♫♪ My little cheap Circlophone© ♫♪

[danielwritesbac]

Originally Posted by kenpeter

Bizarre rant makes no sense whatsoever, stealing my stolen thunder.

I didn't mean to offend. And, I found that resistance between wiper and diode (diode current) was the other adjustment that I was searching for. YAY!!!

[Elvee]

Originally Posted by danielwritesbac

Well, I apologize. I didn't want to second guess or guess at all; however, the BAT diodes are in short supply except for BAT86, BAT85, BAT54 and a few similar. So, that topic is about parts availability and avoiding the hindrance of discontinuances or the expense of rarities. Also, BAT54, although highly available, the SMD part doesn't fit through hole board easily.

No problem, but you have to understand that derating indiscriminately at every component of the amp can only bring trouble.

Derating makes sense for heavily stressed components, typically OP devices.
In the case of D7, the working current is 1.5mA, and Imax is 300mA: that's already a 200x derating factor.
The maximum reverse voltage is 0V, and the Vmax is 30V: for voltage the derating factor is 30/0=∞.
Improving on that is ludicrous and can only cause problems

??? I could not make that work in all conditions, for all transformers, with all power supplies, in all locations, so I changed the constructor's chart to real DC measures because managing transformer variety was too far beyond the scope of a little chart.

How do I calculate R21 from DC? I've used your guidelines posted here: ♫♪ My little cheap Circlophone© ♫♪ for some of the examples and then attempted to extrapolate the rest.

Ohm's law: for 1.5mA current, R21=Vtot/1.5, expressed in kilohm.
The DC voltage is a little under 1.4 times the AC voltage, thus R21=Vac(1.35/1.5)~=Vac*0.9
All that is approximate, a current comprised between 1 and 2mA is fine.

So, here is the chart by itself (temporarily large), along with a new question of how much DC voltage can we use when specifically 8 ohm speakers?
Apparently a fun 75 watt version of the original has appeared with the MJ15015 being the higher voltage version of 2N3055. But, for some of the higher voltage selections I worry about overheating some of Circlophone's resistors. Which chart selections aren't suitable?

Joule's law: U=1.5+√(2*P*R)
For 75W/8Ω, that's 1.5+√(2*75*8)=36V (absolute minimum)
No resistor risk overheating, even at total supplies in excess of 120V.
The 250mW rating remains adequate in all cases, though I would advise a larger rating for R17, in the interest of linearity.

[Elvee]

I have tested the idea of darlington and CFP inputs.
Darlington doesn't look promising: there is a notable degradation, whatever the configuration.
CFP is much more attractive, but is also more demanding on compensation.

The amp was stable with a simple plug-in replacement, but it was marginal, and I doubt the physical circuit could be made to actually work.

I redesigned the compensations, at least the differential ones: as I took care not to interfere with the bias loop, it should remain more or less OK.

With the redesign, the stability margin is huge, even better than the original Circlophone, and now exceeds 110°.

The improved loop gain has had a very favorable impact on the linearity, and the THD is now under 10ppm, with a remarkable profile, almost pure 2nd and 3rd, and negligible higher order.
The bias current is now well under 100nA, meaning source resistances of up to 100K are no problem.

The increased loop gain also means very high closed loop gains are achievable without loss of quality.

It just needs to be physically tested to confirm the good auspices.....

[Bigun]

It's unusual to see such high gain (CFP) in an LTP without any degeneration. My own experience of a CFP LTP was that a) the sound was very clean, b) the bass was very powerful, c) the treble was a bit harsh.

[kenpeter]

Though I mentiond that it would increase open loop gain,
you had already declared that removing R1 was unstable.
All it would have taken to be CFP...

Input was already in CFP with Q5 Q6. You have now insert
new transistors that also Darlington with Q5 Q6. Drawing
them over by the input pair doesn't change this ambiguity.
CFDCFCFP with a sideways loop back to the beginning....

Add more stages toward infinite open loop gain, soon you
will have another fine oscillator to beat in stereo with mine.
I'm not saying it might not work, only that after you comp
the hell out of it, will it have improved anything?

[Elvee]

Originally Posted by kenpeter

...

Input was already in CFP with Q5 Q6. You have now insert
new transistors that also Darlington with Q5 Q6. Drawing
them over by the input pair doesn't change this ambiguity.
CFDCFCFP with a sideways loop back to the beginning....

Not not really. It is actually a CFP in the input, not a darlington with Q5 Q6: this is because the collectors of Q7 and Q8 are tied to the LTP, not the collectors of Q5 Q6

Removing R1 has no benefit for the differential gain, but it increases hugely the common mode gain, which is problematic

[Tony]

Hi Elvee,

how about a cascode using jfet and trannies in input stage?

Like Bigun i am also wondering how come there is no degeneration in the input ltp stage.....

[Elvee]

Originally Posted by Tony

Hi Elvee,

how about a cascode using jfet and trannies in input stage?

Yes, I think you are not the first to propose the option:
♫♪ My little cheap Circlophone© ♫♪

Like Bigun i am also wondering how come there is no degeneration in the input ltp stage.....

OK why not, but what specific benefits could we expect in this case?

[Tony]

Originally Posted by Elvee

Yes, I think you are not the first to propose the option:
♫♪ My little cheap Circlophone© ♫♪


OK why not, but what specific benefits could we expect in this case?

ok, how about input stage linearity, do you really need very high open loop gain?

[terranigma]

Do seem 2N5858 (80V, 200Mhz, 12pF, 750mW) another candidate as Q5/Q6 despite its very high fT?

Supply regulation.. could circlophone benefit 3pin regulators like LM338K? Is it worth?