Multichannel Stereo System wattage per channel

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I'm building a sound system, to be all run from one unit.

I want enough power to where a subwoofer doesn't bring the rails down much so the other channels also get good power.

I want my watts per channel in the 4 front channels to be outputting the full RMS when loaded with 4 ohm speakers.

I want to design the front channels and subwoofer amps differently to get the best sound quality.

Idea is 60W x 4 min RMS @ 4 ohm (2 left, 2 right)
240W RMS @ 4 ohm BTL for a 350W Subwoofer.

To get this I plan on using a 500VA 25-0-25 Toroid. (they run cooler than EI) Is this a big enough transformer?

The highest peak I want of the rails is less than 40V because my caps I have are 40V. I want to get 60W on all 4 channels (240W) while the sub is playing at full 240W power. I'm also debating whether I should cut some of the bass out of the front channels, or not. 240W Music + 240W Bass = 480W RMS

I have a few paper schematics, but I wanted some input on power before I get started.
Hello EWorkshop, You will want a bigger transformer than that. While 480 watts will be the planned RMS output, the amplifier components are not going to be that close to 100% efficiency. It's not uncommon to have an amp running in class AB to be running 60% efficiency. If you use the National Semi LM series power opamp chips, they are more efficient, but still not going to be even 90%, much less 100% efficient. I would think you are going to need at least an 800VA transformer, preferably 1000VA or more. Your voltages should be good, but you are going to need more current to run 4 ohm drivers.


System Configuration

4 x 60W @ 4 ohms SE *Main Channels*
2 x 120W @ 2 ohms SE or 1x 240W BTL @ 4 ohms *Subwoofer channels combine for single Subwoofer*

If the bass was filtered a bit from the main 4 channels, would I be able to use a smaller transformer? The sub will play only bass, and the 4 main speakers don't have the best bass response anyway, that's where the sub will fill in the lower end.

For output transistors I'm using MJL4281/4302 1 pair per channel. Same for the two sub amps, but 2 pairs each amp. I was planning on using a higher bias for the main channels, and low bias for the sub channels. I want good clean treble and voice, I won't mind a bit of extra bias with the big heatsinks I just got :D

I also have some NJL4281/4302 (Thermaltrak Transistors) and I'm thinking of possibly using them for the main channels instead..

*As an extra bonus, I want to run the front and rear channels out of phase so I can have the option of using bigger 8 ohm speakers (BTL 120W x 2) if I'm not using the 4 4ohm speakers. So I still get the same watts with 2 x 8 ohm or 4 x 4 ohms. :D
Hello EWorkshop. The next question here would be at what frequency do you plan on crossing over from the bass to the mid/high. Bass is the most power hungry wavelength, however some midbass can still require some power. It's hard to say exactly how much. Another thing to consider is while you are planning on 4x 60w RMS and 240w x1 RMS, what are your listening habits. If you listen at lower volumes and crossover the majority of the bass to the subs, then you may be able to get away with the transformer you have, presuming that you rework the power sections for the mid/high to be a lower RMS power, and maybe work towards better peak power. This would mean that you can't sustain high output currents to your speakers, for long periods, but if you have large enough capacitors for the power supply rails, then you will be able to handle some good loud bursts.
Things to consider when crossing over bass, as far as being unable to audibly locate a bass driver, then the crossover freq. needs to be 100Hz or lower. Above 100Hz, most people can detect the location of the source. Below this the wavelengths are usually too long. The problem with this is that signals of 100Hz and above going to the mid/high freq drivers will require more power, since you are still sending some pretty power hungry wavelength signals to the mid/high amps. A question that would need to be considered here is that if you do a biamp design, will the low bass drivers be mounted in the same cabinet as the mid/highs, or will be it more of a sub/satellite design. If it is the former, then you could do a crossover frequency of 300Hz between the bass driver and the mid, this would alleviate much of the high power demand from the mid/high amplifier channels. Check out Rod Elliotts website He has some pretty good designs, and he is a fan of active crossovers between the preamp and power amps. Thus he has some good designs for this, and good explanations of how to go about this sort of design.



P.S. Please don't take the recommendation of Mr. Elliotts website as a brush, I will still offer my input as I am capable. I just think that Mr. Elliott has a lot more experience than I in this area!:D ;)
My listening habits push the limits of the system. Sometimes I listen low, but when I want to, I crank it up! :eek:

The goal of this system is to have all channels together, and use small speakers, but high wattage, to sound like a full size sound system. Yes, I will turn it up for long periods of time.

The last speaker I blew was a 1200W (350WRMS) 12 inch Sony sub. :hot: I blew it with my 350W DIY sub amp when listeing to heavy rock.
Voice Coil was melted, had loose turns, and was also shorted to 0.9 ohm! All it did to the amp was blow the fuse. Works fine still.

I figured 240W would be a bit tamer for a 12" sub. It's set up like a sub-satellite system, but uses 6x9 and 6.5 inch speakers in small sealed boxes as satellites. Small enough to move around and not take up the floor, but big enough to do some midbass and handle high power to have the sound of a full-size system.
This was a transformer I was looking at:

They don't make the higher VA in lower voltages. With a toroid, I wonder how much extra current you can get with these if you sag the rails slightly below spec voltage....

BTW, I'm using 40V, 70,000uf caps. I have to use these caps, so I don't want rails that peak over 40V. I don't mind if the rails sag too bad as long as all channels get nearly full RMS. I think the 70,000uf per rail may be big enough to make it sound strong.
Hey EWorkshop, The transformer you listed is definitely better for the power range you are looking at, especially when combined with the capacitance you are planning for your rails. Speaking of the caps, they are going to have a pretty huge inrush current draw when you first power the amp on, I would suggest that you put a NTC thermistor into the circuit on the primary side of the transformer. A CL-60 should work. They are available from Digikey, among others. This should keep the instantaneous current down to safe levels for the windings of the transformer. Otherwise you could cause current levels that will destroy the transformer windings.


Hello Again EW, One more thing to think about, what is the surge voltage rating on your caps. The input of the transformer is rated at 115v for 25v output on the rails. This will give you approx 35.4vdc after filtering. If your incoming ac is 120v, then your output will be 26vac on the trafo. The vdc after filtering will then be approx 37vdc. If there are any major surges on the incoming ac, it could beat up the main filter caps pretty bad. Just something else to think about.


25Vac will take your 40Vdc caps almost to their limit.
+5% regulation
+6% mains voltage tolerance
*1.414 peak voltage
-0.7Vdc for rectifier Vdrop.
gives max cap voltage of 38.6Vdc without a load and no ripple.

Five channels totalling 480W will require around 500VA to 1000VA if all require maximum power at the same time. You may get away with a 500VA transformer, certainly worth trying.
But your planned smoothing capacitance is far too low.
For good well extended bass you need ~+-20mF for an 8ohm speaker load. that equates to +-40mF for a 4ohm load.
But you have 4 off 4ohm loads and 2 off 2ohm loads. That is equivalent to 1ohm on the surround channels and since you don't require low bass from these you may get away with +-40mF for the 4 surround channels.
The sub-bass will require around +-160mF.
Go and build the sub-bass amp with it's own PSU properly scaled to not require bridging.
Are your 5 speakers distributed around the room?
Then power each from a monoblock located right next to the speaker terminals.
BTW, the capacitors are 40VDC Rated, 50VDC Surge

I've noticed commercial whole 5 channel receivers using just a pair of 4700 uf caps before :eek: I'm thinking 70Kuf may do, but I have some more 50V 10000's if I have to add more.

The speakers will be around the room. Two Front and Two Rear Speakers. The purpose of this system is full-size sound, but taking little space.

I want the amps and the Subwoofer to be part of the same unit, powering the small speakers from there. Like a Computer Sound system on a larger scale. That way the whole sound system only takes the floor space of just a single sub, instead of having space used for 2 large floor speakers, amps, and a subwoofer.

I'm also wanting to save space by using only 1 transformer, which is why I want to run all channels from the same rails, to keep it simple and reliable. I want to use toroid because they don't get hot.
EWorkshop, how many of each cap do you have and what sort of config were you thinking? Are you just planning on putting an enormous amount of capacitance in parallel on each rail, or were you thinking of using the transformer to feed separate rectifier bridges and filters for each channel. This will have some influence on how to set things up.
For instance, you could use the 70 KuF caps for the sub channel, then use a pair of 10 KuF in parallel on each rail for each mid/high channel. Would require a pretty big bass cabinet, since you are planning on putting the amp in the cab with the driver.
I would also definitely recommend using an NTC thermistor, otherwise you will have some enormous current surges on power up, possibly enough to trip the breaker for the outlet the amp is plugged into. Also make sure to use the proper amperage fuse on the incoming AC power.
If you still don't get enough output from this sort of setup, you will need either a bigger transformer, or multiple transformers so you can feed power to channels separately.


Well, I don't want it to get too large. To keep it small, I'm building the amps together on the heatsinks. Amp & heatsinks mounted on the outside of the box on the back.
I can't take up a bunch of space with a bunch of paralleled caps like in a big enclosed amp. Also, I must stick with only 1 transformer.

I was thinking of using the 2 70Kuf caps after the 50A bridge rectifier for main filtering of the two rails, and 2 10Kuf on the amp PCB after them, so there's some capacitance after the power wires, close to the amplifiers. 80Kuf rails total. An NTC thermistor IS a must to keep from blowing the fuse. I plan on using one. My DIY Sub amp has a very big thermistor I got from a computer PSU.

I plan to use 12 gauge Wires for all power wires.
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