ISC and ISC2 are the same thing, but are for each channel correct? I see that ISC2 which you found is right off of the collector of Q601. ISC is off of the collector of Q701 in the opposite channel. So are you wondering if the collectors of Q601 and Q701 are connected? If so, there is no continuity between them. If that isn’t what you’re asking my apologies.
I wish I had another large pair of 8 ohm resistors, I would drive the amplifier at forms and see if the voltage drops to 58v at a certain point. There isn’t a reason I can’t drive just one channel right? I can parallel up my loads and check it.
Dan
I wish I had another large pair of 8 ohm resistors, I would drive the amplifier at forms and see if the voltage drops to 58v at a certain point. There isn’t a reason I can’t drive just one channel right? I can parallel up my loads and check it.
Dan
OK, I see where you're coming from with that 🙂 I wasn't looking down the page at the other channel. ISC and ISC are the same (Q701 and 708) and ISC2 and ISC looks like an error on Q608. It should say ISC2 not just ISC.
So are both channels supplies at this high 74 volt level all the time?
What is the voltage on Q601 collector and Q701 collector?
And is this adjustment OK?
OK, I see where you're coming from with that 🙂 I wasn't looking down the page at the other channel. ISC and ISC are the same (Q701 and 708) and ISC2 and ISC looks like an error on Q608. It should say ISC2 not just ISC.
So are both channels supplies at this high 74 volt level all the time?
What is the voltage on Q601 collector and Q701 collector?
And is this adjustment OK?
Oh okay, yeah that screwed me up then I guess.
They are indeed both at 74v all the time, which the exception of when I removed all of the boards.
As soon as I get the amplifier boards back in I will check that voltage adjustment. Just finishing up replacing those outputs with the broken leads.
Dan
Fair enough 🙂 if both are the same. It just doesn't sound right to do that though, to run it all the time at the very high voltage.
It would be worth checking the ISC line for either channel and seeing what is setting it to high and try and work back from that and see if anything makes sense. You have to begin by verifying the ISC line voltage, then look at those two opamp outputs and see what voltage each is at and also look at the input from over temp sensing and see if that seems correct.
Its a case of piecing all the evidence together to see if it really is normal or not. Maybe it is normal but it doesn't sound right to be so for several reasons, heat generated, efficiency, lower reliability and so on.
It would be worth checking the ISC line for either channel and seeing what is setting it to high and try and work back from that and see if anything makes sense. You have to begin by verifying the ISC line voltage, then look at those two opamp outputs and see what voltage each is at and also look at the input from over temp sensing and see if that seems correct.
Its a case of piecing all the evidence together to see if it really is normal or not. Maybe it is normal but it doesn't sound right to be so for several reasons, heat generated, efficiency, lower reliability and so on.
NAD's Powerdrive circuit does not work in the way you might expect. See https://www.stereophile.com/content/nad-c-372-integrated-amplifier-what-powerdrive for an explanation of sorts. Not much technical depth there but it seems that running at the higher voltage most of the time is normal. There may be a patent for Powerdrive but I haven't searched for one yet.
OK, I will see what I can dig up. So basically just measure the voltage where it’s marked ISC on the schematic?
This thing is an absolute pain to work on, but I was able to measure the power drive voltage and make adjustments. it says to adjust for 100 mV or less, it was already under that when I checked both channels, but I’ve got them both down to around 10 mV.
I really appreciate you commenting, helps me kind of understand that this could possibly be running correctly. There were two things that made me think it was running this way, one is that the wattage output at 4 ohms is equal to 8 ohm. I’ve really only seen that in McIntosh where they use an output transformer, so in this sense, it would have to be because of a lower rail voltage that would cause no gain in output. The second thing is the person I was speaking with was telling me that for the given output, and how many pairs of transistors there were he didn’t think the C5200 and A1943 were very good choices and didn’t think they would last with a 4 ohm load, but said they would likely be fine into a 4 ohm load of the rails were reduced to 58v. That’s really the only thing that makes sense to me right now.
I definitely want to do a test into a 4 ohm load though to see if I can get the rails to drop as I’m driving it.
If all is good, the only thing left to do is to fix the display.
It doesn’t look it in person, but the display is fairly dim. It’s a vacuum display. at first I thought it was because it was wearing out, but it was a known issue with the firmware. So you just download the new version and install it on the amplifier and it does this two day burn and then all of a sudden you’re display is bright again. Run it through the RS 232 port.
Dan
Well, this is the amp that keeps on giving, it felt like it wasn’t quite ready for this world yet. The issue I am having is a fluctuation in output, I was thinking it might be the relays considering there’s about 1000 of them in here, but that isn’t making sense anymore. Here is the schematic
It looks like after the very first input select relay the signal goes through, after that point all share the exact same circuit.
When I first powered it up, I input a 500 mV rms signal into it and the very most output I could get was roughly 12 W from each channel. Thinking it might be a muting thing I hit the mute button a couple of times which triggered a relay. All of a sudden I could get about 80 W on one channel and 12 on the other. Flipped it a few more times and now the channels are even.
The issue is input are use gives me a different amount of output and it seems to change. These are all measurements I took after increasing the input signal to 1.0 vrms. Also each input had a digital input trimmer level
I started with disc. It was set to an input level of -0.5 db and I could get the amplifier into clipping, but this is right before clipping
So about 190w.
The next input I tested was CD, this was the volume up to max. I couldn’t get any more than about 20 W per channel.
I did see that the trimmer level was lower so I brought it up to 0.0 db. After adjusting that I could get 174w per channel, but could not drive it into clipping.
The other four inputs were set to -0.5 db and I could not get any of them to clipping. They all had different output levels.
Tuner - 153w per channel
Input 4 - 144w per channel
Input 5 - 144w per channel
Input 6 - 153w per channel
I thought it was weird how they were changing a little bit so I went back to input number one (disc) and on this channel, I was able to easily get it to clipping previously, now I could no longer get to clipping. It would hit about 185w and then no more volume control.
I was thinking it could possibly be dirty relays, but what a dirty relay just strictly reduce the signal, and it’s not likely that they would cause the same reduction in both channels. As in every input, the channels were balanced.
So I went into the input trimmer level of “disc” and turned it up from -0.5 to +2.5. I could now once again clip the signal.
Does one volt RMS input seem reasonable? In vintage gear integrated amps can generally go into clipping with 1v. Should I turn up the trimmer input for each channel? I know that they can go to at least + 10.0, I didn’t go any higher than that.
I just want to make sure that I am able to at least go into clipping with each input I use. I’m not sure why each input has a different output level if they have the same input level and their trimmer is set the same. At least it is listenable to. Is there any negative to cranking them all to +5.0? Could I potentially be clipping the input if say I hook up a CD player or a streamer and they happen to output 2.5v rms vs the 1.0 my HP generator is outputing?
Dan
It looks like after the very first input select relay the signal goes through, after that point all share the exact same circuit.
When I first powered it up, I input a 500 mV rms signal into it and the very most output I could get was roughly 12 W from each channel. Thinking it might be a muting thing I hit the mute button a couple of times which triggered a relay. All of a sudden I could get about 80 W on one channel and 12 on the other. Flipped it a few more times and now the channels are even.
The issue is input are use gives me a different amount of output and it seems to change. These are all measurements I took after increasing the input signal to 1.0 vrms. Also each input had a digital input trimmer level
I started with disc. It was set to an input level of -0.5 db and I could get the amplifier into clipping, but this is right before clipping
So about 190w.
The next input I tested was CD, this was the volume up to max. I couldn’t get any more than about 20 W per channel.
I did see that the trimmer level was lower so I brought it up to 0.0 db. After adjusting that I could get 174w per channel, but could not drive it into clipping.
The other four inputs were set to -0.5 db and I could not get any of them to clipping. They all had different output levels.
Tuner - 153w per channel
Input 4 - 144w per channel
Input 5 - 144w per channel
Input 6 - 153w per channel
I thought it was weird how they were changing a little bit so I went back to input number one (disc) and on this channel, I was able to easily get it to clipping previously, now I could no longer get to clipping. It would hit about 185w and then no more volume control.
I was thinking it could possibly be dirty relays, but what a dirty relay just strictly reduce the signal, and it’s not likely that they would cause the same reduction in both channels. As in every input, the channels were balanced.
So I went into the input trimmer level of “disc” and turned it up from -0.5 to +2.5. I could now once again clip the signal.
Does one volt RMS input seem reasonable? In vintage gear integrated amps can generally go into clipping with 1v. Should I turn up the trimmer input for each channel? I know that they can go to at least + 10.0, I didn’t go any higher than that.
I just want to make sure that I am able to at least go into clipping with each input I use. I’m not sure why each input has a different output level if they have the same input level and their trimmer is set the same. At least it is listenable to. Is there any negative to cranking them all to +5.0? Could I potentially be clipping the input if say I hook up a CD player or a streamer and they happen to output 2.5v rms vs the 1.0 my HP generator is outputing?
Dan
Well, never mind on that theory. I couldn’t increase the output by increasing the trimmer for Disc. But I just swapped it to the CD input and anywhere from 0.0 up to + 10 db the output is the same 38 vrms. So that trimmer does make no difference. At least for that input.
I increased the input voltage to 1.5 vrms and then of course I could go into clipping easily
Does this seem like a fault? It sure does seem like a fault. In reality I don’t think that this would ever be a noticeable issue unless you’re coming like straight from headphone output that puts out less than a volt. It would still get loud though I’m sure. It’s just bugging me lol.
Dan
I increased the input voltage to 1.5 vrms and then of course I could go into clipping easily
Does this seem like a fault? It sure does seem like a fault. In reality I don’t think that this would ever be a noticeable issue unless you’re coming like straight from headphone output that puts out less than a volt. It would still get loud though I’m sure. It’s just bugging me lol.
Dan
That brings me to another point. I want to measure this with a Quantasylum QA403. Every other amp I’ve measured I set the chirp for 500-600 mV input and can measure the amp into distortion. I found that as I increased the input the output distortion went down. So I was told 500 mV was a good medium. For this amp I’ll need to set the chirp for like 1.2v. Curious how that will affect the distortion measurements.
I did hook up a small pair of bookshelf speakers and an old Philips CD player that uses a TDA1541A and it did get plenty loud at -10 on the volume control. So I imagine at 0 it would be uncomfortably loud. So I guess my theory was right that the lower output isn’t really an issue in real world use.
Dan
I did hook up a small pair of bookshelf speakers and an old Philips CD player that uses a TDA1541A and it did get plenty loud at -10 on the volume control. So I imagine at 0 it would be uncomfortably loud. So I guess my theory was right that the lower output isn’t really an issue in real world use.
Dan
Thanks Bill, that seems to confirm it then 🙂 It's normal (and the opposite) to the way most switched rail designs operate.
I'll look at the rest later 🙂
At rest the low rail is engaged, at any input the voltage will be any where between the low and higher rail. At near clipping the rail will be a high rail. If heat or over current rail be low. Mute the inputs and measure again
I hit mute on the remote and the rails were still at a +/- 74v.
I ran it all night from about 4 pm PST and still going now. Have it playing fairly loudly and it’s doing great. On my bench when I was driving a sine wave into it I got the heatsinks pretty warm. Currently just playing music the only things warm are the class A low noise line driver amp modules you see there on the top board. Those are pretty toasty.
Dan
Bill's link to that article suggests that it doesn't work like you describe (but which would be the more logical way tbh). It sounds like it runs at the high rail voltage all the time except when extreme loading would push it outside its performance envelope at which point it switches to a lower rail.
In practice a domestic user will never get close to any of these points as most listening happens at just a few watt output.
Here is something that is odd and may explain some of what I was experiencing yesterday. So I have a CD player hooked up to “disc” input and it’s sounding great. I use the remote to switch inputs to CD, it does and there is still music play, though at a reduced volume. And by reduced I mean just slightly deduced, still plenty loud. I go through all of the inputs and they are all playing the music coming through the disc input. Including balanced in.
Now shouldn’t these relays along the right side prevent anything from coming through?
It’ll be a pain to get to the bottom side of the board, but I’ll have to see if every single coil on both input boards have voltage across them. I wouldn’t suspect every single relay to be stuck closed.
It weird because as you change the inputs you can hear relays engage and disengage.
Curious if anyone had any thoughts.
Thank you,
Dan
Now shouldn’t these relays along the right side prevent anything from coming through?
It’ll be a pain to get to the bottom side of the board, but I’ll have to see if every single coil on both input boards have voltage across them. I wouldn’t suspect every single relay to be stuck closed.
It weird because as you change the inputs you can hear relays engage and disengage.
Curious if anyone had any thoughts.
Thank you,
Dan
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I measured and the voltage across the coils of the relays are behaving correctly, get about 22v across the relay of the input that is selected. The odd thing is that with every other relay that is supposed to be open measures 142 ohms to 135 ohms across those open contacts.
So measuring across the red mark when the contacts are supposed to be open I’m seeing 142 ohms. Hmmm where could that be coming from. Am I right in my thinking that the only thing that all of the inputs have in common at this portion of the circuit is ground?
I just checked the relay contacts to ground and pins 1 and 4, the pivot points on all of the relays including the relay engaged measure about 78 ohms to ground. That tells me why I was having a difficult time driving the amplifier. The signal coming through is being pulled the ground through a 78 ohm resistor.
Dan
So measuring across the red mark when the contacts are supposed to be open I’m seeing 142 ohms. Hmmm where could that be coming from. Am I right in my thinking that the only thing that all of the inputs have in common at this portion of the circuit is ground?
I just checked the relay contacts to ground and pins 1 and 4, the pivot points on all of the relays including the relay engaged measure about 78 ohms to ground. That tells me why I was having a difficult time driving the amplifier. The signal coming through is being pulled the ground through a 78 ohm resistor.
Dan
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It looks that way.
All the inputs also feed into U109 (a TC9274N) which might be faulty. It looks like that chip selects the record out signals. You could try removing the chip.
(I have recollections of these TC chips giving issues but that was like 25 years ago. They were used in lots of equipment if I'm remembering correctly)