Just blew a NAP250 clone while attempting to add something similar to global feedback, except this is for a guitar amp. The intent was to reproduce the tube rectifier "droop" as the output current increases, which is pleasing to some ears, and also introduces some compression. A schematic is not available for this "improved" clone, but I know it is different for the following reasons. 1. The electrolytic capacitor off the 1k resistor connected to the TR2 base has its positive terminal connected to ground, and 2. There are a number of low value SMD capacitors presumably to prevent oscillation that do not appear on the attached schematic. Surely, there are other modifications as well.
Looking at some solid state power amps with "droop" feedback, it appears the usual way to do this is to ground the speaker's negative terminal through a sub-ohmic resistor, while running a wire from the speaker's negative terminal to the junction of the said capacitor and resistor (where the + sign is). Probably the one major mistake I made is the models I saw were complimentary designs, and not quasi-complimentary like the NAP250, and the capacitor in question was connected with the polarity as is usual--negative to ground.
Would anyone have an idea as the best way to do this, without blowing the second amp? Any comment cast will be appreciated.
Looking at some solid state power amps with "droop" feedback, it appears the usual way to do this is to ground the speaker's negative terminal through a sub-ohmic resistor, while running a wire from the speaker's negative terminal to the junction of the said capacitor and resistor (where the + sign is). Probably the one major mistake I made is the models I saw were complimentary designs, and not quasi-complimentary like the NAP250, and the capacitor in question was connected with the polarity as is usual--negative to ground.
Would anyone have an idea as the best way to do this, without blowing the second amp? Any comment cast will be appreciated.
hello,
i think this "droop feedback" is called current feedback
and you will find some threads here about this modification...........
i think this "droop feedback" is called current feedback
and you will find some threads here about this modification...........
Regarding the cap sitting upside down: remember GND is +40V relative to the negative supply rail, so from the view point of a cap GND can be the positive terminal.
I think on my side that there are at least 50 other amp schematics compatible with a guitar amp before thinking of this one, but that's just my opinion.
@mjf: Thanks, the correct nomenclature will help in this endeavor. No threads yet, but a comment about a Music Fidelity A1 mentioned a base of a particular transistor is slightly negative was caught. Maybe mine blew because the circuit did not appreciate the wrong polarity with low impedance being sent there. Pretty sure there are fused transistors, and a damaged voice coil on my test speaker. I now think it's time to check polarities amplitudes on the running amp.....
@jpk73: Just verified the cap is installed according to the silkscreen and that it is, in fact, power ground. But thanks for the reminder on the need of being aware.
@huggygood: I agree. Rod Elliot's P27A is specifically designed for guitar, has the current feedback option, and is good for 100w into 4 ohms (and it works good, too). I got this clone because it was cheap and I'm concentrating on the tube preamp and active EQs which are taking an inordinate amount of time, and require point to point breadboarding.
@jpk73: Just verified the cap is installed according to the silkscreen and that it is, in fact, power ground. But thanks for the reminder on the need of being aware.
@huggygood: I agree. Rod Elliot's P27A is specifically designed for guitar, has the current feedback option, and is good for 100w into 4 ohms (and it works good, too). I got this clone because it was cheap and I'm concentrating on the tube preamp and active EQs which are taking an inordinate amount of time, and require point to point breadboarding.
You probably got it to oscillate - and those BDY58’s are fragile. I would NOT use those in a guitar amp. Fitting MJ15003’s is a good idea if you use the amp for that purpose. If it does oscillate with 15003’s or sees a bad load it’s not usually fatal.
It is not going to have its characteristic ‘sound’ with current feedback so using low fT more rugged outputs won’t cause you to ‘lose’ anything.
It is not going to have its characteristic ‘sound’ with current feedback so using low fT more rugged outputs won’t cause you to ‘lose’ anything.
Good info. It has 2SC5200 outputs. So far, after sending all sorts of inputs including a long recording of death metal chords, it has never oscillated, and also took quite a temperature beating once and still stood up. The bias drifts a lot as is mentioned in many articles, but this and the quality of reproduction has no value because "no one" will notice it. Reliability definitely is the highest priority; oscillation and smoking voice coils are not permitted.
Then how did you blow it? C5200’s should be rugged enough (far more than the fragile types you’re supposed to use to follow the original design). Unless you got burned by fakes or actually shorted the speaker wire. If your board takes flat pack types, I’d go for MJL3281 - IF you have access to a real ON Semi distributor, that is. If you can fit TO-3’s, the 15003. Those will withstand local output stage oscillations and will take a 1 or 2 second short. At least on those supply voltages. Both are a little more rugged that 5200’s and orders of magnitude more than BDY58’s or BUV21’s. For guitar amp duty one might consider putting in a basic current limiter circuit at say 6-8 amps. 15003s will run with a shorted output like that - at least above 82 Hz they will (will stay within pulsed SOA).
The C5200 isn’t really a bad compromise for following the original design intent either, but the more expensive and less-universally-available Sanken types tend to be preferred for hi-fi use. They are a little faster, more rugged, and many have better linearity. It’s all splitting hairs, but if you’re into the philosophy that every little bit helps, there is your little bit that can help.
The C5200 isn’t really a bad compromise for following the original design intent either, but the more expensive and less-universally-available Sanken types tend to be preferred for hi-fi use. They are a little faster, more rugged, and many have better linearity. It’s all splitting hairs, but if you’re into the philosophy that every little bit helps, there is your little bit that can help.
@wg_ski: Great info. I have not yet determined what is blown, if anything. With something like this, I would tend to replace all the transistors at once and hope for the best, but purchasing a new one would be less expensive with no work. The 2SC5200 that came with it are genuine ST. I am under the impression the NAP250 already has some current limiting but never got into it. My power supply is already current limited SMPS; it cuts out somewhere over 3A, which sould never be reached since the maximum power is 80W. The rails are 38V. I might eventually go for a 100W unit with 2 transistors per rail that will handle 4 Ohms--all on one side of the board for space reduction. I like philosophy and apply it all the time in every field of life. I have been treating the power section as a black box with hair-splitting attention on the overall signal chain where it counts, for now at least.
Naim amplifiers have large power supply capacity the short circuit protection has a delay about 10uS before it cuts in which allows very high current for peak outputs. This is well in excess of your SMPS power supply capacity which cuts out at 3A which I think would be instantaneous and would result in a spike in the supply rail voltage in your amplifier if you overdrive this. There are reverse connected diodes in parallel with the output transistors to contain spikes but if the overdrive is continuous the spikes could be manifold and continuous.
You should also be aware Naim amplifiers are not guaranteed to be stable if you use the wrong speaker cables. There is no output coil and the level of inductance required for stability is given by 3.5 metres of their own brand of cable. You can make your own coil and err on the side of more inductance than required. If the amplifier has oscillated due to this cause that would explain the failure of the output transistors.
You should also be aware Naim amplifiers are not guaranteed to be stable if you use the wrong speaker cables. There is no output coil and the level of inductance required for stability is given by 3.5 metres of their own brand of cable. You can make your own coil and err on the side of more inductance than required. If the amplifier has oscillated due to this cause that would explain the failure of the output transistors.
@mjona: Excellent heads up. I've seen those; air coil, very low inductance, with a resistor in parallel I recall. Well, it would be senseless not to use this protection. I do go about 3.5m to a cabinet for testing sometimes, although usually a small speaker for local listening for noise, which serves as sort of an oscilloscope. Well, I don't have to turn my head at least.....