B-17A initial test results

The B-17A prototype arrived safely at the South Lab this week. Initial musician testing reveals a need for more treble – here’s how we can accomplish that.

First, let’s go over the predicted vs measured frequency response of the prototype. Below is a table summarizing the results.

The predicted voltage and AC analysis values came from the LTspice program, and as you can see they align nicely with the measured values on the prototype. The values with the level switch open vary a little more but still are pretty good overall. So this shows that we can use the LTspice program to predict the frequency response with reasonable precision.

Here is the frequency response of the B-17A prototype as predicted by LTspice – you may need to click on the image for a larger view:

The roll-off at low frequencies prevents amplification of any stray beat frequencies – which is a good thing to do. Low B on a 5-string bass is around 31Hz so no need to go much below that. On the higher frequencies you can see that by 600Hz the preamp is down to 0dB gain (no gain) and after that starts attenuating the signal. There are definitely harmonics that go well above that, and clearly our resident bass expert would like to hear more of those.

So our first modification will be to remove C2 completely. The circuit diagram was presented in a recent post, and if you refer to that you can see that C2 goes from the signal path to ground and thus filters the higher frequencies. Without C2 in the circuit the frequency response should look like this:

Now the 0dB point has been moved out to around 3300 Hz. If this adds too much treble, we can substitute a smaller value capacitor for C2. The voltage at that point shouldn’t exceed about 16V so a high voltage cap is not needed. If there is still not enough treble, we can look at more extensive modifications.

As a reminder, here is the circuit board layout so you can see which cap is C2. Looking forward to the results!