NAP-140 Clone Amp Kit on eBay

DC Offset output voltage

Has anyone experimented to solve the DC offset output voltage problem. Mine was about 70 mv with Q1 & Q2 as 2N5551 (the supplied transistors). These have an hfe of about 195. I have swopped them for BC550 which have an hfe of about 600 and the offset drops to below 20mv.

The only problems that I can see with the BC550 are lower max VCE voltage (50 versus over 100) and frequency (100 MHz versus 200 - 300).

Anybody else had any experiences here, or indeed any alternatives for Q1 & Q2 ?

Regards

Chris
 
Re: DC Offset output voltage

cdswift said:
Has anyone experimented to solve the DC offset output voltage problem. Mine was about 70 mv with Q1 & Q2 as 2N5551 (the supplied transistors). These have an hfe of about 195. I have swopped them for BC550 which have an hfe of about 600 and the offset drops to below 20mv.

The only problems that I can see with the BC550 are lower max VCE voltage (50 versus over 100) and frequency (100 MHz versus 200 - 300).

Anybody else had any experiences here, or indeed any alternatives for Q1 & Q2 ?

Regards

Chris


20mV is good result.


BC550 is OK for +/-40VDC operation!
 
Finally

One of the two apms, waiting for the PSU to be ready (at least ready enough for testing).
 

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Thanks. I know about transformers working only for AC. I just economized on my basic and largely inadequate drawing skills. :)

But I was wondering, after the rectifiers converting to DC of course and after the caps doing their business, if there is a preference for either option.
 
ttan98 said:



A few suggestions:

1. use 1-2amp fuses on the +/- 35Vdc power supply.
2. use a variable transformer at the main and slowly increase the voltage, eg apply say 1/3-1/2 of the main voltage and measure relevant/key bias voltages on resistors and transistors. that way you can quickly detect incorrect orientations of transistors or incorrect values of resistors.

cheers.

Recently my power blew up, and took the power
transistors as well.

I will outline the procedure below I took to fix it, similarly it can be followed when a power amp kit is first power up. It avoids replacing expensive power transistors if wiring or soldering are incorrect performed or defective components.

method 1
======
less dangerous, put a fast blow 2A fuse on the +/-power supply voltage. If there is a short the fuse will blow.

method 2
======
I use this method, if you don't take precaution(can be dangerous) don't try it. I put 2*100ohm(*), 1/4W carbon resistor on the +/- power supply terminals on the PCB. If there is short, the carbon will burn quickly hence protecting the power transistors. You got to be quick, on power up if there is a short, the resistor will burn and quickly turn power off. If there is no short, the supply voltage on the PCB will be much less than the voltage at the power supply because of the drop across the 100ohm resistor, eg power supply is 45V and PCB is about 20V.

* values depend on the total quiscent current the amp is taking.

Now my amp works.
 
Help

I hope this thread is still alive. I assembled the two boards and connected them to a temporary PSU (just transformer, rectifiers and 2x2 10.000uF caps). The PSU measures just fine, about 44,3V plus and minus. I did connect the signal ground to the PSu ground.

But, no matter how I adjust the bias pots, it seems to make no difference. Q4 gets rather warm (not hot) and both amps sort of stabilize around 25-30mA in the V+ lead. This happens with both boards, ruling out (I hope) the more simple explanations like a transistor that got to hot or an error in soldering resistors etc.

Any ideas any one?
 
Re: Help

ratchet said:
I hope this thread is still alive. I assembled the two boards and connected them to a temporary PSU (just transformer, rectifiers and 2x2 10.000uF caps). The PSU measures just fine, about 44,3V plus and minus. I did connect the signal ground to the PSu ground.

But, no matter how I adjust the bias pots, it seems to make no difference. Q4 gets rather warm (not hot) and both amps sort of stabilize around 25-30mA in the V+ lead. This happens with both boards, ruling out (I hope) the more simple explanations like a transistor that got to hot or an error in soldering resistors etc.

Any ideas any one?

Do both amps behave the same way?

Have you tried measuring the resistance across the bias pot (with power off of course) to check that the resistance changes. Note that this is a multi-turn pot and takes many turns to go from one end to another - also there is no end-stop so the screw keeps turning even when the pot is at one end or the other.

My other suggestion is that Q5 might not be connected properly (reversed).

Regards
Chris
 
Re: Re: Help

Do both amps behave the same way?

Yes, they behave identical

Have you tried measuring the resistance across the bias pot (with power off of course) to check that the resistance changes. Note that this is a multi-turn pot and takes many turns to go from one end to another - also there is no end-stop so the screw keeps turning even when the pot is at one end or the other.

Checked by measuring across R17, increases from 0 to 775 ohm when I turn clockwise

My other suggestion is that Q5 might not be connected properly (reversed).

Q5 checked and double checked including the check for making proper contact. Al resistors, caps, diodes and transistors checked & measured (but I realise that if you overlook something once, you will overlook it again and again).

Both R15 and R18 carry an 8mA current (225mV and 585mV respectively) at any bias setting. This to my limited knowledge suggests a problem with Q5 but Q5 seems to be OK.
 
Re: Re: Re: Help

ratchet said:


Yes, they behave identical



Checked by measuring across R17, increases from 0 to 775 ohm when I turn clockwise



Q5 checked and double checked including the check for making proper contact. Al resistors, caps, diodes and transistors checked & measured (but I realise that if you overlook something once, you will overlook it again and again).

Both R15 and R18 carry an 8mA current (225mV and 585mV respectively) at any bias setting. This to my limited knowledge suggests a problem with Q5 but Q5 seems to be OK.

Is the voltage across R16 & 17 change during calibration?
 
Re: Re: Re: Re: Help

Kan said:


Is the voltage across R16 & 17 change during calibration?


The current thru R15 and R18 should remain steady as Q6 is a constant current source. What matters is that the voltage across the base/emitter of Q5 should change as the pot changes. If this is OK check the voltage across collector/emitter of Q5 - this should also change - if it doesn't Q5 is either misconnected or dud. If the voltage does vary, next check the voltage between the bases of Q9 and Q10 - this should also vary. If it doesn't, one of the resistors R23 or R14 is not connected. If it does, check the base/emitter voltage across each of the output transistors to check they are both biased on.
 
Re: Re: Re: Re: Re: Help

Thanks for all your help. I did some measuring with the fllowing results:

Is the voltage across R16 & 17 changing during calibration?

R17 yes, R16 yes

The current thru R15 and R18 should remain steady as Q6 is a constant current source. What matters is that the voltage across the base/emitter of Q5 should change as the pot changes.

Nope, constant at 579mV

If this is OK check the voltage across collector/emitter of Q5 - this should also change - if it doesn't Q5 is either misconnected or dud.

Changes

next check the voltage between the bases of Q9 and Q10 - this should also vary.

It does

If it does, check the base/emitter voltage across each of the output transistors to check they are both biased on.

Q11 and Q12 start at around 500mV and slowly decrease to 0 no matter what I do with the BIAS pot. That makes it a NO to this question.

Sigh
 
Re: Re: Re: Re: Re: Re: Help

ratchet said:
Thanks for all your help. I did some measuring with the fllowing results:



R17 yes, R16 yes



Nope, constant at 579mV



Changes



It does



Q11 and Q12 start at around 500mV and slowly decrease to 0 no matter what I do with the BIAS pot. That makes it a NO to this question.

Sigh

Your symptoms sounds like the protection transistors Q7 & Q8 are turning on and the delay feels like its caps C3 /C4 charging up. If any of the resistors R32, R33, R35 or R36 are the wrong value, this could cause the protection tranisitors to switch on even when there is no/little current running through the output transistors. I would check that the resistors are of the correct values and then monitor the base/emitter voltage on Q7 & Q8 to see if that starts of at near 0v and then rises to .6v approx (which it shouldn't under normal conditions - it should stay close to zero)

Let us know how you get on

regards

Chris
 
Re: Re: Re: Re: Re: Re: Re: Help

Your symptoms sounds like the protection transistors Q7 & Q8 are turning on and the delay feels like its caps C3 /C4 charging up. If any of the resistors R32, R33, R35 or R36 are the wrong value, this could cause the protection tranisitors to switch on even when there is no/little current running through the output transistors. I would check that the resistors are of the correct values and then monitor the base/emitter voltage on Q7 & Q8 to see if that starts of at near 0v and then rises to .6v approx (which it shouldn't under normal conditions - it should stay close to zero)

Hi Chris,

All resistors checked and OK but: Q8 starts at around 250 mV and decrease independent of BIAS pot. Q7 behaves as you predicted (starts at 0 and rises to I don't know how high (didn't want to risk blowing any transistors). Hmmm...
 
Re: Re: Re: Re: Re: Re: Re: Re: Help

ratchet said:


Hi Chris,

All resistors checked and OK but: Q8 starts at around 250 mV and decrease independent of BIAS pot. Q7 behaves as you predicted (starts at 0 and rises to I don't know how high (didn't want to risk blowing any transistors). Hmmm...

I'm beginning to run out of suggestions, although if R29 was open circuit or much higher than .22 ohms - this would explain some of the behaviour on the base of Q7.

Regards

Chris
 
salesmonster said:
I've followed this thread for a while with a lot of interest. Maybe it's just my perception, but has anyone been successful at getting the current revision of the NAP-140 kit to make music? I really like the Naim sound, but the thread has been discouraging so far.

My amps works and sound pretty good in a temporary hookup. I am in the process of building a decent OSU & case & will report back when I have finished.