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Precise Volume Control Regulator AU/RA D7A

A very high quality solution for volume control

[Italian version]


Product: Volume Control regulator D7A
Producer: AU/RA v.o.s - Czech Republic
Price: Model 1 (built-in, without case) 137 $/Euro, Model 2 (standalone, with the case) about 175 $/Euro
Reviewer: Giorgio Pozzoli
Reviewed: August 2001

AU/RA (the name derives from the initial letters of AUdio RAcionalne, that is Rational Audio) is a Czech company based in Praha. It was founded in 1988. They produce several different audio components including turntables with tangential reading system, power amps, pre-amps, integrated amps, loudspeaker systems.

The D7A is a separate volume control.


First of all, what is a separate volume control? In this case it is just a ultra high quality, step potentiometer. It is completely passive, no power supply is required. Two models are available.

Model 1 (built-in) is a version designed to be included in any industrial product or a DIY kit as a stereo volume control, in place of the normal potentiometer. The dimensions are 45x45x116mm including the metal shaft with 6mm diameter. It can be mounted using 4 front screws and 6 side outlets to me soldered for example to a motherboard.

Model 2 (standalone), the one I tested, is a fully functional standalone unit based on Model 1, with ultra high quality RCA connectors for input and output. It is designed to be inserted between a source and an amplifier: typically, it could be used in very high quality, minimal systems, composed of only a CD player and a power amp. The look is quite attractive: it is a square-fronted, black cuboid of 88x88x270mm, with a very pleasant black, lucid front panel with yellow inscriptions and a single large knob with a (precise) scale in dB. Note that the knob of the unit I received is completely black with a short white line: it looks much better than the one in the photo. On the back panel, only four RCA pins (one input and one output for each channel) and a grounding screw. The foots are very normal rubber foots. The weight is surprising: 2.3Kg!!! If you have stiff interconnects you'll never appreciate this enough.

All details are taken care of. The volume regulator itself is a masterpiece. According to available documentation, it has 33 steps of 1.666dB each, with a matching between the channels better than 0.1 dB. The change in level in 3 steps is therefore 5dB, so that the front scale indication can be very precise. The structure is dual mono (as far as the concept can be applied to this case) and as a consequence crosstalk is better than 110dB at 10KHz.

The dividers are produced using SMD (surface mounting devices) technology which accounts for much more compact assemblies with far lower strand capacitance. All contact surfaces are gold plated. Each divider is composed by 32 low noise, metal resistors with 1% tolerance. The nominal impedance is 22Kohm, a good average value, but in case of high quantity orders any custom value can be made available.

The design is really interesting. The number of components (and of interconnections between components) is the minimum possible. The unattenuated signal and ground directly go to a small, square, very high quality gold plated PCB which is the static part of the selector (contains on its surface the 33 static contact points); on this same board the SMD resistors are directly soldered. The attenuated signal is extracted by a sliding, gold plated contact, and transferred to the static PCB through a short length of thin wire, loosely rolled up in a few turns around the shaft, in order to reduce mechanical stress on its ends; there is therefore only 1 "open air" contact for each channel: everything else is connected by soldering!!

Model 2 has even more. It is also a de-coupling unit, in that it contains a couple of high quality non-inductive polypropylene capacitors in order to block any DC that might be present on any of the two interfaces. RCA connectors are non-magnetic, gold electroplated, with polyethylene insulation.

The mechanical structure of the cabinet is really sound. The front panel and the back one are connected by long and strong metal beams on which all the other components are mounted. To take the internal structure out of the external cover, which is a very stout square tube (the wall is about 3mm thick!!!) you just need to unfasten two screws. Very practical: a dream for any DIYer....

One note. Both models seem to have been able to combine the best of industry and craftsmanship: the electronic and mechanical structure is perfectly designed and engineered, but the design approach is such that, apart from dividers, most of the other components are commercial grade. Everything must then be assembled manually, with the highest precision and craftmanship. The tolerances are such that the shift is perfectly stable, solid. Really impressive.

A few observations

The first one is only a matter of philosophy connected to the idea of a stand alone volume control: I cannot see so many ways of using it. Personally, I would add a very simple input selector and transform it into a complete passive pre-amp. But I am a reviewer: probably for anyone more sane than me it could solve some problem.

The second is about the technical implementation. In the configuration used by AU/RA in the divider, the selector has only one section for each channel; the audio signal must go through one "open-air", gold-to-gold contact and a number of resistors, variable from 0 to 33 depending on the attenuation required, plus 2 soldered joints for each resistor. In other current solutions, instead, another solution is preferred: the selector must have 2 sections for each channel, and the audio signal must go through two contacts, but only two resistors and 4 soldered joints. But there are others that follow the same design as AU/RA, and this solution becomes mandatory if you want to use special quality resistors and keep the cost acceptable. Which is the best one? Really difficult to say: a matter of philosophy again, in the end.

I would have appreciated a remote control, provided that it does not have any negative effect on sound. A competitors volume control with remote is anyway sold for 1400USD, just to give you an idea...

There also is a last point: some people love passive pre-amps, others hate them. Before buying it, take into account that in the end the Model 2 is a minimal passive preamp: if you have no experience with this kind of object, it will be better for you to listen to one.

The sound

Listening tests have not been easy at all to set-up, as I was really interested to compare the behaviour of the volume regulator with a standard volume control. Using any other active pre-amp would have been not correct, as they have an input selector and an active stage. I decided finally to compare this unit with Pressive, which is a passive pre-amp using carbon potentiometers selected through listening tests based on their musicality. Even so, Pressive has an input selector, so comparison have been a little unfair for Pressive.

This said, the superiority of this kind of volume control is evident. More detail, a more stable sound, no strange noise when regulating the volume even after years, and most of all no tracking problems even at very low levels are the major advatages. No compatibility problem and no RF noise pickup. By the way, all this is well known and widely accepted, so this is no surprise.

The step distribution is perfect: I have never found any problem in regulating the volume, even late at night. By the way, the first step mutes the output.

The mechanism seems to be very reliable, no problem whatsoever has arisen.


Model 2 is a very good product, assembled in a very stout way, with an attractive and professional look, requiring a very modest amount of real estate and at a cost for which no high quality passive pre-amp is available. If you just need to connect a CD player to a power amp it can solve the problem definitely. Only check that the CD output level and the amp input level match and avoid power amps with low input impedance.

Model 1 is a really good step potentiometer, that can be included in any industrial or DIY design high level product. As such the limitations related to the Model 2 passive nature are no longer there, and there really is no limit in using it. The 22Kohm value seems ideal for most situations, making it acceptable even as a volume control to be placed downstream with respect to the amplification circuit, as in our TNT MW Pre 01 for example. The only evaluation then is about its quality and price, and in this the low production numbers plus the craft of construction does not help in keeping down the cost, which seems anyway more than acceptable.

© Copyright 2001 Giorgio Pozzoli - http://www.tnt-audio.com

HTML & proofreading: Tom Browne

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