Aleph-X with common-mode oscillation

[DACMan1]

I built Grey Rollins' version of the amp and now I get 13.3V common mode oscillation of about 500Hz. Anyone familiar with this problem? I havent had any time to investigate further. I will statr with powersupply decoupling first, as I don't have a clue where else this type of problem could originate.

Thanks

 

[Netlist]

What boards did you use?
Try adding a 1nF cap between Collector and Base of Q3 and Q8 or a 10nF between Collector and Emitter of the same transistors. As described in the Wiki under 7: More unsorted info from The Big Thread.

/Hugo

 

[DACMan1]

I believe those caps are for high frequency oscillations? The boards are ones I did myself.

 

[Netlist]

Yes, HF. As for the 500Hz, no idea. I'm sure someone with more experience will help you out.

/Hugo

 

[Babowana]

Before I do anything else, I would carefully check the connections
of the PSU capacitors whether they are tight.

 

[DACMan1]

Solved

Seems that grounding the inputs solved the problem (even though I thought I grounded them previously). Now the only problem is heatsinking for the diodes on the powersupply which is inadequate but there is not enough space for bigger heatsinking

 

[Babowana]

Re: Solved

Now the only problem is heatsinking for the diodes on the powersupply which is inadequate but there is not enough space for bigger heatsinking

For your info. I often fix the bridge diodes on the main heatsinks.
In this case, we need to be generous with somewhat lengthy wire arrangement.

 

[DACMan1]

That won't work with my layout and the fact that I am using 4 BYV diodes. Also, I want to minimse the wires to and from the diodes, as they carry large current spikes to top-up the reservoir caps. I have other heatsinks that are suitable, just don't know how much better they will be. I will look into it tomorrow.

 

[EUVL]

You can estimate the heat dissipation by multiplying you total bias in Ampere by 3V (2 diode drops). Then choose the heatsink to give you max. 30 degC temperature rise.

Patrick

 

[DACMan1]

In other words, if I have individual diodes, Multiplying half the current by 1.5V per heatsink.

Which gives 1.5*2.25 = 3.375W

30degC/3.375W = 8.88degC/W

 

[DACMan1]

Ones I have is 15.5degC/W which means a temp rise of 52degC but its the best that will fit. However, forward voltage drop on these will only be less than 1V(worst case 1.15V and will drop at higher temperatures. Thus:

2.25*1=2.25W
2.25W*15.5degC/W=35degC

Worst case:

2.25*1.15*15.5 = 40degC

This is acceptable to me.

PS, they will be mounted directly on the heatsink, so lets say a 0.8degC/W case to heatsink junction temp. They have 1.6degC/W junction to case drop.

So the die will run at: (above ambient in case)

(1.6+0.8+15.5)*2.25=40.25degC
*1.15(worstcase) = 46.3degC

at worst case internal temp = main heatsink temp

gives realitic to worst case ratings of die temp 90-110degC.

These diodes will live quite happily at a die temp of 100degC.

Hopefully internal temp will be a bit lower

 

[DACMan1]

absolute dc mode offset

I cant seem to find this in the main thread, the absolute dc offset of my amp starts at around 10V when cold and then goes down to -1V after about an hour. Is this normal?