BCD Audio's New ALT 2 Mic Amps

History of the BCD Audio ALTs
In 1993, the BBC at Millbank were looking for a simple microphone amplifier for use in Radio journalism, and on the back of a 10 off order, the ALT-1 was born. Over time, variants were introduced and nearly 4000 units were sold, many of which are still in service.

Economics and the obsolescence of key parts meant that I had to redesign the unit, and so the BCD ALT-2 was born in 2016.

BCD ALT-1 – 1993
The main requirement was to create a top-quality mic preamplifier with transformer balanced XLR input with switchable phantom power, but with a minimum of user controls and powerable from a plug-top power supply.

Creating the diverse power supplies required was difficult enough, but finally a design was established that has lasted till today. User controls were reduced to two buttons, one of which could be disabled. A 'TAKE' switch was used, so that the unit could learn what audio level was present from the user, the front end gain optimised and the final gain set by a VCA. After the level was taken, the front end gain was fixed, but a compressor, limiter and AGC operated on the VCA to maintain optimum level. The compressor was optimised at 2:1 ratio, starting at just above 4dBU, and the limiter with fast attack and brick-wall ratio limited at +8dBU. The idea is that the compressor gently makes the sound a little fuller, and compensates for level variation, and the limiter blocks gross overload and clipping. Every time the limiter operates, an AGC slowly removes some gain, so that the level does not change very much, but the limiter effect is gently removed. The AGC can only remove gain; after sustained overload it gently backs out, leaving the gain back at the wanted setting.

BCD ALT-1R – 1995
There was a general requirement for voice-over booths and unattended studios, and the ALT was perfect for the application; all that was required was to remote the TAKE switch and its indicator.

Initially this was achieved by a 9way D lead, and units were built into panels and products supplied by installation companies. A matching headphones unit, the HPN-1 was created, and power stolen from the ALT to power this unit.

BCD ALT-1S – 1997
There is a requirement in newsrooms for a simple, good quality mic amplifier, but the journalist also needs to listen back to the recording and other sources at modest cost. The ALT-1S added a stereo line level balanced input to unbalanced mini-jack, that could drive PC style active loudspeakers. A larger case was needed to accommodate the metal XLRs used at the time. The ALT-2S uses the same case for both variants.

Solid-state input – 2001
The original units had a costly mic-transformer and gain setting relay, and was designed when solid-state microphone amplifiers were not outstanding. Both units were re-designed to use solid-state front ends when they became freely available, and of good quality. The majority of units out there are the solid-state variety, and the ALT-1R remote option was simplified to a mini-jack. A 2U rack mount panel was introduced that can take our BCD range of black boxes, apart from the ALT1-S.

BCD ALT-2R, ALT-2S – 2016
A total redesign was required. Advances in micro-controllers meant that most of the metering and compressor limiter could be achieved in software, but the output side VCA was retained. Care was taken to ensure that the operation and 'feel' of the unit was similar to the original and I incorporated most of the custom variants that had been requested over the 20 years of manufacture.

The ALT-1 and ALT-1R are replaced by the BCD-ALT-2R
The new ALT-2R retains the balanced XLR input and outputs. A 9way D connector provides the remote TAKE and LED, a new soft MUTE (cough switch), and an unbalanced output.

DC power can be supplied at this connector, and our HPN-1 can also be powered from there. Two pins can be customised at the factory to accommodate user requirements. The unit is intended for use in voice-over booths, unattended studios, direct to camera installations, emergency studio microphone points, and talkback.

The new ALT-2S is in the same case design as the ALT-1 and adds the Stereo balanced XLR input to mini-jack for PC type loudspeakers. The unit is intended to be used in journalist news rooms.

www.bcd-audio.co.uk

(JP/MH)