Dagogo - Preamplifier Review - March 2012
Slide rules and pocket protectors
In the subjective audio universe, where gross amounts of 2nd harmonic distortion, horrendous phase shift, and totally inadequate frequency response gets euphemistically labeled “bloom” and “harmonic richness,” the purveyors of audio accuracy must hide their devotion to truthfulness like it’s a cardinal sin. “Zero phase shift?! Grab the matches and gasoline, we have a scientist in our midst.” There are negative connotations to marketing with specifications. Anyone old enough to have heard the first transistor amps will agree they sucked, regardless of what the specs said. Engineers in the ‘60s, ‘70s, and ‘80s, with their use of feedback as a blunt-force-instrument, forever tainted integrated circuits, feedback, and solid state. Though the equipment measured good, it sounded bad. Real bad. In a way, we should be thankful the big corporations made bad sounding equipment. Had they not, we wouldn’t have the audio underground and cottage industries that sprung up to satiate the appetite for good sound.
So, what’s wrong with electronics that have no audible distortion, no audible phase or frequency response issues, no oscillations, no noise and no problem driving any set of cables or amps or speakers? Nothing. Nothing is wrong with that. Are you so hung up on tube rolling and tweaking that you reject the hypothetical existence of an integrated circuit that would do a better job, and keep doing it the same way for 20 or 30 years? Are you that afraid of your music collection?
Though I admit to enjoying tubes, analog, open reel tape, and single driver loudspeakers, I will be the first to admit that there are significant limitations with all the above. If the primary job of an audio system is to be truthful, then does the current crop of high performance digital, and high performance integrated circuits deliver the goods? Increasingly, the answer is yes. We are advancing the state of the art in baby steps. Some of the advancements come from improved manufacturing techniques, giving us higher performance through better matched transistors and purer materials. Some of it has been new ways to accurately measure distortion, or the discovery that the ear can hear things that are hard to measure with simple harmonic distortion analyzers. Some of it is trickling down from medical, aerospace and military technology. The same circuits used to hunt down submarines or find tiny tumors have applications in audio. Billions of dollars have been spent to beat the competition, whether it is another chip manufacturer, stage IV cancer, or enemy combatants.
For all audio, I ask a few simple questions. Is the frequency response wide enough, and flat enough, to allow us to hear all the fundamentals and overtones, and in the correct relationship? Is there audible distortion? Does it have enough power to avoid clipping? After those criteria are met, can it recreate an acoustic space (imaging)? Finally, is it reliable? Those criteria are hard to satisfy, and when a piece meets the criteria, it usually satisfies musically. Sure, there are very subtle nuances, almost impossible to measure, that are the difference between the good and the great, but I have a hard time hearing those nuances over audible harmonic distortion and hum. Almost all the equipment I’ve reviewed had issues, usually minor ones, in one or more categories. Some people don’t mind scads of harmonic distortion, limited power or limited bandwidth, and you know who you are. I do mind, but there is a niche product for everyone.
So, what has that to do with the latest preamplifier from Sanders Sound Systems? Everything. Roger is an actual engineer, in an industry with many “gurus” and copycats. Let me offer a few interesting excerpts from the Sanders website in the following:
“Our previous preamps (line stage and phono) were $4,000 each, for a total cost of $8,000 for our customers who needed a phono preamp. Since both preamps are now combined, customers who need a full-function preamp can now save several thousand dollars as the new preamp sells for half the price of the previous two.
The goal of a true audiophile grade preamplifier is to offer gain, switching, and other conveniences, while at the same time passing the original signal downstream without adding distortion, noise, or a sonic signature of its own. The Sanders Preamp does exactly that but includes many ergonomic features for convenience and ease of use that are not available on even far more expensive preamps.
The levels of each individual input can be adjusted to get them all matched so that you don’t get “blasted” or have to turn up the main level each time you switch sources. A stereo/mono switch remains standard equipment. The overall gain, individual gain between devices, and channel balance can be adjusted in precise, 1 dB increments. Muting by remote control is standard. A video readout makes it easy to monitor the levels.
… Front panel controls are done through micro-touch electronic switches. Internal switching is done by miniature, sealed, gold relays. Conventional rotary volume controls have channel matching error of around 20%, which causes the left/right balance to shift as you change the level. To solve this problem, some preamp manufacturers use discrete, precision resistors on a multi-step switch.
While this solves the channel tracking problem, they introduce new ones. Specifically, they have very limited resolution due to too few steps (typically 31 steps of 2 dB each). These “stepped attenuators” produce very annoying switching transients at each step.
The Sanders Preamp solves these problems by using the “volume control” knob to drive an optical comparator circuit. The optical circuit operates a microprocessor that controls an electronic gain system. This controls the level using one hundred, one dB steps, with precision of greater than 0.1% between channels.
The microprocessor monitors the signal voltage and waits for it to cross the zero voltage point between waves before switching to the next level. This prevents any switching transients. The volume control knob has detents at each 1 dB point and it rotates continually. So it as an infinite number of detents and you can feel each 1 dB change in level.
There is a digital display with beautiful, blue, light emitting diodes (LEDs). The display continually shows the output level of the unit and switches automatically to show level differences between channels, when you adjust the balance, or when you adjust the input levels. You no longer have to guess at the levels or try to see fine gradations on a knob to know the levels, since you can see them from several feet away.”
The heart of the new preamp is an extremely sophisticated chip that matches the gain to within .1dB and can remember the various level adjustments for each input. I asked Roger to elaborate:
“The volume control in my preamp is an electronic level control designed and manufactured by Burr-Brown (now owned by Texas Instruments). This chip has many excellent features. It is actually built like a stepped attenuator (using 400 microscopic resistors to obtain 200, 1/2 dB steps, in stereo).
The accuracy of the channel balance is determined by the precision of the resistors. Modern chip-manufacturing technology can now produce resistors with tolerances of better then 0.1% — which is far better than discrete resistors, which are limited to about 1%. Therefore, these Burr-Brown chips offer essentially perfect channel balance.
Conventional stepped attenuators often make “pop” sounds as the circuit changes to different resistors. This is due to the short delay as the switch moves to the next set of resistors, and during this delay, the voltage from the music changes. If the voltage change is significant, you will hear a “pop” when the next contact is made.
The Burr Brown chip solves this problem by constantly monitoring the voltage of the signal. When a change in volume is requested, the electronic resistor switching circuit waits until the voltage monitor shows that the voltage crosses the zero point. It then switches. Since there is no voltage present when the next step in the chain occurs, there is no “pop.”
Of course, music is changing voltage very rapidly, so any delay only lasts a few microseconds at worst. As a result, no human can detect any delay in the process, even when switching through the resistors very rapidly.
The chip has a video driver circuit so that I can have it show its levels on an LED digital display. It also has a microprocessor so that it can be programmed to operate in many different ways. It includes an opamp so that the gain can be controlled.
In my preamp, I control the microprocessor using an optical comparator “pot.” Therefore there are no conventional analog potentiometers to wear out and fail. This also makes it possible to operate the unit by remote control.”
The rest of the preamp includes input and output buffers (discreet) along with regulators. The parts quality is excellent including silver mica caps for the RIAA, Mills wire wound resistors and Mundorf caps. The thickness and quality of metal work are first rate: The top cover of the preamp is very heavy, good for damping vibrations and blocking EMI/RFI. Everything is of very high quality, whether a passive device, cutting edge integrated circuitry, potted toroidal transformer, faceplate, controls with good tactile “feel”, etc. All the components are mounted on one board, with a minimum of connectors and wires that would add noise.
I’ve been researching the dreaded op-amp lately, and it’s kind of a blanket term that should be avoided. You should rather think “integrated circuit”. When I asked Roger about using ICs he responded this way:
“Of course, audiophiles generally believe that ICs sound horrible. Like most audiophile beliefs, this is a myth. The truth is that modern ICs easily outperform any discrete circuit.
If you doubt that, just look at the IC’s measurements and specifications. Don’t believe the specifications? Then listen to ICs using valid listening tests. You will quickly discover that they sound absolutely transparent and that you cannot hear any difference between a modern IC and the perfect reference (a short, straight piece of wire)… I have abandoned discrete circuits like I used in my dedicated phono preamp because they simply could not match the performance of a modern IC.
This should not be surprising. After all, there is no magic. A transistor is a transistor regardless of where it is housed.
The sound of what?
The Sanders Preamplifier is a piece that many reviewers will hate, because it doesn’t have much sound of its own. Cartridges and speakers are easy to review. All mechanical transducers have ridiculous failings, though we are accustomed to those artifacts. Electronics, though, can be difficult to describe, and the Sanders Preamp (and the rest of Sanders electronics) is near the top of the list. It’s hard to describe any peculiar sounds or character produced by the Sanders Preamplifier.
So, what does it sound like? As a preamp, it has zero noise, and zero audible distortion. Whatever distortion is there, is so low that it’s impossible to point to it and say “A-Ha”! Take, for example, the fine tube preamp from Melody that I just finished reviewing. While I loved the looks and sunny disposition, it wasn’t as quick as the Sanders, plus it had audible 2nd harmonic distortion, audible noise and phase shift. Many listeners might not notice these but if you switch between the Sanders and Melody preamps, and if your speakers have good frequency extension, you can hear the difference. Plus, the Melody doesn’t have the perfect channel-to-channel match and volume tracking of the Sanders.
When I say perfect channel match and volume tracking, I mean it. This isn’t reviewer hyperbole. The volume tracking offered by the precision chipset gives .1dB accuracy, something a traditional volume control or unconventional transformer volume control can’t match. The frequency response of the two channels sounded indistinguishable: a nearly impossible feat by a tube preamp. Regardless of volume setting, the two channels were exactly the same. What that gives you is a world class center image “lock” and stereo spread better than anything I’ve used. In that regard, there might be products that are as good as the Sanders Preamplifier, but I doubt you’ll find anything audibly better. There might be other preamps with better imaging outside the speakers due to an even lower level of distortion or better transparency, but I don’t know of one; and it wouldn’t be because they had better volume tracking. It would be interesting to hear the Sanders compared to other transistor preamps using a similar volume control setup. I’ve always heard similar strengths when comparing transistor with tube pieces, but this preamp takes it to a new level.
The phono stage is as accurate, tonally, as any I’ve heard. There are hundreds of phono stages that have enough RIAA accuracy that I expect all phono stages to be similar. If they aren’t, it’s because the designer goofed. I’ll argue that perfect RIAA deemphasis is a waste of time, for a number of reasons that will make up a separate article of the subject. But, it’s not a bad thing to be accurate to less than a dB. What is more audible and more important is matching the EQ of the two channels. It’s the Achilles heel of tube phono stages. The Sanders LP playback channel-balance is as good as other transistor units.
When compared to tube phono stages, mono records don’t sound as smeared, left-to-right. Playing back mono records with tubes can give you a misshapen center image, with the highs stretched to one channel, the mids to the other channel, and the bass back to the other channel, making them sound like they were recorded in a house of mirrors. With the Sanders, mono records were locked in the center. I recommend using the mono switch if you are using a stereo cartridge to play mono recordings. Stereo cartridges will smear mono images for a number of reasons: skating forces, unequal coil windings, cartridge misalignment, warps and off-center pressings. Also, if it’s a mono recording cut with a stereo cutter head driven by stereo electronics, the two channels will be slightly different because cutting heads aren’t perfect; the cutting electronics and tape machines aren’t perfect either. Roger should be thanked for including the mono button. Thanks Roger.
There is enough flexibility in cartridge loading and gain to make a good match for the majority of cartridges. Is it my favorite? Not quite, but it is more linear, with less noise, than my favorite phono stages. This comes down to personal preference because LP playback will never be as “perfect” as playing back a CD. There are too many variables with vinyl play that can affect the sound. The added noise and distortion of my favorite tube LCR phono stages might be covering up mastering deficiencies, cartridge mistracking and/or misadjustment, along with the possibility of complex interactions of cartridge, tonearm, tonearm cable and phono loading which serves to “enhance” the music.
Compared to my favorite LCR phono stages, the Sanders didn’t have as much image depth (see my caveat below), and large scale dynamics seemed to be slightly suppressed. There are records that sound better on the Sanders. There are records that sound better on a tube LCR. The odd thing is that there are records that sound better on the budget Rek-O-Kut Professional Moving Magnet Preamp and, on a few rare occasions, there are records that sound better on a Dynaco PAS, although the PAS is not accurate). What does it mean? Vinyl mastering and playback is as much art as science, so it cannot be “perfected.” I suppose a company could master and press the vinyl, then put together a package of cartridge, tonearm, tonearm cable, and phono preamp, comparing and tweaking things to sound as close to the master tape as possible, but I don’t know of any such setup.