I was using a TA2021B based Tripath amp hooked up to my speakers which use a 1" titanium dome tweeter. They're 8 ohms and around 90dB @ 1W/1m. I had the amp turned up more than normal, but the Tripath amp is limited to around 8-9W.
I don't know the answer to the ultrasonic frequency question. I do know a graduate student who was working with high power ultrasonic transducer arrays and he felt some pain in his ears while working around an array that was putting out about 140dB @ 1m. He didn't know it was on because it was set around 40kHz but he definitely felt its presence! He thinks he may have damaged his hearing...
I don't know the answer to the ultrasonic frequency question. I do know a graduate student who was working with high power ultrasonic transducer arrays and he felt some pain in his ears while working around an array that was putting out about 140dB @ 1m. He didn't know it was on because it was set around 40kHz but he definitely felt its presence! He thinks he may have damaged his hearing...
140dB is quite a bit. Perhaps he was experienceing heating in his ears, or some other kind of tissue denigration?
But I am concerned about the felt presence. I know that the very highest frequencies which can be heard, are experienced in this way.
I know what i want to do. But I don't want it to be something which is a physiological hazard.
The 8 or 9 watts on the tweater is quite a bit. A 1" dome tweeter will work at 21khz or more, but it will put out a beaming pattern. Straight on axis will be fine.
That much power is a strong signal.
I once made smoke come out of a pair of tweeters with sustained ultrasonic once. 8 or 9 watts continuously could over heat many tweeters.
But I am concerned about the felt presence. I know that the very highest frequencies which can be heard, are experienced in this way.
I know what i want to do. But I don't want it to be something which is a physiological hazard.
The 8 or 9 watts on the tweater is quite a bit. A 1" dome tweeter will work at 21khz or more, but it will put out a beaming pattern. Straight on axis will be fine.
That much power is a strong signal.
I once made smoke come out of a pair of tweeters with sustained ultrasonic once. 8 or 9 watts continuously could over heat many tweeters.
If you don't mind my asking, what was the 140dB 40khz for? How much power did it have, and what was being used as a transducer?
There is a 12dB/octave crossover network and resistive divider between the amp and the tweeter. I'm sure I wasn't feeding it 9W but I'd bet it was putting out in the realm of 100dB @ 1m.
The 140dB 40kHz was produced with a very efficient array of piezoelectric transducers - I think there were 16 of them on a clear plexiglass panel if I remember correctly. Not sure how much amplifier power was used and I don't know exactly what they were using the array for. At the time the grad student was an instructor for one of my acoustics classes and he was showing us some of what he was doing for his doctoral thesis.
The 140dB 40kHz was produced with a very efficient array of piezoelectric transducers - I think there were 16 of them on a clear plexiglass panel if I remember correctly. Not sure how much amplifier power was used and I don't know exactly what they were using the array for. At the time the grad student was an instructor for one of my acoustics classes and he was showing us some of what he was doing for his doctoral thesis.
Thanks.
100dB at 21khz is still a very strong signal.
For the more typical kind of class D amps, running at higher frequency, how many orders of filtering do they have, and starting at what frequency?
As I see it, a very big subwoofer could approach 100 dB/ 1 meter.
Say I had 2000 watts, that would make it 133 dB.
But then I have 156dB of filtering. Ok, but that is a 1khz filter.
Going out above 20khz, there are parasitic capacitances in the filter, so it might not continue to roll off that far out .
So there could perhaps be some 30dB of signal remaining??
Maybe?? I still think that could not be heard, even at 20kHz, and especially with a subwoofer.
But at 23.5 it really should be unhearable. But then I have my foam rubber grill cloth too.
I believe there must be precedents of people making bipolar subwoofer amps. There must be precendents for the lower switching frequency. People must have done this.
Even an MOS amp will be able to do more power per output device if the switching frequency can be lowered.
100dB at 21khz is still a very strong signal.
For the more typical kind of class D amps, running at higher frequency, how many orders of filtering do they have, and starting at what frequency?
As I see it, a very big subwoofer could approach 100 dB/ 1 meter.
Say I had 2000 watts, that would make it 133 dB.
But then I have 156dB of filtering. Ok, but that is a 1khz filter.
Going out above 20khz, there are parasitic capacitances in the filter, so it might not continue to roll off that far out .
So there could perhaps be some 30dB of signal remaining??
Maybe?? I still think that could not be heard, even at 20kHz, and especially with a subwoofer.
But at 23.5 it really should be unhearable. But then I have my foam rubber grill cloth too.
I believe there must be precedents of people making bipolar subwoofer amps. There must be precendents for the lower switching frequency. People must have done this.
Even an MOS amp will be able to do more power per output device if the switching frequency can be lowered.
zenmasterbrian said:
Most high power class d amps intended for audio use switch up in the 100-500kHz range and most all use a simple second order LC filter on the output.
Parasitics don't have much effect up until the RF range for the smaller components. Larger componets will obviously have more parasitics which lower the frequency at which the component starts to stray from ideal operation.
Using only a 2nd order filter, even with such high switching frequencies, will still result in significant switching signal going to the driver. Of course it is a very very high frequency.
There must be precedent for subwoofer class D amps in the frequency range I have in mind. These issues must have been dealt with.
There must be precedent for subwoofer class D amps in the frequency range I have in mind. These issues must have been dealt with.
If you use a high enough switching frequency and design the LC filter to have a cutoff in the 30-50kHz region then there will be enough attenuation of the switching frequency to not worry about anything that does make it through.zenmasterbrian said:
There is a precedent, but this precedent is what gave class d amps a bad rep in the first place! Many people still think class d amps are only suitable for subwoofer use because of effective marketing and a general misunderstanding of how class d works.zenmasterbrian said:
If your filter is set to 30khz, so it is 3db down there, and it is 2nd order, Then at 120khz you will be 40dB down, and at 240khz, 60dB down.
This amount of signal is not a serious hit on power efficiency, but it is significant. Rember it is a rail to rail square wave. So it is about double the amps rated sine wave power.
Now, some of those upper harmonics will be more attenuated.
What really drives this for me is not an interest in BJTs, in Class D per se, or a belief that Class D is only suitable for subwoofers.
The first consideration is wanting to do the whole system multi-amped, with active crossovers.
So, you end up having a subwoofer amp separate.
For the tweeter amp, I don't see any reason to go class D, even though I'm quite convinced that it could work.
So for the subwoofer I want class D, at a modest frequency.
Have you ever seen any published info on such Class D subwoofer amps?
I can manage the 8th order filter and closed loop issues.
This amount of signal is not a serious hit on power efficiency, but it is significant. Rember it is a rail to rail square wave. So it is about double the amps rated sine wave power.
Now, some of those upper harmonics will be more attenuated.
What really drives this for me is not an interest in BJTs, in Class D per se, or a belief that Class D is only suitable for subwoofers.
The first consideration is wanting to do the whole system multi-amped, with active crossovers.
So, you end up having a subwoofer amp separate.
For the tweeter amp, I don't see any reason to go class D, even though I'm quite convinced that it could work.
So for the subwoofer I want class D, at a modest frequency.
Have you ever seen any published info on such Class D subwoofer amps?
I can manage the 8th order filter and closed loop issues.
An 8th order filter is pretty rediculous at the frequency and power you're looking at. Do you have any idea how large the coils will have to be to have low winding resistance? Then you want to put 4 of them between the output stage and the driver? I don't see any advantage in doing that.zenmasterbrian said:
An LC filter has a 12dB/octave slope or 20dB/decade. For a filter with a cutoff frequency of 30kHz that would have attenuation figures of 12dB at 60kHz, 24dB at 120kHz, 36dB at 240kHz, 48dB at 480kHz, and so on. Sure some ultrasonic stuff gets through the filter, but it doesn't have anything to do with power efficiency. Just like 20kHz will not be reproduced by a subwoofer driver, the switching residual will not be reproduced by woofers, midranges, or tweeters (maybe if you have tweeters with real extended frequency response...). It will mainly be wasted as heat in the voice coil. What most people don't mention is that there may be some serious advantages to the high frequency residual. Have you ever heard of an interesting technique they used to get better performance from magnetic tape? They summed the audio signal with a high frequency signal before recording onto the magnetic tape. Thie high frequency bias signal reduced the magnetic memory effect of the tape. Something similar may be happening in the conductive paths subject to high frequency residual. Just a theory really, but I know other people who have pondered this as well.
Have a look here: http://www.hypex.nl/ and check out the UCD700. I suggest you buy those modules instead of trying to build your own with BJTs. They can get 700W from only 3 pairs of mosfets and a simple LC filter with 92% efficiency. Not too shabby eh?
The 8th order filter does not require bigger total reactances than the 2nd order filter. They are just broken up into smaller units.
It is about the same as a 1khz speaker crossover. ( you mention speakers with 1" dome tweeters. That is about where their cross over probably is. )
As far as coil resistance being a problem, the loop gain works to divide that down.
Again, because this only has to work to 300hz, I can compensate it so I can get incredible loop gain.
I want to fully take advantage of what making it subwoofer only makes possible. It really is designed for that application.
So it is all the advantages of the lower audio bandwidth on the closed loop dynamics, a lower switching frequency, and I hope, the advantages that bipolar offers at such frequency.
I am aquainted with the high frequency bias used in magetic tape.
That works because tape recording is nonlinear, and stores info by hysterysis.
I am not aware of any benefits of high frequency in an audio system. That sounds like metaphysics to me.
I will look at the MOSFET module link you show. I am always interested to see such links.
As I see it, for 3 pairs of devices on good heat sinks, Class D, 700Watts is not impressive. I believe bipolar at modest frequency can do much better. But I will look.
I know that 23.5khz will not be well reproduced as it goes into a subwoofer. But it is still a cause for concern if it is perceptable.
I'm still hoping to find precedent for a subwoofer amp designed in the switching frequency range I speak of. All the better if it was designed with bipolar.
It is about the same as a 1khz speaker crossover. ( you mention speakers with 1" dome tweeters. That is about where their cross over probably is. )
As far as coil resistance being a problem, the loop gain works to divide that down.
Again, because this only has to work to 300hz, I can compensate it so I can get incredible loop gain.
I want to fully take advantage of what making it subwoofer only makes possible. It really is designed for that application.
So it is all the advantages of the lower audio bandwidth on the closed loop dynamics, a lower switching frequency, and I hope, the advantages that bipolar offers at such frequency.
I am aquainted with the high frequency bias used in magetic tape.
That works because tape recording is nonlinear, and stores info by hysterysis.
I am not aware of any benefits of high frequency in an audio system. That sounds like metaphysics to me.
I will look at the MOSFET module link you show. I am always interested to see such links.
As I see it, for 3 pairs of devices on good heat sinks, Class D, 700Watts is not impressive. I believe bipolar at modest frequency can do much better. But I will look.
I know that 23.5khz will not be well reproduced as it goes into a subwoofer. But it is still a cause for concern if it is perceptable.
I'm still hoping to find precedent for a subwoofer amp designed in the switching frequency range I speak of. All the better if it was designed with bipolar.
It is not the same because you're looking to use the filter on the output of a 1kW amp. The crossover on my tweeter is around 2KHz.zenmasterbrian said:
Precisely. You don't think there would be any benefit to having the magnetic fields of a conductor constantly changed by a high frequency bias signal?zenmasterbrian said:
700W with 3 pairs of mosfets on a heatsink that small and you aren't impressed? You know that module is capable of 20Hz-20kHz right with low distortion? You clearly don't fully understand how difficult it is to accomplish such a task.zenmasterbrian said:
The UDC web site is very interesting. I have started to read from it, and will continue.
Clearly these are very well designed, very high quality, and very practicle devices.
Help me know a little more. Am I seeing correctly that the big T shaped blue part is a heat sink, and then there are three power tab transistors on each side?
If you had data sheets for these transistors, and anything else it uses, I would be interested.
They also say you can use only partially filtered DC power. You almost have to allow that at these kinds of power levels. When you allow big power ripples, you are placing additional demands on the output devices, although this increase is less for class D than for linear.
What seems to be the heat sink is not what I would consider to be a big heat sink. It is a practical heat sink.
I'm glad to know about this UDC company. It looks like their things are well made and very interesting.
Again, my starting point here is multiamping, and the opportuities afforded when each amp in the system can be designed differently.🙂
Clearly these are very well designed, very high quality, and very practicle devices.
Help me know a little more. Am I seeing correctly that the big T shaped blue part is a heat sink, and then there are three power tab transistors on each side?
If you had data sheets for these transistors, and anything else it uses, I would be interested.
They also say you can use only partially filtered DC power. You almost have to allow that at these kinds of power levels. When you allow big power ripples, you are placing additional demands on the output devices, although this increase is less for class D than for linear.
What seems to be the heat sink is not what I would consider to be a big heat sink. It is a practical heat sink.
I'm glad to know about this UDC company. It looks like their things are well made and very interesting.
Again, my starting point here is multiamping, and the opportuities afforded when each amp in the system can be designed differently.🙂
Mr. BWRX, we have never met. So we don't know each other. Online communication is slow and sometimes difficult. But I think it worth the effort.
The 8th order filter that I would use does not have to be much bigger than what is in your speaker cross over, even though the power level is higher.
The inductors would be air core. So there is not iron or ferrite core that could saturate if not big enough. So its just a matter of the wires being big enough to not over heat.
That won't be a problem. Also my high power is a peak power. The physical size of the inductors will not need to be much bigger.
Also, I could tollerate much more DC resistance than you passive speaker crossover could because I am inside a close loop, which will be well designed.
That is something I have studied well and in excruciating detail.
The capacitor merely needs to have a high enough voltage rating. Since it is not electrolytic, this will not be a problem.
Remember, this amp will be part of a big subwoofer enclosure.
The total capacitance and total inductance will be about the same as the speaker crossover parts.
TO BE CONTINUED
The 8th order filter that I would use does not have to be much bigger than what is in your speaker cross over, even though the power level is higher.
The inductors would be air core. So there is not iron or ferrite core that could saturate if not big enough. So its just a matter of the wires being big enough to not over heat.
That won't be a problem. Also my high power is a peak power. The physical size of the inductors will not need to be much bigger.
Also, I could tollerate much more DC resistance than you passive speaker crossover could because I am inside a close loop, which will be well designed.
That is something I have studied well and in excruciating detail.
The capacitor merely needs to have a high enough voltage rating. Since it is not electrolytic, this will not be a problem.
Remember, this amp will be part of a big subwoofer enclosure.
The total capacitance and total inductance will be about the same as the speaker crossover parts.
TO BE CONTINUED
Clearly the UDC amp is well designed.
I am impressed with the good performance all the way up to 20khz.
For my application though, that frequency range is not imporant.
I am impressed with how small it is, but that is also not an objective for my appilcation.
So of this is the difference between hobby and DIY stuff, versus mass market stuff.
Again, multi-amping is the premise I am starting with, not class D, or small and compact.
I am impressed with the good performance all the way up to 20khz.
For my application though, that frequency range is not imporant.
I am impressed with how small it is, but that is also not an objective for my appilcation.
So of this is the difference between hobby and DIY stuff, versus mass market stuff.
Again, multi-amping is the premise I am starting with, not class D, or small and compact.
You may call me Brian, Brian 🙂
What value will the inductors in your output filter be?
More importantly, what class d topology will your amplifier be?
What value will the inductors in your output filter be?
More importantly, what class d topology will your amplifier be?
As far as the high frequency on the conductors?
No, I don't go along with that. I will have some of that myself. If that was really desired, linear amps could be made to do that.
The basic premise is that the wires are linear. If they are not, we are in trouble.
Its not like magnetic recording tape.
I beleive that some of this stuff is more metaphysics, and typical of what you see in "High End" Audio circles.
Do you have one of these UDC amps? I'm sure it works very well.
As far as heat sink size, I feel that what they have is practical. It is more of a thermal mass. I'm sure it is a practical way of meeting their objectives, small size being one.
When I speak of a big heat sink, I mean one that lets you come closer to the data sheet disapation level. For home audio, I see this as being a large aluminum heat sink, positioned to promote convection. But also put the transistors on live copper carriers. Then the mica insulator surface can be about 3 square inches. Often these mica insulators are the biggest thermal resistance.
I know that for may applications such a bulky approach would be undesired. Again, DIY audio is often done for different objectives.
Big heat sinks are one of the things solid state can have to make it showy like tube amps.
No, I don't go along with that. I will have some of that myself. If that was really desired, linear amps could be made to do that.
The basic premise is that the wires are linear. If they are not, we are in trouble.
Its not like magnetic recording tape.
I beleive that some of this stuff is more metaphysics, and typical of what you see in "High End" Audio circles.
Do you have one of these UDC amps? I'm sure it works very well.
As far as heat sink size, I feel that what they have is practical. It is more of a thermal mass. I'm sure it is a practical way of meeting their objectives, small size being one.
When I speak of a big heat sink, I mean one that lets you come closer to the data sheet disapation level. For home audio, I see this as being a large aluminum heat sink, positioned to promote convection. But also put the transistors on live copper carriers. Then the mica insulator surface can be about 3 square inches. Often these mica insulators are the biggest thermal resistance.
I know that for may applications such a bulky approach would be undesired. Again, DIY audio is often done for different objectives.
Big heat sinks are one of the things solid state can have to make it showy like tube amps.
Brian, I haven't figured out the inductor values.
I've designed filters before. I've made tables for filter design. I've written progams for filter design.
This was all years ago.
There would be four inductors and four capacitors. They will not be the same sized.
But the total of all the four inductors and the total of all the capacitors would be about the same as a second order LP speaker cross over for the same impedance, at the same 1000hz.
Some people sell foil wound inductors for speaker cross overs.
But I can tollerate much more resistance because of the feedback loop, and because of its narrow bandwidth.
Lets say for 4 ohms, the inductance would total 0.7 milli henrys.
The capacitance would total 40uF. ( these values are sold in non-electrolytic from specialty audio sources for speaker cross overs.
As far as class D topology, I assume you mean the output transistor configuration?
I do not know . First I wanted to get the minimum frequency down, these physiological issues.
I have only thought about this a bit. It will be influenced by switching power supplies.
But this is different because I can't bias the speaker.
Perhaps it would use two NPN transistors stacked, with the output to the filter between them.
So the top one is an emitter follower, and the bottom one is common emitter.
This is usually called quasi complementary.
Maybe?
The biggest bipolar I know of is in the Motorola Catalog.
350 Watt dissapation, in a TO-3 package.
What this means is that if you could keep the case at 25 deg C, and were willing to tollerate 175 deg C junction temperatures, you could dissapate 350 Watts continuously.
In practice, using the kind of big heat sinks I describe, you could dissapate about a 3rd of that continuously.
Now, this is for music power, not steady power. So how much peak power it could do is very subjective. Best to have thermal shut offs.
The biggest bipolars for linear audio are usually 250W each, rated in the same way. Two of them, on the kinds of heat sinks I describe, can do about 200W musical peaks.
I would like to think that two of the 350 Watt bipolars I specify could do 1000W in switching, but only at modest frequency.
Just for comparison, as I know, the high power MOS transistors used for linear audio are rated at lower dissapation.
But the dissapation numbers are not the real issue when comparing MOS and Bipolar for class D. Then it is the on resistance, and the switching speed relative to the switching frequency being used.
I know that many designs don't use the scale of heat sinks I am wanting here. That is their right, and often entirely appropriate to their overall design objectives. Transistors are often cheaper than heatsinks.
Brian
If you have links to big transistors, I'd like to see.
I've designed filters before. I've made tables for filter design. I've written progams for filter design.
This was all years ago.
There would be four inductors and four capacitors. They will not be the same sized.
But the total of all the four inductors and the total of all the capacitors would be about the same as a second order LP speaker cross over for the same impedance, at the same 1000hz.
Some people sell foil wound inductors for speaker cross overs.
But I can tollerate much more resistance because of the feedback loop, and because of its narrow bandwidth.
Lets say for 4 ohms, the inductance would total 0.7 milli henrys.
The capacitance would total 40uF. ( these values are sold in non-electrolytic from specialty audio sources for speaker cross overs.
As far as class D topology, I assume you mean the output transistor configuration?
I do not know . First I wanted to get the minimum frequency down, these physiological issues.
I have only thought about this a bit. It will be influenced by switching power supplies.
But this is different because I can't bias the speaker.
Perhaps it would use two NPN transistors stacked, with the output to the filter between them.
So the top one is an emitter follower, and the bottom one is common emitter.
This is usually called quasi complementary.
Maybe?
The biggest bipolar I know of is in the Motorola Catalog.
350 Watt dissapation, in a TO-3 package.
What this means is that if you could keep the case at 25 deg C, and were willing to tollerate 175 deg C junction temperatures, you could dissapate 350 Watts continuously.
In practice, using the kind of big heat sinks I describe, you could dissapate about a 3rd of that continuously.
Now, this is for music power, not steady power. So how much peak power it could do is very subjective. Best to have thermal shut offs.
The biggest bipolars for linear audio are usually 250W each, rated in the same way. Two of them, on the kinds of heat sinks I describe, can do about 200W musical peaks.
I would like to think that two of the 350 Watt bipolars I specify could do 1000W in switching, but only at modest frequency.
Just for comparison, as I know, the high power MOS transistors used for linear audio are rated at lower dissapation.
But the dissapation numbers are not the real issue when comparing MOS and Bipolar for class D. Then it is the on resistance, and the switching speed relative to the switching frequency being used.
I know that many designs don't use the scale of heat sinks I am wanting here. That is their right, and often entirely appropriate to their overall design objectives. Transistors are often cheaper than heatsinks.
Brian
If you have links to big transistors, I'd like to see.
At the end of the thread Chip Amps-> Chip amps with built in heat sinks, we are talking about such heat sinks.
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