Going beyond the typical stompboxes, there are some effects that utilize clever analogue filters to shape the sound. 
 
This niche category includes everything from basic wahs to pitch shifters, envelope filters, and even more complex effects like synths and ring modulators. 
 
The Electro-Harmonix Doctor Q Envelope Follower falls into this category, alongside its larger counterpart, the EHX Zipper—both potentially inspired by the iconic Musitronics Mu‑Tron. 
 
While often called auto-wahs, these pedals offer much more than that. The filter activates once the input signal hits a specific threshold, sweeping through until it fades out. 
 
Envelope followers aren’t for everyone, but when used thoughtfully and in moderation, they can create captivating sonic textures, especially when paired with other effects like echo or fuzz—think pure ‘70s vibes. 
 
Although the Doctor Q has only two external controls, it can deliver some excellent vintage filter tones with just the press of a footswitch.