Sugden A28 amplifier

The Sugden A28 once again gives you the unsurpassed quality of pure Class "A"

This is how we regard the Sugden A28 amplifier. Based upon well proven Sugden output circuitry, we now bring you an Amplifier which is second to none in its sound reproducing quality.

Let's start with the input circuitry, accessed via easy to use phono plugs and sockets and selected by four simple push-on, push-off switches. On the disc input not only do we have the facility for an internal plug in Moving Coil Module (the A 28 CTHA) deriving its power from within the Amplifier, but also a brand new "series-shunt" disc Amplifier with precision RIAA equalization and two level gain adjustment. In fac, everything to suit the cartridge of your choice.

Next the Tuner, Auxiliary and Tape inputs. All these three have identical input impedances, allowing a wide range of ancillary equipment to be used without interface problems.

The Tape output signal can, of course be fed from any of the input signals. For those people with 3 head Tape Machines, the Tape button will allow immediate post tape recording/replay through the Amplifier, i.e. Tape Monitor.

 

No tone controls here to marr distortion figures, just volume and balance controls with simple but effective filter circuits for hiss and rumble attenuation. Don't forget the Mono Button. Still essential for much recorded material.

Observe the neat, unfussy and easy to clean case style. Front and rear sections are matt black extrusions, whilst the side and top plates are covered in grey textured "Nextel" paint, giving a very attractive two tone finish.

As to how it sounds, well the specification, amy give you some clue, but we believe you will be delighted at the clear transparent sound.

The very low harmonic distortion of a class "A" Amplifier gives fatigue free listening to please even the most discerning ear. All this and more. Sugden expertise brings you the "S.Q.C" circuitry to give an amazingly compact pure class "A" output stage with up to 35 watts in class "B" allowing all but the most inefficient loudspeakers to be driven. Backed by the Sugden workmanship for life guarantees, we are justifiably proud to present this high quality British product.

 

Features:

20 watts pure Class "A" per channel

Continuous reserve capability of 35 watts per channel

Budget price (top quality sound at a price you can afford)

With additional module, can be used with Moving Coil cartridge

Variable gain disc input

Very low distortion

New precision RIAA equalisation network

Built to Sugdens uncompromising standards

Carries Sugden workmanship "for life" guarantee

 

S.Q.C. is short for Switched quiescent Current

And its development gives new potential to the cless "A" amplifier. We at Sugden have always believed in the class "A" amplifier and , in fact, were the first company in the world to produce the all transistor class "A" amplifier in 1968. it was only the demand for more output power which forced us away from the 12 watt A21 pure class "A" amplifier into the higher power "AB" amplifiers, such as the A48 and the P51 in the majority of casses were music is played through reasonably efficient 8 Ω  loudspeakers, the average power used is probably not more that 3 to 4 watts with excursions to higher levels depending upon the dynamic rrange of the recorded material.

Also what is the point of burning off power when it is not required. This then is the philosophy around which the S.Q.C. circuit has evolved. The availability of high speed, low level, switching components enables us to control the "AB" circuit to its very best advantage.

The A28 operates continuously at a quiescent level of 10 watts in Class A, i.e. with a current in the output transistors of about 800 mA. By looking at the incoming signal level, after the potentiometer, we can determine whether the 10 watt output level will be exceeded and should this be the case increase the quiescent current level up to 1,2A or 20 watts. The current will stay at this level for as long as the heavy signal exists and also for a short time afterwards, before reverting back to the lower quiescent level. There is, however, a finite switching time of the S.Q.C. circuit, admittedly it is only about one millionth of a second, but under some very fast transient signal conditions there is just a possibility that if the switch sensing point is set at the same level as the quiescent current level, the Amplifier may slip momentarily into class "B" before reverting back to class "A". This is obviously unacceptable and in order to prevent it, we set the sensing a elactronics to switch at a level of 7 watts allowing a 3 watts reserve, giving time for the output stages to react to the increased demand.

Using this technique we can absolutely guarantee a full 20 watts in class "A" at all times.

The additional overhead level between 20 and 35 watts in class "B" allows for the wide dynamic range of some recorded material and since it is far into the linear range of the devices, there is little increase in distortion right up to 35 watts.

 

Specifications

Disc Input Magnetic

Sensitivity :  Internally Selectable to 1,7 mV or 3,5 mV for 35 watts into 8 Ω

Signal-to-Noise Ratio :  better than 65 dB unweighted

Frequency Response:  Equalised to within ±0,5 dB of RIAA characteristics 20 Hz to 20 kHz

Input Impedance:  47 k Ω  in parallel with approx. 50 pF

Overload :  margin better than 35 dB at all frequencies

Moving Coil Amplifier

Provision is made with the A28 to lift the A28-CTHA Moving Coil Amplifier Module.

Inputs sensitivities for this unit are 40 µV, 90 µV, 180 µV depending upon the internal sensitivity switch position in the Module and also in the main Amplifier.

This head amp. Uses a novel input circuitry which obviates the need for impedance matching.

Tuner, Aux, Tape Inputs:

Sensitivity :  100 mV into 180 k Ω  for 35 watts into 8 Ω

Tape output

Dependent upon input selected Typically 100 mV from disc Amplifier

Tape Monitor

Allows direct "AB" comparison from Tape Recorder on 3 head Machines

Filters

Low Filter:  is a rumble filter with attenuation of 18 dB /octave below 30 Hz

High Filter :  is a hiss filter with an attenuation of 18 dB/octave above 10 kHz

Mono Button:

Allows the incoming signal to be mixed and applied equally to both channels

Loudspeakers:

The Amplifier is suitable for any 8 to 16 Ω  loudspeaker - 4 Ω  speakers can be driven with reduced power output.

Connection is via 4 mm "banana" plugs

Headphones

Any headphones of 8 Ω  to 2 k Ω  impedance may be operated. Connection is by standard 1/4" stereo jack socket on the front panel. The loudspeakers are automatically muted when headphones are used.

Frequency Response:  20 Hz to 20 kHz ±1 dB referred to 1 kHz

Cross Talk (stereo):  better than -35 dB at all levels

Load Stability:  Unconditional

Damping Factor: better than 35 into 8 Ω

Power Output (Both Channel):

20 watts in Class "A" into 8 Ω

35 watts in Class "B" into 8 Ω

Distortion

Class "A" :  0,04 at 1 kHz (20 watts)

Class "B" :  0,06% at 1 kHz (35 watts)

Power Supplies:  Internally adjustable to 110, 120, 220, 230, and 240 V AC

Mains Supply Frequency:  48 / 60 Hz

Total Power Consumption:  220 watts

Auxiliary Power Output:  a 3 pin IEC socket with mating plug is provided. The output is switched live via the Amplifier power on switch. Max. Load from socket is 250 watts

Power ON Lamp:  indication that the Amplifier is operating is given by a light emitting diode on the front panel adjacent to the power on switch.

Dimensions (W x H x D):  400 x 88 x 267 mm

Weight:  6,5 kg