The modern world has far more radio frequency interference (RFI) than just twenty-five years ago – thanks to our far greater use of mobile phones, wireless routers, home computers and switching power supplies. RFI can affect the clarity and transparency of your system, masking dynamics and detail and preventing your equipment from delivering its full sonic potential. It's one factor as to why cables sound different, because some shield it out better than others.

connected-fidelity has come up with a simple-to-fit device designed to go across your amplifier speaker terminals and/or the terminals of your loudspeakers, which acts as an RFI blocking Faraday cage. The RF Blocker is entirely passive and has no effect on impedance. Each one is housed in a diecast aluminium box measuring 55x60x20mm. A short pair of leads terminate in a choice of spade lugs, banana plugs, or bare wire. A set costs between £235 and £249 depending on connectors.

No technical details are given as to what's inside but, apparently, it's a Zobel filter network tuned to remove RFI at the frequencies relevant to home Wi-Fi and mobile phones. The blockers are extremely easy to install. Just connect them across the speaker output terminals of your amplifier or the connection terminals of your speakers, and you're away.

THE LISTENING

I tried my review samples with several amps – solid-state, tube, and a tube/transistor hybrid. In each case, they created a cleaner, more transparent sound. There was a small but noticeable reduction in glare. The top-end seemed crisper, cleaner, more open and detailed with the RF Blockers in place. Subjectively, it also seemed like the volume level had increased slightly. The music sounded fresher, and voices and instruments projected a little more strongly into the room.

I began by using the RF Blockers with a Consonance M100S Plus valve amp which uses paired 300B output tubes. Having a transformer-coupled output (like most tube amps), I'd wondered if this amp might be less prone to RFI – with the output transformer possibly having a filtering effect of its own. However, the improvement in clarity and dynamics was similar to that with my old Musical Fidelity Nu Vista 800 tube/solid-state hybrid and Copland's pure solid-state CSA 70. This is significant, as these three represent most amplifier types out there.

Playing a 1976 DG recording of Shostakovich's Second Cello Concerto with Rostropovich, the percussion in the second and third movements had greater immediacy with RF Blockers fitted. The snare drum, for example, seemed more tactile with increased bite. Everything was firmer and better focused. The RF Blockers also had a beneficial effect on the Musical Fidelity Nu Vista 800, helping to clarify the sound, making it more incisive, articulate, and better-able to stand comparison with the Consonance.

I tried removing the RF Blockers from the amp and putting them across the speaker terminals. The improvement seemed broadly similar to what I'd heard with the Blockers on the amp. Possibly it wasn't quite as good, but honestly, there wasn't much in it. I have 8m long speaker cables and wondered if a second set of RF Blockers might be beneficial. This would allow you to fit one set on the amp and another on the speakers. I tried it and heard further benefits along similar lines; reduced glare, greater focus, increased clarity and more presence.

I played Elgar's Piano Quintet with Bernard Roberts (piano) and the Chilingirian Quartet on EMI from 1986. I chose this CD partly because I'd never much liked EMI's early digital recording; it's always sounded bright, glassy and ill-focused. Could the RF Blockers help with this? Yes, they could!

Sadly, the RF Blockers didn't totally eliminate the bright tonal balance; however, they noticeably reduced the glassiness while improving clarity and fine detail. For example, the individual character of the low notes of the cello and piano became clearer - much better-separated and distinct.

Using two sets of RF Blockers, this difficult recording suddenly sounded more solidly focused and enjoyable. Notably, things became a bit smeared and slightly muddy when I removed them. Furthermore, I compared with and without a few times and heard improved focus each time the RF Blockers were connected.

I tried an LP recorded in the early nineteen eighties of Mozart symphonies with Christopher Hogwood. This usually sounds bright and thin, but the sound became more full-bodied and rich. I also felt able to play it at quite a low volume level without loss of presence or impact. However, each time I removed the Blockers, I felt that I needed to increase the volume a smidgen. So, if you do try a set of these, it's important to keep levels identical when making A/B comparisons.

THE VERDICT

Connected-fidelity's RF Blockers made a subtle but worthwhile improvement to my system. However, designer Michael Osborne points out that amps and speakers vary in their sensitivity to RFI, with some being better-shielded than others. So improvements will depend on your situation. I already run RFI-cancelling Kimber mains cables in my system, so I thought these RF blockers might not make that much difference – but they certainly did. Well worth trying then, catch them if you can.

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