SKA sponsored Audio Forum - Naim Power Amp Mods

repeet Ex Member Country: Australia	Re: Naim Power Amp Mods Reply #50 - Feb 14^th, 2010, 9:16am The clone kit uses 2SA1145 & 2SC2705 here. I wonder if these are actually more suitable?
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Greg B.

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Re: Naim Power Amp Mods
Reply #51 - Feb 14^th, 2010, 9:31am

Hi Pete,

Yes, the 2SA1145 looks closer for the Vas PNP. Have a look at the Ft /Ic how it peaks to 200MHz around 10-20mA. thumbsup

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It's still only 800mW so a heatsink would be nice. Gain is also a bit low so I would select a high Hfe one or go for the Y grading.

The 2SC2705 is fine also for the CCS. One caution though, these are only 50mA rated and the PNP Vas could easily be blown in the Vas position under overload conditions, without a current limiting emitter resistor as I've added.

Greg

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repeet Ex Member Country: Australia	Re: Naim Power Amp Mods Reply #52 - Feb 14^th, 2010, 9:52am Excellent, the kit even has the y graded ones already. Also, what wattage resistor should be used there please? Thanks Greg.
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Greg B. Administrator Offline audio hostage Posts: 3292 Kota Kinabalu, Sabah Country: Malaysia	Re: Naim Power Amp Mods Reply #53 - Feb 14^th, 2010, 10:13am Hi Pete, 1/4W metal film is fine. With that, the 1K on the base of TR4 needs to increase to 1K5 to keep the balance. Greg
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repeet

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Re: Naim Power Amp Mods
Reply #54 - Mar 8^th, 2010, 8:44pm

I have gone back to this project and am just about finished and ready to power up. Just a few Q's after re-reading this forum.

Should Tr1 and Tr2 now be matched instead of the 10% difference as in the Avondale circuit?

Where should a trim pot be added for adjusting DC offset and value?

Are there any further changes required before power up from your current modifications, as I'm not sure if it is yet complete?

Thanks,
Pete.

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Greg B. Administrator Offline audio hostage Posts: 3292 Kota Kinabalu, Sabah Country: Malaysia	Re: Naim Power Amp Mods Reply #55 - Mar 8^th, 2010, 11:47pm Hi Pete, The TR1 and TR2 can be matched. Will need a filtered +/-DC for the trimpot. Have to think. Should be OK otherwise.
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mikesnowdon SKA Owner Offline Posts: 346 London Country: United Kingdom	Re: Naim Power Amp Mods Reply #56 - Mar 12^th, 2010, 9:15am Hows it goin Pete?
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repeet Ex Member Country: Australia	Re: Naim Power Amp Mods Reply #57 - Mar 12^th, 2010, 11:02pm I have just powered it up and set the bias to 5.5mV, DC offset is about 40mV. Played some music through it but sounds terrible, really tinny and a bit distorted. At least it works though. Might need to play around with different cap values. I used the exact values from the last posted diagram.
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Greg B.

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Re: Naim Power Amp Mods
Reply #58 - Mar 13^th, 2010, 8:07am

Hi Pete,

There's no reason for that. I would check that the biasing is OK at a number of points around the circuit. Really tinny sounds like thin bass so check the caps for sure. Also the voltage across the 1K5 resistors will tell if the input stage is balanced up. You can trim the DC offset on the 1K5 at the base of TR4 for lower offset. Put a 100K across it, if it drops, you're heading in the right direction. Check you have around 300mV across the 33R at the driver emitter.

Greg

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repeet

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Country: Australia

Re: Naim Power Amp Mods
Reply #59 - Mar 14^th, 2010, 4:18pm

Hi again,

I checked the voltages across boht 1.5k resistors. One was .5v the other .9v The 33r had 295mv. the DC offset was actually
-40mv, adding the 100k resistor made it worse.

Tried some music again and the left channel popped then made loud static sound so I powered off straight away.

I've put the original modules back in and will shelve this project for a while, as my Gb150D repair parts have arrived.

Pete.

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