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Measuring bias voltage in NAPs

Well just replaced the PSU caps on my NAP140 today (same make/values as originals), and decided to check the bias. Prior to my doing the PSU it's been an untouched unit.

From cold, the bias measured 14mV across BOTH resistors, on each channel, rather than 7mV!

Could this be because I have refreshed the PSU, did somebody at Naim make a mistake (even though it's been working fine for 21 years!), or do the NAP140s with the Sanken transistors need twice as much juice?

Really not sure what to do now - leave it or adjust to 7mV per board?

Thanks,

- John
 
Just put the bias pots to zero (anti-clockwise) and started bringing it up gently. Measured 179.3mV on one channel!!!!!!!!! Quickly turned off!

Have I somehow screwed up the 140 or is it a normal turn-on surge? Or should the pots be anti-clockwise when starting from scratch?

HELP!!!
 
Good plan - being as it was measuring a steady 14mV on each channel BEFORE I moved the pots seems to suggest I didn't bugger anything up by recapping the PSU at least.

But perhaps one of the adjustment pots is buggered. I'll unplug the board that went to 179mV, and try the other one, with the pot at the mid position as you suggest.

Cheers (and keep 'em crossed haha!)

- John
 
valveheaduk said:
Just put the bias pots to zero (anti-clockwise) and started bringing it up gently. Measured 179.3mV on one channel!!!!!!!!! Quickly turned off!

Have I somehow screwed up the 140 or is it a normal turn-on surge? Or should the pots be anti-clockwise when starting from scratch?

HELP!!!
Click to expand...

I think you simply opened up the flow as opposed to turning it down...fully clockwise should reduce the flow to next to nothing....
 
neiljadman said:
I think you simply opened up the flow as opposed to turning it down...fully clockwise should reduce the flow to next to nothing....
Click to expand...

I'm sure I read in several places that turning the pot anti-clockwise reduces the bias?

Still I guess if I start with it in the middle position this should prevent any catastrophe. I just hope I turned the amp off fast enough before any damage was done :S

Hitting the sackerino now but will try again with it tomorrow. Thanks for the suggestions folks much appreciated :)
 
valveheaduk said:
Any comments on post #41 folks? 14mV on both channels.....
Click to expand...

In doing the work you've done many things may have changed. It could just be coincidence that both channels have gone to 14 mV.

I thought we were now all agreed that the bias setting used by Naim was 7.0 mV across both resistors together? One of the few exceptions was the older style (CB?) 140 where it was 7.5 mV.

You can go higher than those figures but will run the risk of thermal runaway for little, or no, benefit.

My advice is to reset for 7.0 - 7.5 mV as previously discussed.

malcolm
 
valveheaduk said:
I'm sure I read in several places that turning the pot anti-clockwise reduces the bias?

Still I guess if I start with it in the middle position this should prevent any catastrophe. I just hope I turned the amp off fast enough before any damage was done :S

Hitting the sackerino now but will try again with it tomorrow. Thanks for the suggestions folks much appreciated :)
Click to expand...

There are ways to test the boards before the power transistors are installed during which the bias trimmer can be set to a safe level.

Starting up with the trimmer at one end or the other is NOT recommended.

The advice here would be to start up with the trimmer at mid position, the meter firmly connected across both resistors, and a screwdriver in the slot of the trimmer ready to move it as soon as you power up,

malcolm
 
neiljadman said:
I think you simply opened up the flow as opposed to turning it down...fully clockwise should reduce the flow to next to nothing....
Click to expand...

Just had another look at it today and you're right! I think I got confused because the Naim clone I'd built previously was the opposite in terms of bias adjustment.

Both channels set to 7.1mV across both resistors (from cold) and sounding fine so far - those new PSU caps will probably take a few hours to burn in or whatever it is they do. It's already sounding more 'punchy' however - those 21 year old PSU caps where obviously slugging the sound quite a bit!

MANY thanks for all the help everyone :D

Should have a Hicap arriving tomorrow or Wednesday so will be interested to see how much further that takes the sound of my 72/140.

- John
 
Given the bias seems to be pretty sensitive to temperature change, how much difference does a change in ambient room temperature make? My listening room can be as much as 20-25 degrees (F) warmer in the summer than in the winter. I ask because I just double checked my bias and it was around 13mV (after twenty minutes) and I'm pretty sure I set the bias to 7.5 last winter. (One reason I checked the bias was that the case was warm at idle.)

Edit: Had a nagging feeling that this question had been asked and answered already so I did a search. And, yes, the question had been asked...by me! And Martin answered it. See here.
 
misterc6 said:
Martin is absolutely correct. Measuring at the emitter resistors tells you exactly what quiescent current is flowing through the output transistors.

TR11 and 12 and the two 0R22 resistors are connected in series directly between the positive and negative rails and, under no load and no signal conditions, the same d.c. current flows through all of them.

It is preferably to measure the current across both resistors together because your DVM is going to be slightly more accurate at the higher reading.

The article found here: http://www.acoustica.org.uk/ and then clicking 'bias-setting in naim amps' from the menu gives some interesting information. In particular thd does not vary much over a wide range of bias settings so the value set is not excessively critical. Naim set most of their NAPA boards to between 7.0 and 7.2 mV across both resistors together which gives between 15.9 and 16.4 mA.

Tony, you are correct, the voltage measured across each of the two resistors should be the same within the tolerance of the resistor values. If not, something is wrong. I've seen several where constructors have inadvertently put the wrong value in one or both positions.

If it's of any help to anyone I have photos of exactly where to connect your meter (across both resistors) on NAPA series 5, NAPA series 6 and NCC200 boards:

http://picasaweb.google.com/misterc6mg/BiasSetting?feat=directlink

malcolm
Click to expand...

Hello Malcolm, hello all, I am new here, sorry if resurrecting this old bias discussion but I think I need some help with my Nap 250 cb.
The amp is fully serviced and working fine.
The voltage is set at -39,9 and +39,3, the bias at 7 mV.
In the beginnig I set the bias at 7mV with the amp cold but after several hours, with case removed, I noticed an increase up to 7,8- 8 mV.
I then listened to the amp and noticed that after several hous the was a strong softening of the high frequencies and reduction of PRAT.
The magic was gone.
I then set the bias at 7 mV after 2 hours and I had the impression it remained stable over time.
On listening the problem occours later after 8-10 hours.
Any advice please?
Thank you
Andrea
 
The advice from Naim is to set the bias to 7.0 mV (measured across both emitter resistors) with the amplifier cold. Obviously, this figure will rise as the amplifier warms up and will change again when it is cased up. Naim have taken this into account and you should resist the temptation to alter it back down to 7.0 mV with the amplifier warm.

That said a change from 7 to 8 mV should make hardly any discernible difference to the sound of the amplifier and I suspect that there is some other problem with your 250.
 
Did the full service include the caps on the power supply boards? Most 250 problems are related to the 10uF caps on the output of these boards. Symptoms vary but they all sound 'off'.
 
MJS said:
Did the full service include the caps on the power supply boards? Most 250 problems are related to the 10uF caps on the output of these boards. Symptoms vary but they all sound 'off'.
Click to expand...
Yes, all caps replaced, even the 10 mF 63 volt (Vishay 105° C).
If I set the bias at 7mV warm the voltage is stable, if I set the 7 mV with the amp cold the tension slowly rises over time (several hours).
Am I missing something?
 
Andrea Pignanelli said:
Yes, all caps replaced, even the 10 mF 63 volt (Vishay 105° C).
If I set the bias at 7mV warm the voltage is stable, if I set the 7 mV with the amp cold the tension slowly rises over time (several hours).
Am I missing something?
Click to expand...

Bias rising from cold is entirely normal for the amp's topology. Don't worry about it. It should be set from cold however.
 
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