Does the Direcstream DAC have OP amps in the signal path/output stage?


#1

Hello,

As I’m reading more about DACs. I’ve noticed that the higher end ones describe as not having “OP” amps in the signal path. eg the Meitner and EMM Labs products.

Does the DS have these amps?

Thanks


#2

The DS Sr uses a passive output filter that includes a transformer. In the Jr, an opamp takes the place of the transformer.

Both have a passive output section driven from full scale digital outputs. This differs from most DACs which instead take a low level current output, convert it to voltage, and then amplify this output with opamps.

This is significant. In the DirectStreams, any noise on the output rails is filtered out by the output circuitry. In a typical DAC, the noise on the output rails is amplified by the output opamps.


#3

Thanks for your reply. I’m indeed looking at the SR as it’s good to know that this product doesn’t have OP amps. I assume the SR is a full Class A output stage?


#4

You might want to check out this thread: http://www.psaudio.com/forum/directstream-all-about-it/directstream-snr-output/


#5

The answer is slightly more complicated. There are opamps, but they aren’t being used for audio, per se, they are used as digital switches. That is they are the devices that select one power rail or the other depending on the digital input. Also they aren’t audio opamps, they are video opamps so that they have the bandwidth to deal with the 5.6448MHz or 11.2896MHz inputs.

After that Elk’s answer is correct - the premise is that the DS (and DS Jr) take a full level (rail to rail) input and filter that to get the outputs rather than using some sort of amplifier on a low level audio signal. The significance is that instead of amplifying a low level signal (including it’s noise) they filter their the analog signal including filtering noise.


#6

From PS Audio’s DirectStream DAC SR product page features tab regarding the Class A output question.

Purely passive output stage

Most modern DACs use some form of active tube or solid state output stage to amplify, filter and drive the DACs output. DirectStream takes a different approach: passive. In one of the simplest, elegant and musically correct executions of an output stage, high speed class A video amplifiers are used as the final switch for the single-bit, double DSD output of DirectStream; feeding a high performance passive audio output transformer. The transformer is specially designed to act as both the galvanic isolation between the DAC and the outside world as well as the primary low pass filter required to present a perfect, low distortion, low noise output signal. When you listen to DirectStream you never are accosted by added bright, hard glare in the music as happens with many active designs. Instead, clean, pure music flows from the DirectStream directly to your preamplifier or power amplifier.