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Is adding preamp outputs / digital bitstream out to cheap-O AV Receiver impossible?

AwesomeSauce2015

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From what I can tell, nobody has asked (here) whether or not it is possible to add in preamp (post-DAC) or digital (post-DSP) outputs to a modern AV Receiver.

Given that stuff with a full set of preamp outputs costs a fortune (ie: Denon X3800), as well as the benefits of active speakers like the JBL 308 (and DIY).
Does anyone have any input on this? I'm not in a position right now where I could buy something to experiment, but I have considered the possibility.
Also, looking at teardowns of some of these budget receivers, it seems that the DSP chip in some models is on a separate board from the amp section, which means that either a digital or analog signal is exposed somewhere post-DSP.

The reason I would need to use a receiver is because they can handle the rendering of the fancy object-based surround formats like Atmos. However, many cheap receivers don't have good amps or room correction, so I would use my own DSP for that. - And also that's why I don't want to just use a line output converter: Bad amps -> more noise -> worse sound quality.

So basically: Does anyone know anything about the modern, super budget receivers that would make what I'm proposing impossible?
 
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AwesomeSauce2015

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Something like this?
Yeah, basically. If basically all AVRs use I2S then that would greatly simplify things...
I guess the question I had is if we know if cheapo AVRs use a single integrated DSP / DAC chip, or if basically all AVRs use separate dac / DSP chips...
But chances are that with all the different models out there I could find something that would work...

Thanks!
 

Chrispy

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Yeah, basically. If basically all AVRs use I2S then that would greatly simplify things...
I guess the question I had is if we know if cheapo AVRs use a single integrated DSP / DAC chip, or if basically all AVRs use separate dac / DSP chips...
But chances are that with all the different models out there I could find something that would work...

Thanks!
Curious, if capable of adding such, how would you tackle the mounting of the connectors etc? Would it be a permanent switch or could you add an option for pre-outs in the avr menu or just remain live always?
 

Rick Sykora

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AwesomeSauce2015

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Curious, if capable of adding such, how would you tackle the mounting of the connectors etc? Would it be a permanent switch or could you add an option for pre-outs in the avr menu or just remain live always?
I'd probably just add some kind of multipin connector(s) to take the i2s signal out from the receiver to my own box with my DSP and amplification.
I wouldn't mess with the avr firmware. Adding a switch like that to something like the Denon S760h would be quite hard. I'd need to add hardware relays and a way for the relays to be switched by the AVR.

Lol, cool video. He needs to add AVR schematic to the requirements list though!
Yeah... Getting schematics seems to be the difficult bit... Of course, reverse engineering isn't that hard... lol
 

alex-z

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Yeah, basically. If basically all AVRs use I2S then that would greatly simplify things...
I guess the question I had is if we know if cheapo AVRs use a single integrated DSP / DAC chip, or if basically all AVRs use separate dac / DSP chips...

I can't speak for literally every model, but all the ones I have seen used independent chips for surround decoding and the actual DAC section, and the two communicate over I2S or TDM. The models with more than 8 channels (7.1) just have a pair of 8 channel DAC's.

The service manual is nice, but all the chips usually have public datasheets anyway, so you can see the pin layout.

Although in some models there is no convenient tap point. Which means soldering directly onto the chips, which is risky. If you just want pre-outs, the safe option is to just tap the existing inputs of the amplifier section. Worst case scenario you solder too close to a transistor leg, and transistor damage is cheap to fix.

Been mulling over doing this mod to an Onkyo NR555 so I can run a centre channel which is a 2 Ohm load. Did a bunch of research and AV receivers are shockingly simple devices, the only "black box" is the surround decoding. Those datasheets (like for the Cirrus Logic CS49834) are available by request only. AFAIK, they are custom ARM cores, and Dolby + DTS provide them the "magic" software. Some of them even have exposed I2C for programming and debugging, but nobody has bothered to hack an AVR, because there is no big black market, or bounty program.
 
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Trouble Maker

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This is analog, OPs looking for the digital signal.

I can't for the life of me imagine why one would solder on the back of the board there rather than re-pin the connector or splice into the wires on the ribbon cable. There would be much less chance for damage with either of those methods.

Edit: Sorry, I did just see the OP was asking for digital or analog.
 
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Rick Sykora

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Unless you are an experienced electronics designer and are equipped to measure your work, getting a good result tapping into an existing analog design is a long shot in my opinion. Along with changing loading, potentially introducing noise and/or heat damage to existing circuits, this is risky. Otherwise, it is just a hack job.

Given the lack of decent surround processors, I get the motivation here. But ripping into a cheap AVR and hoping to add multiple analog pre-outs reliably seems extremely optimistic to me. If you mess up, you could screw up downstream amps or speakers too. Dump the old AVR and buy something with pre-outs...

The Weeb Labs approach seems much more likely to have a successful outcome, but needing an external DAC for multiple channels (LCR at least?) is added time and cost too. Finding a used AVR with pre-outs seems a better investment to me.
 
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TonyJZX

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while I'm of the opinion that sp/dif out on an affordable avr is a good thing... no one is doing this... and if you beleive you can do better on dacs just buying a pile of cheap but good dacs for your HT setup is a good use of your time then i dont see the problem

i think sp/dif out on all channels of an avr is something the industry flat out doesnt want

but analog pre-outs is another thing.... a lot of mid range avrs have it already so the money and effort you spend upgrading an entry level avr would be better served just buying a mid range unit that has full preouts anyway?
 

Alexium

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I want to add front L and R pre-outs to my Denon X2500H. Wouldn't it be a good idea to also power off the power amplifiers when using the pre-outs? Or the amps should not affect the pre-outs when working with no load?
 

Plcamp

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With my Yamaha Aventage 3030, there is an SPDIF out, but it is only active when the input is also SPDIF…they deliberately disallow conversion from hdmi to SPDIF.

Which is weird, because all TV’s do exactly that.

This idea looks good to overcome that.
 
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AwesomeSauce2015

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Yeah, my original idea basically revolved around the idea that with active (custom) DSP speakers, I can feed them directly from either analog or digital outputs, once the special dolby / DTS sauce is applied.
I also just can't justify paying the 1-2k that manufacturers charge for their receivers with full preouts. Especially when there are models with 7.1 decoding for literally 1/10th the price. I get that 11 channels is more than 7, but still.

WRT the value argument of getting something like an X3700h:
If I managed to get the digital bitstream outputs, my added cost would be basically zero. I would send the bitstream to the DSP I already have, and send that out to the amps / DAC I would already have. -- This argument assumes that I would be using DSP / active speakers, which I would be doing.
If I was to just use an X3*00H from denon:
I would still need the DSP, and amps. However, I'd have to add a suitable number of analog-> digital converters to my design, making the cost definitely not zero.

So yeah, if someone is just trying to add more power to a cheap AV receiver, than get something better and use the preouts. However, since I have designs in the works for a fully-active DSP speaker, getting the digital bitstream outputs is highly preferable.

But chances are if I ever do do something like this, I will probably get a box like the Denon 3800h with Dirac and a full preout section...
 
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