DCDIY2008 test fixture (?)

[neildavis]

I got this Spherex Xbox amp mostly tamed, and it is very cool. There are 6 full channels, with 2 other channels that are "buried". Each channel has 10 digital biquads. Each biquad can be programmed to be one or two-pole low pass or high pass, peaking (positive or negative gain), notch, bandpass or all-pass. You can combine the biquads to build high order filters or crossovers or use the biquads for EQ.

The screen shots show some of the software screens. There is still a lot I need to finish off, but this shows some of the features. You can set the amp up to be 3-way stereo or 6-way mono, and any smaller combination. The screen shot shows a 3-way stereo configuration using 10-pole LR filters. And it works...you can hear the crossover change as the biquad coefficients get calculated and loaded into the amp. Right now it takes a long time (several seconds on a dual-core machine) to crunch out all of the numbers, so the changes aren't instantaneous. However, with a little more work I can store some pre-calculated values to make it change quickly.

So if we use this as a test fixture for DCDIY2008 we could compare crossover slopes, and evaluate exotic configurations such as 6-way speakers. Combined with the volume-compensated relay box that Dennis has, we could do an apples-to-apples comparison of drivers, where each driver has been tweaked to be "flat" and at the same volume. And I'm sure there are other possibilities...

Any ideas/suggestions?

[gregm]

Me likey...

I won't be attending DCDIY (I have yet to start even a single project), but Active XO's are interesting to me. And the idea of having participants submit various XO designs for a single set of drivers and be able to compare them sounds cool.

Was this something you got at the Tent Sale? Or can mere mortals acquire one of these? It sounds like the software is something you wrote yourself.

--Greg

[neildavis]

Was this something you got at the Tent Sale? Or can mere mortals acquire one of these? It sounds like the software is something you wrote yourself.
--Greg

I bought two of the complete Spherex Xbox 5.1 systems way back when CompUSA folded up the local stores, but I just recently got around to working with the amps. They are the amps that go in the Spherex subwoofer boxes that PE is selling. I suppose there were too many licensing issues for them to sell the amps as well, which is a shame. The amps are very nice digital amps, with about 40-50W per channel, and they use the ST308 digital PWM controller. Occasionally they pop up on eBay.

The code I wrote lets me cut the traces from the on-board controller in the Spherex amp and put on my own controller that has a USB interface. That way I can write to the registers in the amp from Windows, although there is enough storage in the controller so that the amp will work on its own using the downloaded configuration. I know there are other home theater systems that use the same chipset, so this mod should work for any of those other amps as well (JVC and LG have a couple of 5.1 systems that use the same power amp circuitry). With some software mods the same application will control the amps that use the TI Purepath chips (Panasonic SA-XR55, etc). Or, I can build my own version, which is what I wanted to do initially.

I've been impressed by these digital amps for a long time, and always wondered why they aren't used for active speakers. I asked a sales rep at Apogee, which originally developed the ST308 series, and he didn't have an answer. He said Apogee expected them to be used for active speakers, but they just never caught on.

I'll have one extra Spherex amp that I could make into a test fixture for DCDIY2008. But I need to know whether there is enough interest to make it worth my while. Also, this will end up being a group project, to build baffles, set up a reference system, decide on a test method, etc. So this is the first post to see if there is interest...

[mikebw]

I may not be full grasping the potential of this project, but so far it seems like this would be a perfect way to evaluate the individual "sound" or timbre of any particular driver.

We all are familiar with the terms used to describe certain types of drivers, from ribbon tweeters to metal or fabric domes, the sound can be quite different from the same test material. With an automated system for leveling factors such as frequency response and SPL curves this opens doors for direct A-B testing for the true sound of a driver.

Did I just repeat what you said Neil? Either way, very cool.

[neildavis]

I may not be full grasping the potential of this project, but so far it seems like this would be a perfect way to evaluate the individual "sound" or timbre of any particular driver.

In some this project it is nothing new, because you can buy a powerful EQ unit, six channels of amplification, and manually set up this equipment to compensate for volume and frequency response to compare drivers or crossovers. However, this amp probably has more EQ capability than most DSP systems, and I can set it up to allow quick A/B testing between two sets of stored settings. Also, I can import FRD files from a measurement system to help with setting the levels. My Windows app provides an accurate math model of the biquad processing, so you can see the effect of the EQ on the imported FRD data. So there are some nice possibilities here for experimentation.

This amp is small enough to embed in a semi-permanent test fixture. This one didn't cost me much (about $130 for the complete Spherex system, including the speakers and shipping), so I don't mind donating it for driver or crossover testing for DIY events. But there is still a lot to do to make this a useful test fixture, including woodworking, relay switching, and a good bit more software. If we can come up with a set of drivers that people want to evaluate, or if there is a lot of interest in comparing crossover types, I'll keep plugging away at this as a test fixture. If there's not much interest in this type of testing I'll work on my omni/dipole that also uses the Spherex amp.

The picture shows the collection of drivers that I was thinking about bringing to DCDIY2003. I had a crude switching system back then and it became clear after some initial tests that I needed something like this Spherex amp to compensate for volume and frequency response and to provide more flexibility in setting the crossover points. I still have a lot of these drivers.

[davidroberts]

What is the software that you are using to make the changes?

[neildavis]

What is the software that you are using to make the changes?

It's my own. The amplifier is actually controlled by a slave board that has a Freescale micro. The crossover designer program is written in VB.NET, and it sends commands to the slave board via USB. So you can model the operation of the biquads with the VB.NET program and then program the amplifier to hear the results in real time.

[philiparcario]

I have an interest In it. I have a few questions. How big is the amp can it fit in a 12 inch by 12 inch by 4 inch box? Can it drive 4 ohms? It sounds like I could have two boxes of amps 1 for 5 tweeters and 1 for 5 woofers for home theater. With a spare channel for each amp am I right?

[neildavis]

I have an interest In it. I have a few questions. How big is the amp can it fit in a 12 inch by 12 inch by 4 inch box? Can it drive 4 ohms? It sounds like I could have two boxes of amps 1 for 5 tweeters and 1 for 5 woofers for home theater. With a spare channel for each amp am I right?

The spherex amp is about 7" by 10" by 4", so it would probably fit in the box you describe. And yes, it would provide 10 independent amps with just about crossover you could imagine (including TP, because the board I add in has delay). 4 ohms isn't a problem, but these digital amps can be optimized for a given load by adjusting the output filter.

The mod I make is to add a board to the amp that ties into the I2C bus to control the biquads that are in the STA308. Then my biquad crossover designer (BCD) program sends data to that board to program the STA308. So I'm not tied to a Spherex amp--that is just the one that I've used so far. There are other amps that use the STA308/STA508 digital amplifier circuitry that could work. The Harman Kardon DPR 1001 uses the same chipset, and Hitachi, JVC and LG/Sylvania have used it, also. Eventually I'd like to build my own amplifiers and adapt the code to control the TI Purepath amps as well. I'm getting there, but first I had to develop all of this code.

So this approach would work, but you probably don't want to use the Spherex amp for your application. Right now my code waits for the Spherex amp to power up and use its own controller to initialize the A/D chip, the SPDIF circuitry and the STA308. I have to use the Spherex remote control to select the SPDIF input (or analog) and then I can listen to the amp and take control of the biquads. That's OK for a test fixture, but probably not OK for a home system that you use all of the time. I could write the code to initialize all of the chips on the Spherex amp myself, but I probably won't, because I don't intend to buy any more of these amps. I'll save this effort for my own boards.

Actually, the Spherex amp sounds like a force-fit for your application. If I understand it right, you just need two high-powered amps with two channels of biquads rather than 6 (or 12) medium-powered amps. A design using the STA308 controller with some higher powered output devices would be a much better match--and that's something I'm planning to build. Or, you could use a preamp board with A/D, D/A, SPDIF and biquads along with power amps of your choice. I've got a prototype of the preamp board like this already built...debugging the hardware and writing the software for that board is coming up in the queue. Either of these options would use the same BCD program to control the biquads.