I sold Armstrong receivers in a shop called Westwoods in Oxford . Recently I took a 625 out of hibernation . As all have said a lovely sound . I also have a Kenwood KR 750 which is equally loved .
Westwoods was connected with Armstong as he ( sorry I forget the name ) bought his components from Leslie Westwood when a student . This man visited me not long before his death to show the 800 series ( fantastic and advanced design ) . He named the company after his favourite motor car an Armstrong Siderley if I remember it correctly .
I find a remarkable quality in this design apart from those stated . It never is loud . The sound is to be heard everywhere in the house thus is loud .
Technically, the power amplifier design uses a single input transistor having a double feedback loop with double NPN quasi complimentary outputs. The ouput is capacitor coupled . The driver load is by bootstrap capacitor ( A high grade here might be an idea . 50 uF from memory . 47 uF 100 v now ) I remember Armstong saying testing would have to be with a dummy loudspeaker network to get the stated distortion . Hi Fi news retested and confirmed that . The drivers are TV CRT transistors. A good choice as the requirements are similar . I wonder if this real loudspeaker loud situation explains the above average sound ( many tube amps are not this good ) . Also it is rare to take feedback after the feedback capacitor . It can make an amplifier unstable , Armstrong made it work .
The only modification I would recomend is to replace the power amplifier input coupling capacitor with a 1 uF film type . This will reduce volume control noise and is big enough to do the job . Armstrong use 10 uF from memory . Check the big power supply capacitor from time to time . Any increase in hum will be a danger sign . WD 40 is excellent for restoring switches etc , I suspect it is sonically the best of such treatments .
The FM is as said a reference for other designs .
Westwoods was connected with Armstong as he ( sorry I forget the name ) bought his components from Leslie Westwood when a student . This man visited me not long before his death to show the 800 series ( fantastic and advanced design ) . He named the company after his favourite motor car an Armstrong Siderley if I remember it correctly .
I find a remarkable quality in this design apart from those stated . It never is loud . The sound is to be heard everywhere in the house thus is loud .
Technically, the power amplifier design uses a single input transistor having a double feedback loop with double NPN quasi complimentary outputs. The ouput is capacitor coupled . The driver load is by bootstrap capacitor ( A high grade here might be an idea . 50 uF from memory . 47 uF 100 v now ) I remember Armstong saying testing would have to be with a dummy loudspeaker network to get the stated distortion . Hi Fi news retested and confirmed that . The drivers are TV CRT transistors. A good choice as the requirements are similar . I wonder if this real loudspeaker loud situation explains the above average sound ( many tube amps are not this good ) . Also it is rare to take feedback after the feedback capacitor . It can make an amplifier unstable , Armstrong made it work .
The only modification I would recomend is to replace the power amplifier input coupling capacitor with a 1 uF film type . This will reduce volume control noise and is big enough to do the job . Armstrong use 10 uF from memory . Check the big power supply capacitor from time to time . Any increase in hum will be a danger sign . WD 40 is excellent for restoring switches etc , I suspect it is sonically the best of such treatments .
The FM is as said a reference for other designs .