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Old 16th April 2002, 09:55 AM   #11
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Thanks; and should the Vbe multiplier be in contact with the heatsink for thermal tracking? How many output devices and what rail voltages would take me to about 500 watts into 8 ohms?

Any measurements yet? Any comments on stability issues while operating at very high power levels? Sorry-too many questions, but thanks.
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Old 16th April 2002, 12:26 PM   #12
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The Vcbo of the 2SD669/B649 is 180V, Allowing 10% for line voltage regulation means we cannot use this part with more than +/- 81V. This kind of voltage is what we see in 250W/8R amplifiers. An Adcom GFA585 is a good example of this with 5 pair of outputs per channel. The QSC USA1310 is rated at 400W/8R, but only at 1Khz. It uses 8 pairs of outputs on +/- 95V. The Crown MA2400 will do 525W/8, 20hz~20Khz. It is a bridge amp with only 6 pair of outputs on +/- 57V. I recommend you build a Leach with 3 pair of outputs and bridge. This is a tried and true design with all the parts values specified. Substituting parts in a high voltage, high power design will lead to frequency compensation problems and quite possibly total failure at some point down the road due to instability/oscillation at various load/temperature/drive level combinations. With the correct VA rating in the transformer this will drive over 800W/4R. With 5 pair of outputs per half, 10 pair per bridged channel, up to 1500W/2R is possible. The limit here is how much power the mains socket can deliver.
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Old 16th April 2002, 07:01 PM   #13
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I agree with djk that 500 watts into 8 ohms is probably too demanding for this amplifier due to rail voltage limitations because of VCE parameters. It can easily be done into 4 ohms however. If you are still interested let me know and I'll forward you some calcs, the FFT, distortion and freq. response curves, ect...
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Old 16th April 2002, 07:12 PM   #14
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This is frequency response (line is nearly flat) and 2nd and 3rd order distortion with the marker at 1k which calculates the % instead of DB there. The amp is driven to near full power of my prototype (300W) into a 4 ohm load. The abrupt increase in HF distortion is due to the ADC used in the Clio system with has a sampling frequency of only 50kHz. It also cannot measure distortion much lower than 0.05%... this curve was used to demonstrate there were no nasties in the sound.
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Old 17th April 2002, 09:18 AM   #15
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McAnally,

The distortion figures show that 3rd harmonic distortion is slightly more than 2nd harmonic distortion. Hence, is the sound a bit sharp and nasal at higher output levels?

djk, thanks for your input. After a lot of study, I have come to the conclusion that trying for power levels such as 500watts into 8 ohms is rather expensive and risky. My requirement is to drive 8 Nos. 18" EV woofers (horn loaded), either each with its own power amp (400 watts of the woofers + a bit of headroom for the amps) or to be able to drive 1kw bridged into 4 ohm or 1.6kw bridged into 2 ohms. This means four bass bins per side, all paralleled into one bridged amp or 2 each paralleled into 2 bridged amps.

Hence, can this design be operated at lower voltage and more output devices to drive typical low impedance loads as I have stated above.

djk, by the way the 2SD669/B649 devices are used by Randy Slone running at 85volt rails in his OPTIMOS 400 watt designs. Is this sailing too close to the wind? I ask, since this is another design I am considering - 8 mono amplifiers, one for each bass bin.

Thanks again for further inputs,
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Old 17th April 2002, 09:32 AM   #16
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As for "nasal" sound I don't know... it really sounds quite transparent in my opinion. It isn't uncommon for 3rd order to be a little higher than 2nd order in solid state designs. In the case of driving bass bins, 3rd distortion isn't really applicable. Assume your bins will operate from 40-150hz (for example). 3rd distortion will appear in the 120-450hz range which should naturally roll off by the response of your enclosures. Also, less than 0.1% is pretty hard to hear considering the enclosures are sure to add much more distortion than that.

The amp I presented would be well suited for low impedances given enough output devices are used. I would use a total of 16 (eight top, eight bottom) to drive a 2-ohm load continuously. This keeps the limiting circuitry from even thinking of operating before it should and also gives a good reliability/safety margin.
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Old 18th April 2002, 05:23 AM   #17
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McAnally,

Thanks. I agree with what you say about 3rd Harmonic Distortion. I will try out your design with more output devices as suggested by you. I suppose 8+8 output devices should give about 600 to 800 watts continuous into 2 ohm loads with a dual supply of 55-60 volts. This suits me fine as I can bridge the amp into 4 ohms.

One last question. Instead of MJL devices suggested by you, would it be possible to use 2SC3281/2SA1302 devices, adding up more devices to handle low impedance loads. Ofcourse, it would require 14 or 15 devices per bank to handle 2 ohm loads. But availability and cost are factors that prompt me to ask this question.

Overall this is a great contribution by you to the DIY community. Keep it up!
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Old 18th April 2002, 06:02 AM   #18
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I'm not quite sure. I have only a single pair of those and they work fine, however I haven't tried with multiples yet. I would suggest using a higher value for the compensation cap to start since I think the 2sc/2sa's are a little faster and you dont wanna get unstable with that much power =). Another tip, build a test circuit on a breadboard with the power devices on a heatsink seperate from the board and if that works you should be set since this is a worst case setup. I haven't tested the circuit at the power levels you are suggesting... but it should work in theory. I would be very interested to know about your results.





Quote:
Originally posted by Samuel Jayaraj
McAnally,

Thanks. I agree with what you say about 3rd Harmonic Distortion. I will try out your design with more output devices as suggested by you. I suppose 8+8 output devices should give about 600 to 800 watts continuous into 2 ohm loads with a dual supply of 55-60 volts. This suits me fine as I can bridge the amp into 4 ohms.

One last question. Instead of MJL devices suggested by you, would it be possible to use 2SC3281/2SA1302 devices, adding up more devices to handle low impedance loads. Ofcourse, it would require 14 or 15 devices per bank to handle 2 ohm loads. But availability and cost are factors that prompt me to ask this question.

Overall this is a great contribution by you to the DIY community. Keep it up!
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Old 18th April 2002, 08:27 AM   #19
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McAnally,

That was a quick reply. You are right regarding the compensation cap. I will buy a bunch of the 2SC/2SA devices and get started off. These require a smaller base bias than the MJL devices, so the driver stage shown in your schematic should handle this well. Since each transistor would be contributing a lower power than as shown in your schematic, the protection circuit may need some adjustment of values. Keep me posted if you have any calculated values. I will post the results here. Thanks again.
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Old 28th April 2002, 08:19 AM   #20
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Quote:
Originally posted by djk
Some local feedback in the voltage gain stages would be nice too.
If the amp is to be used for higher power output than originally spec'd (300w) I recommend 100-200 pF of capacitance from collector to base of Q6 and Q7 and a small (22 ohm) degeneration at the emitter of Q4. This will ensure stability with certain loads and nasty circuit layouts. I've now got eight pairs of MJL21193/4's at the output with excellent performance and no signs of instability with almost no turn on thump and none at turnoff. I have yet to boost the rails up to +- 80V but as soon as the new caps and toroid come in I'll post the results.

As per the recommendation of Nelson Pass I've included a seperate LED for the voltage reference on Q10 just in case. Thanks for all the input guys.
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