Finalized and powered up my stereo Aleph-X (with single PSU, 800VA 12V sec, dual rect.bridge and 2xCRCRC with C=47,000uF and R=0.1ohm per side, given about 11.0-11.5 V rail, which is a bit low but acceptable for now). I used the PCB's from the 2003 group buy:PCB)
using this schematic: Schematic
with option: J1a, D1a, and jumper from pin 1 to 3 on Q12a. Used IRFP044 biased at about 2.25A each.
Aleph-x settles in quite nicely, with absolute DC of about 50mV and relative DC of about 4mV on both channels using RCA with input caps.
The problem I'm facing is this:
When using open RCA I hear a clear hum from the xformer through the speakers, seems 50Hz but I'm not sure. I also hear some noise in the tweeter and midrange. Fiddling with the wires doesn't help.
When shorting +input to ground gives me a completely quiet amp at the speakers not hearing any noise or humming.
When connecting the interconnects to the RCA also gives me a completely quiet amp.
Question is what is causing the noise? Could it simply be a floating +input? Cause it seems to me that it has nothing to do with ground loops or so since the shorting of the RCA solves the problem.
Luckily the noise is only present when the amp is connected, so I've already had the pleasure of listening to this amp and it sounds marvelous!
Hope someone can help here.
Cheers.
using this schematic: Schematic
with option: J1a, D1a, and jumper from pin 1 to 3 on Q12a. Used IRFP044 biased at about 2.25A each.
Aleph-x settles in quite nicely, with absolute DC of about 50mV and relative DC of about 4mV on both channels using RCA with input caps.
The problem I'm facing is this:
When using open RCA I hear a clear hum from the xformer through the speakers, seems 50Hz but I'm not sure. I also hear some noise in the tweeter and midrange. Fiddling with the wires doesn't help.
When shorting +input to ground gives me a completely quiet amp at the speakers not hearing any noise or humming.
When connecting the interconnects to the RCA also gives me a completely quiet amp.
Question is what is causing the noise? Could it simply be a floating +input? Cause it seems to me that it has nothing to do with ground loops or so since the shorting of the RCA solves the problem.
Luckily the noise is only present when the amp is connected, so I've already had the pleasure of listening to this amp and it sounds marvelous!
Hope someone can help here.
Cheers.
Hi there,
noise measurements are always done using shorted inputs as there might be garbage picked up otherwise.
Keep in mind the X-amps are typically sensitive to the DC-impedance at its inputs, so if you want the same small output offset for balanced and unbalanced inputs, you can try AC-coupling.
noise measurements are always done using shorted inputs as there might be garbage picked up otherwise.
Keep in mind the X-amps are typically sensitive to the DC-impedance at its inputs, so if you want the same small output offset for balanced and unbalanced inputs, you can try AC-coupling.
Not as far as I know. Amp is about dead quiet except for a very very slight hum when getting almost into the woofer.
Any reason to assume oscillation with this setup?
Any reason to assume oscillation with this setup?
Resistor on the gate is not very close. It is there for the purpose of preventing oscillation. IF you got a DMM, plug it up to outputs and see.
Ah, I was under the impression that with this setup oscillations wouldnt be that big a problem. But who knows. Nevertheless, I doubt if a conventional dmm can measure any hf oscillation. But I'll try with C and t wo anti parallel leds at the outputs to see. But the very low level of output noise makes we wonder if it is actually oscillating. It might sound like a dc component on the xformer or perhaps some overloading of the xformer... although Im not sure if overloading can.actually result in output noise...
jeekee, you are using a combination of star and bus ground in this amp (bus is a red wire from bridge rectifier through ElKos to half-star point). Make a clean star and shorten the red wires.
like this
in some cases you'll also need bridge directly between Audio gnd left and Audio gnd right , made of thick wire , laid out in shortest path
in some cases you'll also need bridge directly between Audio gnd left and Audio gnd right , made of thick wire , laid out in shortest path
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I think that hum is better suppressed if the first capacitor pair have their "ground" connection tied together before the common wire is brought to the output star.
I have not built a 3stage rCRCRC PSU, so I am not sure where the second pair of capacitors should be ground connected.
I have not built a 3stage rCRCRC PSU, so I am not sure where the second pair of capacitors should be ground connected.
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Guys, thanks for all the input. I've managed to rewire the bung and the hum improved nicely. It's still there but it's almost not audible anymore. Best guess is some ripple getting through to the output.
Perhaps changing the R's for L's might reduce ripple enough. On the other hand perhaps it's worth the effort to rewire the xformer to output 24V, this will give me room to reduce ripple rigorously and perhaps use an input choke to dampen input current swings. Might also benefit the xformer hum... when aiming for a 15V rail the only problem is the current rating for the choke. If I remember correctly the RMS current would be around 10A but the peak current at about 16A. Most if not all chokes will go to about 10A, so I'm not sure whether the 16A peak will be problem... Any ideas?
Thanks again for all the GOOD help and fast answers!
Perhaps changing the R's for L's might reduce ripple enough. On the other hand perhaps it's worth the effort to rewire the xformer to output 24V, this will give me room to reduce ripple rigorously and perhaps use an input choke to dampen input current swings. Might also benefit the xformer hum... when aiming for a 15V rail the only problem is the current rating for the choke. If I remember correctly the RMS current would be around 10A but the peak current at about 16A. Most if not all chokes will go to about 10A, so I'm not sure whether the 16A peak will be problem... Any ideas?
Thanks again for all the GOOD help and fast answers!
Zen Mod, good point. No hum with inputs grounded... so guess ripple isn't much of a problem. Any options left untouched to further reduce the last bit of hum?
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