NAP90 Distortion Problem

[Specul8tr]

Can anybody help with a problem with my Naim NAP 90 power amp?

The amp has recently started to distort once the volume reaches around 2pm on the pre-amp. Both channels are affected, and the distortion increases with volume. The amp is around 10 years old and probably should have a service, but I'm curious as to what could be causing this. Many thanks.

[ingenieus]

Taking a wild guess - the biasing could be out of kilter.

Another guess: are you sure that it is the power amp and not the pre amp?

[Ian Finch]

There are not many things in amplifiers that will fail similarly in both channels apart from the Power Supply or protection circuits. For that and Naim service - read expensive, if you wish to keep it Naim.

I assume this is a modernised version of a NAP80 in concept at least, with 45W/channel or so. At 2 o'clock on the preamp, any amplifier could be playing moderately at say, a few watts peak or flat out and clipping, depending on the load, design and programme source settings. You will have to measure that or at least describe how loud it plays into what impedance/sensitivity/type speakers. Otherwise, we can only guess what is meant there. For example, a couple of my integrated amplifiers designed back in early CD days will clip before even 1 o'clock with standard CD inputs, so knowing the level is relevant to what problem. if any, you have.

[sakis]

for a Naim ...it can be all shorts of things soldering, capacitors, tantaliums ,thermal stress , power supply or tuning issues

it could be nice to isolate pre and main to know where the trouble starts from

[Specul8tr]

Firstly, many thanks for the replies.

My system is :

CD 3.5 CD Player+ Flatcap Power supply
NAP 92R Preamp + Flatcap
NAC 90 Power Amp
Proac Response 1.5 Speakers. (8 ohm)

I hadn't considered the possibility of the problem being with the preamp, as I wouldn't have thought there was much to go wrong, and the problem occurs on all inputs.

I have eliminated the power supplies as being the cause of the problem by removing them from the system, but the clipping still remains.

The power amp I believe is rated at 30w per channel, but I have only noticed clipping over the last couple of weeks, so shouldn't be wattage related.

I have fed a sine wave into the preamp, and clipping occurs when a certain threshold is reached. If I lower the input level into the preamp, I can turn the volume pot up more until clipping occurs, and vice versa, if I raise the input level into the preamp, clipping occurs at a lower volume pot level. Once clipping begins to occur though, it increases dramatically as the volume is raised. And the same happens on each input.

[wintermute]

just an off the wall suggestion. Have you tested your mains voltage (of course this assumes you know what it was when the problem wasn't occuring)? If it's dropped then your rail voltage on the amp will also have dropped. What you describe in this last post sounds exactly like what I would expect if you were simply driving the amp to the point of clipping. The only logical thing I can think of is that the voltage rails on your amp are lower than they used to be.

I don't know which part of the world you are in, but you could possibly be transitioning to winter or summer, and either way the demands on the electricity grid could be increasing resulting in lower voltage at your wall outlet.

edit: alternatively your PSU caps might be going, resulting in significantly less capacitance, which would also lead to a lower rail voltage (though they would have to be pretty caput to make a significant difference, and then you would probably be noticing other issues. .......

Tony.

[Specul8tr]

Originally Posted by wintermute

just an off the wall suggestion. Have you tested your mains voltage (of course this assumes you know what it was when the problem wasn't occuring)? If it's dropped then your rail voltage on the amp will also have dropped. What you describe in this last post sounds exactly like what I would expect if you were simply driving the amp to the point of clipping. The only logical thing I can think of is that the voltage rails on your amp are lower than they used to be.

I don't know which part of the world you are in, but you could possibly be transitioning to winter or summer, and either way the demands on the electricity grid could be increasing resulting in lower voltage at your wall outlet.

edit: alternatively your PSU caps might be going, resulting in significantly less capacitance, which would also lead to a lower rail voltage.......

Tony.

I'm in the UK... The mains supply voltage is about the same as usual, allowing for the usual variances.

I did wonder about the PSU caps though. Is there any way to check these ? I believe the NAP90 runs from a 24v rail.

[Ian Finch]

OK, 'sounds like you need a check on the preamp first. Does it have a headphone out that you can check the clipping there with? I guess you have no test instruments but it's always a good start to measure what is happening rather that guess. It's flying blind.

Connect the preamp to the inputs of any other amplifier with its own volume control or your PC sound to hear if clipping still occurs at the 2 o'clock level. This is so you don't deafen yourself or cause other damage but still hear what the pre does. This assumes it does have standard leads or you can adapt as required. Then if you do hear the distortion at comparable levels, you do have a preamp issue.

Even that revered Naim is made with parts that will fail in time and if its main regulated supplies are low, it will clip early, just like any amplifier where the power supply is low. Regulated power fails without mains fault but it can be difficult to repair to original quality. Service it properly.

Re: 24V rail for 30W/8R power - don't think so. with twin rails, +/- 30V more likely,
Just recalled - Naim uses a 24V single supply to its old discrete preamps. Not sure what is used now.

[Specul8tr]

Originally Posted by Ian Finch

OK, 'sounds like you need a check on the preamp first. Does it have a headphone out that you can check the clipping there with? I guess you have no test instruments but it's always a good start to measure what is happening rather that guess. It's flying blind.

Connect the preamp to the inputs of any other amplifier with its own volume control or your PC sound to hear if clipping still occurs at the 2 o'clock level. This is so you don't deafen yourself or cause other damage but still hear what the pre does. This assumes it does have standard leads or you can adapt as required. Then if you do hear the distortion at comparable levels, you do have a preamp issue.

Even that revered Naim is made with parts that will fail in time and if its main regulated supplies are low, it will clip early, just like any amplifier where the power supply is low. Regulated power fails without mains fault but it can be difficult to repair to original quality. Service it properly.

Re: 24V rail for 30W/8R power - don't think so. with twin rails, +/- 30V more likely,
Just recalled - Naim uses a 24V single supply to its old discrete preamps. Not sure what is used now.

Thank you - and it is a 30v supply. I'll follow your advice here regarding the preamp and post the results later. I'm assuming the power supply to the preamp is functioning correctly though as interchanging the flatcaps, or running from the power amp makes no difference.

And no, I don't have any sophisticated test equipment, but do have a basic working knowledge of electronics. Thanks again.

[martin clark]

With that source and preamp and insensitive speakers I'm pretty sure you are just running into clipping as already suggested. The preamps have quite a lot of gain and usually in an all-Naim setup with a line level source you can expect clipping much past about '12 oclock' on the dial.