Need comments on amp design

[mightydub]

R1 is just a dummy load to get some current flowing in the output transistors. - I omitted the output network for simplicity, mainly because I was just trying to understand the amp circuit.

I have been playing in LTspice with the mods suggested, and also a more extensive mod of changing the output driver pairs to complementary pairs (inspired by reading Rod Elliot's articles on amp design in general and his 60W amp in particular.) I'll post it if there's interest.

[mightydub]

Today I had the time to get all the new parts into the amp - turned it on, tweaked the DC and idle current, let it warm up thoroughly and then checked/adjusted again, then put it all back together and hooked up the speakers. It's running and making music again after 10 years in a box.

Thanks to all for the comments and suggestions.

[Bonsai]

Mightydub, did you put those emitter ballast resistors in the output devices as suggested earlier in this thread? If you have not, your amp will be going back into a box in the garage sometime soon.

I'd suggest 0.33 or 0.47 Ohms 4W - you can get them from Digikey.


Also, I'd check that the pot used to adjust the quiescent current is ok - in fact, double check this part of the circuit. If you have a dry joint or a crack on any of the PCB tracks around this part of the circuit, the output will probably blow (might be this problem only manifests itself when the amp gets hot).

Happy listening

[mightydub]

Originally posted by Bonsai
Mightydub, did you put those emitter ballast resistors in the output devices as suggested earlier in this thread? If you have not, your amp will be going back into a box in the garage sometime soon.

I'd suggest 0.33 or 0.47 Ohms 4W - you can get them from Digikey.


Also, I'd check that the pot used to adjust the quiescent current is ok - in fact, double check this part of the circuit. If you have a dry joint or a crack on any of the PCB tracks around this part of the circuit, the output will probably blow (might be this problem only manifests itself when the amp gets hot).

Happy listening

yes - I added 0.33 ballast resistors, as well as 6R8 between the drivers and the bases of the output transistors. And I went over the pcb and the pots with a magnifier looking for cracks.

Thanks for the advice on this.

[negromo]

Originally posted by mightydub


yes - I added 0.33 ballast resistors, as well as 6R8 between the drivers and the bases of the output transistors. And I went over the pcb and the pots with a magnifier looking for cracks.

Thanks for the advice on this.

Hi guys, I live in Argentina, and have a 10 years old NAD701 with one channel blowed.
I change all the transistor in the output stage, (the BDs and the 2Ns) twice. Both times the same result: works well in the preview with headphones and blow when the speaker was drived.
One issue I discovered after talk with experimented audio technicians is the fact that those 2N3055 and MJ2955 that cost one dollar each and supossed to be made in Japan by Toshiba (as is printed in the transistor case) are fakes made in China.
So I looked for an originally replace, and find the MJ15003 and MJ15004 made in Mexico by Motorolla , that cost 7 dollars each.
I put this and made the suggested modifications posted in the forum, add the .47 emiter metalfim resistor and the 4R7 carbon base resistor.
I turn on the 701 and drive the speakers near its full power, it works well with a reasonably warm in the internal dissipator.

I adjusted the 0 VDC , but I cannot adjust the quisquicent current because I havent got the calibration procedure.

I am very glad of put this amp again to work.
If any of you can post the quisquicent calibration procedure I`ll be thankfull.

You could see here photos of the circuit modifications:

http://www.mediafire.com/imageview.p...vumdmz&thumb=4

http://www.mediafire.com/imageview.p...2mjzn2&thumb=4

(sorry for my english)
Tnx. Peace.

[lineup]

Originally posted by negromo



Hi guys, I live in Argentina, and have a 10 years old NAD701 with one channel blowed.
------
I adjusted the 0 VDC , but I cannot adjust the quisquicent current because I havent got the calibration procedure.

I am very glad of put this amp again to work.
If any of you can post the quisquicent calibration procedure I`ll be thankfull.

You could see here photos of the circuit modifications:

http://www.mediafire.com/imageview.p...vumdmz&thumb=4

http://www.mediafire.com/imageview.p...2mjzn2&thumb=4

Here is a topic with NAD 701:
Nad 701

If you search forum for: NAD 701
you may find more ....

[riku888]

I've just been asked my a mate to fix one of these amps.
Does anyone on here have the service manual so that I can do the bias when I've replaced all the transistors?
my e-mail address is
riku888 @ hotmail.com
I'm going to do the mod suggested above to see if it will make it a bit more stable.

Thanks in advance
Richard

[mightydub]

riku888, check your email.

[riku888]

Thanks for the mail Mightydub. Unfortunately my MSN account went nuts and deleted a load of e-mails, including the service manual before I had chance to rebias the NAD. Please could you re-send to me at RQS @ dolby.co.uk

many thanks
Rik

[riku888]

as soon as I wrote the above post I found the manual. I'd been sensible and saved a copy on my hard drive.

Rik