adding pre-outs to an inexpensive HT receiver.

[Nigel Goodwin]

Originally posted by montreal


Nigel,

So you have 3 CXD9634Q chips labled Stream Processors where each drives 2 CXD9646Z dual power amplifier chips with 33 ohm resistors in series in each path which connects a Stream Processor chip output to a power amp chip input.

Actually I don't 'have' one, only the manual, I was replying to someone elses post. But that connection is correct.

Any chance of you posting an attachment that shows one of the three 9634 processor chips?

Yes, here it is

[montreal]

Thanks Nigel for posting the diagram.

We can see that there is are four 22 ohm resistors following the 2 pairs of non-inverted and inverted outputs for the 2 channels.

That means that anyone who owns such a receiver need only attach a decoupling capacitor (say 10 microfarad) on each non-inverting output (pins 10 and 24) after the 22 ohm resistors.

Since these pins are at 2.5 volts, the polarity of the capacitor must respect this. The other side of each decoupling capacitor could lead to a RCA socket attached to the back of the case. A 100k ohm resistor could be placed between ground and the center pin of the RCA socket so that the capacitor can charge up and discharge as needed.

I can't say if there will be a DC pulse sent to the downstream equipment at turnon, but it should be easy to make sure this doesn't happen.

I guess it's time I went shopping for a Sony receiver given that Creative Labs can't seem to reveal the specs on the analog audio output of their DDTS-100 decoder.

[Nigel Goodwin]

Originally posted by montreal
We can see that there is are four 22 ohm resistors following the 2 pairs of non-inverted and inverted outputs for the 2 channels.

That means that anyone who owns such a receiver need only attach a decoupling capacitor (say 10 microfarad) on each non-inverting output (pins 10 and 24) after the 22 ohm resistors.

Yes, it's simple to do - except it's COUPLING capacitors you are adding, not DECOUPLING ones. As the 22 ohms are insignificant, I originally posted the amplifier circuit, so it could be connected there - on the basis it's probably easier than a SM chip?.

Personally I would probably like to see a buffer after the capacitor, but it depends what you're feeding to?

[montreal]

Originally posted by Nigel Goodwin


Personally I would probably like to see a buffer after the capacitor, but it depends what you're feeding to?

I agree we probably need an opamp to follow each output.

Good luck and thanks for your postings.

[montreal]

Originally posted by Nigel Goodwin



Personally I would probably like to see a buffer after the capacitor, but it depends what you're feeding to?

Nigel,

It has been a few months since we let this thread go to sleep.

I have just learned that the CXD9646 is not a pure analog dual power amp chip, but a digital swtiched dual power amp which is driven by a stream processor (typically a CXD9634) which generates (an inverting and non-inverting) pulse width modulated (PWM) signals for the CXD9646.

Therefore, if my goal is to tap pin 8 on the CXD9646 to derive my PRE-OUT signal for my external power amp, then I had better recognize that the signal at pin 8 is not analog, but rather a digital signal with a variable pulse width and the pulse widths may be measured in micro-seconds.


If the two amps in the single CXD9646 package are to operate in a bridged configuration using a single polar power supply, then I imagine that for the bridge to generate the maximum possible signal (full on), then that would call for sending a 100% duty cycle pulse to one power amp and a 0% duty cycle to the other amp.

When idling, that would imply that each amp receives the same 50% duty cycle signal (with each signal in phase with the other) which would set the voltage at the each of the speaker terminals at 13.6 volts, this being half the supply voltage.

It is not clear to me how this 13.6 volts at each speaker terminal would appear on a scope. Would we see a steady DC voltage as the result of low-pass filtering or a steady stream of rapidly and constantly switching between 27 volts and zero volts (50% duty cycle)?

[Nigel Goodwin]

You're talking about 'class D', between the output of the amplifier and the speaker is a low pass filter, this removes (most!) of the high frequency switching, and leaves the audio to feed the speaker.

[montreal]

Originally posted by Nigel Goodwin
You're talking about 'class D', between the output of the amplifier and the speaker is a low pass filter, this removes (most!) of the high frequency switching, and leaves the audio to feed the speaker.

Do you agree that the CXD9646 is a class D dual amp?

In the following link, I've learned how a pair might be bridged and filtered and the impact of any high frequency residue that remains after the low pass filter.


http://www.audioholics.com/techtips/...ierBasics2.php

However, my interest remains the creation of the PRE-OUTs and now I realize that if I build a new LC low pass filter to clean up the signal that I plan to steal from pin 8 of CXD9646, then if there is a residue that enters my external power amp, then we are talking about noise in the 350 khz. region.

I'm not sure how my external power amps will react to such noise and if I should be now looking for a Surround Sound receiver which does not use class D power amp chips.

Thanks for answering.

[Nigel Goodwin]

I imagine they are class-D, as there are LC filters on the outputs.

[montreal]

Thanks Nigel for your opinion.

I posted a new thread in the Class-D forum hoping that someone can advise me on how to adequately filter out the switching noise in the signal I will be trying to steal from the input of the CXD9646.

[Nigel Goodwin]

Originally posted by montreal
Thanks Nigel for your opinion.

I posted a new thread in the Class-D forum hoping that someone can advise me on how to adequately filter out the switching noise in the signal I will be trying to steal from the input of the CXD9646.

Assuming it's just the same signal that's coming out of the speaker outputs?, then you just need a low pass filter - but as it's at a low level you can filter it much more effectively using an opamp filter.