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Pbassred 14th November 2003 11:09 PM

Mj15003 And Mj15024
Straight out I have to admit a severely limited knowledge. I'm a user, not a builder. I aquired an old Studiomaster 400C power amp. I can't get details on it anymore.

While I was trying to establish if it is 200W into 4ohms or 8, I noted that one channel uses 4 x MJ15024, The other 4 x MJ15003. What is the difference?
I understand mached pairs but why all the same on each channel. Why not use all the same type?
Lastly both of these device are rated at 250w@ 55V, so why would I need 4 per side?

Confused of london

mrfeedback 15th November 2003 11:06 AM

It sounds like one channel has been rebuilt at one stage.
Go to On-semi to get specs of these two transistor types.
The output stage is quasi complimentary design - not uncommon in older amplifiers.
Multiple output transistors ensure SOAR limits are not exceeded.


Centauri 15th November 2003 11:28 AM

The MJ15024 is higher voltage rating transistor (250V vs 200V). For the amp power you are describing, the 15003's would probably be ok. The 4 transistors per channel could be two parallelled pairs, or could be 2 series pairs using 4 power supply rails, which keeps heat dissipation low (a few high end amps use this arrangement).

You can get a bit of an idea about power rating by measuring the DC power supply rails: +- 55volts or so would give 200W into 4ohms, whilst around +-67volts would be needed to produce 200W into 8ohms.


Pbassred 15th November 2003 12:18 PM

So I could heat up the amp just by increasing the voltage? One of my hastles is the 4 ohm load whereas most comertial cabs are 8ohm.

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