Need comments on amp design

[ostripper]

wow , negromo really gives that old NAD amp overkill
(Mj15003/4) They can do 200w+ !! the old NAD will last
till doomsday. (with a 2 ohm load)

[negromo]

Jajj ... my friend Ostripper, I cant find originals 2955/3055 anywere, all are fakes (chinesse) that blows @ 5 % volumen. Thats why I put that powerfull original Motorolla pair. And with very good result, I always use the NAD near 80 , 90 % of power and all I have to worry about is to keep the unit aireated.

Sorry about my english.

Peace.

[jaycee]

FAKE 2955/3055 ? Wow, that's normally what they try and sell as MJ15003/4 and other higher power motorolas!

[philoneous]

Hey, great thread... just acquired and old 701 (guess they are all old). Fired it up and the outputs are driving speaker loads ok. If one of you has that pdf manual, I'd appreciate a copy, as I'm going to give it a good going over before putting it into service.

[negromo]

Hi, this is the link to the file in PDF

http://www.mediafire.com/?trzthzyjzzm


Peace.

[philoneous]

Excellent! thanks very much.

[Johan Potgieter]

Originally posted by djk
No emitter resistors. Causes thermal runaway in the drivers. Cut trace going to the base of each output and put in anything from 4R7~10R, a 1/4W is OK.

In retrospect, I am somewhat puzzled by the fact that this was such a successful amplifier - without the mentioned emitter resistors which, if such a certain cause of blown output transistors, would not have earned that reputation.

I accept the point of the early 2N3055s (used them but did not realise the R of the emitter lead). But how many knew this? I must first ask whether the transistors Q408, Q411, Q413, Q416, Q417 (and perhaps also diode D406) were all mounted on the heat-sink. If so, thermal runaway could have been controlled by thermal feedback without said emitter resistors. I omit such emitter resistors in a full complimentary topology for another reason, and find that I can easily keep current in the output transistors constant simply by thermal feedback and correct compensator transistor (Q408) circuit resistance values. In fact one can over-compensate by the wrong resistors.

[mhifi508]

My first post to this forum.

I have a used NAD701 when power up, there is a soft "pop" sound heard from the speakers, is it normal? A meter on the spk terminal shows a low voltage swing from pos to neg and back to zero. When can I do?

I am surprise that the Amp do not have speaker protaction relay for the OCL output! Anyway it has been operation for more than ten years before pass it to me so I guess NAD must has good design.

[mightydub]

Originally posted by Johan Potgieter


I accept the point of the early 2N3055s (used them but did not realise the R of the emitter lead). But how many knew this? I must first ask whether the transistors Q408, Q411, Q413, Q416, Q417 (and perhaps also diode D406) were all mounted on the heat-sink. If so, thermal runaway could have been controlled by thermal feedback without said emitter resistors.

The output pair (Q415, Q417) and the bias transistor (Q409) are mounted on the heat-sink. The drivers (Q411, Q413) are not. I found that with the new output transistors, the amp was not thermally stable without emitter resistors - the quiescent current would drift higher as the amp warmed up, I'd tweak it down, come back in 10 minutes and it had gone up again, tweak it down, repeat... Once I put the emitter resistors in it was immediately stable.

I agree that theoretically, thermal feedback can work. In reality, component variations and thermal lag make it very difficult. See the thread on the OnSemi ThermalTrak parts.

Originally posted by mhifi508
My first post to this forum.

I have a used NAD701 when power up, there is a soft "pop" sound heard from the speakers, is it normal? A meter on the spk terminal shows a low voltage swing from pos to neg and back to zero. When can I do?

This is normal for this design. On mine, I don't get an audible pop but I can easily see the woofer cone move. I agree that muting relays would be nice, but this design was obviously done to a tight budget.