@Jos has some notes on the Bissell envelope follower:
and @klausklaerwerk talks about Eddy Bergman’s ARP2600 envelope follower here:
Any other recommendations?
There is a design published by Nathan Ramsden as referenced here from 2016 which I used in my 14-envelope follower build as part of my analog vocoder.
It works nicely, but has a drawback that the input voltage must exceed the threshold voltage of diode D1, otherwise nothing happens. So for small signal levels this could be problematic. A Schotky diode would lower that, or a bias current could possibly help. The attack and decay pots are handy although I have found that the attack pot gives you only little range to work with. This in fact is to be expected: if the attack time is set to a large value, the input signal might have lowered in voltage before the attack time passes and then there will not be much signal you can control with the decay pot. This is especially so with impulsive signals like drum beats etc.
2 Likes
I find the Barton Musical Circuit one
http://www.bartonmusicalcircuits.com/envelope/index.html
and the Skull & Circuits one
https://www.skullandcircuits.com/envelop-follower-follow-moe/
I’ve made the ARP2600 @EddyBergman’s version and this module work very well, several input for different impedance (mic,ligne,synth) and easy to build.
3 Likes
There seem to be (at least) 2 approaches:
1: rectifying the signal and using a filter or RC-pair to follow the envelope.
Pros: easy to implements, only few components necessary
Cons: if the filter is too slow or the RC-time is too large, it won’t follow the envelope if that drops quite suddenly in amplitude.
2: rectify the signal (Bissell) and ‘sample’ the maximum amplitude and use a sample and hold kinda approach to set the output voltage.
Pros: the sampling makes envelope following easier even if the amplitude of the signal drops suddenly
Cons:
- the sampling leads to step wise following the contour of the input signal.
- (sightly) more complicated circuitry
One thing I think is important to consider when choosing between types and that is what part of the signal do you want the envelope follower to follow. Only some low frequency component, or all of them? And to what aim does it have to do that?
The answer to those questions may make the choice of the type easier, I think.
3 Likes
This is designed as an envelope generator; are you saying it also works as an envelope follower? I guess if you squint at it it’s kind of a half wave rectifier followed by a low pass filter, so maybe it can. Edit: No, it’s a comparator followed by a filter, isn’t it?
Bissell’s design also has several diodes in the audio path. How does it handle small signals?
It looks pretty much the same as Sam’s simple envelope generator, except for the input circuitry.
1 Like
Yep, the Ramsden device can also be used as an envelope follower. I’m waiting for some Schottky diodes I ordered. I expect they will make the threshold effect in the Bissell approach smaller.
2 Likes
I’m so glad that it all went smoothly, building the Envelope Follower. Excellent!!
1 Like
You can see as a filter, but it’s probably easier to look at it in terms of charging (attack) and discharging (release) the capacitor.
Yeah, I’m just trying to understand how it could function as an envelope follower. Not succeeding.
The signal is half way rectified, the comparator will output a high voltage if the signal rises above a certain level en return to 0 if it drops below that. The density of the pulses depends on the number of zero crossings of the signal and the overall amplitude of the signal. This series of pulses can be smoothened using a resistor / capacitor combo. The pulses can thus be converted into a slope that rises or falls. If the signal falls it does so following an RC / exponential curve and not so much following the falling contour of the input signal. But if you squint at the signal, it is not a bad envelope follower.
1 Like
@Jos Did you make any progress on this? I am also looking in the vocoder topic and would like to use the simple envelope follower.
I took some of the Bissel design and simplified it because I found that some of the circuitry was superfluous / not necessary.
I wrote about the final design here:
I own a B2600 which has an envelope follower and I’ve been meaning to compare that one to the one I implemented. Maybe next week I will get round to that.
3 Likes
Oh, I need to take a look at that! A part of my polysynth approach is a 8 channel VCA and I like to reuse that for a vocoder. Next step is to look into the filters, Vocoder Analyzer www.haraldswerk.de seems to be quite interesting. I would be quite happy if you could share your progress and thoughts on this topic!
I have build a vocoder based on two Behringer 914 filterbanks, 14 envelope followers and two Doepfer eight channel VCAs (of which I use 14), but haven’t come round to writing about it because the circuit is quite complex. Complex in the sense that it contains quite a few modules to realize it and explaining what they all do and how they need to be interconnected is quite a task. Furthermore I want to make some music with it first so that I can include it in the write up.
After using the hardware vocoder for a while I found that the attack pot of the envelope follower did not bring much and in a next rendition of the device could best be left out. The remaining part of the enveloper follower however suits its purpose and you probably do not need the more refined Follow-O-Matic.
As you may know I created Vocode-O-Matic (D31) for Daisy Seed which has 31 filter bands and a very flexible modulation matrix / patch matrix. This version sounds soooooo much better than the hardware version, and is soooooo much more flexible in its configuration, that it is more worth my while to invest some more time in making that into a module than to write about the hardware version. But if there are any specific questions, I’m happy to answer those.
3 Likes
ARP Envelope follower
This is an envelope follower (EF) module in Kosmo format with mic and instrument preamplifiers.
It is a design based on the ARP 2600 — there’s a schematic in the service manual. Aside from changing the op amps to TL074 and reducing the output stage gain (which it seemed to need, maybe because it uses ±12 V instead of the ARP’s ±15 V) the EF core is essentially identical to the original.
The ARP EF also has a preamplifier section for input of external, non-synth level signals which, of course, are what you’re going to use an EF on a lot of the time. Rather than try to adapt the Teledyne 1339 based design I replaced the ARP preamp with two different preamps: One to boost instrument levels to synth levels, and one to boost mic levels to instrument levels — which then goes through the other preamp as well. This is inspired by @EddyBergman’s stripboard build, though I used different preamp circuits. The mic preamp is from a design by Andy Collinson (with a mod from here) while the instrument preamp is based on the one in Ken Stone’s Stomp Box Adapter.
If nothing’s plugged into the synth input it uses the output of the instrument preamp, and if nothing’s plugged into the input of the instrument preamp it uses the output of the mic preamp. And if nothing’s plugged into the input of the mic preamp it uses the condenser mic capsule mounted on the front panel! Yes, this module is always listening, so be careful what you say. Besides going to the EF input, the instrument preamp output goes to a front panel jack, so you can use this module to feed external signals (or the front panel mic) to other modules too.
This module isn’t really a plug and play kind of thing. The output levels you get depend not only on the amplitude of the input signal but the waveform — square waves give bigger outputs than ramp waves, for instance. With an external signal source you will want to use the gain knob to get the preamp output up to around 10 Vpp, and then use the signal level knob to get the output to the right level. It’s probably a good idea to use a scope to check your settings.
Photos
GitHub repository
9 Likes
Stunning work mate. The functions, board, logic and this post itself. Bravo!
4 Likes
Hey, superb work, as always @analogoutput !
I was wondering why you have different preamps for instrument and mic in. Are they more different than only in the needed gain? I have looked at mutable ears and there is only a single opamp preamp and it is supposed to work with guitars as well as microphones. I guess the difference is the type of microphone that you can use?? I have no clue about microphone types, so maybe you can enlighten me 
Ask an expert, not me. My understanding and experience is that mic levels are lower than instrument levels, which is why the mic preamp output feeds into the instrument preamp input here, but that seems at odds with the Ears thing.
I do know the mic preamp design here is supposed to be for an electret condenser mic, which needs to be supplied with power to work. So it needs phantom power, it can’t just be connected to an unpowered amp input like on Ears, unless the mic has a battery built into it. Whereas I guess a dynamic mic doesn’t require power and would work with Ears? Or so I think I understand. Again, ask an expert.
Added: From @EddyBergman’s post, which inspired this design (in general though not in specific):
The next input is the one at switch S1 which bypasses the transistor microphone pre-amp and inputs straight into the LM386. This can be used for a guitar for instance. I also tested it with a run of the mill dynamic microphone (600 Ohm) and that works very well too but the volume knobs all had to be fully opened up.
4 Likes