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Old 26th October 2005, 06:35 AM   #1
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Default 10 pair of MJ15024-25 + 1000Va 2x60V +4x 100000uF 50V...

Hi Everyone, i'm new here!.

I have this material that i bought for cheap price. And i would like to built a sub amplifier.

I have :

- 10 pair of MJ15024-25
- 1x 1000Va 2x60V toroid transfo
- 4x 100000uF 50V...caps(i know it would be better with 100V version..)

Have you some circuit to suggest to me?
(assuming that i can modify the transfo into 1000VA 2x30V to have a high current amp..).. it would be better to drive 1 or 2ohms subs...

I checked the SOA curve and they would be ok for current at 42V per rail.. like 6A..

Any suggestions?.. comments..

Thanks to all helper!

Doc
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Old 26th October 2005, 07:45 AM   #2
djk is offline djk
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60-0-60 will give 85V no load. Run your 100mF caps in series with a balancing resistor and you will be set.

10 pair of MJ15024/25 will do close to 1KW at 2 ohms with that kind of rail voltage, about 600W at 4 ohms.

I would build a Leach v4.5, only a few minor changes are needed to run the higher voltage.

http://users.ece.gatech.edu/~mleach/lowtim/

The board is available for a nominal fee from Leach.

I have a Leach with 3 pair of outputs that runs at 63V into 2 ohms (needed to be re-built after 22 years).

Another is running 1 ohm with 10 pair at 80V (since 1989).
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Old 26th October 2005, 02:39 PM   #3
K-amps is offline K-amps  United States
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Dennis,

Mind pointing us to what mods are needed for a +/-90v supply for the leach?

TIA.
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Old 28th October 2005, 07:56 AM   #4
djk is offline djk
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I'll see if I can dig up my notes this weekend.

One of the 63V ones I used MJ15011/12 as drivers, the other I used MJE15030/31 to drive 3 pair of MJ15011/12 for outputs.

The 80V one I used MJ15024/25 as drivers for 10 pair of MJ15024/25 outputs, and 2N6421/3584 as pre-drivers. I used no current limiting in this amp.

For 90V I would go to the MJE15032/33 to drive 10 pair of MJ15024/25 for outputs down to 2 ohms.

If a guy didn't already have any outputs, I would buy MJ21195/96. They have 50% more SOA at 80V than the MJ15024/25, higher gain, and are the same price.
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Old 28th October 2005, 09:33 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
for a sub you will need lots of capacitance in the smoothing section.
I recommend you allow 2mF to 3mF per amp of peak output current.
For 70Vpk into 2ohms that's 35Apk. You will need 70mF to 105mF on each supply rail.
Your 100mF caps would have been perfect except for the wrong voltage. As pointed out earlier you can series connect them to obtain 100V working but the capacitance reduces to 50mF per rail.
This would be enough for 4 ohm speakers.
Buy some smaller 100V caps to parallel with the main smoothing caps. This will lower the ESR slightly as well.

The newer MJE15034/5 make really good drivers for multiple output pairs. Don't worry about them being only 4A, you will never use this part of the SOAR. Just keep them and the outputs cool
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Old 28th October 2005, 11:37 AM   #6
K-amps is offline K-amps  United States
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Quote:
Originally posted by djk
I'll see if I can dig up my notes this weekend.

One of the 63V ones I used MJ15011/12 as drivers, the other I used MJE15030/31 to drive 3 pair of MJ15011/12 for outputs.

The 80V one I used MJ15024/25 as drivers for 10 pair of MJ15024/25 outputs, and 2N6421/3584 as pre-drivers. I used no current limiting in this amp.

For 90V I would go to the MJE15032/33 to drive 10 pair of MJ15024/25 for outputs down to 2 ohms.

If a guy didn't already have any outputs, I would buy MJ21195/96. They have 50% more SOA at 80V than the MJ15024/25, higher gain, and are the same price.
So basically replacing the bjt's with higher voltage ones and re-bias... is that all? Would there be a need to regulate/ lower the voltage to the first stage?
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Old 28th October 2005, 01:02 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi Kamps,
most conventional power amps run the VAS at the same voltage as the output stage. With this topology you MUST NOT run the voltage amp stages at a lower voltage than the output stage or you will lose the volts drive ability to the final stage. e.g. run the volt amp from +-25Vdc and the output stage on +-50Vdc and your maximum output drops from 100W into 8ohms down to 25W into 8ohms.
There are a few amps that have some voltage again immediately preceeding the output stage and these deliberately run the voltage amp at a lower voltage. This is common when an opamp is used at the front end because they are often limited to +-15Vdc upto +-21Vdc.
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Old 28th October 2005, 03:21 PM   #8
K-amps is offline K-amps  United States
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True, thats why my post said first stage and not VA stage.
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Old 28th October 2005, 06:09 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi Kamps,
that whole voltage amplifier stage has to run off the same voltage supply. Have a look at the voltages around the VAS transistor and the collector of the input transistor and/or it's cascode if it has one.

There is an exception for the tail supply to/from the LTP where many designers use a heavily decoupled or zener stabilised tail supply voltage. I can be only a few volts away from the emitters but I believe it performs better if it has a few tens of volts to play with.
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Old 28th October 2005, 07:30 PM   #10
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Quote:
Originally posted by djk
Run your 100mF caps in series with a balancing resistor and you will be set.
Please can you draw a schematic how did you mean with this!


Regards,

Audio
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