Leach VAS runs 5mA with 2mA per leg, a bit less if using 12k ohms..
Leach thoght that increasing tail resistor to 12k makes the VAS run a bit cooler...
with the values shown in his latest drawing, this amp runs VAS at around 44mA...the MJ15033/15032 are running at about 1.5watts each so carefull heatsinking is required..
Leach thoght that increasing tail resistor to 12k makes the VAS run a bit cooler...
with the values shown in his latest drawing, this amp runs VAS at around 44mA...the MJ15033/15032 are running at about 1.5watts each so carefull heatsinking is required..
raitraak said:
I got transistors as samples from ON Semi, I hope they are not a questionable source
Onsemi is a great source. You are assured REAL transistors unlike some.
Their transistors have high ratings, and live up to them. I've overheated and run high current through MJL4281/4302 and they won't burn out. Very rugged. I even had a pair of MJE15034/35 run so hot, that they had a heat trail rise off of them, and you could smell the running hot, and they survived. I even damaged a 6V Lead-acid battery with a MJL4302 by drawing excessive current, and the transistor still survived.
The Onsemi transistors are really good.
I hear you I'll decrease vas current to 15-20 mA.
Just got back from little shopping tour. My transformer was ready! 24h after the order. 800VA 2*60V 2*6.66A for 100$. Nice Much cheaper than in the store!
Also bought 4 10000uF/100V caps for the supply. Hope that 20 mF per rail is better than 9.4 mF
Aq
I'll decrease vas current to 15-20 mA.Just got back from little shopping tour. My transformer was ready! 24h after the order. 800VA 2*60V 2*6.66A for 100$. Nice
Much cheaper than in the store!Also bought 4 10000uF/100V caps for the supply. Hope that 20 mF per rail is better than 9.4 mF
Aq
raitraak said:I hear you
Just got back from little shopping tour. My transformer was ready! 24h after the order. 800VA 2*60V 2*6.66A for 100$. Nice
Also bought 4 10000uF/100V caps for the supply. Hope that 20 mF per rail is better than 9.4 mF
Aq
good on you!
there are two ways to go about it, but i would prefer to lower R1 and R4 from 3.3k to something like 1.5K..good luck.....
Nice amplifier. I like high power designs. Your input stage it is good with the thermally compensated current sources. But it is very complex IMHO. Because i am familiar with high power projects, you can obtain a very small rise time (about 1,6 to 1,8ìs) with simplest input stage due to high level of supply voltage. Anyway, the choice is yours.
Some suggestions from my experience:
1) You can obtain a better low frequency response by increasing C6 from 220mF to 470mF at least. Also it is most safe for this cap the parallel connection of one more diode forward biased this time for full voltage swing clamping.
2) Q3 it is a high Vce device and thus very slow in thermal reaction in the place of the Vbe multiplier. Because the voltage accross the c-e junction of Q3 never exceeds the 3,5 to 4 Volts, it is better the use of a device with lower Vce but much faster in thermal compensation of Iq such as BD137. Trust me in this point. You can make an experiment for this, it is not so difficult.
3) The number of output devices it is small for so high voltage supply level (and thus output current). Commercial class AB amplifiers use at least 7 pairs of TO3 output devices (like 15024-15025) in such supply levels. Make a comparison of SOA given by OnSemi between the most rugged TO3 MJ15024-15025 and the most delicate 340G (TO-3PBL) MJL21193-21194. For the first type it is reffered 80V/2A safe margin, instead for the second type 80V/2A safe margin ONLY for 1sec! Be carefull in this point.
Because in your circuit incorporated a VI limitter you can adjust the values of resistors in this network, so the output does not exceeds the 500VA which is as i think a safe margin for the 5 pairs of 21193-21194.
Regards
Fotios
Some suggestions from my experience:
1) You can obtain a better low frequency response by increasing C6 from 220mF to 470mF at least. Also it is most safe for this cap the parallel connection of one more diode forward biased this time for full voltage swing clamping.
2) Q3 it is a high Vce device and thus very slow in thermal reaction in the place of the Vbe multiplier. Because the voltage accross the c-e junction of Q3 never exceeds the 3,5 to 4 Volts, it is better the use of a device with lower Vce but much faster in thermal compensation of Iq such as BD137. Trust me in this point. You can make an experiment for this, it is not so difficult.
3) The number of output devices it is small for so high voltage supply level (and thus output current). Commercial class AB amplifiers use at least 7 pairs of TO3 output devices (like 15024-15025) in such supply levels. Make a comparison of SOA given by OnSemi between the most rugged TO3 MJ15024-15025 and the most delicate 340G (TO-3PBL) MJL21193-21194. For the first type it is reffered 80V/2A safe margin, instead for the second type 80V/2A safe margin ONLY for 1sec! Be carefull in this point.
Because in your circuit incorporated a VI limitter you can adjust the values of resistors in this network, so the output does not exceeds the 500VA which is as i think a safe margin for the 5 pairs of 21193-21194.
Regards
Fotios
fotios said:
Comments:
1/ Bear in mind that output transistors are only switched on for half of an audio cycle. I use 3 pairs of MOSFETs on output of my 1000WRMS amp and they dont get hot at full bore.
2/ Dont do what I did with a VI limiter and forget the diodes to the supply to discharge the inductance from the speakers when VI limiter swicthes off the output transistors !
nigelwright7557 said:
Sorry for that i am so reserved, due to my previous expertise - for 20 years - with P.A. sound equipments. As you know mosfets don't used in such type amplifiers and i can't explain you the reason because the limitted space here. I don't know if you can obtain 1000WRMS (in which load??) for half an hour of continuous operation in this level with 3 pairs of mosfet. If you can do this, then please accept my compliments!
For the second, i think in the circuit incorporated 2 shottky clamping diodes between output and voltage rails. Moreover, a VI limitter never switch off the output devices absolutelly. Simply it suck up current from the bases of drivers when reached the VI limit in output. Thus the output devices drive current reduced in an exponential manner. The VI limitter offers only some protection. For heavy loading of output it does not offers full protection; in this case an optocoupler that monitors the output current flow to load and activates the output protection relay it is the absolute solution.
Fotios
fotios said:
I do use a large heatsink and a fan to keep the MOSFETs cool.
Sometimes people forget that the output transistors are only on for half a cycle so the current will be only half of what you might think.
Of course you are right about the VI limiter not switching off the output transistors. But they might square off the waveform at the cut off point which can produce nasty spikes on the output.
I am happy with cheap vertical MOSFET devices in my amplifiers.
I am sure you can probably design better with more expensive MOSFETs or BJT's.
I find so long as the crossover distortion is tuned out that the amps sound very good.
Hey!
Thanks for the replies!
Tony
I'll do that.
fotios
1. I will use 1000uF/25V paralleled with pair of diodes in both directions (as in Self's designs).
2. I chose MJE15032 because I already had it and it was in to220, which is mountable on the heatsink. Hope BD135 will do the job (BD137 is harder to get).
3. Because the VI-limiter measures only one output transistor, it does not care of the total output level, only about not exceeding one's limits. I thought of adding 2 pairs of output transistors, but leve the places empty for now and add them later. VI-protection shuld look something like this now:
nigelwright7557
Do you mean D2 and D9?
Updated Main Schematics
Aq
Thanks for the replies!
Tony
I'll do that.
fotios
1. I will use 1000uF/25V paralleled with pair of diodes in both directions (as in Self's designs).
2. I chose MJE15032 because I already had it and it was in to220, which is mountable on the heatsink. Hope BD135 will do the job (BD137 is harder to get).
3. Because the VI-limiter measures only one output transistor, it does not care of the total output level, only about not exceeding one's limits. I thought of adding 2 pairs of output transistors, but leve the places empty for now and add them later. VI-protection shuld look something like this now:
An externally hosted image should be here but it was not working when we last tested it.
nigelwright7557
Do you mean D2 and D9?
Updated Main Schematics
Aq
raitraak said:
The better you use BD135 because it has lower Vce from BD137 and then it is more faster in thermal variations from BD137. The only difficulty it is the inverse place of B-E junctions between TO225 and TO220.
Me also i use 1000mF cap with 4 diodes for clamping.
For the VI limiter, if your output transistors are matched then it is enough the current flow monitor from only two output emitter resistors as i think. Don't worry about this.
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