NAP-140 Clone Amp Kit on Ebay

[nigel pearson]

You can solder some 10K 0.6 Watt resistors across the capacitor . The discharge rate is approximately 5CR . If 1000 uF that is 5 x 1000 x 10 000 / 1000 000 . That is slightly less than 1 minute . 4k7 would be OK and would do it is less than half the time . Power rating is VxV/R = 42 x 42 /4700 = < 0.4W . I have 10 K on my power amps to aid discharge . To discharge between 10 seconds to a minute is usually good practice .

In mains power suppression < 2 seconds is required in regulations ( e.g. the time for a child to remove a mains plug and touch the terminals ) . Douglas Self recommends 470 nF for a maximum value mains suppression capacitor . That would require a resistor of < 850 K > 0.1W . Ideally 2 x 390 K 2 W in series for voltage reasons mostly ( gives a 1000 V rating , 2 x 0.6 W gives 700 V typically which should be fine , the cap will protect the resistors a little so not as critical as when no capacitor ) . Not an area I would encourage for experiments , I only say because D S brings it up .

[Evenharmonics]

Thank you for the tips.

Originally Posted by nigel pearson

You can solder some 10K 0.6 Watt resistors across the capacitor .

Shouldn't something like that have been part of standard feature in every power supply? I'm somewhat disappointed that this one didn't come with it.

[Ruwe]

Originally Posted by Evenharmonics

Shouldn't something like that have been part of standard feature in every power supply? I'm somewhat disappointed that this one didn't come with it.

It is standard option in the high voltage power supplies, as a safety measure. In our case, with the Naim, the caps will discharge pretty quick through the amplifier circuits because of the quiescent current. But, if the power supply is not an integral part of the amp, some higher value resistors added across the capacitors are good idea.

[Ian Finch]

Originally Posted by Evenharmonics

I'm somewhat disappointed that this one didn't come with it.

I've not seen any prebuilt or kit PSU assemblies for solid state audio amplifiers fitted with bleed resistors - on Ebay or elsewhere - ever!
They aren't intended as freestanding power supply boards, but to remain connected to loads, amplifiers etc. where they will then discharge
as detailed by Ruwe. Once fitted to the amp, there's no problem or need for any resistor - just as there isn't in most commercial amplifiers.

I imagine you weren't aware of any such convention and that's probably half the annoyance.

[Evenharmonics]

Originally Posted by Ian Finch

I imagine you weren't aware of any such convention and that's probably half the annoyance.

I realized it soon after I posted above.

Anyway, I finished assembling NAP 140 board, connected the power, heat sink and it works. I've read this thread from the beginning and the only check out info I found (I might have missed others) was about checking 0.22 ohm resistor voltage. Mine gets 4 mV DC. Where do I adjust to make it higher?

[Ian Finch]

There is only one variable potentiometer. This is to adjust the bias (idling current). Note that the setting will vary because it is the air temperature in the amplifier that regulates the bias current. Closed up in a small case like the original amplifier, is fine, as after about 30 mins it stabilises and you get steady measurements. Out in the open air, it never stabilises and drifts about - sometimes dangerously.

For safety, I would keep the measurement to 5 mV across each resistor until it is enclosed in its case (measuring across both resistors for 10 mV is more accurate with typical DMMs) If you have the amplifier enclosed, you can quickly recheck after it has warmed up to operating condition and reset. Naim amplifiers are infamous for their sound quality improvement as they warm up.

Hope you enjoy and get to make some useful improvements on the basic kit. There's always something to learn and tinker with

[Spuds]

I use some LED's to bleed off the caps slowly. That way you know things have totally discharged and don't end up burning holes in your hands.

[nigel pearson]

If wanting to use cheap capacitors for valve amps the discharge resistor also makes certain the capacitors are equally changed . For example 1 bought 3 x 150 uF 200 V for about $1 recently ( Rapid Electronics UK clearance ) . I used 3 x 1M resistors ( one for each capacitor ) . This gives me 50 uF 600 V on a 480 V supply . This is to a capacitance multiplier . 1 M is about the optimum . 3 x 350 V resistors at 3 cents doing 3 vital jobs , equalizing charge , discharge and giving a bias current . As you can imagine if 480V it is useful to have it discharge . Equally if too aggressive the ripple voltages climb . Ripple is seldom a serious problem with transistor amps . With valve amps it is almost Malaria .

I recently had to build a laboratory standard mains voltage detector which I am proud to say came in at well below $1 ( US ) . It even works - 0.3 % at 100 C , -20 C 0 % ( 10 to 40 C required ) . It took me 10 minutes to conceive and 3 weeks to make easy to build ( I told my boss if a football match it went to penalties , he is very happy as the cost per unit is low , I had said $3 ) . My only problem was matching the discharge rate to the regulations and not screwing up the measurements .

[Evenharmonics]

Originally Posted by Ian Finch

There is only one variable potentiometer. This is to adjust the bias (idling current).

R17 on this image?


I tried to adjust mine but nothing changes. Could the trim pot be defective?

[kclusa@yahoo.co]

You can measure the voltage drop between Collecotr and Emitter of Q5.
If R17 is defective, Q5 Vce will not change.
R17 is multi turn resistor, it may requires several turns before idle current change.