NAD 3020 Tone Bypass

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I'm thinking about modding my NAD 3020 to bypass the tone control section. There's a description of one version of this mod here:

295: Audio/NAD/3020/Kimber/Cardas

But this describes, basically, a bunch of mods all taken together. My idea is to do just the tone controls, and maybe work on other stuff later. As we all know, sometimes the character of an amp is a bigger influence on listenability than a sort of "objective" clarity, so I'm trying to mod slow, and test a lot.

The design of this amp is...well...very unusual. As far as I can tell (which may not be very far) it seems like the preamp is actually an op amp (ish) made from discrete components, with the tone controls being placed in the negative feedback path. As such, obviously one can't simply "remove" them without driving the theoretical amplification factor to infinity. So here's my idea: if one were to just disconnect C520, bridge all the terminals of VR3b, and disconnect the wiper of VR2b, one should get (close to) the feedback designed for the midrange, but without filtering, as well as removing the excess loading. In comparison with all the mods on the PeaK site, this would be very simple, very quick, and easily reversible. Though obviously, conversely we wouldn't be cleaning up the signal path to quite the same degree.

What stops me at the moment, though, is that I can't figure out what on earth R534 is for. At 68K, it seems like the voltage drop would be enough to make the signal at the C524 end very weak, and therefore I should just not worry about it. But then I don't know why NAD put it in here. This entire circuit is pretty confusing for my limited electronics knowledge, honestly.

Anyone know this amp and want to help a little?

The schematic I'm looking at is in the service manual, here:

http://www.thegreatbear.net/nad-3020-service-manual.pdf

Thanks!
 
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The 3020 is a good sounding, basic and inexpensive integrated amp from way back. Unless it is failing due to wear, damage or corrosion, I would avoid messing with it because much of the sound character is in what you have now, warts and all. If you hope to resell some day, bear in mind that the next guy won't appreciate hacks, even if if they plan to do them for themselves.

You will be better off studying Baxandall tone controls if you want to get to grips with what the function of its components are.
Here's just one simplified reference from Wikipedia. enjoy the maths!
The James-Baxandall Passive Tone-Control Network
Note: this isn't the active version we see in the NAD but that's easy enough to locate. Try the attached simulator from the Duncan Amps link.
That looks like more fun.
 
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Sorry I do not have a 3020 but a c352
Anyway I share your worry
Just a push on the defeat of tone controls on my amp has a beneficial effect
I think that tone controls are a real disaster
I was thinking to mode my unit to use it as a better preamp but after looking the schematic I think I will look for another Nad model without these damned tone controls
please excuse my outburst
Good luck anyway
Regards,
gino
 
Well it's not failing, or anything. The pots for the tone and balance are corroded/dirty/something to the point where they're pretty much not usable. But as I know, that should be easily fixable, and the fix for that should probably clear up the sound a bit (and will be the first step before modding further, but I'm thinking ahead).

But although it's not failing, this amp has always had a curious mix of really "good" and really "bad" for me. It sounds both tight and warm, with superb focused bass and superb focused treble---but with a weak, muted midrange. That's the part I'm trying to make up for. Turning down the bass and treble and turning up the volume don't fix it, so I'm trying to figure out if there's something else going on here. Hence, I'm thinking about screwing with the tone controls. The nice thing about the proposed mod is that it's very easily undoable. Just remove the short between the three terminals of Vr3b, then reconnect the wiper of Vr2b and C520. I suppose you could be even simpler and just disconnect the wiper and the signal-side part of Vr2b, instead of disconnecting C520. This all still seems like it would work (ish?), anyway; and if it's just messing with connections to pots, maybe its worth an experiment.

As for understanding the circuit itself, I think I mostly get the James-Baxandall Tone-Control stuff. That's a super helpful reference, thanks. But this just confirms my suspicion that R534 is doing something fishy here, as I don't see anything like that in the couple of James-Baxandall references I looked at. Of course, I could just not be seeing the part where this crept into the general design of something like this...
 
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The potentiometer wipers are actually the output node of the filter, connected by caps from points A and B. So resistors R533,4 shunt the filter which limits loading according to the ref. (R5) , That doesn't seem right but perhaps it does apply a shelving effect but I haven't tested it. In the James version, you see this resistor shunted to ground.
 
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Also, ginetto, I believe the 3120 is essentially the same as the 3020 but without the tone controls, if you don't want to risk modding something

Hi ! sorry for the delay in the reply ... I was visiting parents.
You are right and actually I think about that unit
To be honest I am always perplexed
I find a lot of nice sounding power amps ... even vintage ! even power amp sections of integrated
When i look for a nice solid state preamp starts the nightmare :(
By the way if you have any suggestion please feel free to email me ;)
Thanks a lot and kind regards,
gino :)
 
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