Hi Puffin!
Fuses:
Well, I like 4A or 6A fast-blo on the speakers, and have never noticed any sound effects from them.
I also like a standard fuse on the mains.
If using fuses on rails, please do also use fuses on speakers.
The speaker fuse can release the load (zero load) and prevent the nightmare scenerio from fuses on rails (possible infinity gain with simultaneous dc offset at full rails voltage).
Other problems from fuses on rails--Namely, fuses on rails puts inductance into your power, which can blur audio from chipamps.
Hi BWRX!
There are some odd little exceptions, like the "T" model can take it, but the "TF" can't. The PDF is for the "TF"
Alone of the basic overture power amps, LM3875T takes 45v with grace. Although, I'd be tempted to use some 60A05 diodes or a 6B6CTA bridge to cut that voltage down to 42v, just in case. Maybe a pair of bleeder resistors too, so there's not surprise voltages if there's big caps involved. What do you think?
The LM1875's also seem to negotiate quite a few of the rules and seriously bend some of the laws of physics. Have you built yours yet? I like those a lot!
Fuses:
Well, I like 4A or 6A fast-blo on the speakers, and have never noticed any sound effects from them.
I also like a standard fuse on the mains.
If using fuses on rails, please do also use fuses on speakers.
The speaker fuse can release the load (zero load) and prevent the nightmare scenerio from fuses on rails (possible infinity gain with simultaneous dc offset at full rails voltage).
Other problems from fuses on rails--Namely, fuses on rails puts inductance into your power, which can blur audio from chipamps.
Hi BWRX!
There are some odd little exceptions, like the "T" model can take it, but the "TF" can't. The PDF is for the "TF"
Alone of the basic overture power amps, LM3875T takes 45v with grace. Although, I'd be tempted to use some 60A05 diodes or a 6B6CTA bridge to cut that voltage down to 42v, just in case. Maybe a pair of bleeder resistors too, so there's not surprise voltages if there's big caps involved. What do you think? The LM1875's also seem to negotiate quite a few of the rules and seriously bend some of the laws of physics.
Have you built yours yet? I like those a lot!
BWRX said:
Also (and the data sheet is clear in that; doesn't anybody READ anymore?), if you run it at or above Absolute Max, you can permanently damage the chip in such a way that it no longer meets spec, even if it doesn't blow. The chip may already be 'bad' in that it distorts more, has more noise, whatever, than when you first put it in. No going back, I'm afraid.
Jan Didden
Imteresting reading guys. As always on this forum you get a divergence of opinion which is good. I have now made 4 monoblocks. 2 using the "dangerous" 30v trannys and 2 using the 25v trannys. I cannot really discern that the chips using the higher voltage sound any worse ( or have been cooked). IMHO they sound better.
Don't valve amps measure badly because of the distortion levels, but in practice may sound good ? Maybe I am hearing distortion and like it !?
Don't valve amps measure badly because of the distortion levels, but in practice may sound good ? Maybe I am hearing distortion and like it !?
Puffin said:
The effect is similar between LM3875T and LM1875. There's a point at the middle ground for voltage where observed performance is mediocre; however, at above and below these points the chips can sound wonderful, albeit more difficult power supply issues at maxvolts.
Say, what are you using for power supply and local decoupling that sounds so good and keeps the chips cool? I think that this must be the reason for the success, so I'm quite curious for details.
There can be no 'special' supply voltage for running the chips cool.
We need to know the ambient temperature, the thermal resistance of the heat sink ( or a picture ? ) and the actual output level ( average voltage across the load ) and supply voltage.
The chip data shows that maximum chip dissipation and so its temperature peak is at a certain power output for a specified supply voltage. This is not at maximum output and will vary depending on the actual supply voltage used. So it shouldn't be possible to run the chip "cool" with a small heat sink and low supply voltages and try to get max output from it.
Average listening levels with a decent speaker sensitivity will be about 1 to 3 watts ! peaks will of course be higher but average dissipation will be low. With a low ambient temperature the heat sinks should remain 'cool'.
Better to focus on using a supply voltage that appears to 'sound' best ( if there is one such point ) and use a heatsink that will dissipate the maximum power dissipated by the chip with that supply. If you will never continuously use the maximum ( just clipped ) power from the amp but only on musical peaks , you can get away with a much smaller heat sink. For use in colder countries with low ambient temperature you will probably have to design the heat sink keeping summer ambient temperatures in mind as these appear to be rising globally !
I don't believe that it is essential to keep the chip very ' cool' to get good sound out of it. Running it at 60deg C on the heatsink didn't seem to sound any worse than when it was cold at start up. Maybe it was a shade better ! At 60 deg C you cannot touch the heat sink for more than a couple of seconds without discomfort.
We need to know the ambient temperature, the thermal resistance of the heat sink ( or a picture ? ) and the actual output level ( average voltage across the load ) and supply voltage.
The chip data shows that maximum chip dissipation and so its temperature peak is at a certain power output for a specified supply voltage. This is not at maximum output and will vary depending on the actual supply voltage used. So it shouldn't be possible to run the chip "cool" with a small heat sink and low supply voltages and try to get max output from it.
Average listening levels with a decent speaker sensitivity will be about 1 to 3 watts ! peaks will of course be higher but average dissipation will be low. With a low ambient temperature the heat sinks should remain 'cool'.
Better to focus on using a supply voltage that appears to 'sound' best ( if there is one such point ) and use a heatsink that will dissipate the maximum power dissipated by the chip with that supply. If you will never continuously use the maximum ( just clipped ) power from the amp but only on musical peaks , you can get away with a much smaller heat sink. For use in colder countries with low ambient temperature you will probably have to design the heat sink keeping summer ambient temperatures in mind as these appear to be rising globally !
I don't believe that it is essential to keep the chip very ' cool' to get good sound out of it. Running it at 60deg C on the heatsink didn't seem to sound any worse than when it was cold at start up. Maybe it was a shade better ! At 60 deg C you cannot touch the heat sink for more than a couple of seconds without discomfort.
Power supply inductance as a product of:
Trances
Wires
Large caps versus small caps
and noises, such as diode noise
All of those above have a great effect on chipamp temperatures.
Since the high voltage supply needs large capacitance (a cause of inductance that makes heat), yet his chipamp still runs cool, this must be a miracle! So that's why I'm curious.
Trances
Wires
Large caps versus small caps
and noises, such as diode noise
All of those above have a great effect on chipamp temperatures.
Since the high voltage supply needs large capacitance (a cause of inductance that makes heat), yet his chipamp still runs cool, this must be a miracle! So that's why I'm curious.
I like the way Daniel-WB keeps plodding on with his explanation of how things happen ( work ) no matter what anyone else might say. He doesn't seem to ever get offended neither does he throw tantrums or flames at those who say otherwise !
He sticks to his pet explanations and apparently is 'happy' with the way things are.
That's the KEY to this whole exercise and he REALLY seems to have it !
EVERYONE has a lesson for each one of us in their own subtle way. That's why I love these forums . The great Global teaching ground in more ways that one !
Like they say .. "Good" doesn't happen to us , it's there all around us to take if we want to !
He sticks to his pet explanations and apparently is 'happy' with the way things are.
That's the KEY to this whole exercise and he REALLY seems to have it !
EVERYONE has a lesson for each one of us in their own subtle way. That's why I love these forums . The great Global teaching ground in more ways that one !
Like they say .. "Good" doesn't happen to us , it's there all around us to take if we want to !
Daniel. I suspect the fact that the chip runs fairly cool is to do with high sensitivity speakers, with no crossover. I use a short piece of twisted Cat5 cable (12inches approx) so the speakers are effectively being run actively.
The power supply is as posted originally a bog standard 160va toroid and an over specced bridge rectifier (25amps 400v)
Only capacitance is 1,000 uf on eacn rail and no decoupling caps.
The power supply is as posted originally a bog standard 160va toroid and an over specced bridge rectifier (25amps 400v)
Only capacitance is 1,000 uf on eacn rail and no decoupling caps.
Hi Ashok!
Why thank you! I do try a lot of things in actual applications. Sometimes the figures match the documentations. Sometimes I win at bingo. That's so much less expensive.
This time, its more reasonable. For me, I'm curious about the reported operating temperature.
I would like to try Puffin's power circuit (supply and onboard decoupling) with LM1875 on 32v rails. $2 is cheap for an experiment.
I want to measure the temperature differences.
Since his isn't fan cooled, then the only sure way they're running acceptably is because of their power circuits.
Inquiring minds want to know. . .
Some of what I was trying to say is that improving the tolerances on heat (running barely warm like Puffin's amp), may (may!) increase tolerances in other areas, like MTBF.
As for me, I want to save $$$ on heatsinks while using normal voltages. So, I'm after learning a new thing after all.
P.S. Oh, well, and probably shouldn't mention that seriously rocking out to an overvolted LM1875 is just too tempting.
Why thank you! I do try a lot of things in actual applications. Sometimes the figures match the documentations. Sometimes I win at bingo. That's so much less expensive.
This time, its more reasonable. For me, I'm curious about the reported operating temperature.
I would like to try Puffin's power circuit (supply and onboard decoupling) with LM1875 on 32v rails. $2 is cheap for an experiment.
I want to measure the temperature differences.
Since his isn't fan cooled, then the only sure way they're running acceptably is because of their power circuits.
Inquiring minds want to know. . .
Some of what I was trying to say is that improving the tolerances on heat (running barely warm like Puffin's amp), may (may!) increase tolerances in other areas, like MTBF.
As for me, I want to save $$$ on heatsinks while using normal voltages. So, I'm after learning a new thing after all.
P.S. Oh, well, and probably shouldn't mention that seriously rocking out to an overvolted LM1875 is just too tempting.
The National datasheet for the 1875 shows typical Iq=70mA and maximum =100mA.
I suspect minimum could be as low as 50mA.
These variations in quiescent currents will determine the running temperature of the chipamp when playing normal music and using 20db headroom for transient peaks.
It's plain arithmetic not magic.
I suspect minimum could be as low as 50mA.
These variations in quiescent currents will determine the running temperature of the chipamp when playing normal music and using 20db headroom for transient peaks.
It's plain arithmetic not magic.
AndrewT said:
Hi Andrew! Your arithmetic is more talented than most engineers. However, I do always need more of a "where does the bolt go?" approach.
In my opinion, the magic is the cool running temperatures at the stated voltages if also in combination with quality sound.
He's running LM3875's. I'm going to try his approach on LM1875's because I have a whole box of them, their actual limitation is heat, and they do run very hot.
I'd like to try exactly the same caps and bridge, or at least use some with a near-identical nature.
If I had to guess, it would be that the cap is perhaps a model designed for multipurpose duties that happen to include speaker output for car amps. Either that or it contains cermaic powder content. Its certain that the inductance is very, very low.
The bridge could possibly be one of the lightspeed models by Motorola. Those are just guesses, but hey! We'll find out soon.
Thanks Puffin!
AndrewT said:
This is why I appreciate your calculations so much! Even though I do have the hardest time getting a grasp at them, its well worth the effort because your calculations actually work directly onto the application. . . scalable, and without fail. Bravo!!!
I'm not at that step yet. The step I'm on its identifying what's going on. That's a chip amp running enough voltage to get it roasting hott at idle.
Yet, Puffin's amplifier stays cool without a readily apparent means to do so. The really beautiful speakers, while possibly a light load, couldn't keep the amp cool unless there's also a resistor in-series to the amplifier or perhaps 16 ohm speakers. That isn't the case, so the temp difference isn't happening on the speaker output side (although they do look wonderful).
So, the most reasonable explanation for the cool amplifiers is still the power circuit, including component choices and layout.
Pics of the amp. not pretty, but if I box it up no-one will know !
http://i73.photobucket.com/albums/i239/saxonsex/30v-3.jpg
http://i73.photobucket.com/albums/i239/saxonsex/30v-2.jpg
http://i73.photobucket.com/albums/i239/saxonsex/30v-3.jpg
http://i73.photobucket.com/albums/i239/saxonsex/30v-2.jpg
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