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Old 22nd August 2004, 06:51 PM   #11
cunningham is offline cunningham  United States
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I would think it would be easier and more productive to just buy or build another amplifier circuit and cut the bandwidth so that each amp has its own bandwidth and correct type of speaker attached... Lows, mids, and highs. This will be more effecient for recreating full range bandwidth for certain. or just lows and highs. Use the one you have for lows, and build a smaller more simple circuit for the highs and tweeters. You can then push up the gain and still have low distortion overall.

As was mentioned before, it would require an almost complete redesign of the circuit to make it more powerful, and then you would have a completly different amplifier afterwards.
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Old 24th August 2004, 01:25 AM   #12
bowdown is offline bowdown  Australia
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Ok thanx for your info, i might just see which way i go.This is the chassis that im in the process of finishing off to house all the circuitry let me know what you guys think.I should have more pics later on, oh by the way the pic quality isnt too good as it was taken by a camera phone.Cheers.

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Bowdown
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Old 24th August 2004, 04:32 AM   #13
kilowattski is offline kilowattski  United States
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It is impossible to tell if the heat sinks you are planning to use are sufficient. You will have to calculate how much power your transistors are going to dissipate and select a proper heat sink. If you plan to build a large power amp your heat sinks may be quite insufficient.
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Old 24th August 2004, 12:17 PM   #14
traderbam is offline traderbam  Europe
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Bowdown,
Here are some power dissipation estimates.

Power rails are +/-90V
Assumes one transistor per half, class B

8-ohm resistive speaker, single-ended (not bridged)
max avg output power = 500W approx.
avg transistor power = 102W @ 57V Vo peak
peak transistor power = 253W @ 45V Vce
peak Ic = 7A

The MJ21193 is rated at 250W average at 25C and 16A average current. But the power limit reduces with increasing case temperature. You sort of have to put a stake in the ground for the case temperature and then try to find enough heatsinking to maintain it. A good start is to set the case temperature at 70C - where the MJ21193 is rated at 186W.

You need to divide the avg transistor power by the transistor rating to get the number of transistors per output half. In this case 102/186 = 1. The Ic and peak power are ok so your amp needs 2 transistors in total (1 npn, 1 pnp). The total dissipation of all transistors will be 204W - your heatsink must remove 204W and keep the case temperatures less than 70C. For a 25C air temperature this means a heatsink rated at 0.2W/C or less.

When you bridge two amps everything basically doubles, except the output power which quadruples:

8-ohm resistive, bridged
max average output power = 2000W approx.
avg device power = 203W @ 57V Vo peak
peak device power = 506W @ 45V Vce
peak device Ic = 14A

Now you need 2 transistors per half per amp to meet the 203W average power and you need to check the peak power is ok - two devices at 506W (254W each) is just about ok since it is a short duration event. So you should use 4 transistors per amp (2 npn, 2 pnp). The total power is now 406W requiring a heatsink for each amp rated at 0.1W/C or less.

Now you have to be really carefull with speakers. They are not resistors and can exhibit wildly varying impedances at different frequencies. Therefore, for high reliability, a good rule of thumb is to design for half the nominal speaker impedance. So for an 8-ohm speaker design the amp to work ok into a 4-ohm resistor. You use less transistors and design special power limiting circuitry that dynamically measures the transistor power but this is quite complex.

When the load is halved you basically double all the numbers. In bridged mode with 4-ohms resistive load, the avg device power is 406W, thus requiring 406/186 = 3 (rounded up) transistors per half. Peak power is 1012W which is too high among 3 devices, so I would use 4. Each amp should have 8 transistors (4 npn, 4 pnp). Each amp could dissipate 812W so the heatsinking must be rated at 0.05W/C or less.

In summary, for high reliability, for an 8-ohm nominal speaker you should use 4 transistors (2 npn, 2 pnp) and a 0.1W/C heatsink. If you will operate in bridge mode, you should use 8 transistors (4 npn, 4 pnp) and 0.05W/C heatsink per amp.

I would think your 5kVA transformer is ample and 200,000uF ('u' is for micro, not 'm' which is for milli) capacitors are ample.
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Old 26th August 2004, 02:26 AM   #15
bowdown is offline bowdown  Australia
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Thanx heaps for helping me out guys .I was thinking of using 8 pairs of output devices per channel (8 x mjl21193 + 8 x mjl21194) what load can i use this down to???Im still trying to get the hang of working out what type of outputs to use on amplifiers, ive been trying to understand the data sheets for the mjl21193/4 and im not sure if im right but i think they are only good to about 1 amp @ 100v, now is this correct? I really want to use to247 packages as it is easier to fit them.Please help me understand this stuff.

Regards

Bowdown
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Old 26th August 2004, 03:35 AM   #16
mgmopar is offline mgmopar  United States
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I have used active crossovers in the past for automotive to achieve what cunningham stated and limited input bandwidth each amp had to deal with.

Quote:
I would think it would be easier and more productive to just buy or build another amplifier circuit and cut the bandwidth so that each amp has its own bandwidth and correct type of speaker attached... Lows, mids, and highs. This will be more effecient for recreating full range bandwidth for certain. or just lows and highs. Use the one you have for lows, and build a smaller more simple circuit for the highs and tweeters. You can then push up the gain and still have low distortion overall.
I have been running multi amps (mostly resurrected receivers with bass and treble controls) on my home system but am in the process of trying to upgrade quality of the components. I have been using the bass and treble as well as the volumes on the amps to achieve a fairly flat response. I am now planing on building some chip amps to replace the weaker amps at first. Eventually having mostly diy amps. I am using a Sony sdp e800 surround sound processor for my pre amp (I don't think it is capable effectively balancing the system on its own. I also don't want to add variable bass and treble to the new power amps. Does anyone have suggestions to help limit added circuitry but still give some tonal control/filtering to the inputs or outputs?
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Old 27th August 2004, 01:25 AM   #17
bowdown is offline bowdown  Australia
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So im guessing that its fine to use 8 x mjl21193 and 8 x mjl21194 running of +/-90V approx and i should be able to run the amp down to approx 2ohm with no hassles?

Cheers
Bowdown
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Old 27th August 2004, 12:36 PM   #18
sajti is offline sajti  Hungary
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Quote:
Originally posted by bowdown
So im guessing that its fine to use 8 x mjl21193 and 8 x mjl21194 running of +/-90V approx and i should be able to run the amp down to approx 2ohm with no hassles?

Cheers
Bowdown

For 2ohm real load, I would Youse 10 pairs at least.

sajti
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Old 27th August 2004, 12:46 PM   #19
sajti is offline sajti  Hungary
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Just check out this topic!

sajti
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Old 27th August 2004, 03:44 PM   #20
traderbam is offline traderbam  Europe
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Using the same calculation as before, 16 devices (8 npn, 8 pnp) will drive 2.2 ohms nominal (1.1 resistive). 18 devices will drive 2 ohms nominal ok. This is only if you keep the cases at 70C or less.
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