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Old 3rd June 2004, 06:55 PM   #1
rmgvs is offline rmgvs  Netherlands
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Default NAD preamp 1240 distortion

I just bought a secondhand NAD preamp type 1240 (probably made around 1985-1990). I have no schematics and have a problem with the amp. The amp distorts prematurely and does not have enough gain/amplification. The problem is in the line-stage on all inputs and in both channels. First guess was that the power supply was not okay: but here I measure a nice plus/minus 20 Volts after rectification and including stabilization.

In the signal path there is an opamp type JRC 2043D around the tone controls and further there are a number of normal BC5xx small transistors. Maybe the opamp is the problem but why then should the problem be there in both channels? Not so likely. But if one of the small transistors is the problem, the same applies. Passive components? They look okay to me, no burnt resistors and the like, no loose solder joints.

Who has any idea what can be the cause of this problem?
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Old 3rd June 2004, 07:08 PM   #2
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Does the preamp have a "mute" circuit?
If so, it may be switched on. Check for a bad "mute" switch or something gone wrong in the "mute" control circuitry.
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Old 3rd June 2004, 07:23 PM   #3
rmgvs is offline rmgvs  Netherlands
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Default mute

Hallo Frank,

yes, the circuit has a mute function. But this seems to work all right: switching it on does mute the signal nicely and there is no crackle of other nuisance by pushing the button. With the mute on there is hardly any distortion but in this case there is hardly any signal to speak of.

When I play with a power amp I can easily put the volume knob on the 13 or 14 hours position and it is easy listening. Turning the knob to 15 hours and further introduces big distortion. The power amp is not a difficult load to the preamp (47k impedance, needs around 1 V sensitivity for full output, so I guess the amp distorts onwards from 200 mV output?).

Also the head phone output is distorting (this is logical since it is fed from the output of the preamp) not matter the impedance of the phone (a simple walkman has much greater output levels with these same phones).
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Old 3rd June 2004, 07:52 PM   #4
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Is it possible that someone has made changes to the preamp?
Clipping at an output of 200 mV?
Where did you purchase this preamp? Who was using it?
For what purpose?
I'm betting that the problem is something simple and easily repaired.
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Old 3rd June 2004, 08:01 PM   #5
jaycee is offline jaycee  United Kingdom
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That opamp is a dual opamp chip, so if it's gone bad it is likely it will be affecting both channels...

Try temporarily shoving an NE5532 or so in there. You may also want to check the voltages on pin 4 and pin 8 of the chip.
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Old 3rd June 2004, 08:06 PM   #6
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Good idea.
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Old 3rd June 2004, 09:56 PM   #7
rmgvs is offline rmgvs  Netherlands
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Default supply voltages

I guess that the power transformer has been changed. It is a type and make that I do not associate with Nad-taiwan electronics. The previous owner put in a transformer with 15-0-15 V windings at 0,2 A per winding. This gives around 20 V of symmetrical DC supply.

Could this be a voltage too low for the amp? I think these values are enough for a decente voltage swing at the output. Looking at several electrolytics I see that a number of them is rated at 35V, but this is not a proof of higher voltage per se.

I bought the unit via-via (including a story that he never used it, thinking he had bought a normal integrated amp etc etc).

I could change the opamp for some substitute, just for the experiment. The voltage at the pin numbers 4 and 8 are -14 and +12 Volts respectively. Indeed it is a dual opamp, it could be the reason ... but could I hear ANY music coming out then?
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Old 3rd June 2004, 10:18 PM   #8
jaycee is offline jaycee  United Kingdom
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Hmm, -14 and +12 would indicate that the supply regulation for the opamps is a bit off, should ideally be identical. It was probably supposed to be -14 and +14, the drop on the +ve rail could indicate a problem with the opamp causing it to draw more power there.

Try removing the chip and soldering in a socket. Then power up and test pin 4 and 8 voltage with no chip in there. If they still don't match attempt to find and adjust the +ve regulator to bring it in line with the -ve. Then replace with another chip.

Preamp transformers are usually 18-0-18, this takes care of voltage drops should the mains be low. In practice I doubt it'll be a big problem though.

The 35V capacitors are probably just used for reliability and lower ESR. You don't see many op amps for audio use that accept over 18V symmetrical.

It does not always follow that when a component fails it fails totally. I repaired an integrated amp experiencing severe distortion at anything over 2 on the volume slider. It turned out that one TL072 in the preamp stage was causing it. Replacing the part cured it. It was the part connected to the input terminals so I can only assume it experienced some kind of static damage.
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Old 4th June 2004, 03:55 PM   #9
rmgvs is offline rmgvs  Netherlands
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Default not the opamp

As the advice on this thread goes, I soldered the opamp out and soldered a 8-pin socket into place. I measured the voltages without the opamp and measured +19,6V and -20,4V resp.

Then I put in the NE5532 (AN) and measured +6,4V and -9,9V resp. Listening showed that the problem remains the same.

Without opamp there is no signal at all.

The 2043D is okay, that must be the lesson.

What's next, any new ideas?
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Old 6th June 2004, 03:40 PM   #10
rmgvs is offline rmgvs  Netherlands
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Default built preamp of mark hennessy into nad

Hello Mark,

you probably know me from some discussions we had in the past about the MF A1 family.

I built your replacement amp yesterday and today and the reason is a coincidence of factors.

Firstly I bought a secondhand A100 that was broke about 3 months ago. I restored the thing and split the pre- and main amps. I was so enthousiastic about the main-amp (again, I must say) that I decided to built the amp from scratch myself into monobloks. These are playing now for 6-8 weeks and do sound very special too (though exactly the same sound you will never get due to other components etc: I discovered that just using separate transformers per channel does change the sound, it becomes more clean and better defined, in some way you lose some of the woolyness or magic of the integrated amp!!). I use the amp with a tube preamp.

Secondly I bought a secondhand NAD preamp 1240 for only 25 Euro. I posted a threat on diyaudio because here are very strange things happening, distortion prematurely etc, see the postings if you like. I have not resolved the matter but thought: why not using the infrastructure of this amp to fit in your NE5532 preamp. I use the chassis, the power supply, the inputs and -selector, the mono facility, the volume and balance.

On a vero-board I put together the stuff in 2 hours and discoverd the following:

- the amp is hum sensitive and needs quite a power supply. Originally I used 1000 mF per side, a stabilizer and 220 mF again plus a 12V zener-diode. This gives some hum (bases on the original power supply of the NAD which is hum free). When I use 4700 mF in the first stage most of the hum is gone ( the A1 uses 2 times 10.000mF so here it is no problem).

- when turning the amp off after some 4 seconds there is a loud whistle coming from the speakers (when the power amp is still on). Something to do with some HF-instability, maybe using too long wires?

- the sound is much to my liking. very clean and airy and musical. very good for an IC I would say. I'm used to tube preamps but this has its own special qualities.

- the control range is very smooth and easy, just enough amplification for most of my poweramps, including the A100/MA50's.

Thanks for your preamp, and maybe you have some comments on my remarks.

Rudy

More info about the Hennessy preamp at http://www.mhennessy.f9.co.uk/mf_a1/
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