The B-17A prototype with the enhanced midrange modification was tested in the south lab and was declared to sound much better. The midrange is now sometimes too strong – ideally there would be a midrange control in addition to the bass and treble controls.Otherwise its sounding pretty good, and that is probably as far as we will go with that version.
So now we move on to initial design of the next iteration – the B-17B.
First, let’s start with the addition of a midrange control. Here is the circuit updated to include a mid adjustment.
You can see that there is now a lot of passive tone control here, and as you might guess the controls are not entirely independent – each control has some effect on the others.
This got me thinking about equalizer circuits. I found this circuit at http://gilmore2.chem.northwestern.edu/projects/equal_prj.htm and entered it into LTspice.
As you can see this is a tube equalizer. The center frequencies are 28Hz, 2Khz, and 20Khz. These are easily adjusted by changing the capacitor values, for instance C2 and C3 for the bass control.
So how does it work? Here is the effect of the treble control stepped through its range as simulated in LTspice:
As usual you can click on the image for a closer view. So far this looks pretty good. How about the mid control?
Now we are starting to see some limitations – the mid control does indeed affect the frequencies around 2 kHz, but also has some effect on all the frequencies above that.
Let’s try the bass control:
Here we see the same thing – the control does change the response at the bass frequencies but also affects all the frequencies above that. This is not exactly ideal.
So, I have some more thinking to do on tone control design.
The second aspect under consideration is the inclusion of a power amplifier in the next iteration. One possibility is to simply include a standard push-pull class A tube amplifier with an output transformer – this design is well understood. The main drawback as always with this design is the output transformer – large, heavy, expensive, and not exactly distortion-free. An example of a good transformer that gets down in the bass region and is capable of 100 watts output is the Hammond 1650RA. That transformer retails for $186 and weighs 12 pounds.
So what if we used those evil semiconductors in the output stage? 100 watts for a pair of power MOSFETs is a walk in the park. But what about the hard clipping you get when you overdrive them?
My proposal is to design the circuit so that it is impossible to overdrive the output stage – set it up so the preamp tubes will saturate and limit the signal to a level that the output stage can easily handle without clipping. That way you get that nice soft tube clipping even with the solid state output stage.
One question remaining is – how much of that “tube sound” comes from the distortion inherent in the output transformer? One way to find out would be to design a solid state output stage, build it, and see how it sounds. But I can’t believe I’m the first one to think of this, so a little more research on those who have tried this before is in order.
That’s all for today !