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diy didi 11th February 2012 09:19 AM

HELP WITH P3A AMPLIFIER
 
Hi All!!
Just reading through Rod's paper on his P3A Amp. He mentions that for a 4ohm load, one should not exceed +/-35V supply.
I have an old amp board with a previous traffo and rectifier, capacitors etc. When this amp was still operational, it delivered 112Watts into a 4ohm driver before the onset of clipping. That means the supply then dropped to +/-21V or so. This is what I actually measured too.
So my question is, what is the 35V Rod refers to?? Is it the measured supply when loaded with 4ohm load or what?? Will my traffo then work??
I'm really keen on building his amp! I'm gonna mount Vbe multiplier transistor to driver transistors heatsink.(CFP output stage).

currentflow 11th February 2012 11:08 AM

Quote:

Originally Posted by diy didi (Post 2901623)
Hi All!!
Just reading through Rod's paper on his P3A Amp. He mentions that for a 4ohm load, one should not exceed +/-35V supply.
I have an old amp board with a previous traffo and rectifier, capacitors etc. When this amp was still operational, it delivered 112Watts into a 4ohm driver before the onset of clipping. That means the supply then dropped to +/-21V or so. This is what I actually measured too.
So my question is, what is the 35V Rod refers to?? Is it the measured supply when loaded with 4ohm load or what?? Will my traffo then work??
I'm really keen on building his amp! I'm gonna mount Vbe multiplier transistor to driver transistors heatsink.(CFP output stage).

Hi,

If your old amplifier previously delivered 112W rms into 4 ohms, the peak output voltage was around 30V and the supply rails were a few volts higher than this due to the losses across the output transistors. Your DC rails were probably around 35V DC.

The 35V supply Rod refers to is the maximum recommended supply voltage that may safely be used with a 4 ohm load. Your supply should be fine and would still give you around 112W into 4 ohms, as before. You can measure your supply rails (with no amplifier connected) to verify this.

Regards,
currentflow

discrete 11th February 2012 11:22 AM

+/-21 V into 4 ohm gives ~55 W....the ~110 W is peak power, not 'rms' power.

(Note that 'rms power' is a misnomer ... power is calculated using the rms values of voltage/current, but should never be called 'rms power', just 'power')

currentflow 11th February 2012 12:39 PM

Quote:

Originally Posted by discrete (Post 2901743)
+/-21 V into 4 ohm gives ~55 W....the ~110 W is peak power, not 'rms' power.

+/-21V (DC) rails would produce around 45W into a 4 ohm load if ~ 2V is lost across the output transistors (BJT) with their respective series resistors. If the original amplifier used MOSFET outputs then this loss would have been higher. However I did say "if" the original power was 112W (rms).

discrete 11th February 2012 12:46 PM

Quote:

Originally Posted by diy didi (Post 2901623)
When this amp was still operational, it delivered 112Watts into a 4ohm driver before the onset of clipping. That means the supply then dropped to +/-21V or so.

Sorry cf, should have quoted didi....

And you are of course correct, there is the dropout voltage of the amplifier to contend with....

diy didi 11th February 2012 03:24 PM

You are all right! I meant the rms voltage on output was 21v. The supply was indeed in the 30's. So will my traffo with filter cct unloaded +\-46v be fine with p3a project?

sgrossklass 11th February 2012 03:39 PM

Amp supplies are usually specified at idle. What do you get when loaded with about 100 mA? I suppose you're still well over +/-40 then, i.e. outside the recommended range for 4-ohm loads.

Incidentally, commercial amps with power devices like that (or even smaller ones) are commonly run at over +/- 50 V. They tend to be correspondingly easy to kill by accidental shorts on the output (2nd-order breakthrough at work).

currentflow 11th February 2012 03:54 PM

Quote:

Originally Posted by diy didi (Post 2902023)
You are all right! I meant the rms voltage on output was 21v. The supply was indeed in the 30's. So will my traffo with filter cct unloaded +\-46v be fine with p3a project?

Is it possible the original amplifier was fitted with a limiter, causing the output clipping at 21V as you had observed? If, on the other hand, the rails drop to around 25V under load due to the supply alone, then I wouldn't be too concerned about proceeding with your P3A :)

Tot siens,
currentflow

diy didi 12th February 2012 07:13 AM

So, If i added more output transistors, would his advice on +/-35 supplies still hold true?????

currentflow 12th February 2012 09:11 AM

Quote:

Originally Posted by diy didi (Post 2902999)
So, If i added more output transistors, would his advice on +/-35 supplies still hold true?????

The concern is that with too high a supply rail voltage and with a 4 ohm load, the safe operating area of the output transistors will be exceeded. Before considering modifying the P3A design, it might be better to examine your existing power supply to see exactly what it is capable of delivering. Do you have a circuit diagram / service manual for your original amplifier?

Regards,
currentflow


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