Hey folks,
I've been lurking and reading here for a few weeks trying to assimilate as much information as possible on these Nat'l Semi based chip amps / gainclones.
What I'm looking to do is build a high density 12-zone (24-channel) whole house amplifier which can drive 4 and 8 ohm loads at approximately 40 and 30w, respectively. Perhaps a bit lower.
Some other requirements / thoughts / questions:
1) I'd like to use a prefab PCB available out there, or would love assistance in getting one designed.
2) I'm trying to do this on the cheap, but still have great sound. I could go buy two SpeakerCraft BB1235's for $1400 total, so if I can't come significantly under that, or have significantly better sound quality, there's no point.
3) I'd like to use LM1875's, but am a bit concerned by the amount of power I can get out of them. Or shouldn't I be?
4) If I did use LM1875's, would anybody think it too dangerous to use a 22-0-22 transformer and thus push the rail voltage up to say 31v? Again, I'm trying to squeeze as much power out of the chip to drive these 90dB @ 1w/1m efficiency in-wall / in-ceiling drivers.
5) The inputs will all be coming from line level outputs of external USB sound cards. Anybody see an issue with that? Think I'll need pre-amp circuit in front of the main amp circuit? If so that is where I would probably place the gain control if needed, but right now I'm planning on managing volume within the PC WHA controller itself.
6) Again assuming LM1875 and trying to squeeze all 24 channels into one chassis, should I be looking at 1, 2 or 3, or ? many transformers and at what VAC?
7) I need a design that is very power efficient when the volume controls are off, thereby leaving no circuit path through the speaker. This will be going in basement network / AV closet so it needs to essentially be in "standby" and consume little to no power when all of the zone volume controls are off.
8) Am I crazy trying to fit all 24 channels into one chassis? Would it be better to simply to make two identical, 12 channel / 6 zone units?
9) Any recommendations on any kits or components are most welcome. In particular I'm looking for LM1875 PCB's that contain two or more stereo pairs per PCB set. Clearly the higher density the better, but SMD's are a no-no. I'm too light on patience and skill for those. ;-)
Thanks in advance for any thoughts / questions / concerns.
- Rhino
I've been lurking and reading here for a few weeks trying to assimilate as much information as possible on these Nat'l Semi based chip amps / gainclones.
What I'm looking to do is build a high density 12-zone (24-channel) whole house amplifier which can drive 4 and 8 ohm loads at approximately 40 and 30w, respectively. Perhaps a bit lower.
Some other requirements / thoughts / questions:
1) I'd like to use a prefab PCB available out there, or would love assistance in getting one designed.
2) I'm trying to do this on the cheap, but still have great sound. I could go buy two SpeakerCraft BB1235's for $1400 total, so if I can't come significantly under that, or have significantly better sound quality, there's no point.
3) I'd like to use LM1875's, but am a bit concerned by the amount of power I can get out of them. Or shouldn't I be?
4) If I did use LM1875's, would anybody think it too dangerous to use a 22-0-22 transformer and thus push the rail voltage up to say 31v? Again, I'm trying to squeeze as much power out of the chip to drive these 90dB @ 1w/1m efficiency in-wall / in-ceiling drivers.
5) The inputs will all be coming from line level outputs of external USB sound cards. Anybody see an issue with that? Think I'll need pre-amp circuit in front of the main amp circuit? If so that is where I would probably place the gain control if needed, but right now I'm planning on managing volume within the PC WHA controller itself.
6) Again assuming LM1875 and trying to squeeze all 24 channels into one chassis, should I be looking at 1, 2 or 3, or ? many transformers and at what VAC?
7) I need a design that is very power efficient when the volume controls are off, thereby leaving no circuit path through the speaker. This will be going in basement network / AV closet so it needs to essentially be in "standby" and consume little to no power when all of the zone volume controls are off.
8) Am I crazy trying to fit all 24 channels into one chassis? Would it be better to simply to make two identical, 12 channel / 6 zone units?
9) Any recommendations on any kits or components are most welcome. In particular I'm looking for LM1875 PCB's that contain two or more stereo pairs per PCB set. Clearly the higher density the better, but SMD's are a no-no. I'm too light on patience and skill for those. ;-)
Thanks in advance for any thoughts / questions / concerns.
- Rhino
RhinoDude said:
Hi,
Should be able to do it cheaper, but not by much if you haven't been doing this kind of thing. Parts from many different sources add up and in no time you are looking at a bigger than expected expense.
Don't do it to save - do it for fun. It's relatively cheap entertainment.
Is the idea for all of these amps to be operating at the same time? If so, you need a big power supply, capable of keeping up with the current demand.
FWIW, I'd use 12 LM4780, run them on a 1000VA, 25-0-25 transformer (About $190.00). This will give you headroom, wattage wise and each LM being a stereo chip, you will save space in the chassis.
Should be nice boards or kits available for these, like here
Thanks for the reply John. I agree, if it wasn't for the fun-factor of diy, I wouldn't even consider it. So I'm definately factoring that in.
THere is one uncommon scenario where all zones will be on at once: walk-through if we ever sell the place. In my last place we had this happen as the realtors would often turn on the music in each room, but not turn them off. But, back then I had an impedence matching Volume Controls, so it was no big deal. My current VC's are jumper selectable whether they are straight or impedence matched, and I'm using the former.
During parties a few times a year it is extremely likely the 6 of the 12 zones will be playing at once (12 channels total), with 3 of them playing potentially quite load (outdoor zone, 3 season hot tub zone, living room zone). Those are also the three largest zones.
However, half of the zones are very small bedroom / bathroom zones, and while the bathroom zones may be on during parties, it's unlikely they'll be turned up very loud due to the hard surfaces / reflections and small spaces. The basement and garage zones will also rarely be used, and when used, at low volumes (at least for the basement). I guess I could see the garage zone getting cranked as it's a large volume of space, plus could also be used during larger parties as our gambling area.
As a result of this, I was figuring a power supply that would meet somewhere around 50% to 75% of the max draw of the chip amps should suffice. What is the standard here in traditional amp design? Is the power supply always sized so that it can provide 100% of power to clip all of the contained amps?
I'll have to dig up the spec sheet on that chip and see what it draws...to tell you the truth I hadn't really considered the higher powered stereo chips.
Thanks,
Ryan
THere is one uncommon scenario where all zones will be on at once: walk-through if we ever sell the place. In my last place we had this happen as the realtors would often turn on the music in each room, but not turn them off. But, back then I had an impedence matching Volume Controls, so it was no big deal. My current VC's are jumper selectable whether they are straight or impedence matched, and I'm using the former.
During parties a few times a year it is extremely likely the 6 of the 12 zones will be playing at once (12 channels total), with 3 of them playing potentially quite load (outdoor zone, 3 season hot tub zone, living room zone). Those are also the three largest zones.
However, half of the zones are very small bedroom / bathroom zones, and while the bathroom zones may be on during parties, it's unlikely they'll be turned up very loud due to the hard surfaces / reflections and small spaces. The basement and garage zones will also rarely be used, and when used, at low volumes (at least for the basement). I guess I could see the garage zone getting cranked as it's a large volume of space, plus could also be used during larger parties as our gambling area.
As a result of this, I was figuring a power supply that would meet somewhere around 50% to 75% of the max draw of the chip amps should suffice. What is the standard here in traditional amp design? Is the power supply always sized so that it can provide 100% of power to clip all of the contained amps?
I'll have to dig up the spec sheet on that chip and see what it draws...to tell you the truth I hadn't really considered the higher powered stereo chips.
Thanks,
Ryan
With all amps in one place you will have long speaker cables. You might be better off with a 100V setup, if you don't have the speakers already.
If you have the speakers your choice may be the LM1876 instead of LM1875. The LM1876 is a stereo amplifier with the same amount of output power as the LM1875. It has the standby function you want for energy saving, which the LM1875 has not.
On the other hand you could always switch the unused channels off. That is the most efficient way to save energy.
If you have the speakers your choice may be the LM1876 instead of LM1875. The LM1876 is a stereo amplifier with the same amount of output power as the LM1875. It has the standby function you want for energy saving, which the LM1875 has not.
On the other hand you could always switch the unused channels off. That is the most efficient way to save energy.
I would build powered speakers and make a distributed system instead. This implies carrying mains power (usually already available), balanced line-level audio and a turn-on signal to every room. There are compact 24V or +/-24V low-cost SMPS that can be used for local powering of several channels in a room per room basis.
Putting all the 24 channels together results in a massive waste of speaker wire and a true wiring mess. Long runs of speaker wire (non-100V systems) result in substantial attenuation, noise pickup and potential amplifier instability (see the thread about output inductors in the solid state forum).
Putting all the 24 channels together results in a massive waste of speaker wire and a true wiring mess. Long runs of speaker wire (non-100V systems) result in substantial attenuation, noise pickup and potential amplifier instability (see the thread about output inductors in the solid state forum).
Thanks for the feedback Eva. The boat sailed months ago with changing the design; but no worries I've implemented this approach several times effectively. ;-)
There are 12 sets of CL3 rated wire are already home run to the centralized av/network closet. I ran 12 gauge for the few runs I have over 80', and 14 gauge for everything else. The speakers are already on hand (and a few zones installed to test things out). The IR control system is all installed, and volume controls are all installed as well (both all Buffalo; I work with them exclusively, as their stull just *works*).
Currently I'm using an old Yamaha RX-V2095 receiver just to test things out the two zones for which I've actually installed the speakers. One of these is my absolute longest run (approx 140') and it sounds fantastic given how cheap the speakers are that I'm using.
Having installed quite a few large WHA installations in the past, I'm familiar with the typical pitfalls to avoid when pulling wire. So noise pickup won't be a problem. Yes, there will be some non-linear attenuation due to the amount of copper on longer runs, but that's why a) I used 12 gauge for longer runs and b) I have software equilization I can apply on a per-zone basis. So I can cancel out the non-linear aspect, and use pure watts to overcome the rest.
That is one of the reasons why I am a bit hesitent with the LM1875/6. As I understand it I'm looking at 20 may be 25 watts. Well, today with my RX-V2095, I have 100 watts per channel, and a listening position of 5 results in a nice sound level that I, even in a party, would rarely exceed. And, that room is rather large (approx 380 sq ft). But I'm not sure how much that signal is being attenuated. As I said, it's 140' of 12 guage wire. To get the LM1875/6 to push that loud without the protection circuitry kicking in may be a real challenge...
- Ryan
There are 12 sets of CL3 rated wire are already home run to the centralized av/network closet. I ran 12 gauge for the few runs I have over 80', and 14 gauge for everything else. The speakers are already on hand (and a few zones installed to test things out). The IR control system is all installed, and volume controls are all installed as well (both all Buffalo; I work with them exclusively, as their stull just *works*).
Currently I'm using an old Yamaha RX-V2095 receiver just to test things out the two zones for which I've actually installed the speakers. One of these is my absolute longest run (approx 140') and it sounds fantastic given how cheap the speakers are that I'm using.
Having installed quite a few large WHA installations in the past, I'm familiar with the typical pitfalls to avoid when pulling wire. So noise pickup won't be a problem. Yes, there will be some non-linear attenuation due to the amount of copper on longer runs, but that's why a) I used 12 gauge for longer runs and b) I have software equilization I can apply on a per-zone basis. So I can cancel out the non-linear aspect, and use pure watts to overcome the rest.
That is one of the reasons why I am a bit hesitent with the LM1875/6. As I understand it I'm looking at 20 may be 25 watts. Well, today with my RX-V2095, I have 100 watts per channel, and a listening position of 5 results in a nice sound level that I, even in a party, would rarely exceed. And, that room is rather large (approx 380 sq ft). But I'm not sure how much that signal is being attenuated. As I said, it's 140' of 12 guage wire. To get the LM1875/6 to push that loud without the protection circuitry kicking in may be a real challenge...
- Ryan
Pacificblue, when you say "switch the unused channels off", do you mean to imply having a separate switchable power supply for each channel? Or simply having a switch to break the PS to amp circuit? If the LM chip is in standby / muted, how much power savings would there really be between those two solutions? I wouldn't thin that it would be much, but I'm far from am electronics expert.
Of course, I can always have the *entire* amp turned off when no zones are calling for music, if that is what you mean. I can do that easily enough with a triggerable 20A @ 120v relay. Then when no zones need music the amp uses absolutely 0 watts until the WHA controller triggers it.
Then, when the amp starts up, I would have each LM steroe pair muted by default. However, I am torn as to which would be a better way to use the mute feature on the LM chips.
a) expose a 12V trigger jack per zone, and allow the WHA controller to directly mute or clear each zone as needed.
b) implement some form of input detection circuit which automatically unmutes a muted LM chip whenever a signal is detected, and re-mute it if no signal has been detected for an extended period (say 1 minute or so). The SpeakerCraft Big Bang amps actually use this in order to turn the entire amp on when configured for "sense" mode.
Also, on a related note, if all of the twelve speaker loops are open circuits (because the speaker volume controls are "off"), would there be a difference in power consumption by a LM amp chip that is muted vs. one that has no load attached? If so anybody have a clue how much we are talking about? Which spec in the product sheets should I be looking at to determine this?
Thanks,
Ryan
Of course, I can always have the *entire* amp turned off when no zones are calling for music, if that is what you mean. I can do that easily enough with a triggerable 20A @ 120v relay. Then when no zones need music the amp uses absolutely 0 watts until the WHA controller triggers it.
Then, when the amp starts up, I would have each LM steroe pair muted by default. However, I am torn as to which would be a better way to use the mute feature on the LM chips.
a) expose a 12V trigger jack per zone, and allow the WHA controller to directly mute or clear each zone as needed.
b) implement some form of input detection circuit which automatically unmutes a muted LM chip whenever a signal is detected, and re-mute it if no signal has been detected for an extended period (say 1 minute or so). The SpeakerCraft Big Bang amps actually use this in order to turn the entire amp on when configured for "sense" mode.
Also, on a related note, if all of the twelve speaker loops are open circuits (because the speaker volume controls are "off"), would there be a difference in power consumption by a LM amp chip that is muted vs. one that has no load attached? If so anybody have a clue how much we are talking about? Which spec in the product sheets should I be looking at to determine this?
Thanks,
Ryan
You could base the decision for several small power supplies or a big one with switches/relays per amp on the average amount of active amps. The big one has less losses, when all amps are on. The small ones have more power saving potential, when several amps are off. The advantage grows with the number of switched-off amps.
Or decide according to investment. Several small power supplies should be more expensive than a big one.
With a big power supply you have to keep in mind that you deal with high currents. So the cable and wire diameters have to be big enough. And it will be bulky. Sometimes it is easier to fit many small things rather than one big bulk, even if the smaller things sum up to more space.
The muting trigger should come from the amps power supply. An external 12V trigger might easily become a hum source.
http://sound.westhost.com/project38.htm could be what you need.
The LM1876 datasheet gives a typical quiescent current of 50mA, max 80mA with standby off and typical 4.2mA, max 6 mA with standby on. Multiply that with 1.5 times your power supply voltage to know approximately what you can expect to save. Several tens of W using standby plus a few more, if you switch off.
Or decide according to investment. Several small power supplies should be more expensive than a big one.
With a big power supply you have to keep in mind that you deal with high currents. So the cable and wire diameters have to be big enough. And it will be bulky. Sometimes it is easier to fit many small things rather than one big bulk, even if the smaller things sum up to more space.
The muting trigger should come from the amps power supply. An external 12V trigger might easily become a hum source.
http://sound.westhost.com/project38.htm could be what you need.
The LM1876 datasheet gives a typical quiescent current of 50mA, max 80mA with standby off and typical 4.2mA, max 6 mA with standby on. Multiply that with 1.5 times your power supply voltage to know approximately what you can expect to save. Several tens of W using standby plus a few more, if you switch off.
Because I am a big conservative I'd recommend you make a 6 channel or 8 channel amplifier first. Then if it works as you want build 3-4 more just like them. This allows you to switch off amplifiers completely for unused zones and makes an amplifier design a little easier to do (less floorplanning). Cost will be more because you have to duplicate things like cases and power supplies.
For high density channels look at the LM4782. The power might not be where you want it but close. It also has the Standby function to cut power use. Also keep in mind than National runs all channels at worst case power dissipation (Pdmax) when they spec a power range. This is almost always with a passive heat sink so since your amps will be in a closet you can have cooling fans. If you can keep it cool the LM4782 can push more power. Or go for the LM4765, only 2 channels but a bit more power.
BTW, when putting an Overture chip into Mute mode there is pretty much zero power savings. The Mute mode only disconnects the input signal internally.
-SL
For high density channels look at the LM4782. The power might not be where you want it but close. It also has the Standby function to cut power use. Also keep in mind than National runs all channels at worst case power dissipation (Pdmax) when they spec a power range. This is almost always with a passive heat sink so since your amps will be in a closet you can have cooling fans. If you can keep it cool the LM4782 can push more power. Or go for the LM4765, only 2 channels but a bit more power.
BTW, when putting an Overture chip into Mute mode there is pretty much zero power savings. The Mute mode only disconnects the input signal internally.
-SL
Thanks for the reply SL.
I noticed not many of the overture chips have Standby mode. That kinda bites, I'm glad you pointed out that there is indeed a big difference between mute and standby. THe LM4780 which I had all but decided upon doesn't support standby. However, I guess I could always place a relay in there and provide no power to the chip when there is no signal. Then standy mode should be irrelevant.
I agree the LM4782 would be nice, not only for standby, but also for the extra 50% density. However, I haven't read much about the performace of this chip compared to others, and according to Nat'l Semi's site, it's specs are much worse; relatively speaking. Now, I know specs aren't the end-all-be-all, but the listed THD is almost 7x the other chip, and SNR is a whopping 16db lower.
To use the standby since I'd need either a trigger or signal sensing circuit, I'm thinking that I would just use those to control a relay. When the relay is open, the amp circuit simply has no power.
If I use this approach, will I have to worry about turn-on pops/clicks, or does the Overture chips have some form of soft-start in them? If I remember correctly, relays can introduce a lot of noise briefly when they are first turned on. I'd rather not amplify and send this noise to the speakers.
Thanks,
- Ryan
I noticed not many of the overture chips have Standby mode. That kinda bites, I'm glad you pointed out that there is indeed a big difference between mute and standby. THe LM4780 which I had all but decided upon doesn't support standby. However, I guess I could always place a relay in there and provide no power to the chip when there is no signal. Then standy mode should be irrelevant.
I agree the LM4782 would be nice, not only for standby, but also for the extra 50% density. However, I haven't read much about the performace of this chip compared to others, and according to Nat'l Semi's site, it's specs are much worse; relatively speaking. Now, I know specs aren't the end-all-be-all, but the listed THD is almost 7x the other chip, and SNR is a whopping 16db lower.
To use the standby since I'd need either a trigger or signal sensing circuit, I'm thinking that I would just use those to control a relay. When the relay is open, the amp circuit simply has no power.
If I use this approach, will I have to worry about turn-on pops/clicks, or does the Overture chips have some form of soft-start in them? If I remember correctly, relays can introduce a lot of noise briefly when they are first turned on. I'd rather not amplify and send this noise to the speakers.
Thanks,
- Ryan
The mute pin can work as a delay useing a cap and resistor.... see the datasheet... which will also provide you with the current consumption in idle mode...
The LM4782 can be compared to three LM1875s you started this thread with.
For lower THD choose the LM3886 or LM4780 or copy AN-1490 or AN-1850. Cheap speakers in-wall or in-ceiling, garage or bathroom PA and party music seem to indicate that THD is not too critical. 😉
Where did you read that SNR is 16 dB worse? And how good do you want SNR to be? How many of your speakers will play so loud that noise below -97 dB (LM4782@1W) becomes an issue? 😎
The drawback of LM4782 could be its limited capacity for heat dissipation. At least during parties and especially with 4 Ohm speakers. If you stick with the 22-0-22 V transformer from your first thread, use LM3886 for all 4 Ohm speakers and take the LM4780 for the 8 Ohmers. Be aware that you'll get much more power than you initially wanted, nearly 40W into 8Ohm and more than 70W into 4Ohm with double that figure as peaks@10%THD. If you reduce the voltage to somewhere from 15-0-15V (for 8 Ohm) to 18-0-18V (for 4 Ohm) you get around 20W@1%THD per channel. Watch your heatsinking.
To avoid pops and clicks use capacitors across each relay contact, something like 10 nF should do.
For lower THD choose the LM3886 or LM4780 or copy AN-1490 or AN-1850. Cheap speakers in-wall or in-ceiling, garage or bathroom PA and party music seem to indicate that THD is not too critical. 😉
Where did you read that SNR is 16 dB worse? And how good do you want SNR to be? How many of your speakers will play so loud that noise below -97 dB (LM4782@1W) becomes an issue? 😎
The drawback of LM4782 could be its limited capacity for heat dissipation. At least during parties and especially with 4 Ohm speakers. If you stick with the 22-0-22 V transformer from your first thread, use LM3886 for all 4 Ohm speakers and take the LM4780 for the 8 Ohmers. Be aware that you'll get much more power than you initially wanted, nearly 40W into 8Ohm and more than 70W into 4Ohm with double that figure as peaks@10%THD. If you reduce the voltage to somewhere from 15-0-15V (for 8 Ohm) to 18-0-18V (for 4 Ohm) you get around 20W@1%THD per channel. Watch your heatsinking.
To avoid pops and clicks use capacitors across each relay contact, something like 10 nF should do.
Thanks for the additional pointers.
Yes, while I have cheap speakers today, I do plan on upgrading key rooms as money permits in the future. The kids bedrooms, bathrooms, garage, basement, etc. will probably always keep the el cheapo's, but the rooms like MBR, study, kitchen, living room, etc. will likely get upgrades to definitive tech in the future. While I don't mind throwing away a $60 pair of speakers, I don't want to have to rebuild this $1000+ amp. ;-) So quality is important...
Where I read the 16dB SNR difference was at the Overture summary page on Nat'l Semi's site:
http://www.national.com/cat/index.cgi?i=i//286
The LM4780 is rated at 114dB, the LM4782 only 98dB. I find it interesting that the numbers are so wildly different from the LM1875. It makes you wonder if the standby feature has anything to do with it...as all models with that feature tend to have worse specs. It probably has more to do with the density though than anything else.
That's a great point re: heat dissipation; that's another thing that was worrying me about the 3 amp package. However, I doubt very much I'll ever be using 4 ohm speakers. My current speakers are 8ohm, and the units I'm likely to replace them with in the future are also 8ohm. So maybe it wouldn't be a concern.
Thanks Nordic, I had read about that in the datasheet. Since I plan on using a triggered relay on each individual amp pair, and snice I have mute control at the source, I do plan on wiring up the mute pins exactly like this.
At this point I'm leaning towards doing this as two amps / two projects, each with 6 zones / 12 channels. The primary benefit to doing so is I don't have to use identical amps. 6 of the rooms are much smaller then the other 6. The kids rooms, bathrooms, and laundry room are all small, very small, compared to the other 6 zones. the LM1875/LM4782 will be fine for them (and in fact, may be fine for all of them). So the plan is I'll build that amp first, and can test it everywhere. If it suffices for the larger rooms, I'll just build another. If not, I'll build a larger LM4780 version to handle MBR, Kitchen, LR, Study, Outside, Basement.
So now it really comes down to deciding between the LM1875, LM1876 and LM4782. At the end of the day, it will probably matter which boards I can get. If anyboay has any recommendations for the last two, that would be great.
Thanks,
Ryan
Yes, while I have cheap speakers today, I do plan on upgrading key rooms as money permits in the future. The kids bedrooms, bathrooms, garage, basement, etc. will probably always keep the el cheapo's, but the rooms like MBR, study, kitchen, living room, etc. will likely get upgrades to definitive tech in the future. While I don't mind throwing away a $60 pair of speakers, I don't want to have to rebuild this $1000+ amp. ;-) So quality is important...
Where I read the 16dB SNR difference was at the Overture summary page on Nat'l Semi's site:
http://www.national.com/cat/index.cgi?i=i//286
The LM4780 is rated at 114dB, the LM4782 only 98dB. I find it interesting that the numbers are so wildly different from the LM1875. It makes you wonder if the standby feature has anything to do with it...as all models with that feature tend to have worse specs. It probably has more to do with the density though than anything else.
That's a great point re: heat dissipation; that's another thing that was worrying me about the 3 amp package. However, I doubt very much I'll ever be using 4 ohm speakers. My current speakers are 8ohm, and the units I'm likely to replace them with in the future are also 8ohm. So maybe it wouldn't be a concern.
Thanks Nordic, I had read about that in the datasheet. Since I plan on using a triggered relay on each individual amp pair, and snice I have mute control at the source, I do plan on wiring up the mute pins exactly like this.
At this point I'm leaning towards doing this as two amps / two projects, each with 6 zones / 12 channels. The primary benefit to doing so is I don't have to use identical amps. 6 of the rooms are much smaller then the other 6. The kids rooms, bathrooms, and laundry room are all small, very small, compared to the other 6 zones. the LM1875/LM4782 will be fine for them (and in fact, may be fine for all of them). So the plan is I'll build that amp first, and can test it everywhere. If it suffices for the larger rooms, I'll just build another. If not, I'll build a larger LM4780 version to handle MBR, Kitchen, LR, Study, Outside, Basement.
So now it really comes down to deciding between the LM1875, LM1876 and LM4782. At the end of the day, it will probably matter which boards I can get. If anyboay has any recommendations for the last two, that would be great.
Thanks,
Ryan
The SNRs on the Overture Summary Page are not comparable, because they were not measured at the same power output.
The datasheets tell that the SNRs are not so different at 1 W. The advantage for the LM4782 at high output power is due to the fact that it has more power.
LM4782
97 dB @ 1W
114 dB @ 50W
LM1876/LM4782
98 dB @ 1 W
108 dB @ 15 W
You can also derive from the datasheets that LM1876 is the two-channel and LM4782 the three-channel version of the same chip. LM1875 is the oldest of them all and has consequently slightly worse data. Your decision will be space (LM4782) vs. heat dissipation (LM1876).
I remember having seen kits for the LM1876 on a Polish and an Argentinian site, but I don't recall either name.
How about using the layouts from http://www.national.com/analog/audio/pwramps_evaluation_boards and based on them have some PCBs made? Maybe with a few modifications. Once you're into DIY, DIAY (= do it all yourself) 😀
The datasheets tell that the SNRs are not so different at 1 W. The advantage for the LM4782 at high output power is due to the fact that it has more power.
LM4782
97 dB @ 1W
114 dB @ 50W
LM1876/LM4782
98 dB @ 1 W
108 dB @ 15 W
You can also derive from the datasheets that LM1876 is the two-channel and LM4782 the three-channel version of the same chip. LM1875 is the oldest of them all and has consequently slightly worse data. Your decision will be space (LM4782) vs. heat dissipation (LM1876).
I remember having seen kits for the LM1876 on a Polish and an Argentinian site, but I don't recall either name.
How about using the layouts from http://www.national.com/analog/audio/pwramps_evaluation_boards and based on them have some PCBs made? Maybe with a few modifications. Once you're into DIY, DIAY (= do it all yourself) 😀
Nice catch! I didn't even notice that; I should have looked at all of the columns more carefully.
When you said "LM1876/LM4782" in your post, did you meant "LM1876/LM1875" or "LM1876/LM4780" for the second set of stats? Because LM4782 is listed twice...
I didn't realize the gerber files were available as a starting point; I thought just the pictures. That is a thought... ;-)
One interesting thing I just noticed is that, shy of the standby feature, that not only are the LM1876/LM4765/LM4766 pin-out compatible, they should be design-compatible. Nat'l Semi uses the same reference PCB and parts list for all three chips. So in theory, I could pick-a-power at 20w, 30w, 40w, and the only real difference is the standby pins on the 40w chips are NC's. Have I got this right? Could I really use one design for all three of these chips, assuming the input voltage is still within range for all three? Of course, any standby circuitry would be "dead" on the LM4766 version.
If I have this correct, I think this heavily tips the scales in favor these 2-channel chips over the 3-channel. Given a 25v to 30v input voltage, in theory I could mix and match chips within the same chassis. This is especially nice as it goes extremely well with the idea I mentioned before of splitting this into two (6) zone chassis separate projects. If I have this correct, all I'd really have to do is change the chips and increase the transformer size in the second chassis. I could even use identical rectification and filtration in both chassis.
I'll build my own boards if I have to, but I gotta think there are LM1876 boards out there somewhere, and if so, they should work for both cases (I think ;-).
- Ryan
When you said "LM1876/LM4782" in your post, did you meant "LM1876/LM1875" or "LM1876/LM4780" for the second set of stats? Because LM4782 is listed twice...
I didn't realize the gerber files were available as a starting point; I thought just the pictures. That is a thought... ;-)
One interesting thing I just noticed is that, shy of the standby feature, that not only are the LM1876/LM4765/LM4766 pin-out compatible, they should be design-compatible. Nat'l Semi uses the same reference PCB and parts list for all three chips. So in theory, I could pick-a-power at 20w, 30w, 40w, and the only real difference is the standby pins on the 40w chips are NC's. Have I got this right? Could I really use one design for all three of these chips, assuming the input voltage is still within range for all three? Of course, any standby circuitry would be "dead" on the LM4766 version.
If I have this correct, I think this heavily tips the scales in favor these 2-channel chips over the 3-channel. Given a 25v to 30v input voltage, in theory I could mix and match chips within the same chassis. This is especially nice as it goes extremely well with the idea I mentioned before of splitting this into two (6) zone chassis separate projects. If I have this correct, all I'd really have to do is change the chips and increase the transformer size in the second chassis. I could even use identical rectification and filtration in both chassis.
I'll build my own boards if I have to, but I gotta think there are LM1876 boards out there somewhere, and if so, they should work for both cases (I think ;-).
- Ryan
Oops. The first set of stats is for the LM4780, the second for LM1876/LM4782.
Yes, LM1876/LM4765/LM4766 are design compatible, because OEMs have the same thoughts like you. They want one PCB to fit all.
You can check output power and heatsink size with the Overture Design Guide that you can download from http://www.national.com/analog/audio. Depending on your power supply smoothing capacity you can assume something like 1.27 to 1.41 times the nominal transformer voltage as power supply voltage. The truth is somewhere in between. To be conservative calculate the power with factor 1.27, and to be on the safe side calculate the heatsink with factor 1.41.
About heatsinking AN-1192 is interesting to read for applications, where the output power is really used, like party PA. The conclusion in AN-1192 is that the heat dissipation capability of the LM3886 is limited to 40W for the TA type and 30W for the TF type, if you use a heatsink without fan. 60W for the TA and 45W for the TF type could be achieved with fan. The LM1876/LM4765/LM4766 have a similar casing, so you can assume a similar situation. That means, watch the "Total PD/IC" field in the Overture Design Guide to make sure you don't pass these values, even if a sensible heatsink size is given.
Yes, LM1876/LM4765/LM4766 are design compatible, because OEMs have the same thoughts like you. They want one PCB to fit all.
If you want more power, you need higher supply voltage. Different chips at the same supply voltage will still give you more or less the same output power. You could use the same IC for all channels and use transformers with different voltages to select different output powers. 18-0-18V with any of the ICs is enough for the channels, where you only need 20W. From about 20-0-20V upward (less than 30W/ch) you could run into heatsinking limits with the stereo ICs. So for all places, where you need more power it is better to choose the LM3886.
You can check output power and heatsink size with the Overture Design Guide that you can download from http://www.national.com/analog/audio. Depending on your power supply smoothing capacity you can assume something like 1.27 to 1.41 times the nominal transformer voltage as power supply voltage. The truth is somewhere in between. To be conservative calculate the power with factor 1.27, and to be on the safe side calculate the heatsink with factor 1.41.
About heatsinking AN-1192 is interesting to read for applications, where the output power is really used, like party PA. The conclusion in AN-1192 is that the heat dissipation capability of the LM3886 is limited to 40W for the TA type and 30W for the TF type, if you use a heatsink without fan. 60W for the TA and 45W for the TF type could be achieved with fan. The LM1876/LM4765/LM4766 have a similar casing, so you can assume a similar situation. That means, watch the "Total PD/IC" field in the Overture Design Guide to make sure you don't pass these values, even if a sensible heatsink size is given.
You seem to be ignoring Eva's very good point earlier. I can't imagine any of the nat semi chips being very happy trying to drive speakers on the end of 140' of 12 gauge wire...
I'm not completely ignoring it, that's why I'm concerned about power output; otherwise I wouldn't be concerned at all about a 20 or 25 watt amplifier (whether chip or discrete).
I believe these chips are up to the task given that I'l l be using a very large heatsink and actively cooling it. As was pointed out earlier in the thread, since these amps are going in a network closet in a basement, the noise of active cooling is a non-issue. I have several delta fans that push 60cfm, one of those across both of the large heatsinks should keep these cool enough. I'll easily be able to dissapate many times the thermal load the spec sheet is calling for.
Of course, I'll want to build just a single dual-channel pair and test that open board prototype first to ensure they are up to mustard before I finalize on a design and buy enough parts to build 24 channels. That way if these chips really aren't up for the task, I can alter my plans.
- Ryan
I believe these chips are up to the task given that I'l l be using a very large heatsink and actively cooling it. As was pointed out earlier in the thread, since these amps are going in a network closet in a basement, the noise of active cooling is a non-issue. I have several delta fans that push 60cfm, one of those across both of the large heatsinks should keep these cool enough. I'll easily be able to dissapate many times the thermal load the spec sheet is calling for.
Of course, I'll want to build just a single dual-channel pair and test that open board prototype first to ensure they are up to mustard before I finalize on a design and buy enough parts to build 24 channels. That way if these chips really aren't up for the task, I can alter my plans.
- Ryan
As long as you take care while building it, I dont see there being much problem with heating, power requirements, or even length of wire..... I used to work for Matrix Audio Designs in Canada a few years ago and their main product was a 16 channel whole house audio unit that was based on the overture 1876
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