Direct Digital out From Lexicon - AVS Forum
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post #1 of 9 Old 11-19-2013, 11:36 AM - Thread Starter
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I am working on a project to convert my HT system to 100% digital audio up to the power amps. I am building a DSP engine (a DIY Trinnov) based on MiniDSP boards - four MiniSharcs total!

My Bluray player has a similar modification to the Lexicon MC8 shown here. This is basically what the SPDIF Oppo mod is doing. That is to bring out the digital audio streams in a standard format. I choose balanced 110ohm AES versus SPDIF or 75ohm AES-3.

The Lexicon runs 96khz internally so this mod brings out the 7.1 as four digital streams. This also bypasses the MC8 volume control but more importantly I have found Lexicon tweaks the analog VCAs to produce some of the mock surround modes. So I'll have to duplicate that function elsewhere. More on that later, I do have it nailed.


This is the AES output board. There is a large surface mount chip on the backside of that board.


Here is the modified rear panel. I have standardized on 15pin VGA connectors for 8 channel digital audio. I need 4 pairs plus grounds so that's 12 pins. This is especially helpful on the DPS engine which is in a PC chassis. You can't put an XLR connector on a PC card slot let alone four of them.

In my new setup the MC8 is no longer the audio centerpiece but just an AC3 / DTS decoder that provides additional surround modes like the famous Logic 7. The BluRay player completely goes around the MC8. The DSP engine has four 8 channel inputs of which now two will be used, the MC8 and the BluRay. The third input will someday be a 4K player.

*Note that when I say 8 channels it's really only four physical digital channels or streams because AES and SPDIF carry two channels per stream or cable.

*For you engineering types, the interface chip is an AKM4101. That is a neat 4 channel AES / SPDIF digital output formatter chip. I obviously use the hardware configuration mode hence the jumper strips on the board. Also note the transformer coupling. We're doing this right!

More images on the way.

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post #2 of 9 Old 11-19-2013, 12:57 PM
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Originally Posted by Glimmie View Post

I am working on a project to convert my HT system to 100% digital audio up to the power amps.
You're using "true" digital amps e.g. TacT or NAD?
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post #3 of 9 Old 11-19-2013, 01:09 PM - Thread Starter
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You're using "true" digital amps e.g. TacT or NAD?

LOL! No the LCR power amps are in fact biampped and use TUBES! It's digital up to the power amps. The surround amps are good old SS class B as well.

Actually the volume control is also analog. The DACS are Lucid 88192's x2. I will have 16 discrete channels of DACs and the DIY level controller is also 16 channels but fully balanced I/O. In fact I will have absolutely no unbalanced analog audio in the entire system.

The MiniDSP products do not have a good digital volume control feature wise and as I said above, I need to re-apply the gain changes the Lexicon did in their analog section. My "volume master" box is also where I tweak the speaker gains based on room setup via REW. Good digital volume control is difficult to do. The MiniDSP may in fact be very good signal quality as it's 40bit resolution but the adjustment and remote control features I need are not possible in that product. So I went with a VCA based volume control based on Burr Brown VCA chips which are very good.

I'll post some picture of that sub project.

BTW, there is technically no such thing as a "digital audio amplifier". The class D topology has been around since the 1970s and still is an analog process. There have been attempts to make true digital RF amplifiers, a power DAC, but these have yet to become feasible. You just can't tame the switching noise enough at high power levels. Same issue with a power DAC even at audio frequencies and class D is an ANALOG PWM technology.

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post #4 of 9 Old 11-19-2013, 05:39 PM
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Originally Posted by Glimmie View Post

BTW, there is technically no such thing as a "digital audio amplifier". The class D topology has been around since the 1970s and still is an analog process. There have been attempts to make true digital RF amplifiers, a power DAC, but these have yet to become feasible. You just can't tame the switching noise enough at high power levels. Same issue with a power DAC even at audio frequencies and class D is an ANALOG PWM technology.
Yup, I was referring to "power DAC's", sometimes referred to as "true digital" amps - not to class D. Developed by Toccatta and used by TacT/Lyngdorf. Toccatta sold the technology to TI who named it Equibit. IIRC, NAD developed their own, similar technology, used in the M2 integrated and a couple of other devices, Only "amps" which will accept a digital signal and AFAIK 150W is max achieved in product so far.

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post #5 of 9 Old 11-20-2013, 03:46 PM
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" This also bypasses the MC8 volume control but more importantly I have found Lexicon tweaks the analog VCAs to produce some of the mock surround modes. "

They also adjust the VCAs based on the Mode Adjust settings you choose within the modes. For example the various settings within Vocal Enhance will cause the Lexicon to adjust the VCAs to account for the --3dB or -6dB that is applied to the Center channel (when using vocal enhance) to give headroom for the processing to avoid clipping at 0dBfs. Same thing with some of the settings that adjust the surround channels.

Due to all those level changes it would be easiest to tap into the control signals to the Lexicon VCAs and feed that volume control data for your own VCAs. That way the Lexicon volume control (and speaker balancing and all the other tweaks) would happen in your volume controls.

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post #6 of 9 Old 12-01-2013, 11:33 AM - Thread Starter
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Originally Posted by sfogg View Post

" This also bypasses the MC8 volume control but more importantly I have found Lexicon tweaks the analog VCAs to produce some of the mock surround modes. "

They also adjust the VCAs based on the Mode Adjust settings you choose within the modes. For example the various settings within Vocal Enhance will cause the Lexicon to adjust the VCAs to account for the --3dB or -6dB that is applied to the Center channel (when using vocal enhance) to give headroom for the processing to avoid clipping at 0dBfs. Same thing with some of the settings that adjust the surround channels.

Due to all those level changes it would be easiest to tap into the control signals to the Lexicon VCAs and feed that volume control data for your own VCAs. That way the Lexicon volume control (and speaker balancing and all the other tweaks) would happen in your volume controls.

Shawn

That's a great idea. And I have already been there in a sense. My old rear center was done at line level with an old Lexicon CP1. So I built a small VCA box that tapped into the MC8 VCA SPI control buss. I brought out the four VCA enables and the SPI clocks and data through a TTL buffer to a connector. I'll just put that mod back in.

I was going to get the offsets just by measuring the audio levels with sinewaves as I really only use a few of the MC8 modes. But your approach would be exact, automatic, and work with the enitre MC8 effect profile.

But I'm going to need a bigger boat! The 8K embedded porocessor in my new VCA box is about 85% full. I'll need a lot more code space for this update!

Thanks.

P.S. You must have worked at Lexicon. You know deep details of the MC8 design .

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post #7 of 9 Old 12-03-2013, 08:57 AM
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Originally Posted by RUR View Post

Yup, I was referring to "power DAC's", sometimes referred to as "true digital" amps - not to class D. Developed by Toccatta and used by TacT/Lyngdorf. Toccatta sold the technology to TI who named it Equibit. IIRC, NAD developed their own, similar technology, used in the M2 integrated and a couple of other devices, Only "amps" which will accept a digital signal and AFAIK 150W is max achieved in product so far.

FWIW, the correct max figure is 200W in 8 ohms. The TacT Boz 2200 is 200W @ 8 ohms and 400W @ 4 ohms
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post #8 of 9 Old 12-03-2013, 11:11 AM
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FWIW, the correct max figure is 200W in 8 ohms. The TacT Boz 2200 is 200W @ 8 ohms and 400W @ 4 ohms
Turns out the NAD M2 is 250W into 8 ohms, but is, curiously, 250W into 4 ohms, as well.

Peter, do you know if volume for the Boz amps be controlled using anything other than the Boz preamp? I'm familiar only with the S2150.
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post #9 of 9 Old 12-04-2013, 09:15 AM
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Peter, do you know if volume for the Boz amps be controlled using anything other than the Boz preamp? I'm familiar only with the S2150.

The 216/2200 lets you adjust the volume with the supplied remote control. Volume can also be adjusted via RS-232. (When combined with a TacT preamp, for example the TCS, the preamp will control the volume over RS-232).

To clarify a common misunderstanding: Boz 216 is the power supply for the 2200 amplifiers. Boz 216 contains two toridal transformers and a computer that is used for programming the 2200 amplifier modules. The 216 does no audio processing whatsoever and only serves as a big step-down transformer during normal use. (If you use the remote to adjust the volume, or turn the volume control wheel, all it does is send RS-232 command codes to the 2200 amplifier modules).
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