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 Post subject: Another Part 15 AMTX idea. Would step down transformer work?
PostPosted: Sep Mon 27, 2021 12:28 am 
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Driver driving final class C PA.
20ohm bass impedance of PA stage is stepped up to 720ohm for the driver which is biased class A 14ma current. AM modulation is a little unusual. Base modulation is now very rarely in use. Primary of an audio transformer has an inductance of around 0.7H. Would a normal power supply step down transformer work here? Their primary inductance is around 3H or henries, enough to prevent audio signal shorting to ground but develop a positive voltage through the in4007 diode in accordance with the audio signal to bias the PA stage transistor, varying its power output and generating AM. The collector tap should be close to the +12 supply to preserve the high Q of the tuned circuit build around a coil on PVC pipe and a variable capacitor for peaking for maximum range.

Normal power step down transformer would work like 110V 12V . We only need to use primary.

Anyone has ever used a step down power transformer to check its modulation properties? They are common bur adio transformers are slowly disappearing where as step down transformers are everywhere.


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 Post subject: Re: Another Part 15 AMTX idea. Would step down transformer w
PostPosted: Sep Mon 27, 2021 1:17 pm 
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Dare4444 wrote:
Anyone has ever used a step down power transformer to check its modulation properties?
By modulation properties I suppose you mean the transformer's performance at audio frequencies.

I have not measured one, but I have used a small filament transformer in place of an audio output transformer in a communications radio and that worked just fine. The original transformer developed a shorted turn. It didn't burn up, but it greatly loaded down the audio output tube and severely reduced the available audio output from the radio. The small "filament transformer" did fine as a replacement and is still in that radio today (50 years later).

Curtis Eickerman

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 Post subject: Re: Another Part 15 AMTX idea. Would step down transformer w
PostPosted: Sep Mon 27, 2021 1:42 pm 
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Just use an eight ohm resistor. Any inductor of that magnitude will have far more DC resistance.

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 Post subject: Re: Another Part 15 AMTX idea. Would step down transformer w
PostPosted: Sep Tue 28, 2021 11:48 pm 
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Peter Bertini wrote:
Just use an eight ohm resistor. Any inductor of that magnitude will have far more DC resistance.


1.7K resistance for 230V primary winding.


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 Post subject: Re: Another Part 15 AMTX idea. Would step down transformer w
PostPosted: Sep Wed 29, 2021 2:00 pm 
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There are good reasons to include a transformer, or an inductor as a load, compared to a resistor.

The basic difference is that with the inductor , or transformer (in a class A application) the dynamic range is doubled.

The driving device (typically a transistor or tube) can have its collector (or plate) voltage held near the B+ rail. Due to energy storage in the core of the transformer, the voltage can swing above and below the power supply rail voltage. You cannot do this with a resistor where the collector or plate voltage has to be at about 1/2 the power supply voltage for maximum dynamic range. If you halve a voltage, or dynamic range of it, the power drops to 1/4. In addition the I^R power losses in a resistor, drastically exceed those in the transformer case.

However, transformers are expensive and have a higher mass than resistors. Over the years designers have sought to eliminate transformers in signal process circuits. They are difficult to get right and can also result in peaks in the frequency response in audio amplifiers. However, despite all that, many circuits with transformers cannot be beaten by those without them in terms of electrical efficiency and performance. Hobbyists though often prefer circuits without transformers because they often do not have the core materials, bobbins and wire & winding machines to make transformers, especially for audio work. So its becomes easy to convince yourself you are ok without them or don't need transformers. This is ok if you are prepared to settle for average.

In addition in electronics history there have been some extremely creative circuits made based on the magnetic properties of an inductor or a transformer's core. Some examples:, if you have magnetic material with a very sharp edged B-H curve, it makes an excellent core for a DC DC converter. Tek used such a toroidal core for the 12V power option board in their 466 scope.
In transistor TV work, you will often see a magnetically saturable coil in series with the H deflection coils to correct the intrinsic non-linearity of the H scan. Nearly impossible to do with any other method. Early light dimmers used magnetically saturable cores.

One really creative circuit uses Alnico as the core of a pulse coupling transformer, which couples pulses to the gate of an SCR. If you magnetize the core in one direction with an auxiliary winding you can make the transformer couple a pulse or not, making it possible to create a shift register just from coils and SCR's.

So the list of interesting things that can be done with electro-magnetism and various core materials is amazing, but often outside the reach of home experimenters due to lack of the core materials at most electronics shops, where they will happily sell you an Arduino shield. Before experimenting with transformers and core materials, it is worth reading Ampere's theory of magnetism.


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 Post subject: Re: Another Part 15 AMTX idea. Would step down transformer w
PostPosted: Sep Wed 29, 2021 3:23 pm 
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It is a Class C amp being base modulated. How much voltage swing is required?

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 Post subject: Re: Another Part 15 AMTX idea. Would step down transformer w
PostPosted: Sep Wed 29, 2021 10:06 pm 
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Peter Bertini wrote:
It is a Class C amp being base modulated. How much voltage swing is required?


In this case very little, but I was referring to the general principles, say when a modulator transformer or audio amplifier output requires a large dynamic range, better results are achieved with transformers, especially if the available power supply voltages are limited, but they are more bulky and costly. Transistor radios always benefited from them, typically a driver and output transformer, especially those running from a low supply voltage, like two or three AA cells. But these days, transformers appear to be avoided by designers.

One pcb I'm working on, for a DC motor controller, has two variants, one where the +/- 12V supplies for the OP amps on it are created by a small line to 15V-0-15V pcb mounted 1W rated transformer. Despite the DC resistances of the windings on a line transformer this small it is much more efficient than the other pcb version that uses a series dropping resistor from the line that generates around 4W of heat. The other way to avoid the heat loss could have been to use an X2 rated capacitor as a dropper. But these parts have a very bad habit of losing their uF capacity over time.

There was also another thread recently where a transistor circuit replaced a vintage tube radio audio inter-stage transformer, but it limits the dynamic range, prior to heavy distortion of the tube driving it, by lowering the tube's plate voltage to half the B+, unlike the transformer case.

So what I was getting at generally, is that it is often very hard to beat a transformer for many applications, even though they are costly & time consuming to make, causing them to be avoided in many cases, especially in audio work. In other cases they are a simple, lightweight and cheap magic cure for issues of isolation where you only want to couple a transient and have isolation, the best example would be the trigger transformer for an SCR, or high frequency RF work where the transformers and inductors are often very small with low turns numbers.


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