Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

Re: Mistake?

Posted by BBeck (A) on July 12, 2006 at 11:05:47

In Reply to: Mistake? posted by Gordon Rankin on July 12, 2006 at 07:17:30:

Gordon,

Allen is absolutely right (and Wright). I think there must have some misunderstanding in your discussion with BB/TI people. The PCM1704 is truly intended to drive a current into a dead short.

Open the link to the data sheet, and go to Figure 5 on page 9. It shows an opamp (an OPA627) with the inverting input connected directly to the Iout pin of the converter. There is no series resistor. Negative feedback forces the inverting input to act like a virtual ground, zero ohms, that the DAC happily dumps current into. The feedback resistor (2.5K you mentioned) converts that current into a voltage on the output of the opamp. But the opamp is really doing the work here. For the DAC to work to full specs, there should be almost no signal voltage on the Iout pins. Don’t let the 1000 ohm output Z throw you; that’s actually pretty low for a current source. In any case the load resistance should be much, much less than 1000 ohms. The BB/TI guys were probably just giving you their standard answer about how the passive approach is not recommended. If you were to load the opamp with a 100 ohm feedback resistor, that would lower voltage output and increase opamp distortion (overload). In a passive use, a 100 ohm shunt load gives a very low voltage (about 25 times lower than what they’d want), and you’d add noise amplifying this back up later; hence the BB/TI remarks. Furthermore, there would then be +/- 0.12 volts of signal swing at Iout that may or may not be too much for the DAC to handle and still meet spec. Now, you may like how it sounds, but it’s not how it was designed to work. There’s no mystery here; this is how current output DACs have been used for decades, and for a lot of applications other than audio too.

PS: Calling an MC step-up transformer an I/V device, while not strictly incorrect, is very misleading.

This post is made possible by the generous support of people like you and our sponsors:

The Cable Cooker

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2025, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---