• Carlsbro Cobra 90 Keyboard C...
  • Carlsbro Cobra 90 Keyboard C...

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Carlsbro Cobra 90 Keyboard Combo


Carlsbro's Cobra series is aimed for use in small venues and for home studio use: Aside from a power (and price!) tradeoff, they're essentially identical to corresponding models in the well known Marlin range. The 3 channel combo we're concerned with here features a high impedance input on one channel for Fender Rhodes and the like, FX routing switches, and integral reverb to which the channels are routed by push buttons. There's also a control labelled 'Attack' and a socket for headphones. In the absence of the latter, sound comes from a 15" driver in a vented enclosure, together with a horn tweeter. Alternatively, auxiliary amplification can be hooked up to the line output socket - or you can simply connect up extra speakers, 2 sockets being provided.

The first problem encountered was 'Bi-FET' hiss when the master gain control was wound up. This might not matter much when the combo is used in it's own right, but as an item of backline gear hooked up to a PA, this HF noise could prove quite a nuisance, although, in some cases, it'd probably be swamped by hiss from keyboards!

Testing



The Combo was tested with a Transcendent 2000 as it seemed a likely partnership. Overall, the sound was full, but perhaps a little brittle and lacking in warmth, certainly in comparison to a valve instrument amp, though the application of a little reverb helped matters considerably. Bass response was smooth and well extended. But at the same time, the 15" driver's cone didn't 'flap' on very low notes, which is a common problem with Theile alignments. This suggests considerable care has been taken to optimise the driver's loading - it's certainly not an easy task. So, full marks on the bottom end performance.

Now, although the top - in terms of handling audible harmonics - was reasonably sweet, problems arose when the synth's attack setting was wound up. The Combo began to sound uncomfortable and there was a distinct nastiness in the high midrange to the extent of hurting the ears at high sound levels.

Moving on to other instruments, the combo handled a drum synth competently. Electret and high impedance moving-coil mics also made a good partnership with any of the 3 inputs. Putting all three instruments together (Keys, vocals and the drum synth), the combo proved itself up to handling a one man band without the intermodulation distortion (giving rise to ear wrenching nastiness) that characterises some combos in this mode.

As channels 2 and 3 have a pair of inputs, the combo can potentially handle 5 instruments, assuming the extra two can make use of the existing EQ and gain settings. We tested the mixing of the two inputs on each channel and couldn't detect any tonal colouration arising from the mixing of two sources.

In all cases, it was easy to get a 'good' sound because, with the unit's response (with the tone controls at their central position) being basically flat, the EQ was nearly wholly available for creative purposes rather than being used up in a struggle to achieve an acceptable sound from the speaker.

The reverb uses a standard 350mm springline; sonic differences in comparison to other gear arises principally from the mounting of the spring and the drive circuitry. As implied earlier, a small amount of reverb greatly improved the combo's keyboard sound, and moreover, the reverb depth control showed pronounced changes in FX over most of its range - unlike some! One worry here is the absence of any foam rubber to restrain the spring from bouncing around excessively. Moving the amp around with the reverb turned fully on and the volume turned up is definitely not recommended for people who jump out of their skins easily!

Construction



Moving onto the mechanics in general, the design of the knobs is particularly well thought out. It's possible to spot your control settings from twenty feet. Also, the pots feel good, although they're set a little close together. The speaker grille aesthetics are good but how long it'll stand up to abuse on the road without tears appearing is open to debate. A rather more annoying feature is that whilst both drivers are front loaded, the grille and associated frame aren't readily removable - without, it seems, breaking apart glued-up seams. Another niggle is the On/Off switch. This has an easily broken plastic toggle. The panel recess is unlikely to give adequate protection in transit, whilst experience suggests a metal toggle, adding a few pennies to the cost would survive unscathed. Perhaps these points will be corrected in production versions?

Circuitry



The physical design and assembly of the unit, in common with other items from Carlsbro, is exemplary. To begin with, the electronics is tightly packaged. The PCBs have screened legends. Moreover, the two assemblies that make up the amp (built around the front and rear panels) are exceedingly easy to gain access to without the need to dismember enormous cable looms. Instead, the two sections are coupled together with a length of standard 4 core cable, as used by BT and in domestic security loops, both ends being pluggable. The remainder of the wiring (there isn't much) consists of very short, point-to-point links. This is elegant design work, and in view of the fact that wholesale wiring looms are prime scapegoats for mechanical failures, RF burnout and finnicky performance, the minimal use of wire deserves praise. Again, the pots (which are PCB mounted) deserve special mention in that they're supported by brackets. This eases the strain from the electrical connections whenever the combo is banged around, so avoiding the hairline fracture and consequent hassle with pots going intermittent that plagues other equipment. In fact, with the bracket supports, the rear of the PCB is held so rigidly you could almost drive a vehicle over it without breaking anything!

Turning to the power amp assembly on the rear panel, the power rails pass via PCB mounted fuses. In theory, these should blow only if something calamitous happens to the amp, thus the fact that they're hidden away isn't any real cause for concern; if they do blow, then the amp will almost certainly need repair - it'd be no use just replacing the fuses. However, fuses do have a habit of developing metal fatigue and breaking spontaneously - or even falling out - and if this occurs, DC can appear across the 15" driver. Result? Dead speaker for no justifiable reason. This potential problem could be easily avoided by fitting anti-surge fuses (which will still protect the transformer from overload, but are much less prone to metal fatigue) and tiewrapping these in place.

Finally, the combo's electrical performance was checked out in view of the minor shortcomings noted earlier. With the master gain wound fully up, noise was 59dB below maximum output between 1Hz and 3MHz-3dB points. In view of the unit's very audible dislike of signals with a large transient content (i.e. with fast attack) the frequency response was inspected. With the tone controls set flat, it was 5dB up at 20kHz, 0dB at 30kHz and only 6dB down at 65kHz. The Attack control provides a bell curve response, adding about 5dB centred on 10kHz at its maximum setting. However, with a channel treble control turned to maximum boost, the response rose another 8dB at 20kHz, 11dB at 30kHz (!) and didn't fall back to 0dB until 150kHz... With a frequency response like this, it's clear that the amp will be easily grossly overloaded by ultrasonic energy. In view of the power amp design being an RCA bipolar derivative, and thus not exactly excelling at frequencies above 20kHz, it's no wonder that lots of transient energy from the test synth caused problems! Fortunately, this problem is nothing that can't be put right with some judiciously applied HF filtering to tame the treble control's boost above 20kHz, and ruthlessly attenuate ultrasonic energy. Returning to the noise, this fell to a more respectable -76dB below maximum output when the measurement bandwidth was reduced from 3mHz to 10kHz, so this problem appears to relate directly to the mixer's over-extended response and the characteristic very 'toppy' noise of Bi-Fet op-amps. A 10kHz low pass filter on the line output should ensure that the noise is reasonably well cleaned up for DI feeds.

Conclusions



Overall, once a few easily corrected shortcomings have been sorted out, this amp is strongly recommended forthose who don't need enormous amounts of power. The standard of construction and the aesthetics are both first class and should be a lesson to other UK manufacturers!

For further details contact: Carlsbro Sales Ltd., (Contact Details). Please mention E&MM when doing so.

Specification

Input Sensitivity CH1 26mV 1M
CH2 & 3 46mV 47K
Tone Controls Treble +12dB, -16dB at 10kHz
Bass +10dB, -12dB at 100Hz
Presence +10dB at 8kHz
Front Panel Effects Loop -10dBV send and return
Rear Panel Effects Loop -6dBV send and return
Output Power 90 watts into 4 ohms at 1kHz for 240V AC supply. 5% THD


Also featuring gear in this article



Previous Article in this issue

The MC-202 MicroComposer

Next article in this issue

Talking Shop


Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.

 

Electronics & Music Maker - May 1983

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Gear in this article:

Amplifier (Combo) > Carlsbro > Cobra 90 Keyboard

Review by Ben Duncan

Previous article in this issue:

> The MC-202 MicroComposer

Next article in this issue:

> Talking Shop


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