm load loudspeaker is hard enough, making it fully class A capable into 4 ohm loads too is almost a fools errand, which is probably why very few do it.
Another option of course, would be to bi-amp the B&Ws this will make the load on the class A amp a lot easier if it's job is only to drive the FST and the Diamond tweeter.
I like class A amps, but the massive heat dissipation is a bit of a dampener when it comes to practicality.I think the recommendations made here to actually measure the voltage peaks at your loudspeakers under normal listening, then calculate the worse case current necessary to deliver that peak into the hardest phase angle/impedance is a very good idea.
The general concept of building a massive class A amp is commendable, but simply diving into a huge design without actually knowing how much you really use is a bit premature. You might find out that you actually need more power then you've currently got, which would mean a class A amp for all occasions is really out of the question, or you might find that for 95% of your listening you only actually need say less then 50 watts give or take and most of the time might only need say 25 watts. If you measure and see that you only need actually a low amount of power for a lot of your listening, then it would make far more sense to build a 30 watt class A amp and see if it sounds how you want. If it does what you want then you could go for something more extravagant.
That is one of the issues with the B&Ws and almost all expensive loudspeakers, the manufactures don't seem too concerned with making them all 4 ohm loads or tougher with horrible phase angles. This is alright if your idea of the perfect power amp is some Krell monster, but if you're a fan of class A amps you're on a hiding to nothing. Pure proper class A into a nominal 8 ohm load loudspeaker is hard enough, making it fully class A capable into 4 ohm loads too is almost a fools errand, which is probably why very few do it.
Another option of course, would be to bi-amp the B&Ws this will make the load on the class A amp a lot easier if it's job is only to drive the FST and the Diamond tweeter.
so 40A is the maximum current the Amp can deliver in Class AB, correct?
How do you evaluate this number?
Based on the amount of filter capacitance? Obviously the mosfets are capable of delivering a huge amount of pulse current.
Is that 40A rated for a short period of time or continuos?
Also to bump back my previous point, still, how do you get 700W idle from the XA200.5 if the topology is balance PP?
5th,
you mentioned very good points.
Measuring what is the actual requirement is very custom and very smart.
However I want to make a bigger amplifier for the following reasons:
1) it is more challenging
2) it allows to basically be able to connect any speaker you want and now I have a 92dB efficiency, tomorrow who knows
3) I am planning ongetting a big audio room which could significantly change the required power.
Bi-Amplification is probably the ideal solution and I am very appealed by this concept.
The only thing is that I already have a bi-wire cable and although you are not a big fan of fancy hi-end cable, buy double set of speaker cable would just get extremely expensive but it is something I will give it a thought in the future.
My other concern with the Bi-amp is with the input impedance that can get pretty low thus hard to drive with a low feedback preamplifier.
In the next months I will be developing the amplifier. I will buy heatsinks online and build a simple chassis aroung for testing only.
I would like to keep this thread going to discuss topologies, solitions, simulations, testing results and then when the amplifier starts shaping I will post my build on another specific thread.
Meanwhile I appreciated all your inputs//
you mentioned very good points.
Measuring what is the actual requirement is very custom and very smart.
However I want to make a bigger amplifier for the following reasons:
1) it is more challenging
2) it allows to basically be able to connect any speaker you want and now I have a 92dB efficiency, tomorrow who knows
3) I am planning ongetting a big audio room which could significantly change the required power.
Bi-Amplification is probably the ideal solution and I am very appealed by this concept.
The only thing is that I already have a bi-wire cable and although you are not a big fan of fancy hi-end cable, buy double set of speaker cable would just get extremely expensive but it is something I will give it a thought in the future.
My other concern with the Bi-amp is with the input impedance that can get pretty low thus hard to drive with a low feedback preamplifier.
In the next months I will be developing the amplifier. I will buy heatsinks online and build a simple chassis aroung for testing only.
I would like to keep this thread going to discuss topologies, solitions, simulations, testing results and then when the amplifier starts shaping I will post my build on another specific thread.
Meanwhile I appreciated all your inputs//
Melon,
thanks for your concern, but I am not in need of much help at this stage.
It was just interesting to talk about the different topics like differences between X-XA-SE...and so on.
I already told you what I am roughly going to build:
200W class A with Class A peak of 400W, which is basically matching the specs of the XA200.5 which is tha amplifier I would like to buy, so I just try to make an attempt to make something comparable and then in the future audition it to see if I am still in need of that beautiful unit!
thanks for your concern, but I am not in need of much help at this stage.
It was just interesting to talk about the different topics like differences between X-XA-SE...and so on.
I already told you what I am roughly going to build:
200W class A with Class A peak of 400W, which is basically matching the specs of the XA200.5 which is tha amplifier I would like to buy, so I just try to make an attempt to make something comparable and then in the future audition it to see if I am still in need of that beautiful unit!
40 amps in 1 ohm makes 1600W continuous.
Transformer VA in the XA200.5 is more than that, and max dissipation of 80 output devices is 7.5 times as high.
(continuous is a somewhat strange word in the audio world)
700W idle is 175W dissipation per quadrant.
175W divided by 35Vdc rail makes 5 amps bias current.
Positive rail side ; 5A total drain current for the parallel IRFP240 bank.
Negative rail side ; 4.25A total drain current for the IRFP9240, 0.75A single-ended bias.
Effectively a net max class A output current of ~8.5A
I mean, by saying 40A NP maybe should have specified for how long, because the Heatsink's compound obviously cannot handle much more heat in top of the IDLE condition.
Maybe he intended to say that the Tranformer/ Rectifier/ Filter and Output stage can handle up to 40A for a short period of time.
Anyway, going back to your calculation I am not understanding something.
If the idle is 700W and the output devices are 80, this means that power dissipation for each device would ONLY be 8.75W.
Also for a 35V rail, assuming that output swings up to 30V, power on the differential load will be roughly 60 * 5 = 300W and not 400W class A as specified.
What do you mean by "Effectively a net max class A output current of ~8.5A "
I also have this curiosity:
when you say negative side 4.25A and 0.75A SE bias, isn't it more like 5A negative side plus 0.75A?
I mean with SE Nelson controls the amount of bias on each quadrant?
This is probably something I haven't understood yet. Check out my sketch and please tell me where I am getting wrong.
Thanks.
Maybe he intended to say that the Tranformer/ Rectifier/ Filter and Output stage can handle up to 40A for a short period of time.
Anyway, going back to your calculation I am not understanding something.
If the idle is 700W and the output devices are 80, this means that power dissipation for each device would ONLY be 8.75W.
Also for a 35V rail, assuming that output swings up to 30V, power on the differential load will be roughly 60 * 5 = 300W and not 400W class A as specified.
What do you mean by "Effectively a net max class A output current of ~8.5A "
I also have this curiosity:
when you say negative side 4.25A and 0.75A SE bias, isn't it more like 5A negative side plus 0.75A?
I mean with SE Nelson controls the amount of bias on each quadrant?
This is probably something I haven't understood yet. Check out my sketch and please tell me where I am getting wrong.
Thanks.
Attachments
Personally I think you're crazy. Class A isn't something you build with a what if on the future, you build as per your needs now. Don't think that the 200 watt class A is going to sound better then the 30 watt class A, it won't. If the front end of the amp is the same, the only difference between one and the other is the number of devices in the output stage, the bias current and the voltage the amp is run at, these in themselves do not alter the way the amp sounds. That is, they will sound the same if both are used within their class A limits and are not forced into clipping.
You might have 92dB speakers, but that, in this regard, isn't really that great. Sure they are reasonably sensitive but they are not exactly easy to drive. It's the ease of drive with class A that is more important then how sensitive the loudspeakers are. In other words you could have a pair of speakers that are 89dB and a doddle to drive with the impedance not dropping below 6.5 ohms with the worst phase angle of say +-10 degrees and you might only need 2 amps of standing bias vs the B&Ws which have bad phase angles combined with low impedance dips and you might need 8 amps, to ensure that the amp remains in class A for the same SPL requirements.
There's a reason that most people who like class amps and tube stuff go for sensitive, easy to drive loudspeakers. Not because they want to as per say, but because they need to, to ensure that the most is got out of their amplifiers.
You are looking at this from sort of the wrong way round. You don't build a class A amp with the thought in mind that you might attach something ridiculous to it at some point and want it to be able to power that without dropping out of class A. You build the class A amp and then if you're going to change the speakers, you pick the speakers to match the amplifier.
I mean you say...
But this is DIY, not I want to buy Pass labs biggest most expensive pure class A amp. With DIY you very much DO build to suit your needs, that's the point, that's why it's DIY. Not I'll build something then if I like it, audition the real Pass amp and then buy it if I like it...The point is you build it so you don't have to buy it.
Considering the scale of such a project though and your seeming reluctance to measure what your requirements actually are. I recommend you just go and audition the XA200.5 and see if you like what it does. If you've got the expendable income to just buy it then I'd just buy it, as it would save you one heck of a headache as the build wont be trivial. If you like what it does, but don't have the money to buy one, at least now you will know that it's worthwhile doing, however, I would still measure to see what your requirements actually are, if your an EE this is what makes logical sense too.
You might have 92dB speakers, but that, in this regard, isn't really that great. Sure they are reasonably sensitive but they are not exactly easy to drive. It's the ease of drive with class A that is more important then how sensitive the loudspeakers are. In other words you could have a pair of speakers that are 89dB and a doddle to drive with the impedance not dropping below 6.5 ohms with the worst phase angle of say +-10 degrees and you might only need 2 amps of standing bias vs the B&Ws which have bad phase angles combined with low impedance dips and you might need 8 amps, to ensure that the amp remains in class A for the same SPL requirements.
There's a reason that most people who like class amps and tube stuff go for sensitive, easy to drive loudspeakers. Not because they want to as per say, but because they need to, to ensure that the most is got out of their amplifiers.
You are looking at this from sort of the wrong way round. You don't build a class A amp with the thought in mind that you might attach something ridiculous to it at some point and want it to be able to power that without dropping out of class A. You build the class A amp and then if you're going to change the speakers, you pick the speakers to match the amplifier.
I mean you say...
But this is DIY, not I want to buy Pass labs biggest most expensive pure class A amp. With DIY you very much DO build to suit your needs, that's the point, that's why it's DIY. Not I'll build something then if I like it, audition the real Pass amp and then buy it if I like it...The point is you build it so you don't have to buy it.
Considering the scale of such a project though and your seeming reluctance to measure what your requirements actually are. I recommend you just go and audition the XA200.5 and see if you like what it does. If you've got the expendable income to just buy it then I'd just buy it, as it would save you one heck of a headache as the build wont be trivial. If you like what it does, but don't have the money to buy one, at least now you will know that it's worthwhile doing, however, I would still measure to see what your requirements actually are, if your an EE this is what makes logical sense too.
No I think you are crazy....go tell papa and the other people here!
The basic thing you learn from output Mosfet is that their BIAS DEEPLY AFFECTS THE SOUND.
It's like saying that a 5.5Liter Mercedes AMG engine will drive worse at 50mph than a VW 1.8T Jetta just because it is bigger engine.
It is the other way around, while the former will go at 1800rpm the latter will drive at 3000rpm.
That is a basic concept. Obviously to create a powerful engine you need to spend more resources upfront!
Same goes for amplifier!
The fact I want to build a 200W Class A has nothibng to do with what I need.
I think it is an interesting project and a good power target for my needs and like you said B&W has deeps on the impedance which requires a more powerful amplifier.
Now what you don't understand is that having deeps doesn't mean lower quality, in the other hand means that they choose a simpler crossover which yealds to better sound in their and my opinion but needs to be matched to a monster amp like a Krell or big Pass.
Which also makes sense since you spent so much on speaker you are not going to buy a cheap amp to match with.
They claim that final result will be batter than having a speaker with more complicated crossover and simpler power amp and if you think through this makes total sense.
But anyway, enough with speakers this is not a thread for debating on speakers, you can open up a new thread and I will be happy to join it if you want me to.
I don't need to audition the XA200.5 to know that it is a wonderful amplifier.
I trust NP master work and I am sure it is capable of doing just anything and at the moment I can't spend 20+ grand on this, maybe next year.
In the meantime I take the opportunity to build something fun and hopefully very sound rewarding to not feel the need to spend this amount next year and instead use it for something else or save it.
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