NAIM NAP250 Original clone build thread

[cristobool]

Glad to see your finished amplifier!

I'm surprised what you say in comparisons with the original Naim. Is it the output protection cards that could degrade? Are the MJ15003 output transistors worse than the Naim Originals?

For my part, I have not yet reworked the ground plane in order to reduce the hum of my amps as much as possible. I admit that I was a little tired after all this work and above all I am now enjoying my new amp coupled with my brand new speakers ^^

But I still have my old amplifier to be able to do tests on the ground plane later when I have the time and the courage.

 

[jpk73]

Both my amps have the same protection modules etc., and layout is same, but the Naim has Holden+Fisher transformers and Mundorf caps, the clone has Toroidy transformers and Kemet caps. I think the clone is too tightly stuffed, hence things disturb each other. Also it sounded much better when it was wired outside the case with flying leads. I will try to disable the separate front end PSUs and see if that changes things. BTW the H+F transformers have 2 center tapped windings each, the Toroidys only one, so the rectifier arrangement is a bit different as well.

 

[mdardeniz]

1k resistor at the ZTX653's base, is overheating. I don't understant that. do you have any idea

 

[jpk73]

Is that a 0.75W resistor? The ZTXses become fairly hot, that's normal, but I never noticed the base resistor becoming hot...

 

[jpk73]

For my part, I have not yet reworked the ground plane in order to reduce the hum of my amps as much as possible.

Grund plane...? iirc Naim philosophy is minimal ground. Never saw a ground plane in a classic power amp...

 

[mdardeniz]

Is that a 0.75W resistor? The ZTXses become fairly hot, that's normal, but I never noticed the base resistor becoming hot...

No 0.25W resistor.

 

[jpk73]

Mine is 0.6W iirc...

 

[mdardeniz]

Voltage on the resistor 0.047v. I think it's my fault measurement

 

[jpk73]

Here pictures of the finished amps:

 

[mdardeniz]

which one is better

 

[jpk73]

When I had them wired outside the case with crocodile clip wires my clone sounded better, but mounted in the cases I am not sure. I have to test some more configurations to see if it's because of the front end PSU which was powered differently: I shared the transformer/smoothing caps of the output stage, now it has a dedicated (but smaller) transformer with other types of caps. Also the original boards were powered from the same arrangement, but now there are vintage Holden+Fisher transfomers as you can see in the picture. It seems the original sounds more dark and smooth, the clone more open and textured, but somehow a bit flat in comparison... Any thoughts?

 

[mdardeniz]

Is 3.3uf cap for input? Did you test the sound with tantal cap. and decide for it? (ok not important). Perhaps source and pre amp may cause the difference? it's very difficult to decide if the all system are not the same (for me .

 

[jpk73]

3u3 is decoupling of CCS, a small mod taken from the HackerNAP. Signal path caps, preamp, source etc. are identical. Biggest differences are the transformer and the smoothing caps...

 

[traderbam]

I gather this is the HackerNAP schematic.

 

[jpk73]

Yes, C25 is the added cap which is not present in the original circuit...

 

[Bigun]

I've seen that C25 used in many designs with current sources where it's designed to improve PSRR.

 

[traderbam]

I would expect the HackerNAP to sound quite different to a Naim NAP. The circuit has important differences.

It looks like Hacker tried to change the stability of the circuit by adding LTP emitter resistors, eliminating the phase lead compensation and adding an output coil.

 

[Bigun]

Indeed.

Mind you, I like having the option of the output inductor to eliminate the constraints on the speaker cables etc. It can always be left out.

 

[jpk73]

Yes, it's optional, also the degeneration resistors can be linked out and are only needed in case the BC239s are substituted with BC246s...

 

[Bigun]

I didn't use any LTP degeneration in my clone (which uses a pcb of my own design) but in other projects they have the very helpful function of allowing you to measure the standing current through each leg of the LTP so perhaps low value resistors in that position have some reason to be.