NAD 3020 Series 20 mk-3 MOSFET Output Conversion

[poundy]

So this is the doner amp that is going to be used for the project, its in almost mint condition and i dint want to break it(unless i have to lol)

Its what i call a mk3, this is the same board they used as they moved onto the 'A' version

So things that need to be done

Modify the base of the heatsink so it will accept the FETS
Modify the print so it maches the pin out

so this is the scheme that needs to be used for this one

ive never been able to find an official 'A' service manual, but this is close

Its easily recognised by the fact the regulated supply is now on the main pcb

 

[jacques antoine]

Laterals?

 

[poundy]

Laterals?

Oui (yes)

 

[jacques antoine]

Just a simple bjt <-> mosfet swap?
Have you already seen it done successfully ?

 

[poundy]

Just a simple bjt <-> mosfet swap?
Have you already seen it done successfully ?

not personaly no

 

[wg_ski]

The same basic design often works with little or no modification other that the adjustment range on the vbe multiplier. You get a few watts less with the mosfets, unless the drive voltage can be enhanced to drive the gates a bit above the rails. Even thats not essential, you can just live with 3 volts or so less output swing.

 

[jacques antoine]

Beyond mod fever , I fail to see any improvement except the negative tempco of the laterals.

 

[wg_ski]

Some people just like lateral mosfets. Another cult which you can join .
They can also handle a momentary overload which would normally kill 2N3055’s.

 

[jacques antoine]

If I remember well the 3020 is only a 20W amp: difficult to abuse 2N3055's with that amount of 'raw' power .

 

[wg_ski]

No SOA protection + shorted speaker wire. 3055’s won’t always tolerate it. Laterals don’t either, but your chances of winning that lottery are MUCH higher.

 

[jacques antoine]

Still not convinced that a NAD3020 in 'almost mint condition' deserves such a butchering.

 

[huggygood]

let's not kill the bear before we sell the skin.
I'm waiting to see the final result and anyway, it will surely advance the debate on the "for or against" in general 😜

 

[Mooly]

Some people just like lateral mosfets. Another cult which you can join .

In case anyone is wondering... so you're not OK then

Well it started back here:
https://www.diyaudio.com/community/...ct-strip-down-and-upgrade.386581/post-7084088

Uncharted territory.

 

[jacques antoine]

let's not kill the bear before we sell the skin.

I prefer 'if it ain't broke, don't fix it'.

 

[wg_ski]

Given the choice, I’d start with one that needed work anyway if I was doing this. Like the one that got stripped down. Or at least one that needed new output transistors.

 

[jacques antoine]

Fixing? Nah!
Mod extravaganza is the issue here.

 

[huggygood]

I prefer 'if it ain't broke, don't fix it'.

I keep 😁

 

[mlloyd1]

i've never looked at the NAD3020 sch before. does it really use the 2n3055 and no emitter resistors?

 

[Ian Finch]

Yes, but those are original hometaxial type 2N3055s, not the present day epitaxial type. Certainly, we'd need to add the emitter resistors and re-arrange the bias to use more readily available epitaxial product. There are actually many 3020 product series repair threads here that cover or at least draw attention to this.

 

[jacques antoine]

Yes, but those are original hometaxial type 2N3055s, not the present day epitaxial type.

Hi Ian,
Can you discriminate between the two types using some measurement (without opening them) ?
I got a bunch of different brands recycled from old equipement.