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W4S STP-SE DIY mods

Greetings,

I've got my STP-SE inhouse and researching my upgrade mods. I understand both Underwood and W4S sell mods as a service but I would like to keep costs down and exercise my soldering iron.

Does anyone have specifics on values/locations for updating the volume ladder resistors, diodes, and inductors that would be awesome.  

I believe I have details around the PS caps, voltage regs and opamp chips. 

Anyone have specific recommendations for parts to swap vs leave as is? Any specific pitfalls or gotchas I should be aware of?

Thanks much,



vfrpilot
vfrpilot OP
13 posts
 
Since we have no takers I will ask a different question. With regard to the Underwood Hifi mods they describe replacing the input and output buffers with Dexa Opamps. I am curious what people think the value is of installing opamps which are even better than Dexa in these postions. Burson, Sonic Imagery, Sparkos for example.
auxinput
2,920 posts
 

Replacing the input/output op amps will change the sonic signature of the device. Choice is dependent on what you’re looking for. I have tried the dexa Special Edition op amps, which have a very fast 60V/us slew rate. While better than some other choices, these had somewhat of a shouty midrange and the sound was somewhat flat. Also, it has somewhat of a lean bass (just a little bit lean). I don’t know if the normal 20V/us version is any better.

Burson’s are FET devices and have that somewhat rich flavor. I didn’t really like them. While they had good resolution, they just didn’t "sing" and I found myself losing interest in a song very fast with these. I did put 175 hours on them, so they were definitely burned in. Others love the Burson. Personally, I think the best FET opamps are the OPA827. They are slow enough where you get bass/midbass, but fast enough where it’s not smearing the sound (like opa2134).

Sparkos was the best discrete I have found, but be careful. Too many Sparkos in a circuit and the sound could become too bright/rich or it could become too slow (if not enough power supply). They are awesome sounding Class A op amps if used correctly and convervatively. Two dual-channel op amps in the STP should be fine.  The Sparkos definitely have that "Class A" goodness/magic.

I have no experience with Sonic Imagery. That being said, these discrete Class A op amps require a huge power supply. The STP has more than enough to handle these discretes, so go ahead and experiement!

The idea of the STP is to be very transparent. It is meant to act more as a passive preamp, but with the ability to add voltage/gain if required on the output impedance side. If you want to keep this passive/transparent character without adding any sonic signature, the best "transparent" op amp I have used is the AD797. You can get a dual-AD797 on an adapter from Brown Dog. Make sure to solder a 47pf ceramic between pins 6 and 8 of each AD797 mono op amp for the high frequency compensation (reduces high frequency distortion).

I have also had benefits when soldering a 47uf Nichicon Muse + 0.1uf to the +/- power pins of an opamp.

Another thought is you could try replacing those green Nichicon Muse bipolar opamps with Elna Silmic. These would make a better DC blocking signal capacitor.

vfrpilot OP
13 posts
 
Very interesting and enlightening feedback Auxinput. Much appreciated. I am surprised to see that you would reference a monolithic opamp as being more transparent than a discreet one much less the SOTA ones.

STP-SE contains two input buffers and one output buffer. any idea the role and relative importance of the two input buffers? one SE and the second for Balanced? Two stages?
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