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NAP250 - The Mojo Mod

Jo Sharp said:
You have changed several caps in the signal path. I suggest you leave the amp running for a few days and then listen again- letting the caps burn in, especially in the feedback loop which has tiny voltage and current to deal with, can make a surprising difference.

If after a week you are still unhappy, then make changes.
Click to expand...

Hi,

sure, I'm aware of possible changes - and I still want to swap the main caps as well to a pair of fresh ones.

However, the whole exercise has shown to me how important it is to service the NAIM amps. Surprisingly enough that nobody else is asking customers to do this to other amps. With idle current of 10mA, the amp could have never been running that hot and so it's surprising to learn that the 10uF changed that much. As this have been Vishay BC and not Philips, I'm sure, they must have been changed some time ago.



ATB

KH
 
finkaudio said:
Hi,
my NAP250 that arrived yesterday (bought via the forum) had those 10uF completely broken. One was only 2uF, the best one only 7uF. According to the previous owner, the unit was serviced in 2008! So checking those is always a good idea.
On top of it, the idle current was set too low with 2.3mV across the 0.22 Ohm resistor.

Fortunately, I got those 125° VISHAY caps in 10uF, 47uF and 100uF - so I replaced the ones on the regulator board, changed the feedback capacitor to 47uF MKT (AVX) in parallel with a 47uF Cerafine, changed the Tantalum to fresh originals and added the 100uF Mojo capacitors.

Did a quick listen (as I did when it arrived, but it sounded awful) and decided to let it run over night before doing more.
To be honest, I thought my NAP160 was sounding more open with more lower midband The NAP250 had the better dynamic for sure, but sounded a bit too hot in the upper midband that made it a bit lean.

I did not change the main capacitors yet - just waiting for a fresh set of Felsic.
As alternative, I have a set of KEMET here as well - maybe I'll try them out first and add the bar across the terminals same time.

ATB KH
Click to expand...

Would echo Jo Sharp's comments 100%.

Don't even think about doing the 'Mojo Mod' unless you are absolutely sure the 250 is working as it should - and that would be at least good enough that all but the most picky of audiophiles would be perfectly content with how it performs. Giving it 'Mojo' is the place you go after years of agonising over why a standard (and healthy) 250 can't beat a little old 110 for raw emotion ...

Mr Tibbs
 
Mr Tibbs said:
Would echo Jo Sharp's comments 100%.

Don't even think about doing the 'Mojo Mod' unless you are absolutely sure the 250 is working as it should - and that would be at least good enough that all but the most picky of audiophiles would be perfectly content with how it performs. Giving it 'Mojo' is the place you go after years of agonising over why a standard (and healthy) 250 can't beat a little old 110 for raw emotion ...

Mr Tibbs
Click to expand...

Hi Mr. Tibbs,

it's all up and running since almost 2 days. Amp is stable, idle current as well, no high temperature and the Mojo Mod did not change that.
Added the link between the caps as well - you never know :D
The Kemets are sounding pretty nice I must say. A bit more chest compared with the French caps ( the old ones from 2007).

Best regards

KH
 
Sorry if this is a little off topic- however I'd appreciate some expert advice for my 30 year old 250 please.
I've replaced the mains caps and other electrolytics, however I have read conflicting opinions on whether or not the tants need replacement, would appreciate your good advice on this issue
thanks
bryan
 
After 30 years I would not hesitate to change the input caps. In a 250 they in a much warmer environment than in a pre-amp which will usually accelerate any aging.

And if you don't like the result, you can always put the original ones back in.
 
Jo Sharp said:
After 30 years I would not hesitate to change the input caps. In a 250 they in a much warmer environment than in a pre-amp which will usually accelerate any aging.
Click to expand...

I just finished my 250 and I bypassed the input cap in my amp. I use a NAIM preamp and they have a output cap. No need for 2 ;)


Best KH
 
Before I disappear back into my own Audio world, I would like to report about the mod that I think makes a big difference in imaging and soundstage on my NAP-250.
I replaced the 0.22Ohm and 8.2Ohm resistors on my amp with a TO-220 style low inductance type from a German company called Isabellenhütte.

isabelle.jpg


I have used the PBH range a lot in High-End speaker designs on tweeter crossovers. A 0.22Ohm and a 8.2 Ohm are sitting in the output stage in the NAIM amp - just on the opposite side of the loudspeaker cable. Did the two other 0.22Ohm as well. I know, there are similar resistor types from Caddock, but I found the Isabellenhütte to be the best ones on speakers, so I did not test others here.
For me now, the 250 is finished and can stay as it is. I still have a pair of FELSIC original parts sitting here, but I don't have the feeling, I need a
change.

nap250.jpg


With fresh caps and the mojo mod, the 250 sounds really nice now. So thanks to everybody for the ideas and help.

Best regards and thanks for the fish

Karl-Heinz Fink
 
Very interesting, Karl-Heinz - so I take it the low inductance of the new resistors is in keeping with the original Naim spec?

I see you've added an earth coupling cable on the reservoir caps, what gauge did you use? I was going to try this but can't source a copper link.

mat
 
mmaatt said:
Very interesting, Karl-Heinz - so I take it the low inductance of the new resistors is in keeping with the original Naim spec?

I see you've added an earth coupling cable on the reservoir caps, what gauge did you use? I was going to try this but can't source a copper link.

mat
Click to expand...

The 8.2Om is in series with the 0.22uF cap, both in parallel with the output. One 0.22 Ohm is in series with the speaker cable, the others on the Emitter I checked and found no oscillation on the amp. Not sure what makes the sound of the resistors so special - I don't think it's the low inductance and I don't think the original had high inductance either

I don't have a copper bar, so I used the thickest cable sitting around - guess 4qmm or 6qmm.

Next week, when my electronic engineer is back, we will do a full set of AP measurements. I can post it if you like ;)
 
finkaudio said:
The 8.2Om is in series with the 0.22uF cap, both in parallel with the output. One 0.22 Ohm is in series with the speaker cable, the others on the Emitter I checked and found no oscillation on the amp. Not sure what makes the sound of the resistors so special - I don't think it's the low inductance and I don't think the original had high inductance either

I don't have a copper bar, so I used the thickest cable sitting around - guess 4qmm or 6qmm.

Next week, when my electronic engineer is back, we will do a full set of AP measurements. I can post it if you like ;)
Click to expand...

Yes please, that would be good, thanks.

mat
 
I use 1/6 watt metal film resistors in parallel for the emitter resistors they are cheap, sound good and blow very quickly in the event of a fault.

Pete
 
finkaudio said:
Before I disappear back into my own Audio world, I would like to report about the mod that I think makes a big difference in imaging and soundstage on my NAP-250.
I replaced the 0.22Ohm and 8.2Ohm resistors on my amp with a TO-220 style low inductance type from a German company called Isabellenhütte.

isabelle.jpg


I have used the PBH range a lot in High-End speaker designs on tweeter crossovers. A 0.22Ohm and a 8.2 Ohm are sitting in the output stage in the NAIM amp - just on the opposite side of the loudspeaker cable. Did the two other 0.22Ohm as well. I know, there are similar resistor types from Caddock, but I found the Isabellenhütte to be the best ones on speakers, so I did not test others here.
For me now, the 250 is finished and can stay as it is. I still have a pair of FELSIC original parts sitting here, but I don't have the feeling, I need a
change.

nap250.jpg


With fresh caps and the mojo mod, the 250 sounds really nice now. So thanks to everybody for the ideas and help.

Best regards and thanks for the fish

Karl-Heinz Fink
Click to expand...

Good stuff. When a 250 is sounding 'really nice' you've got a front row seat at whatever concert takes your fancy :)

Mr Tibbs
 
So spurned on by the positive comments in this thread, i decided to try out the "mojo" mod on my 135s.

At the same time I serviced the amps, so cannot entirely say where the improvement came from, but net result was "OMG, that is unbelievable!"

Thanks for the great tip!
 
Good stuff and thanks for the feedback. Should really have titled this thread as '250/135 Mojo Mod' as no reason why the 135 won't benefit in the same way the 250 does since they both share the same regulator.

Mr Tibbs
 
I have read with very much interest the mojo mod and I am really impressed.
As the issue is to clean up the supply rail noise at mid/high frequencies why not using a low value plastic film cap in the 0'1 mF/1mF range?
It would be faster and more efficient at high frequencies than a polarized electolytic.
Thanks to Mr. Tibbs for the idea.
I would very much appreciate feedback on my idea.
Thank you.
Andrea
 
Using low value film caps will almost certainly risk turning the PSU into an oscillator - not recommended in other words!

The cap type and values chosen were based on getting worthwhile performance gains without risking amp damage.

Mr Tibbs
 
I used to have s pair of 2.2uF film caps across the output of my reg boards in my 135 clones, cleaned things up nicely.

Pete
 
Hmm... 4 pages discussing something which will have no effect and is predicated on a lack of understanding of voltage regulators...
 
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