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Bose F1 'Studiocraft' Amplifier



In the past, the UK subsidiary of Bose had (like their parent company in the US) commissioned BGW to produce their amplifiers, but being a company with interest in things innovative, the amplifier reviewed here is a break with tradition. It's a UK design, a version of the 'Doctor Thomas' amplifier from Musical Fidelity, and a MOS-FET design to boot.

Power output is a nominal 120 watts into 8 ohm speakers, or 200 watts into 4 ohms, stereo. Or there's an add-on bridge-mode unit; using this, output levels can be raised to 400 and 600 watts respectively, in mono only of course.

Exterior Facilities



F1 is a utilitarian unit, after the minimalist fashion in up-market hi-fi circles. On the front panel, there's simply an on-off switch; deeply fluted heatsinks occupy the remaining space, in a manner vaguely reminiscent of another, well known studio amplifier. Moving to the rear, inputs are via gold-plated phono sockets. As a cynic you may scowl at the gold, but these phonos certainly offer the essential tight fit. On a less voluptuous note, they're also isolated from the chassis; there will be no grounding hassles with this amplifier. Parallel XLR inputs are also provided, wired pin 2 hot, and unbalanced; it's also good to see these pinouts clearly identified on the panel.

Outputs are solely via gold-plated binding posts. Again, the plus point is not so much the gold, but the inherent quality of the terminal. Unlike conventional 4mm binding posts/terminals, you can clamp 4mm plugs, as well as very heavy-gauge speaker cables! And whether it's a plug or wires you're clamping, it's easy to make a very firm, gastight seal - and this is all that really matters. Put simply, the F1 has the best output terminals I've seen on any amplifier, and this fact mitigates for the absence of XLRs - who needs them on the F1?

Thermal Design



Fan cooling is apt to be too noisy in many control rooms, but the need for it can be avoided by placing the heatsinks on the outside of the amplifier. The back panel is normally ruled out (unless the connectors are on the front), and most manufacturers stick the heatsinking in the wings - that's on either side of the case. This is fine for free-standing amplifiers, but not so smart when the unit is rack-mounted, especially if there's lots of heat emanating from other equipment, usually of the digital variety.

Mounting the heatsinks on the front panel is an obvious conclusion, and the F1 has gone one step further than other designs: the rack-mount tabs are placed so the fins are well proud of the rack. In addition, the case design produces a well defined chimney around the fins, for superb cooling by natural air convection.

Coupled with this arrangement, the interior of the F1 involves convoluted metalwork which gives the chassis great rigidity and basic strength, without the massive weight common to many amplifiers of this power rating. This has positive ramifications should you choose to take your F1 on the road, where excess weight adds to trucking costs and complicates packaging.

Turning now to the components, as the review model (a prototype) stands, it's unsuitable for road use: screws aren't locked and the mains earth connection - in particular - can loosen easily. Also, the mains connections aren't sleeved, and the reservoir capacitors have inadequate support. Bose have assured us, however, that all these points will be put right in production, and being a dynamic company, there should be no worries on this score.

The amplifier cards are neatly bolted directly to their respective heatsinks via their output devices, which are F1-PAK (plastic encapsulated) power-FETs. These are thermally more efficient than the conventional 'T03' metal-can types that HH were using in their 'V' series amplifiers (reviewed last month). And ironically, the elegant sturdiness of the PCB mounting puts many macho PA amplifiers to shame; the mechanical design is very good in this area.

One further niggle concerns the output fusing. It shouldn't be necessary to fuse the output of a MOS-FET amplifier, but fusing can protect bass drivers in the unlikely event of a DC fault, so it's worth having. However, the output fuse's position on each of the F1's two channels leaves much to be desired - it's mounted on a skimpy holder on the amplifier card, and replacement entails removing thirty (30) screws and stripping down a major portion of the amplifier.

This is acceptable if the dead fuses are exclusive to dead amplifiers, but more often than not, fuses die of old age, vibration or simply spuriously, and the need to dismantle the F1 will prove very annoying under these circumstances. However, we hope Bose will take our advice on providing a panel-mounted fuseholder on the rear panel. Short of this, a heavy-duty fuse lightly soldered in place should suffice, although this would compromise any protection offered to the speaker(s).

Evaluation



Measurements proved the quoted output powers to be conservative, as is often the case; the table shows output power into various load or (nominal) speaker impedances, and the accompanying sensitivity. As the F1 has no metering, you should ideally set up peak-level metering on the desk - if available - to 'go into the red' at the levels quoted, ideally according to the lowest impedance to which your speakers will reach. Otherwise, go by their nominal impedance.

Table 1
F1 sensitivity versus power into impedance

Input Sensitivity (at clipping) Power* Output Load Impedance
Volts dBu 1 Watts Ohms
1.35V +5 96W 15
1.20V +4 157W 8
1.00V +2 240W 4
800mV 0dBu 270W 2.5 ohms

* Mains voltage at 240V
1 Approximate: rounded up.


Whilst on the topic of speaker impedances, note that like some other MOS-FET designs, there's no protection in the shape of crude power-limiting into low impedances. Thus the F1 will drive speakers with impedances dipping below 2 ohms without distortion, albeit with a reduced headroom. With this in mind, and providing the input level is kept below, say, -2dBu, the F1 can drive virtually any speaker without getting into impedance related troubles.

Residual hum and noise were very low, as too was the DC offset. All this one can expect from a top MOS-FET design, but one disappointing feature was an acoustic 'buzz' from the toroidal transformer, which is uncomfortably obvious in a quiet room, if not exactly audible with the system wound up! The hum arises from saturation of the toroidal transformer's primary windings, and Bose are currently looking out for improved transformers. Pending modifications here, it would be wise for you to assess this potential annoyance when auditioning, allowing about 15 minutes for the amplifier to warm up, for the acoustic noise will diminish after a while.

As is to be expected from a UK audiophile design of this calibre, the F1 scores very highly on sound quality, particularly at the top-end, where percussion sounds are better focused and much cleaner. Indeed, the F1 is one of the few that excels in all three frequency ranges - it would be equally at home as the bass amplifier in a bi-amped set-up, for example.


Conclusion



The F1 appears to be expensive when viewed in terms of its sparse physical contents, but as a proven audiophile design with high power capabilities and commensurate heatsinking, it will meet the needs of many readers admirably. For instance, the power available from the F1 (in bridge mode) is at a maximum into 4 ohms and below, at 600 watts, in contrast to the majority of Bi-polar (non MOS-FET) amps, where the bridge mode only provides useful extra power into 8 ohm or higher impedances.

This makes the F1 invaluable when you need to drive power into low impedance speakers. Bose's own speakers for instance... Moreover, the simplicity of the electronics assembly should translate as intrinsic reliability, and in low servicing costs.

On a practical note, the F1 has no metering, no delayed switch-on and no gain controls, but in this respect it's no different from, say, the Quad 405, and the absence of these features shouldn't detract from the F1's ability to perform usefully in the studio control room, and provide an accurate sound picture: (1) the thump from the F1 when it's initially switched on is negligible (some amplifiers make large bangs - they shall be nameless...), (2) gain adjustments and metering can be achieved via the console; or you could readily add your own.

The F1 amp sells for £495 inc VAT.

Further information from Bose UK Ltd., (Contact Details).



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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Apr 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Amplifier > Bose > F1 Studiocraft

Review by Ben Duncan

Previous article in this issue:

> Fender Microphones

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