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Quad 405 Amplifier Opamp Upgrade With OPA627

Continue to my restoration process of this Quad 405 amplifier in part 1 & part 2, I found that this little amplifier is a very good sounding amplifier.

I should tell you this amplifier rival, also my previous reference amplifier is the Adcom GFA-535II is easily beaten by this Quad 405 in term of everything!

Don't believe that? Me either! After I completely restored this amplifier to it's potential, then I am speechless with the result. I think the last upgrade right now is changing the op-amp to the famous OPA627 from Burr-Brown.

Let's check it out.
First of all, beware of the counterfeit components.

This single op-amp OPA627 is very famous and the price is unreasonable high. That is why so many counterfeit OPA627 on the market right now.

Before you buy, make sure you get them from reputable seller or I prefer using the original and cheap LM301 rather than any fakes op-amps.

This OPA627 upgrade is actually not on my upgrade list. Who gonna buy US$30 op-amp each when a dollar op-amp will do the same?

But this is the fun side of DIY.

I really want to know on how much difference in sound by only replacing the op-amps.


 This OPA627 is the same pin out as LM301. I know this should be the last upgrade for this amplifier, then no need to keep the op-amp in the 8 pins socket anyway. This op-amp is solder directly to the board for the best signal transfer.

 It is done and the amplifier is back to my listening room.

First things first, you will notice the turn-on and off thump is now very loud now. Quite scary that this thump could possibly broke your precious speakers. It is strange that this problem is not present with the LM301.

Then this is how I solved this problem.


 The simple method is by using the additional filter caps on the positive and negative voltage pin of the op-amp to the ground. I am using Elna Silmic II rated 10uf/ 35v. On pin number 4 is for the negative and pin number 7 for the positive. The remaining leg from the caps are joining together into the ground point.

The photo bellow is show the connections.




 It is completely done now. By adding those capacitors, the thumping noise reduce very significant and almost no noise occur when turning on/off this amplifier.

Did I have mentioned about the sound improvement yet?

 With the same system and same music played, I notice that the mid to high frequency is more spacious, airy, and detailed. It is more engagement in overall music presentation. If I have to compare the LM301 with this OPA627, then the LM301 sound dull and narrow.

I believe there is so many improvement in sound quality by using this OPA627. It is only the matters of the over-priced chips that you should afford for this kind improvement.

Disclaimer: Any statement and photos in this article are not allowed to copy or publish without written permission from the writer. Any injury or loss from following tips in this article is not under writer responsibility.
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