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Old 20th October 2003, 04:16 PM   #1
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Default Seperate power supply for pre-amp section in integrated amp?

Hello,

I have a Rotel RA-930AX integrated that I have completed a lot of modifications to.

My question is: Would a seperate power supply for the pre-amp section improve the sound?

The pre-amp section is using OPA627 op-amps.

I could get another transformer and bridge and some caps and have an unregulated supply or I could throw in a voltage regulator for good measure.

The current pre-amp power supply section is using a zener to achieve the needed + & - 18v.

Would anything be gained by using the existing transformer in the amp and adding a new bridge/caps & regulator?

Any input or experiences would be appreciated.

Thanks

KevinLee

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Old 20th October 2003, 04:40 PM   #2
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Yes, an extra ps will certainly improve the sound. More than a regulator with the existing supply. If you decide to regulate be sure to listen to several regulators - i'd probably prefer the sound of zeners to the typical 3-terminal regs.
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Old 20th October 2003, 04:54 PM   #3
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Thanks for the info analog_sa!

The secondary voltage of the amps transformer is 28 volts, once I go through the bridge it is like 39 volts.

The pre-amp section currently is supplied with + & - 18 volts.

Most regulators will not accept this kind of input voltage.

Maybe I would be best to but a transformer of say 20 volts and then build a bridge, add caps & 18v regs & more caps & call it a day.

Or would I be better to use an unregulated supply using a 12v secondary transformer and bridge & caps and supply my pre-amp section with 16-17 volts unregulated.

What will change by supplying the pre-amp section with 16volts instead of 18volts? Will I need to change some resistors values?

As always, any input is appreciated.

Cheers

KevinLee
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Old 20th October 2003, 05:03 PM   #4
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Nothing much changes at 16v - no need to change any resistors. You will severely limit your choices with a low voltage transformer though, good regulators sound better than no regulation at all and some experimenting is a must. I just don't see how you can decide everything in advance and 'call it a day' without critically listening to all changes. After all you want your amp to sound better, right?
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Old 20th October 2003, 05:12 PM   #5
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You are 100% right!, I do want my amp to sound better.

From what you are saying, I understand I should have a regulated supply.

I will source a couple different voltage regs and get to work.

The reason I was thinking of supplying the pre-amp section with a lower voltage than 18v is that the OPA627 have a limit of 18volts and I would rather not work any electronic part at its extreme.

Thanks

KevinLee
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Old 20th October 2003, 06:04 PM   #6
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If you get anywhere between 20-25v raw dc you should have ample voltage to drop accross the regulators; 18v is a bit marginal if you aim for 16 regulated. In my experience you may get better audible results using a simple emitter follower with a zener in the base than any 3-terminal but the opposite opinion is also popular It's largely a question of taste: if you value air and dynamics more than control and bass solidity the emitter follower is likely the winner.
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Old 28th April 2005, 05:49 AM   #7
Fin is offline Fin
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Default Any results??

Kevin

Did any of this work for you?

I'm interested in doing something similar with my Denon PMA350.
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Regards
Fin
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Old 2nd February 2008, 11:30 AM   #8
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I have a 930ax too

on the pre amplifier:
I would like to replace the ne5532 by a lm6542, must I replace the dr18e by dr16e to have + - 16v (or dr14e +-14 v) or, then I can use the lm6542 with his tension max (+-18v)

others tweak on this amplifier?

I have the schematic

thanks
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Old 2nd February 2008, 10:20 PM   #9
jaycee is offline jaycee  United Kingdom
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If it's like the RA-931AX (which i have a service manual showing PCB layout and schematic for), the regulators are basic transistor zener followers.

Simply replace the zeners with a slightly higher voltage zener, for example BZX85C24, then use LM317/337 to regulate down to 18V (or whatever) before the opamp.

Cheap, easy, good

The only issue here is that it may involve cutting the track. Again i'm going by the RA-931AX service manual. Another issue might be the dissipation in the pass transistors (they are only TO-92 devices !) - I would probably replace them with TO-126 packaged devices (the pinouts match). BD139/140 would do.
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