Most modern linear 16 through 20 bit converters provide a current output option. Examples of this would include the 20 bit Burr Brown PCM63 found in the Parts Connection (Sonic Frontiers) Assemblage DACs (All models as of this date.) Other devices from Analog Devices include the AD1856, a 16 bit DAC, and the AD1860 18 bit DAC which was used in Sony CD players, the AA Dacman, and early Monarchy products amongst others.
The circuit illustrated has been in use in my Sony CDP-790 cd player for about 5 years now with good results. The design is extremely simple and fairly none critical in construction. The high voltage supply is outboard due to a lack of room in the cd player for the required transformer and tube based voltage regulator.
The I/V conversion is actually performed by a 22 ohm resistor in my design. This low a value was chosen to prevent linearity problems with the DAC's employed (AD1860) which do not want to see much audio voltage present on the current output. Linearity in the converter begins to suffer when as little as 50mVpk is present at the output terminals, a value which is not approached in this design as the converter sources no more than 1mA in either direction.
To maintain a low noise figure both halves of a 5751 or 12AX7A are paralleled for each channel input from the DACs, these in turn drive a simple cathode follower. No post filtering is employed as the AD1860 is run 8 times oversampled in the CD player, and the bandwidth restrictions in the voltage amplifier circuit provide adequate filtering in this application. A simple first order filter can be inserted between stages if desired, components required will be designated Rx and Cx.
De-emphasis when required is accomplished in the digital domain in this model CD player. This is apparently also true in the Assemblage DAC and most others as well. If this is not true in your application a relay can switch an additional capacitor across CX as required for de-emphasis - the source impedance here is typically around 34K ohms.
Output muting is accomplished using a relay operated off of one of the existing muting transistors in the CD player. The muting relay is wired so that the N.0. contacts are across the output circuit, and energizing the transistor (original mute operation in player) causes the the relay to short the output to gnd - essentially any time the player is on, and not playing a CD.
This design requires well regulated and ripple free +300Vdc at 6mA, and 12.6vdc at 450mA for its operation. A suitable power supply design is as found in the phono pre-amplifier project article.
This admittedly simple approach is works well with any current output dac from BURR BROWN, TI or AD. I however don't recommend its use with any voltage output devices as it may overload
and certainly has excessive gain. Again this was designed in the very early 1990's, but from a simplicity standpoint it is hard to beat. Later versions have employed 6SL7 as well.
KRK 11/27/2005
Tube Based I/V Converter Circuit
As promised this schematic is now available as unrestricted/printable PDF 11/27/2005.
©1998, 2001, 2005 By Kevin Kennedy