Post by antigua on Mar 28, 2021 18:00:45 GMT -5
I was inspired by this thread guitarnuts2.proboards.com/thread/9488/kent-armstrong-tone-choke-tc1us about the Kent Armstrong inductor , and I've looked at the Bill Lawrence Q Filter guitarnuts2.proboards.com/thread/8288/bill-lawrence-filter-analysis . On various forums on the Internet, the Xicon 42TL021-RC, a small high ratio transformer, has been recommended to act as a stand in as a 1.5 henry inductor.
Even though these inductors are recommended for the same purpose, I had the impression that the Bill Lawrence sounded a lot better. The clarity seemed to be retained with the Q filter, but the Xicon 42TL021-RC caused a loss of highs and lows alike. I didn't think it worked much better than a rolled off volume control. The only difference I can see between the Xicon 42TL021-RC and Q Fitler is that the Xicon 42TL021-RC has a higher impedance, around 5k ohms at 1kHz, where as the Q filter shows only about 50k ohms impedance at 1kHz.
So I've tested the Xicon 42TL021-RC and the Q Filter with the bode plotter and integrator with an SSL-1 for an "in situ" demonstration.
This first plot shows the SSL-1 by itself, then the Q Filter in parallel, then the Xicon 42TL021-RC in parallel:
As can be seen right off the bat, the Q Filter, with its super low Z, produces are very high Q factor, but more than that. the overall signal is higher and much more treble it preserved in the output.
This plot below is the same thing, but now a 10k ohm resistor and a 22nF cap twisted together are in series with the inductors, which is how Bill Lawrence recommends the Q Filter be implemented, for a mid-scoop control:
The Xicon 42TL021-RC performs pretty badly with the Bill Lawrence wiring scheme, it acts more like a high end attenuation than a mid range attenuation. Difference resistor and cap values might improve the curve slightly, but the underlying problem is the high Z of the Xicon 42TL021-RC, so no cap or resistor pair will permit it to match the prominent effect that's seen with the Q filter.
Lastly, the same plot as above, but this time with the cable and control pots simulated load of 470pF and 200k ohms in parallel:
Note that the resonant peak and Q of the SSL-1 alone and the Q Filter are lower, as is expected, but notice that the Xicon 42TL021-RC remains unchanged, because the impedance of that transformer is so high that the additional load is swamped out. There is a mid scoop effect, but it's very slight and the DB of the high end has dropped by 10 to 15dBV, so "mid scoop" would not be the first adjective that comes to mind.
In conclusion, the Xicon 42TL021-RC is not real useful as an inductor in the context of a high Z passive guitar circuit. The Q Filter is more pricey at around $35, but they can be made at home for cheap, it's just about 100 turns of wire around a ceramic pot core.
Bill Lawrence Q Filter and a Xicon 42TL021-RC
Even though these inductors are recommended for the same purpose, I had the impression that the Bill Lawrence sounded a lot better. The clarity seemed to be retained with the Q filter, but the Xicon 42TL021-RC caused a loss of highs and lows alike. I didn't think it worked much better than a rolled off volume control. The only difference I can see between the Xicon 42TL021-RC and Q Fitler is that the Xicon 42TL021-RC has a higher impedance, around 5k ohms at 1kHz, where as the Q filter shows only about 50k ohms impedance at 1kHz.
So I've tested the Xicon 42TL021-RC and the Q Filter with the bode plotter and integrator with an SSL-1 for an "in situ" demonstration.
This first plot shows the SSL-1 by itself, then the Q Filter in parallel, then the Xicon 42TL021-RC in parallel:
As can be seen right off the bat, the Q Filter, with its super low Z, produces are very high Q factor, but more than that. the overall signal is higher and much more treble it preserved in the output.
This plot below is the same thing, but now a 10k ohm resistor and a 22nF cap twisted together are in series with the inductors, which is how Bill Lawrence recommends the Q Filter be implemented, for a mid-scoop control:
The Xicon 42TL021-RC performs pretty badly with the Bill Lawrence wiring scheme, it acts more like a high end attenuation than a mid range attenuation. Difference resistor and cap values might improve the curve slightly, but the underlying problem is the high Z of the Xicon 42TL021-RC, so no cap or resistor pair will permit it to match the prominent effect that's seen with the Q filter.
Lastly, the same plot as above, but this time with the cable and control pots simulated load of 470pF and 200k ohms in parallel:
Note that the resonant peak and Q of the SSL-1 alone and the Q Filter are lower, as is expected, but notice that the Xicon 42TL021-RC remains unchanged, because the impedance of that transformer is so high that the additional load is swamped out. There is a mid scoop effect, but it's very slight and the DB of the high end has dropped by 10 to 15dBV, so "mid scoop" would not be the first adjective that comes to mind.
In conclusion, the Xicon 42TL021-RC is not real useful as an inductor in the context of a high Z passive guitar circuit. The Q Filter is more pricey at around $35, but they can be made at home for cheap, it's just about 100 turns of wire around a ceramic pot core.
Bill Lawrence Q Filter and a Xicon 42TL021-RC
