Neither me as I have never listen them (basically any tubes) in my audio setup. But I got a little bit interested in them. So, I decided to give a try 🙂
To buy or not buy Chinese tube buffer
If you go to eBay, there are probably thousands of tube buffers available in different price categories for different buyer 🙂 Some of them are used, other are brand new. A bigger part of them are from Chinese manufacturers and one of the most affordable for the beginners is 6J1 tube based pre-amplifier buffer (via eBay). If you chose a fully assembled unit with the case with 6J1 tubes buffer, you would end up by paying around 40 Eur. Not very expensive. But I know myself, I would start tweak everything starting from replacing the tubes (to Americans RCA?), the capacitors, the transformer and something else 🙂 That means I would pay for a case + a pcb. But I think it is still a pretty good bargain for some people who are not interested in tweaking 😉
It’s in the blood of every DIY’er to squeeze max juice from poor 6J1 tube buffer (via eBay) and it looks that I am not alone with such attitude. There are plenty of the guys on diyaudio or similar forums discussing the ways & possibilities of doing this.
But after some reading in different places, I decided not to buy a Chinese buffer. Instead, I chose to do few experiments with tubes and tube buffers/pre-amps in order to understand either I like or not their sound in my audio setup.
Initial idea of tube buffer project
To implement this circuit, you would need three subminiature tubes 6Н16Б-В (6N16B-V) tubes (via eBay), where two of them would work as SRPP and the last one as a cathode follower. The circuit looked quite easy to implement, so I simply ordered the tubes on eBay for less than 20 Eur for four tubes and ordered other parts as well. Here is the datasheet of 6Н16Б-В (6N16B-V) tube.
What I liked in this circuit – a bit innovative approach to tubes voltage power supply. In this case, the tube circuit requires 140V (DC) from a tube transformer (via eBay). There are two transformers connected in reverse direction. The first gives 6V (AC), the next one has a transforming coefficient of 25.5, which I think would result in 25.5×6=153V (AC).
Changing the plans
All parts were travelling in my direction from different places in the world and I came across the discussion on diyaudio forum about the circuit above. The short conclusion – most likely I will not enjoy the sound 🙁
But that is a DAC not a tube buffer!!!??? That’s correct. There are two versions of the circuit – one for current output DACs and another for voltage output DACs. Let’s use the second 🙂 Here is the circuit for voltage output DAC without the resistors R101-R104, which I expect are used for converting the current into the voltage:
You can use the capacitor C102 or not, as it most cases similar capacitor is already is installed in your sound source (DAC, PC audio card or whatever else).
What is still missing? Power supply
As first try, I decided not to build the voltage regulator (Q101 etc.) but simply use voltage multiplier with RC filter consisting of 100 Ohms resistor and 220uF capacitor for each channel. I found such circuit of the voltage multiplier on Aikido website:
The circuit should give around 150V voltage, which required for our 6Н16Б-В (6N16B-V) tube buffer. That is pretty close to 145V mentioned in the link above. More on how the voltage mutiplier works, you can read here.
In green color, the connection for 18V(AC) transformer is shown. I chose as R12 100 Ohms and C11 680uF/220V (via eBay). I dublicated R12 and C11 for each channel (this supposed to improve channels separation). If you skip this RC filter, you will end up with some buzz 😉 You can skip the parts marked as R11 and C10 as 6Н16Б-В (6N16B-V) tubes (via eBay) require 6.3V each for the heater. In my case, I used LM317 chip according to the datasheet. For C4-C9, you could use Nichicon 680uF/63V capacitors (via eBay). I chose old Russian 2uF 250V paper in oil (PIO) capacitors MBGCH1 as output capacitor C107 (via eBay).
The sound of this tube buffer
If I am not wrong, around 0.6V audio signal is feeded into original circuit after converting the current to voltage. But the level of distortions depends on the input voltage too. Feel free to experiment what you like or not (it’s quite easy if you connect to the output of your PC audio card), as the perception and preferences of the sound are quite personal.
What I think about the sound of this buffer? It’s really “tubby”, the tube buffer produces quite warm sound compared to original. How this is expressed? It’s more like 3D sound and it separates better the instruments and the vocals. I really like it. I start to think that I need to have the tubes in my audio setup 😉