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Old 19th August 2008, 12:13 AM   #1
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Default making 25w constant class A amp

hi,

i saw this circuit for 25w class a. but problem is VR1 have to set for class-A 1.5amp quiescent current. if the current is low it will switch to class AB. i want to make it only for class-A. how can do this? how to i check my quiescent current is 1.5a? i only have multimeter.

http://sound.westhost.com/project3b.htm

i posted the circuit design.

thanks
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Old 19th August 2008, 01:59 AM   #2
lineup is offline lineup  Sweden
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Good choice, space2000

As usual, no problems to find the transistors, when go for Rod Elliott Projects.
BC546 and MJL4381A and MJL4302A should be easy.
And if you do not find those MJL you may use several other similar:
2SC1381 and 2SA1302
even popular
2SC5200 and 2SA1943 would work well, I think

1.5 Ampere is not a bad choice for 2x25 Volt Class A into 8 Ohm
As this output is push-pull, the max theoretical output is 3.0 Ampere (2x1.5)

25 Volt/ 8 Ohm ~ 3 Ampere
I = U / R
Ampere = Volt / Ohm

When you have 0.5 Volt across R13
or
1.0 Volt across R13+R14 .. then is 1.5 A idle Class A current

The heatsink must take 50 Volt x 1.5 Ampere = 75 Watt
A good heatsink size would be something like 1-2 kilo in weight .. or more.

Lineup new website = CLICK
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Old 19th August 2008, 07:49 AM   #3
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Quote:
Originally posted by lineup
the max theoretical output is 3.0 Ampere (2x1.5)
You meant to say : the maximum output current for CLASS A operation is 3A.
25W continuous in 8 Ohm is 50W peak, which corresponds to an output current of 2.5A=> requires a quiescent current level of 1.25A.

1.5A quiescent is 3A peak Class A output current for a push pull output stage=> in a 4 Ohm load the output stage then operates in Class A to an output level of 3x3x4=36W before it crosses over to Class AB.
Where Mr Elliot gets the 9W/4 figure from is beyond my grasp of reality.

Full Class A operation in a 4 Ohm load requires a quiescent current level of 2.5A.
=>50W continuous output in 4 Ohm is 20V theoretical peak output voltage, 20/4=5.
For 25V rails, a quiescent current level of 2.5A corresponds to an output stage dissipation of 125 Watts.

The regular plastic 2-device output stage of the P3A will be unable to handle 125W of heat.
The only option for that would be to use Sanken MT200 devices, my ExtremeA power amps dissipate a total of 520W with 8 devices.
But that also requires Extreme fan operated heatsinks to keep the die temperature of the output devices at a safe level.

With "normal" heatsinks, you'd have to double the number of output devices of the P3A to 4 per channel and use 0.25C/W heatsinks.
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Old 19th August 2008, 11:51 AM   #4
awpagan is offline awpagan  Australia
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Rod Elliott is measuring across both resistors.

Quoted from ESP

"When you are satisfied that all is well, set the bias current. Connect a multimeter between the collectors of Q7 and Q8 - you are measuring the voltage drop across the two 0.33 ohm resistors. The correct quiescent current is 1.5A, but I strongly suggest that you use a lower current to start with! The voltage you measure across the resistors should be set to 500mV ±5mV"
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Old 19th August 2008, 12:03 PM   #5
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500mV across two 0.33 Ohm resistors is 0.75A.
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Old 19th August 2008, 01:18 PM   #6
awpagan is offline awpagan  Australia
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Quote:
Originally posted by jacco vermeulen
500mV across two 0.33 Ohm resistors is 0.75A.

Yes, but those giong to tell Rod?

He state's he measures between the collectors.....
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Old 19th August 2008, 01:35 PM   #7
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Mr Elliot likely hasn't cleaned up that page yet.
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Old 19th August 2008, 01:40 PM   #8
awpagan is offline awpagan  Australia
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Jacco
that's what you call an "old" ooops.
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Old 20th August 2008, 09:52 AM   #9
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hi,

i was curious about the sound quality. its impressed me very much...wow...wow. i just made a dummy model and checked the sound quality. ofcourse it is not final. just made a vb circuit. so cute and small....ha....ha......its really works no hum and distortion at all...like a little baby in hand.


here is the pic.
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Old 20th August 2008, 10:36 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
the amplifier section of this will cost just a few pounds.
The heatsink and PSU will each cost far more.

1.5A bias current is just about right for a push pull ClassA amplifier driving an 8ohm load. It should give around 25 to 30W into 8r0 and stay in ClassA.

If you demand more current (>3Apk) it automatically moves into ClassAB on the high current transients, if the PSU is properly scaled to deliver such transient currents.
I would suggest 300VA and >=+-30mF/channel for 8ohm speakers.

1pair of outputs dissipating ~75 to 80W will run very hot,
fitting a temperature cut-off switch may be a good idea. Latching rather than reset on cooling.

As for circuit mods, I would prefer to see emitter resistors added to Q1, Q2 & Q4. Add a trimmer to either R6 or R7 to ensure the LTP is actually balanced. I would change C1 to 3u3F or 4uF and C3 to 150uF.

Q.) is 2.8mA through the drivers sufficient? 10mA or maybe even as high as 30mA seems more suitable.
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