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Old 15th July 2008, 11:51 PM   #1
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Default 12-zone whole house audio chip amp

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
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Old 16th July 2008, 12:35 AM   #2
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Default Re: 12-zone whole house audio chip amp

Originally posted by RhinoDude

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.

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
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Old 16th July 2008, 06:42 AM   #3
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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.

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Old 16th July 2008, 07:13 AM   #4
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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.
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Old 16th July 2008, 07:20 AM   #5
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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).
I use to feel like the small child in The Emperor's New Clothes tale
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Old 17th July 2008, 01:51 AM   #6
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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
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Old 17th July 2008, 02:48 AM   #7
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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?

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Old 17th July 2008, 08:37 AM   #8
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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.
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Old 17th July 2008, 08:30 PM   #9
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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.

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Old 17th July 2008, 10:29 PM   #10
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Have you looked Here

These are good product and I have read somewhere that the IC are gainclone IC.

Good luck,
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