Buffalo III DAC Tube Output Stage

[beau2317]

I recently managed to order a Buffalo III DAC board from Twisted Pear. This is a top-rated kit that uses the ESS9018 Sabre32 chip, one of the most advanced DAC chips currently available.

I wanted to make a "digital preamp" - replace my current preamp with the DAC, but also have input switching (all my sources are digital) and remote control volume.

The Buffalo III does not provide an output stage, simply exposing the Sabre's outputs directly. These are differential outputs, which can be used in current-mode or voltage-mode.

Twisted Pear also supply two different I-V (current to voltage) output stages : the op-amp based IVY and the discrete Legato.

However I like the look of glowing bottles, so I wanted to make a tube output stage. I talked with my friend John Broskie about this, which basically involved me floating ideas to him, and then he would shoot them down..

After some thought I decided to use his "Unbalancer" circuit and kit.

See here for full details of the Unbalancer.

The basic circuit is


The Unbalancer contains two stages. The first is a differential amplifier that uses a pair of triodes with their cathodes tied together in typical fashion. The cathodes are connected to a constant-current source. This leads to a "broskie cathode follower" - a cathode follower-like stage that converts a balanced signal to single ended.

The kit is very good value; the basic PCB is $45. The bullet proof PCB contains the basic Unbalancer circuit plus power supplies for the heaters and B+, so that you just need to supply AC at 6 or 12 volts, and HT AC at about 240V. John's GlassWare store also sells kits with all the components, with or without tubes (for example, a kit with no tubes or coupling caps, but all the resistors, diodes, supporting caps, heatsinks,tube sockets, etc, comes to $130).

Building

I had a few weeks spare before the Buffalo III kit arrived so this game me time to build everything else. I used three separate transformers for (a) heater supply (b) tube B+ supply and (c) Buffalo and other things like remote control volume control and fan.

The Buffalo supply was taken from one of two windings on the low voltage transformer. This had a simple pre-regulation circuit consisting of RC filtering and LM317 voltage regulator.

I decided on the tubes to use with the Unbalancer. The input tubes would be 6GC7, a 9-pin version of the 6SN7. I had some RCA NOS examples. For the output stage I planned to use 6H30pi-DR tubes (some call them "supertubes"). The -DR prefix are the "good" ones apparently, and I had a couple from a few years ago when you could still buy them for a reasonable price.

The input stage requires a small modification. In the standard kit, the input impedance is set by a set of 1M resistors. I wanted a much lower input impedance, as I wanted to run the DAC output as much in current mode as I could. Initially I was hoping that I could use 22R resistors but in practice I found that 50R was the best value.
The 50R input resistors were bypassed with 0.01uF cap and 5R resistor to provide HF filtering.

I managed to get the Unbalancer working and tested, and the Buffalo power supply all OK before the Buffalo arrived. When it did arrive, which was at 10am one fine morning, I set to work getting it installed. By 2:30pm I had a working Buffalo!

Special Components

I put some good quality coupling caps in the Unbalancer. There are two required on each channel. The 0.1uF cap was a Cardas cap that I had from an old project. The main output cap, 1uF, is an Obbligato gold. I'm impressed by this cap, especially it's low price - $12 versus $68 for a Mundorf Silver/Gold. It seems to have a very good sound.

The most critical component (imho) is the IV resistor. I ordered some Vishay bulk foil types for this position as I'd found first hand that these sound really good in an IV position.

But before these arrived I purchased some "precision wirewound" resistors from RS components, made by NeoOhm. I replaced the stock metal films with the Neohms, and the sound immediately went up two or three notches.
When the Vishays arrived I replaced them right away but was dismayed to hear the sound quality had gotten worse - it sounded colored and slow compared to the Neohms. I figured they might need burning in, so I left them running for 6 hours...Still no improvement. The Vishays sounded better than cheap metal films, but just nowhere near as good as the Neohms. Looks like I need a new tagline..

The Neohm resistors are RS part number 217-3166 and the datasheet is here

Overall Sound

Prior to this I'd used a NOS DAC based on the DDDAC kit. It used 60 x TDA1543 chips, and was a very musical sounding DAC with good "get up and go", but no digital fatigue. I'd tested this DAC against a good range of DACs and only one, a DSP based thing that cost about $15k, bested it. And even then, the only thing really better was the low level details (echoes, hall ambience, etc).

When I first built the DAC I used it via my old preamp (an Aikido). I could switch easily between the old and new DACs.
The new DAC was easy to pick as the best. It sounded very detailed and pure, but with great bass and drive. I was pleased.

But when I connected it up as a preamp, it went from good to great. I wasn't expecting such a big improvement so I was amazed. It wasn't a case of listening to old music and hearing the differences and new things, but more a case of marvelling that this was the same recording that I'd known all these years.

The treble is something else. Until you hear it you can't hear how much is missing from normal treble. It's powerful and detailed (typical Sabre words) but not overpowering.

And the bass ain't bad either; I normally use subs with my Magnepan 3.6R speakers, only the really low frequencies, below 40Hz. But with this DAC it's pretty much not required. The bass is tuneful and, yes, powerful.

The mids are just gorgeous. Sorry but I am not a wordsmith, so I can't properly describe just how good this sounds.

Yesterday a friend brought over his new DAC, a NuForce DAC-9, their flagship DAC (retail:$1700). The Sabre DAC made this sound pretty awful. There was no competition (In fact I would rather listen to my old DDDAC than the Nuforce).

Volume Control

The Buffalo kit lets you use the Sabre's on-chip volume control by using a 10k linear pot. I had a remote control kit that I had bought for another project (from ebay). But this used a log pot.
After some searching I found a motorized Alps pot, 10k linear, also from ebay.
The volume control works great. Even at the lowest levels (ear against speaker) it's impossible to hear any graininess or noise .

Picture Notes:

You can see the (almost) finished thing below. Some notes:
-- I need two outputs, one for the power amp and one for the subs. The yellow caps, top right, are 5uF Sidereal caps for the subs. Some vintage caps these, I used them originally in a Ben Duncan DAC design from more than 20 years ago.

-- I put a small fan into the box, blowing over the tube board. The 6H30 tubes sure do pump out some heat, especially when they are passing 30mA. Since this pic I also have some Pearl Tube Coolers. The fan is a 12 VDC fan running on about 5V so it hardly makes any noise. In fact the volume control motor makes more noise.

-- the red board, bottom left, is the remote control board. If you get one like this just note that you only need two connections for the motor drive - just the two motor connections at the back. There's no other limit switches or anything.

-- it's not quite finished yet so the wires are not all neatly dressed, etc.

[grubyhalo]

Thanks for your post, very informational. I am considering the un-balancer in a slightly different application. Will PM you later if I have any doubts, if it's OK with you...

[beau2317]

Sure no problem. John tells me that he uses the Unbalancer on his main system (along with a phono stage that makes balanced output.)

[lil' knight]

Do you know if John has any balanced tube output stage?

[beau2317]

He doesn't have any specific balanced kits but I know that you can use a pair of aikidos in a balanced configuration.

[regal]

With that tube stage you will have one heck of a high output, not only is the 2V buffalo output multiplied by 2 but also the gain of the first stage. I'm guessing you are saved by the 32 bit-input bit depth digital volume control but the output is so low coming out of the buffalo then overamplifyied in the tube stage that I would be very intereted in seeing your SNR, I estimate 80dB tops (~13 bit resolution.) Hope I am wrong.

Maybe I don't fully understand the Sabre digital volume control (no public datasheet) and maybe you are feeding an amp that benefits from a >10VRMS DAC output. Or maybe you are using a low value I/V resistor?

Don't get me wrong its a decent way to use tubes on S_D dac's (probably the only good way.) If you throw an OPT on the cathode you could have a neat headamp.

Do you have DC bocking caps on the input?

Thanks for sharing your experience with this. Been following broskies S-D DAC analog stage evolution for years. I could also be very usefull with R2R DAC's if you use enough for balanced output and then the gain is an asset.

[beau2317]

With that tube stage you will have one heck of a high output, not only is the 2V buffalo output multiplied by 2 but also the gain of the first stage.

I tuned the output so maximum volume is around about max power on most recordings. I use a 50R IV resistor (as I said). This means the input voltage is around about 4oomV. The gain is about 20, including the x2 factor, so the output is about 8V max peak to peak. But bear in mind that many recordings don't go anywhere near max output. I had initially used a 22R but this didn't give enough gain on some recordings.

Anyway, this produces a good situation where soft listening is about 11-12 o'clock, with loud being 2-3o'clock. (Remember it is logarithmic)

One thing I know about the sabre volume control is that the math for it is done using 48bits, so that gives a lot of overhead. And it seems to work.

Do you have DC blocking caps on the input?

No, it doesn't need it. The input is a true differential amp, so the approximately 1 volt on each input balances out.

As for signal/noise: I cannot hear any hiss or hash through the speakers. I can hear a slight hum with my ear close, but that sort of thing is quite hard to kill completely. Overall it's quieter than my previous Aikido preamp.

Anyway, thanks for your comment.

[Russ White]

Good work on your project. I am truly gratified that you are enjoying it!

One of the first published B3 builds to be sure.

Cheers!
Russ

[regal]

Thanks, that DAC does make a good match to this stage then. If you decide to take some measurements with a soundcard I'm sure many would like to see them.

Again thanks for sharing your impressions/success. Job well done.

[glt]

Very nice! thanks for sharing.