High DC offset in just-recapped CB NAP 135s...

[hifiaf]

Hi everyone,

I recently took possession of some CB NAP 135s. They needed a service, but they came to me with a recap kit. I plugged them in on receipt, and there was some crackling in one channel, so I proceeded straight to the service.

The recap kit is all standard Naim-approved components. Everything was a like-for-like substitution. I only serviced the regs and amp boards.

I completed the service yesterday, and have spent this afternoon checking the bias (fine), the power rails (fine) -- and the DC offset: not fine!

I've got 190mV in one amp and 130mV in the other.

Any ideas on what might be up? I've checked I've got the right values of tants and caps everywhere and that the orientation is all correct.

I'm hoping it's some obvious thing I've done wrong but naturally I'm a bit worried it's the transistors. But it seems strange that both amps would be affected. Unfortunately I didn't measure DC offset before servicing.

From other threads I see some recommend getting the amp in its case and warmed up to see if it stabilizes.

 

[hifiaf]

Reading around some more, it seems likely I’ve done this. We’ll see if some replaced feedback tants sort me out... and no more setting bias that way! Emitter resistors forever!

Whatever you do DON'T turn the thing on with the red + power lead disconnected! That's what I did, to measure the Amps drawn, and I buggered up one or more of the tantalum caps, and was then getting high DC offset at the speaker terminals.

 

[hifiaf]

Okay, yes, that seems to have been exactly what happened. No 47uF 6V tants kicking around, so I swapped in a 100uF 6V in the feedback cap position (yellow circle below), and that sorted things. The VBE cap appears to have been unaffected, but I may swap it anyway...

Anyway, to anyone following the 135 Service guide in the reference section -- as I was doing -- I'd strongly recommend not following the primary bias procedure outlined there, but rather doing the 0R22 emitter resistor route. Much safer.

... and conversation with self complete

 

[Loz828]

Just so you're not talking to yourself and feeling lonely - Yup..... I had a NAP250 once with a DC offset of 450mV on one channel and was thinking it might be the BC239C long tail pair, but after a full recap it turned out to be just the feedback tant like yours. IIRC the reason that Naim used 2x47uF in some amps was only because they had problems with leakage current of 100uF tants back in those days - not sure if that's true or not.

 

[MJS]

The 100uF tants did tend to go leaky and they were also a one-off part when the 47's were used elsewhere, so it made sense just to use the 47s.

 

[Gervais Cote]

Can’t this 100 uf tant be replaced by a more robust/stable 47 uf capacitor like a polyester or a metal film something to get rid of the risk or problem on the long term ?

 

[Pete MB&D]

Yes and it will sound better as well as being more reliable.

Pete

 

[hifiaf]

I'll see how the 100uF behave for now. My recap kit came to me with 4 x 100uF tants, when I only needed two, so I've even got backups for when they go leaky I read about their tendency to fail, so put in the 2 x 47uF since I had some of those around anyway. Of course I then forced my hand by destroying the 47uFs during the bias testing procedure.

I've heard going to something other than a tant may move you away from the "Naim sound" but am also curious to test that theory out at some point. In colasblue's service thread here he swaps in two giant 47uF MMKs in the feedback position...

 

[Gervais Cote]

I replaced the coupling capacitors and the feedback capacitors in my NAP 110 and it didn’t change the signature of the amplifier.
Bass is still powerful, PRAT is all still there and the amp is still very punchy on bass drums.
The improvements are the human voices that became more natural and the treble became more detailed and softer.
In fact, I was not able to listen to my NAP 110 for several hours in a row because of listener fatigue but now I am able and it’s a pure pleasure.
The coupling cap has been replaced by a Kemet SMR 15475J50B14L16 and the feedback cap by a AVX FFB44E0476K.
The biggest improvement comes more from the feedback cap than the coupling but worth for both IMO.
Highly recommended.

 

[Pete MB&D]

Don't worry about the naim sound it will have been forced through a multitude of tants before it gets to the Poweramp.

Pete

 

[Loz828]

I put film caps in one of my NAP250s (feedback only), it didn't change the sound substantially but it did make the higher freqs more delicate - a bit less bright, it's difficult to expain sound using just words, and what I like might not be what you like, and it also depends on your source and speakers. Give it a try because the only judge that matters is your own ears, it's not a permanent or expensive change and if you don't like it then just put the tants back.

 

[hifiaf]

Definitely will try it out at some point. I'll live with the dead stock setup for a bit, though, to let my ears acclimate to it. (I've finally got the 135s into my system after sorting out the DC offset problem and letting the caps form a bit, and they are definitely sounding very nice so far!)