My friend is planning a preamp suitable for pedal steel guitar. We would love to hear your opinions. You can read his description of the project below.
Thanks in advance. Carl
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The main part of the circuit is pretty much based on the pre-stage in an Ampeg B15.
The curves in the pictures show how the EQ knobs work when you sweep the respective knob from zero to max.
The Mid control is in the circuit separated from the bass-treble and works "independently", so it therefore has its own image.
As can be seen on the curves, the mid control loses a bit of treble when it is set to max, which is due to the output impedance from the first tube stage before it, but if it is at all noticeable in practice, it is easy to compensate either with the treble control, Bright -switch or somewhere else in the circuit.
As a starting point, I have limited the Bright switch a bit with a resistor so that the treble increase is not so strong, that's something we can tweak according to how it sounds later.
An advantage (as I see it) of this so-called James EQ plus the mid control is precisely that all three knobs work independently.
In many guitar amplifiers, the knobs are more interactive, for example when you turn the knobs on the bass, the cutoff frequency of the mid range is moved, etc.
Here, the mid-range around 1kHz is kept fairly constant at the same level and in the same frequency range, regardless of how you turn the bass and treble knobs, and the idea is then that you can reduce the mid-range in just those frequencies with the independent Mid-knob.
A problem with Fender's EQ in particular, both those with 2 and 3 knobs, is that the more middle register you pull on (with the middle knob on a 3-band or with a fixed resistor on a 2-band), the more ineffective the bass becomes - and the tweeters.
To get a straight frequency response in a Fender, you instead have to turn off all the bass and treble, but then you end up with no volume.
If you want roughly the same frequency response as in a Fender EQ in this preamp, you set the bass and treble to 12 o'clock and turn down the Mid knob.
The biggest question mark for me in the basic circuit itself here is the first half of the 12AU7 tube (labeled V2A on the schematic).
If it turns out that it does not add anything other than an unnecessary increase in volume, you simply have to bypass it. Otherwise, the idea with it is that it should to some extent simulate what happens with the sound in the so-called phase inverter and the output stage in an amplifier.
The second half (V2B) is mainly for lowering the output impedance, which is good when connecting to mixers, power stages, etc. It is a so-called cathode follower that does not add any volume.
//Tom
