Three-way Project

[WasteRunner]

I'm working on a three-way speaker system that uses Scanspeak drivers powered by an actively crossed-over LM3886 power stage. The electronics include a Linkwitz Transform and a 24db/octave crossover that can be switched to 12dB/octave. There is a remote power on section, balanced input and a couple other cool things going on.

The board has seven LM3886T devices on it: one for high end, two in parallel for midrange, and basically a BPA-200 for low end. The first iteration uses smaller format drivers and will use lower Vcc, resulting in only one mid range opamp and one BTL pair being needed (plus of course the tweet power). The next design will use a larger driver set using the same board, but will operate at +/- 35V and will need all seven opamps.

I'm currently working on the power supply section and am looking forward to auditioning several different designs, as it is my view that the power supply is the determining factor in overall sound quality. I'm starting with a 600VAC toroidal and paralleled 2,200uF caps bypassed with films.

[macboy]

Very nice; I have a project planned to convert my existing 3-way mains into a self-powered, actively crossed system with 4th-order LW aligned crossover as well. Your idea sounds very similar to mine. I was also planning to use balanced inputs (differential signalling over CAT-5 cable) to supply their input signals.

You may be overestimating your power requrements, but that is not for me to judge. Anyway, you may find that even the larger drivers perform well with the smaller initial set of amps that you have planned. Try and see I guess (that's half the fun).

I am curious to hear your ideas on power supply design. My intent was to use a single transformer per channel (one each Left and Right) but to have the rectifiers and caps individual per amp-speaker (tweet, mid, woofer each have their own rectifiers et all). I believe that this would substantially reduce the interaction/coupling between the power supply rails for the different frequency bands' amps, vs. just using one set of rails for all amps. The PSRR of these chips is, after all, merely good, not exceptional.

[WasteRunner]

Thanks macboy.

I am also concerned about inter-amplifier power coupling effects. I have a 600VAC xformer mounted in the bottom of each speaker so there are individual supplies for left and right by design. I was also considering seperate bridges for the bands but after thinking about it, I don't see that multiple rectifiers is a substantial benefit regarding isolation. I hope to get appreciable isolation gains through individual banks of capacitors for each band, possibly combining mid and high (not a lot of space in the speaker cabinet). What interests me the most is if there will be any audible difference in using reasonable caps and grossly expensive ones.

The main advantage of having several chips in parallel is that you are able to get the output impedance way low, and that's a good thing. I will be impressed if I find that i don't need more power than the single set of BTL's on 25V rails.

I did a short listening test of the Linkwitz Xform on a set of mockup cabinets to see what it sounded like before having a case company builf the real deal. I consider low-end directional and, more importantly, it has to be created in the same physical manner that the rest of the spectrum (or low end part at least) is. When 80-250Hz is coming out of one size box and driver and everything below that is coming out of a sub, I don't think that you can get the imaging right. So I wanted to try to get a three way system to be flat all the way down, understanding that the output wasn't going to be all that. I have to say that I am more than pleased with the overall response of the proto's. I do concert sound for a living and am used to tons of low end that isn't necessarily well-imaged with the rest of the system. The proto's are vastly different, and they are using a 7.5" woofer with something like 7mm Xmax! Of course there's no sternum crushing going on, but they still get loud, and they go way dooowwwwnnnn.

[jleaman]

Originally posted by WasteRunner
I'm working on a three-way speaker system that uses Scanspeak drivers powered by an actively crossed-over LM3886 power stage. The electronics include a Linkwitz Transform and a 24db/octave crossover that can be switched to 12dB/octave. There is a remote power on section, balanced input and a couple other cool things going on.

The board has seven LM3886T devices on it: one for high end, two in parallel for midrange, and basically a BPA-200 for low end. The first iteration uses smaller format drivers and will use lower Vcc, resulting in only one mid range opamp and one BTL pair being needed (plus of course the tweet power). The next design will use a larger driver set using the same board, but will operate at +/- 35V and will need all seven opamps.

I'm currently working on the power supply section and am looking forward to auditioning several different designs, as it is my view that the power supply is the determining factor in overall sound quality. I'm starting with a 600VAC toroidal and paralleled 2,200uF caps bypassed with films.

nice board. : O ) Do you got the gerber files or the board / schematic files ?

[WasteRunner]

Yes jleaman, I have the files, although I would prefer to send them to you instead of posting them. I'm also trying to pull together a website with better pitcures and project description.

[macboy]

WasteRunner,
I don't know if youv'e considered this, but for a bridged amp, you do not need gobs of capactiance between each rail and ground, since the speaker current will always flow from the V+ rail to the V- rail (never from + to gnd or gnd to -). So your big caps need only to be directly across the V+ and V- rails, and in fact this is preferred, because it keeps the speaker currents from influencing the ground voltage. This also gives you the benefit of greater energy storage in the caps for a given uF rating (because you doubled the voltage across the caps, and Energy is proportional to Voltage squared). You will of course need caps of appropriate voltage rating. You will still need small caps on each rail to gnd (I'd use maybe a couple hundred uF, more if the pre-amp opamps shared these rails).

[mwmkravchenko]

Mighty interested in your board to. If that is possible.

Mark

[WasteRunner]

I hadn't considered that macboy. It sounds like a good idea to decouple the bridged pairs to the supplies instead of the ground, since that is essentially were the laod current is ending up anyway. As far as doubling the effective onboard capacitance, I wonder how sharing the big EL caps between the two chips will combine with decoupling to ground. They share them in the design because, being out of phase, they draw off opposite caps and thus don't need individual ones. I wonder if that quadruples the effective capacitance or if the effects cancel. I'll have to think about it. To me, tricks like this is what makes superior performing devices.

What power supply arrangements have you tried and what differences have you noticed?

What is the safest way to share files in this forum without posting my email?

[WasteRunner]

Today I was able to get everything going. The circuitry performance is pretty much what I was expecting, although I'm not certain that LM3886's sound the same as LM3875's. The relays that disconnect the AC mains from the bridge that feeds the LM3886T's work as expected. The crossover, processing and servos stay powered while the power section is disconnected when not in use to lower Iq draw. I've archived my CD collection in iTunes (lossless) and send digital out of my laptop to a Theta DAC, so the plan is to only have to turn on the laptop and get music (easiest for the G-fry, too). Connecting the power section is quiet, but powering it down causes an audible transient. I've built LM3875 amps that don't pop, so I need to figure out why this one does. Also, the noise floor is not low, and this is probably due to there being no true shielding for the circuit board, maybe lining the cabinet with foil will help, and/or grounding the heatsink.

The cabinet design and performance is somethign else entirely. I plan on using SMAART to get the actual measured acoustic response, but just by listening I can tell several things. The low end is not balanced in the sense that there is too much from 20 up to 60Hz or so. I would guess that this is either because the parameters of the driver are inaccurate or, more likely, the proto cabinet volume is larger than the target volume. The proto cab used scraps so I was somewhat limited to certain sizes, the resultant volume being different from the one the Linkwitz Transform values were chosen for. I shot for larger thinking that it would be easier to reduce the internal volume until it was right for the circuit.

I expected the midrange to require the most tweaking and that is proving to be the case. The goal is to get it to be flat from 300 to 6k. I cut a hole in the back of the cabinet which basically makes the space behind the driver a square tube, the plan being to cut out a long block of various materials to try to get a happy medium between a sealed back and open baffle. The circuit board has a stage of passive EQ that is probably going to be needed to reduce the output above 1k. Hopefully this little driver will be able to do 300Hz well enough to keep up with the top of the woofer.

I plan on letting the drivers break in before doing too much tweaking. I'll do a moderate amount of updating to this thread as things progress.

[mwmkravchenko]

Any pics of the prototype as a unit?

Mark