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Elvee 24th May 2011 05:27 PM

♫♪ My little cheap Circlophoneİ ♫♪
1 Attachment(s)
Here is something different for a change:

It is a small, unassuming amplifier project, but with some unusual features:
It combines a circlo output topology with an active bias servo.

The servo stabilizes the quiescent current, but it goes further: it actively controls the output stage so that both sides always remain active in a "warm" class AB, having most of the attributes of the full class A, including a complete absence of crossover artifacts.

It is extremely easy to build, offers excellent performance even with 2N3055's, requires no adjustment, no matching, no thermal compensation and yet has a rock-solid thermal stability.

It is also incredibly healthy, and behaves gracefully, even with loads lower than 2R.

In short, a nice little amplifier, not in the top-class, but with excellent all round performances.

Meet the Circlophoneİ!!!

jacco vermeulen 24th May 2011 06:20 PM

Don't mind me asz-king, but what does the "Circlo" part refer to ?

Elvee 24th May 2011 07:16 PM


Originally Posted by jacco vermeulen (
Don't mind me asz-king, but what does the "Circlo" part refer to ?

It refers to the same-sex output transistors. It doesn't include the folklore of the original Circlotron, the floating supplies: it resembles more later versions, by Philips f.e., because there is no output transformer to worry about.
The drive is similar to that of a circlotron, and the power supplies and transistors could be rearranged in a more familiar order, but there is no advantage to be gained, rather the contrary.

Bigun 25th May 2011 12:41 AM

I likes it very much !

Elvee 25th May 2011 07:36 AM

5 Attachment(s)
Here are some waveforms illustrating the operation and performances of the Circlophoneİ:

First, the output voltage together with the collectors currents of the output transistors: one can see that Ic never falls below ~75mA, and that the crossing current (Iq) is ~150mA (warm class AB).
Note that the "idle" transistor remains fully active: the active bias servo encompasses the whole amplifier from the input pair, and keeps all the transistors always active in a linear mode.
Next is the response to a 16V/10KHz squarewave on an infinite load impedance, followed by the same signal on a 3Ω load.
Then, there is a 16V/50KHz triangle wave, again on infinite and 3Ω loads.

Note: these pictures have been made with actual 2N3055's as output devices

padamiecki 25th May 2011 07:57 AM


Originally Posted by Elvee (
Here are some waveforms


Nico Ras 25th May 2011 02:41 PM

It is nice to see some different topology. Nice!

Elvee 26th May 2011 08:54 AM

3 Attachment(s)
Here are some additional characteristics and information:

First the clipping behaviour, see picture.

It is shown with 20V supplies, the clipping occurs to within <1.5V of either rail.
It is a simulation, but the real life waveform is strictly identical.

The supply voltage can range from 15 to 40V with no or minor modifications:
The current through R21 should ideally be kept between 1 and 1.5mA, and the total voltage of the zeners should approximately match the supply voltage.

The input resistance is ~10K, but the source impedance is preferably kept below 1K, because of the base current modulation caused by the bias servo.

Output impedance is <3mΩ @1KHz
THD @1KHz and 0.9*Vmax is <0.005%
Full power bandwidth is >100KHz
Slew rate: positive ~24V/µs, negative ~20V/µs

Here are some pics of one of the prototype:

blakkshepeaudio 26th May 2011 09:41 AM

Looks very nice. Are you planning a PCB?

parb 26th May 2011 11:02 AM

It looks interesting!

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