You can hear it in the street, see it in the dragging feet

Gosh, but I’m cynical. I was just reading a review of a very audiophile-ish amplifier where the remote control can only “trim” the volume – any major volume change involves getting up and turning the knob on the amp itself. This apparently results in a simpler signal path. What..? I was trying to think how and why this might be. Apparently the amp uses electromechanical relays to control the volume.

I have become so cynical about this whole business that I actually came up with this thought: maybe the manufacturers were having trouble with the remote control receiver and were worried that it might go haywire and go to full volume so they limited it to “trimming”. Then I came up with the rather more charitable view that it was simply a question of ‘bits’. Maybe for some reason their remote receiver only has a few bits, so they have to choose between coarse control over a wide range or fine control over a narrow range; they chose the latter. In the end, my final guess was that maybe the remote control applies to a separate attenuator in series with the one controlled by the knob rather than being ‘OR’-ed into the control of a single relay-based attenuator. This means that a quiet volume setting from a wide-ranging remote control would restrict the range achievable by the knob, and vice versa, hence the necessity to limit one of the controllers to trim-only duties.

You see, that’s what I have become. I read about a perfectly innocent and highly-desirable audiophile design feature such as a “trim-only” remote control, and my mind wanders off, coming up with these sceptical thoughts. This is not an isolated example of the way I regard the ‘high end’ audio industry…

I, too, am an electronics designer. The way my mind works, I would have identified the control of volume as absolutely the central function of my pre-amplifier and noted that the interaction between remote control and front panel knob was important. If one of my marketing ploys (careful of that cynicism…) was to have no nasty, dirty software in the box then it would certainly be a design headache; less so if I was ‘permitted’ to use a microcontroller (we are talking audiophile prejudices here and some people would hate to have a microcontroller in the same box as the audio signal). But I would have completely failed to realise that a trim-only remote is a perfectly viable product in high end audioland, and can even be marketed as a feature!

Other aspects to consider would have been volume resolution (how many steps do I have available) and logarithmic vs. linear response. If my gimmick (damn, it’s that cynicism again) was going to be relay attenuation then I would have to consider finite switching time and contact bounce. A clever arrangement of relays and resistors giving me high resolution with few relays, but where more than one relay changed state at the same moment, might result in some pretty ugly volume changes. That would need thinking about carefully.

The relay attenuator is one of those ideas that really appeals to the audio ‘high end’. It has everything: the ability to choose expensive resistors, the steampunk-ness of relays and the fact they make a mechanical noise when operated! They are necessarily large, physically, so in a long chain of them the signal is forced over quite a long, convoluted path with many solder joints. I imagine that no one worries too much about shielding – do they put the attenuator in a tin can? How would the punter see the lovely expensive resistors if they did?

And another thing that seems less than desirable: people used to go to great lengths to avoid putting switch contacts in series with the signal. If we are talking about micro-diodes being a noteworthy factor in cables, then here is a case where a slightly oxidised, damaged or contaminated contact really could have an effect (and of course even if the relay is just shunting to ground it is still in the signal path). Relays for small voltages and currents need to have gold contacts otherwise oxidation builds up and is not burned off by arcing (yes, controlled arcing is desirable in power relays) but even gold-plated contacts are not immune from atmospheric damage working its way under the gold. An alternative that might have worked quite well, the mercury-wetted relay, is banned in the EU. It is probably desirable that the contacts are in airtight chambers to prevent ingress of dust, moisture and, that great enemy of switch contacts, silicone which can “creep” over long distances and reduce the effectiveness of electrical contacts. Such arrangements may be found in reed relays, or hermetically sealed conventional relays.

So which has the more signal-degrading effect: an expensive relay attenuator or a £2 interconnect cable? If I had to bet…

In the rational world we now have electronic attenuators such as the ones that live in the multi-channel amplifier that I use for my system. These are a natural result of the need for digitally-controllable volume, and have many advantages (high resolution, high precision, high reliability, low distortion, low noise, small size, low cost). Their disadvantage is their high end non-marketability: they are hidden away in integrated circuits and therefore immune to being infused with musicality by skilled designers and artisans. Nor are they are well-suited for low depth-of-field photographs in brochures.

The fact that the ‘high end’ unquestioningly prefers the steampunk version with all its obvious faults to the ‘perfect’ modern alternative is today’s hi-fi industry in a nutshell.

____________________________________________________

An example of the difficulties of designing a relay attenuator:

I just looked at a beautifully-presented DIY project on the web. If we delve into the data sheet for the miniature relays specified by the designer, we find the following stipulation:

“Min. permissible load: 10uA at 10mV”

with an added note:

“This value was measured at a switching frequency of 120 operations/min and the criterion of contact resistance is 50 Ω. This value may vary depending on the switching frequency and operating environment. Always double-check relay suitability under actual operating conditions.”

To me, selling this as a commercial product would look like a bit of a minefield. The contacts in these relays are not in an airtight chamber and so are exposed to the atmosphere. While my prototype might work OK (as far as I could hear, but measurements might reveal a different story), what if I left it for several months without operating the contacts?

It would be fascinating to know which relays are used in commercial products. I wouldn’t bet much on them being radically different. The ones in a photograph of the interior of a certain high end amplifier certainly look very similar…

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s