A "Transparent" Buffer?

[Transmogrifox]

The answer is "no", a buffer isn't essential for splitting in all cases.  There are controlled cases in which a passive splitter is the right tool for the job.  To generalize this is like saying "I used a 3/8" 12-point socket once and it just stripped the bolt.  12-point sockets are useless.".  It may very well have been you were using a 3/8" 12-point socket on a 5/8" bolt or you were working in a situation where the torque was extremely high and a perfect fit was needed.  This wouldn't have meant anything about whether a 12-point socket can be useful for certain tasks.

In other words if you identified one situation in which a passive splitter wasn't the right tool for the job it doesn't follow that every situation in which you could potentially use a passive splitter is the same as the one that didn't work.

If making a generic splitter where you don't have any control over the input impedance of the 2 ends and if you want to drive any arbitrary cable length, your statement about needing a buffer would be accurate.  A generic splitter may have 20 ft cable from guitar to splitter + 20 ft cable from splitter to amp 1 + 20 ft cable from splitter to amp 2.  That kind of passive splitter would present the same capacitance as a 60 ft cable AND the 2 amps likely have 1M input impedance, so now input impedance is down to 500k.

For this special purpose you DO have control over the input impedance of the Surfy Bear.  You can make it somewhere around 10 Meg then it's not much different than putting  a pair of cables through a female-female coupler (double cable length).  Then the only "signal loss" you have is double cable length.  For that you could use a short patch cord from SB to amp and then that effect is of small consequence.

IF the double cable length is an issue and you want it to sound exactly the same as having a 20' cable going straight into your amp then you will need the buffer. 

If you really want to use the buffer: 
1)  Remove R4 and take that to the output jack.
2)  Connect pot grounded leg to ground instead of VB.

Hypothetically you don't need the mixer stage but you might want to keep it the way it is.

I also noticed something else re-reading your prior post:

I settled for a 50pf cap between pins 5 and 6. Sounds pretty close!

From schematic above adding a 50p cap between 5 and 6 compromises gain and phase margins on the op amp at high frequencies.  In an ideal op amp it would not have an audible effect but simulating with the LT equivalent op amp shows a significant +39 dB resonant peak at 200 kHz which results in about 0.1 dB boost above 10 kHz.  My opinion is this would be inaudible, but maybe in a different op amp and a different board layout other things combine.  Either way doing something like this makes the circuit extremely dependent upon op amp IC imperfections.  Even swapping a different TL072 might sound different if the 50p cap between 5 and 6 makes an audible difference.  The main problem is the op amp is on the verge of going unstable at a high frequency -- and if it does you will hear a lot of noise in the audible spectrum.

If you were going to sell these to a customer it would be in your best interest to eliminate the cap between pins 5 and 6 and try to get it to sound right in a way that is not dependent upon a specific IC at a specific room temperature.

Adding a cap between pins 3 and 4 is what I had in mind (input to ground to simulate a cable). 

Also if adding a cap to the mixer then putting it between 6 and 7 may be useful.  That would also help the op amp to tend toward stability instead of instability.

Either way I'm glad it's working out.

[Loose]

My bad, I meant pins 6 and 7

I'm gonna try experimenting a bit with splitting the signal into two channels, i'll update how it worked out.

Thanks for the help!

[Transmogrifox]

Ok.  In that case if you keep the buffer you may want to just do this:

If you really want to use the buffer:
1)  Remove R4 and take that to the output jack.
2)  Connect pot grounded leg to ground instead of VB.

I still think the passive tap will be the most simple but either will work fine. 

If you stuck with the buffer + mixer then you could make a version you can switch to either mode of operation with a DPDT toggle or stompswitch.

[PRR]

> trans·mog·ri·fy

OFF-topic, sorry!

I know the word; never saw it used in a sentence.

"Wizards are usually bizarre in their appearance, and this was a wizard among wizards. Even at that very moment, the fellow was bringing some fearful-sounding incantation to a close, which I had no doubt would have transmogrified us right proper. Mind you, I've no use for their extravagant theories, mages, but there's no denying the better ones can wreak havoc on a man's morphology."

The Philosophical Strangler
by Eric Flint
http://www.baen.com/the-philosophical-strangler.html 
Read online, chapter P, halfway down

[Loose]

So I tried splitting the signal with the buffer still in the circuit. The reason I wanted to try it is because the dry signal gets washed by the wet signal when using reverb, and you lose the punch. Splitting the signal didn't make much of a difference, it just gives you extra control over the volume and eq with the amp knobs.

I guess this would work better by splitting into two amps with two cabs, making a true wet\dry rig.

[Transmogrifox]

Yeah, something like reverb going into a preamp is hard to manage.   Especially if you're using the amp at the sweet spot where you're making the tubes compress.  If you mix verb into the nonlinear response of the amp it makes things a bit dull.

There's where I would have thought your experiment splitting to different amp channels would help but apparently the problem is more like you said.

You might get a little more punch if you started tweaking the reverb effect itself.  The surfy bear is generally more treble so maybe that's the worst thing to wash your signal because it decorrelates the high end attack right where you need the tightness in the sound.

Maybe re-voicing the reverb to focus on mid-range and cutting the highs might help get some punch back.  Just a wild idea but possibly worth consideration.

I agree the best bet would be 2 separate rigs.  The main benefit is you can locate the 2 amps to get an initial delay which delays the attack from the reverb past the attack on your strumming and probably would not rob the punch as much.

If you play in a band where a PA system is available then your buffered split version would be useful because you can send the reverb to the PA system (although you lose control over the mix level if the sound guy is a jerk and mixes it the way he likes it).

[Loose]

If you play in a band where a PA system is available then your buffered split version would be useful because you can send the reverb to the PA system (although you lose control over the mix level if the sound guy is a jerk and mixes it the way he likes it).

That was my thought exactly.
I was also thinking of a line out box that will go between the amp and the cab, that would probably produce a better sound, but could be more of a hassle.

[bluebunny]

> trans·mog·ri·fy

OFF-topic, sorry!

I know the word; never saw it used in a sentence.

[Transmogrifox]

This very strip was my first introduction to the word -- see, comic strips are educational reading material