adjustable XO / amps ?

[jimbo1968]

Be gentle with me, I haven't searched very hard but that's only because I don't really know what to look for.

I'm excited by the idea of building different speaker designs but a little daunted by the XO design. Plus my personality is such that I don't like to set things in concrete but prefer to keep things flexible.

What I want is an adjustable set-up that I can quickly attach new speakers to and try them out. At first I'll be experimenting on just the L and R channels of the 5.1 output from my AV amp but later may expand to include centre and rear channels.

Starting with a 2-way speaker design, I'm thinking of a single box per channel which allows me to adjust the crossover frequency, and adjust the gains of the split signals, and then power amplifies them (3 dials on the front).

Is this at all sensible/possible. If so, I expect it's been done before?

Or is their some other way to do what I want?

TIA, Jim

PS. What sort of power split is typical for a 2 way design, ie would the 2 power amps be the same or would one be bigger than the other?

[theChris]

most designs for adjustable power supplies involve adjusting resistors via a potentiometer. some designs have more frequency dependent resistors then others. other designs will not retain the desired Q as resistance changes.

the most flexible design would be a DSP algorithm, as you can set all the parameters to exactly what you want.

another method to do what you want involves digital potentiometers and a microcontroller.

the lower order the filter the easier it is to design.

look into subtractive crossovers as well, as they can help simplify the design by requireing you to only make a lowpass.

generally the power supply is the same for convience and peak power -- a highpass can theorerically require 2x normal peak voltage (on a square wave). unless there is a real capacitor on the output that can charge to Vcc, you would have to simulate one...

[jimbo1968]

I've been looking at all the gainclones out there, very nice many of them are and it makes me want to build something stylish as well as functional. So I'm happy that the power side is inexpensive and quite easy to do.

If I'm going to build general purpose amps for each channel they should be 3-way I guess. Which if I have understood means 1 transformer, 1psu kit, and 3 amp kits per channel. Which looks like it'll cost about 100USD plus box, connectors, switches.

Now I just need to understand the crossover/dsp bit. Should I go to the speaker area for that?

[johnf]

Originally posted by theChris
look into subtractive crossovers as well, as they can help simplify the design by requireing you to only make a lowpass.

State variable filters are also good for minimizing the number of tuning resistors.

[jimbo1968]

I'm going to have a go at using a dcx2946 to give me my 6 filtered channels for the 2x 3-way speakers.

So that means I want to build 6 channels of amplification. One decision I'm having trouble with is how many transformers and how many boxes. I want a nice clean layout with good separation of psu and amps (separate compartments if not boxes). Also I want course stepped attenuation on each channel (because I may need to use high levels through the dcx).

My favoured options are:

3x 2ch amp with 1 transformer in each box. These boxes could have many other uses, and are easy to design nice and symmetric.

2x 3ch amp with 1 transformer in each box. This is a neat match with the intended purpose of driving 2 3way speakers. Gives the option of locating the amp with (inside) the speaker later.

1xpsu , 2x 3ch amp. A 3 joined box design could look very smart, unfortunately it'll be sitting under the dcx so maybe I shouldn't be aiming for great looks).


What do you think ?

What sort of wire/connectors should I use between psu and amps?


I'm thinking of plate aluminum designs but someone said al was not good for RFI, so do I want steel box with al for the heatsink only?


Jim

[Nuuk]

I gave a lot of thought to a similar project and finally decided that I would use 3 dual mono amps with each channel having it's own PSU (in the amp housing).

Of course this is the most expensive option but whatever you decide on will have to take cost into consideration!

[richie00boy]

Take a look at the MOX active crossover by Jens and Tiroth.

[jimbo1968]

Nuuk - Ah so you are the decibel dungeon. I have read quite a few of your pages. Did you actually build an active 3way speaker?

I like the looks of your wood tube monoblocks, but don't you have to veneer your CD player to match?

I'm pretty decided now that this first project will be 6 channel in one box. The box will be functional rather than pretty (probably stainless steel as I can get that fabricated for free). 1 or 2 transformers will sit in the centre compartment.

Is there any advantage to putting each amp in it's own compartment ? Or will 3 of them share the same space happily?

Big unknown for me is heatsink. Can I just screw each amp to a piece of al angle, and screw that to the steel box floor? Or should I put each amp on a thick piece of al which is also an external wall?


hmmm, I may have changed my mind already. If I can get small neat looking boxes made, then 6 monoblocks could be good. You can't get more flexible. I could build an A/C power strip with 6 power plugs on it that each monoblock sockets onto (otherwise I can see that it will be a full time job keeping the 6 blocks neatly lined up)

[jimbo1968]

Richie - I will take a look. I don't yet know what is meant by a MOX active xo. My target is easily adjustable xo ( by easy I mean stand up, twiddle a knob or pot, sit down again - not swap resistors etc).

My aim is to be able to experiment with drivers and enclosures without having to understand the workings of crossovers.

And using DSP I'm expecting that the xo design tradeoffs will be removed (not that I know what the tradeoffs are!).

Jim

[Nuuk]

Nuuk - Ah so you are the decibel dungeon

That's right, and just down the road from you in Burnham-on-sea (and not too far from Richie!).

Did you actually build an active 3way speaker?

No but I was considering a three amp set up for my current speakers (TL subwoofer, full-range and tweeter).

I like the looks of your wood tube monoblocks, but don't you have to veneer your CD player to match?

I have considered a makeover for the CD but it would be a tricky veneering job with the rounded edges. Perhaps solid wood for top and bottom and veneer for the front!

Big unknown for me is heatsink.

I have found my LM3875 amps to run only luke warm. Even the LM3886's don't get very hot. It depends how hard they ahve to work but if you can get a big piece of angle and add a PC heatsink for each chip you should be fine, specially if you put vents in the base and top of the case.

If I can get small neat looking boxes made, then 6 monoblocks could be good.

Those cylindrical enclosures are not too difficult to make! You could paint or veneer them and if you can find somebody with a lathe you can easily knock up the front panels