I am planning to build a LM4780 chipamp to use in my dorm room. My main music is instruments and new age, so I often play at moderate volume. The speakers are just some cheap bookshelf from Amazon. Either of these:
Amazon.com: Micca MB42X Bookshelf Speakers with 4-Inch Carbon Fiber Woofer and Silk Dome Tweeter, Black: Electronics
Amazon.com: Monoprice Premium 5.25-inch 2-Way Bookshelf Speakers (Pair), Black Finish: Electronics
Right now I only have a 2x22v 130va transformer to power the amp. My question is whether it is sufficient to drive those speaker, or I should buy a higher ratings transformer?
Thanks
Amazon.com: Micca MB42X Bookshelf Speakers with 4-Inch Carbon Fiber Woofer and Silk Dome Tweeter, Black: Electronics
Amazon.com: Monoprice Premium 5.25-inch 2-Way Bookshelf Speakers (Pair), Black Finish: Electronics
Right now I only have a 2x22v 130va transformer to power the amp. My question is whether it is sufficient to drive those speaker, or I should buy a higher ratings transformer?
Thanks
Hi Rol,
Apologies in advance for not directly answering your question. I do have a completed dual mono LM4780 for sale in the swap meet section. I have a 500va in mine, which is about 20% more than what is recommended but why not have it, right? Anyway, if you haven't purchased any of the parts for your build I could part mine out and you would recieve all the necessary boards (already populated), transformer, power supply, speaker protection and soft start. I would even throw in my gold plated rca and speaker connects. This way you would only have to wire it all together in your own chassis. Otherwise, if you like the look of my chassis, I could sell it as a working amp - just plug it in.
If you decide your own route, good luck. It is not too difficult of a build and sounds excellent!
Apologies in advance for not directly answering your question. I do have a completed dual mono LM4780 for sale in the swap meet section. I have a 500va in mine, which is about 20% more than what is recommended but why not have it, right? Anyway, if you haven't purchased any of the parts for your build I could part mine out and you would recieve all the necessary boards (already populated), transformer, power supply, speaker protection and soft start. I would even throw in my gold plated rca and speaker connects. This way you would only have to wire it all together in your own chassis. Otherwise, if you like the look of my chassis, I could sell it as a working amp - just plug it in.
If you decide your own route, good luck. It is not too difficult of a build and sounds excellent!
.
The flip side to your situation is that you'll never listen to more than 5 watts at the very most in your dorm room. Much more and you'll suffer permanent hearing loss before they throw you out, which will be soon. Actually a couple of watts is probably more like it.
So you could hook up the transformer you have and run it, sure. If you turn up the volume toward max something bad will happen, but you'll never have the volume that high anyway--unless you attend the University of Georgia, in which case all bets are off.
But actually, two 60 watt channels is a wee bit more power than your setup seems to call for. I'm going to get all busybody and suggest that a pair of LM1875s would be more appropriate. They go for something like 2 bucks apiece on eBay, and the audio is first class. You'd be interested in the "Typical Applications" circuit diagram in the data sheet, which is here. http://www.ti.com/lit/gpn/lm1875
Your dual 22 volts rectifies to about +/- 30 volts, which pushes the LM1875 up to around 30 watts per channel. This will still shake the walls of your dorm room, and the room next door, and the room next to that. But it seems to be a somewhat better fit for what you're doing than that 120 watt monster amp. So just thought I'd mention.
As for your concerns about whether the amp will drive the speakers, can a 16-wheeler pull a garden tractor? Unless a speaker specifies "RMS power," (true power) then it's very likely rated at "music power," which is generally 1/2 to 1/3 of true power.
So take speaker ratings with a grain of salt. And remember that even if a speaker won't burn out at it's rated power, distortion will be extreme as you approach the upper part of its rating.
.
The flip side to your situation is that you'll never listen to more than 5 watts at the very most in your dorm room. Much more and you'll suffer permanent hearing loss before they throw you out, which will be soon. Actually a couple of watts is probably more like it.
So you could hook up the transformer you have and run it, sure. If you turn up the volume toward max something bad will happen, but you'll never have the volume that high anyway--unless you attend the University of Georgia, in which case all bets are off.
But actually, two 60 watt channels is a wee bit more power than your setup seems to call for. I'm going to get all busybody and suggest that a pair of LM1875s would be more appropriate. They go for something like 2 bucks apiece on eBay, and the audio is first class. You'd be interested in the "Typical Applications" circuit diagram in the data sheet, which is here. http://www.ti.com/lit/gpn/lm1875
Your dual 22 volts rectifies to about +/- 30 volts, which pushes the LM1875 up to around 30 watts per channel. This will still shake the walls of your dorm room, and the room next door, and the room next to that. But it seems to be a somewhat better fit for what you're doing than that 120 watt monster amp. So just thought I'd mention.
As for your concerns about whether the amp will drive the speakers, can a 16-wheeler pull a garden tractor? Unless a speaker specifies "RMS power," (true power) then it's very likely rated at "music power," which is generally 1/2 to 1/3 of true power.
So take speaker ratings with a grain of salt. And remember that even if a speaker won't burn out at it's rated power, distortion will be extreme as you approach the upper part of its rating.
.
Hi,
Thanks a lot for your offer, but I already populated the LM4780 board, and about to test it very soon. Would love to try a dual mono, but my room is quite limited in space, so I try to things as small as possible.
My amp is only a single LM4780 and as mentioned, I don't play loud music, only moderately to fall asleep. Never built a speaker amp so I am pretty much clueless regarding the specs. Looks like I will give it a try with this 130va transformer. Seriously, buying and shipping a 300va or 400va traffo internationally is very costly and it would be great to be able to use what I got on hands.
If you're not chasing maximum output and just want something that works, you'll do fine. I routinely test all sorts of amplifiers with 25V rails off a 36VA transformer and a very basic power supply (sort of a test mule) and it works fine, plenty loud for plenty people.
Do remember to size your capacitance bank according to the transformer VA and rectifier capacity. My rule of thumb in most cases is power supply = 2.5x power output, which means 25 watts or so out of that transformer, per channel. This would suggest to me that a cap bank of 10,000 to 15,000uF per rail should be sufficient. You can be very happy with 25 watts in a small room, but I would heed the heatsink warnings carefully.
Normally when we want reduced power output we also reduce the supply voltage to keep the dissipation in check. You have a pretty healthy supply voltage, which means the idle dissipation and that at about 20 watts will be quite high, so you will have to deal with a bunch of heat. Class AB amps including chipamps will dissipate maximum power at 1/3 power output, which you can calculate from your supply rails and the National datasheet. These 4780s can run really hot.
Do remember to size your capacitance bank according to the transformer VA and rectifier capacity. My rule of thumb in most cases is power supply = 2.5x power output, which means 25 watts or so out of that transformer, per channel. This would suggest to me that a cap bank of 10,000 to 15,000uF per rail should be sufficient. You can be very happy with 25 watts in a small room, but I would heed the heatsink warnings carefully.
Normally when we want reduced power output we also reduce the supply voltage to keep the dissipation in check. You have a pretty healthy supply voltage, which means the idle dissipation and that at about 20 watts will be quite high, so you will have to deal with a bunch of heat. Class AB amps including chipamps will dissipate maximum power at 1/3 power output, which you can calculate from your supply rails and the National datasheet. These 4780s can run really hot.
I reported a couple of weeks ago on the heatsink temperatures of a newly built 2channel amplifier.
Two days ago I did a similar test on a lower power version.
Two 3886 fitted to a common 0.9C/W heatsink.
15-0-15Vac transformer giving a quiescent voltage at the chipamps of +-23.7Vdc
Attached two 4r0 dummy loads to the outputs and sent a 1kHz signal to the input.
Room Ta=22°C
with each chipamp sending ½W to the 4r0, the heatsink reached about 52°C and the chips were too hot to hold my finger on either of them.
The sink backplate was just too hot for a prolonged finger test.
BTW, the 15-0-15Vac allows 24W into 8ohms/8r0, that leaves ½W @ ~ -17dB ref maximum.
Two days ago I did a similar test on a lower power version.
Two 3886 fitted to a common 0.9C/W heatsink.
15-0-15Vac transformer giving a quiescent voltage at the chipamps of +-23.7Vdc
Attached two 4r0 dummy loads to the outputs and sent a 1kHz signal to the input.
Room Ta=22°C
with each chipamp sending ½W to the 4r0, the heatsink reached about 52°C and the chips were too hot to hold my finger on either of them.
The sink backplate was just too hot for a prolonged finger test.
BTW, the 15-0-15Vac allows 24W into 8ohms/8r0, that leaves ½W @ ~ -17dB ref maximum.
.
<< I already populated the LM4780 board, and about to test it very soon >>
To jump in with another 2 cents, you're fine. Watts = volts x amps, so your 130VA transformer is essentially a 130 watt transformer. You're using it to power a 120 watt amplifier. You'll be fine as long as you don't crank up the volume.
The trick is that transformers don't work at 100% efficiency. More like 60-80%, with emphasis toward 60. This is why power supplies are usually made oversize. My opinion is 150% oversize, or thereabouts.
Additionally, when the drummer or bass player go to work--or the cellos or tympani--there's need for additional power. The capacitor bank (the smoothing capacitors) helps with this, but the capacitors can't supply what's not there in the first place.
But all of this comes into play at higher volume levels. Normal room listening level is perhaps 2-5 watts, so 2-5 watts is all the amp is putting out, so 2-5 watts is all the power supply has to provide. You'll be perfectly OK in an environment like a dorm room.
And again, don't worry about being able to drive the speakers you mentioned. The amp you're talking about can melt them.
<< buying and shipping...internationally is very costly >>
Have you looked at Mouser (mouser.com), or Jameco (jameco.com), just to mention two? Jameco is more DIYer oriented, neither has a minimum order. Disclosure: I have no association with either, just mentioning.
.
<< I already populated the LM4780 board, and about to test it very soon >>
To jump in with another 2 cents, you're fine. Watts = volts x amps, so your 130VA transformer is essentially a 130 watt transformer. You're using it to power a 120 watt amplifier. You'll be fine as long as you don't crank up the volume.
The trick is that transformers don't work at 100% efficiency. More like 60-80%, with emphasis toward 60. This is why power supplies are usually made oversize. My opinion is 150% oversize, or thereabouts.
Additionally, when the drummer or bass player go to work--or the cellos or tympani--there's need for additional power. The capacitor bank (the smoothing capacitors) helps with this, but the capacitors can't supply what's not there in the first place.
But all of this comes into play at higher volume levels. Normal room listening level is perhaps 2-5 watts, so 2-5 watts is all the amp is putting out, so 2-5 watts is all the power supply has to provide. You'll be perfectly OK in an environment like a dorm room.
And again, don't worry about being able to drive the speakers you mentioned. The amp you're talking about can melt them.
<< buying and shipping...internationally is very costly >>
Have you looked at Mouser (mouser.com), or Jameco (jameco.com), just to mention two? Jameco is more DIYer oriented, neither has a minimum order. Disclosure: I have no association with either, just mentioning.
.
Thanks all.
My amp has 15,000uf per rail so quite sufficient for the task. I will surely watch out for the heat. I don't actually use a real heatsink, just a chunk of CNC'ed solid aluminum block.
I am really not looking for a monster amp that puts out hundred watts, just something that fits perfectly in my environment.
I did order many times from Mouser. What I actually meant is the exorbitant cost to ship a huge, heavy transformer to my place. Shipping cost would be easily more than that of the transformer.
My amp has 15,000uf per rail so quite sufficient for the task. I will surely watch out for the heat. I don't actually use a real heatsink, just a chunk of CNC'ed solid aluminum block.
I am really not looking for a monster amp that puts out hundred watts, just something that fits perfectly in my environment.
I did order many times from Mouser. What I actually meant is the exorbitant cost to ship a huge, heavy transformer to my place. Shipping cost would be easily more than that of the transformer.
agreed, there are losses in the transformer that prevent 100% efficiency
Wrong.
Small transformer say around 1VA to 10VA will have efficiencies around 80% to 90%
Medium transformers from 12VA to 200VA will have efficiencies increasing with size to around 95%.
Large transformers, 225VA to 1kVA. will have an efficiencies around 96%.
It seems to be either expensive, or technically difficult, to get much over 96%.
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The reason you need to oversize your power supply is mainly because of the efficiency of the amp itself!!
For an example, Leaving the transformer losses out of the equation, if you had a single ended Class A amplifier that is capable of 25 watts of power output to the load then you must supply the amplifier with a power source that is capable producing 100 watts of power.
This is because such an amplifier of this design is only 25% efficient.
A Bridged Class A design is a bit more efficient at 50% therefore it is possible to get 50 watts of output power to the load with the same 100 Watts of power being supplied to the amplifier under the same conditions as the previous example.
For class B amplifiers the theoretical limt factor is 78.5% and less for a Class AB types.
Amplifier - Wikipedia, the free encyclopedia
So therefore you must overate your power supply for the type of amplifier that you are building.
This has nothing to do with transformer efficiency as that is another factor of the difference in power going into the transformer vs the power coming out.
What we are concerned about is that we already know that our power supply is capable of 100 watts of output power regardless of how much it is using in order to make that power, But is 100 watts enough to power our 100watt amplifier?
The answer is No!!
It is not enough due to the amplifiers efficiency rating.
66% is usually a good ballpark figure for a common class AB amplifier therefore you would need about 150 watts of power from your supply to properly supply your 100 watt amplifier without it starving the load for current when it is run at full power as this is one of the main causes for clipping.
Then if you want to consider actual power usage then you figure in your transformers efficiency rating on top of that.
Considering the same 100 watts of power for a 100 watt amplifier is okay if you don't plan on running the amp at full power 100% of the time and will be sufficient for the transient peaks under normal conditions and listening levels (however it will still be under powered).
This depends of course on what you consider a normal listening level is!!
FWIW
jer
For an example, Leaving the transformer losses out of the equation, if you had a single ended Class A amplifier that is capable of 25 watts of power output to the load then you must supply the amplifier with a power source that is capable producing 100 watts of power.
This is because such an amplifier of this design is only 25% efficient.
A Bridged Class A design is a bit more efficient at 50% therefore it is possible to get 50 watts of output power to the load with the same 100 Watts of power being supplied to the amplifier under the same conditions as the previous example.
For class B amplifiers the theoretical limt factor is 78.5% and less for a Class AB types.
Amplifier - Wikipedia, the free encyclopedia
So therefore you must overate your power supply for the type of amplifier that you are building.
This has nothing to do with transformer efficiency as that is another factor of the difference in power going into the transformer vs the power coming out.
What we are concerned about is that we already know that our power supply is capable of 100 watts of output power regardless of how much it is using in order to make that power, But is 100 watts enough to power our 100watt amplifier?
The answer is No!!
It is not enough due to the amplifiers efficiency rating.
66% is usually a good ballpark figure for a common class AB amplifier therefore you would need about 150 watts of power from your supply to properly supply your 100 watt amplifier without it starving the load for current when it is run at full power as this is one of the main causes for clipping.
Then if you want to consider actual power usage then you figure in your transformers efficiency rating on top of that.
Considering the same 100 watts of power for a 100 watt amplifier is okay if you don't plan on running the amp at full power 100% of the time and will be sufficient for the transient peaks under normal conditions and listening levels (however it will still be under powered).
This depends of course on what you consider a normal listening level is!!
FWIW
jer
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The current and voltage:
Because of a fortunate choice of transformer, the current into the amplifier doesn't exceed the amplifier's maximum specs with either 4 ohm or 8 ohm speakers. The output also exceeds the requirements of most compact speakers and most 2-way speakers. No problem found.
However, the heatsink:
For these conditions, that much current, that much voltage and 4 ohm speakers, the LM4780 manufacturer's calculation spreadsheet said 0.72C/W is the minimum size heatsink if when the room temp doesn't run beyond average indoor environment (they did not include an enclosure--that figure is an exposed heatsink in an air-conditioned room). Generally, we'd aim for more safety margin by using a heatsink larger than the minimum. If used within an enclosure, excellent ventilation both above (hot air output) and below (cool air intake) is needed.
Summary:
You don't need a bigger transformer. A bigger transformer would make the heatsink concern get much worse and probably unmanageable with the load of 4 ohm speakers.
However, you do already need a bigger heatsink. That's what you actually need--a generous heatsink (a proper heatsink with wide-spaced big size fins and good ventilation too).
One advantage to using an LM4870 in parallel mode is that dual mono is possible, if you have two of LM4780 chips.
The audible advantage to running the LM4780 in parallel mode, is that you get a mono amplifier with higher current handling, easier thermal management and much finer quality output linearity, which means nicer tonality and less coloration.
Also, no two overture chip cores emit the spike system hard clipper noise at precisely the same moment in time, and thus the racket can be caught in the big white output ballast resistors (used when paralleling outputs) instead of sent into your speakers and ears (so you hear less limiter noises).
Basically, a chip amp no longer sounds like a cheap chip if run parallel, but rather a lot more like a discrete amp--note the physical size of the output devices when comparing.
With 2 chip amplifiers, Dual mono is possible. However for a more compact build, virtual dual mono can be done, inexpensively, with just one transformer, just one power supply board, if given either 4 regulators Or just 4 inexpensive 6a05 diodes added (a pair for left and a pair for right, such as a pair goes onto the edge of each amplifier board, as series elements, in series to the V+ and V- cable). Dual mono sound is a decrease of crosstalk, and that does not require separate left-right power, but what it actually requires is different left-right power, and installing a difference is neither difficult nor expensive.
Also, not bulky. For the sound of dual mono (less crosstalk) the main size increase is the additional LM4780, its own pair of power decoupling caps, 4 big white resistors and, optionally, 4 more diodes used as series elements to decrease crosstalk. The workload of the transformer and the workload of the heatsink does not increase significantly. Even so, a very generous heatsink would be a good thing.
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Andrew is right about this; I've had the same experience -- high dissipation begins at remarkably low power levels into 4 ohm. See also Fig. 16 in LM4780 datasheet.
That is not a 4ohms test.
I did a 4r0 (resistor load test).
Every 8ohms rated amplifier should easily drive a 4r0 load.
A 4ohms rated amplifier should easily drive a 2r0 load.
Effectively I was testing an 8ohms rated amplifier and proved again that a 3886 chipamp needs a big heatsink to help avoid early triggering of the temperature sensitive protections.
What we are concerned about is that we already know that our power supply is capable of 100 watts of output power regardless of how much it is using in order to make that power, But is 100 watts enough to power our 100watt amplifier?
The answer is No!!
It is not enough due to the amplifiers efficiency rating.
66% is usually a good ballpark figure for a common class AB amplifier therefore you would need about 150 watts of power from your supply to properly supply your 100 watt amplifier without it starving the load for current when it is run at full power as this is one of the main causes for clipping.
Then if you want to consider actual power usage then you figure in your transformers efficiency rating on top of that.
Considering the same 100 watts of power for a 100 watt amplifier is okay if you don't plan on running the amp at full power 100% of the time and will be sufficient for the transient peaks under normal conditions and listening levels (however it will still be under powered).
This depends of course on what you consider a normal listening level is!!
FWIW
jer
You're right, but only if you listen to full power sine wave.
I do not know why everybody recommends oversized transformers but nobody listens pure sine wave power.
Music, according to stile, has a crest factor of 2 to about 10.
This means that for a factor of 5 (lets say) using an amplifier of 100W without clipping, the average power is only 20W.
A class AB amplifier efficiency it is around 60% and then the power consumption from transformer will be only 34-36W. In this case a transformer of only 40W it is enough and you do not need a 200W transformer.
Lets consider the worst case of a crest factor of only 2.
Maximum output power 2*60W=120W
Maximum average power = 60W (crest factor of only 2)
Amplifier efficiency around 60% result a power consumption of around 100W
My conclusion is that a transformer of 120W it is enough if you do not listen a high clipped signal (distortion over 1%-10%) or pure sine wave at maximum power.
the overall efficiency of the chipamp can be worse than 60%. for example, the LM4780 datasheet fig 16 shows us that with +/- 25v rails and a 4 ohm load, there will be over 60 watts dissipation at 30 watts output per channel -- ie: less than 50% of the power consumption goes to audio output. At 6 ohms things are not much different.
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