May 2013

Bent Audio AVC-1 Modular Remote Control Autoformer Preamplifier
Oodles of resolution, excellent imaging and spatial effects!
Article By Clive Meakins

  This review has been something like six years in the making. I've been a long-time fan of transformer volume controls (TVCs); I've been using a home-brewed S&B transformer preamp since 2003. What I was missing was not the fantastic sound transformers can give; it was the remote control functions I wanted. John Chapman, a.k.a. Bent Audio, sold control modules and chassis for TVCs for years. When S&B decided to move away from supplying the DIY market, John upgraded to new transformers but needed to find the time to develop a new chassis to match his modules. It's taken a while (six years!) but John's been busy with all sorts in between.

John uses a particular type of transformer called an autoformer, this has a single winding; they do not have primary and secondary windings as normally found with transformers. So instead of TVC we have AVC -- Autoformer Volume Control. The AVCs John uses are from the highly respected designer, Dave Slagle. John has rather neatly named his relay switched module with Dave's AVC at its heart the "Slagleformer". Even though AVCs can use a smaller core than a traditional transformer the Slagleformer uses a core of similar size to most TVCs. The Slagleformers are optimized for core size, signal level handling, and bandwidth. The original design has stood the test of time, not needing any revisions. Right first time is always best! A closely associated Dave Slagle AVC to the ones John Chapman is using was reviewed for Enjoy the Music.com by Dick Olsher. What Dick reviewed was a very affordable hair-shirt volume control kit which ultimately sounds very similar to the AVC-1 but with coarser volume control steps and no remote options. John Chapman developed the switching arrangement for Dave Slagle. You can find more details about the cost effective AVC at Dave Slagle's Intact Audio site.

What's all the fuss about using TVCs and AVCs for volume controls? For me they are synonymous with uncompressed dynamics, resolution and imaging. Most volume controls are resistive; when you turn down the volume a proportion of the voltage and current is thrown away along with dynamic contrasts. One of the reasons often given for transformers sounding for good at low volume as well as high volume is that they only attenuate the voltage, not the current. With AVCs more potential current becomes available as you turn down the volume. Nothing is perfect in this hi-fi lark so what are the downsides? The inductive load transformers present can cause problems for a very few output high impedance tube phono stage designs but these types are intended to feed into active preamps in the first place. In truth they would fare worse into a typical resistive passive preamp. The vast majority of tube sources will drive the Slagleformers with ease. Any solid-state phono stage or CD player should drive Slagleformers with supreme ease too. Here's the reason; AVC's lower the impedance seen by the source. With the level turned down to a typical setting, the source(s) are able to drive the power amplifiers vastly better than if they were directly connected to the amplifiers. The Slagleformers can be thought of an impedance helper.

Before I'm jumped upon for calling the AVC-1 a pre-amplifier when it's a passive device I should point out that it will provide 7dB of gain so it can amplify. Some folks get picky about passive devices being called amplifiers when they don't amplify – well in this case they can amplify, so there!

Specification
The AVC-1 is not a hard-core kit and it's not a finished product either. It is a set of modular options and a chassis which you can assemble into a preamp according to your required specification. All the modules are pre-populated with components. Should you want to include your own active modules within the chassis there is plenty of room, likewise it can be built as balanced too. The combination of modules I chose is what I would expect to be the most commonly requested specification:

Single-end (balanced can be spec'ed and requires 4 Slagleformers)
6 pairs of RCA inputs
2 pairs of RCA outputs
Remote control of input selection, volume and balance, all relay operated
Volume control from -54dB to +7dB in 1dB steps
Numeric display of left & right volume settings
LED indication for mute, selected input, t ape
Stealth mode option to turn off the entire display

Once you unpack your prized delivery, spend plenty of time understanding what everything does and how to complete the assembly. The extremely well executed circuit boards for the relay operated input and control sections need to be bolted to the chassis as does the small control power supply board. You then have the all-important Slagleformers. These are top and tailed with circuit boards providing really slick relay switching and they also eliminate virtually all wiring. Typically these sorts of volume controls have wires flying everywhere and there are many wires to prep plus solder too. Not here! All that needs to be done once the modules are in place is to prepare and solder seven wires in place. The wiring taken at an easy pace took me around 2.5 hours to complete. It might sound like a long time but I wanted to do a decent job. I used OFC magnet wire, you can use whatever you favorite wire is. The cost of the chassis, modules, and remote is $1600. You can choose to invest $1300, yet this comes without the chassis should you decide to use your own box.

The AVC-1 chassis designed by John Chapman is sourced from a chassis supplier to several well-known brands. It's a quality chassis; it it' or $1, been something like 6s all aluminum too which is great for sonics. The chassis is solid without being crazily hewn out of a solid aluminum billet. John's struck the perfect balance between heft and affordability. The fastenings, control knob, buttons and phono sockets all have a really good fit and finish. Then there's the remote control….John went totally over the top with this. He tells me he couldn't find a suitable off-the-shelf item so he designed the extruded case, chromed end-plates, circuit board and chromed buttons. The result is a remote control which would not disgrace a $20,000 preamp. It weighs half a pound and is brilliant to hold and use. The remote is neatly named the Bent Metal Handset – it's anything but bent. Seeing as you will be touching the handset when operating the AVC-1 it seems John has used thinking from the automotive industry. What I mean is that high-end vehicle brands focus on the parts you touch to enhance the feeling of quality, i.e. door handles and controls all need to feel good when operated if you are to feel good about the whole vehicle.

Performance
As I had previously borrowed an Intact Audio AVC from a friend I knew what sound to expect from the AVC-1 and I wasn't disappointed. Borrowing the low cost AVC was valuable as I didn't want to move to a nicely remote controlled AVC which didn't quite match up sonically to my S&B TX-102mII TVC. Both Intact Audio AVC and the Bent Audio AVC-1 at a minimum match the TX-102 and even out-perform it in some aspects. I should set some context about the S&B TX-102; the successors to these are found in the Music First Audio preamp. TX-102 was available as a DIY component in mk I, II and III forms. I'm advised the II and III were the more consistent sounding meaning that they behaved well with the widest range of systems. Arguably my mkII was the most truthful sounding version, though some like the extra top-end with the mkIII.

The AVC-1 sounded great from initial switch on, it does improve with a hundred or so hours use, getting smoother over time though it never sounded remotely harsh to me. Used within the context high quality system you will hear what a great preamp the AVC-1 is. My system comprises 2 record decks (Garrard 301 and Trans-Fi Salvation), Computer Audio with various DACs, 300B SE power amps feeding Open Baffle speakers (hybrid Bastanis / Trans-Fi). I also brought a couple of T-amps and Valvet Er1 SE solid-state power amps into the mix. With Open Baffle speakers including two actively driven and DSP'ed 15" bass drivers per side I'm easily able to pick up on spatial clues and as for bass; if it's deep, tuneful and powerful I can easily hear it -- and I do with the AVC-1. Operation is faultless; the remote is especially wonderful and the relay operation of inputs and level is really slick!

In doing a straight comparison it was clear the AVC-1 shared most traits with the well renowned TX-102. Oodles of resolution, excellent imaging and spatial effects, silent noise floor, subtlety and almost visible texture. Dynamics... AVCs should preserve dynamics valiantly and do so at any volume. Yes, dynamics with the AVC-1 are every bit as good as you'd hope for and probably better than most people are used to. So what does it do better? The area which struck me immediately was the treble. It is more extended, that's not to say brighter because it isn't. The treble simply extends further upwards. In harmony with this is significantly better decay of high-frequency sounds. One of my favourite tracks for testing bandwidth and treble balance is Jennifer Warnes "Bird on a Wire". The tinkly treble in the background can on some systems be a little recessed, for me it's quite obvious when the balance is correct and it is with the AVC-1.

The AVC-1 costs $1600, this provides you all the parts to DIY build a high-end sounding preamp with excellent functionality and appearance. Unless you need a preamp that warms up a cool sounding system or need high gain then I can't think of a more ideal preamp, what more could you want?

Manufacturer Comment
Many thanks to Clive for his endless patience while I sourced a new case option for DIY builders. For many years now folks have been building projects using these same internal modules but often they were stuck for what to do for a chassis. It is nice to now have a chassis option for them. Also I'm glad that Clive found the DIY part of the project easy - the pre-assembled and tested modules were made with just that in mind. He's an experienced builder but the project should be fun for anyone with a bit of DIY experience. Finally (and most importantly!) it's great to hear that he found the pre-amp musically satisfying. Credit for that performance must go to Dave Slagle as it's his AVCs that are the sonic heart of the AVC-1

Specifications
Type: Single-end preamplifier (balanced spec requires 4 Slagleformers)
Inputs: 6 pairs of RCA inputs
Outputs: 2 pairs of RCA outputs
Remote control of input selection, volume and balance, all relay operated
Volume control from -54dB to +7dB in 1dB steps
Numeric display of left & right volume settings
LED indication for mute, selected input, tape
Stealth mode option to turn off the entire display
Price: $1600 as complete kit or $1300 without chassis.

Manufacturer
Bent Audio
800 - 15355 24th Ave
Suite 444
Surrey, British Columbia
Canada V4A 2H9

E-Mail: johnc@bentaudio.com
Website: www.BentAudio.com