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Smoothing The Audio Jaggiesan...
Easy CD tube output stage you can build!
From VALVE Volume 2 Number 10

  Last week I invited audio buds over at various times to hear the final version of the Super Whamodynes. One of these friends, upon hearing my moaning over my cheapo Onkyo DX-1400 CD player, offered to bring a $900 solid-state output CD player for audition. That was all I needed to light the tweak fire. I had to try the tube output mod for my CD player that had been itching my brains for about a year. First I pulled out the three articles I had seen two from Glass Audio and one from' Positive Feedback, which discussed tube output stages in various CD players and DACs.

Two of the three articles dealt with Burr Brown 28 pin DAC chips, PCM58P's2 and PCM63P's3. I opened my Onkyo to find a pair of PCM61 P's, so I thought I ought to be able to reverse engineer the pinout, even though the PCM61 Pis a 16 pin IC. The Onkyo also contains the same digital filter, a Yamaha YM3433, that the Audio Alchemy DAC-in-a-box uses, so I figured the digital parts quality was probably pretty decent, and therefore, the unit was worth hacking.

First, I had to figure out where to tap into the player. The Norman Tracy article in Glass Audio, as well as the Sheldon Stokes article in Positive Feedbag3 both use a resistor as the 1/V converter which converts the dinky current output at the analog output of the DAC chip to a dinky voltage output. Both articles also run this converted signal directly to the grid of the tube output stage. Shoot, it can't get any easier than that! I took my trusty scope lead and started touching it to DAC pins while I played a CD. At pin 9, I got signal. Whoops, at pin 10 I got signal too. Damn, they were jumpered together!

Well, looking back at the Tracy article, I discovered that the PCM58P did the same thing. One pin was the analog current output, and the other pin was a "bipolar offset point", which sources !mA of current to convert the 0-2 mA current swing of the DAC output to -1 to + 1 mA. This way the voltage conversion swings about 0 volts instead of +.2 volts. Anyway, I knew that I could tie in at this pair of pins and hook up my IIV resistor. I decided to use Sheldon Stokes' output stage fora couple reasons. First I hadn't fooled with an SRPP circuit yet, and, secondly, he offered resistor values for using either 12AT7's or 6DJ8's.

So I pick a couple of sockets, dig out some resistors and a couple of caps, and go looking for an old chassis to build on. Can't find a darned thing, but I turn up a little cardboard box, upon which I glue two ceramic sockets, and build up the circuit using 12AT7WC's. Oh, this is too cool!

I add in and out jacks and attach a Lambda tube regulated power supply and adjustable DC filament supply. I then clip the circuit into the left DAC. I found a 1000 ohm resistor between the DAC and the original I/V converter chip (a 5532) which, when unsoldered at one end, disconnected the 5532 from the left DAC.

In goes a CD. Wow, obvious difference between the solid and vacuum sides. The tube side is way smoother, has much better bass, and seems a bit more transparent. It also has gobs of gain, which I need to drive my passive line stage. Awesome!

I cobble together the other channel about two hours before my friend shows up with the $900 player. We put on the Muddy Waters, Blues Singer CD and compare the two players. With the tube Onkyo Muddy's voice is OBVIOUSLY smoother and the bass is better too. The only plus with the solid stale player is a slightly better leading edge to transients. The next day I altered the circuit to use 6DJ8's, which Sheldon Stokes favors in his DAC. A pair of Telefunken 6DJ8's gave me better transient attack, but bass was a bit lighter. When I switched to early 80's Sylvania 6922's, I got what seemed to be a good compromise between the 12AT7 bass and the Telefunken 6DJ8 transient response. And so they have stayed plugged into the cardboard box.

A good argument for this type of 'in the player' conversion over construction of a whole new outboard DAC is that there's no digital interface between transport and DAC to add jitter. So hacking a medium priced player might sound better than some tube DAC / transport combinations...

Lout from pin 9 of BB PCM61 P. For6DJ8,

B+ is 260VDC,. R31- 33.2Q, R32,R33-

1. Painter, Richard, "A Valued Vacuum