Can a PRE-OUT be easily created with a Class-D HT receiver? - diyAudio
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Old 6th August 2006, 05:51 PM   #1
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Default Can a PRE-OUT be easily created with a Class-D HT receiver?

Hello,

A few months ago I created another thread which tried to determine if I could use an inexpensive HT receiver and create PRE-OUTs so that I could drive my stand-alone power amps in my existing home cinema.

The thread focused on using an inexpensive Sony HT surround receiver which used the CX9646 dual power amps chips. We concluded that by taping pin 8 of this chip and adding a coupling capacitor, a PRE-OUT for each channel could be created.

Today I discovered that CXD9646 (a Sony S-Master chip) appears to be a class-d switching amp.

Therefore if I am going to steal the input signal from this power amp, then I had better be prepared to filter out all the high frequency residue so that this noise does not get into my external power amps and mess up the electronics.

I have read that even the best LC filter placed between a Class D amp and the loadspeaker does not always get rid of all the residue. In most cases, any residue getting into loadspeakers has no impact. Perhaps nearby AM radios might be affected.

But in my case, any high frequency residue getting into my external power amps might cause oscillation and heat them up.

Does anyone have any experience creating PRE-OUTS from a class d amp or in converting a PWM stream to a clean analog audio signal?

Thanks

Thanks
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Old 8th August 2006, 12:37 PM   #2
lucpes is offline lucpes  Romania
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Default Re: Can a PRE-OUT be easily created with a Class-D HT receiver?

Quote:
Originally posted by montreal
Hello,

A few months ago I created another thread which tried to determine if I could use an inexpensive HT receiver and create PRE-OUTs so that I could drive my stand-alone power amps in my existing home cinema.

The thread focused on using an inexpensive Sony HT surround receiver which used the CX9646 dual power amps chips. We concluded that by taping pin 8 of this chip and adding a coupling capacitor, a PRE-OUT for each channel could be created.

Today I discovered that CXD9646 (a Sony S-Master chip) appears to be a class-d switching amp.

Therefore if I am going to steal the input signal from this power amp, then I had better be prepared to filter out all the high frequency residue so that this noise does not get into my external power amps and mess up the electronics.

I have read that even the best LC filter placed between a Class D amp and the loadspeaker does not always get rid of all the residue. In most cases, any residue getting into loadspeakers has no impact. Perhaps nearby AM radios might be affected.

But in my case, any high frequency residue getting into my external power amps might cause oscillation and heat them up.

Does anyone have any experience creating PRE-OUTS from a class d amp or in converting a PWM stream to a clean analog audio signal?

Thanks

Thanks
I don't know your particular case but if you take the signal before any PWM processing is done you should be safe from any troubles....
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Old 8th August 2006, 02:54 PM   #3
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Quote:
Originally posted by lucpes


I don't know your particular case but if you take the signal before any PWM processing is done you should be safe from any troubles....
Thanks for suggesting.

It would be great if where ever we find the CXD9646 which is fed with PWM, we could simply go far enough upstream to eventually find a pure analog signal which could serve well as a source for a DIY pre-out.

The reality is that the CXD9646 is part of the S-Master family which takes pride in the fact that receivers which use it are essentially all digital from front to back, except for the analog inputs which are quickly converted to the digital domain and processed and switched thereafter.

Often the CXD9646 is proceded by the CXD9634 which is responsible for converting PCM and DSD into PWM for the benefit of the CXD9646 chip.

In my particular case, the SPDIF output from my cable box is already digital and in a receiver which uses the CXD9646, this signal would remain digital right up to the LC filter following the Class D output stage. The only purpose for me purchasing an inexpensive receiver and adding pre-outs is to obtain the Dolby decoder and surround processor that is not included in my HD cable box the way it is included in my SACD/DVD player.

Otherwise, I don't need the switching facility, nor the FM radio, nor the 6.1 power amps which all come in the receiver package.

Unfortunately, the stand-alone Dolby decoders available in the marketplace are either professional devices used in recording studios costing thousands of dollars, or re-packaged computer sound cards destined for use with small computer loadspeakers.

The S-Master approach is sure to become the standard in the years to come and I have the highest respect for the more expensive receivers which already incorporate this technology. But for my modest budget and limited desire to build high performance low pass opamp filters to convert PWM to noise free analog, I will now be lowering my sites to receivers such as the STR-DG500 which use good old fashion analog power amp circuitry.

I suppose that my decision to abandon receivers with S-Master technology effectively makes this thread obsolete.

There are a few receivers which have S-Master technology, but not pre-outs, and their owners might be interested in the challenge of designing a high performance low pass filter/buffer so that they can introduce pre-outs on two or more channels.
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