This thread is going to be dedicated to the specific issues encountered in the design and construction of higher power amplifiers using the Circlophone topology.
The original Circlophone is not naturally suited to high power topologies, because they require paralleling, meaning emitter resistors, which the normal C does not use.
With a single pair of output transistors, it is difficult to go very much beyond ~100W, and a BTL configuration doubles the complexity, which is not very advantageous.
I could build one and not use it, but it would be a serious waste of resources, time and space.
I may test critical sections and schemes in isolation, and I will provide guidance for candidate builders, but that's about it.
Now, let's get into the heart of the matter:
The most intriguing aspect are probably the low value PTC's.
Using PTC's is advantageous, because they behave in an opposite way compared to the transistors. When the current increases, they heat up and increase their value, just where it's needed.
Thanks to that compensating behavior, they can be much smaller than a regular resistor and still have a good balancing effect.
The 1,000$ question is: where to source those miracle components?
This is DIYaudio, and we are going to make them ourselves, in the true DIY spirit, and it is much easier than you might think.
That's what I am going to describe in my next post.
The original Circlophone is not naturally suited to high power topologies, because they require paralleling, meaning emitter resistors, which the normal C does not use.
With a single pair of output transistors, it is difficult to go very much beyond ~100W, and a BTL configuration doubles the complexity, which is not very advantageous.
No, I am afraid I won't: I have no use for such a monster, and my speakers are only 100W anyway, and if I tried to use them at 1/10th of their power, I would have serious problems with my neighbors.Great! It looks like you're going to actually build it?
I could build one and not use it, but it would be a serious waste of resources, time and space.
I may test critical sections and schemes in isolation, and I will provide guidance for candidate builders, but that's about it.
If the matching is too imperfect and there are no compensation measures, the transistor having the highest gain/lowest Vbe will take more than its share of dissipation, which will lead to an even higher Hfe and lower Vbe, etc. etc.: a thermal runawayIt seems like beside mechanical issues (heatsinking), the only more complicated thing would be correct matching of the output devices.
Question - what will go wrong (beside the load being distributed not evenly between all devices) if they are not matched perfectly?
Slight differences will be absorbed by the balancing measuresNot thinking about extreme cases when devices are totally unmatched,
but just slight differences?
Yes, multiple devices can turn into a local push-pull oscillator (or a multi-push, multi-pull one)Never tried paralleling BJTs. Only Mosfets, with mixed results.
Either it worked perfectly well (matched devices of course), or
I was getting oscillating amp, with no visible way to fix it.
Tried little zobel networks, different gate stoppers, different PCB designs, etc...
Same amp with one pair would work OK.
Now, let's get into the heart of the matter:
I have examined the question, and I think I have found an acceptable solution: one that is safe and effective but also preserves the Circlophone sound character.
Not everyone likes the sound of the C, but if someone builds a bigger version, he probably wants to keep all of its peculiarities, including the sound character.
It would of course be possible to just add largish emitter resistors for a safe paralleling, but the sound would be "anonymized", just like any other class AB amplifier.
The solution has three components:
-Matching of the paralleled transistors
-Use of special, PTC emitter resistors
-Mounting of the transistors on a common heat spreader to minimize inter-transistor Rth compared to the thermal resistance of each transistor to the ambience
These measures go against the Circlophone spirit of simplicity, absence of matching, tolerance and ease of mounting, which is why I shall develop them in a dedicated thread, because I do not want to confuse would-be builders and lead them into thinking that a 1kW C is going to be as easy as a 30W one.
In the meantime, here is a first preview of the method:
The most intriguing aspect are probably the low value PTC's.
Using PTC's is advantageous, because they behave in an opposite way compared to the transistors. When the current increases, they heat up and increase their value, just where it's needed.
Thanks to that compensating behavior, they can be much smaller than a regular resistor and still have a good balancing effect.
The 1,000$ question is: where to source those miracle components?
This is DIYaudio, and we are going to make them ourselves, in the true DIY spirit, and it is much easier than you might think.
That's what I am going to describe in my next post.