Level Detectors, Absolute Value, Peak and RMS

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
User avatar
mediatechnology
Posts: 5737
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Level Detectors, Absolute Value, Peak and RMS

Post by mediatechnology »

Improved Absolute Value and Peak Detector Circuit Needs Only One Op Amp

Adding a single diode, D6, provides a peak detector output.
D5 compensates for D6's forward drop.
I raised R6 to 100KΩ to reduce gain errors at low level from D5's dynamic impedance.
R6 can be.

Image
Improved Absolute Value and Peak Detector Circuit Needs Only One Op Amp

Tip: Adding a 6K49 in series with the absolute value output along with a 10µF to ground provides a VU-like ballistic.

Next Up: I'll make D5 and D6 emitter followers to increase output and peak charging current.
User avatar
mediatechnology
Posts: 5737
Joined: Sat Aug 11, 2007 2:34 pm
Location: Oak Cliff, Texas
Contact:

Re: Level Detectors, Absolute Value, Peak and RMS

Post by mediatechnology »

Current-boosted Absolute Value, Average and Peak Detector Circuit Needs Only One Op Amp

Adding two transistors lowers the output impedance and provides a high-charging-current peak output.
D5 in the previous drawings is replaced by Q1, an emitter follower.
Q2, which replaces D6, provides a high charging current peak detector that can capture a single-cycle 20 kHz burst. (C1=0.47µF)
R8 and C2 provide an average VU-type response.

Ideally D1-D3 are matched and thermally track.
Q1 and Q2 Vbe can also be matched.
The maximum peak output is limited by the maximum reverse Vbe of Q2. Input voltages of 5 to 6V peak (12V p-p) seems to be a reasonable limit.
I used some matched ZTX1053 which appeared to operate to about 8V.
The op amp is a TL072A.
An input coupling capacitor, not shown, is required for best low-level performance to block the prior stage's offset.

Image
Current-boosted Absolute Value, Average and Peak Detector Circuit Needs Only One Op Amp

Image
Current-boosted circuit. Absolute Value Waveform 3V/Div

Image
Current-boosted circuit. Input versus Peak Output Waveform 3V/Div

Note that D1 and D2 are unmatched so there is some slight asymmetry.
Jacko
Posts: 10
Joined: Sat Dec 16, 2017 12:02 pm

Re: Level Detectors, Absolute Value, Peak and RMS

Post by Jacko »

MXR Dynacomp. This rectifier is common in guitar stompbox compressors.

Image
Post Reply