I've built 2 gainclones before with no problems and thought I'd have a go at a BrianGT kit I bought about a year ago. I made it dual mono with 2 seperate rectifier bridges and 2 transformers. Tested the voltage and got +/- 35 on each side, so went on to connect up the amp boards (basic design with just the resistors and 1500uF caps. Tested the offset and got a few millamps on each side, but when I connect a source and speakers up, I got a loud buzzing from both speakers and then a high pitched whine from one channel. This stopped when I turned the source off.
However, I have 2 burnt out mid/bass Kef B110 drivers which aren't cheap to replace.
I assume the amp was oscillating but I daren't use it again and don't have access to an oscilloscope. Shall I throw it in the bin?
However, I have 2 burnt out mid/bass Kef B110 drivers which aren't cheap to replace.
I assume the amp was oscillating but I daren't use it again and don't have access to an oscilloscope. Shall I throw it in the bin?
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Joined 2004
ericallan said:
Sorry to hear about that.
The KEF B110 drivers are not only expensive but they are increasingly rare these days.
I hate to say I told you so but you really should have tested the amps into a dummy load first and then into some cheap drivers that you don't care about.
Only when you've tortured the amp for a good while should you connect the expensive stuff.
Thats amp building 101.
richie00boy said:
I've had about a dozen pairs of B110s go through my hands - but this were the last. KEF were repairing them not too long ago, so its just the repair bill that stings.
I didnt use a zobel or DC cap - thats a lesson learnt. It didnt sound like DC though - more like a very bad earth.
One question - I assumed that the earths being tied at the rectifiers was enough, but do they need to meet later in the circuit as a star ground?
Cheers for the info so far - I'll try to get pics here tomorrow.
A pop of DC as you plug in a source (and a lack of protection from this at the input) can be all that is needed to knock the chip amp into oscillation, especially as it will be over sensitive to this with the lack of zobel network.
If you have used a twin bridge PSU the centre of the caps is your star ground and should be the only ground return point in the system. Were your input sockets isolated from the case? Was your case connected to mains earth? Was mains earth connected to the star ground?
If you have used a twin bridge PSU the centre of the caps is your star ground and should be the only ground return point in the system. Were your input sockets isolated from the case? Was your case connected to mains earth? Was mains earth connected to the star ground?
richie00boy said:
The pop of DC would have come from the cheap portable cd player as it was turned on - it was all quiet before that. The input sockets are insulated but no earth connection to the case yet and the mains earth was not connected to the star ground.
And to the earlier question - it should have read millivolts.
I dont like to moan about such nice pcbs, but the build instructions still need some way to go to make a foolproof amp.
Anyway - managed to get some piccys;
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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