Autoformer Volume Control AF-28PC/PB

The AF-28 – an introduction 

The AF-28 is a high performance core component for a magnetic volume control. It is a multi tapped autoformer designed to be used with a 28 position switch or with relays in order to create a piece of equipment. It is not a complete product ready for use in an audio system. 

The primary application is in so-called passive preamplifiers, more precisely passive control units. As such it allows a number of functions, the most important being the ability to change the output level of the passive control in multiple steps to control the volume of a high fidelity system. Applied in a passive control unit the AF-28 by applying autoformer technology offers a material improvement over all previous such schemes based around resistive attenuation. 

The so-called passive preamplifier appeared on the map of High Performance Audio with the emergence of the CD Player. Modern Sources, such as CD-Players, DVD Players, SACD Players and others 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 “passive preamplifiers” initially created notable interest as a sonically extremely pure method of controlling volume and selecting inputs, they have soon faded back into obscurity, not surprisingly as most of these devices suffered from substantial impedance mismatches with either sources or loads.  

If for example a 5k Ohm resistive volume control were to be employed in a passive control unit the source would be required to drive all the time a quite severe load of 5k Ohm. If combined with a 1k Ohm source impedance the worst case output impedance of the combination would be 1500 Ohm at -6dB attenuation, while at -20dB attenuation the output impedance would still be around 540 Ohm. If combined with around 1nF load capacitance (easily found in longer, high capacitance interconnects), this leads to a 0.3dB attenuation at 20kHz for a 20dB attenuation setting, practically showing the absolute permissible limit for load capacitance.  

The worst-case attenuation at 20kHz almost reaches 1dB!!! If, to provide our source with an easier load we choose a 50 kOhm resistive volume control we must either accept drastically higher levels of roll off at 20kHz or we must limit the load capacitance to less than 100pF. Such a level of capacitance (100pF) can easily be found with only 1m of high quality interconnect cable and is often exceeded by the input capacitance of many amplifiers!  

The so-called transformer volume controls that became fashionable around the year 2000 use separate primary and secondary windings. While this has some advantages (isolation between primary and secondary windings) it leads to problems with resonances and non-flat frequency response (especially at lower level settings) with many of such units. 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 and frequency response. Also, as only halve the winding space is used for the input winding the primary inductance and hence the low frequency response is limited.  

An autoformer volume control instead has only one winding, used for both input and output. As such resonances are much better controlled, any that are present in the AF-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 transformer volume control 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 transformer volume controls. 

The introduction of the autoformer volume control AF-28 makes possible 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 AF-28 whose specifications and measurements are covered in this document. 

The autoformer is available with either a full permalloy core (PC suffix) or with a 50% permalloy & 50% steel core (PB suffix).  

The full permalloy core version can handle all sensible line level signals (up to around +20 dBu) however level handling is limited. The 50% permalloy core version can handle a lot more more level (more than we can measure) and is hence ideal as volume control after an active gain stage.  

AF-28PC Specifications 

Nominal Impedance:100KOhm  

Maximum Attenuation:60dB 

Primary Inductance (@20Hz):1000H 

Impedance (@1KHz):> 400KOhm 

Maximum Level (@20Hz):>7.75V (+20dBu) 

Frequency Response -20dB Tap:<5Hz – > 200KHz (+0dB/-3dB, 100R Source) 

AF-28PB Specifications 

Nominal Impedance:20KOhm  

Maximum Attenuation:60dB 

Primary Inductance (@20Hz):150H 

Impedance (@1KHz):> 200KOhm 

Maximum Level (@20Hz):>31V (+32dBu) 

Frequency Response -20dB Tap:<5Hz – > 200KHz (+0dB/-3dB, 100R Source) 

AF-28 Technical details  

The autoformer is housed in a soft steel shielding can that measures 60mm (23/8in) in diameter x 50mm (2in) 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 (output) 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, when compared to continuous controls.  

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 sucesffully by most sources. 

Realistically 6 – 10dB of Gain form the maximum for the AF-28PC (100% permalloy core). 

The AF-28PB (50% permalloy core) should NOT be used with gain. 

The full winding has around 600 Ohm DC Winding resistance.  

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

The 50% permalloy core version has 150H primary inductance and provides a load impedance of around 20kOhm across the audio range. 

Input considerations  

The input impedance of the AF-28 is strongly dependent upon the load impedance and selected attenuation. 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. As the AF-28 should not be used with loads lower than 10kOhm the use of a 2.2uF or larger coupling capacitor in equipment preceding the AF-28PC should suffice in all conditions. 

The AF-28PB 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 10kOhm load on the output the worst-case load impedance on the source 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 load be used only with relatively high impedance loads (> 40kOhm) for the AF-28PC, 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 should be no smaller than 10uF.  

As soon as the signal is attenuated (which is the way the AF-28PC 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 AF-28 based device can become VERY HIGH, with a 10k load and 14dB attenuation selected the midrange input impedance will approach 250kOhm. Hence, in the subjectively critical midrange, the loading of the source is drastically lowered, resulting in most cases in drastically lowered distortion from the source.  

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

For a source impedance of 50 Ohm and the unit gain connection a maximum level of +20dBu (7.75V RMS) is permissible above 20Hz for the AF-28PC. The AF-28PB can handle much more level, well over +20dBu are handled at 10Hz without problems (this is the limit of our measurement gear). Permissable levels scale with gain, so with 6dB passive gain maximum levels are 6dB lower. 

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

Output considerations  

The output impedance of the AF-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 AF-28 is around 600 Ohm. If the AF-28 is used with 6dB step-up the source impedance is “stepped up” too, namely by the square of the step-up ratio (2), 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 linestage is 1200 Ohm. The highly reviewed Conrad Johnson Premier 17LS linestage has around 850 Ohm output impedance. 

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

The table shown illustrates the input impedances of the AF-28PC at 1kHz and 20Hz and the Output impedance in unity gain mode when driven from a 1kOhm source and a with a 10kOhm 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 AF-28 when driven from a sufficiently low impedance (1kOhm or less) will provide a usable bandwidth of at least 5Hz – 200kHz (+0/-3dB) or better. 

Tagged