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?
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?
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).
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).
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?
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?
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.
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.
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?
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?
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/
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/
Hi Rudy!
Right - these are classic signs of HF instability.
I must admit that I didn't try powering the preamp from anything other than the A1 or a bench power supply, but even poor op-amps have decent PSRR to work ok from smoothed (not regulated) DC. The hum is probably down to HF oscillation. The dying squeal is proof of this…
Would you be able to send me a picture of your setup? It could be down to an earthing problem… Also, have you a schematic of the bits of the NAD that you're retaining? There could be some issue there…
As for the power supply, you won't need anything more elaborate than simple 7815/7915 regulators. Smoothing caps of around 470-1000uF, 15-0-15 transformer...
Thanks. I've had a surprising amount of positive feedback about the preamp - I'm surprised that people are willing to go to the effort of modding their A1/100's so extensively...
Thanks for your comments. The preamp really is nothing special, but I think it shows you that basic solid engineering gets you a long way, even using something as unfashionable as a NE5532. BTW, if you're thinking of trying different op-amps here, it might be an idea to scale up the feedback resistors to reduce the load on the op-amp.
Best regards,
Mark
Right - these are classic signs of HF instability.
I must admit that I didn't try powering the preamp from anything other than the A1 or a bench power supply, but even poor op-amps have decent PSRR to work ok from smoothed (not regulated) DC. The hum is probably down to HF oscillation. The dying squeal is proof of this…
Would you be able to send me a picture of your setup? It could be down to an earthing problem… Also, have you a schematic of the bits of the NAD that you're retaining? There could be some issue there…
As for the power supply, you won't need anything more elaborate than simple 7815/7915 regulators. Smoothing caps of around 470-1000uF, 15-0-15 transformer...
Thanks. I've had a surprising amount of positive feedback about the preamp - I'm surprised that people are willing to go to the effort of modding their A1/100's so extensively...
Thanks for your comments. The preamp really is nothing special, but I think it shows you that basic solid engineering gets you a long way, even using something as unfashionable as a NE5532. BTW, if you're thinking of trying different op-amps here, it might be an idea to scale up the feedback resistors to reduce the load on the op-amp.
Best regards,
Mark
Rather than start a new post for one question I hope it was okay to dig this one out. I dusted off a 1240 pre that I've never used before it sounds good, except for a problem. The volume/balance pot might need cleaning. I rested it on its back with the face upright and tried working some deoxing spray down in there but it didn't help much, its still cutting out in the right channel if I adjust it. Then I have to fart around with the balance a little to right it. Once I do it doesn't cut out as its playing as long as I leave it alone. The volume functions fine its just the balance that's acting up. Is there a better way to clean these? I'm not experiencing any distortions or other odd behavior other than that. Quite the contrary it sounds very clean and it's caught me off guard too. Once I could of sworn another stereo was turned on in the other room almost 90 degrees of my left. I started hearing an instrument chiming in a good 5 or 6 feet away from the L speaker. The staging is more spread out over all, glad I gave it a spin.
NAD 1240 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
The service manual shows VR201, the volume control, has the usual dual volume control wafers, one behind the other for each channel. Usually you can flush one or both of these wafers from the front panel but the balance control is a special symmetrical tapered resistance wafer, outboard at the rear, so it's unlikely your "mechanic-in-a-can" can get the fluid back there. The stuff will likely run out of the gaps in the housing before that happens.
To get serious, assuming you can't get to the small gaps in the pot. cases, you'll have to remove the front panel and unsolder the pot by carefully removing all solder on the PCB terminals and pads with solder-wick or similar fluxed braid product. Extract it carefully so as not to lift the copper traces and flush/clean the pot thoroughly, through the small gaps. I admit, I can't check whether they should exist on this model, but older equipment usually had them, as part of the rotation stop limits, located on the lower side. of the pot.
Hopefully though, you can access this gap in the balance control case and, with the preamp vertical to avoid spills on the PCB, just flush it properly - several times! It's always possible this treatment is not effective or not for long. You may even have to lose the balance control or make other mechanical arrangements to splice in a replacement as it's unlikely these products are still available as stock items.
The service manual shows VR201, the volume control, has the usual dual volume control wafers, one behind the other for each channel. Usually you can flush one or both of these wafers from the front panel but the balance control is a special symmetrical tapered resistance wafer, outboard at the rear, so it's unlikely your "mechanic-in-a-can" can get the fluid back there. The stuff will likely run out of the gaps in the housing before that happens.
To get serious, assuming you can't get to the small gaps in the pot. cases, you'll have to remove the front panel and unsolder the pot by carefully removing all solder on the PCB terminals and pads with solder-wick or similar fluxed braid product. Extract it carefully so as not to lift the copper traces and flush/clean the pot thoroughly, through the small gaps. I admit, I can't check whether they should exist on this model, but older equipment usually had them, as part of the rotation stop limits, located on the lower side. of the pot.
Hopefully though, you can access this gap in the balance control case and, with the preamp vertical to avoid spills on the PCB, just flush it properly - several times! It's always possible this treatment is not effective or not for long. You may even have to lose the balance control or make other mechanical arrangements to splice in a replacement as it's unlikely these products are still available as stock items.
Spare parts
I had a look at replacement parts a few years back. Not available and nothing that looks like it since it is a custom made special arrangement.
If cleaning does not work out okay, then you have 2 options:
1. Replace the combo by a normal volume pot and forget the balance.
2. Same but drill another hole for a standard balance pot. Maybe even at the back side of the amp for cosmetic reasons.
And you can buy another second hand copy and mix parts of course.
I made a separate power supply for this unit and did a cap replacement and some minor fiddling. This is a damn fine preamp.
Rudy
I had a look at replacement parts a few years back. Not available and nothing that looks like it since it is a custom made special arrangement.
If cleaning does not work out okay, then you have 2 options:
1. Replace the combo by a normal volume pot and forget the balance.
2. Same but drill another hole for a standard balance pot. Maybe even at the back side of the amp for cosmetic reasons.
And you can buy another second hand copy and mix parts of course.
I made a separate power supply for this unit and did a cap replacement and some minor fiddling. This is a damn fine preamp.
Rudy
I'm trying a little more cleaning. This time I got some compressed air to use with the deox thinking it may dislodge the more stubborn particles. I'm letting it soak before giving it another round. I don't feel confident enough to take it apart and perform the work. I'd leave that until I decide to stop using it (may take a while). I read good things about the phonos but I'm not able to try it.
People often call nad parts ugly. I find it ugly but in a charming sort of way.
I was reading about the pot in the 1300 pre amp prior to posting,
"I was only glad to get rid of the balance potentiometer, I have never used them. The picture suggests some other changes, as you can see. The only active circuit in use in the amp is the opa627. This simplification has allowed me to throw out, or shorten, most caps and switches in the signal path. (I have lowered the supply voltage to the opa somewhat)."
Would the same apply to the 1240? I never use balance faders either and if doing away with it would further refine its sound I'd be all for it. I would pay for the labor if that's the case. Again if I don't feel confident enough. It seems to posses high sleeper potential and I like that too.
People often call nad parts ugly. I find it ugly but in a charming sort of way.
I was reading about the pot in the 1300 pre amp prior to posting,
"I was only glad to get rid of the balance potentiometer, I have never used them. The picture suggests some other changes, as you can see. The only active circuit in use in the amp is the opa627. This simplification has allowed me to throw out, or shorten, most caps and switches in the signal path. (I have lowered the supply voltage to the opa somewhat)."
Would the same apply to the 1240? I never use balance faders either and if doing away with it would further refine its sound I'd be all for it. I would pay for the labor if that's the case. Again if I don't feel confident enough. It seems to posses high sleeper potential and I like that too.
1300
I have had the whole lot of NAD preamps. The 1300 did not live up to my expectations as a few other types, but the 1240 did.
And sure, you can ripp off a huge deal of the amp. Speaking of the line stage you can take out the tone circuit and make it a simple linestage a la the A1 Mod of Hennessy as mentioned above. Actually I did that mod around the time of first posting this stuff. But for one reason or the other, the preamp lost some magic and did not appeal to me anymore. From then on I gradually went back step by step to the original design and guess what: old sound again and more to my taste.
I guess there is some deviation from the straight line picture that causes some pleasant distortion and/or loudness effect in the original preamp. So this can't be better you say? Probably not according to theory and purists, but in the end I have to live with it and am the judge.
One such thing: I discovered only recently that the standard mode of operation is INFRA ON (pushing the button means INFRA OFF) so some filter is in the signal path all of the time. Try putting it OFF and you will notice small differences in the perceived sound, with INFRA ON sounding fatter and more relaxed (?? don't know why, before cutting the lows (below 20hz) I guess there's a mild upswing of midlows (20-50 Hz)?).
I have had the whole lot of NAD preamps. The 1300 did not live up to my expectations as a few other types, but the 1240 did.
And sure, you can ripp off a huge deal of the amp. Speaking of the line stage you can take out the tone circuit and make it a simple linestage a la the A1 Mod of Hennessy as mentioned above. Actually I did that mod around the time of first posting this stuff. But for one reason or the other, the preamp lost some magic and did not appeal to me anymore. From then on I gradually went back step by step to the original design and guess what: old sound again and more to my taste.
I guess there is some deviation from the straight line picture that causes some pleasant distortion and/or loudness effect in the original preamp. So this can't be better you say? Probably not according to theory and purists, but in the end I have to live with it and am the judge.
One such thing: I discovered only recently that the standard mode of operation is INFRA ON (pushing the button means INFRA OFF) so some filter is in the signal path all of the time. Try putting it OFF and you will notice small differences in the perceived sound, with INFRA ON sounding fatter and more relaxed (?? don't know why, before cutting the lows (below 20hz) I guess there's a mild upswing of midlows (20-50 Hz)?).
Yes, simply clipping the balance pot leads and substituting a couple of 25k resistors for the pot. resistances is as simple and close to the original in centre position as it can get. There are those who compulsively swap or "roll" anything removable in amplifiers but that is often a way of degenerating rather than improving quality unless you hear benefit in the sound effects produced by increasing distortion. Removing caps from the signal path exposes amplifiers to DC where it shouldn't be. Those who can do this with individual amplifiers carefully adjusted to null errors, may do this with success but I suspect that a lot of amps are binned by owners who tackle these projects with zero technical competence. Like mechanically modifying an engine; you need to know exactly what you are doing when unravelling an engineer's careful design. i
As described, you won't do much for the balance pot from the front panel. You do have to get out the screwdriver and get to work. If the gaps in the pot casing are visible, the whole operation is only 5 mins and you don't need any more equipment, so no drama and no further disassembly or soldering will be required. At least try what will become painfully obvious once you see the pot assembly or look at manual.
@ rmgvs
mhennessy's website appears to have been taken down. 'Not sure if an archive is accessible. It's a shame if we can't see his projects but perhaps he has moved on.
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mhennessy's last post was just over a year ago and 07' prior to that. Try your luck with an email
Since I'm not using phono I just left the infra off and haven't played with it. The bass eq is a nice feature, especially with certain recordings.
Thank you thank you, appreciate your helpful knowledge. External cleaning did not completely fix it. Over the weekend I will open it up. While I'm at it try to give all the controls a cleaning and inspect caps. As far as changing any of the original pathways I'll take a pass and leave well enough alone and just enjoy it.
Since I'm not using phono I just left the infra off and haven't played with it. The bass eq is a nice feature, especially with certain recordings.
Thank you thank you, appreciate your helpful knowledge. External cleaning did not completely fix it. Over the weekend I will open it up. While I'm at it try to give all the controls a cleaning and inspect caps. As far as changing any of the original pathways I'll take a pass and leave well enough alone and just enjoy it.
@ss4927: how intelligent! Wish I had that knowledge a few years ago. Enjoying the stuff and music is the best you can do.
@ian finch. Surprised to see the disappearance of Hennessy's A1 stuff. About the preamp he fitted into the A1: was a basic and universal 5532 opamp with feedback and gain of around 3 (3k / 1k low impedance divider in the feedback loop), nothing to it, only (from memory) a coupling-cap before and after the stage. Clean sounding, after some time I missed the 'warmth' (which you will call distortion I guess) and put the whole thing back in original shape.
@ian finch. Surprised to see the disappearance of Hennessy's A1 stuff. About the preamp he fitted into the A1: was a basic and universal 5532 opamp with feedback and gain of around 3 (3k / 1k low impedance divider in the feedback loop), nothing to it, only (from memory) a coupling-cap before and after the stage. Clean sounding, after some time I missed the 'warmth' (which you will call distortion I guess) and put the whole thing back in original shape.
My website...
From time to time, my website gets taken down by Force9/Plusnet (who are my ISP) because it gets too much traffic. I'm only allowed 250MB per day. Averaging over just a day sucks, because it only takes one link to it from a busy site to effectively cause a DoS attack on the site. This time, it was Instrucables.com: Make your first Serious Amplifier
So, I've moved the website to a paid-for host - you can find it at http://www.markhennessy.co.uk/ - please update your bookmarks.
When Force9 restore my access to my original webspace, I'll add redirect pages that point users to the new location. So old links will continue to work for as long as I get my internet from them.
Apologies for the inconvenience, but hopefully the new host will work out better.
Mark
PS: I am responding to enquiries from the website, and generally within a few days (although please note that the email contact form won't work until Force9 restore my access to the site - in the short term, I'm planning to use that until I decide how best to accept messages). People can always contact me via here, or any of the other forums I frequent. I've had other priorities over the last 8 years (since the kids came along), but I'm still here
From time to time, my website gets taken down by Force9/Plusnet (who are my ISP) because it gets too much traffic. I'm only allowed 250MB per day. Averaging over just a day sucks, because it only takes one link to it from a busy site to effectively cause a DoS attack on the site. This time, it was Instrucables.com: Make your first Serious Amplifier
So, I've moved the website to a paid-for host - you can find it at http://www.markhennessy.co.uk/ - please update your bookmarks.
When Force9 restore my access to my original webspace, I'll add redirect pages that point users to the new location. So old links will continue to work for as long as I get my internet from them.
Apologies for the inconvenience, but hopefully the new host will work out better.
Mark
PS: I am responding to enquiries from the website, and generally within a few days (although please note that the email contact form won't work until Force9 restore my access to the site - in the short term, I'm planning to use that until I decide how best to accept messages). People can always contact me via here, or any of the other forums I frequent. I've had other priorities over the last 8 years (since the kids came along), but I'm still here
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