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Old 14th November 2001, 01:01 AM   #1
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Hi,

I've built the amp you can find at:

http://home.t-online.de/home/rat.moeller/280watt.zip

This amp works only if there's an input connected, if there's nothing connected the transistors will blow up or burn out; can anyone help me with this problem???

Thanks anyway,


Hugo.
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Old 14th November 2001, 11:34 AM   #2
djk is offline djk
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Check with your ohm meter to see if R1 is grounded properly.Make sure the center tap of the power supply is grounded to both the board and the chassis.
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Old 14th November 2001, 09:36 PM   #3
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I don't think it's R1, otherwise the amp would
not function with an input.

This is a very high gain amp at 83X, and I suggest
that the output is bleeding back into the input
and causing oscillation. You can correct this by
a low pass filter at the input, say at 50 - 100 KHz,
or by reducing R1 to 10 Kohm, or by attenuating the
input with a resistive divider, or by not running the
amp without an input.

Alternatively, check for proximity of input and output
hot leads, and make sure any shielded cables have the
shield properly grounded.
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Old 15th November 2001, 12:13 AM   #4
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I agree with Mr Pass.

Furthermore, I think that the open loop gain of the amplifier is too high, this is unreasonnable. I suggest to connect in series with each emitter of the input differential pair a small resistor (33...68 ohms) in order to keep the open loop gain at usual value. This will improve the stability of the amplifier at high frequencies.

Also, R1 must be equal to R5, i.e. 8.2 kohms, for correct ac/dc balance of the differential amplifier.

Film capacitors of 0.47 uF or more must be connected across input supply terminals (one between +48 and Gnd, the other betwwen -48 and Gnd), directly on the PCB, with shortest possible wires, in order to avoid disastrous feedback from high frequency output currents into input stages.

Regards, P.Lacombe
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Old 15th November 2001, 05:30 AM   #5
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Default Amplifier High Frequncy Instability

I would have to agree and say that say that the amplifier is going into oscillation when there is no audio source connected to the input and thus making both output devices conduct at the same time. This would indicate very high frequency oscillation where the speed of the output devices will not allow one device to turn off before the other starts conducing heavily.

I would suppose that the original designer had no problem with this so there is likely to be some small difference between your unit and the one the designer built. It is also very likely the circuit is somewhat unstable under the best of conditions. Excessive gain along with enough phase shift at the frequency the amplifier gain is more than 1 will turn the amplifier into a oscillator under some conditions. The frequency this occurs at can exceed 30 Mhz (30,000,000 HZ) quite easily in some designs.

Very likely is problem is aggravated when the input impedance for the audio input is raised by disconnecting the signal source. You can lower the resistance on the input but this is not a good cure. The real problem will still be there.

The best way to attack this problem may well be to simulate the circuit with computer software and make the required circuit changes to make it stable. In lieu of this the fix may very well depend on how your particular unit was built. However I would say that reducing the gain by adding the resistors in the emitters of the input differential pair is a very good place to start. The devices you used may well have more gain than those used by the designer.

I would also suggest that you fuse the positive and negative supplies to the output devices with some small value fuses, perhaps 1 amp to help prevent blowing output devices while you are trouble shooting. You may also have to add some small value feedback capacitors in the circuit somewhere to help reduce the amplifier gain at the low RF frequency range.

John Fassotte
Alaskan Audio







[Edited by alaskanaudio on 11-15-2001 at 12:32 AM]
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Old 15th November 2001, 07:03 PM   #6
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Smile thanks

Hey, thanks to all of you. I'll start right now to improve my amplifier. I'll let you know if I've made any improvement or not.

greets,

HB
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Old 16th November 2001, 07:28 PM   #7
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Wink This might be the solution?

Maybe I found the right solution for the problem of oscillation in case of no input. Can anybody this will work properly?

you can find my solution at:

http://hugobross.tripod.com/improvement/improvement.jpg


best regards,

hugobross.
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Old 16th November 2001, 11:32 PM   #8
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hugobross,

You cann't obtain good results with this op amp, which is of poor quality for audio.

Please, try the modifications gived in previous posts...you have to correct some design errors, it's the simplest way to obtain correct operation of the amplifier. Adding more circuitry will cause more problems, the simplest is the best.

Regards, P.Lacombe.
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Old 17th November 2001, 01:22 AM   #9
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Question please explain this

dear P.Lacombe,

Today, at highschool, I've tested this op-amp during my laboratory sessions and I've seen that this op-amp can amplify all audio-frequencies without any distortion visible on the oscilloscope. Can you please explain me, why using this op-amp can reduce the quality? Imagine that this op-amp is perfect, do you think that the problem of unwished oscillation will be solved by reducing R1?

regards,

HB.
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Old 17th November 2001, 12:27 PM   #10
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Hugo,
There's more...far more to listening to music than specs, particularly a simple frequency sweep, will reveal. Opamps are convenient and (usually) cheap. There are some that are designed specifically for audio use. If you must use an opamp, choose one of these. Be prepared to pay for a quality opamp. Personally, I avoid them like the plague, although I admit that they've gotten better over the years.
There have been a few threads here about which opamp is the 'best' sounding. Use the search function to locate them.

Grey
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