there is no standard voltage.
You can use any sensible supply voltage, but you need to check the currents through all the stages. A few component values will need to change with big voltage changes.
You can use any sensible supply voltage, but you need to check the currents through all the stages. A few component values will need to change with big voltage changes.
Reference...background
Dr. Marshall Leach of Georgia Tech began providing this information a long, long time ago.
http://users.ece.gatech.edu/~mleach/lowtim/part3.html
See his mention of the wattage rating for resistors.
I bring forward his paragraph regarding value adjustment for resistors.
Note these resistor designators do not match those on the 12 device board we are using.
Dr. Marshall Leach of Georgia Tech began providing this information a long, long time ago.
http://users.ece.gatech.edu/~mleach/lowtim/part3.html
See his mention of the wattage rating for resistors.
I bring forward his paragraph regarding value adjustment for resistors.
Note these resistor designators do not match those on the 12 device board we are using.
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Hello Ed and Andrew,
Thank you very much for the info. I had already read Mr. Leach's website, but wasn't sure if Jen's circuit was that flexible.
Well, I have a few questions:
1) By using the transistors listed on Jen's BOM (1.03.12) is it safe to use +/-65V rails?
2) By using +/-65V rails and only 6 output transistor, is it safe to run the amplifier on 4 ohm load?
3) By using +/-65V rails and 12 output transistor, is it safe to run the amplifier on 2 ohm load?
Sorry for all that questions, and thanks again for the attention!
Ed, your gradchildren are very lucky to have a grandfather that drives 3 1/2 hours to see them. Very nice! 🙂
Thank you very much for the info. I had already read Mr. Leach's website, but wasn't sure if Jen's circuit was that flexible.
Well, I have a few questions:
1) By using the transistors listed on Jen's BOM (1.03.12) is it safe to use +/-65V rails?
2) By using +/-65V rails and only 6 output transistor, is it safe to run the amplifier on 4 ohm load?
3) By using +/-65V rails and 12 output transistor, is it safe to run the amplifier on 2 ohm load?
Sorry for all that questions, and thanks again for the attention!
Ed, your gradchildren are very lucky to have a grandfather that drives 3 1/2 hours to see them. Very nice! 🙂
I run the 3pair Jen's clone on +-58.5Vdc and it drives 8r0 and 8ohm well within it's capabilities.
The model I have used to investigate the temp de-rated SOAR with various reactive loads says that it can drive a 6ohm speaker load, but using my criteria it cannot drive a 4ohm load from +-58.5Vdc, unless the 4ohm speaker is only of moderate reactance.
It can easily drive a 2r6 load but that is only a test load not a speaker load.
Thank you Andrew,
I guess I'll assemble the board will all the 12 output. This way I can safely run it on +-65V rails and use 4 ohms load, correct?
Thank you!
I guess I'll assemble the board will all the 12 output. This way I can safely run it on +-65V rails and use 4 ohms load, correct?
Thank you!
I'll switch on the spreadsheet and come back.
Which output devices are you using?
What will be the heatink temp at quiescent operation?
What smoothing capacitance are you fitting?
What is the transformer voltage & regulation?
What value of output Re are you fitting?
Which output devices are you using?
What will be the heatink temp at quiescent operation?
What smoothing capacitance are you fitting?
What is the transformer voltage & regulation?
What value of output Re are you fitting?
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Hello Andrew,
I'm using the MJL1302/3281 pair.
The transformer is ywt to be bought, but I it's very easy to obtain a toroid here - the factory can make a special order with any voltage I want, and the proce is fair.
I guess I'll order a 47-0-47 VAC (unloaded) 400VA for each channel (I'm going for the dual mono approach).
I'll build a capacitance bank - about 44.000uF for each channel.
I'll certainly use a soft start ...
I haven't bought the heatsink yet, but again I have another factory that can cut about any size I want, and their catalog is very large.
Thank you very much!
I'm using the MJL1302/3281 pair.
The transformer is ywt to be bought, but I it's very easy to obtain a toroid here - the factory can make a special order with any voltage I want, and the proce is fair.
I guess I'll order a 47-0-47 VAC (unloaded) 400VA for each channel (I'm going for the dual mono approach).
I'll build a capacitance bank - about 44.000uF for each channel.
I'll certainly use a soft start ...
I haven't bought the heatsink yet, but again I have another factory that can cut about any size I want, and their catalog is very large.
Thank you very much!
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using +-65Vdc from a 4.5% regulation transformer (~45Vac), with +-23500uF, Re=0.33, Tsink~50degC (Tc<60degC) I see ~370W into 4ohm 60degree phase angle and >800W into 1r3, if you can keep the Tc from rising above 60degC.
Wow, that's very nice Andrew!
Seems a good goal to me. Actually, using loads below 4 ohms is out of the question. At tops, I'll use a 4 ohms nominal loudspeaker, that could have a impedance curve that goes down to about 3 or 2.5 ohms worst case.
I've just checked my heatsink supplier, and they don't have a large enough heatsink to fit the board. The solution would be to buy a 15cm width heatsink and have it cut to about 35cm long. The problem is that this way the fins wouldn'tbe in the best position to heat transfer. I don't know if I made myself clear.
That's the heatsink I intend to use. It's rated at 0,78ºC/W/4". As you can see, it's about 15,5cm width. This would be the chassis height. I would have it cut to about 35cm long, what would be the chassis depth.
Do you think it would be better to take two of this heatsinks and join them somehow with a aluminum plate, making a larger heatsink? This way the fins would be in the best position to heat transfer, because the air could go up through the fins ...
What's the best choice?
Seems a good goal to me. Actually, using loads below 4 ohms is out of the question. At tops, I'll use a 4 ohms nominal loudspeaker, that could have a impedance curve that goes down to about 3 or 2.5 ohms worst case.
I've just checked my heatsink supplier, and they don't have a large enough heatsink to fit the board. The solution would be to buy a 15cm width heatsink and have it cut to about 35cm long. The problem is that this way the fins wouldn'tbe in the best position to heat transfer. I don't know if I made myself clear.
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That's the heatsink I intend to use. It's rated at 0,78ºC/W/4". As you can see, it's about 15,5cm width. This would be the chassis height. I would have it cut to about 35cm long, what would be the chassis depth.
Do you think it would be better to take two of this heatsinks and join them somehow with a aluminum plate, making a larger heatsink? This way the fins would be in the best position to heat transfer, because the air could go up through the fins ...
What's the best choice?
Miguel,
Do you mind sharing the heatsink catalog with us? It sounds like you have some good audio equipment suppliers close at hand.
Tad
Do you mind sharing the heatsink catalog with us? It sounds like you have some good audio equipment suppliers close at hand.
Tad
0.78C/W might get too hot on an optimum bias ClassAB 6pair.
The optimum bias for Re=0r33 would be approximately 22mVre to 25mVre.
This will result in 12 * 65 * 0.022 / 0.33 = 52W.
delta Ts ~ 41Cdegrees.
Factor up for Ts-Ta<75Cdegrees, try about 1.1
Ta = 30degC
Ts ~ 75degC.
Tc~ 80degC
Worst case Tc for one device running hotter than the others say Tc<=85degC.
That is a lot worse than the 60degC that I input into the spreadsheet.
Two of those @ 6inches tall will result in 0.78 / sqrt(6/4) / 2 = 0.32
Delta Ts ~ 52 * 0.32 = 16.5C degrees.
Factor for low Delta, try 1.3
Ta =30degC
Ts ~ 51degC
Tc ~ 56degC
Worst case Tc ~61degC very close to what I assumed.
I reckon you need two pieces of 155 wide by 150 tall to take a 12pair with Re=0r33
Running a 350mm length on it's side will not be as effective and will force Ts higher.
The spread of devices for both schemes is towards the middle. This too will force the Tc higher.
I suggest you look for a 6inch length of 300mm to 350mm wide 0.3C/W for your sink with a 10mm to 12mm thick backplate.
Or blow it like all the PA gear.
BTW,
if you double the height of the 4inch length to 8inch high, still 155mmm wide Rth s-a becomes ~0.55C/W
If you double it again to 16inches tall the backplate will not all be at the same temperature as the device contact points. The Rth s-a can never be as good as 0.39, more likely 0.45.
Turn it on it's side and this could rise another 10% to 0.5C/W.
Add in the de-rating factor for low temperature rise of DF=1.15 and you have your proposal giving about 0.58C/W at 16 inches deep from back to front.
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Re and optimum bias
Hi, just in case you are not aware, changing the Re value changes the bais current.
If the Re=0r33 were changed to 0r68 then the quiescent power dissipated by the 6pr output stage would reduce from ~52W to ~28W.
Now the 0.78C/W sink might just be cool enough.
You would have to fit a temp monitor to the heatsink and decide whether that gives a flashing indication or a latched power off, or some action in between.
Hi, just in case you are not aware, changing the Re value changes the bais current.
If the Re=0r33 were changed to 0r68 then the quiescent power dissipated by the 6pr output stage would reduce from ~52W to ~28W.
Now the 0.78C/W sink might just be cool enough.
You would have to fit a temp monitor to the heatsink and decide whether that gives a flashing indication or a latched power off, or some action in between.
Miguel,
Try the link below for a good deep heatsink. The fins are 3+ inches in height.
HEATSINK ALUMINUM EXTRUSION 10.08 WIDE 1-12" PC. - eBay (item 170478322033 end time May-03-10 16:22:42 PDT).
This guy is a member at DIY audio and has these for sale continually.
Tad
Try the link below for a good deep heatsink. The fins are 3+ inches in height.
HEATSINK ALUMINUM EXTRUSION 10.08 WIDE 1-12" PC. - eBay (item 170478322033 end time May-03-10 16:22:42 PDT).
This guy is a member at DIY audio and has these for sale continually.
Tad
Heatsinks
Here is an Austrailian link for conrad heatsinks, They have a wide selection.
Conrad Heatsinks
Here is an Austrailian link for conrad heatsinks, They have a wide selection.
Conrad Heatsinks
Hello everybody,
I've been offline since thursday, but now I'm back on 😉
First of all, thanks for the good info you've posted here.
Tad,
Duda's got it right. The link for the catalog is correct: http://hsdissipadores.com.br/catalogo.pdf
If any of you need anything I can get here, it's my pleasure to send you! Just tell me what you need, and I'll check the costs.
Andrew,
The heatsink I posted here is rated at 0.78ºC with 4" long. The supplier has a calculator that shows us that with the 14 inches long heatsink it's actually rated at 0,39ºC/W. So, If use these heatsink as the lateral of my amplifier (one for each channel), isn't it enough?
The supplier cuts the heatsinks from 20 feet long pieces. The supplier can cut any size I want, so I was thinking to cut it with 14 inches, to cover the whole lateral of the amp (one for each channel/lateral). This way, the heatsink is rated at 0,39ºC.
linvalb,
Thank you for the links. I am not sure if importing those heatsinks would be a good idea, because of the shipping costs. But I thank you anyway. I'll check that!
I've been offline since thursday, but now I'm back on 😉
First of all, thanks for the good info you've posted here.
Tad,
Duda's got it right. The link for the catalog is correct: http://hsdissipadores.com.br/catalogo.pdf
If any of you need anything I can get here, it's my pleasure to send you! Just tell me what you need, and I'll check the costs.
Andrew,
The heatsink I posted here is rated at 0.78ºC with 4" long. The supplier has a calculator that shows us that with the 14 inches long heatsink it's actually rated at 0,39ºC/W. So, If use these heatsink as the lateral of my amplifier (one for each channel), isn't it enough?
The supplier cuts the heatsinks from 20 feet long pieces. The supplier can cut any size I want, so I was thinking to cut it with 14 inches, to cover the whole lateral of the amp (one for each channel/lateral). This way, the heatsink is rated at 0,39ºC.
linvalb,
Thank you for the links. I am not sure if importing those heatsinks would be a good idea, because of the shipping costs. But I thank you anyway. I'll check that!
Andrew,
I read it. I'm sorry if I appeared to have misunderstdood your explanations. Actually, I was only correcting the heatsink rating, because the whole calculations (and even the Re change suggestion) was based on the assumption that it was a 0,78ºC heatsink (well, it still is, but not with the size I was aiming).
What are the audible cons of changing Re value?
Regarding the temp monitor, I can use a turn on delay/DC protection that includes provision for a thermal switch. Maybe a 60ºC switch would be ideal, or even 70ºC.
Thanks a lot again for your patience!
I read it. I'm sorry if I appeared to have misunderstdood your explanations. Actually, I was only correcting the heatsink rating, because the whole calculations (and even the Re change suggestion) was based on the assumption that it was a 0,78ºC heatsink (well, it still is, but not with the size I was aiming).
What are the audible cons of changing Re value?
Regarding the temp monitor, I can use a turn on delay/DC protection that includes provision for a thermal switch. Maybe a 60ºC switch would be ideal, or even 70ºC.
Thanks a lot again for your patience!
Yes that is why I doubled it's length and gave the calculation for the new Rth s-a. Then doubled again and pointed out why the Rth s-a would be optimistic.
BTW,
for a 14inch length of sink that has an Rth s-a of 0.78C/W for a 4inch legnth is ~0.78 / sqrt(14/4) ~0.42C/W.
But this only holds true provided two conditions are met.
condition 1.
The backplate surface of the heatsink must all be at the same temperature.
condition 2.
The difference in temperature, delta(Ts-a), of the backplate surface temperature and the ambient air at the finned side must equal the value quoted by the manufacturer in his data.
A low delta T requires the Rth s-a to be de-rated according to the formula that the manufacturer has found by testing his heatsinks. i.e. model for a different deltaT.
A very large backplate that is not all at the same temperature must also be allowed for by de-rating the modified Rth s-a from the last simulation. This de-rating is rarely given in the data.
That's where I have adopted an empirical rule.
The longest distance from a corner to the nearest heat dissipating device <= 10times the thickness of the backplate.
Audibility of Re changes? no idea. Some suggest very low values are better, eg 0r1. Some suggest that higher values eg. 0r47 are better.
Some tell us that the thermal stability of the output stage is decreased with low Re.
BTW,
for a 14inch length of sink that has an Rth s-a of 0.78C/W for a 4inch legnth is ~0.78 / sqrt(14/4) ~0.42C/W.
But this only holds true provided two conditions are met.
condition 1.
The backplate surface of the heatsink must all be at the same temperature.
condition 2.
The difference in temperature, delta(Ts-a), of the backplate surface temperature and the ambient air at the finned side must equal the value quoted by the manufacturer in his data.
A low delta T requires the Rth s-a to be de-rated according to the formula that the manufacturer has found by testing his heatsinks. i.e. model for a different deltaT.
A very large backplate that is not all at the same temperature must also be allowed for by de-rating the modified Rth s-a from the last simulation. This de-rating is rarely given in the data.
That's where I have adopted an empirical rule.
The longest distance from a corner to the nearest heat dissipating device <= 10times the thickness of the backplate.
Audibility of Re changes? no idea. Some suggest very low values are better, eg 0r1. Some suggest that higher values eg. 0r47 are better.
Some tell us that the thermal stability of the output stage is decreased with low Re.
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