Any ideas for a digital/analog 6-channel pre-amp switcher/mixer? - diyAudio
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Old 27th April 2009, 02:12 PM   #1
Ball is offline Ball  United States
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Join Date: Apr 2009
Talking Any ideas for a digital/analog 6-channel pre-amp switcher/mixer?

Hey guys,
I've recently acquired a van which opens awesome possibilities entertainment-wise. The short-and-sweet version is: I'd like to have a pre-amp (mixer or switcher) of some kind which can take analog inputs from the in-dash stereo (Sony POS with 4-channel out and iPod adaptor) and/or a laptop (digital toslink source, or possibly USB/firewire?), then output to the van's various amps (1000W sub I'm building, and possibly six mids). The tricky thing is when the digital input is used, four of the channels have to be 'rotated' and preferably volume adjusted so they're aligned for the rear entertainment center, which faces to the van's right (not forward).

So, the configuration would be like:

DRIVING ANALOG vs PARKED DIGITAL

F-L F-R (dashboard) unused

M-L M-R (roof or behind seat) R-L F-L

R-L R-R (roof or rear doors) R-R F-R

subwoofer (under custom futon) subwoofer

(another possibility is to use the front speakers for the rotation, but that would probably hamper quality)

I've mulled over how I can get this to work. One possible method is to use one 2-channel amp for the front, one 4-channel for the rear four, then use a 6pdt switch (like this) to switch the inputs for the four-channel amp and sub (possibly adding some 10k pots to fuss with the levels). This would still require some kind of toslink-to-4.1 converter which I haven't come across (for cheap, anyway). I think a home stereo pre-amp source switcher might work (like this), but they're like $800 and would require a clean inverter.

Even if I did find a cheap toslink-to-4.1 converter, I would have to fiddle with my preamp channels somewhat since I only have four to begin with. Originally, I was simply going to go with one 4-channel amp and a sub, running the sub off the 4-channel amp's preamp output. What I may end up doing is run the 4-channel off the rear outputs in the head unit and a separate 2-channel amp off the front outputs. The downside to this is if I fade out the rear, I lose the subs. A custom pre-amp would solve all of this, of course

I do have some experience with electronics (mostly digital, though) and am game to solder something custom for the job. I'm also game for modifying existing equipment, or even building a custom amp (although having things modular would be more reliable). I would be most grateful for any suggestions, creative or otherwise.
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Old 27th April 2009, 08:28 PM   #2
Ball is offline Ball  United States
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I believe I found another unit which could help: the Technics SH-AC500D.

It's a surround processor and audio switch. The manual:

http://service.us.panasonic.com/OPERMANPDF/SHAC500D.PDF

seems to indicate that it has unmodified "line in terminals" for all six channels (5.1 system).

To me this suggests I could wire the outputs from the head unit 'rotated' for driving (analog), and the toslink would then be 'normal'.

The only problem then, besides having to hack the PSU for 12-14v DC, is the preamps from the header or front amp (which only has four with fade control). Ideally, I'd want to convert this to five or six out (two not being fade controlled). Originally, I was going to do this using a 4 channel amp which has a preamp output, but obviously I need the unfaded channels prior to amplification. Is there a simple way to do this, or do I need to find a slightly less crappy head unit?

If I don't use fade control, I could try to split the rear channel from the head unit into all six channels. That sounds less than ideal, to me.
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Old 27th April 2009, 08:28 PM   #3
Ball is offline Ball  United States
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PS: I suppose another possibility is to convert the rear channels from the head unit into digital and hope the preamp unit is smart enough to propagate all the channels.
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Old 30th April 2009, 06:59 AM   #4
dangus is offline dangus  Canada
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Join Date: May 2004
Location: Vancouver Island
There were or are some autosound surround processors, like the Panasonic CY-AC300 (something like that). I scored two of those off the bargain table of a local car stereo store for $25 each. New prices are in the "if you have to ask..." range, but if you cruise car parts swap meets and the bargain tables at car audio stores you might get lucky. Be aware that some models don't run standalone and require a particular head unit to control them.

A better option may be the Soundblaster Extigy. Word on the street is that it can run standalone as a Dolby Digital decoder. Those are comparatively cheap, and run off an external power supply, so converting to 12V should be trivial.

There's also various digital surround processors made for computer speaker systems. There's a hunger for them on eBay which results in unrealistic prices, but you might get lucky at a local store or computer swap meet.

As for getting from 4.0 to 5.1, what I did when I used a SH-AC500D in my home system was to put the active crossover (which sent bass to the subwoofers) after the Technics, and set the processor for "large" front speakers and no subwoofer, so the LFE bass would be directed to the front channels.

I've considered something similar for my van, although I'd just need to swap front and rear. I might use relays, since I have a heap of little surface mount ones.
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Old 30th April 2009, 02:01 PM   #5
Ball is offline Ball  United States
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I don't think a 5.1 head unit would be much better, even if I could acquire one on my budget. They don't have 5.1 inputs, and I'm only using 5.1 when I'm in the back, anyway.

What about using a home receiver amp with 5.1 pass-though (input)? They're easier to come by than DTS pre-amps like the AC500D.

The downsides to this idea are A: I'd have to use an inverter or more powerful voltage regulators, B: the 2ohm car midrange speakers would likely fry the home amp if I hooked them up directly.

However, I could possibly get away with using one of those 120W 12V ATX PSUs (used to run miniATX PCs in a car) and feeding a low volume audio out to the high-watt inputs of a 4ch car amp (many have this option instead of RCA jacks).

It would be a lot more feasible, in my mind, if I could find a stand-alone processor for 5.1 with pass-through (like the AC500D) and hook it up to a ATX PSU or some voltage regulators.

Things are moving forward regardless. I've ordered the rest of the speakers and Friday I will acquire two 2ch amps (for fronts and subs). I'm also trying to buy a 2nd hand Pioneer head unit with 6 preamps so I don't have to daisy chain the amps. The current Sony one blows.
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