Radford Electronics
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Radford Electronics was a renowned British audio manufacturer of the 1960s, 70s and 80s started by Arthur Hedley Radford in 1961, when the STA range of Amplifiers was launched along with the DSM pre-amplifier. The series 3 STA25 along with other Amplifiers in the STA range won critical acclaim. An expert in transformer design, Arthur Radford was able to reduce distortion by a factor of ten over the best performing amplifier of the day. The MK11 and Series 3 Amplifiers used a unique pentode phase inverter that was a joint venture between Dr A R Bailey and Arthur Radford and was the subject of a detailed article in Wireless World.
However Arthur Radford’s greatest love was for his loudspeaker designs and the rear termination of drivers. After many years of work he eventually took out a patent in conjunction with Dr A R Bailey for the first transmission line loudspeaker. This system was the basis for all his larger designs and is still used by many well-known manufacturers.
Although today the valve amplifiers attract the most attention from enthuiasts, the pinnacle of Radford designs was, without doubt, the "Zero distortion" series of transistor power amplifiers produced in the mid 1970s in which THD was reduced to less than the circuit noise level. These (and other products such as the Quad 405) achieved the near perfect measured performance for which designers had been striving since the earliest days of the industry. Left with no potential for objective improvements in quality, the industry either regressed to valve designs (such as the hybrid Radford TT100) or lost it's way in subjectivity or "black magic" such as OFC uni-directional interconnections.
Another well known Radford product was the "Labpak" low voltage power supply, used in schools and colleges both in the UK and overseas
At the peak Radford had four factories employing 160 personnel producing metalwork, painting, printing, transformer and PCB manufacture all in house.
As Arthur approached retirement the company was wound down as there was no one to take it on. After he retired in 1989 Woodside Electronics was formed by John Widgery, who had been apprenticed to Radford Electronics and was responsible for many of the classic designs. Mike Davis joined John as a partner in 1989 and they worked together to launch the CD player and update the range of valve equipment as part of the Renaissance series. Arthur died on the 21st of November 1993 but Woodside continued for a while and ended in a small way in Llanbedrog, Pwllheli, North Wales in 2004.
Radford valve amplifiers are rare today and change hands for hundreds of pounds or even more. Much sought after and enthusiastically rebuilt, they are particularly known for a most fortunate and restrained physical design and outstanding and listenable sound quality - including a particular ability to drive, very sweetly, the notoriously difficult Quad electrostatic loudspeakers. 2006 brought hope of a rebirth for the brand, with news of plans to begin manufacture of classic valve based designs like the STA 25 Mk III in 2007 [1].
The Radford forum, a Yahoo group of enthusiasts from all over the world, maintains substantial files of schematics, pictures and discussions.
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[edit] Radford amplifiers
Below is a (incomplete) list of Radford designs in an attempt at chronological order. The prefix STA indicates a stereo amplifier. The prefix MA indicates a mono amplifier. The number indicates the nominal power rating.
Original Radford amplifiers:
| Model | Information | |
|---|---|---|
| STA12 | (1960) Used EL34 output tube and EF86 with ECF82 driver tubes. The rectifier was a GZ34. Produced just over 12 Watts per channel and based on a Mullard 5/20 with good output transformers and some careful LF and HF phase compensation to keep the unit stable. | |
| MA15 | power Amplifier. Single channel version of the STA12 with slightly higher power output. | |
| STA7 | (1960) Very rare dual channel amplifier giving about 8 Watts from a pair of push-pull ECL86’s. | |
| STA15 mk2 | (1962) As above but output = 15 wpc | |
| STA15 series 3 | (1962) As above. | |
| STA25 series 3 | (1965) The first stereo version of the MA25 gave a conservative 25w per channel from EL34 output tubes and EF68 with ECF82 driver tubes. This amp is sometimes confused with the STA15. It can be identified by its manual bias adjustment and solid state rectifier both of which are not present on the STA15. | |
| ISTA60 | 60wpc. | |
| STA100 | driver tubes were ECC88 and EF184. A solid state rectifier was used. | |
| MA15 | (1961) First monoblock, valve line up = EF86 ECC83 EL34X2 EZ81 rectifier | |
| MA15 Series 3 | (~1961) As above but uses EF86 ECF82 GZ34 Valve rectifier. | |
| MA25 | (1962) Valves used = EF86, ECF82, EL34X2, Diode bridge rectifier. | |
| MA25 mk3 | (1962) As above. | |
| K25 | A modified version of the MA25 for use with a BBC monitor loudspeaker manufactured by KEF. It featured a frequency shaping network, to flatten the response of the speaker, and a balanced input. |
Radford pre-amplifiers/control units
- DSM(~1961) Beautifully made control unit with 3 EF86’s and 1 ECC83’s per channel, a lot of electronics in a very small box.
- SC2 A pre-amplifier with passive tone control system.
- SC3 A three channel version of the SC2.The third derived channel was for a separate centre loudspeaker.
- SC22 Feedback controlled tone controls, transistor disc stage and selective frequency RIAA equalisation network. There was also a switchable feedback rumble filter incorporated around the valve stage.
- SC22p As SC22 with power supply.
Radford "Zero distortion" (distortion below circuit noise level) transistor amplifiers
- ZD50 & ZD100 stereo power amplifier
- ZD22 preamplifier
- HD250 stereo integrated amplifier
Later Radford hybrid valve & transistor amplifiers
- TT100 stereo power ampliier
Woodside Electronics amplifiers
- STA25 renaissance
- STA35 renaissance
2007 Radford amplifiers
- STA25 mk6
 STA15 MK3 and SC22 preamp |
 A glowing example of a STA15 MK3 |
 A Radford STA15 MK3 amplifier with its top cover removed |
[edit] Radford loudspeakers
Radford also manufactured Loudspeakers, Tuners and other audio gear. A (incomplete) list of these is below:
Tristar T90
Studio S90
Monitor M180
Studio S270
Studio S360
Monitor 90
[edit] Radford Tuners
FMT1
FMT2
FMT3
FMT4
FMT5 prototype produced, but never manufactured
[edit] Radford Lab power supples (specifications approximate)
Model ?? 0-24V output switchable in 0.2V increments @ 2.0A and 240V @ 100mA
Model ?? Continuously variable 0-24V output with current limit variable up to 2.0A
[edit] Radford test gear
Arthur Radford was designing such high specification amplifiers for the time period that he was having problems measuring their performance due to limitations of the test gear. Radford set about designing new ultra-linear measurement and test tools to solve this problem.
| Model | Information |
|---|---|
| DMS Series 3 | LF Total Harmonic Distortion (THD) analyzer, to 0.003%, 10Hz-50k, 1mV-50v, portable. |
| LDO series 3 | Low Distortion Oscillator. The instrument harmonic distortions are about 120 dB below the fundamental. |
