Hi Chris
I can agree with that.
But everytime a solution to something is introduced, it should be evaluated if the solution causes a cost on something else. And if so, is the cost higher than the benefit? (Normal cost/benefit analysis).
Otherwise it's like driving to your neighbor city to buy cheap gas for your car. It is undispuatbly true that you save 50c per gallon, and a total of 8 $. But you overlooked that you spent 10 $ driving there to get it.
Mostly the latter part is overlooked by women though
In this case it can be determined that almost all loudspeakers are self linearizing because of the voice coil inductance is higher than the choke of the Class D amplifier. Look here for an example:
http://www.tymphany.com/datasheet/printview.php?id=58
This tweeter from Scanspeak has 80 uH voice coil inductance, much higher than any modern Class D amplifier filter. And so the resonse is ruler linear, even if you don't use post filter compensation.
So you are solving a problem, that is in almost any case not important, but at a cost. The cost is a complex load component in the control loop. The question now is: is it an overall benefit?
I am not saying one or the other type is b**ls**t.
In some few cases the compensation of post filter feedback can be a benefit.
What i am proposing is why not make pre or post filtering switchable, so the user can switch back and forth as he pleases,
to find the sound of preference? It's definitely possible on almost any Class D system.
I can agree with that.
But everytime a solution to something is introduced, it should be evaluated if the solution causes a cost on something else. And if so, is the cost higher than the benefit? (Normal cost/benefit analysis).
Otherwise it's like driving to your neighbor city to buy cheap gas for your car. It is undispuatbly true that you save 50c per gallon, and a total of 8 $. But you overlooked that you spent 10 $ driving there to get it.
Mostly the latter part is overlooked by women though In this case it can be determined that almost all loudspeakers are self linearizing because of the voice coil inductance is higher than the choke of the Class D amplifier. Look here for an example:
http://www.tymphany.com/datasheet/printview.php?id=58
This tweeter from Scanspeak has 80 uH voice coil inductance, much higher than any modern Class D amplifier filter. And so the resonse is ruler linear, even if you don't use post filter compensation.
So you are solving a problem, that is in almost any case not important, but at a cost. The cost is a complex load component in the control loop. The question now is: is it an overall benefit?
I am not saying one or the other type is b**ls**t.
In some few cases the compensation of post filter feedback can be a benefit.
What i am proposing is why not make pre or post filtering switchable, so the user can switch back and forth as he pleases,
to find the sound of preference? It's definitely possible on almost any Class D system.
Lars?
I'm sorry, but I simply don't subscribe to your claims at all. Every class-D amplifier, but for UcD, that I have measured exhibits a change in the frequency response due to a loudspeaker being attached. This is purely due to the reactive load that a typical loudspeaker represents which is not compensated for by the amplifier.
I'm not just talking about minor, tenths of a dB, variations, but pretty extreme variations up to +/6dB, and usually worse at higher frequencies. UcD is the only topology that does not exhibit this behaviour and as a result has the same tonal character and balance with every loudspeaker I attach it to.
To me this is as close to the proverbial 'piece of wire with gain' one can get at the moment. As effectively the load invariant character of the UcD concept takes the amplifier out of the equation. There's no, or by comparision several magnitudes less, interaction between the (complex and often highly reactive) load the loudspeaker represents and the amplifier.
If you think, and your post expresses this, that a load variant character is an asset for an amplifier to have I suggest you rethink what we're trying to achieve here. Ideally we'd like every component in the chain from the recording to the soundwaves that finally reach the ear to have as little influence on the reproduction. Only then we can properly reproduce the recording how the artist orginally intended it. This obviously includes anything from error correction in the CD-player to room-gain.
An amplifier however is a big factor in this equation as it provides the highest amount of gain and thus its contribution to the sum of the parts is not insignificant. Therefore an amplifier should have very low THD and a flat FR curve (vitually) independent of frequency and load. Currently the only class-D topology that comes close to meeting this goal is UcD. And you still think pre-filter feedback is the way to go?
The moment you show me a class-D amplifier that uses pre-filter feedback that's able to match or surpass UcD's load invariant behaviour I'm willing to explore this further, up untill now however all attempts to design one (which I know of) have failed.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
I'm sorry, but I simply don't subscribe to your claims at all. Every class-D amplifier, but for UcD, that I have measured exhibits a change in the frequency response due to a loudspeaker being attached. This is purely due to the reactive load that a typical loudspeaker represents which is not compensated for by the amplifier.
I'm not just talking about minor, tenths of a dB, variations, but pretty extreme variations up to +/6dB, and usually worse at higher frequencies. UcD is the only topology that does not exhibit this behaviour and as a result has the same tonal character and balance with every loudspeaker I attach it to.
To me this is as close to the proverbial 'piece of wire with gain' one can get at the moment. As effectively the load invariant character of the UcD concept takes the amplifier out of the equation. There's no, or by comparision several magnitudes less, interaction between the (complex and often highly reactive) load the loudspeaker represents and the amplifier.
If you think, and your post expresses this, that a load variant character is an asset for an amplifier to have I suggest you rethink what we're trying to achieve here. Ideally we'd like every component in the chain from the recording to the soundwaves that finally reach the ear to have as little influence on the reproduction. Only then we can properly reproduce the recording how the artist orginally intended it. This obviously includes anything from error correction in the CD-player to room-gain.
An amplifier however is a big factor in this equation as it provides the highest amount of gain and thus its contribution to the sum of the parts is not insignificant. Therefore an amplifier should have very low THD and a flat FR curve (vitually) independent of frequency and load. Currently the only class-D topology that comes close to meeting this goal is UcD. And you still think pre-filter feedback is the way to go?
The moment you show me a class-D amplifier that uses pre-filter feedback that's able to match or surpass UcD's load invariant behaviour I'm willing to explore this further, up untill now however all attempts to design one (which I know of) have failed.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Of course i don't think that:
But you might say that i think the load variant character is of minor importance, and too expensive (in terms of sound quality) to get rid of.
After all you are placing the...
... in the middle of the amplifier, instead (as i propose) on the output. The reactive load is in other words a (rather unpredictable) component in the amplifier's control loop. I don't think this can contribute to anything good.
But of course this is only my personal opinion.
Your website suggests you have someting to do with testing, measurement etc. Let me suggest you a test, ok?
Take your PC soundcard, and add a couple of resistors in the input to dampen the signal. Then connect the input of your soundcard to the output of your UcD modules, and
the output of the soundcard to the input of the UcD modules.
(or any other Class D module for that matter).
Make a frequency / THD sweep with loudspeakers connected as a load. (Not a resistor). Just sweep at say 1-2W or so.
(Can download free software here: www.ymec.com )
Then i think you will see, that post filter feedback will in fact not solve your reactive load problems, like you think.
But don't take my word for it, try it out yourself
May i ask which Class D amplifiers you are referring to, i want to replicate your findings.
All the best
Lars
But you might say that i think the load variant character is of minor importance, and too expensive (in terms of sound quality) to get rid of.
After all you are placing the...
... in the middle of the amplifier, instead (as i propose) on the output. The reactive load is in other words a (rather unpredictable) component in the amplifier's control loop. I don't think this can contribute to anything good.
But of course this is only my personal opinion.
Your website suggests you have someting to do with testing, measurement etc. Let me suggest you a test, ok?
Take your PC soundcard, and add a couple of resistors in the input to dampen the signal. Then connect the input of your soundcard to the output of your UcD modules, and
the output of the soundcard to the input of the UcD modules.
(or any other Class D module for that matter).
Make a frequency / THD sweep with loudspeakers connected as a load. (Not a resistor). Just sweep at say 1-2W or so.
(Can download free software here: www.ymec.com )
Then i think you will see, that post filter feedback will in fact not solve your reactive load problems, like you think.
But don't take my word for it, try it out yourself
May i ask which Class D amplifiers you are referring to, i want to replicate your findings.
All the best
Lars
Hi Lars,
I don't just think this is the case, I know so, as this is what I've been investigating over the past two months or so. I have already posted such measurements in the past, hence the reason I'm replying, here's a few examples of what I'm refering to.
hyst. osc. 8-ohm loudspeaker
sodfa 8-ohm loudspeaker
UcD 8-ohm loudspeaker
I think this properly illustrates what I'm trying to get across. As you rightfully pointed out these measurements can be duplicated by anyone with the right software (for example the freeware RightMark Audio Analyzer) and a PC equipped with a decent soundcard.
As you can see both the hyst. self. osc. and the sodfa topology that use pre-filter feedback see a rise in their FR at higher frequencies due to the reactive character of the attached loudspeaker. It is a simple two-way configuration, with no impedance correction network on the woofer or tweeter and hence has a rising impedance curve and due to the passive crossover a reactive component as well which is not compensated for.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
I don't just think this is the case, I know so, as this is what I've been investigating over the past two months or so. I have already posted such measurements in the past, hence the reason I'm replying, here's a few examples of what I'm refering to.
hyst. osc. 8-ohm loudspeaker
sodfa 8-ohm loudspeaker
UcD 8-ohm loudspeaker
I think this properly illustrates what I'm trying to get across. As you rightfully pointed out these measurements can be duplicated by anyone with the right software (for example the freeware RightMark Audio Analyzer) and a PC equipped with a decent soundcard.
As you can see both the hyst. self. osc. and the sodfa topology that use pre-filter feedback see a rise in their FR at higher frequencies due to the reactive character of the attached loudspeaker. It is a simple two-way configuration, with no impedance correction network on the woofer or tweeter and hence has a rising impedance curve and due to the passive crossover a reactive component as well which is not compensated for.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Hi Sander
Thank You for posting these examples. Very illustrative!
The green curve is the no-load situation and the white curve is with the speaker connected?
Here are my own findings. First the no-load condition. My soundcard is the one on a Dell laptop, so it's not perfect in the top
region, but will do anyway.
And here with a 8 Ohm speaker load.
As can be seen there is a rolloff of around 1 dB at 20 kHz, caused by the load. As i see it not critical.
Thank You for posting these examples. Very illustrative!
The green curve is the no-load situation and the white curve is with the speaker connected?
Here are my own findings. First the no-load condition. My soundcard is the one on a Dell laptop, so it's not perfect in the top
region, but will do anyway.
An externally hosted image should be here but it was not working when we last tested it.
And here with a 8 Ohm speaker load.
An externally hosted image should be here but it was not working when we last tested it.
As can be seen there is a rolloff of around 1 dB at 20 kHz, caused by the load. As i see it not critical.
Hi Lars,
With the risk of insulting you and yourr hard work, but that's far from a flat curve, even with no load connected. Especially the 'wobble' at the end of the frequency curve strikes me as problematic. The UcD180 or 400 for that matter have an almost exemplary flat curve, as do a number of UcD prototypes I've recently built.
Just realize that I've been trying my hand at a class-D design of my own over the past two months (with no commercial interest I might add) and I have learnt a great deal by often trying the hands-on path rather than the pure theoretical evaluation of different concepts. I ended up deciding to go with UcD for a class-D amplifier of my own design exactly due to this load invariant behaviour, and believe me I'd done the ugly tests as well, by putting a 10uF capacitor in parallel with the output for example.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
With the risk of insulting you and yourr hard work, but that's far from a flat curve, even with no load connected. Especially the 'wobble' at the end of the frequency curve strikes me as problematic. The UcD180 or 400 for that matter have an almost exemplary flat curve, as do a number of UcD prototypes I've recently built.
Just realize that I've been trying my hand at a class-D design of my own over the past two months (with no commercial interest I might add) and I have learnt a great deal by often trying the hands-on path rather than the pure theoretical evaluation of different concepts. I ended up deciding to go with UcD for a class-D amplifier of my own design exactly due to this load invariant behaviour, and believe me I'd done the ugly tests as well, by putting a 10uF capacitor in parallel with the output for example.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Ohh yes I shouldn't have used the laptop soundcard. Here you see the curve of the laptop only. (No Class D amps connected!)
So that's where the wobbling is coming from.
Anyway you win the argument that pre filter feedback may give deviations of maybe 0-1 dB (as shown in both yours and my measurements. However i dont think +/- 1dB is a significant problem. Loudspeakers can deviate much more, and anyway the ear will automaticly compensate a few dB without you noticing it.
I shouldn't have used the laptop soundcard. Here you see the curve of the laptop only. (No Class D amps connected!)An externally hosted image should be here but it was not working when we last tested it.
So that's where the wobbling is coming from.
Anyway you win the argument that pre filter feedback may give deviations of maybe 0-1 dB (as shown in both yours and my measurements. However i dont think +/- 1dB is a significant problem. Loudspeakers can deviate much more, and anyway the ear will automaticly compensate a few dB without you noticing it.
Jan Peter Thanks for posting these interesting curves!
As i said back in post #105 there can be cases where post filter feedback can be a benefit. I guess if your loudspeaker has an impedance of 1 Ohm at 20 kHz, anf you are not going to make a freq compensation for it, this is one of those cases.
As i said back in post #105 there can be cases where post filter feedback can be a benefit. I guess if your loudspeaker has an impedance of 1 Ohm at 20 kHz, anf you are not going to make a freq compensation for it, this is one of those cases.
Hi Lars, Jan-Peter,
Thanks for sharing. From my experience it is not uncommon for the 'wobble' in Lars' plot to be shifted to the left of the curve, depending on the 'reactiveness' of the loudspeaker connected. This effectively puts it in the midrange and hence it is very audible giving the amplifier a tonal character and balance that is specific to that combination of loudspeaker and amplifier.
I know Lars stated that that wobble is due to his soundcard, and I have no reason to doubt that, yet I've seen similar behaviour from pre-filter feedback class-D amplifiers that display such behaviour with reactive loads, so I wasn't surprised to see it shown up in the plot.
As for the plot Jan-Peter showed, that's pretty dramatic and exactly what I was referring to. Unfortunately in the real world loudspeakers are not a simple resistive 4-ohm or 8-ohm load I'm afraid, and no two loudspeakers are the same, hence having a load invariant behaviour (whether implemented pre- or postfilter) is something I'd put high up on my list of design goals for any amplifier.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Thanks for sharing. From my experience it is not uncommon for the 'wobble' in Lars' plot to be shifted to the left of the curve, depending on the 'reactiveness' of the loudspeaker connected. This effectively puts it in the midrange and hence it is very audible giving the amplifier a tonal character and balance that is specific to that combination of loudspeaker and amplifier.
I know Lars stated that that wobble is due to his soundcard, and I have no reason to doubt that, yet I've seen similar behaviour from pre-filter feedback class-D amplifiers that display such behaviour with reactive loads, so I wasn't surprised to see it shown up in the plot.
As for the plot Jan-Peter showed, that's pretty dramatic and exactly what I was referring to. Unfortunately in the real world loudspeakers are not a simple resistive 4-ohm or 8-ohm load I'm afraid, and no two loudspeakers are the same, hence having a load invariant behaviour (whether implemented pre- or postfilter) is something I'd put high up on my list of design goals for any amplifier.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Hi Jaka,
The UcD amplifier I used has slightly less gain than the two others, but at the time of these measurements that's not the of any concern, I was looking at the behaviour with different loads attached, so that's why the plot is slightly lower than the others, that's it.
Cheers,
Sander Sassen
http://www.hardwareanalysis.com
The UcD amplifier I used has slightly less gain than the two others, but at the time of these measurements that's not the of any concern, I was looking at the behaviour with different loads attached, so that's why the plot is slightly lower than the others, that's it.
Cheers,
Sander Sassen
http://www.hardwareanalysis.com
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