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~ Sugden A48 (Mk1?) Integrated Amplifier repair ~

Above picture shows a pre 1979 J E Sugden A48 integrated amplifier ~ So many variants of this amplifier appear to have been made ~ This one suffered from bad hum and the volume control mains switch had all but disintegrated and would not turn off

Later models were fitted with a separate mains switch ~ Because I could not find a dual 250KΩ log pot with mains switch the neon mains indicator below the volume control was changed to an illuminated mains switch allowing a standard dual log pot to be used

The A48 has separate supplies for each channel ~ The supply voltage to each power amplifier is a single rail +77V taken across C14 and C15 in series ~ hum on both channels was cured by replacing C14 and C15 along with C16 which was the same age and type

All 6 large capacitors are marked on the schematic as 2500µF but in practice 2200µF were fitted and these were replaced with the 6 PCB mounting 4,700µF 100V shown in the picture which unlike the originals are now mounted the correct "way up" for their type

The double pole neon indicator mains switch fitted neatly once the hole of the original orange indicator was enlarged and reshaped

The mains wiring becomes simplified and can be shortened to keep it away from the volume control ~ although hum pick-up at this point was not found to be a problem

After replacing the faulty the C14 and C15s hum was very low but there was a bit more distortion than the spec suggested

The old volume control was working okay and the mains switch could have simply been removed or left disconnected

Although the switch no longer "clicked" it left a bad dead spot when the volume control was turned counter clockwise and at low level the control did not track well

I managed to get an ALPS 27mm pot which was 250kΩ dual log like the original ~ It was held centred in the large original hole using a 10mm shake proof washer



Click on image for positions of C14-C16 and Vr2
The A48 power amp PCBs like the QUAD 303 and other similar designs using a single supply have a control ~ Vr1 ~ to set the d.c. of the output transistors to mid rail ~ 38V

There is also a control on each board to set the quiescent or standing current with no signal ~ Sugden say to remove the fuse and adjust Vr2 for 150-170mA current between the fuse terminals

The above measurement method includes current to the rest of the power amp stage and assumes no faults ~ The setting is higher than many other amps and tends to make the amp run hot so adjustment this way should be done slowly to allow the temperature to settle

Rather than remove the fuses the quiescent current was set using my audio spectrum analyser while checking the voltage across R32 or R33 did not exceed about 50mV indicating about 150mA through VT9 and VT10

A high quiescent current should in theory give low distortion but setting Vr2 for lowest distortion at about 2Vrms output into an 8Ω load allowed the quiescent current to be set lower than 150mA keeping the amp cooler

The A48 power amps are connected to the pre-amp section by a 4 pin plug which can be removed to inject signals to test the power amps separately ~ With the quiescent current set the whole amplifier was tested via the pre-amplifier and found to be working well within spec and sounding very good

Sugden are still going strong and provide excellent support for their new and older products unlike some other "British" manufacturers ~ The Sugden 21 series is a thing of beauty ~ check it out


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