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.
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.
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?
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.
Your right. i mixed it up. :-)
"We add 1 DEXA Technologies “discrete” DUAL op amp module to the input buffer.
Two additional DEXA Technologies “discrete” DUAL op amp modules. One for each output buffer."
thanks again for your recommendations, they seem inline with my thinking and research. sounds like you have listened extensively to what i have only read and thought about. Cheers!
Quote: "I have also had benefits when soldering a 47uf Nichicon Muse + 0.1uf to the +/- power pins of an opamp."
Question: would i add this to each opamp individually or just to the dual DIP8 pins on the adapter? It seems obvious that it would be a single .1uf mkp cap across the adapters pins itself and not each opamp but just checking.
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:
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:
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).
Another thing I would mention here is that I am making a big assumption that those green Nichicon Muse bipolar are being used for DC blocking signal caps (why else would you use a bipolar?). If he is actually using these for power supply filters, then Elna Silmic is not the best choice. The best I found is the normal Nichicon Muse (UKZ) series. If you do this, better make damn sure you put them in the correct polarity (put in an electrolytic reverse polarity will probably blow the cap pretty quickly). Might have to use a multimeter to determine which pins are positive/negative.
Also, I don’t know if you have looked at this, but you can also upgrade the voltage regulators (unless they are already upgraded). I have used the Sparkos discrete regulators in several applications and they are just excellent. Much faster and smoother than monolithic 78xx/79xx regulators and they also shield noise from main power supply better. I have not tested the Dexa regulators, but the Dexa are much taller and I never had the space to place them. The Sparkos are same physical size as the monolithic 78xx/79xx, but a bit thicker. The Dexa uses a NE5534 op amp for error correction. The Sparkos uses a discrete Class A circuit based on their own discrete op amps.
"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."
So upgrading the input buffer would only improve the sound of the RCA inputs and have no affect on the balanced input signals?