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Ocean Audio Signature Three

Dual-channel Equaliser
Published January 2015
By Hugh Robjohns

Ocean Audio Signature Three

The Signature Three offers something no other EQ does — four inductor-based bands per channel.

Ocean Audio is the latest venture of Malcom Toft, who is perhaps best known for his involvement in the Trident A-Range and Series 80 consoles of the ‘70s and ‘80s, and their subsequent reinvention with MTA, Trident-MTA, and both Toft Audio Designs and Trident Audio Developments within the PMI group. Ocean Audio’s flagship product is another large analogue console, the Ark, available in various frame sizes and in two forms, with and without 500-series module slots. However, I suspect the bulk of the company’s business will relate to its other product lines, which include various 500-series preamps, EQs and routing modules, and three ‘Signature Series’ rackmount products. This Signature Series currently comprises a channel strip, a stereo FET-based compressor, and a stereo inductor-based four-band equaliser — the last being the subject of this review.

Overview

In the world of electronics, frequency-selective circuitry generally relies upon two kinds of reactive components: capacitors, which pass high frequencies but inhibit low frequencies, and inductors, which do the exact opposite. Although inductors are a fundamental element (mainly in the LF and LMF sections) in most early equalisers, including the classic Pultec EQs and Neve channel strips, they tend to be physically large and expensive things. Modern devices tend to use electronic trickery to avoid using physical inductors, reducing both size and cost, so it’s interesting and unusual that Malcolm Toft has built the Signature Three equaliser with inductors employed throughout all four of its EQ bands. The handbook explains that inductors lend a certain distinctive sound character, and the peaking responses obtained allow precise frequency control.

Another unusual aspect of the Signature Three is that the rest of the circuitry comprises seven all-discrete five-transistor gain stages in each channel, constructed entirely with conventional components — there are neither op amps nor SMDs here! Again, the handbook lists the benefits as being a simpler, cleaner signal path, and the ability to run at higher power-rail voltages for increased headroom. That theory is perfectly valid — and there’s certainly no disadvantage in this approach — but there are, in fact, countless examples of high-end, op amp-based products that deliver exceptional technical and sonic performance. And while some classic transistor-based equipment does indeed run on elevated power rails (eg. early Calrec and modern GML preamps run on ±24V rails), a peep inside the Signature Three suggests it actually generates ±18V rails — about the same as most op amp-based products. Arguably, the only step towards modernity is the use of an OEM switched-mode power module, which means the unit works on all mains voltages between 80 and 260 V AC. I was pleased to note that the mains safety earth is taken straight to a chassis bolt. Whatever the arguments for and against the old-school design, the Signature Three certainly delivers the required results.

Connectivity & Controls

The rear-panel electronically balanced input and output connections comprise paralleled quarter-inch TRS jacks and XLRs, and there’s a central IEC mains inlet and an illuminated on-off rocker switch. A large red LED on the front panel confirms the unit’s powered status. Two more red LEDs, one per channel and labelled ‘Peak’, illuminate when the output signal exceeds 15dBu — a good 18dB below the clipping level. Below these peak LEDs, each channel has its own independent push-button, which illuminates green when the corresponding EQ is in-circuit.

The rear panel includes some thoughtful touches — such as duplicating the legend upside down for anyone who is leaning over the top of the unit when plumbing things in!The rear panel includes some thoughtful touches — such as duplicating the legend upside down for anyone who is leaning over the top of the unit when plumbing things in!The remaining controls are logically arranged, with the four band’s continuous cut/boost gain controls positioned directly above their corresponding frequency selection rotary switches. The gain controls span a nominal ±15dB range, and all have centre detents at the unity gain position. All four EQ bands are fixed peaking types — there are no shelf options here — and all four are provided with six selectable centre frequencies. The handbook provides no specifications for the filter bandwidths, but rough and ready calculations suggest that all four bands are the same, with a bandwidth of just over one octave (Q=1.2) at high boost/cut settings, and progressively wider bandwidths at lower gain settings. The centre-frequency selections offered on each band overlap nicely, and are spaced to allow meaningful musical tweaking. For the record, I’ve listed the values. LF: 50, 80, 100, 150, 300 and 600 Hz; LMF: 125, 250, 500, 1000, 2000, and 4000 Hz; HMF: 2.5, 3, 5, 6, 7, and 8 kHz; and HF: 8, 9, 10, 12, 14, and 16 kHz.

Bench Test

After running a set of Audio Precision bench tests on the Signature Three, I was impressed with the technical performance. The frequency response at the -3dB points extends between 10Hz and 50kHz, and there’s a subtle ‘air’ lift of about +1dB above 10kHz, which helps to make the unit sound very open and airy when switched into circuit, even with all the band EQs at their unity positions. The THD+N figure was slightly different for the two channels, achieving 0.0025 percent in the right channel and 0.004 percent in the left (at 0dBu) — but both are better than the claimed 0.005 percent. With increasing input levels the THD+N figure increased, naturally, reaching 0.01 percent on both channels at +12dBu, and 0.06 percent at +24dBu. This last is slightly higher than the claimed 0.05 percent at +27dBu, but nothing to worry about.

My Audio Precision system runs out of puff at +26dBu, but there was no sign of clipping at that level, and I’m inclined to believe the specified +28dBu limit, which is far more than anyone would require in practice. The signal-to-noise ratio was, again, slightly different for the two channels, but measured around 94dB (A-weighted) and 88dB flat — comfortably better than the claimed specifications, and implying a potential dynamic range in excess of 120dB when driven to its fullest extent. With more conventional peak levels, the dynamic range still exceeds 115dB, which is excellent. The crosstalk between channels at both 1kHz and 10kHz measured around 109dB — normally the figure is higher at 1kHz, but these are still good results, and the only test which disappointed slightly was the common-mode rejection ratio, which delivered 53dB at 1kHz and 36dB at 10kHz, although I doubt that will cause anyone any problems.

In Use

The Signature Three is perfectly logical and straightforward to use, thanks to a clear and spacious control layout, although it perhaps suits dual-channel use more readily than true stereo applications, simply because of the inherent difficulties of accurately matching the individual band gain settings on the two channels, as well as the practical aspect of switching the two channels in and out simultaneously to assess the effectiveness of the current settings. That said, when using it across a stereo bus during the review tests I achieved good results.

Ocean Audio Signature ThreeThe gain range and frequency selectivity of each band is more than sufficient to allow quite precise surgical and corrective equalisation, when required. But when used with more modest gain settings the broader bandwidths and generously overlapping bands allow subtle and very musical creative tonal shaping. I didn’t mind the absence of shelf-equalisation options at the top and bottom of the frequency range, and in fact the peaking response characteristic proved more useful when trying to shape and control the low end. At the high end, the relatively wide peaking response bandwidth still allows gentle air lifts to be introduced without any difficulty.

Verdict

There is something very attractive in the sound character of inductor-based equalisers that’s hard to describe — they just sound more natural and ‘right’ to my ears, especially at the bass end of things. It’s probably tied up in the mysteries of phase shifts and waveform distortion, but whatever it is, the Signature Three delivers it well. Overall, this is an unusual inductor-based dual-channel equaliser with all-discrete transistor circuitry that delivers a classy and attractive sound, with well-sorted EQ options. It is very well made — here in the UK — and achieves excellent technical performance. What’s not to like?

Alternatives

I can’t think of any other EQ that offers four bands, each featuring inductors, but there are plenty of other inductor EQs offered by the likes of Neve, Rupert Neve Designs, Cartec and A-Designs, as well as the companies with which Malcolm Toft has been involved in the past, Trident and Toft Audio Designs.

Published January 2015