KMJ,
Play the sound file of the sine wave into your scope from your PC or CD player and fiddle with the verticle and horizontal controls till you can see the sine wave very clearly.
then feed the sine wave into your amp without changing any settings and probe the outputs from the board with your scope. You should see the same sine wave and hopefully it will be bigger!
You will then know for sure that your amp is amplifying and its working.
For temperature, it takes a few min to get really hot. High temps can be withstood but life is lower. So for testing don't worry so much about heat, 60-70 C is fine but make sure that in everday listening you don't go much above 60. If your output devices are plastic you would want a lower temp than 60, maybe 55. You can set the heat by changing the bias.
Play the sound file of the sine wave into your scope from your PC or CD player and fiddle with the verticle and horizontal controls till you can see the sine wave very clearly.
then feed the sine wave into your amp without changing any settings and probe the outputs from the board with your scope. You should see the same sine wave and hopefully it will be bigger!
You will then know for sure that your amp is amplifying and its working.
For temperature, it takes a few min to get really hot. High temps can be withstood but life is lower. So for testing don't worry so much about heat, 60-70 C is fine but make sure that in everday listening you don't go much above 60. If your output devices are plastic you would want a lower temp than 60, maybe 55. You can set the heat by changing the bias.
or adding another pair of transistors (have som leftovers) and lowering the bias
And thanks for the pointers regarding the scope, i'll be sure to test it all soon. Since I have one I might as well learn how to use it after all and it takes something like this to supply proper motivation.
Oh, the devices are metalcan TO-3's
haha, no chance of that.
It's just that I got the recommendation to add an extra pair of transistors to lower the temperature a month or two ago (if not changing the bias which ofcourse is the easiest way) . When it comes to knowing and building this amp I need all the luck and well wishes I can get
So, I connected the 6,8ohm resistor between the feedback from the driverboard and ground and turned the amp on. The DC-offset was at this point about 100-150mV and since I just tested it for a few seconds I didn't try to lower it. The ting is this, when I turned it off again after some seconds the offset climbed to about 6-700mV before it started to get lower again, like a discharge across the speaker. What can cause this and what do I do about it? I assume that this could cause problems in the future.
Input still shorted.
Input still shorted.
Ok, I did as you said and turned the bias all the way down, started the amp and set the offset as close to zero as possible. Then I turned the bias up a bit again and checked so that the offset didn't drift. When I turned the amp of again the offset climbed yet again to approx 200mV where it turned and started to descend towards zero again.
I did however notice that the PSU-cap for the positive rail discharges much faster than the one on the negative rail (a few seconds while the other takes minutes). Could it be that one is flawed?
I did however notice that the PSU-cap for the positive rail discharges much faster than the one on the negative rail (a few seconds while the other takes minutes). Could it be that one is flawed?
I have a 10K 2W resistor in series with a LED at this time so that I can be sure that they are discharged when I'm testing. And I changed the cap for another one and that one also discharges faster so I quess that they are OK after all.
There's still the thing with the offset drifting on turn-off thou.
Hi,
the different discharge characteristic could be due to capacitor tolerance, but equally likely, is the different current that each rail of the amplifier draws.
If you use bleeder resistors you can adjust them to equalise the current draw between the two supply rails to compensate for the unequal amplifier current draw.
The output pulse is more likely to be due to a capacitor in the voltage amplifier that is holding significant charge as the rest of the amplifier is shutting down.
the different discharge characteristic could be due to capacitor tolerance, but equally likely, is the different current that each rail of the amplifier draws.
If you use bleeder resistors you can adjust them to equalise the current draw between the two supply rails to compensate for the unequal amplifier current draw.
The output pulse is more likely to be due to a capacitor in the voltage amplifier that is holding significant charge as the rest of the amplifier is shutting down.
So I downloaded a sound file and connected my laptop to the scope and adjusted it to show a clear curv when playing a 1KHz signal.
Then I connected a 13,3kohm resistor between the feedbackwire of the driverboard and the speakerground, connected the probe in parallell with the resistor and fired everything up but no curv could be seen.
Well, I just thought that the resistor was needed to give the amp a kind of load to get it stable, like when measuring offset and such.
The scope ground is connected to the speakerground, at the starground between the PSUcaps.
When measuring without a resistor I still get nothing from the feedbackwire but I do get a reading from the R101, a nice wave/curve