Recently I've got an old NAD 3020. It works but the high frequencies are not good enough. I plan to replace some of the caps and VR. Also some very low noise comes out, apparently from the transformer, and potential appears on the chasis. Any suggestions for improving the amp?
I am also searching for the schematic of this amp.
I am also searching for the schematic of this amp.
hi you could look at replacing the archaic 2n 3055 o/p devices with more modern ones ie toshibs you would find in aksa amps etc, the same thing goes with the small signal devices along with the usual thing with caps etc,,,,,cheers
TCThere's another site with various mods to the 3020, seems to be borked ATM but starts here.
(Google may have it cached).
From memory, several of the electrolytics were paralleled with film or pp and I think the 3055s were changed, but other mods were more drastic - bypass of tone controls, etc.
On my list of things to do...
(Google may have it cached).
From memory, several of the electrolytics were paralleled with film or pp and I think the 3055s were changed, but other mods were more drastic - bypass of tone controls, etc.
On my list of things to do...
Chassis isn't connected to mains earth through the mains lead, but there's an earthing terminal on the back of the amp if you need to fit a non-looping earth.NamWong said:
The modding page noted above is archived at http://web.archive.org/web/20030417235855/http://www.angelfire.com/ca/rchau/audio.html
I'm only guessing at this so take my suggestions as ideas not facts. If someone posts that I'm nuts, listen to them as they might be right.
I would suggest focusing mostly on the input section. Except for the rather old-fashioned 2n3055/2n3055's, the output looks like a faily decent EF design. Given the rail voltages and the power output, the output transistors are perhaps not all that much of a limitation. However, if R641 is a reostat or something to adjust bias, I would suggest you do the same as I suggest for VR5, below, VR5 probably has to be adjusted first. If that's not what R641 is, then it looks to me like you have a fixed bias which is almost certainly suboptimal by now, so depending on you skill and experience or availablity of someone you can pay to do it, try to put a trimpot in there and have the bias optimised. A larger, 47uF cap parallel to C627 might furthther help protect the bias circuit from rail noise.
Since you are not going to change topology, there are only some incremental things that can be done. The input section is (I believe), by it's nature, the primary source for noise and distortion in this design.
--A first step is to find a service manual (or maybe someone here can post the info) and adjust VR5 since after 20 years or so there is a good chance something has drifted.
--As previously stated replacing resistors with 1% metal film might be good, especially if you can find datasheets and go for ones with lowest noise rating.
--C601 and C603 seem awfuly small to be in series at the input. **IS there some reason they need to be this small?** They are almost certainly a low frequency noise/distortion mechanism. I don't know how much physical space is available, but to the extent you can replace them with higher values, that should help. If nothing else fits, even bi-polar electros might be better (use Black Gate "N"s if the idea worries you).
--Maybe I'm missing something, but I don't see any large (~220uF) caps from rail to ground. If there is a convenient place to mount these near where the supplies connect to the PCB that would help keep rail born noise and ripple out of the signal.
By the way, I had one of these years ago and it was very good at the time. If you do nothing else but confirm the adjustments mentioned above are correct, you will have a pretty decent unit. If you want to go further but avoid some of the more difficult suggestions, just replacing the electrolytics is not a bad idea since they most definaely go bad with age, if not already then eventually.
I would suggest focusing mostly on the input section. Except for the rather old-fashioned 2n3055/2n3055's, the output looks like a faily decent EF design. Given the rail voltages and the power output, the output transistors are perhaps not all that much of a limitation. However, if R641 is a reostat or something to adjust bias, I would suggest you do the same as I suggest for VR5, below, VR5 probably has to be adjusted first. If that's not what R641 is, then it looks to me like you have a fixed bias which is almost certainly suboptimal by now, so depending on you skill and experience or availablity of someone you can pay to do it, try to put a trimpot in there and have the bias optimised. A larger, 47uF cap parallel to C627 might furthther help protect the bias circuit from rail noise.
Since you are not going to change topology, there are only some incremental things that can be done. The input section is (I believe), by it's nature, the primary source for noise and distortion in this design.
--A first step is to find a service manual (or maybe someone here can post the info) and adjust VR5 since after 20 years or so there is a good chance something has drifted.
--As previously stated replacing resistors with 1% metal film might be good, especially if you can find datasheets and go for ones with lowest noise rating.
--C601 and C603 seem awfuly small to be in series at the input. **IS there some reason they need to be this small?** They are almost certainly a low frequency noise/distortion mechanism. I don't know how much physical space is available, but to the extent you can replace them with higher values, that should help. If nothing else fits, even bi-polar electros might be better (use Black Gate "N"s if the idea worries you).
--Maybe I'm missing something, but I don't see any large (~220uF) caps from rail to ground. If there is a convenient place to mount these near where the supplies connect to the PCB that would help keep rail born noise and ripple out of the signal.
By the way, I had one of these years ago and it was very good at the time. If you do nothing else but confirm the adjustments mentioned above are correct, you will have a pretty decent unit. If you want to go further but avoid some of the more difficult suggestions, just replacing the electrolytics is not a bad idea since they most definaely go bad with age, if not already then eventually.
qwad said:
Here is one good sub...
I have a 7020 with this mod...
When i sold these we used to add some C to the power-supply. It was particularily effective on the poweramp (a friend still has one we added 2x68,000 uF to).
I cooresponded with a fellow who replaced almost every cap & resistor in his 7020 (more room to work inside a 7020 than a 3020). The one place where you had to be careful changing values was on the power supply regulator board. This one ended up a REAL giant killer.
I have a 3020 in the lab, that i needed to borrow parts from to fix another, so i went thru it and stripped out all the extraneous circuitry (tone controls/loudness/soft-clipping etc). Results aren't back in from that one yet -- i missed something and haven't got it working yet.
There are a number of revisions of the 3020 so your unit may not exactly match the scematics you find... i have a pdf of the older units service manual.
dave
"--C601 and C603 seem awfuly small to be in series at the input. **IS there some reason they need to be this small?** They are almost certainly a low frequency noise/distortion mechanism. I don't know how much physical space is available, but to the extent you can replace them with higher values, that should help. If nothing else fits, even bi-polar electros might be better (use Black Gate "N"s if the idea worries you)."
How can you give advice about a circuit you don't know what it even does?
Better parts, OK, but don't mess with the values.
How can you give advice about a circuit you don't know what it even does?
Better parts, OK, but don't mess with the values.
"How can you give advice about a circuit you don't know what it even does?"
I've seen caps in series with the input before. Normally they are simply for AC coupling. But I thought I wrote the comment with enough uncertainty to express cautuion if need be.
A 1uF cap in series with the input will roll off the low frequencies well into the audio range. This demonstrated by somevsimple arithmetic, by a simple spice simulation and by listening. In equipment from that period, this (low value AC coupling caps) seems pretty common. I'm guessing the reason is that the primary signal source was a turntable. Although very good for its price an NAD 3020 was still a budget item, and likely to be teamed with a budget turntable. Thus rolling off lows like this supressed "rumble" , and as there was less LF content on the typical LP of the day than CDs or modern vinyl, little hard was done. I suspect it was also justified by the expectation that speakers that were likely to be teamed up with the NAD didn't go too deep as well.
I presume the posdter will be listening to CDs so the effect of the low values caps would have a greater impact on him than they would in 1980. In addition to the above, a 1uF electro at that point does more than just roll off the lows, it will add distortion as high up in the spectrum as 100Hz. This isn't just "capacitor lore", you can demonstate it with SPICE and it can be measured. Individual people vary as to whther they can hear it.
I've seen caps in series with the input before. Normally they are simply for AC coupling. But I thought I wrote the comment with enough uncertainty to express cautuion if need be.
A 1uF cap in series with the input will roll off the low frequencies well into the audio range. This demonstrated by somevsimple arithmetic, by a simple spice simulation and by listening. In equipment from that period, this (low value AC coupling caps) seems pretty common. I'm guessing the reason is that the primary signal source was a turntable. Although very good for its price an NAD 3020 was still a budget item, and likely to be teamed with a budget turntable. Thus rolling off lows like this supressed "rumble" , and as there was less LF content on the typical LP of the day than CDs or modern vinyl, little hard was done. I suspect it was also justified by the expectation that speakers that were likely to be teamed up with the NAD didn't go too deep as well.
I presume the posdter will be listening to CDs so the effect of the low values caps would have a greater impact on him than they would in 1980. In addition to the above, a 1uF electro at that point does more than just roll off the lows, it will add distortion as high up in the spectrum as 100Hz. This isn't just "capacitor lore", you can demonstate it with SPICE and it can be measured. Individual people vary as to whther they can hear it.
I'd be grateful for a copy. I've no firm idea of the build date, it was bought s/h in '82, the manual is dated 1980. It does have MJ29655 PNPs, not the 2N2955 shown on the schematic, I've not compared any deeper than I can see through the grille. My main thought is to replace the electrolytics, which must be past their sell-by date, though there's no serious problems with hum.planet10 said:
"I've seen caps in series with the input before. Normally they are simply for AC coupling. But I thought I wrote the comment with enough uncertainty to express cautuion if need be."
Have you ever seen an active filter?
Do you know how to figure the frequency and Q?
Blindly messing with the RC time constants affects the PRAT of the amplifier.
"Except the C for the output stage."
There are limits to that too. How big do you think you can go before the rectifier will blow? Do you know how to calculate the % of ripple? What is the point of diminishing return? Considering that the voltage gainstages of the PA and the first current gain stage run off a fully regulated power supply, hows does that change things?
Have you ever seen an active filter?
Do you know how to figure the frequency and Q?
Blindly messing with the RC time constants affects the PRAT of the amplifier.
"Except the C for the output stage."
There are limits to that too. How big do you think you can go before the rectifier will blow? Do you know how to calculate the % of ripple? What is the point of diminishing return? Considering that the voltage gainstages of the PA and the first current gain stage run off a fully regulated power supply, hows does that change things?
Banned
Joined 2002
How did you measure this ? Maybe it is just capacitive coupling between primary and secondary of the transformer and therefore harmless. But it could also be a dangerous insulation failure.
How does it behave when you turn the mains plug by 180 degrees ? How much current flows from the chassis to earth when you measure with a multimeter (use a series resistor to avoid fireworks or worse things !!!) ?
Regards
Charles
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