My Wave Isn't Square.

[DF96]

10kHz ringing either comes from a poor OPT or loop instability.

 

[gwalters]

I posted several pics of the ring but they did not show up. Perhaps the files were to big. Each was about 700k. Her is just one to see if that will show up. It's the ring with the feed back cap of .001uf. The freq is 1K.

 

[gwalters]

Here it is with .0022uf. Freq still 1K.

 

[gwalters]

And with .0033uf and freq still 1K. It is over damped in this case. Note that the square wave has the ring that is getting amplified in each case.

 

[DF96]

Sorry, ringing is 1kHz or square wave is 1kHz?

If the ringing is on the input then it should be on the output too!

 

[gwalters]

What you're looking at is the square wave frequency is 1kHz. The lower one is the input and the upper is output.

 

[DF96]

The square wave frequency is almost irrelevant. Without knowing the timebase setting I can't estimate the ringing freq, except to say that it is probably greater than 25kHz. Have you tried changing the feedback, as I suggested?

 

[gwalters]

Here is a pic of the ring using a 10kHz square wave. The freq seems to be about 1/4 of that. Does this make sense to you?

 

[gwalters]

I have not done your resistor swap yet. I have to dissemble the unit, take the tubes out etc to get to the place I can the unsolder the resistor. I"ll do that this weekend.

 

[DF96]

Yes, it looks like 40-50kHz. OPT resonance is most likely. Nothing to worry about.

 

[gwalters]

Thanks

 

[gwalters]

Humm

I have finished the amp, thanks to all who helped. I have found one lingering problem which has me puzzled. The amp has no hum except when I plug in my iPhone (any iPhone) then it is annoying. I've changed connecting cables but no change. Does anyone know how to fix this or what the problem is?

 

[DF96]

With or without external power supply? If with, could be hum loop.

 

[gwalters]

With or without external power supply? If with, could be hum loop.

I have a doc station and I can plug in the iphone directly or through the doc station. Both are the same. Each hum.

 

[gwalters]

I understand that the push-pull negative feedback amplifier design was to remove distortion. Can someone explain where the distortion exists that it minimized and how it minimized it?

BTW I found the source of the hum in the previous post. It was in the PCB groundplane, put there by the pwr supply. Hooking up an input signal from the iPhone (or anything) connected the ground with the humm to the input. I just took the pwr supply ground off the PCB groundplane and the problem went away.

 

[SY]

Many books have been written about that! Start with the amp books from Doug Self and Bob Cordell for solid state p-p negative feedback amps. For tubes, Morgan Jones's book.

 

[gwalters]

I have read Morgan Jones book about this push pull topic but do not find where he discusses how it solves the distortion issue. He goes into a great deal of detail how it works, which I understand, but not how it solves any distortion problem.

 

[SY]

Did you see the part about cancellation of even order distortion?

 

[gwalters]

No I didn't see that. Do you have a page number? If not I'll look harder.

 

[DF96]

For a given device, even-order distortion stays the same even if you invert the input signal. This is elementary maths (powers and trigonometry). Push-pull has two devices, with one input signal inverted. The output transformer subtracts the two signal it sees: wanted plus odd-order are antiphase so add, even-order is in phase so cancels.

Alternatively, look at combined push-pull valve curves. The two sets of curves are bent in opposite directions so cancel out. Either way of looking at it, P-P cancels even-order distortion but leaves odd-order unchanged. Fortunately even-order is higher for all devices so the total is reduced.