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In Reply to: RE: Ultrasonic heaters! (again) posted by Triode_Kingdom on October 27, 2014 at 21:19:27
The "oriental" (Chinese) products are actually a rather simple elaboration to exploit a generic oscillator circuit - the half wave bridge. And, they are not doing it for electron tubes, but halogen light bulbs: otherwise it would be much more expensive (an up-market affair)?
As TK has noted his "fear" of the audibility of artefacts (after 10 years of use?! no way...) at 65kHz, as suggested by Lynn Olsen... and mentions David Berning and his allegedly 250kHz solution, this is a chance to point out the difference between "good idea" (stroke of genius?) and "over-engineering" (necessary if you are in the Hi-End business, for the marketing department to use for your advantage).
The circuit itself is rather simple. We could enhance it by including a regulation stage (with LM317 or TL783) that would cancel virtually all 100Hz ripple before feeding the oscillator stage. Adding a few elements like NTCs in the right spots, and a couple additional elements to provide fail-safe mechanisms, all that is left would be to increase further the oscillation frequency (different ratio in the first small transformer) and use better (more expensive) IGBTs... at that point we can design a new board and incorporate it in our amps, while the marketing department has a field day with our "enhanced, innovative, and unique" features -- while in practice the original Chinese unit was good enough after the necessary modifications that make it suitable for the purpose.
Even more so, the cost of the necessary added parts (mod - cap, NTC) can easily exceed the cost of the unit itself! If we were to source the various parts on our own (ferrite cores, wire, etc.) we would end up with a much higher price than if we buy the unit.
Being sort of isolated here in Serbia is a blessing in disguise. There is no Lynn Olsen to freighten me with the potential audibility of HF AC if frequencies are not at least 100kHz... so I can just do it - and once I am aware that I cannot hear any of it, nor can the people I ask to listen with me hear anything - at that point it's too late for the "Lynn Olsens" to make me change my mind, because I consider it a proven fact (for me and others who were satisfied with the result).
Thus: are multiplexing and heterodyne by-products possible? Well, most probably they are. Can those by-products be found on a scope? Well, probably they can. But can they be heard? I cannot hear them with 30-50kHz, TK has not heard them with 65kHz... why would we need 100kHz or 250kHz? Only for marketing purposes, the best of the best, the non-plus-ultra... no small wonder that DIY equipment more often than not sounds better than coveted High-End stuff.
******
http://rh-amps.blogspot.com/
Follow Ups:
Thanks Alex for your interesting reply.
I do have read your blog on the subject, and am interested in trying, but there were a few points that were unclear to me. I'd like to take this opportunity to ask you a couple of questions to make sure my understanding is correct.
When you say that the output of these transformers is between 30 and 60 kHz but “modulated” by the mains - do you mean that the output is a rectified sinewave at mains frequency that is "chopped" by some square-wave generator afterwards? So adding a cap at the bridge output basically gives a DC supply that is then chopped? Wouldn't an LC stage after the rectifier limit the raise in DC voltage?
And second point, once you have reduced the number of turns on the output toroid top compensate for the higher voltage, can you adjust it down to the required heater voltage as well or you still need to drop voltage with power resistors?
These questions might seem basic or downright dumb, but then i am just a self-taught hobbyist. HF in particular, is a bit out of my league - but it defenitely interests me in the goal of HF heating.
Thank you in advance.
Joris
"do you mean that the output is a rectified sinewave at mains frequency that is "chopped" by some square-wave generator afterwards? So adding a cap at the bridge output basically gives a DC supply that is then chopped?"
Yes, that's how these modules are configured. With no filter cap, the output from the module is essentially 120Hz chopped at the HF rate.
"Wouldn't an LC stage after the rectifier limit the raise in DC voltage?"
There are two issues with that. First, the choke would have to meet critical inductance at 120 Hz without incurring excessive loss. A value of 0.1 to 0.2H is probably about right for this. A more serious issue is the initial voltage overshoot when power is first applied to the LC. Depending on the DCR of the choke, that might be 50% or more of the static voltage. Personally, I wouldn't want to connect such a circuit to a filament. If regulation is your goal, a small SS device might be a better choice.
Remember that everything ahead of the HF output transformer is attached to the mains. These components must be well isolated from the chassis. Meeting that requirement will be more difficult as the circuit becomes physically larger.
"...you still need to drop voltage with power resistors?"
If the transformer is toroidal, and you don't mind rewinding both the primary and secondary, the exact ratio can probably be created for the filament. Otherwise, if you only unwind turns from the secondary, resistors will probably be required to "trim" the voltage.
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Buy Chinese. Bury freedom.
Thanks TK,
I read most of the threads you posted on this subject. Just ordered three 20W units to play with. I hope their transformers will be of the toroidal variety.
Well TK has already given the answers. A few additional points might be added, but I guess at the moment we need not complicate further with the "what if" list ;)
Never heard of 20W units?! If there are any, I expect the transformers to be very tiny, which in turns means more difficult to work with.
There are two important ratings: maximum power, which is mostly related to the "primary side capability", or what is the maximum that you can get from it: I suggest to take at least 30% more powerful units than needed; and minimum power, i.e. the minimum load required for oscillation. For a 50W unit this is usually 20W minimum, but from experience I can say they will operate on 5W automotive bulbs without apparent flicker although the light may appear dim which means that ooeration is not stable, and 10W perfectly stable and shining with adequate light intensity... but this varies from make to make. You should always test this ahead with automotive bulbs (5, 10, 20, 40/45, 50/55 W units are available for car lighting. I use H7 bulbs as 813 filament simulation.
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http://rh-amps.blogspot.com/
Thanks Alex for the car light trick! I take good note. I was thinking of using a 8 ohm power resistor as a load, as it has the same resistance as the target tube, and observing with my good ol' 50Mhz analog scope. Do you think it is ok? I do have a true RMS DMM, but i'm not sure if it's capable of reliable measurements up to those frequencies.
I think it will be quite educative for me, but when poking around with a scope probe I'll make sure to stay away from the input transformer primary :-) It'd be safe to use an isolation transformer as well, although the only one i have is only about 25VA. I'll try to rig some back-to-back trannies.
If the model I bought is too small i can always find another use for the units. I might even connect them to halogen lamps :-) Anyways, it's just 12$ for the three. On the picture for the 40W model we can see that the case has a screw for removal, so like you did i will put back the case on after modding, passing the capacitor's wires through the vent holes.
Thanks for your input!
Joris
Joris,
The resistor is a good option if it has the same resistance as the filament. The only problem might be power dissipation and the fact that it does not light-up (hopefully!).
The units look very similar to the ones I like to use: the case is "clip on" and the inside is actually upside down (i.e. the bottom is actually the cover). The screw has a different purpose, it is bolted to a piece of metal so the output transistors are cooled by the piece of metal and the box! In order to access the board you will need to undo the screw, of course - but you can open it without a screwdriver. Hopefully the transformer is a tiny toroid.
The scope is the best option, and the DMM needs to be capable of measuring at least 100kHz frequency. A Fluke 87 III and upwards will do. Once you start taking measurements and doing some math, you will see that the tolerances can be made pretty tight by calculation only, even without measurements.
Let us know!
******
http://rh-amps.blogspot.com/
Joris, don't forget that the capacitor you add is connected to the AC mains through the rectifier diodes. There's no isolation. The only safe way to use these units in an amplifier chassis is to mount everything inside another (larger) plastic box.
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Buy Chinese. Bury freedom.
After the modification, the cap which is too large remains outside of the box of the original electronic transformer unit. I use connectors, and before installing it inside an amp isolate those with heat-shrinking tube, and of course fix everything in place so it does not move.
Another part that might protrude out of the box is the NTC that you are going to install in series with the AC input (where mains AC comes in). Eventually you can connect it instead of the small fuse / low power resistor inside the unit which is going to blow anyway once you connect the cap across the bridge (if such fuse/resistor is applicable to your unit).
Once the unit is fixed inside, you can treat it just like any other transformer - connect the mains connection to your mains input via the main switch or however you were planning to do it.
The cap to be connected across the rectifier bridge should be rated 400V for Europe and UK (220-240V mains) and 200V for US/Japan (120V mains approx.). Since the current draw of US/Japan units is double in the primary, the cap needs to be twice the capacity. Thus, if you would use a 200uF/400V cap for Europe, in the US you should use 2 200uF/200V caps in parallel.
The above cap values are what I feel to be a reasonable minimum, but the quality of the results (ripple cancellation for the initial rectification) obviously depends on the current draw. It's not the same when supplying 2x 813 with 100W output power, and supplying 2x 300B with 12W of output power: the first case obviously has almost 10x more current draw on the primary, thus more capacity is necessary.
The increase in output voltage after inserting the cap is a result of the ripple cancellation and subsequent DC RMS increase for the oscillating stage: this poses no problem to the oscillator or the output transformer primary, since peak voltage remains the same (for the same mains input). A byproduct of the cap is the "redundancy" of the diac in the oscillator circuit, since the voltage never falls below the level where the diac is activated again restarting the oscillation - while in normal halogen bulb mode the diac is active in every cycle.
While testing and fine-tuning this circuit, you must observe the usual rules when working with lethal voltages. Isolation is necessary, and careful what you do with the probes...
******
http://rh-amps.blogspot.com/
Alex, I agree with much of what you've said, but Lynn has many years' experience in this field, and I respect his opinions. Just to clarify the issue, Lynn's concern was not that the system would be problematic with 20-20kHz audio. Rather, he was considering the fact that digital sources can output spurious, low-level products above 20kHz. The concern is that these products might mix with the filament's AC frequency and the incoming baseband audio to create audible artifacts. The fact that this doesn't appear to have occurred in your system or mine doesn't mean we're not vulnerable, or that it might not happen to someone else who uses this technique.
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Buy Chinese. Bury freedom.
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