♫♪ My little cheap Circlophone© ♫♪

[danielwritesbac]

Camel-free zone: Building Elvee's Circlophone: Documentation, Parts, Accessories, & beginner friendly

[Elvee]

Originally Posted by danielwritesbac

By same factor, the clipping performance will also be cleaner if a case exists where the power amp clips but the preamp doesn't, then the nesting loop will wash clean some of the clip.

This will always be the case: a preamplifier will normally have an ample overload margin, like 20dB, and the amplifier will clip well before the preamp (it should anyway, unless internal levels are set with no regard for common sense).
But! No feedback in the world can clean a clipping: the previous stages will simply push the output into deeper saturation.

Originally Posted by The Prof

Elvee, you are the best that has happened to DIY in a long while, if only it is for the reason that yours is a highly tolerant design and lends itself to implementation by the hobbyist with limited knowledge, resources and test equipment, while at the same time it takes him/her to a higher level of audio 'nirvana' with considerable ease and confidence!

Thanks! You caught the exact spirit in which the Circlophone was designed.

[The Prof]

@Dan:
You've got another great 'first' coming your way--you are the first that I know of who has gone to the time/trouble of organizing a 'camel-free' build zone... ;-)
All of us are proud of your efforts..... and thanks a zillion!

@Elvee:
The pleasure is mine/ours, Elvee!!

Perhaps you would think of posting a more detailed 'explication' of the circuit topology and its unique aspects of design. I personally would very much welcome such a post.

Could somebody suggest a way of saving the entire set of posts into one big file?

Now let us, the DIY bunch, get to the build/listen/compare stage and post our results.

I would like to see some observations about the quasi-comp 'quality' factor by the more experienced.

[kenpeter]

I want to know more about the mixed compensation.
Where is the differential dominant pole each half cycle?
What dominates QFB during the aB crossing?

Do the .AC Bode plots look different if we offset DC
to see what happens in each half cycle of Class B?
Note: they are only in B because QFB loop is closed,
so any phase shift of QFB might also be relevant.

One side of the differential driver sees a swinging load,
the other side sees a load standing still. They can't
possibly Bode the same. Yet both obviously stable...

If all three differential Bode's (+/-/crossing) all look
stable with NFB open and QFB closed, does that mean
QFB is stable? Or there is some other test of stabilty
in the common mode that QFB needs to pass?

[danielwritesbac]

Originally Posted by The Prof

@Dan:
You've got another great 'first' coming your way--you are the first that I know of who has gone to the time/trouble of organizing a 'camel-free' build zone... ;-) All of us are proud of your efforts..... and thanks a zillion!

Elvee put in a LOT of work on that as well--much device testing.
Terranigma's very compact board also serves up CFP version documentation.

Thank you so much for your kind words, and the fun terminology as well.
I might have exaggerated a fraction though, so perhaps the builder's thread is 99% camel-free.
There is one exception.
See the attachment.
That soft clip circuit didn't quite work as intended, but rather it makes noise quite similar to a triode (as shown, although it may be set differently). The effect has a use for MP4, AAC types such as Digital TV, Itunes, HD radio or for any source that has duller treble that needs some help (those would blare if cranked up were it not for the funny little soft clip accessory). If someday you need some fake clarity, remember this little thing--especially good with TV and HD radio, and surely lower distortion than a baxandall. It is actually a treble harmonic enhancer not much of a soft clipper. Thanks for the opportunity to fix the documentation for it.

[The Prof]

@Dan
Who said DIY forums were not fun? Let me have a big laugh with you....hahahaha...hohoho... this is good for the spirit!!

[Elvee]

Originally Posted by The Prof

The pleasure is mine/ours, Elvee!!

Perhaps you would think of posting a more detailed 'explication' of the circuit topology and its unique aspects of design. I personally would very much welcome such a post.

I think you could figure out a complete explication from all the informations I have given along the thread, but it is in bits and pieces scattered more or less randomly, and not very convenient I have to admit.

I'll try to come up with something more structured.

I want to know more about the mixed compensation.
Where is the differential dominant pole each half cycle?

The differential low frequency pole (not exactly dominant) is set by C6, and is at ~10KHz.
It varies little with the output excursion.

Do the .AC Bode plots look different if we offset DC
to see what happens in each half cycle of Class B?
Note: they are only in B because QFB loop is closed,
so any phase shift of QFB might also be relevant.

One side of the differential driver sees a swinging load,
the other side sees a load standing still. They can't
possibly Bode the same. Yet both obviously stable...

Your observations would apply to a real class B amplifier. Here, all transistors remain always active, which means the amplifier behaves essentially as a class A regarding the Bode plots, with only minor differences related to the current magnitude/polarity, caused by the transconductance modulation (*but see also below).
But this is a second order effect.

If all three differential Bode's (+/-/crossing) all look
stable with NFB open and QFB closed, does that mean
QFB is stable? Or there is some other test of stabilty
in the common mode that QFB needs to pass?

No, absolutely not. The QFB loop must also be stable under all current polarity/magnitude conditions, which is trickier, because a secondary pole moves when the polarity is changed: in one case, the feedback loop is direct from R8, in the other, the signal also has to transit through Q13, which increases the phase shift.
I have to admit this is a tricky issue, and if a "New Circlophone" was to be designed, this would be high on the priority list.

*In addition, the two loops interact parametrically with each other, and this causes additional effects, but I do not know how to handle it mathematically or in simulation: this is an advanced level of the control theory, much more advanced than my own (humble) level.

[danielwritesbac]

Elvee, I have a simpler question: How do we adapt the inputs of Circlophone for supporting single rail operation?

[The Prof]

Yeah, please show us, Elvee!

[mickeymoose]

That is easy. An even simpler question: Why? E