Need comments on amp design
Here's the background - I have a NAD 701 receiver that I'm trying to resurrect. After it was out of warranty the left channel blew. I had moved away from the shop where I had purchased it and I didn't have the time to deal with it myself so I took it to a local repair shop. The tech replaced the output transistors and pronounced it cured. About a month later the left channel blew out again. I took it back to the shop and the story the tech gave me is that he tried replacing the output transistors again, and as soon as he powered it up they blew out again. So he threw up his hands and gave me my money back, and made some comments about the design not being very stable. It went into a box and sat in the garage for about 10 years.
A few weeks ago I dug it out and started to work on it. I acquired a copy of the schematic and started checking. Sure enough, the ouput pair (2n3055/mj2955), the driver pair (2sd669/2sb649) and the bias transistor (bd139) are all blown. The service manual has a procedure for setting the DC offset and quiescent current, I have no idea if the tech had the manual and followed it.
I have ordered new parts, and while I'm waiting for them to arrive I started playing around with LTSpice and decided to model the circuit. (see pdf of LTSpice circuit.)
A few notes:
- DC output level is set by R411, a pot in the actual circuit. I played with the value in simulation to get it close to 0V.
- Quiescent current is set by R443 (another pot, currently set to 75 ohms.)
- Q409 is mounted on the heatsink with the output pair.
- The output filter and circuit breaker have been omitted for simplicity, and yes that's just a dummy load on the output.
I'm new to amp design - that EE degree was a long time ago, and I concentrated in digital. My questions:
1) Any comments on the design?
2) Any thoughts about what might have made it blow up in the first place?
3) what is the purpose of R453 (between the emitters of the driver pair) - when I powered on the amp, it started smoking since the drivers were shorted. Removing it doesn't seem to make any difference in simulation.
Again, I'm coming back to analog design after decades away from it and I have been learning (and re-learning) a lot the last few weeks, thanks to this forum and also Rod Elliot's articles. Your comments and feedback are appreciated!
Many thanks in advance.
This is an interesting design in that it has a single-ended input stage. Different, but no worse in principle than differential input stages, and with some advantages. It probably is more linear of itself so distortion products with feedback may be more benign. It does make it a bit harder to get zero DC offset at the output, but careful adjustment should take care of that.
The resistor between the emitters of the drivers is there to help to take charge out of the power amp bases quickly, improving the hf performance of the relatively slow power transistors.
The bootstrap shows this is a bit older amplifier.
As well as some transistors used.
I agree with most janneman says.
And also I will say is interesting to see some different ideas.
It always is.
As a vacation from the 'boring' main stream of amplifiers
- MOSFET me this and that, until I throw up
- Show me the usual LTP pair with a CCS current source, one after the other - identical as clones.
What makes me extra happy, is to see something
I often have posted about in this audio forum.
A power amplifier construction with two power supply :)
40 Volt for input and VAS
and 36 Volt for Power Stage.
NAD is good Audio, People :cool:
... but this is no secret ... you knew this before, didn't you?
And very good price, too.
Re: Need comments on amp design
This coments from the tech is a little amusing , because without knowing it ,he was talking about the power section of the famous NAD 3020 , one of the most acclaimed budget amplifiers of all times.
Compare the schematic that you post , with the schematic of the 3020 and "find the differences "... ;)
Put new drivers, outputs and enjoy !
Those blew up repeatedly for a couple of reasons:
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.
Outputs. Not high enough voltage. Use the high voltage versions of the 3055/2955, the MJ15015/16. These are also rated at 180W.
Add flyback diodes across the outputs.
The resistors and the diodes will cost you about $0.50, a set of outputs about $7. The drivers are probably shot too, and there may be other damage.
The guy that repaired it probably didn't know how to set the bias either (unless he had a shop manual). There is a cut trace with a jumper across it. Remove the jumper to monitor the bias, adjust, then put the jumper back.
Thanks for all of the replies so far.
djk - the idea of adding the resistor to the base of each output is to swamp the change in base current as Vbe decreases with temperature, thereby preventing thermal runaway, correct?
I have the service manual with the procedure for setting the bias, the spec for idle current is 28mA. (28mV across a 1 ohm resistor between the + supply and the C of the top output transistor.) I doubt the guy who tried to repair it had the manual.
tube_dude, lineup, janneman - NAD's reputation for great sound at a budget price was one of the reasons I bought this unit - but the quality and finish of the pre-amp/main amp board is poor, the power supply bulk capacitance seems rather low (6800uF/side) and the interior is a rat's nest of wires. Overall I'm not impressed with the build quality. On the other hand, it has provided a great project and learning experience.
i worked at NAD for a year...... build quality varied a bit within product lines. unit assembly was by Fulet/Proton, design and test/inspection was done in the U.S. by NAD. the first few shipments of a new product were "prototype" units.... i.e. they were built according to the design, but Fulet was "working the bugs" out of the new assembly line, and so the internal "fit and finish" wasn't all that great. you will probably notice one or more colored dots on the back of the unit (unless somebody scratched them off). those identify the factory modifications that were done on the unit. also be aware that some of the required amp mods dealt with ground loops and oscillation in the amp. you may want to call NAD and talk with an amp tech, and find out whether this amp has the correct mods in it. when i was there i think we did the mods for free, even after expiration of the warranty..... but that was a long time ago when NAD was based in Norwood MA.....
Re: Need comments on amp design
Unfortunately the technology evolve and old 2N3055s were superseded by new, better devices which *need* external emitter ballast resistors, otherwise they go in thermal runaway and blows.
If you wish to recover your equipment, you have to make some minor but *essential* modification in output circuitry to improve its reliability.
1) Add in series with the bases of 2N3055/MJ2955 a 10 Ohm/1 Watt reistors.
2) Add in series with their emitters a 0.33 Ohm/ 3-5 Watt resistor.
3) Retrim queiscent current for about a 40-50 mA and monitor it's behaviour to avoid increasing when equipment is working. If quiescent current tend to increase, you must retrim, with *warmed amplifier*, quiescent current to the 40-50 mA as told before.
This will bear to a more "crossy" amplifier immediately after a power-up (i.e when it's cold) but with a more thrustable behavior once it's well warmed up (i.e. in the normal working condition for listening).
No comments about the rest of circuit. It's a bit idiosincratic but this was done in order to obtain a relative predominance of even harmonics in THD, for someone a "must" for achieving a "quasi-tube" sound.
I don't agree much with this kind of "sorceries" (tube sound, in my opinion, has not much to do with even or odd harmonic distortion but with other aspects of tube circuits - different supply filtering strategies, presence of output transformers and, generally speaking, totally different working impedances of active devices with equivalent output power - but its just a personal view about the tube/transistor "sound affair") but, in this case, the essential is that the circuit work in a reliable fashion: the rest of the question is just philosophy.
I am worried by just one matter in Mightydub's diagram as posted. I see R1 = 8 ohm across the output. Has the usual series capacitor (Zobel network), accidentally been omitted, or is half of the available output going up in heating R1?
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