Autotransformer Volume Control
AVC-28. 28-step permalloy core

Autotransformer Volume Control (AVC) using a 100% permalloy core and soft iron shielding case. Easy to install on top side of chassis and looks really good. Suitable for use in passive pre-amplifiers (like a TVC). 28-step with maximum attenuation around 60dB.

28 step AVC, 100% permalloy core.

Hand crafted using a 100% permalloy alloy for the transformer core.
28-steps with maximum 60dB attenuation.

Dimensions: (dia.x h) 60 x 50 mm. Weight: 700gr.each

not available anymore

per pair

Autotransformer Volume Control

The AVC-28 is a multi tapped autotransformer made from a high-performance 100% permalloy core. Its
primary application is in so-called (transformer) passive preamplifiers. The AVC-28 by applying autoformer technology offers a material improvement over all schemes based around resistive attenuation.

The so-called passive preamplifier appeared on the map with the emergence of the CD-player. Modern sources generally offer output levels sufficient to drive power amplifiers to full power (usually 2V RMS or more for digital full scale) and also offer sufficient drive for external devices and cables.

Many CD-players and similar devices have output impedances lower than 1kOhm, some are materially lower. Whilst (resistive) passive preamplifiers initially created notable interest as a sonically extremely pure method of controlling volume, they have soon faded back. These (resistive) passive preamplifiers can sound very "pure", but they lack dynamics. The sound becomes boring. Problem with the resistive preamplifier is that the attenuated portion of the signal is "thrown away" as heat. The (resistive) volume control can't supply enough electrons to drive the capacitance of the interconnect.

Volume controls applying transformer technology, don't throw away any energy. When the voltage is transformed down, the output current is transformed up. This current capability gives the transformer passive preamplifier the dynamic sound of an active preamplifier.

A Transformer Volume Control (TVC) uses separate primary and secondary windings. While this has advantages (isolation between primary and secondary windings) it can also lead to problems with resonance's and non-flat frequency response (especially at lower level settings). Some or the more budget minded units show ultrasonic peaks of more than 6dB just slightly above the audio band, with notable impact on sound quality. Also, as only halve the winding space can be used for the input winding the primary inductance is limited and hence the low frequency response is limited.

An Autoformer Volume Control (AVC) instead has only one winding, used for both input and output. As such resonance's are better controlled, any that are present in the AVC-28 are shifted to beyond 200KHz and are well damped. Also, as the whole winding space is used for the input winding a greater primary inductance can be attained which allows a better low frequency response with a given source than a TVC using the same core. Additionally the better coupling and greater number of turns allows a larger amount of attenuation to be build in, compared to TVC controls.

The introduction of the autoformer volume control AVC-28 can make passive control units that work more effectively and in a much wider range of environments than comparable resistive or transformer volume controls. Considerable research and development has resulted in the current model AVC-28 whose specifications and measurements are covered in this document.

The full permalloy core can handle all line level signals (up to around +20 dBu, 7.75V)

AVC-28 Specifications

• Nominal Impedance:100K Ohm
Maximum Attenuation:60dB
Primary Inductance (@20Hz):1000H
Impedance (@1KHz):> 400K Ohm
Maximum Level (@20Hz):>7.75V (+20dBu)
Frequency Response -20dB Tap:<5Hz – > 200KHz (+0dB/-3dB, 100R Source)

AVC-28 Technical details

The autoformer is housed in a soft steel shielding can that measures 60mm in diameter and 50mm in height, excluding the connecting pins. The threaded holes are included to allow mounting the autoformer to a suitably drilled metal plate.

The single winding winding offers a number of taps allowing the following attenuation values:

0 dB, -2 dB, -4 dB, -6 dB, -8 dB, -10 dB, -12 dB, -14 dB, -16 dB, -18 dB, -20 dB, -22 dB, -24 dB, -26 dB, -28 dB, -30 dB, -32 dB, -34 dB, -36 dB, -38 dB, -40 dB, -42 dB, -44 dB, -48 dB, -52 dB, -56 dB, -60 dB

With these values the steps by which the volume is changed over the majority of the range is smaller than the commonly acknowledged limit of audibility (3dB), giving consistent fine control over the volume.

It is possible to achieve some gain using this autoformer, by connecting the input to a tap that is marked as attenuation. In principle the gain can be very high, however using any gain considerably increases the load on the source and high gain produces potentially punishingly low loads that cannot be driven successfully by most sources.

Realistically 6 – 10dB of Gain form the maximum for the AVC-28

The full winding has around 600 Ohm DC Winding resistance.

The primary inductive reactance at 20 Hz is in excess of 100K Ohm (950 H Primary Inductance) and thus provides an input impedance of more than 100K Ohm across the audio band if the secondary loading is infinite.

Input considerations

The input impedance of the AVC-28 is strongly dependent upon the load impedance and selected attenuation. (Load impedance = the input input impedance of the power amplifier behind the AVC-28)
The worst-case input impedance occurs with no attenuation selected, in this case the input impedance will equal the load impedance plus the inductive losses. The AVC-28 should not be used with loads lower than 10K Ohm. The use of a 2.2uF or larger coupling capacitor in equipment preceding the AVC-28 should suffice in all conditions.

The AVC-28 is best used with a 1…2.2uF coupling capacitor in parallel with an RC “snubber” of 10uF+3.3K.

In the case of operation with 6dB of gain and with a 10K Ohm load (the power amplifier) on the output, the worst-case load impedance on the source (the thing driving the AVC) will be 2500 Ohm plus copper losses. This is a very severe loading and many items of consumer electronic will not be able to drive such a load adequately! It is thus recommended that the 6dB step-up connection be used only with relatively high impedance loads (> 40kOhm), like for example Valve Amplifiers. If the 6dB step-up connection is to be used with a 10k load on the output the preceding coupling capacitor (in the thing driving the AVC) should be no smaller than 10uF.

As soon as the signal is attenuated (which is the way the AVC-28 will be operated in most cases, most of the time) the input impedance rises and is ultimately only limited by the Inductive and Capacitive reactance of the primary winding near the outer edges of the audio band. In the midrange the input impedance of the AVC-28 based device can become VERY HIGH (which is a good thing :-). With a 10K load and 14dB attenuation selected, the midrange input impedance will approach 250K Ohm. Hence, in the subjectively critical midrange, the loading of the source is drastically lowered, resulting in most cases in lowered distortion from the source.

The limit of the input voltage to the AVC-28 depends upon the exact frequency and also the source impedance.

For a source impedance of 50 Ohm and the unity gain connection a maximum level of +20dBu (7.75V RMS) is permissible above 20Hz for the AVC-28. Permissible levels change according with gain, so with 6dB passive gain maximum levels are 6dB lower.

The input to the AVC-28 must be COMPLETELY free of DC offset; the presence of DC current materially degrades the performance of the AVC-28 Autoformer, both with respect to level handling and frequency response. The source impedance (= output impedance of the source driving the AVC) in general should be as low as possible, the lower the impedance from which the AVC-28 is operated the lower the distortion and the wider the effective bandwidth.

Output considerations

The output impedance of the AVC-28 follows a fairly complex patterns but the worst case is found with no attenuation. In this case the source impedance and the copper losses determine your output impedance for the unity gain connection. Thus for a CD-Player with 200 Ohm output impedance the worst case output impedance after the AVC-28 is around 600 Ohm. If the AVC-28 is used with 6dB step-up the source impedance is “stepped up” too, namely by the square of the step-up ratio (2*2=4), so our 200 Ohm source impedance becomes 800 Ohm, which is added to the 400 Ohm copper losses, for a total worst case output impedance of 1200 Ohm.

For reference, the output impedance of the popular Audible Illusions Tube Preamp’s line stage is 1200 Ohm. The highly reviewed Conrad Johnson Premier 17LS line stage has around 850 Ohm output impedance.

Attenuation (dB)
Input Impedance
@ 1KHz (Ohm)
Input Impedance
@ 20KHz (Ohm)
Output Impedance
@ 1KHz (Ohm)
0 9.75K 9K 1K
-6 36K 28K 320
-12 114K 58K 94
-18 246K 80K 26
-24 346K 88K 6
-40 398K 92K 0.2

The table shown illustrates the input impedances of the AVC-28 at 1kHz and 20Hz and the output impedance in unity gain mode when driven from a 1K Ohm source and a with a 10K Ohm output load at different attenuation settings. This should give some general idea of the relevant relationships.

With all autoformers the ultimately realized output bandwidth is very much system and application dependent, but the bandwidth limited AVC-28 when driven from a sufficiently low impedance (1K Ohm or less) will provide a usable bandwidth of at least 5Hz – 200KHz (+0/-3dB) or better.