[JohnR]

I've been thinking about building multi-channel amps for a while. The obvious application is active speakers, but I need one for home theater as well. And I need some mundane things like volume control so it's not just a matter of bunging six modules into a box. I guess what I'm after is a sort of "reference" design that I can use in multiple situations.

Here's the overall block diagram. It's not as elaborate as the diagram makes it look. More details on the individual blocks later.

[JohnR]

Gain trim: for 5.1 usage, used to adjust levels for center, surrounds and sub. For active speaker usage, use to adjust levels (post-DAC) for the three sets of drivers.

Circuit on the left gives around +/- 12 dB of gain. On the right, up to 6dB of attenuation (assuming 10k for the 6-ch attenuator):

[Æ]

So, are you asking for ideas or suggestions? Or just meandering?

I'd hate to think of the power supply you are going to need for six channels of amplification, especially if you are building with the intent of high power output. Is this amplifier going to get hot or not?

I see nothing wrong with shoehorning six modules into one enclosure, just as long as you do it competantly.

I have a couple of large toroids with five center tapped secondaries, I've kind of sort of considered building a five channel amp for myself.
Years ago here at my local electronics surplus store there were some ready four channel amp boards, with no documentation though.

[JohnR]

So, are you asking for ideas or suggestions? Or just meandering?

All of the above I still have a few more blocks to draw from the scribbles in my notebook, but suggestions and ideas are welcome too! At some point I'll need help with physical layout of the line-level section of the prototype, it's been well over 20 years since I made anything solid-state from scratch

Hopefully I'll get the rest drawn up today.

I'll be using Class D and SMPS for the power section. I've got a few configurations noted down. You can get anything from 10 to 600 W per channel. The mechanical challenges of Class A/B would be too much for me, although I do have some unused modules lying around.

[JohnR]

The 6-channel attenuator. This part I have struggled with for a long time. Here are the options that I know about (prices not including postage):

• 6-gang ALPS pot from ebay seller familygate - roughly $20
• 6-gang ALPS pot with a remote control from analog metric - roughly $60
• Goldpoint 6-channel attenuator - $349
• Above plus remote control from Bent Audio - +$190 = $539
• Full monty Bent Audio system with resistive or transformer volume control - fantastic gear but probably overkill for a project like this

I would have liked to include a cheap-ish digital remote system able to drive 6 analogue channels, perhaps situated in price between the ALPs and the Goldpoint, but I can't find one - any ideas?

[JohnR]

Buffer/EQ

There are two reasons for this block. The first is that many Class D amps have quite a low input impedance, so a buffer between it and the volume attenuator is a good idea. Second is that it's a chance to introduce some EQ for baffle step compensation. This could also be used for dipole EQ, at a pinch.

At left, 10:1 ratio in BSC center frequency. Perhaps too much. Gain of BSC is assumed to be 6dB max, but again varies with frequency.  These are intended to be "set and forget" pots. Of course, if you don't need any EQ, do a simple buffer as at right.

[Russtafarian]

Does the MiniDSP 8X8 with rotary/remote volume card work for what you're trying to do?  Probably overkill for just volume but if the xo/filtering capabilities will help you it might be worth considering.

[Tyson]

Yeah, the miniDSP seems like it would be a good solution.  Overkill?  Maybe, but when in our hobby is overkill ever a bad thing? 

[JohnR]

It's a bit large though, physically. Also it would add to the cost of the project, almost doubling it. I did originally have a balanced 2x4 on channels 5 and 6 but eventually realized that it would just be simpler to have a separate box if digital EQ was needed.

[JohnR]

If one were feeling sufficiently ambitious, here's a 6-channel volume control schematic:

  http://bygselvhifi.dk/51volcontrol.htm

Haven't found an off-the-shelf one yet.

[JohnR]

The phase/driver block is used on channels 5 and 6 only. It serves to provide balanced outs to drive a more powerful amp. In addition, if amps 5 and 6 can be bridged, a switch is used to select this mode. This would be used only for 5.1 systems, and is most likely to be useful with low-power versions of the amp (so you could get more power for the center channel, and since you would be using an external amp for the sub anyway).



I did come up with more complicated versions of this, but have decided they are unnecessary...

[JohnR]

OK, lessee.... amps and power supply. Lots of choices, here are some example combos that (in theory) should work well. Cost figure is just amp modules and power supply - add the chassis and other hardware plus preamp circuity. And postage.

This will take a few posts....

Config 1

Hifimediy T1-M TK2050 low voltage x 3.

Connexelectronic SMP300R, at 24V with +/-15v aux supplies.

~$130. This is the "silly cheap" option. "Low power" still means 50W/ch. While I haven't seen any reports on the sound of the T1-m, I think I'll probably use this config for the first prototype.

As far as I can tell, you can't bridge this module, so in a 5.1 system, channel 6 goes to waste. Oh well.