Modding a Pre/Pro.

[McLoki]

As many of you know - I have an NAD pre/pro (T163) that I purchased to replace my old Onkyo777 AVR. While I am happy with the increased clarity and features that the NAD offers me over the Onkyo, it is just not overwhelming me paired with the LSi speakers. (I am mostly HT and the NAD and LSi combo is just to laid back for my tastes.)

I have been doing some research and have thought about changing out the OpAmps in the pre-out and DSP boards in the NAD. I have purchased the service manual for the NAD and know people with the skills and equipment to swap out parts like this.

Right now all the OpAmps in the NAD are NJM5332M (A solid but very inexpensive opamp). From the research that I have done - something more on the lines of a AD823AN will provide me with the sound that I am looking for.

What else do I need to replace other than the opamps? I know it can get overboard really quickly, but I am just looking for a little more detailed - less warm sound from the NAD. The OpAmp change will end up costing me about $70 + labor. (labor maybe another $50 or so) - I can always change it back if the sound doesn't improve in the way that I expect it to.

Just looking for other inexpensive mods that can be done while the case is open. (or if just changing out the opamps can give me the change I am looking for)

Thanks,

Michael

[GV#27]

You might want to increase the amount of capacitance in the power supply and make sure that the new opamps are well bypassed.Using a combo of Panasonic .1uf stacked films paralleled with Panasonic FC series 100uf works real good.
Also if it has cheap electrolytic coupling caps at the pre out you may want to change those to film types or if DC offset is very low you can remove them.

[Ferres]

You can look at this rotel thread regarding similar modifications.
http://www.htguide.com/forum/showthr...3&page=1&pp=35

[McLoki]

Originally Posted by Ferres

You can look at this rotel thread regarding similar modifications.
http://www.htguide.com/forum/showthr...3&page=1&pp=35

Oddly enough - that is the thread that got me thinking about doing this with my NAD. He used the ad825 in the thread. (for only the front channels) I was thinking of using the ad823 (it is much cheaper) but for all channels.

What I am looking for is

• better details than I have now.
• Less of a laid back / warm sound. (not to get rid of all of it, but to reduce it from where it is now)
• A more cohesive front and side stage than I have now. (I have heard it with my speakers when using Opus's preamp so I know it is possible.)

I know I could just purchase another preamp, but the WAF on that is pretty low and from the reading I have done - it sounds like an OpAmp swap may be able to give me what I want for about $100 total.

I will keep you all informed as to how it goes.

Michael

[Early B.]

Originally Posted by McLoki

I know I could just purchase another preamp, but the WAF on that is pretty low and from the reading I have done - it sounds like an OpAmp swap may be able to give me what I want for about $100 total.

You should budget at least $100 for labor if a professional electronic tech is gonna do the mods for you. IMO, if the cost to do this mod is over $200, you may as well sell what you have and buy another pre. Even if you do the mods, I doubt you'll be happy with the NAD in the long run. I'd never mod a product that I didn't already like a lot.

[Ferres]

It is a nice option when the gear passes it's warranty years. Cheaper than getting a whole new one.

[McLoki]

Prior to attempting to mod my preamp - I have decided to just sell it and get something else. If it does not sell, I may still mod it I don't know.

Here is a link to the sale thread if interested. I have not even taken the cover off at this point - (I do have the service manual though).

Michael

[McLoki]

Ok - my sale fell through (for the pre I was wanting to purchase and selling my NAD to gain funds for) so the mod is back on.

Through an odd twist of fate - I have gotten all the opamps I need to replace for free. (they will be here in about 4 days.) I have a few questions about what to replace though.

To start with - all the opamps in my pre are NMJ4553m's.

There are 6 opamps on the DSP board and 5 opamps on the pre-out board.

On the preout board the opamps are as listed:

• Front right/left
• Surround right/left
• Surround back right/left
• Center/Sub
• the last one is labeled MS L and MS R - I have no idea what this one is for.

On the DSP board the opamps are not as well defined what they are actually used for. (I can only find 5 of the 6 on the schematic - There are defineately 6 on the board though.)

My question is, should I only replace the ones on the pre-out board that I know I use (front, Surround, Center/sub) or should I go ahead and replace all of them.

On the DSP board - should I wait to replace them until after the pre-out board is done? Should I just do them all at once?

Like I said - I am getting enough opamps to replace all of them and they should be here in about 4 days.

What do you think?

Michael

[Ferres]

You could ask the guys at the Rotel forum. Atleast they have actually gone on to do this even though it was not a NAD.

[GV#27]

I would only change the ones for the front left and right and center/sub because these will yield the biggest benefit.
Can you put a pic of the DSP booards schematic ?Are there any electrolytic coupling caps at the outputs?If so you might want to change them.

[McLoki]

I have the .pdf file of the service manual that I would be glad to e-mail you (it is 10 meg in size). The schematic of the DSP on a normal 8.5x11 sheet of paper is so small it is useless.

PM me your e-mail address and I would love to have you take a look for me.

Thanks,

Michael

[GV#27]

pm sent

[McLoki]

.PDf sent. Please let me know if you get it or not. (mail program was acting odd due to the message size - It left me aware of whether it sent or not)

Thanks again.

Michael

Edit - I tried to send it twice. I need to know if they are getting through before I try it again. (I don't want to fill up your mailbox.)

[GV#27]

Yep got it all 58 pages.I will take some time and go through it.

[McLoki]

Awesome - thanks again for your help. I hope the AD823 will work out to be a good opamp for it. (I have 11 of them showing up in 4 days or so)

Let me know what else you would consider changing out while it is open.

Michael

[GV#27]

I have a list in the works.
I usually use BB OPA 2134's or OPA 2604's so I am not familiar with the AD823 but it should be fine,anything is better than the 4560's.A word of caution though, I assume you where aware that these are SMD (surface mounted devices) and are very difficult to remove and replace with an ordinary solder iron.

[McLoki]

Actually I work in an electronics manufauturer. I am going to have one of the people on our SMT line that fixes (removes and replaces) smt parts all day do it for me. They are estimating about 1/2 hour to replace 11 chips. (the AD823 that I ordered are all SMT parts)

They have the correct solder, Irons and most importantly - experience to replace the parts correctly.

I just have to provide the parts and tell them what to replace.

Michael

[GV#27]

Awesome.

[McLoki]

Here is a pic of part of our facility. The top picture is of our SMT lines. We use My9 and My12 machines as well as a chipshooter to place the SMT parts. They go from there to wave ovens to inspection lines.

There is a window in our department (right outside the computer room) that overlooks these lines....

[McLoki]

Originally Posted by GV#27

I have a list in the works.
I usually use BB OPA 2134's or OPA 2604's so I am not familiar with the AD823 but it should be fine,anything is better than the 4560's.

I noticed that the schematic listed that they were 4560's, but the parts list at the back of the service manual lists all opamps as NJM5532's. I have not opened the NAD yet, so I am not sure what is actually in there.....

Michael

[McLoki]

Also there is one opamp on the main board that is not a 5532 - I am not sure what it is used for either....

[GV#27]

In the schematic the ones on the pre out board are shown as 5532's and the ones on the dsp board are 4560's but the parts list says they are all 5532's.Strange.

[GV#27]

Originally Posted by McLoki

Also there is one opamp on the main board that is not a 5532 - I am not sure what it is used for either....

what part number ?

[McLoki]

Part U708 - NJM4558 opamp.

On the main board schematic - it is right in the center. (cant miss it on that one...)

[GV#27]

Originally Posted by McLoki

Part U708 - NJM4558 opamp.

On the main board schematic - it is right in the center. (cant miss it on that one...)

Nope not hard to find when it is the only one. EDIT: It is the headphone amp.
The opamps that I would recommend changing are U609 and U612 on the pre out board and U1011 and U1013 on the DSP board .But I can't seem to find any local bypass caps near the opamps which is very odd.If you put in a faster opamp like the 823 it may oscillate with out them.
A nice touch though is that the pre outputs are driven by a discrete opamp.

[GV#27]

I also have list of caps that could be changed but they are all SMD so it would be a royal pain in the back side to bother .

Your facility looks very hi tech and very clean.

[GV#27]

Michael,Here is a good article about the importance of using bypass caps on the power supply pins of opamps. http://www.elecdesign.com/Articles/I...leID=1478&pg=3

[McLoki]

So can they be added to an existing board? The traces are already there and you can't really add more at this point. I am unsure if the board is multi layer or not. (most now adays are) so I would think drilling for through hole would be out.

While bypass caps may be important - what can I do to change the design now?

Michael

[GV#27]

Originally Posted by McLoki

So can they be added to an existing board? The traces are already there and you can't really add more at this point. I am unsure if the board is multi layer or not. (most now adays are) so I would think drilling for through hole would be out.

While bypass caps may be important - what can I do to change the design now?

Michael

I hate Surface mounted parts it makes it real pain in the butt to do mods.
The only way it could be done is by carefully soldering one leg of the cap to the appropriate pin of the opamp and the other leg of the cap to a ground point near by.If no ground point is near it is possible to scrape some of the solder mask off of a ground trace and solder to it.Not recommended for the inexpierienced though.

[McLoki]

So I assume that the opamp will be soldered just like normal with one pin also connected to the leg of a cap. (the other leg of the cap would run to a ground location.)

In that case, couldn't I have the opamps replaced and if I have issues with occilation - add the bypass cap at that time? (or maybe my understanding is not quite there yet)

Michael