300b lovers

Love ’em or hate ’em, DACs have gone a long way in the last thirty years, and continue to evolve pretty quickly. The internals of the AKM and ESS converters run at 90 MHz, with stupendous processing power. It’s what makes 4K TV and digicams possible.

That kind of speed makes up for many sins, and lets the noise-shaping algorithms operate much, much better than earliest days of SACD and single-bit MASH converters running at 2.8 MHz. In a lot of ways, it makes the endless upsampling discussions on the forums moot, since the internals are upsampling everything to 90 MHz anyway. Might as well let the chip do it, rather than play games in Roon. (Although converting PCM to high-rate DSD forces the chip to use different algorithms, which will definitely sound different.)

It is a consciously retro decision to use antique Eighties-vintage Philips TDA1541A converters, or late-Nineties Burr-Brown PCM-63 or PCM-1704 converters. Those are true once-through flash converters, with no signal processing or noise-shaping involved. But the least significant bits are kind of marginal, since it took R2R to the limit of what can be done with laser trimming and ultra precise fabrication. Nowadays, speed and good algorithms are the answer.

Which leaves the current-to-voltage converter as the last domain of audio tweakery. Op-amps are way, way better than the 1979-vintage 5532/5534 from Philips/Signetics, but you still find these antiques in consumer DACs. That’s probably where tuning happens in modern DACs, since there is little left elsewhere in the design.

And if you want to "sweeten" things, do it in the power amp or speaker. Much easier to tweak. I think making records sound like ultra-quiet, ultra-precise digital, or making CDs smoothed-out and "analog", is taking away from the strengths of each medium. LPs sound like LPs, and PCM sounds like PCM.

PCM to DSD256 is fair game, though, so why not? It’s what my Marantz SA-KI SACD player does to incoming PCM (it has S/PDIF and Toslink inputs), and an interesting "alternate view" of PCM sources.

lynn_olson

74 posts

07-04-2023 at 02:08am 

Well, enough of the rant on DACs. Addressing the question in the post by lewinskyh01, what does a really good tube linestage bring to the table if the DAC can directly drive the power amps?

A sense of ease, dynamic impact, and sometimes more vivid tone colors. How? Partly better cable drive, partly signal conditioning, scraping off RFI and noise induced in the cables. On paper, op-amps can do an amazing job driving a cable, in practice, not so much. If the preamp passes a quality threshold, yes, it can improve the signal compared to a direct connection to a DAC. Found that out the hard way with first Amity amp.

Great! Thank you for the answer. Would building such a device, passing said quality threshold, be super expensive? This device wouldn't need volume control nor the capability to handle multiple sources. Maybe the device increases gain by a given amount and then listening level gets adjusted down through software volume control.

I agree it's an endless rabbit hole going down into the audibility of DACs vs upstream network settings vs software. And bleeding edge DACs bleed out their value soon after their peak in fame. Yet some more professional-oriented devices (such as Merging Horus/Hapi, Prism Titan, Lynx Hilo) are worth the same today as they were 7-8 years ago (nominal prices are higher due to inflation, price of Cu, etc) and still are the company's reference product. I like to stay among these, which of course are the ones capable of doing 8-ways.

My gut feeling has been there is something else good preamps achieve, and your post helps put this into more specific words. I'm not aware of any commercially-available product that does this and I'm intrigued to explore and maybe DIY. The idea of introducing a tube-driven class A stage to achieve "better cable drive, partly signal conditioning, scraping off RFI and noise induced in the cables" is appealing. How would you recommend I learn about this?

The idea of introducing a tube-driven class A stage to achieve "better cable drive, partly signal conditioning, scraping off RFI and noise induced in the cables" is appealing. How would you recommend I learn about this?

@lewinskih01 You might want to study how balanced lines work. Properly done, balanced lines are the best cable drive available to audio. RFI and noise are rejected due to the low impedance aspect of balanced lines (in the old days the studio line inputs were 600 Ohms; these days its more like 1-2KOhms); weak signals induced in the cable are swamped by the low impedance. In addition the input that is being driven has a high Common Mode Rejection Ratio, which is to say that signals common to both the inverted and non-inverted inputs (such as noise and RFI) get rejection.

In a true balanced line system ground is ignored to eliminate ground loops. If using tubes this is usually done using an an output transformer which can float with respect to ground. Its also possible to direct couple using a Circlotron output, for which Atma-Sphere has several patents.

If you are supporting the balanced line standards (AES48 is one of the standards; the other is the low impedance aspect) these two methods are the only ways to do it.

@atmasphere - If the input of the next component is balanced and not referenced to ground (e.g. transformer coupled), I don't understand why it is necessary to decouple the output in the source component from any ground reference to achieve the full benefits of balanced connections. Can you please help me understand. Thanks.

if the input of the next component is balanced and not referenced to ground (e.g. transformer coupled), I don't understand why it is necessary to decouple the output in the source component from any ground reference to achieve the full benefits of balanced connections. Can you please help me understand. Thanks.

@jaytor Part of the issue driving interconnect cables is how the signal travels in the cable. When the shield is part of that connection, its more likely to pick up noise and the actual construction of the cable (what sort of insulation it uses and so on) becomes more critical. That shield is connected to chassis ground at the input of whatever is being driven- so now you also have the possibility of a ground loop too.

So when the source is referencing ground, such as a pair of single-ended outputs, one of which is out of phase with the other, you have a problem where the ground circuit return is active in the shield of the cable. Suddenly the dielectric in the cable is playing a role that it did not when the shield was only used for shielding with no signal on it.

It is precisely this problem which is why there are 'high end audio' balanced line cables now that might cost up to $1000/foot or more (put another way, most 'balanced outputs' on 'high end audio' equipment actually references ground as if the designers were not aware of the balanced line standard)! If the connection is done properly, you won't be hearing the sort of differences between cables that might convince someone (who might have a touch of audiophile nervousa) to spend that $1000/foot.

I'm saying that an inexpensive cable can sound just as good in every way.

The proof of this is the vast number of recordings that were made in exactly this way- proper balanced outputs and inputs. Its part of why you could have 150 feet or more of interconnect cable between a microphone and the input of the tape recorder in 1958, nearly 20 years before Robert Fulton showed off his first 'high end audio interconnect' cable, yet the resulting recording just gets better and better as you improve your system's ability to winnow more information out of that recording. That can only happen if the cables used to make that recording are absolutely transparent!

Put simply, you have to dot your 'i's and cross your 't's if you want this system to work properly.

But let's look a bit closer at that balanced source that references ground. It may well be rack mounted in a relay rack and through that rack its chassis is grounded to every other bit of equipment in the rack or maybe even in the studio. Some of that equipment might be on the input side or the output side. So a ground loop could easily be introduced! 

You might think that because you're not using a 7' tall steel relay rack at home that you won't have that problem, but keep in mind that the equipment is also grounded into the wall. That's where you get in trouble: you must be sure that ground is ignored with both inputs and outputs; that ground is only used for shielding in cables and never for any kind of signal ground! If you don't do this, the benefit of balanced operation is eroded. It was designed so that exotic cables aren't needed and grounding issues are eliminated.

Think about the advantage of having cables that sound as good as the best out there price no object, but not having to pay that price- for all the interconnects in your system, you might have only a few hundred dollars invested at the most, rather than $1000s or $10,000s. And they don't go out of date or any such thing...