OK, I have been pondering this for days, and haven't managed to make up my mind as to how I will proceed, so here is the problem.
I am trying to use up various bits and pieces to make a small bedroom system. It will consist of two way monitors with a single sub. That part is sorted and no issues there. However, I need to get the amps sorted out, so here is the brief.
1) It has to fit in a 1u rackmount case. Non negotiable.
2) Amps will likely be three LM4780s, two doing mid high duties, and one bridged for the sub, but this is open a little, I'm only using those as I have both the chips and a PCB layout already done.
3) The only traffos I can fit in this case are 100VA 12-0-12-0. Buying new is not currently an option. I have three.
4) Output power is not a huge issue, the mains will unlikely be ever running more than 10W apiece.
I have thought this through so much I am now going around in circles, and can no longer think straight, so I'm throwing it open to you guys for suggestions. I want your unbiased opinions, so I'm not going to post my thoughts just yet.
I am trying to use up various bits and pieces to make a small bedroom system. It will consist of two way monitors with a single sub. That part is sorted and no issues there. However, I need to get the amps sorted out, so here is the brief.
1) It has to fit in a 1u rackmount case. Non negotiable.
2) Amps will likely be three LM4780s, two doing mid high duties, and one bridged for the sub, but this is open a little, I'm only using those as I have both the chips and a PCB layout already done.
3) The only traffos I can fit in this case are 100VA 12-0-12-0. Buying new is not currently an option. I have three.
4) Output power is not a huge issue, the mains will unlikely be ever running more than 10W apiece.
I have thought this through so much I am now going around in circles, and can no longer think straight, so I'm throwing it open to you guys for suggestions. I want your unbiased opinions, so I'm not going to post my thoughts just yet.
I did look at the 41Hz site today, and was thinking of their 4 way Amp9 kit, with our TI design on sub duties, using one traffo for the Amp9 and one for the TI chip, then I thought I could just use two in series for the sub LM4780 and one for the mid/hi's, then I started go round in circles again!
Interesting. I must admit, I hadn't considered half wave rectification. I'm not worried too much about cooling, as I have a nice little tunnel I made up ages ago that was good for 100W classA/B dissipation (in normal use, not sine waves!) with the fan just ticking over, so I could go regulated.
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Hi Pink
what is the problem?
The transformers fit, the cooling tunnels fit.
Is it the amp module that is too tall?
or that the smoothing caps are too fat?
two 100VA transformers give adequate power input for three 60W chipamps.
Is it the voltage?
2*12Vac is a bit low
2*24Vac is a bit high, or could you manage with +-33Vdc?
What if you could split ONE 12Vac winding in half to get a centre tapped 12Vac (6+6Vac). Then you could get 2*18Vac. The slight problem would be using 100VA but only 50VA from the other.
If you split one 12Vac winding on each of two transformers then you can get reasonable balance using about 75VA from each.
And have two spare 6Vac windings (12Vac total) that could run your fan , control circuits and relays.
what is the problem?
The transformers fit, the cooling tunnels fit.
Is it the amp module that is too tall?
or that the smoothing caps are too fat?
two 100VA transformers give adequate power input for three 60W chipamps.
Is it the voltage?
2*12Vac is a bit low
2*24Vac is a bit high, or could you manage with +-33Vdc?
What if you could split ONE 12Vac winding in half to get a centre tapped 12Vac (6+6Vac). Then you could get 2*18Vac. The slight problem would be using 100VA but only 50VA from the other.
If you split one 12Vac winding on each of two transformers then you can get reasonable balance using about 75VA from each.
And have two spare 6Vac windings (12Vac total) that could run your fan , control circuits and relays.
Hi Al:
If the desired power is 10W and the cooling of the 1HE box is not an issue, there a few elegant amp designs you may wish to consider building from the components at hand.
Design 1:
sub --- 1xLM4780 in bridge + 1x100VA/2x12V ---- ca 50W/8ohm
sats --- 2xLM4780 stereo + 1x100VA/2x12V ---- ca 12W/8ohm/ch
act.crossover --- 1x100VA/2x12V + LM317/337 at 12V DC
Design 2:
sub + sats --- 1xLM4780 parallel + 2xLM4780 stereo + 2x100VA/2x12V (2x30V DC)
act.crossover --- 1x100VA/2x12V + LM317/337 at 12V DC
Design 3 (for a sub with a low Z):
sub --- 1xLM4780 parallel + 2x100VA/2x12V (2x30V DC)
sats + act.crossover --- 1x100VA/2x12V + LM317/337 at 12V DC for crossovers.
In my opinion, the proposed Design 1 would be the optimal solution provided that LM4780 is suitable for high frequencies at low PSU voltages (+/-16V), which is something that would need to be tested on the workbench first.
You may experience some trouble with fitting everything (together with the power supply rectifiers and low profile capacitors) in your box, though.
Regards,
Milan
If the desired power is 10W and the cooling of the 1HE box is not an issue, there a few elegant amp designs you may wish to consider building from the components at hand.
Design 1:
sub --- 1xLM4780 in bridge + 1x100VA/2x12V ---- ca 50W/8ohm
sats --- 2xLM4780 stereo + 1x100VA/2x12V ---- ca 12W/8ohm/ch
act.crossover --- 1x100VA/2x12V + LM317/337 at 12V DC
Design 2:
sub + sats --- 1xLM4780 parallel + 2xLM4780 stereo + 2x100VA/2x12V (2x30V DC)
act.crossover --- 1x100VA/2x12V + LM317/337 at 12V DC
Design 3 (for a sub with a low Z):
sub --- 1xLM4780 parallel + 2x100VA/2x12V (2x30V DC)
sats + act.crossover --- 1x100VA/2x12V + LM317/337 at 12V DC for crossovers.
In my opinion, the proposed Design 1 would be the optimal solution provided that LM4780 is suitable for high frequencies at low PSU voltages (+/-16V), which is something that would need to be tested on the workbench first.
You may experience some trouble with fitting everything (together with the power supply rectifiers and low profile capacitors) in your box, though.
Regards,
Milan
Fitting everything in is tight, but do-able. I quickly knocked up a rough layout, the small encapsulated trafo is for the fan.
Milan's option 1 is what I favoured, simply because I had the parts at hand, though I was thinking of using one traffo each for left and right, and fitting another in for the crossover. BTW, the crossover board is the Silicon Chip three way, I built it a year or so for testing, and it wasn't quite up to running the main system but it should be fine for this one, and will only need a few resistors changing to fit my drivers.
Milan's option 1 is what I favoured, simply because I had the parts at hand, though I was thinking of using one traffo each for left and right, and fitting another in for the crossover. BTW, the crossover board is the Silicon Chip three way, I built it a year or so for testing, and it wasn't quite up to running the main system but it should be fine for this one, and will only need a few resistors changing to fit my drivers.
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While I agree that LM4780s work with +/-15V, they don't sound nearly as good as they can at -/+ 30V or more. I run a 4780 amp at +/-38V and an 4766 at +/-35.4V. I have had some very critical audiophiles tell me that these are the best-sounding chipamps they have ever heard.
I have found that with all National Overture chips, they work best when they are run in the top 15% of their supply rating. They use less current, run cooler and sound WAAAAAY better.
If this is still to be a small room system, why don't you use the LM4831 (2 X 20w at +/-10 to +/-28V ) or LM4766 (2 X 40w at +/-10 to +/- 39V) instead of the 4780. They can be bridged or paralleled, just like the 4780, they only have 15 pins per pkg (instead of 27) and are dead simple to implement.
JessG
Alert bay, BC, Canada
'Cos I've got the 4780s! 
I agree about the higher voltage rails, I was running BrianGT's LM4780 boards at the limit with 42V rails, and they rocked. But this isn't the ultimate audiophile project, I just want something for the bedroom, and I have lots of bits I needed to use up. SQ isn't paramount, but I have to say so far it doesn't sound bad on the bench. I've taken it apart again to build into the case, or I'd post pics.
I agree about the higher voltage rails, I was running BrianGT's LM4780 boards at the limit with 42V rails, and they rocked. But this isn't the ultimate audiophile project, I just want something for the bedroom, and I have lots of bits I needed to use up. SQ isn't paramount, but I have to say so far it doesn't sound bad on the bench. I've taken it apart again to build into the case, or I'd post pics.
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