I was just looking at the product page for the DirectStream Mk. 1, and it says this:
“Most modern DACs use some form of active tube or solid state output stage to amplify, filter and drive the DAC’s 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 feeding a high-performance passive audio output transformer.”
Does anyone know, what is a “video amplifier”? And how is any kind of electric amplifier “passive”?
If you open this thread about modding the DS MK1 (Link), you will see one of 7 posts by Ted Smith, the designer. If you enter ‘video opamp’ in a search limited to the thread in question, you will find the 6 other posts. May be this will help you a little?
A video amplifier is an amplification block not unlike an audio amplification block but capable of operating at a very high bandwidth: megahertz rather than kilohertz.
Essentially, a high speed op amp. When the DSMKII FPGA outputs its digital 1s and 0s in the DSD format, they are at a fairly low level of voltage. Let’s call it millivolts. In order to listen to those bits they need to be amplified and then filtered so the high frequency stuff, running at a bit over 2mHz doesn’t get out of the DAC and into your amplifier. Typically, these bits are sent to an audio amplifying circuit. In this case, because of their high speed, we used a video amplifying circuit because it is much more comfortable banging on and off 2.7 million times a second than and audio amplifier with lower bandwidth.
So do the video amplifiers require the transformers to lower the output impedance or something like that, and that’s why they’re there? Or are they serving a different function?
And if so, if I may ask, how does the new PMG DAC do without the output transformers? (I assume it’s because it’s not using the video amplifiers, but doing the amplification some other way.)
I appreciate you’re time, I’m just very interested in how all this works.
No worries. The video amplifiers did not require the transformer, as demonstrated in the DirectStream Junior DAC that was basically the same circuitry but without a transformer. It is there for a couple of reasons, isolation and Ted’s love of its sound were chief among them. It definitely has a tube like sound we’ve all gotten used to.
One of the reasons for Ted’s use of the video amps right after the FPGA output is that he preferred a certain type of silicon that has a very small output level and needed to be boosted significantly before it could be of any value to an audio system. In the PMG 512 we take a different approach and use different silicon that intrinsically has a much higher output level, and then that signal is fed into a multi-stage FIR filter to lower noise (it is still digital PDM at this point), then it gets cleaned up with a simple analog low pass filter, then on through a class A analog gain stage that takes that audio—the DSD stream—and amplifies it up by a factor of about 5, then finally through the PMG DAC analog volume control.
The DirectStream never had an analog volume control so volume was controlled through digital means. One of the downsides of this approach is whatever noise/distortion floor the output stage of the DAC is sitting at, as the volume decreases through digital means, the S/N ratio gets worse and worse as the level goes down (might be hard to imagine but think about a speaker in a room with an air conditioner blowing. If the speaker is playing loudly you don’t hear the hiss of the air conditioner. But as the speaker volume lowers, the signal—the loudspeaker— gets lower and you notice the air conditioner sound. That’s signal to noise ratio). Because one of the unfortunate flaws of the DirectStream was a marginally acceptable S/N ratio of about 80dB, then when folks used it directly into their power amplifiers with the built in digital volume control, low volumes would uncover the intrinsic hiss of the device. For most people it was just fine, but…
In any case, we solved all that in the PMG 512 with a 20dB better S/N ratio of -100dB and an analog volume control at the output of the DAC that turns both the signal AND the noise down with the level control.
I will say that the “tube-like sound” of the DirectStream has a lot to do with why I love mine, and I’m curious how the PMG without the trafos sounds in comparison. But I’m sure it’s fantastic.