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!
The B-17A bass preamplifier prototype is complete! It works as intended and is now ready to ship to the South Lab for some real musician testing. There were some minor modifications to the circuit during testing and the resulting circuit diagram is shown below:
The modifications include removal of R20, and biasing of the heater circuit to approximately +50V DC. R20 was previously a 4.7 megohm resistor in series with the volume control, with a switch across it to choose higher or lower gain. In practice because of its high impedance it was really just a noise source, and is not needed to get the input range we are looking for. Biasing of the tube heaters prevents them from supplying electrons to the cathodes of the tubes which can inject 60Hz noise.
The measured frequency response of the preamp is very close to the target predicted by the LTspice simulation, proving that the simulation models are pretty accurate. The amp is nice and quiet once sufficient time is given for the tubes to warm up, and sounds good when playing recorded bass inputs. The heater elements are running slightly below their rated voltage with this power transformer, so they do take a while to reach operating temperature.
The circuit board was installed by wiring the tube sockets and the rest of the controls as shown below.
So as I said, next step is to get the prototype to the South Lab to test it with real instrument input.
After that goals for the next prototype include:
- Improved circuit layout and a better case
- Use of a commercially available power transformer rather than the scrounged unit used in this prototype
- Perhaps a power amp stage?
Good progress today on the B-17A bass preamp prototype. As you can see from the pictures below the chassis is complete and the power supply is built. I went ahead and put the knobs on and tubes in the sockets so you can see what it will look like.
The power supply is in the enclosure on top, to keep it isolated from the preamp circuitry. Here is what it looks like inside.
The inside of the chassis still needs a few things, as you can see below…
The standoffs are there for the circuit board, so hopefully its just a matter of adding that and getting everything wired up. But this is as far as I’m going today.
For the next version of this preamp we have already chosen a different case which we believe will add to the appeal. But for now the next step is to make sure this prototype works and make any adjustments needed.
That’s it for now!