Femtoverb Build Report

[DaveTV]

Hello everyone. After a 2 year hiatus (which coincidentally is how old my son is now), I finally found some time to return to the Femtoverb, which has spent these past 2 years sitting patiently on a breadboard. First off, I was shocked to discover in my time away that other members of this board had taken interest in this circuit (hello MetalGuy!). I never imagined ANYONE would actually dig up this circuit, let alone try to build it.

My goal with this project was to build a useable digital reverb pedal that didn't take up a lot of space on my pedalboard. At the time when I started thinking about this, there were only a few reverb pedals on the market that looked interesting to me. The EH Holy Grail had a low price and good user reviews, but I didn't like its size (I was hoping more for a BB-sized box) or its limited set of reverb effects. For a short time I tried using my Alesis Nanoverb for a guitar effect, complete with its own home-brewed bypass switch box. The Nanoverb is the precursor to the Picoverb, and they appear to share most of the same reverb effects (except the Nanoverb is based on an older reverb chip). The Nanoverb sounded OK with guitar, but it really wasn't a pedal.

Around that time I discovered a thread started by Peter Snowberg discussing digital circuits based around the Alesis AL3201 reverb chip, now manufactured by Wavefront. After finding the datasheet for this chip, it occurred to me that it probably wouldn't be too hard to build a reverb pedal around the AL3201.

The Femtoverb is taken almost directly from a test circuit described in the datasheet for the AL3201, which, according to Peter Snowberg, is probably the circuit used for the Picoverb. Once I had the Femtoverb circuit set up on a breadboard, I did try a few of my own mods (a lame attempt to try to make it more my own design), but nothing I did seemed to improve upon the original circuit, so I ended up leaving it alone for the most part. The only changes are a higher input impedence for guitar, and a volume boost control at the output to make up for any noticeable loss of gain. Also, the original circuit is for stereo use, whereas the Femtoverb is for mono only (it wouldn't be hard to make it stereo though, the chip's designed for it). On the breadboard, the circuit sounded fine, except for maybe some minor hiss that I attributed to lack of proper shielding.

So after a 2 year break of not working on the Femtoverb, I wanted to finally get it soldered together and into a box so I could use it. Here's a report on my construction, based on the schematic:

First of all, as crazy as it sounds for this type of circuit, I'm exclusively a perfboard builder. I've had decent results using perfboard with other complicated circuits, and besides, it's a great gimmick ("This digital reverb pedal features point-to-point hand-soldered components...").

In order to mount the 3 SOIC Wavefront chips on perfboard, I first soldered them onto surfboard SOIC adapters and then used in-line chip sockets as a way of "plugging in" the surfboards into the circuit board. This worked out well, and its been a convenient way of bringing the chips on and off the board. But doing this meant I needed a taller box, so I settled on the 1590-BB size with 2-inch high walls.

The rest of the soldering work was fairly straightforward. Before starting, I did spend a lot of time designing the layout of the components on paper to ensure everything fit okay. In the end, I was able to get everything to fit comfortably on one of Aron's 3.3" x 2.5" perfboards. One thing that really helped free up space was using surface-mount capacitors on the surfboards for the Wavefront chips.

Here are some images:

View of input section leading to AL1101 chip.


View of AL3201 and AL1201 chips followed by output section.


Underside of perfboard.


In the box.


Topside of box.


So am I satisfied with how the Femtoverb sounds?

Almost. The circuit seems to work as it should. The Mix knob blends the signal between dry and wet, the Boost knob gives just a little bit of extra gain, and the rotary switch selects all 16 of the built-in effects.

My first complaint is that there's a slight hiss in the background. So far, I've only tested the Femtoverb out while plugged into a POD and listening through headphones, so perhaps this hiss isn't as noticeable through a real amplifier (though it probably is). Going over the notes in Wavefront's chip datasheets, I did notice that the filter following the AL1201 chip was designed for a 48kHz lowpass cutoff frequency, which seems unnecessarily high for use with guitar. I was thinking I might try to attenuate the hiss a bit by lowering the cutoff frequency in the filter, but I wasn't sure how low I could safely go before affecting the reverb itself. Anybody have any ideas on this?

The other main complaint I have is that the reverb effect isn't as noticeable as I think it could be, and it isn't until the mix knob is almost 100% wet before you can really hear it. In the Femtoverb schematic, the mix pot's value is 10k, which is higher than the 1k pot shown in the Wavefront schematic. Switching back to the 1k pot seemed to improve how the wet signal blends with the dry. Another thing that seemed to help was slightly increasing the gain at the AL1201's output filter by following the calculations in the Wavefront datasheet. By increasing the AL1201's output gain by a factor of 3, I found that the reverb level seemed to more evenly match the dry signal's level, making for a smoother blending of the two. Anybody have any other ideas on how to improve the Femtoverb's Mix control?

Once I get these issues addressed I'll try to post audio clips of how the Femtoverb sounds. I'll also update the schematic to reflect any improvements.

All input welcome.

[nelson]

Not much from the input side of things. However if steve stocked these chips it would be much easier for us DIYers to approach this project.

Being in the UK I find it near impossible to attempt this project without serious time and effort contacting the manufacturer of the chips.

imo this would be the best introduction project to digital circuits and provide the holy grail of a decent non-spring DIY reverb pedal.

Why hasnt steve @ SB done a deal?

With a reliable supplier and possibly "surfboards" this could be made into a staple DIY project that would answer the eternal newbie question of "Anyone have a schematic for X reverb pedal?".

With reverb becoming a staple in most post rock guitarists rigs, its about time.

[Peter Snowberg]

Very cool, Dave!

That is a really beautiful build.

I hope Small Bear will eventually pick these chips up. They have some amazing potential.

Welcome back.

[MetalGuy]

Hello, DaveTV!

Very cool to see more people building this nice project. Unfortunately the chips are difficult  to get and yes, it's really about time for Steve to stock some, so more people can build the Femtoverb.
As far as my build is concerned I don't hear any noise and can't advise you on that, maybe the component selection also matters. I'm not using the level control on mine - it's a 10k resistor instead but the Mix control seems to work OK - on some effects you need to turn it up more on others less.

Good luck to all !

[puretube]

chipsets here: