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Old 12th January 2012, 05:12 AM   #1
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Default Number of output pairs needed?

Recently built a transistor amp. I'm using a 4ohm load. Its a sub amplifier.
Tried using 2pairs of 2sc5200(Quasi Complimentary).
The amp works great at volume set to 3O'clock for about 30 min.
Heatsink gets hot, but I can still touch it. Then suddenly I loose one of the 2sc5200's. They are all Toshibas and have the same batch numbers.
Am I missing something? How many pairs of these do I need for 4Ohm load and +/- 30V supplies(fully loaded) I measure into 4ohm resistive load, 112W before clipping.
I'm now thinking of using Mjl21194's. But I want to be sure before they go up in smoke too. I'm sure the rest of the circuit is working. I have my Vbe multiplier glued to one of the transistors. Biasing a bit cold.
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Old 12th January 2012, 05:27 AM   #2
MOER is offline MOER  United States
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Default How many pairs

2+2 on a +/-30v rail is fine for 4 ohms. Please post your schematic.

If you are using a double darlington the bias spreader should be set for about 2.2v maximum with typical 0.1- 0.22R emitter resistors.

If the output stage is a triple then set at about 3.3v

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Steve Mantz
Zed Audio

PS How is Pretoria? I have not been there since I left SA 29 years ago.
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Old 12th January 2012, 09:13 AM   #3
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MOER! I like your name. VERY Afrikaans!
Pretoria is just fine.
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Old 12th January 2012, 10:22 AM   #4
Enzo is offline Enzo  United States
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Monitor current through their ballast resistors. WIth voltage and current, then see the data sheet for those transistors, and look at the "SOA" chart - Safe Operating Area. see if you are violating its limits. A 16A 200v transistor can do 16 amps or 200 volts, but not both at the same time.

Also, look into your cooling. Is the heatsink adequate and can it cool itself enough?

Monitor power rail currents on a scope. Any spiking as signal flows? You say bias is cold. You say that from what measurements? If for some reason bias is not tracking, it may be draw gets excessive when the amp is pushed.
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Old 12th January 2012, 11:03 AM   #5
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Quote:
Originally Posted by diy didi View Post
MOER! I like your name. VERY Afrikaans!


Greetings from another Ex SA..... but I do miss my Castles and boerewors.

I recommend three pairs of toshibas for 30v rails, 4 ohm loads especially if amp is going to be driven hard and long periods but make sure the amp is not oscillating when driven hard or when it clips. Speakers of 4 ohms can present a impedance as low as 1 ohm at certain frequencies and the reason I recommend 3 pairs, it could also be reason oscillation occurs and or during clipping.

2 pairs of 21194s are however enough but there will be detoriation of THD specs ect, paper spec not necessarily sound wise.

With a schematic we could better advise.
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Old 12th January 2012, 11:35 AM   #6
AndrewT is offline AndrewT  Scotland
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112W into 4ohm 60degree phase angle with Tc=80degC looks OK.

But how can you get 29.93Vpk at the speaker, when the supply rails are at +-30Vdc?

My model predicts ~+-39Vdc for the supply.
Then the 2pair run into difficulties trying to push 12Apk when @ 20Vce.
Reducing Tc<58degC solves this problem.
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Old 12th January 2012, 11:49 AM   #7
jaycee is offline jaycee  United Kingdom
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For a sub amp MJL21194 may be better. One thing you need to check is that your parts are genuine - 2SC5200 are commonly faked
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Old 14th January 2012, 05:47 AM   #8
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Quote:
Originally Posted by AndrewT View Post
112W into 4ohm 60degree phase angle with Tc=80degC looks OK.

But how can you get 29.93Vpk at the speaker, when the supply rails are at +-30Vdc?

My model predicts ~+-39Vdc for the supply.
Then the 2pair run into difficulties trying to push 12Apk when @ 20Vce.
Reducing Tc<58degC solves this problem.
Andrewt, your right. 39V. Will 2 pairs of mjl21194 do the job?
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Old 14th January 2012, 02:27 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AndrewT View Post
Reducing Tc<58degC solves this problem.
Quote:
Originally Posted by diy didi View Post
Andrewt, your right. 39V. Will 2 pairs of mjl21194 do the job?
the original specified pairs will work if you can ensure they never get hot.
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Old 14th January 2012, 06:00 PM   #10
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Hi Didi,

you can also try the MJL3281A, I found them to be pretty hardy and robust devices. Mantech sells them for around R60 for small quantities.

If you are using a typical LC filter on the speaker and the bass amp runs into clipping (which is very easy at 30VDC supply at a low frequency), the capacitor essentially looks like a momentary short circuit and the transistors are at risk.
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