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Old 24th April 2004, 09:01 AM   #1
NamWong is offline NamWong  Hong Kong
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Default Tweaks for NAD 3020

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.
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Old 24th April 2004, 06:48 PM   #2
Steven is offline Steven  Netherlands
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http://www.tcaas.btinternet.co.uk/index-4.htm

Cheers,

Steven
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Old 24th April 2004, 07:30 PM   #3
qwad is offline qwad  Australia
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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 TC
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Old 25th April 2004, 11:42 PM   #4
cpemma is offline cpemma  United Kingdom
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There'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...
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Old 26th April 2004, 04:04 PM   #5
NamWong is offline NamWong  Hong Kong
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Any suggestion why there was potential on the chassis?
Is it related to the power supply or grounding?
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Old 27th April 2004, 10:54 PM   #6
cpemma is offline cpemma  United Kingdom
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Quote:
Originally posted by NamWong
Any suggestion why there was potential on the chassis?
Is it related to the power supply or grounding?
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.

The modding page noted above is archived at http://web.archive.org/web/200304172...hau/audio.html
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Old 28th April 2004, 12:20 AM   #7
sam9 is offline sam9  United States
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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.
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Old 28th April 2004, 07:48 PM   #8
cpemma is offline cpemma  United Kingdom
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Quote:
Originally posted by sam9
--Maybe I'm missing something, but I don't see any large (~220uF) caps from rail to ground...
Virtually everything is on one big pcb, including the electrolytic caps shown on the nad3020ps.gif from the Class A site linked.

Thanks for the tips, if anyone can suggest a setting-up procedure I'll be a happy bunny.
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Old 28th April 2004, 09:50 PM   #9
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Quote:
Originally posted by qwad
replacing the archaic 2n 3055 o/p devices
Here is one good sub...

Quote:
"The transistors are 2SD733 and 2SB557. They are 30-amp types, but their
small-signal behavior matches the original devices, so no circuit changes
are necessary. I pored through transistor manuals at great length before
finding these types. They would be a perfect match for your souped-up power
supply - even one Farad per channel won't help you if your output stage is
in saturation.
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
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Old 29th April 2004, 08:33 AM   #10
djk is offline djk
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"--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.
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