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NAP250 - The Mojo Mod

mmaatt said:
Thinking about this, and wanting to do something which is reversible, I will try connecting a separate cable from each of the new paired caps 0V points, soldering the other ends to crimp connectors and bolting one to the reservoir cap's common + and one to the other's common -ve. I will try this without a bus bar so I'm only hearing the one mod.

mat
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For testing purposes you could common the two 0V cap leads on a crocodile clip, allowing a quick method of finding the best place to take the 0V connection before making things more permanent (switch off the amp before moving the clip). As all the 0V lines terminate at the speaker 0V terminal junction you might want to try there. FWIW I don't think there is any issue with grounding the 'Mojo' cap's at the Zobel ground on the amp PCB though slightly better performance is (in theory) possible by using a separate ground.

Mr Tibbs
 
The positive side of the dual railled twin cap should be soldered to the underside of the red wire push down connector on the amp board and the negative to the underside where the black wire from the smoothing caps goes.
Am I wrong?

Cheers
 
Harthold said:
The positive side of the dual railled twin cap should be soldered to the underside of the red wire push down connector on the amp board and the negative to the underside where the black wire from the smoothing caps goes.
Am I wrong?

Cheers
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Seems correct. One cap goes between 0V and the +Ve rail, with the neg lead of the cap going to 0V. The other cap goes between 0V and the -Ve rail, with the neg lead of the cap going to the -Ve rail. Get the polarity wrong and you'll be picking up bits of capacitor for days.

A slight refinement in midrange
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What has it brought to the music (if anything) though?

I can clearly hear subtle use of vibrato from guitars that was only hinted at before, for example. There are other subtle improvements going on that collectively add significant enjoyment to the music to my ears. The 401s are very revealing of this so YMMV.

Mr Tibbs
 
I'm using a pair of Saras with 220uf mod and I think I have a bit of bass now. I let it run for a few days and report back.
Cheers
 
Harthold said:
I'm using a pair of Saras with 220uf mod and I think I have a bit of bass now. I let it run for a few days and report back.
Cheers
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You mean you were using a 250 and Saras and didn't have any bass before the mod? Interesting, I never say no to a bit more bass!

Expecting to get the caps tomorrow, I may try putting them in from the top of the board if possible.

mat
 
Mr Tibbs said:
For testing purposes you could common the two 0V cap leads on a crocodile clip, allowing a quick method of finding the best place to take the 0V connection before making things more permanent (switch off the amp before moving the clip). As all the 0V lines terminate at the speaker 0V terminal junction you might want to try there. FWIW I don't think there is any issue with grounding the 'Mojo' cap's at the Zobel ground on the amp PCB though slightly better performance is (in theory) possible by using a separate ground.

Mr Tibbs
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Thanks for the advice Mr Tibbs. I think on reflection I will try the mod as you have done it; if I like it I may contemplate a separate earth using the same spec cable as Naim used. I'm also not sure my 50 watt Antex is up to the job of soldering onto the earth bar efficiently.

mat
 
I meant quite more bass. It reminds me when I started using the Saras with an MC2200 MacIntosh which was a vintage sound but not as fast as Naims
 
Harthold said:
I meant quite more bass. It reminds me when I started using the Saras with an MC2200 MacIntosh which was a vintage sound but not as fast as Naims
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Really? I heard no difference to the low bass after the mod, which is what I would expect given the small amount of extra capacitance involved - chosen deliberately in order NOT to affect the bass region. The whole idea was to clean up supply rail noise at med/high audio frequencies with a view to improving performance there, which has worked nicely for me. Are you absolutely sure you have connected the cap's up correctly?

By the way - usual caveats apply - this worked for me and the amp seems perfectly stable and happy since the mod, but you mod your own amp at your own risk!

Mr Tibbs
 
My 250 is running too hot. Is it safe to use it as such? Have I made something wrong. I've just disconnected it Should I try 100uf instead?
The midrage is gorgeus with 220uf. I have to admit. Pity, for the moment
 
Harthold said:
My 250 is running too hot. Is it safe to use it as such? Have I made something wrong. I've just disconnected it Should I try 100uf instead?
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Best leave it disconnected - you risk damaging it otherwise.

If it's running hotter than normal that would indicate instability. Has the amp had a service in the last 10 years? If not you'd need to replace the 10uF cap's at the output of the reg boards and carefully check the bias current is correctly set.

Mr Tibbs
 
The regulator hasn´t been touched ever. I'll send away for the axial blue Phillips 10uf caps if this is what matters. I'll check the bias on the amp boards later afterwards.
Thanks Mr,Tibbs for the tip and Mr Arkless Repairs for the offering. Unfortunately I'm far away from your place.
Is it any other bit that I must change/check?. The amps boards feature wimas at input and EVOX FILM for feedback caps. All the rest as original. Does it have to be 10uf Phillips or can be any other good brand even if they're not axial?
Could it be that the Mojo mod conflicts with the "BAR" mod recommended by Les at the smoothers?
I´ll also follow Les's suggestion of taking back the 0 of the two 220uf caps to the main smoothers caps bar. Thanks Les too.
But that in due time
Cheers folks
 
Harthold said:
The regulator hasn´t been touched ever. I'll send away for the axial blue Phillips 10uf caps if this is what matters. I'll check the bias on the amp boards later afterwards.
Thanks Mr,Tibbs for the tip and Mr Arkless Repairs for the offering. Unfortunately I'm far away from your place.
Is it any other bit that I must change/check?. The amps boards feature wimas at input and EVOX FILM for feedback caps. All the rest as original. Does it have to be 10uf Phillips or can be any other good brand even if they're not axial?
Could it be that the Mojo mod conflicts with the "BAR" mod recommended by Les at the smoothers?
I´ll also follow Les's suggestion of taking back the 0 of the two 220uf caps to the main smoothers caps bar. Thanks Les too.
But that in due time
Cheers folks
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OK, first thing you need to do is make sure the amp is working properly before attempting any mod's. Replace those blue Philips 10uF caps but use a decent 105C rated cap instead since the originals can dry out in as little as 5 years use. These cap's are absolutely critical to getting stability and good sound, as is the correct bias setting.

Once the above has been done and the amp is sounding good you could try reintroducing the Mojo mod and use the alternative grounding arrangement as suggested by Les, since it would at least rule out any possible interaction with the Zobel network.

Mr Tibbs
 
Mr Tibbs, this is exactly what the HackerNAP boards do. I started with 470uF/ Panasonic FC on each of the +/- rails, with each 0v tied directly back to the 0v point on the PSU board (just like Les suggested above).

Now I use MMK 47uF/63V polyesters and they blow the 470uF out of the water in terms of refinement and bass control. It's a total bodge to make 'em fit, but well worth it.

However, I do not know if the NAP250 reg boards would be stable with 47uF film caps on their outputs (the HackerNAP has an unregulated output stage), so caveat emptor!
 
Mr Tibbs said:
OK, first thing you need to do is make sure the amp is working properly before attempting any mod's. Replace those blue Philips 10uF caps but use a decent 105C rated cap instead since the originals can dry out in as little as 5 years use. These cap's are absolutely critical to getting stability and good sound, as is the correct bias setting.
Click to expand...

All good stuff.

But - wider warning - stick with axial caps for the output of the NAP250 reg boards, don't be tempted to use 'Low-esr' or film types here thinking that less is better.

It can be done, but requires a small value series resistor to trim and a 'scope to check stability ...when the original 10uF Philips / BC blue axial electrolytic , or prefereably higher-temp rated equiv as Mr Tibbs suggests - is just what this old reg requires: the few tenths of an ohm ESR is very, very useful.
 
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Caps soldered up - I desoldered the relevant pins and threaded the cap wires through the hole from the top, taking care that male socket blades are firmly seated while resoldering from underneath with circuit boards removed from case.

Everything is working ok, amp cold after 30 mins switch on - now for a listen!

mat
 
After an evening's listening taking in a Bach Cantata, some Can, Doors and Hendrix, I have to say I'm mighty impressed with this little mod. There is more transparency and three dimensionality and better resolution of the edges of vocals and instruments; bass guitar and individual drums really stand out being more tuneful and solid.

I played at moderately loud volume and the 250 got mildy warm on the bottom edge of the case driving my Isobariks with self-built crossovers. Running idle it feels cool so I don't think there is any problem with these additional caps in the circuit.

Thanks Mr Tibbs!

mat
 
Glad it is working well for you mat and good to see your findings pretty much exactly match what I'm hearing.

Mr Tibbs
 
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