Arcam rDAC output stage schematic

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ellboy

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Joined 2013
#1
Hello!

In the Arcam rDAC service manual I found following output stage design. What is the purpose of TR303, TR304, TR301, TR302 and TR306? Why they use 2 mute types using that parts and RLY300?

I home some one can me help with this schematic.

Regards
 

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#3
If you read the note on the schematic you'll see that not all those transistors are fitted in production. Just looks like two transistors are in parallel but in reality they're there because they want flexibility to change to another transistor with a different pinout.
 
#4
stephensank said:
Those are ALL for muting. And they are, really strangely, redundant with the ground-shunt muting relay. You can simply remove TR301-307, with only positive potential for sound benefit. The grounding relay is all the muting the darn thing should need.
Click to expand...

I believe it fulfils the original red book standard although this design is only partly implemented. You'll remember that Sony CDP790 I refurbished. That uses a similar arrangement (relay and transistors), and I can confirm that the transistors are used under certain conditions (red book test CD). The Sony however drives each channel separately and some test tracks actually mute either L or R channels as required. That obviously can't happen here.

As to the original question, yes by all means try with them removed and if you hear no problems then all is well. If you do, then put them back.
 
#6
C306 is drawn as a bi polar electroylitic and is essential to prevent DC offset reaching the output. Even a few millivolts offset would create noise and thumps when the relay opened and closed.

So yes, its an AC coupling cap.
 
#7
The power supply design is also not clear to me. They use REG405 and TR400 to create a clear power for the analog part of the WM8741(AIC1735 datasheet doesn't have much information about noise and rejection, but I hope its not so bad). The same part is used to get the 5VA but in that case without an additional transistor and its maybe not so clean. On the over side a chrarge pump sp6661 is used to create -5VA without any additional active parts for noise/ripple rejection.

Is it not better to use a pump connected to 6V for creating negative voltage and a low noise LDO after that for -5VA? For 5VA and 5V in analog DAC part in my oppinion one low noise LDO can be used.
 

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#8
Hi All,

Sorry, I have to resurrect this old thread.

I just built a external linear power supply consists of first stage LM317 and teddyreg on second stage to power up my Arcam rDac instead of using supplied wall wart psu.

Below are the PSU diagrams downloaded from Arcam website.


Main DC in:
rdac_psu_main_zpsvyvhaiik.jpg


Main DC into 6 Regulators:
rdac_psu_circuit_zps4kskngzc.jpg


PCB after recapped:
20151230_133713_zpsr2bpdmye.jpg



My questions:

1. L400 & C400 is the LC filter, can I increase the value of C400 from 470uf to 1200uf? I hope to increase "capacitance bank" here.

2. What are the components at Main DC in section can I bypass? To my understanding they were designed to be used with wall wart adapter, I am now using linear psu as mentioned above.

3. Can I change the AC coupling cap C306 & C319 to polarised audio-grade caps there?

Any other comments & suggestions are welcome.

Thanks for your inputs.

allen.
 
#9
1. Going from 470uf to 1200uf is a small increase and so should be OK.

2. If you are using a good linear supply then you should be able to omit L400 and feed straight onto C400. (if I'm understanding you correctly)

3. If there is zero volts DC across the caps then yes. If there is any DC present then fit the caps accordingly (even if one channel is reversed compared to the other due to different polarity offsets)
 
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