AF modulator in Transmitter what is the A2000?

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neazoi

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In this schematic **broken link removed**
I wonder, what is this IC the A2000? I cannot find it anywhere. Any datasheet?

Also how to wind the output transformer?
As far as I can tell it is a quadrifillar winding, but he does not mention anything about the phases (windings starts/ends)
 

Bear in mind that circuit will only operate at low frequencies, I wouldn't try scaling it to HF.
I have no idea what the A2000 is but clearly it is some kind of low frequency buffer or power amplifier. Judging by the other component types, it could be obsolete by many years. It has to supply around 1 Amp at 6V output and around 2A at 12V output and have a self biasing input which makes me suspect it is a high power buffer rather than an audio device. It could well be a module rather than an IC.

That 741 input stage looks highly suspicious!

Brian.
 

English translation of the article is below. (Chrome web browser, Win7). A Google search on 'A2000 op amp' finds discussions stating it's part of a series of Electrodyne op amps designed to improve on the 709 IC.

 

he also has this in his site **broken link removed**
Perhaps I could use the tda2005 as adrop in replacement in the tx modulator?
He says it can provide less power (5W or so) but the TX is 3-5W so I guess it will be ok, modulating it at high levels despite the AF IC tda2005 has a lower power output?
 
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TDA 2005 has cousins, also rated several Watts which are easy to use. Example, Radio Shack carried TDA2002/LM383.

It helps their popularity because they can use a 12V single supply which is convenient in automotive systems.

However your circuit appears to process +/- AC waveforms. You may decide a more straightforward design is to build the circuit around a bipolar split supply. Perhaps you could get by with a simple class B amplifier.
(Edited to add: In other words a half-bridge might substitute for TDA 2005, because a half-bridge is in the output stage of an op amp.)
 

The problem here is the need for the amplifier to provide half rail at about 1 Amp load in it's idle condition. Unlike a normal audio application, it has to provide enough current at half peak carrier level when not driven. To give anything like reasonable audio quality the PA stage has to be operated in class A mode so its quiescent current is around 50% of the peak. The normal rules for driving a load through a capacitor do not apply. I have reservations as to how useful a 'push pull' audio device its at all, it would be just as efficient to use a single series pass transistor and modulate its base current as the lower (ground side) output transistor would serve no purpose.

I also doubt the 741 input stage will work at all, the DC bias conditions are wrong. The design may work reasonably well for audio frequency data but the quality would be bad for voice applications. It is unlikely to have been used for data given the low RF frequency range.

Brian.
 

The transmitter is for broadcast AM music only. He says that he uses the 741 as an 4.5KHz LPF.
Based on the info on the modulator page I showed you, the A2000 seems to be a direct replacement to the TDA2005 (even pin compatible?) but with lower output power of 5-6W, in contrast the A2000 can operate at 10W at mono, as he says.
If these are correct then I could use it in the transmitter, as the TX pwr is only 3-5W, so a 5-6W could modulate it nicely?

I have built the VCO currently with "similar" components values and it works. The output waveform is more like a sawtooth with a curved (upwards) rising plane. But I thought that is ok since the waveform out of the buffer should be more like a square, as he shows in the diagram. In other words far from clear.
 

Output power isn't the issue, it is whether the TDA2005 or even the original IC would be capable of sustained DC output current. They are class AB devices which means they are only designed to carry a small continuous current and it increases with voltage at the output node. In normal use, the loudspeaker would be coupled through a capacitor and the upper/lower transistors in the output stage would charge and discharge that capacitor, passing current through the loudspeaker as they did so.

In the radio application the situation is quite different, the amplifier only has to source current so the lower output transistor is redundant. Being a class A RF PA (for linearity) it produces half RF power under no modulation conditions and at maximum modulation it would go from zero to full power. All that current has to pass through only the top half of the TDA2005. As such, it offers no advantage over a simple series pass transistor with the modulation fed to its base.

So instead of drawing negligible current when there is no modulation, the TDA2005 would have to produce more like 1 Amp continuously to the PA transistor. It might be able to do that but the circuit would be inefficient and rather wasteful of a good audio amp. If you really want to use one, it would be better to use a conventional AF amplifier configuration but use a 1:1 transformer instead of a loudspeaker.

The 741 input stage will not work as intended, there is no reference voltage on the non-inverting input. At the very least it needs a potential divider to set the input to half supply voltage.

Brian.
 
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    neazoi

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Right, I see the point. I think the reason he uses the IC is NOT to have to use a transformer. That is the reason I like it too, despite the lower efficiency. He has many designs with this method and if you look at "the puppet AM transmitter" on the web, they use the same technique.
a modulated PSU could be done with a series transistor like you mentioned but you would need more stages prior to it for enough gain. A single AF amplifier IC saves lots of components, it is small compared to a transformer and it is very cheap, that is why they use it, despite the decreased efficiency.
I do not know if ANY of these TDA amplifier ICs can be used or if one has to stick with the specific IC. MAybe there is something required in the internal configuration of the AF IC, to be used as a modulated PSU.
 

I can see why they did it that way but in my opinion, the drawbacks outweigh the benefits. A transformer would allow double the RF power by adding the AF and RF powers together or alternatively allow the same RF power with half the audio. Even capacitively coupling the existing amp to the base of a series pass transistor would help and it would only need three extra components.

Brian.
 


Thanks Brian
Have a happy new year!
 

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