I have the zendrive and a analogman maxon od9 silver mod ( should be really similar to the keely sound wise I believe, if there is a significant difference I'm interested to know what it's is )
The od9 is way more of a mid only pedal. The zen has a lot of mid, but seems hifi in comparison, fat lows and the highs are present, but not accented by any means. The od9 has a distinct lack of lows, they are not there at all, but it is a ts9 circuit so it is to be expected. In addition the od9 is a bit more compressed sounding, especially if you drive the input on it with another OD.
They are both creamy sounding, and actually stack well.
The Zen Drive is a modded Tube Screamer derivative style circuit. It has no input and output buffers (I'll get to that in a sec). The distortion in a Tube Screamer is a pair of back to back silicon diodes in a feedback loop. We'll call that a "medium" level of distortion. The distortion in a Zen Drive uses MOSFETs and Schotkey diodes arranged asymmetrically. The clipping threshold is much higher, and because of the way MOSFETs work, it also sounds a lot softer and spongier. MOSFETs have been used in other boutique TS designs for a long time to get a more amplike tone from the pedal's own distortion.
The treble control in a Tube Screamer is active. This means that as you turn it up, it's adding volume in the treble--upper mids range. A lot of pedals use this.
The treble (Tone) in the Zen Drive is a passive control. It's actually more like a volume control in between the amplifications stages. A lot MORE pedals have this kind of treble control (in fact, it's almost exactly the tone control in a RAT and it's similar but not identical to the tone in a Timmy).
The bass (Voice) control in the Zen drive is like the bass control on the Timmy.
The Zen's voicing pretty much covers an upper mids to bass frequency, but cuts subs. A "typical" setting on it has it sitting near a tweed amp's frequency range.
A tube screamer covers more of a low-mids to treble frequency, with higher treble frequencies cut out. A "typical" setting on the pedal will pretty much sit in the hole of a blackface Fender's upper mid scoop. That's why they sound good together.
The input and output buffers in the Tubescreamer change how the circuit handles your guitar volume. A Zen Drive has a pretty sharp drop when you turn down your guitar's volume and stops distorting as much, whereas a Tube Screamer will get quieter but won't really change its distortion too much.
Although I wouldn't personally run either on my board (just not my sound), I like the voicing of the Zen better. It never sounds harsh to me and the drive is actually useful, whereas Tube Screamers always sound harsh unless they're actually pushing an amp with the drive turned down. The volume drop in a Zen Drive annoys me a little, but a Tube Screamer's behavior annoys me more. Turn on another pedal in front of the Zen (like a compressor) and it behaves much more linearly. I couldn't get enough treble out of the Zen, but the treble on a TS sounded noisy to me.
YMMV of course. I really recommend trying the Zen if you can. The Keely Mod won't make a Tube Screamer sound like a different pedal, it's more if you already like the Tube Screamer sound and want it to sound more like an early 80's 808.
Zendrive. As a long time Tube Screamer user (I've had the same one on my board since 1982) the Zendrive is the first and only (of many) TS type pedal to make it onto my board. It records well, you can adjust parameters to suit your guitar, and it sounds amazing. Highly highly recommended.
So, to summarize, the Tubescreamer has built in EQ that pushes the mids, it's more compressed (less dynamics available as you turn down the guitar) and less flexibility in terms of adjusting the EQ and amount of distortion available. Both are op-amp based and on the "smooth" side of the OD spectrum. I love a TS into a BF style Fender amp, but into Marshalls and Voxes I prefer the Zen.
Don't misunderstand me, I'm not slagging off the designer or the design. I recommended it and said it sounded good.
All I meant was that it has the anatomy of a Tube Screamer. It's an op amp that creates distortion by putting diodes in the feedback loop. That method of creating distortion can be traced to a specific design, in the same way that it's fair to say that a Tone Bender is a modified Fuzz Face with an input stage. The Zen Drive also has a small capacitor in the feedback loop to soften the distortion and cut upper harmonics. Just like a Tube Screamer. It even has the same value of drive pot.
When people say that something's a tube screamer derivative, they usually mean it has something inside it that looks like that ^
That doesn't mean it is a Tube Screamer and it doesn't mean there's anything wrong with that. It means that he used a time-honored way of achieving what he wanted. There's only so many ways to do something in a small amplification circuit. I sincerely doubt that Hermida independently stumbled on that particular method of creating soft distortion or had never seen a tube screamer before he created it, and I also will go so far as to say that his method of cutting bass in the circuit was probably not an accidental discovery. Again, there's nothing wrong with that. There are plenty of differences elsewhere in the supporting circuitry.
I'm not an electronics expert, I'm just a hack who can recognize patterns.
midwayfair all your posts in this thread have been very eloquently put and informative, I enjoyed reading them and feel like I've actually learned something. many thanks, appreciate you taking the trouble to post.
Not going to get more highs and lows from a TS circuit. I'd recommend the Timmy for that-independent control of bass and treble and a slightly rougher edge to the distortion from the pretty smooth Zendrive sound.
It's probably not fair to call all pedals that clip with diodes in the negative feedback loop of an op-amp as Tubescreamer derived for a couple of reasons. One is, I'm not convinced that the designer of the OD-II 855 has a clear claim to having thought of this method of obtaining distortion first, although I'd love to be sure (actually just found a thread on freestompboxes.org where Bjorn=BJF claims the Boss OD-1 used opamp clipping a year before the ODII/855 and two years before the TS808). Secondly, if you search freestompboxes.org for the Tim discussion with PaulC, you'll find that he designed his pedal from the ground up for different reasons than the TS circuit, and yet if you look at it, it has an op-amp with diodes for clipping. I believe him when he says it's not just derived from the prior work...
I totally understand what you are saying midwayfair, and you are correct. You also clearly know more than I do, so thank you for the detailed response. I am learning more and more about pedal design every day thanks to folks like you.
I think my point was that there are only so many ways to skin a cat and you could say that a new ferrari is just a derivative of a model t ford if you use enough reductionism in your logic. They both just use a combustion engine and four wheels to move a people carrier of some sort.
i have a keeley ts-808 and a zendrive. had them both on my board for a while for solos... with a bassman.
i don't know the first thing about wiring and circuits... but to my ears... the zendrive is crisper but the 808 is harsher (in a good way) and much fatter and bigger. the zen cuts through the mix while the 808 jumps out of the mix... if that makes sense. just my experience.
i finally figured out that the 808 was better with fenders and the zen was better with gibsons. again, just my experience. i started leaning on my strats/teles more and more... so i eventually got rid of the zen to make room for other stuff.
i'd have no hesitation using either one for a lead pedal if i had room... they both sound great.
i eventually want to put together a scaled down version of my board on a pt-2... and i'm holding onto the zen just for that purpose.
Midwayfair, Couldn't have said it better. Another hack that was trained in the navy 30 years ago.
I wouldn't go near the Klon Db. I was slagged for even mentioning a Klon in the same breath as a Tubescreamer.
A Zenn Drive would get my vote though. The Voice/Tone thing is pretty unique. In my pretty biased opinion a much better more versatile box by far.