Is dedicated PreAmp with Oppo 105 needed?

Toslink interface is BAD BAD BAD.  Don't use it unless you absolutely have to.  Digital COAX is always superior for any digital audio.

That being said, the Krell S1200 is a great preamp/processor, but only if you mod the power supply by soldering in huge 15,000uf capacitors.  The stock power supply is undersized, in my opinion.  Stock, it will give good sound, but it's a bit thin/bright in a way.

You are correct in that putting a preamp between the Oppo and the A51 can be done to improve 2-channel audio.  Look for one that has a HT bypass input, so that you can still use volume control from the Oppo.  What is your budget? 

Sorry to go on and on in this.   There are actually a couple of S1200U 3D models up for sale on usaudiomart for $2800. Wow!  low price, actually. 

In most cases I agree with atmasphere's comment that a separate 2-channel preamp will be better (in most cases) than any HT Processor.  However, this Krell S1200 with my power supply mod is pretty freaking amazing, in my opinion.  I would actually put it up against many other 2-channel preamps (unless we are looking at Krell Illusion/Phantom with separate power supply level).

The S1200 does not support 4K video over HDMI (only 1080p).  If you don't care about this, then I would say S1200  would be the best ht processor choice and also great preamp for 2-channel analog sources.

Atmasphere also commented that 5 channel amps have shared power supply.  That is definitely a restricting element if you watch high volume movies.  When you are listening to 2-channel music only, it is less of a problem.  However, a separate Parasound A21 for left/right would definitely be an improvement.  Though, an S1200 preamp/processor would be a better first step.

Oh, and upgrading the main fuse in the A51 or A21 amps to Furutech rhodium will increase the body and make the sound less thin.

If you do the power supply capacitor mods I will describe, I think the S1200 will be superior over any other multi-channel preamp/processor.  And yes, you can connect XLR/RCA analog audio directly to S1200 as well.  The S1200 uses fully discrete Class A analog stages (no op amps).  Below is a picture of the S1200 audio board, you can see the 8 separate audio stages for each of the 7.1 channels.  Each one of these would be a single opamp in another processor.

https://photos.google.com/photo/AF1QipOOZfHv-_iCrjXTkT3JayjSMG8FCYsl1DBGeygF

As far as I know, the only other processor that uses discrete Class A stages in Bryston SP3.  However, the Bryston is not as good in my opinion.  It is voiced differently.  It has a massive main power supply with big caps.  However, their analog circuits do not have any local capacitors.  As a result, the sound is very laid back, but with massive bass.  In the Krell board, you can see tons of local power supply capacitors for each analog stage (SMD caps).  These local capacitors give the Krell enormous amounts of resolution and attack in the midrange and highs.  Unfortunately, the main power supply on the Krell is undersized, which is essentially starving these Class A analog circuits.  The result is weak bass and a thin/bright sound for the mids/highs.

The Krell Foundation would not require any of these mods as it has very good power supply caps.  However, as far as I can tell, it uses op amps for audio stages and would not be as good as S1200.  The Classe SSP-800 also uses op amps.  So does McIntosh.

Below is a picture of the basic internals of S1200:

https://photos.google.com/photo/AF1QipOAbERmjfNnzPMRpWFmk_9qKW84yLNtow4FbGjA

The bottom board has 4 separate power supplies (each marked by a pair of capacitors).  The top HDMI/video board also has its own power supply.  On the bottom board, the first three power supplies are all shunt regulated.  Power supply on the left is +5V for the ESS sabre dac VREF power supply.  The two in the middle are for the +17V / -17V for the analog stages.  The one on the right is for +12V (and probably +3V) for digital sections and uses conventional LM78xx regulators (not shunt).  We really don’t care about the power supply on the right as it is just fine.

My mod does not replace any existing capacitors. It just adds additional caps into the mix.  Below is a picture of my mod.  For reference, the blue module on the bottom of the picture is the potentiometer for the front panel knob.

https://photos.google.com/photo/AF1QipOiGwC8a6DljI_Gy9hD0Dw9Rg8Q8AMVBI4yVEuJ

For the left DAC VREF power supply, I use a Nichicon KW 10,000uf 10V capacitor in parallel with a Vishay 0.1uf MPK (part BFC241641004) for stabilization.  I basically solder capacitor leads directly to the board pins for the AC/DC diode converter block (which is bolted to the case floor).  These are the outside pins of the 4 pins.  Make sure you solder the cap polarity correctly (negative to negative polarity and so forth).  Use a digital multi-meter to confirm polarity of the pins.  This mods basically adds capacitance to the DC sections of this power supply.

For each of the middle two +17V/-17V power supplies, I use a Nichicon KW 15,000uf 35V in parallel to a Nichicon Muse KZ 470uf 50V in parallel with a Vishay 0.1uf MKP.  The 470uf helps with the quick voltage needed for midbass punch while the big 15,000uf ensures the continues voltage required b Class A circuits is never starved and helps with strong deep bass.

In addition, I always recommend a mandatory fuse upgrade.  I like Furutech rhodium fuses for the highest resolution neutral sound.  If you have a very warm sounding amp/speakers, you could use a Hi-Fi Tuning Supreme to increase the attack/resolution.

You can use the S1200 as purely a 7.1 channel analog preamp if you want (using 7.1 audio from Oppo).  However, the Krell is absolutely excellent when doing tv/movie decoding using dolby digital / DTS, etc.  Be aware that the S1200 has had common problems with the DSP board which affects decoding of audio over HDMI (unstable audio with dolby digital, DTS-MA, loud pops when changing modes, etc.).  Krell has engineered a version 2 of the DSP board which fixes all the problems, but requires you to send the unit in for replacement and new firmware.  I’m not sure of the cost, however.

For education, the Krell S1200 can do lossless decoding and playing of the following audio formats, but only over HDMI (which is fine since you're using Oppo 105 as a source):

- DTS-HD MA (bluray)

- Dolby TrueHD (bluray)

- Dolby Digital Plus (such as Netflix streaming).

The S1200 is absolutely excellent in decoding and playing the above formats.  For all other decoding, such as old school Dolby Digital, DTS and 2-channel PCM, it is better to use digital COAX to transmit this.

If you are a looking for a pure analog only preamp, the P7 is the best.  The C1/C2 are older and appear to have switching power supplies and power supply banks are not good enough.  The P7 has superior linear power supply and architecture.  It will not be nearly as good as Krell S1200, but it is very good for the cost!!

If you are looking for HT Processor on the cheap, there are better solutions than C1/C2.  Look at the Anthem Statement D2, which are going for just over $1k.  They are still older, but much better than C1/C2.

Marantz AV7702 or AV7703 are in the $1k to $1.5k range and are very good (better than the P7 because of the discrete analog circuits).  If you can stretch to $2k, the Marantz AV8802a is excellent. (look on ebay for these).

Of course, none of these are as good as S1200, but in the end, you get what you pay for.

Since I cannot post in message, try this link for the photos:

https://photos.app.goo.gl/cgWrZqVL6u1M791P9

hope that works.

At this point, I would use a single 10,000uf 50V Nichicon KW add-on for each of the three power supply sections.