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Audio Sprockets ToneDexter

Piezo Pickup Preamp & Microphone Emulator
By Bob Thomas

Audio Sprockets ToneDexter

This ‘trainable’ convolution‑based device promises to make your piezo‑pickup signal sound like your own miked‑up instrument.

My first attempts to amplify acoustic instruments on stage used stand‑mounted mics, but my frustration at being unable to move more than a few inches from the mic led me to explore the dark arts of piezo pickups, on‑board mics and external preamps. Combining a pickup signal with one from an on‑board mic when on stage requires a blender‑type two‑channel preamp. Although analogue blenders are still available, the two dedicated digital acoustic instrument preamps that came to market have disappeared without trace or replacement. Both units used digital technology to dispense with mics entirely. They attempted to transform the sound of an undersaddle piezo transducer (UST) into a selection of emulations of specific miked acoustic guitar setups. The initial success and eventual failure of both units hinged almost entirely on how closely the response of a user’s guitar and pickup corresponded to that of the guitar and pickup used to create each transformation — a situation that was probably doomed before it began.

Until the recent arrival of the Audio Sprockets ToneDexter, it seemed to me that the concept of digitally transforming the output of a piezo pickup into a miked‑guitar emulation had had its commercial day. But my pessimism was misplaced, because the ToneDexter enables users to create transformation filters for each of their own combinations of acoustic instrument, piezo pickup and mic — and to do so quickly and easily.

What You See

The ToneDexter has a surprisingly petite footprint. The front fascia is divided into two areas, the upper carrying the rotary controls and a rather retro three‑character screen. The lower section contains the Mute/Tune and Boost (+8dB max) momentary footswitches. The height difference between the two areas won’t stop a wayward toe colliding with a knob, but the overall strength of the casing and the controls’ bolted‑through steel shafts guard against damage; I’m less sanguine about the headphone volume and boost gain knobs that protrude from the rear panel, as a heavy end‑on impact could potentially cause damage to their internal PCB connections. Also on the rear are the balanced XLR connectors for a mic input (with permanent 48V phantom power) and a DI output with switchable fixed/variable output levels. TRS jacks handle the headphone output and effects loop. The PSU connector accepts any polarity of 9‑15 Volts DC at 6W. On the left, a slide‑switch inverts the polarity of the headphone and main outputs (to avoid clashes between your guitar’s acoustic output and the sound from your headphones), and a second selects between the 11‑slot A and B memory banks.

On the chassis’ left‑hand side, an unbalanced TRS jack feeds the 1MΩ input. On the right, the output (controlled by the front‑panel Output Level knob) appears on a TRS jack that can run balanced or unbalanced. The SD card slot below the jack is used to load firmware, but in the near future it will also allow the export and import of data from the ToneDexter’s image banks. Three firmware versions are available:

  • General, which covers instruments such as guitars, dobros, autoharps, violas and other instruments in that general frequency range;
  • High, which is designed for multi‑instrumentalists and splits the memory, with the A bank remaining ‘general’ and the B bank made more suited to higher‑pitched instruments like fiddles, mandolins, ukuleles and banjos;
  • Bass, which is designed for acoustic bass guitars and double basses.

What You Do

The ToneDexter has two operational modes, Train and Run, denoted by a T or an R as the first character of the display; the other two characters show the number of the active memory slot. The unit ships with an empty memory, so your first task will be to create a ‘WaveMap’ transformation filter, based on an impulse response (IR). This filter will be unique to the combination of instrument, pickup and mic used to create it. The ToneDexter uses this filter in a linear convolution that transforms the sound of the instrument’s piezo pickup into an emulation of the mic. The WaveMap can be stored in any one of the 22 memory slots.

Unlike the Fishman Aura and the D‑TAR Mama Bear, the ToneDexter is agnostic when it comes to pickup type and position — you can use any make or variety of under‑saddle (UST) or soundboard (SBT) transducer. The creation process is relatively simple, and since its training phase doesn’t pick up any noticeable room noise, Audio Sprockets claim you can even create a WaveMap in a car... though they advise against doing so whilst driving!

Once you’ve powered up the ToneDexter, connected your active or passive piezo‑equipped guitar, headphones and mic (a condenser is recommended, but a good dynamic will work), you use the detented WaveMap Select switch to set the memory slot you’ll use to ‘train’ the device. If you train in slots 1‑21, the WaveMaps will have anti‑feedback processing applied, and this can reduce their apparent bass response. A WaveMap trained in slot 22 has no such processing applied, and results in the most realistic mic emulation. (Though trained via slot 22, the WaveMap can be stored in any of the other 21 slots.)

The next step is to set a healthy input signal from your pickup using the Pickup Level Trim control, since a low‑level signal will result in a low‑volume WaveMap. Having done that, you tap the left‑hand Mute+Tune+Edit footswitch (this mutes the main outputs but leaves the headphones active), check your tuning using the on‑board tuner, and tap the right‑hand Boost switch to enter Train mode. During the first training phase, the display shows ‘LV’ and a rotating bar, indicating that the software is automatically matching the level of the mic to that of the pickup. The manual recommends avoiding your instrument’s lower register during this phase, and suggests, as examples, repeatedly strumming your guitar’s top four strings at around the fifth fret, or playing mid‑range double stops on a violin.

Once the rotating bar stops, the display shows TL, indicating that the training phase has begun and that your headphones are monitoring the ‘training‑in‑progress’ WaveMap emulation of your mic. During this phase, you can compare the emulation with the real mic signal by pressing the Boost switch, at which point the display changes to ML. If you aren’t happy with the sound and decide to change the mic position, you can abort the training process by pressing Mute, and then start over. The manual states that what you play during the training phase isn’t overly critical, but recommends playing either a series of chords up and down the neck or a series of single‑note exercises covering every fret on every string for guitars. For other stringed instruments, the instructions are to play normally and try to cover the entire range of the instrument.

As training progresses, the display increments from T1 to T9 over a couple of minutes, and you should hear the sound of the emulated mic improve with each change. At the end of training, the ToneDexter enters a ‘Freeze Phase’, indicated by ‘T_’ in the display. If you trained in any slot other than slot 22, the timbre of the sound will now change as the intelligent feedback suppression gets to work. You can toggle between the sounds of the pickup (the display shows P for pickup), the microphone (M), any WaveMap in the slot that you trained in (S for stored), and the new WaveMap (T for trained) by tapping the Boost switch. At this point you can either retrain or use the Option A/B and WaveMap Select switches to select a different storage slot. Slots already loaded show a dot in the display — since the unit is still in its Freeze phase, you can audition it and the P, M and T sounds as before. When you’re ready, a press and hold of the Boost footswitch stores the sound and returns the ToneDexter to Run mode. The mic can then be dispensed with: you’re ready to play, and can use the front‑panel controls to refine the performance of your mic emulation.

On the back, the mic input is joined by a headphone output, an effects loop and a line‑level output, making the Tone Dexter easy to incorporate into most setups.On the back, the mic input is joined by a headphone output, an effects loop and a line‑level output, making the Tone Dexter easy to incorporate into most setups.

What You Get

My main live acoustic‑guitar preamp setup consists of a Collings C10 guitar, a Trance Audio Amulet M pickup system and a Grace Design Felix two‑channel blender preamp. An SM Pro P‑Control phase adjuster between the Amulet M and Felix allows me to phase‑align the pickup and mic signals. This combination is, effectively, an analogue equivalent of the ToneDexter. My preference is to use as much mic as possible and blend in a small amount of pickup to add weight and punch, so I was genuinely intrigued to hear how the ToneDexter would compare.

I trained the ToneDexter in slot 22 with a DPA 4099 guitar‑mounted mic around six inches above the bass side of the 15th fret, pointing towards the C10’s soundhole, and with the pickup going straight to the ToneDexter. After a few false starts, my first WaveMap impressed me by being very close to the analogue mic‑only sound but, though usable, this lacked some crispness in the higher mid‑range and treble. Repositioning the 4099 several times eventually resulted in an emulation that was much closer to the miked sound.

In Run mode, the front‑panel controls really came into their own, enabling me to tailor the emulation much more precisely.The most tonally influential control is the continuously variable Character Select, which allows you to morph from the full captured mic sound (knob fully clockwise; ‘CH2’ appears in the display), through a 50/50 blend of a tighter, punchier version of the emulation combined with the unprocessed original (knob at 2 o’clock; ‘CH1’), to 100 percent of the tighter version at the 12 o’clock position (‘CH0’). Between the noon and 8am positions, the unprocessed pickup is progressively blended into the tighter emulation, up to a maximum of 70 percent pickup, 30 percent emulation. The percentage of mic emulation remaining is shown in the display as ‘Bxx’, the ‘xx’ being a value between 99 and 30. To my ears, the area around the B80 position (80 percent tighter emulation, 20 percent pickup) was the closest emulation of my analogue setup.

The ToneDexter’s two‑band EQ worked well. The bass and treble controls have a shelving response that, with the General firmware, deliver ±9dB at 125Hz and 2kHz respectively. With the High firmware, slots 1‑11 remain General, but slots 12‑22 shift the EQ points to 250Hz and 2kHz to handle higher‑pitched instruments, and, in the Bass case, the frequencies are 63Hz and 1kHz.

The range of the Notch filter moves similarly: General, 73Hz‑392Hz; High (slots 12‑22 only), 124Hz‑466Hz; and Bass, 37Hz‑165Hz. Paired with the intelligent anti‑feedback filter for WaveMaps created in slots 1‑21, the combination allowed me more than enough level without feeding back. Usually I rely on my QSC TouchMix’s anti‑feedback filters, but I could see a ToneDexter‑based strategy working well when playing through somebody else’s PA.

I also created WaveMaps for my other piezo‑equipped instruments, including guitars, bouzouki, banjos, mandolin, mandola and even an autoharp. I was always greeted with very usable results. Regrettably, I didn’t have the opportunity to try any bowed instruments, but I can’t see any reason why the technology shouldn’t work equally well there (there are some very happy users discussing their results online).

To get the very best results from the ToneDexter, I think you need to spend some time not only matching instrument to microphone and pickup, but also in positioning and perhaps EQ’ing the mic to achieve the precise responses required for stage and studio use. The more finicky among us might also consider capturing different presets for any different playing styles that result in a tonal change — one for finger‑picking, one for strumming, for instance. My suspicion is that a good strategy may be to spend some time in the studio recording paired ‘training’ mic and pickup tracks, and tweak things later. But the ToneDexter is capable of great results without going to such lengths.

Conclusion

Its convolution‑based emulations of a miked‑up acoustic instrument make the ToneDexter a major step forward in piezo‑equipped guitar preamplification, for both live performance and studio recording. Its biggest market will, I’m sure, be acoustic guitarists who play live, but it’s equally well suited to all manner of acoustic instruments. It’s both easy to use and capable of excellent results, though the level of performance does depend on the quality of the instruments, pickups and mics that feed it, as well as the care the users take in the training process.

I can’t see myself giving up my current analogue setup just yet, but neither can I see myself not buying a ToneDexter! If you’re in search of a good quality, miked‑up sound from your piezo‑equipped instrument, the ToneDexter is the best solution I’ve come across to date.  

Alternatives

There’s no digital equivalent of the ToneDexter, so you’ll be left with trying to duplicate its performance in analogue, which means the not‑exactly‑cheap combination of a Grace Design Felix and a Radial Engineering Phazer or Little Labs IBP phase alignment unit (the SM Pro P‑Control that I use in my rig has been discontinued).

You could also consider feeding your piezo pickup into a TC Electronic BodyRez pedal and combining the result with an internal electret microphone and a blender preamp from companies such as Headway, LR Baggs or K&K.

Alternatively, if you wanted to keep things in one box and get a selection of effects into the bargain, a Boss AD‑10 Acoustic Preamp plus a dynamic microphone might work well for you.

Further Reading

If you are interested in exploring the underlying science, these two papers are worth a read:

Pros

  • It’s unique.
  • Capable of producing accurate piezo-driven microphone emulations.
  • Surprisingly simple to create an emulation of any piezo-equipped acoustic instrument.
  • Good value, given the performance.
  • Works with different types of pickup.

Cons

  • Rear-panel knobs are vulnerable.

Summary

This convolution-based solution to the perennial problem of transforming the sound of a piezo-based pickup into that of a miked instrument gives its users a straightforward way of using their own instruments, piezo pickup systems and microphones to achieve this.

Published September 2018