Powering 4 Components from 2 PSUs

Hi everybody,

I'm in the process of building my first audio setup and have a question about the power supplies. The intended use of this setup will be common room speakers for the suite where I live, but I want to reduce the number of wall outlets I take up with my system so my roommates don't get mad.

I'm planning to have 4 components total: an active preamp/tone control from Rod Elliott; active XO from Rod Elliott; two PA100 LM3886 amps (byalexw88) for a stereo amp; and mono PA100 or BR100 LM3886 sub amp.

(If this is helpful, below are links to the project pages where I'm getting my info)
Preamp - Hi-Fi Preamplifier
Active Crossover - Project 123
PA100 Stereo Amp - PA100 DIY 2x LM3886 in parallel gainclone audio amplifier
Sub Amp - either copy PA100 from above or use BR100 from AN-1192

Could I run my two amps from a single power supply and my preamp and crossover from a single power supply, so that I only need two wall outlets? While I feel that I understand the theory of the supply circuits pretty well, I don't have a grasp of how much power they can be expected to put out or how many of my components they can be expected to drive.

As a parting note, I apologize if this has been discussed elsewhere. I've rummaged through these forums but haven't found anything on my own, but if it is out there feel free to point me to that thread rather than explain it to me.
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
It's possible to run the lot from one power transformer. Reasons for not doing this might include whether you understand the process enough to do it right, making it best to stick to the published projects. There are arguments for and against, but either should work well.

Broadly speaking, the difference between the 30V power amp rails and the 15V supplies for the earlier stages, which don't draw much current, allows you to incorporate enough resistance tapped from a suitable point to drop the voltage and to smooth the pre stage rails, isolating them. This way they can have a relatively independent reservoir with low enough ripple to suit those stages.

It's worth deciding whether you want to use one or more cases, and whether you want to keep the different stages separate but housed with their supplies like commercial offerings, or the amps separate from the supplies. Some considerations here are noise and earthing concerns.

I notice the power amp project suggests regulating the supply. If you choose to go this way you'll have to ensure that the devices can handle the current needed for two amps and can deal with the heat, and similarly with all components, eg the first large capacitors will see more ripple.
 
Hi,

I see no use at all for that active crossover in any hifi setup.

sreten, could you elaborate on why you wouldn't use the active crossover? My reasoning is so that the lower frequencies are only sent to the sub, reducing the excursion of the L/R drivers.

sreten and rayma, I wasn't sure if power strips would be logical to use with a number of high-power devices - I've never used those for more than lamps, so thanks for that advice :)

Broadly speaking, the difference between the 30V power amp rails and the 15V supplies for the earlier stages, which don't draw much current, allows you to incorporate enough resistance tapped from a suitable point to drop the voltage and to smooth the pre stage rails, isolating them

AllenB, are you suggesting that the voltage drop come from resistors - aka use a resistor voltage divider to create ±15V rails between the 30V and ground and put caps between all of the rails - or do you mean use a regulator to drop the voltage? I didn't really follow what you meant when you said "resistance tapped from a suitable point." :confused:

Ideally I want to use as few PSUs as possible for the simple reason that transformers are expensive (the toroidal 500VA 2x25 ones I've found are ~$100 ea). In combining PSUs I guess the individual transformers will get more expensive but I think this would be less than multiple smaller ones. One of my college professors in EE has agreed to lend a hand if I need it so I think I can get the brainpower to wire up a combined power source correctly if I need to.

Besides ensuring the secondary voltages are high enough, do I mainly need to look at the VA rating when determining if a transformer will be adequate to supply multiple components?
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
resistor voltage divider to create ±15V rails between the 30V and ground and put caps between all of the rails - or do you mean use a regulator
Either way, the opportunity is the same.
One of my college professors in EE has agreed to lend a hand if I need it so I think I can get the brainpower to wire up a combined power source correctly if I need to.

Besides ensuring the secondary voltages are high enough, do I mainly need to look at the VA rating when determining if a transformer will be adequate to supply multiple components?
This is handy, as there is much involved in power supply design. Most simply you could add the extra stages required to bring about the pre stage supplies by leading off from the power stage rails (or before the regulators if suitable). It would probably just work, but you should really consider the allowable ripple for each stage, the isolation of the stages, noise injected into the circuit and also due to the customisations, and the stability of the supply especially considering the regulators.

Secondary voltages and VA rating, yes. It may also be necessary to revise the design of the first stages to accomodate the headroom needed for the regulators if you choose to use them, in light of the actual ripple under higher current conditions. There is an interesting program called PSUDII you might want to play with.
 
Ok so I've done some more reading and talked to some people and I'm thinking three PSUs - 1 for my preamp/crossover (I have decided to keep the crossover based on other research), 1 for the stereo amp, and 1 for sub amp.

The preamp PSU seems simple enough, as there will be little current draw there so a small transformer can be used.

For the two power amps, 500VA (like I'd seen in other projects) seems unnecessarily high. I've read that typically 2x(Average Output) is a good rule of thumb for picking a transformer, aka a 100watt amp needs a 200VA transformer. Do any of you have thoughts on that? I don't remember what that person said about whether that was for a single channel or a stereo amp...

I've decided to switch from a PA100 to a BR100 for the stereo amp so I can drive 8 ohms if I want to with more power, and I will probably build the subwoofer amp the same way. So, could I drive one pair of these amplifiers with a 250VA transformer and the other with another 250VA transformer, as output into 8ohms is about 120 watts?

I've also seen a lot about getting the capacitance bank right. I was initially under the impression that "a lot" was as specific as you needed to be, but I now realize the ripple shouldn't be too small or you will draw lots of current. is 10,000 uF typically a reasonable capacitance bank for the rails?
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
Much depends on your circuit. A supply might have a second stage to sure up the DC. It might have a third stage to buffer the rails. It might have a regulator.

Broadly speaking 10,000uF might be considered as a maximum for a first stage and it might be seen as a minimum for a later stage.