HPF-Pre distortion plot
-100
-80
-60
-40
-20
0
20
0
5000
10000
15000
20000
25000
Frequency (Hz)
A
m
pl
itu
de
(
dB
)
1 V p-p
5 V p-p
.
At 1 V p-p, The second harmonic is at
≈
-55 dB, and the third harmonic at
≈
-78 dB. The rest of the
harmonics are consistent with the distortion floor of my measurement system. Pure second harmonic
distortion at -55 dB corresponds to a THD of 0.17%, which is quite respectable for a Class-A circuit. This
is with an input voltage of 1 V p-p.
At a higher input voltage, you see a slew of harmonics. That's serious distortion.
To estimate the dynamic range of the filter, I measured the total harmonic distortion at different input
voltage levels using Virtual Analyzer. Here is the plot:
HPF-Pre distortion versus input voltage
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0
1
2
3
4
5
6
Input voltage (V p-p)
T
H
D
(
%
)
Distortion rises with increasing amplitude. But unlike an op amp circuit, the dynamic range is not a hard
upper limit. You can push beyond this range, at least on the transient at the front end of a plucked note,
with minimal audible effect. I expect this circuit to be quite forgiving.
Acknowledgement
I am indebted to
drurb
from the TalkBass forum, for trying my first prototype, for many valuable
comments and suggestions, and for prodding me to put the preamp on the market.
