Hi Jim,
Had a similar problem with mine. The bias link across R456 had gone o/c, burning out R456. Unfortunatley mine went +VB (33V) at output, therefore burning out the voice coil of the R/H speaker, an old Tannoy HPD315.
Having repaired the amplifier (i still don't really trust it), it works fine. i need to get one of me valve amps going - no chance of DC output from an output transformer, eh ?
The NAD 302 is a nice design but in my humble opinion far too complicated. i far prefer MOSFET to bipolar trannys.
Attached is the service manual.
Neil xx.
Had a similar problem with mine. The bias link across R456 had gone o/c, burning out R456. Unfortunatley mine went +VB (33V) at output, therefore burning out the voice coil of the R/H speaker, an old Tannoy HPD315.
Having repaired the amplifier (i still don't really trust it), it works fine. i need to get one of me valve amps going - no chance of DC output from an output transformer, eh ?
The NAD 302 is a nice design but in my humble opinion far too complicated. i far prefer MOSFET to bipolar trannys.
Attached is the service manual.
Neil xx.
Thanks for the circuit Redbeard --hifi engine thinks I am a bot .
Your right this is no NAD 3020 which I have in my room somewhere .
Yes I too prefer mosfets to BJT,s --as per JLH , you may not know it but that started a "war " on EW between D.Self and JLH --sides were taken .
Its also the preferred output devices of a well known Italian audio design engineer who put in his point of view with various high slew rate power amp circuits.
Your right this is no NAD 3020 which I have in my room somewhere .
Yes I too prefer mosfets to BJT,s --as per JLH , you may not know it but that started a "war " on EW between D.Self and JLH --sides were taken .
Its also the preferred output devices of a well known Italian audio design engineer who put in his point of view with various high slew rate power amp circuits.
No worries, duncan2,
i rather like NAD stuff. Although it is cheap (well, inexpensive), NAD have clearly put in a very great deal of R&D time into their amps.
Their 3020 which you have is a classic. The right amp at the right time for the right price. The first NAD i got (before the ill-fated 302) was a 310 from a local guy off a well known auction site.
A very peculiar power amp circuit, half MOS and half BJT and relying on a giant electrolytic to decouple the output. Intriguing sound, which is strangely involving. Replace supply capacitors and output coupling capacitors if you try one.
Being a valve guy, i only bought the NAD 310 as a temporary replacement for my 6080 power amp which i failed to simplify (removing the first gain stage {ECC82 anode-follower, gain 12}, outside the feedback loop and upping the gain of the phase splitter and driver stages) - not enough gain. Reduced the ammount of feedback to up the gain, which worked but it sounded shiite. No bass control.
So i bought a cheap NAD 310. Sounds great but has an unusual output stage. Bought a NAD 302 due to it having a more symetrical output stage, with results as stated in post #6.
Although it's completely off-topic, and i will be criticised, have you ever considered trying depletion-mode MOSFET's in a power amp ?
i never have but like the way they should bias, kinda like a JFET or a valve ?
Neil xx.
i rather like NAD stuff. Although it is cheap (well, inexpensive), NAD have clearly put in a very great deal of R&D time into their amps.
Their 3020 which you have is a classic. The right amp at the right time for the right price. The first NAD i got (before the ill-fated 302) was a 310 from a local guy off a well known auction site.
A very peculiar power amp circuit, half MOS and half BJT and relying on a giant electrolytic to decouple the output. Intriguing sound, which is strangely involving. Replace supply capacitors and output coupling capacitors if you try one.
Being a valve guy, i only bought the NAD 310 as a temporary replacement for my 6080 power amp which i failed to simplify (removing the first gain stage {ECC82 anode-follower, gain 12}, outside the feedback loop and upping the gain of the phase splitter and driver stages) - not enough gain. Reduced the ammount of feedback to up the gain, which worked but it sounded shiite. No bass control.
So i bought a cheap NAD 310. Sounds great but has an unusual output stage. Bought a NAD 302 due to it having a more symetrical output stage, with results as stated in post #6.
Although it's completely off-topic, and i will be criticised, have you ever considered trying depletion-mode MOSFET's in a power amp ?
i never have but like the way they should bias, kinda like a JFET or a valve ?
Neil xx.
No sorry Neil I have never tried Depletion mode MOSFETS although JLH gave a warning about the Lateral ones as being "tricky " to keep stable .
Yes the bias is different and the bias circuit has to be designed differently and your right "more like a valve ".
An introduction to Depletion-mode MOSFETs - IntroDepletionModeMOSFET.pdf
Yes the bias is different and the bias circuit has to be designed differently and your right "more like a valve ".
An introduction to Depletion-mode MOSFETs - IntroDepletionModeMOSFET.pdf
Hi big jim,
Yes, the 302 was a later circuit but not, i believe, based on the 3020. See attached service drawing. The 3020 used the old TO3 2N3055 and 2955 output transistors and is a far simpler circuit.
More difficult to mount (a TO3-P would have been easier, had they existed back then). The 302 is also significantly more complicated.
All the best,
Neil xx.
Yes, the 302 was a later circuit but not, i believe, based on the 3020. See attached service drawing. The 3020 used the old TO3 2N3055 and 2955 output transistors and is a far simpler circuit.
More difficult to mount (a TO3-P would have been easier, had they existed back then). The 302 is also significantly more complicated.
All the best,
Neil xx.
Attachments
Cheers duncan2,
i'll give that article a read later on. Maybe not biased "like a valve", more like a JFET ? Same thing really. Grid or Gate at Cathode or Source potential = it's fully "on", Grid or Gate at lower than Cathode or Source potential it turns gradually "off".
Perhaps making them good as a constant current source or sink ?
All the best,
Neil xx.
i'll give that article a read later on. Maybe not biased "like a valve", more like a JFET ? Same thing really. Grid or Gate at Cathode or Source potential = it's fully "on", Grid or Gate at lower than Cathode or Source potential it turns gradually "off".
Perhaps making them good as a constant current source or sink ?
All the best,
Neil xx.
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