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Old 13th January 2002, 05:58 PM   #1
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Location: Portland, Oregon
Default DIY 5.1 Preamp

Relating to thread regarding a DIY 5.1 decoder. Posts there indicate licensing problems point to buying a used decoder.
So, if that route is taken, does anyone know of a DIY 5.1 preamp, either kit or plans?
Barring that solution any reccomendations as to model or source of moderately priced prefab 5.1 preamps would be appreciated.
Thanks
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Old 6th February 2002, 05:16 PM   #2
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Off-the-shelf, the Sony P9000ES runs about $500 or so. It's designed as an adjunct to the TA-9000ES to allow 5.1 switching/volume control, but it functions just fine as a 5.1 preamp. This is probably your only commercial option. Performance is supposedly relatively good.

DACT makes a nice-looking 6 channel stepped attenuator. Not cheap (~350 or so, I think). Great if you don't need input source switching. www.aloha-audio.com is a US source. DACT should (?) handle europe directly, I think.

The CS3310 volume control chip is used in a bunch of high-end pre-pros (Sonic Frontiers, TAG-McLaren, Rowland preamps). It's basically an analog-domain stepped attenuator implemented on a chip. The only theoretical drawback is that it does have an integrated op-amp stage to provide gain if needed. Properly implemented, these should be very, very good. Needs a microcontroller, but the serial protocol is really simple - can probably get away with a PIC, or a simple 68HC12 board. These use a cool shift in/shift out approach, which allows daisy-chaining an arbitrary number of units. Available as DIP or SOIC package.
Burr Brown has released/announced an equivalent which might be even better and handler higher voltages, but I think is only available in SOIC.
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Old 6th February 2002, 06:19 PM   #3
tiroth is offline tiroth  United States
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I'm currently working on a roll-my-own 6.1 preamp. However, I'm still designing/prototyping. It really isn't that complicated, but the route I'm taking may not interest you. Let me know if you want more info.
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Old 6th February 2002, 08:32 PM   #4
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Definitely still interested. Don't know yet about the decoder aspect but maybe something will surface there also as it seems there are more new/used decoders than decoder/preamps available at the present. When I started the quest I wasn't planning on an overnight solution.
In the interim I've been acquiring the bits and pieces for the power amps. As I plan to build the amps as monoblocks they can always be used in pairs for some friends' stereo systems if the home theater component DIY doesn't work out.
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Old 7th February 2002, 02:25 PM   #5
tiroth is offline tiroth  United States
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My idea is basically to piggyback off of an existing AC-3 decoder. I'm using a Technics SH-AC300 decoder (cheap-see ebay) but I believe virtually any outboard decoder could be used, with varying degrees of success perhaps. However, instead of simply taking the analog outputs I pull the digital L/R/C/LS/RS/Sub signals out where they interface from the DSP to the AC300's internal DAC (a CS4226).

This digital signal is then routed through a ASRC (AD1896) to the output DACs (AD1853). Volume control is done with hybrid digital/analog attenuator; AD1853 allows analog attenuation via its current reference, so by using a uC to switch relays and set the digital volume one can achieve something like 30dB of cut (1dB steps) without much loss of S/N ratio, or more than 5 relays/channel. If you design this output stage specifically for your amps you can really maximize your system S/N, especially compared to commercial offerings that are forced to be compatible with a wide range of hardware.

I'm also planning on adding a selectable active matrix decoder for the center rear channel, because I'm curious about its value.\

This kind of setup is attractive to me, because it is a basically a high quality DAC/volume control, so it can be used with most any decoder that comes along...if it becomes possible to make my own decoder (or upgrade) in the future I can simply add that to the box. Also, using a differential output DAC means a balanced signal can be routed to the amps, eliminating ground loops and a good deal of common-mode noise. Better (and cheaper) than those $$$ cables.
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