RCD-965BX / OPA627 - Class A Bias & O/P Coupling Caps - diyAudio
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Old 18th June 2006, 02:51 PM   #1
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Default RCD-965BX / OPA627 - Class A Bias & O/P Coupling Caps

Hi folks,
M a noobie to diying and modding.... Currently working on the rcd-965bx. Need some advise/help on some stuff...

Have tried a couple:

1. Replace OPAMP to opa627
2. Removed muting transistors

After searching the web, I'm thinking of the following:

1. Putting the OPA627 into ClassA bias:
-> How do I do it? It is simply putting a 4.7K resistor from pin2(-IN) to pin6(O/P)?
-> Based on your experience, what are the changes that we can expect from this change?
-> How will varying the value of the resistor change the characteristic of the sound?

2. Bridging the O/P coupling caps or replacing with a alternative better quality/difference size caps?
-> Which are the O/P coupling caps?
Which is a better method? Understand that oscillation is a risk if it is bridged. What are the symptons when oscillation happens?
-> If bridging is not recommended, what value should we use?

Thanks in advance!

Cheers!
Poozebear
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Old 18th June 2006, 03:20 PM   #2
Bobken is offline Bobken  United Kingdom
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Hi poozebear,

I have this circuit schematic and the output coupling caps are C227, & C228, both 100uF 25 volt.

I have not worked on this circuit, nor do I have this CD player, so I cannot offer any advice based on experience. However, coupling caps, such as these, are usually only used to prevent any DC voltage from being output to subsequent equipment, and I cannot see any reason why bypassing them should encourage any oscillation.
You would need to check that there is no DC, though, and using an oscilloscope would be the safest way of determining if you have encouraged some oscillation, after you have bypassed these caps.

Regards,
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Old 18th June 2006, 05:33 PM   #3
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Thanks Bobken!

I found some stuff on audioasylum... not sure which is the way to go:

http://db.audioasylum.com/cgi/t.mpl?f=tweaks&m=125985

"....both these ops are bypassed from =/- to ground with 100uf black gates??..try bypasssing each of these with a .01uf polypropylene cap of your choice... "

"....since the opa627 has an insanely low dc offset you can laso safely remove the cheap electrolytic output coupling caps..... Use a wire jumper to rebridge this path sans cap, if you have a voltmeter handy measure the dc on each output, it should be under 1mv after this mod...I found this tweak to give more extended highs and a little more body overall..."

Anyone tried any of the above? Thanks!

Cheers!
PoozeBear
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Old 19th June 2006, 03:00 AM   #4
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Looks like I missed this huge thread on class a biasing...

The OPA627 really sings

"Make sure you put the resistor across pins 4 and 6, not 2 and 6."

Thanks!
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Old 19th June 2006, 05:31 AM   #5
Noak is offline Noak  Singapore
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This link may be helpful for you.

http://www.tangentsoft.net/audio/opamp-bias.html
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Old 20th June 2006, 06:09 AM   #6
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Thanks Noak!
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Old 13th July 2007, 10:59 PM   #7
KP11520 is offline KP11520  United States
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Hey poozebear,

Did you finish modding the RCD-965BX? What did you do about biasing the OPA627AP op amps? Any experience you can share would be greatly appreciated! Hopefully you will check back in to DIYAudio and respond to this post!

Thanks!

Regards//Keith
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