pa 300 amplifier published nov 1995

[funkstar101]

a friend and i decided to build a stereo amp using two of these amps. the amps seem to be unstable in operation viz. one of the amps will release the smoke from the components for no apparent reason. the presence of a "groundloop" could be contributing but all earth points are common to chassis. hav no idea wots causing the groundloop or the blowups. any help in solving this would be appreciated. have already committed myself financially to this project so i have to see this thru unless any1 can suggest a different project using a 63v-0-63v power supply.

[kypros]

Since the circuit assembly is ok then check for self oscillations.
This has to do with the stability of the whole circuit.

Take a measure at the output to see whether high frequencies in the ultrasonic region do exist. Carry out the test using a current limitting method on your PSU. Either use high power series resistors or lower supply voltage.

In case u conclude to the above, u can use decoupling capacitors both on the supply lines and the feedback network (negative feedback) in order to reduce the responce on frequencies outside the audio spectrum.

The value of the capacitor depends on the value or the feedback resistor and the desired cut-off frequency.

I would be on holidays till 15/8/2005 so no answering till then.

Good luck

Kypros

[funkstar101]

hi kypros thanx 4 the reply and advice. wil definately giv it a try. in the meantime been doin mor checkin myself and found the ff: the 5w resistors in line with the emitter on the finals ought 2b a ,27 ohm. usin a dmm with a calibrate feature these r measurin ,1. also herd that using wire wound resistors here is a no no. wil replace these resistors with power metal film ones but gonna series parallel 2 make the value as i cannot find. do u have any other suggestion other than the metal film. thanx 4 the help so far

[Guest]

Ceramic, low-inductance power resistors MUST be used here as advised in the article. Try C-I Electronics on www.dil.nl

EE Labs

[ghadiri]

Hi.
I have built PA300 and this is a fine amplifire.
but there is problem in my amp and I didn't know how can I do for this situation and I want to help me.
I test it with function generator with 20Hz to 20khz frequencies.
But after 5khz(for instance 10khz) and high power the relay is cut off.but in this stage there is not any clipping and the output voltage is still low than 1khz frequency.in this frequency the output voltage is 27V.
but at 1khz and maximum power there is no any problem.so in this stage the output voltage is around 36V and it is more than the 10khz voltage output.
So in other High frequency for instance 20khz the output voltage would reduce to 20V and in this frequency at that voltage relay will cut off.
so the output voltage of opamp(ne5534) on the B point in the schematic circuit is not 0 volt.
I think the output voltage will became DC.
please help me about it.
thanks.