Unity Gain Tube Buffer - Choosing the Right Location


Hi Everyone -

I am about to receive and install a TVC passive in my system and have been considering purchasing a unity gain tube buffer. After a lot of research it seems that the combination of a TVC and unity gain buffer could really be a home run, combining the transparency of a passive with the attack of an active without any of the excess gain. I won't make any decisions before burning in the TVC but I just wanted to start thinking about this and it could be helpful for others.

My question is where in my system chain the buffer should be installed as this would determine which buffer I would buy and the connections it would need. A little system background - My digital chain is a Schiit Eitr > Metrum Acoustics Amethyst DAC > Promitheus TVC > Red Dragon s500 > Kef LS50. The metrum is single eneded and I will be using the TVC to convert to a balanced signal for the input of my power amp.

Should the buffer be placed before or after the TVC for the most sonic impact? I realize many people won't have explicit experience with this so maybe I am just interested in theory.

As far as I've surmised to this point - Observations of the buffer before the TVC:
-I could use the buffer to convert the single ended DAC signal to a balanced signal. The buffer would do the conversion instead of the TVC. I'm looking at units from Space Tech Labs which could do this and would be cheaper than a fully balanced design.
-The buffer would be running with a full signal voltage strength through it from the DAC (How will this affect s/n?)
-I will have an AVR feeding the TVC as well, I could bypass the buffer for home theater which is ideal.
-Unbalanced input to the buffer adds another set of RCA cables in the chain, which I understand have less noise rejection and more cable artifacts than balanced connections. Hopefully this will not outweigh the benefits of the buffer.

Observations of the buffer after the TVC:
-Constant output impedance to my power amp which will be lower than the source with any attenuation.
-The buffer will see an attenuated voltage signal from the TVC (How will this affect s/n?)
-I will have to use the buffer for my home theater if I don't want to keep changing cables (meh... struggles of integrated home theater and 2-channel)
-Balanced connections on both input and output (better I would think)

I've been searching online for some answers to this but there doesn't seem to be a lot of information out there. I wonder if the the buffer has more effect sonically after the TVC as there is a direct interaction between it and the power amp - I can see a lot of advantages to a constant and lower output impedance and fully balanced connections. This will make the buffer more expensive as I will need it to be fully balanced.

Any and all help/insight is appreciated!
ajnackman
The output impedance is 100 ohms from the metrum and the input impedance of the red dragon is 50kohms. I currently a using a schiit sys which is a passive resistive volume control.
You don’t have any impedance mismatch, the Metrum is great at 100ohms, the Sys output impedance is at worst 1/4 of it’s 10kohm pot value which will be 2.5kohms, and the you say thje Dragon amp is 50kohm!, you have great ouput to input impedance ratios all the way through, so you don’t really need a tube buffer.
What you have already will be the most transparent and dynamic sound that true to the source, unless you want to colour the sound somewhat with a tube buffer.

Cheers George
The buffer from space tech labs I've been considering doesn't have a volume control or input switching (I run a TT, DAC, and AVR). Based on what I've been looking at, there arn't many unity gain buffers with volume controls. I like the idea of a non-resistive volume control but maybe that is limiting my options. 
If you have a buffer that can also convert to balanced, why do you need a TVC? Can't you use the volume control in the buffer? If yes, it will work better if the TVC is omitted.
I don't anticipate an impedance mismatch - The output impedance is 100 ohms from the metrum and the input impedance of the red dragon is 50kohms. I currently a using a schiit sys which is a passive resistive volume control.

My understanding is that a TVC will present a lower impedance to the power amp than the source produces at any amount of attenuation.

I guess the shifting impedance would be between the variable output impedance of the metrum > TVC and the power amp and could benefit from the buffer.
The tube buffer will provide the greatest benefit between the two components that exhibit an impedance mismatch.