Buffer Fault - Page 2

PMA · 8th September 2004, 06:14 AM

Some of the 78XX/79XX (depends on manufacturer) exhibit strange behavior: under no load they supply more voltage than Vref. For example not mentioned Korean producer made 7805 with some 7V output under no load. When loaded everything was fine, 5V. The cure was to use load resistor at the output of the regulator to assure load current at least 5mA (i.e. 1kOhm resistor for 5V regulator).

And - do you have center point (0V) defined?.

carlosfm · 8th September 2004, 09:24 AM

Pavel, he has less voltage under no load than connected to the circuit.

pete.a · 8th September 2004, 03:55 PM

Iv'e scrapped the whole build ! I couldnt find out what was going on in that little circuit.

I may try building the heaphone amp just using one op amp as per Carlos design, as i feel thats where i went wrong using zero links for different connections ! between op amps and buffer chips.

As i only had limited time to construct this project i may have overlooked a connection ..who knows..it happens.

But thanks for the help guys.

Carlos..regs gave out 14.98v under load and 18v off load until the circuit went ...Pop !

Pavel do have any pcb graphics for the head amp, as i have a friend how will make a pcb, but isn't that good at pcb design. Has the equipment but not the electronic discpline.

PMA · 8th September 2004, 04:04 PM

Pete,

probably I would release a PCB artwork for audio buffer, that can be used as a headphone amp (see my web). Send me an e-mail, if you are interested in it.

Rgds,
Pavel

PMA · 8th September 2004, 04:13 PM

Pete,

what was the buffer package you have used? DIP8, TO220-5 or another.

Thanks, Pavel

pete.a · 8th September 2004, 04:18 PM

It was To220 -5 and i only got a couple as samples...now theve ran out

pete.a · 8th September 2004, 07:25 PM

Guys Ive found the fault...or at least i think so.

It would appear that i forgot to break the track underneath the board , on pin 5 of the op amp. Due to the orientation of components and links, it connected pin5 op amp to pin 3 buffer ..ouch..

Take a look at the image, it may explain a little more, this is only prototype board. Finnished will look a whole lot neater and bare legs will be sleeved.

Just a thought , could i use slightly smaller rated regs , ie, 500Ma as apposed to 1A as seen before the smoothing caps.
Anyone got a spare Buf634T out there in diy land, cant get anymore for another 14weeks !!

:

PMA · 8th September 2004, 08:10 PM

Pin 5 of OPA627 is connected very close to +Vs (in the inner structure), pin 3 of BUF634T is -Vs .... the faults are usually like this.

8th September 2004, 03:55 PM	#13
pete.a diyAudio Member Join Date: Jul 2002 Location: Nottingham	Scrapped Iv'e scrapped the whole build ! I couldnt find out what was going on in that little circuit. I may try building the heaphone amp just using one op amp as per Carlos design, as i feel thats where i went wrong using zero links for different connections ! between op amps and buffer chips. As i only had limited time to construct this project i may have overlooked a connection ..who knows..it happens. But thanks for the help guys. Carlos..regs gave out 14.98v under load and 18v off load until the circuit went ...Pop ! Pavel do have any pcb graphics for the head amp, as i have a friend how will make a pcb, but isn't that good at pcb design. Has the equipment but not the electronic discpline.

8th September 2004, 04:18 PM	#16
pete.a diyAudio Member Join Date: Jul 2002 Location: Nottingham	package It was To220 -5 and i only got a couple as samples...now theve ran out

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8th September 2004, 06:14 AM	#11
PMA Design engineer, consultant diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Some of the 78XX/79XX (depends on manufacturer) exhibit strange behavior: under no load they supply more voltage than Vref. For example not mentioned Korean producer made 7805 with some 7V output under no load. When loaded everything was fine, 5V. The cure was to use load resistor at the output of the regulator to assure load current at least 5mA (i.e. 1kOhm resistor for 5V regulator). And - do you have center point (0V) defined?.

8th September 2004, 09:24 AM	#12
carlosfm Banned Join Date: Mar 2003 Location: Lisbon, Portugal	Pavel, he has less voltage under no load than connected to the circuit.

8th September 2004, 04:04 PM	#14
PMA Design engineer, consultant diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Pete, probably I would release a PCB artwork for audio buffer, that can be used as a headphone amp (see my web). Send me an e-mail, if you are interested in it. Rgds, Pavel

8th September 2004, 04:13 PM	#15
PMA Design engineer, consultant diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Pete, what was the buffer package you have used? DIP8, TO220-5 or another. Thanks, Pavel

8th September 2004, 08:10 PM	#18
PMA Design engineer, consultant diyAudio Member Join Date: Apr 2002 Location: Prague, Kitakyushu, Fukuoka	Pin 5 of OPA627 is connected very close to +Vs (in the inner structure), pin 3 of BUF634T is -Vs .... the faults are usually like this.

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