Kilowatt and Fresh-T, you guys are definitely taking the road less travelled!
Kilowatt I think I understand that you're building this primarily for your self, but haven't excluded PA use.
The next paragraph is my memory of articles I read in the early 80s. I'm sure that there are those on this forum that are more up to date on psychoacoustics, please jump in and correct the following if you disagree.
Kilowatt, you may have noticed that horn loaded speakers, (mids and highs included) aren't very popular in hifi. They sound spectacular but few people can live with them. Horns produce peaks and valleys in the frequency response, called comb filtering, these occur close together and often don't show on the plots. The brain, cleaver beasty that it is, recognises the response, (after all horn loading occurs in nature) and actively tries to flatten the response. After an hour of concentration you can get very fatigued. I find myself turning down the volume after about 30 mins and I'm not afraid of a few dBs. You don't get fatigued at concerts because of the ambient noise.
I stress that that is my take on horns. I repeat my invitation if more research has been done since then.
Regarding the use of 2N3055s and MJ2955s, I know that they can operate at this voltage but they do fail, often. I speak from experience there, (25 years in the repair business). The manufacturers specify 60v Vce, remember aluminium does burn, (the Audiobahns)!
Guys if we can't coax you down the path usually taken of this type of endeavour, please be careful. Hell you probably only get to go around once, might as well enjoy it this time.
Regards WALKER
Kilowatt I think I understand that you're building this primarily for your self, but haven't excluded PA use.
The next paragraph is my memory of articles I read in the early 80s. I'm sure that there are those on this forum that are more up to date on psychoacoustics, please jump in and correct the following if you disagree.
Kilowatt, you may have noticed that horn loaded speakers, (mids and highs included) aren't very popular in hifi. They sound spectacular but few people can live with them. Horns produce peaks and valleys in the frequency response, called comb filtering, these occur close together and often don't show on the plots. The brain, cleaver beasty that it is, recognises the response, (after all horn loading occurs in nature) and actively tries to flatten the response. After an hour of concentration you can get very fatigued. I find myself turning down the volume after about 30 mins and I'm not afraid of a few dBs. You don't get fatigued at concerts because of the ambient noise.
I stress that that is my take on horns. I repeat my invitation if more research has been done since then.
Regarding the use of 2N3055s and MJ2955s, I know that they can operate at this voltage but they do fail, often. I speak from experience there, (25 years in the repair business). The manufacturers specify 60v Vce, remember aluminium does burn, (the Audiobahns)!
Guys if we can't coax you down the path usually taken of this type of endeavour, please be careful. Hell you probably only get to go around once, might as well enjoy it this time.
Regards WALKER
Walker, The MJ2955 and 2N2955 transistors won't see too much voltage because there is 3 in series, 6 in parallel.
I'm not going to do this any other way but the way I've talked about because I need to do it cheap and fast, so I would love to get some advice on making the 1800W amps. 🙂
[Edited by Kilowatt on 11-19-2001 at 03:17 PM]
I'm not going to do this any other way but the way I've talked about because I need to do it cheap and fast, so I would love to get some advice on making the 1800W amps. 🙂
[Edited by Kilowatt on 11-19-2001 at 03:17 PM]
Kilowatt
You state that you have decided on what you are going do so why ask for advice, particularly as you seem to be prepared to ignore that which has already been given. However, I will make a couple of further observations.
Your design requirement is for 1800Wrms into 6ohm and you intend to use an output stage comprising six parallel banks of three series transistors (per rail), which must be 2N3055/MJ2955.
For 1800Wrms into 6ohm you need a peak output voltage of 147V and a peak output current of 24.5A. The supply rail voltage will need to be somewhat higher, but for ease of calculation I will use 150V, and a maximum current of 24V.
Ignoring any imbalance due to device characteristic variations (which will occur), the maximum voltage across each 2N3055 will be 50V and the maximum current through it will be 4A. You will be feeding an inductive load which can typically have up to 60deg phase shift so the maximum power dissipation in each device will occur at maximum voltage (50V) and half the maximum current (2A). This point is outside the SOA curve for a 2N3055 and reliability will be seriously affected (ie the device will fail).
The average power dissipation in each device will be Vmax*Imax/4 or 50W. With a junction to case thermal resistance of 1.5degC/W and a typical case to heatsink thermal resistance of 0.5degC/W, the junction will be 100degC above the heatsink temperature. To maintain some safety margin, I will assume a maximum junction temperature of 150degC or a maximum heatsink temperature of 50degC. To cater for an ambient temperature of 25degC, a heatsink rated at 0.5 degC/W will be required for each output device. You will require 36 of these for each amp. This is one hell of a lot of metal (not to mention expense), particularly when multiplied by four for your bridged, two-channel arrangement.
Geoff
You state that you have decided on what you are going do so why ask for advice, particularly as you seem to be prepared to ignore that which has already been given. However, I will make a couple of further observations.
Your design requirement is for 1800Wrms into 6ohm and you intend to use an output stage comprising six parallel banks of three series transistors (per rail), which must be 2N3055/MJ2955.
For 1800Wrms into 6ohm you need a peak output voltage of 147V and a peak output current of 24.5A. The supply rail voltage will need to be somewhat higher, but for ease of calculation I will use 150V, and a maximum current of 24V.
Ignoring any imbalance due to device characteristic variations (which will occur), the maximum voltage across each 2N3055 will be 50V and the maximum current through it will be 4A. You will be feeding an inductive load which can typically have up to 60deg phase shift so the maximum power dissipation in each device will occur at maximum voltage (50V) and half the maximum current (2A). This point is outside the SOA curve for a 2N3055 and reliability will be seriously affected (ie the device will fail).
The average power dissipation in each device will be Vmax*Imax/4 or 50W. With a junction to case thermal resistance of 1.5degC/W and a typical case to heatsink thermal resistance of 0.5degC/W, the junction will be 100degC above the heatsink temperature. To maintain some safety margin, I will assume a maximum junction temperature of 150degC or a maximum heatsink temperature of 50degC. To cater for an ambient temperature of 25degC, a heatsink rated at 0.5 degC/W will be required for each output device. You will require 36 of these for each amp. This is one hell of a lot of metal (not to mention expense), particularly when multiplied by four for your bridged, two-channel arrangement.
Geoff
P.Lacombe said:
Hi Pierre,
Check out the QSC webpage.
http://www.qscaudio.com/
These guys make excellent pro sound amps and have full schematics for all of their amps online. Take a look at the PLX3402. This is a 3400 Watt pa amp (bridged into a 4 ohm load, or 1700 watts per channel into 2 ohm loads) in a 2 space rack cabinet. These amps are much lighter than the competition (QSC has really got switching power supplies figured out).
I highly recommend these amps. I have a PLX1602 and am saving up for a PLX3402. These amps are reliable, perform great and are a steal (best price I've found on the PLX3402 is just under $1200). These are full range amps too, not just subwoofer amps.
Phil Ouellette
Geoff,
You misunderstand what kilowatt wants, 1800W into 6ohms is the brdged output .... the load is 12ohms ...... so each amp sees 6ohms and outputs 1800W ........
This will still need an awful lot of devices and heatsinking ..... but its doable.
You misunderstand what kilowatt wants, 1800W into 6ohms is the brdged output .... the load is 12ohms ...... so each amp sees 6ohms and outputs 1800W ........
This will still need an awful lot of devices and heatsinking ..... but its doable.
Geoff
First of all, there is still much planning that I must do, and I am thankfull for the advice I have recieved so far. It is good to know what problems people suspect this project may have so I know what to expect. I have come to the conclusion that my original plan would be best for my purposes, and my budget.
I was considering (actually planning on) not having that much of a heat sink, just 4 plates with 36 devices on each one, and a small heatsink on the top of each device. For added cooling however, there would be some sort of serious refidgeration unit in front of the cooling fan, or even liquid cooling pipes on the heatsink. How would that be?
Slight variations in device characteristics shouldn't be a big problem because there will a low value power resistor in series with EACH power transistor.
What's this you mention in your fourth paragraph about the devices failing because of phase shift? All inductive loads have that and other amps don't spontaniously fail. Why would this one? What should I do about it? I can't just have my amp blowing up every 10 seconds!
Geoff & AudioFreak,
Just to clear things up, the whole amp consists of 4 sections. Each one is 1800W, 6 ohms. Each of two channels has two of these sections in bridge config. for 3600W into 12 ohms.
[Edited by Kilowatt on 11-19-2001 at 06:18 PM]
First of all, there is still much planning that I must do, and I am thankfull for the advice I have recieved so far. It is good to know what problems people suspect this project may have so I know what to expect. I have come to the conclusion that my original plan would be best for my purposes, and my budget.
I was considering (actually planning on) not having that much of a heat sink, just 4 plates with 36 devices on each one, and a small heatsink on the top of each device. For added cooling however, there would be some sort of serious refidgeration unit in front of the cooling fan, or even liquid cooling pipes on the heatsink. How would that be?
Slight variations in device characteristics shouldn't be a big problem because there will a low value power resistor in series with EACH power transistor.
What's this you mention in your fourth paragraph about the devices failing because of phase shift? All inductive loads have that and other amps don't spontaniously fail. Why would this one? What should I do about it? I can't just have my amp blowing up every 10 seconds!
Geoff & AudioFreak,
Just to clear things up, the whole amp consists of 4 sections. Each one is 1800W, 6 ohms. Each of two channels has two of these sections in bridge config. for 3600W into 12 ohms.
[Edited by Kilowatt on 11-19-2001 at 06:18 PM]
yep ..... the inductive load problem is nothing specific to ur amp ....... to get around it, you just use more (or higher rated) output devices.
Kilowatt: You might want to check previous threads in the SS forum. There was a good discussion on water-cooled amplification.
AudioFreak
There is no misunderstanding, at least not on my part. The supply rail voltage, maximum output voltage and maximum output current remain the same for an individual amp whether you consider the power requirements as 1800Wrms into 6ohm or 3600Wrms into 12ohm in bridged mode, so the figures I quoted are correct.
Kilowatt
With regard to the phase shift into inductive loads, if the load were purely resistive the maximum current through an output device would occur when there is the minimum voltage across it or conversely the maximum voltage would coincide with the minimum current, so the maximum power dissipation in the device would be relatively low (assuming a Class-B output stage). When the load is inductive, the current waveform lags the voltage waveform and so this does not apply. It has been found that with typical speakers the phase shift can approach 60deg and with this degree of phase shift the maximum power dissipation occurs when voltage is at a maximum and the current is at half its maximum value.
As AudioFreak said, this applies to all amplifiers and so must be taken into account when sizing the output stage devices. You will need to check the SOA graphs for the devices you intend to use and ensure that the maximum voltage / half maximum current point falls well within the SOA curve otherwise the device will fail. As AudioFreak also said, this can be achieved by using more devices or by using devices with a better specification. I would suggest the latter. Have a look at some of the other MJ series power transistors such as the MJ21193/21194 . The maximum voltage rating is far higher so you will only need two in series and they have a far greater SOA. These devices may be more expensive than the 3055/2955s but you will need fewer of them. An added benefit is that the junction-case thermal resistance is less than half that for the 3055/2955 so heatsink problems will be minimised.
I will admit that in my last post I presented the worst case scenario with regard to heatsinking in that I quoted sizes for natural convection. However, you will still need far more than you are anticipating. With forced air cooling at 2 to 4m/s air speed, the size per output device will reduce from 0.5degC/W to 2degC/W (based on your 18 2N3055s, though this will obviously change as these will not meet the load requirements). Chilled air will achieve a further reduction in heatsink size but there is a limit to how far the surface area of the heatsink (and therefore size/mass) can be reduced whilst still being able to dissipate the heat.
You mentioned in your last post that you will have resistors in series with each output transistor. Please bear in mind that this will increase the voltage drop in the output circuit and you will need to increase the supply rail voltage to suit (or accept a reduced output power).
Finally, I must reiterate my warning about directly rectifying the mains and must again urge you to rethink your design. As others have pointed out, the lack of a transformer will allow any mains borne noise and interference to pass through to the amplifier. Your statement that you still have the smoothing capacitors is not correct in that in a traditional power supply the capacitors react with the transformer impedance to produce a low pass filter. This will be absent in your design. However, I don't give a damn about how the amp sounds but I do care about safety. Just think about the possible component failure situations and the opportunities for a low impedance (high current) mains supply to be present at the output terminals or on the casework of your amps.
Geoff
[Edited by Geoff on 11-19-2001 at 08:25 PM]
There is no misunderstanding, at least not on my part. The supply rail voltage, maximum output voltage and maximum output current remain the same for an individual amp whether you consider the power requirements as 1800Wrms into 6ohm or 3600Wrms into 12ohm in bridged mode, so the figures I quoted are correct.
Kilowatt
With regard to the phase shift into inductive loads, if the load were purely resistive the maximum current through an output device would occur when there is the minimum voltage across it or conversely the maximum voltage would coincide with the minimum current, so the maximum power dissipation in the device would be relatively low (assuming a Class-B output stage). When the load is inductive, the current waveform lags the voltage waveform and so this does not apply. It has been found that with typical speakers the phase shift can approach 60deg and with this degree of phase shift the maximum power dissipation occurs when voltage is at a maximum and the current is at half its maximum value.
As AudioFreak said, this applies to all amplifiers and so must be taken into account when sizing the output stage devices. You will need to check the SOA graphs for the devices you intend to use and ensure that the maximum voltage / half maximum current point falls well within the SOA curve otherwise the device will fail. As AudioFreak also said, this can be achieved by using more devices or by using devices with a better specification. I would suggest the latter. Have a look at some of the other MJ series power transistors such as the MJ21193/21194 . The maximum voltage rating is far higher so you will only need two in series and they have a far greater SOA. These devices may be more expensive than the 3055/2955s but you will need fewer of them. An added benefit is that the junction-case thermal resistance is less than half that for the 3055/2955 so heatsink problems will be minimised.
I will admit that in my last post I presented the worst case scenario with regard to heatsinking in that I quoted sizes for natural convection. However, you will still need far more than you are anticipating. With forced air cooling at 2 to 4m/s air speed, the size per output device will reduce from 0.5degC/W to 2degC/W (based on your 18 2N3055s, though this will obviously change as these will not meet the load requirements). Chilled air will achieve a further reduction in heatsink size but there is a limit to how far the surface area of the heatsink (and therefore size/mass) can be reduced whilst still being able to dissipate the heat.
You mentioned in your last post that you will have resistors in series with each output transistor. Please bear in mind that this will increase the voltage drop in the output circuit and you will need to increase the supply rail voltage to suit (or accept a reduced output power).
Finally, I must reiterate my warning about directly rectifying the mains and must again urge you to rethink your design. As others have pointed out, the lack of a transformer will allow any mains borne noise and interference to pass through to the amplifier. Your statement that you still have the smoothing capacitors is not correct in that in a traditional power supply the capacitors react with the transformer impedance to produce a low pass filter. This will be absent in your design. However, I don't give a damn about how the amp sounds but I do care about safety. Just think about the possible component failure situations and the opportunities for a low impedance (high current) mains supply to be present at the output terminals or on the casework of your amps.
Geoff
[Edited by Geoff on 11-19-2001 at 08:25 PM]
Sorry Geoff ........ Just realised that you meant multiply by 4 because there will be 4 amps 🙂 ..... just put that one down to human stupidity and my abundance of it 🙂
Geoff
Are you sure I'd have to use two MJ21193/MJ21194 transistors in series? They're rated 250V. That should be plenty shouldn't it. Anyway, I'll look into some other transistors and see which ones offer the best Pd/price. About what total Pd do you think I should have for each side of each 1800W output bank?
Also, wouldn't my power supply caps react with the impedance of the power distribution transformer and the cable coming into my house just as they do in traditional power supplies? Of course I have to admit that pole pigs aren't exactly audio grade.
I don't think safety will be much of a hazard since the case is not conductive and people will be staying away from the speaker hookups anyway since they will normally run at a fairly dangerous voltage.
Are you sure I'd have to use two MJ21193/MJ21194 transistors in series? They're rated 250V. That should be plenty shouldn't it. Anyway, I'll look into some other transistors and see which ones offer the best Pd/price. About what total Pd do you think I should have for each side of each 1800W output bank?
Also, wouldn't my power supply caps react with the impedance of the power distribution transformer and the cable coming into my house just as they do in traditional power supplies? Of course I have to admit that pole pigs aren't exactly audio grade.
I don't think safety will be much of a hazard since the case is not conductive and people will be staying away from the speaker hookups anyway since they will normally run at a fairly dangerous voltage.
I'm considering using 16 x MJ15003/MJ15004 transistors for each output bank. That's 64 of each. They are rated 140Vceo, 250W. I would have to use two in series, 8 in parallel. Their thermal resistance is 0.7degC/W. They fall slightly within the SOA taking into account any phase shift. Am I right on that one? Here's a link to the data sheet:
http://www.onsemi.com/pub/Collateral/MJ15003-D.PDF
I'm displeased with the $310.40 price tag this setup carries with it, but it will do, and I may be able to find them for less. If you know of a supplier where I can get these transistors for less, please let me know.
I wish I could test the amp before sticking $310 worth of transistors in it and praying that they won't blow.
http://www.onsemi.com/pub/Collateral/MJ15003-D.PDF
I'm displeased with the $310.40 price tag this setup carries with it, but it will do, and I may be able to find them for less. If you know of a supplier where I can get these transistors for less, please let me know.
I wish I could test the amp before sticking $310 worth of transistors in it and praying that they won't blow.
Kilowatt,
At the rate you're going, they <i>will</i> blow...
Please...do yourself a favor. Take a little time and learn some basic electronics. This is not a trivial undertaking you're talking about. Several people have warned you that you're rushing in where angels fear to tread, yet you've blithely ignored anything that wasn't what you wanted to hear.
You'd be better off buying a few used Crest or QSC amps and go tilting at windmills that way. Furthermore, they'd have some resale value when you're done. This circuit you're talking about building will have no value, particularly if it's prone to self-destructing; very likely taking speakers with it. You may be able to play it at your house (assuming that you're able to come up with a functional circuit), but you won't be able to take it on the road because 240V lines aren't common in halls. Why? Because everything's designed for 120V (at least here in the US--planning on going overseas with this thing?).
Not to mention the danger aspect.
If you don't have any money, go to the library. Check out some books on electronics. Read them. Horowitz and Hill, The Art of Electronics, is a good one to start with. You might even consider an electronics course at a local night school. Get a bit of grounding in electronics and perhaps you'll understand why a quarter of the posts in this thread have been, shall we say, cautionary in nature.
Grey
At the rate you're going, they <i>will</i> blow...
Please...do yourself a favor. Take a little time and learn some basic electronics. This is not a trivial undertaking you're talking about. Several people have warned you that you're rushing in where angels fear to tread, yet you've blithely ignored anything that wasn't what you wanted to hear.
You'd be better off buying a few used Crest or QSC amps and go tilting at windmills that way. Furthermore, they'd have some resale value when you're done. This circuit you're talking about building will have no value, particularly if it's prone to self-destructing; very likely taking speakers with it. You may be able to play it at your house (assuming that you're able to come up with a functional circuit), but you won't be able to take it on the road because 240V lines aren't common in halls. Why? Because everything's designed for 120V (at least here in the US--planning on going overseas with this thing?).
Not to mention the danger aspect.
If you don't have any money, go to the library. Check out some books on electronics. Read them. Horowitz and Hill, The Art of Electronics, is a good one to start with. You might even consider an electronics course at a local night school. Get a bit of grounding in electronics and perhaps you'll understand why a quarter of the posts in this thread have been, shall we say, cautionary in nature.
Grey
Kilowatt, I know you won't want to hear it but, Grey is absolutly right. The path that you are taking is NOT the cheap one. I not a betting man but the result is blindingly obious, especially to those of us who have smoked the occasional silicon in the past.
Even if you do find a suitable schematic, (I don't think anyone on the forum has the time to develop one for you) you will probably have to go through a number of generations of circiut layout to achieve low noise and stability, (an unstable amp destroys it's self in milliseconds).
Some have suggested switching amps or supplies, these are even more likely to self destruct if the tracks are the wrong length. Not what I would suggest for a first amp.
Volume and quality don't come cheap, there aren't any shortcuts, if you build it yourself you may cut the cost to 1/4 if your lucky. You won't build a $40 000 system for under $1000.
Sorry,
Regards WALKER
Even if you do find a suitable schematic, (I don't think anyone on the forum has the time to develop one for you) you will probably have to go through a number of generations of circiut layout to achieve low noise and stability, (an unstable amp destroys it's self in milliseconds).
Some have suggested switching amps or supplies, these are even more likely to self destruct if the tracks are the wrong length. Not what I would suggest for a first amp.
Volume and quality don't come cheap, there aren't any shortcuts, if you build it yourself you may cut the cost to 1/4 if your lucky. You won't build a $40 000 system for under $1000.
Sorry,
Regards WALKER
GRollins,
Actually, I have read quite a bit about electronics, but just reading is no substitute for experience, and I do have some experience. However, all the basic electronic principles (which I already know) won't help me much more than it already has. I need some books and/or some information about designing audio circuits to complete this project, especially if I want it to be reliable and good sounding. I wouldn't go so far as to say that it wouldn't work if I tried it now, which is what some seem to think. I think I have the output stage fairly well planned. I still need to find a circuit to drive that. I also plan on having some sort of output short circuit protection.
Really now, have you ever used a rectified line power supply? It does seem reasonable to think that there may be some interferance from such a thing, but as long as the line is clear, it should work, especially with some inductive filtering. There's got to be a way to do away with the transformer. You must know that car electrical systems are a lot noisier than the AC line, and they use switching regs and filtering to make an audio grade power supply with it, so why need a transformer here? I have no good evidence to suggest that that idea would leave me with a dead amp.
By the way, I haven't ignored all the advice that I didn't want to hear. It's a good thing that someone alerted me to the fact that I did not take power factor into account in my original plan, so I now plan on using different, more expensive output transistors. Among other good advice. I got onto this forum to see what people thought I should do about this project.
There are amps like this out there, and bigger. Harrison Labs makes a 20,000W RMS amp. It runs on batteries, I think. It can be done. It's just like building a smaller amp, only much bigger and more challenging.
I may have sounded like I'm going to start putting this thing together tomorrow, but that is not the case. I do however want to stay fairly close to my original design, because I know it is workable.
Actually, I have read quite a bit about electronics, but just reading is no substitute for experience, and I do have some experience. However, all the basic electronic principles (which I already know) won't help me much more than it already has. I need some books and/or some information about designing audio circuits to complete this project, especially if I want it to be reliable and good sounding. I wouldn't go so far as to say that it wouldn't work if I tried it now, which is what some seem to think. I think I have the output stage fairly well planned. I still need to find a circuit to drive that. I also plan on having some sort of output short circuit protection.
Really now, have you ever used a rectified line power supply? It does seem reasonable to think that there may be some interferance from such a thing, but as long as the line is clear, it should work, especially with some inductive filtering. There's got to be a way to do away with the transformer. You must know that car electrical systems are a lot noisier than the AC line, and they use switching regs and filtering to make an audio grade power supply with it, so why need a transformer here? I have no good evidence to suggest that that idea would leave me with a dead amp.
By the way, I haven't ignored all the advice that I didn't want to hear. It's a good thing that someone alerted me to the fact that I did not take power factor into account in my original plan, so I now plan on using different, more expensive output transistors. Among other good advice. I got onto this forum to see what people thought I should do about this project.
There are amps like this out there, and bigger. Harrison Labs makes a 20,000W RMS amp. It runs on batteries, I think. It can be done. It's just like building a smaller amp, only much bigger and more challenging.
I may have sounded like I'm going to start putting this thing together tomorrow, but that is not the case. I do however want to stay fairly close to my original design, because I know it is workable.
Walker, I know it's not cheap, but this is no $40,000 system. I could buy a system twice this powerful with fair sound quality, ready made, for less than that.
I don't plan on using any switching. I'm not the digital type, but I've never heard of a class AB audio amp failing because of bad layout. You're very right though that it would probably be impossibly hard to achieve very low distortion at high power levels from this.
[Edited by Kilowatt on 11-20-2001 at 01:40 AM]
I don't plan on using any switching. I'm not the digital type, but I've never heard of a class AB audio amp failing because of bad layout. You're very right though that it would probably be impossibly hard to achieve very low distortion at high power levels from this.
[Edited by Kilowatt on 11-20-2001 at 01:40 AM]
Kilowatt, we are trying to be helpful but it isn't easy when there are so many issues, danger and possibly cost in front of you. We can't see all the pit falls you're likely to encounter form here. It would be irresponsible if we didn't warn you.
Regarding class AB amps destroying themselves due to bad layout, it definitely does happen especially with MOSFETS. I know that you are planning on using fairly slow BJTs but they are not totally immune. IT DOES HAPPEN! Experience speaking here mate, I used to do a fair trade in repairing home built amps.
Regards WALKER
Regarding class AB amps destroying themselves due to bad layout, it definitely does happen especially with MOSFETS. I know that you are planning on using fairly slow BJTs but they are not totally immune. IT DOES HAPPEN! Experience speaking here mate, I used to do a fair trade in repairing home built amps.
Regards WALKER
Thanks, I'd probably be wasting a lot of money on blown transistors if it wasn't for the forum, but I know of some basic amp designs that claim very high stability. If I could ever get 1800W out of something like that, I'd be able to do this ok. I hope I can get this project done before too long, but I do have a lot of planning left.
To Kilowatt
You will need a much larger Heatsink
A 1800W class A/B will disspate ca (1800W/4=) 450W heat at max power!! (With "normal" quiesent current)
If you accept 70 degC max temp on heatsink, it would be 45 deg over ambient.
The heatsink must be: 45deg/450w = 0.1deg/watt/ch
Or: 0.3deg/watt/ch with a very large fan!!
If you skip on this, the amp will blow on high output power.
Use no less than 0.47 ohm emitter resistors when you are using that many transistors in parallell.
By the way. Can you explain to me how you get +/- voltage from the mains, by just rectrifieing. Wouldn't you just get a gnd and a Vcc??
You will need a much larger Heatsink
A 1800W class A/B will disspate ca (1800W/4=) 450W heat at max power!! (With "normal" quiesent current)
If you accept 70 degC max temp on heatsink, it would be 45 deg over ambient.
The heatsink must be: 45deg/450w = 0.1deg/watt/ch
Or: 0.3deg/watt/ch with a very large fan!!
If you skip on this, the amp will blow on high output power.
Use no less than 0.47 ohm emitter resistors when you are using that many transistors in parallell.
By the way. Can you explain to me how you get +/- voltage from the mains, by just rectrifieing. Wouldn't you just get a gnd and a Vcc??
- Status
- Not open for further replies.