Sanders Preamplifier Review

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.