W4S STP-SE DIY mods

My initial thought would be that it wouldn’t make sense to have only one output buffer because W4S indicates this preamp is "fully balanced from input to output".

One theory would be that you mixed it up and there is only one input buffer with two output buffers. The one input buffer would only be used to create the "inverted" balanced signal when a single-ended RCA input is used. Then it goes to the discrete FET stage, which should be fully balanced/differential. Then there are two output buffers for both the normal and inverted signals of the XLR connection.

All op amps will have their own sonic signature and introduce some sort of sonic character to the signal. In my experience, the AD797 introduces the "least" amount of sonic change. It is a bipolar opamp and bipolar devices are usually less distortion and cleaner. The Sparkos is also bipolar, but it is biased heavily into Class A. Best way I can explain it is it introduces a "liveness" or "reverberation effect" to the music. However, if used too much you can overdo it and the sound can become harsh/bright (like if used for both input and output stages). The AD797 is more dry/neutral sounding in comparison.

FET opamps (such as OPA827, Burson, etc.) have more of a rich type of sonic signature. However, the richness/warmth creates somewhat of a smeared sound throughout the range. The smeared high frequencies could come across as bright/harsh. Depends on system synergy. Even the ultra-fast OPA627 has this type of sonic signature, and is more lean on bass/midbass (because the slew rate is so fast, it tends to translate waveforms to be faster then they should).

If I am right in my assumption above that there is only 1 input buffer, I would use a dual-AD797 on adapter for the input buffer to keep the input sonic signature as pure as possible. Then you could experiment with the output buffer. Use AD797 if you want the most transparency. Or try some Sparkos if you want to turn this into a Class A preamp. Or something else entirely.

Well, technically, it is best for each single channel op amp to have its own capacitor. However, that is just not practical (AD797 adaptor has the second opamp underneath, and Sparkos have 2 levels of boards). I just put a 47uf + 0.1uf MKP on each DIP8 module. Here’s a pic of how I do it:

http://i824.photobucket.com/albums/zz170/ainami/electronics/AD797_Sparkos.jpg

I use a Vishay MKP416-420 for the 0.1uf on these caps (digikey # BC2054-ND). It’s metallized Polypropylene and the best I could find in this size.

You can see I have the 47pf ceramic caps between pins 6 and 8 on each AD797 for the high frequency compensation. I use AVX Skycap for this (digikey # 478-4850-ND). For SMD work, here’s a really excellent video:

https://www.youtube.com/watch?v=z7Tu8NXu5UA

When putting on the 47pf, it’s absolutely required to use flux. I use MG Chemicals 8341-10ML syringe for this (digikey # 473-1117-ND). I just put a blob of flux on the pins and melt it with soldering iron. Then I use a spring loaded tweezer to hold the 47pf ceramic down on the pins. I put a bit of solder on the tip of the iron and place it down on the pins of the ceramic. The flux allows the solder to flow down between the pins and make a good connection. Without flux, the solder tends to blob on top and you won’t get good connection.

Since the 47pf stick out, other components might get in the way and you may need to put in a riser DIP8 socket, such as Mil Max 116 (digikey # 11693-30841007000-ND). In fact, I recommend putting in a DIP8 socket for each op amp anyway, because you will probably want to swap around op amps for experimentation. Be ware that gold-plated sockets and leads on these will introduce the warm gold-plated type sound. The tin plated are actually more neutral, if you care.

Before you mount the 47uf, check around the op amp on the board to see if there is room before you make a decision on which way to diagonally bend the capacitor leads. This 47uf will provide a very fast power reserve close to the op amp circuit. It will increase attack and clarity in the mids and highs. It will also give just a bit more punch in the midbass. The 0.1uf MKP is critical because it stabilizes voltage fed from the 47uf. Without it, the high frequencies will have a blare/mess quality.

The Vishay 0.1uf can be used in all other places on the preamp as well. If the Vishay is too thick for a position, I use Kemet SMR (digikey # 399-13042-ND), which is polyphenelyne (better than polyester caps).

For op amp negative feedback loops, I upgrade the caps to Kemet PFR series (film-on-foil), or silver mica if the value is small enough and there is enough space for the cap (the silver mica can get physically too large).