Error in my 1073 simulation?

[GreyboxAudio]

Hey,

I tried to simulate a 1073 with BA283/BA284 amps but something seems to be wrong. With each amplifier stage there's a massive drop above 200Hz until my output signal reaches -200dB at 20kHz. If anyone with a better understanding of the 1073 could help me find the source of my mistake, that would be awesome <3 I've attached the simulation.

 

[ruffrecords]

You have T connected directly to U which I think is wrong.

Cheers

ian

 

[GreyboxAudio]

You have T connected directly to U which I think is wrong.

Cheers

ian

Thanks! That fixed the input amplifier but it's still downhill from there:

 

[Khron]

The output stage (bottom right) has no load (in the collector of Q12) - you've drawn the transformer as just a wire, so it's not there, so you'll get diddly-squat on the final output (you'll just be connecting the power rail to ground or not, depending on the polarity of the signal).

R16-C13 node missing connection to Q5 base; same with R5-C5 and Q2 base.

 

[GreyboxAudio]

The output stage (bottom right) has no load (in the collector of Q12) - you've drawn the transformer as just a wire, so it's not there, so you'll get diddly-squat on the final output (you'll just be connecting the power rail to ground or not, depending on the polarity of the signal).

R16-C13 node missing connection to Q5 base; same with R5-C5 and Q2 base.

Good catch, thanks! Unfortunately the two BA284 stages still drop about 60dB each at 20kHz. I only drew in the output amplifier to see if it is maybe required to get a linear frequency response. But I don't think that's the case, so we can probably ignore it for now.

 

[Khron]

Are C3 and C6 really supposed to be 470nF? I know it says "470" in the original schematics, without "p" as the 47p's above them, but... It makes no sense for such a large value to be there, which would explain the low-pass effect.

Must be some sort of typo, either a zero instead of "p", or just missed adding the "p". All other capacitors, even nF-range ones do have the suffix added, so...

Corresponding caps on the other "pieces" of the BA284 seem to be 1.5nF, so there you go - 470p makes all the sense, and explains the symptoms you're encountering.

 

[GreyboxAudio]

Are C3 and C6 really supposed to be 470nF? I know it says "470" in the original schematics, without "p" as the 47p's above them, but... It makes no sense for such a large value to be there, which would explain the low-pass effect.

Must be some sort of typo, either a zero instead of "p", or just missed adding the "p". All other capacitors, even nF-range ones do have the suffix added, so...

Corresponding caps on the other "pieces" of the BA284 seem to be 1.5nF, so there you go - 470p makes all the sense, and explains the symptoms you're encountering.

Awesome! I was assuming nF as implied because only pF & uF were mentioned explicitely. I've changed C3/C6 to 470p and C7/C14 to 680p and this looks a lot more like a 1073 now:

 

[GreyboxAudio]

about the output stage: there should be an unbalanced output available there (point F on the original BA283). Would the correct transformer loading still matter in that case? I was assuming it's not important, which is why I've drawn the transformer winding as a wire.

 

[Khron]

Would the correct transformer loading still matter in that case?

Well, as i said, without a collector load there, there will be nothing for the current flow to develop a voltage drop over (except that series 12 ohm in the supply line).

Why don't you do a transient simulation and see how the waveform looks without a transformer?

 

[Khron]

And/or add a 200 ohm resistor to emulate the transformer primary, and see how the unbalanced out AC plot looks then.

 

[ruffrecords]

The output transformer or an inductor is absolutely essential for proper functioning of the output stage. Easy enough to do in LTspice. Just add a 10H inductor and it should work OK.


Cheers

Ian

 

[Cqwet Dbdfte]

Making a basic transformer in LTspice is also easy. Place two inductors, called say L1 and L2, then and add a SPICE statement K1 L1 L2 1.
K is a transfomer, L are the two inductances, their value ratio makes the voltage transformation, the "1" at the very end is the coupling efficiency, 1 is 100%.
Also use some actual partnumbers for those BJT's, the default models are suspect, maybe IC models.
Use a realistic impedance for the power supply. It is never 0 ohms.
Like it was said before, use the transient sim to see that the circuit actually works.
Making a more realistic transformer is more complicated that includes parasitics and magnetic metal properties.

 

[Cqwet Dbdfte]

.... but you should add some resistance to those inductors. They are seldom 1 milli ohm.
Other parasitic properties are also available.
High voltage/frequency transformers also have serious capacitance considerations.

Simulating with squarewaves gives some idea of freq response, triangle wave show clipping/compression.
Running some FFT on the sim will show harmonic generation. More points and smaller freq step improves picture