You Can DIY! Building a Guitar-Controlled Synthesizer: VCA & VCF

As a part of my ongoing guitar synthesizer mission, this text presents and explains the ultimate two key modules wanted by each analog synthesizer: the Voltage Managed Amplifier (VCA) and Voltage Managed Filter (VFC). The management voltages that set the amount and filter frequency come primarily from the Assault Decay Maintain Launch (ADSR) envelope generator proven within the June, 2022 subject of audioXpress. However the low-frequency oscillator (LFO) from that very same subject can be used to supply a tremolo impact when driving the VCA, or to comb the filter cyclically so as to add animation.
 

I’ll present two completely different VCA circuits as a result of one is far less complicated, and works wonderful for one-off tasks the place exact unit-to-unit repeatability isn’t wanted. The second is just like the VCA within the authentic Moog synthesizers, utilizing a topology just like the Moog VCF. However the VCAs are less complicated so let’s begin there.

VCA #1

Within the Nineteen Seventies, built-in circuits (IC) producer Nationwide Semiconductor printed an exquisite sequence of utility books that includes its ICs and different merchandise. Nationwide’s gross sales reps would give these paperback books away without spending a dime to digital engineers, and I nonetheless have the whole sequence and seek advice from them usually. The 2 I exploit most are the Linear Functions Handbook, and the Audio-Radio Handbook. However others within the sequence talk about TTL and CMOS digital logic, and each junction and field-effect transistors (JFETs and FETs). The primary VCA circuit proven in Determine 1 is customized from Linear Software Notice AP129.

 

Though this circuit is straightforward, it has a acquire vary of 60dB — unity when the FET’s gate is at -6V, to +60dB when the gate is at 0 volts and the FET’s resistance is 30Ω. One draw back is the ratio between management voltage and FET resistance isn’t linear. Additional, the management voltage vary can range between FETs, even from the identical batch, and particularly between FET sorts. So the vary -6V to 0 volts used on this LTspice emulation is likely to be -4V to -1V, or another vary, with the FETs you purchase. Nevertheless, it really works completely effectively because the VCA in a quantity compressor the place the right management voltage is self-seeking based mostly on the present audio output stage (Desk 1).
 

The distortion of this circuit is simply too low to measure reliably within the spice simulation, so it’s doubtless appropriate for all however essentially the most demanding hi-fi purposes. Notice the 60dB enter attenuator that limits the sign stage into U1 to 20mV to keep away from clipping the output at most acquire. For those who don’t want the total 60dB acquire vary, increase R2 to 4.7kΩ and decrease R3 to three.3kΩ. This limits the vary to solely 40dB, however improves the signal-to-noise ratio (SNR) ratio by 20dB.

One drawback with LTspice is that it doesn’t have all the most recent semiconductor fashions in its element library. On this case the PN4391 FET is out of date, so use the newer 2SK932-23-TB-E or virtually some other small sign N-channel JFET. Likewise, noise within the op-amp generally is a issue, so select a present mannequin such because the Texas Devices OPA sequence.

I arrange this circuit utilizing two management voltage sources to supply completely different views into its operation. The file you may obtain is about to point out the output for seven completely different management voltages without delay once you probe the Audio_Output terminal. Every voltage is proven in a distinct coloration utilizing the .step command. With .step, you specify a variety of values for just about any circuit situation, and LTspice runs a sequence of simulations, one for every worth within the vary.

You merely assign a variable identify (e.g., X or Y) to a element worth, then inform .step to increment that variable. On this case, we’re stepping the output voltage from supply V4, however .step can range resistor and capacitor values, and most different element attributes. For those who right-click the .step command line, a dialog pops up letting you specify helpful choices similar to linear or log steps; beginning, ending, and step values; or a listing of discrete values (e.g., -5, 12, or 49).

I modified the LTspice coloration defaults to make use of the resistor coloration code, so the primary hint is brown, then pink, then orange, and so forth. When probing completely different components of a circuit this makes it simpler to maintain monitor of which hint goes with which node within the circuit. With .step the identical coloration sequence can be used, as proven in Determine 2.

 

Setting the waveform show colours to match the resistor code is straightforward: From the Instruments menu choose Management Panel, Waveforms (tab), then click on Colour Schemes. This brings up a dialog that allows you to assign the colour for every oscilloscope hint proven within the order you show them. Or on this case, the order that .step shows.

Sadly, .step doesn’t work in descending order, as an alternative reversing the values to all the time be ascending. After I reported this drawback within the LTspice Customers Group, one of many consultants there, Dave Bell, confirmed a easy work-around. In a circuit unrelated to this text I needed to step resistor {X} backward from 21kΩ right down to 1kΩ in 5kΩ increments. Dave demonstrated that altering {X} to {22k-X} forces the right descending order. Notice 22kΩ for the system’s resistor worth versus 21kΩ for the beginning .step worth, as a result of the final step is 1kΩ (22 to 21).

For those who disconnect CV supply V4 and join V5, LTspice will show the acquire change over time as proven in Determine 3. I set V5 to output a Pulse somewhat than a gradual or stepped DC voltage. When a voltage supply is about for Pulse you may management the preliminary voltage, rise time, vacation spot voltage, variety of cycles, and extra. For those who swap the CV sources as described, additionally change the audio sine wave to 200 cycles of 200Hz, change 0.1 within the .tran command to 1, then convert the .step command to a remark: Proper-click the .step command line, click on cancel, then you may choose Remark to disable it. For those who depart V4 set to {X} that can pop up a warning message you may ignore. Or change {X} to a quantity similar to 3.

 

VCA #2
The core VCA design proven in Determine 4 is a linear present multiplier. It multiplies the enter present into the bases of Q1 and Q2 instances the collector present set by R12. Because the inverted management voltage pulls down an increasing number of on their emitters, the transistors conduct an rising quantity. We often see an NPN emitter linked to floor both by a resistor or immediately, with the enter sign pulling up on the bottom. On this case, the bases are linked to floor, and the emitters are pulled down towards a detrimental voltage.
 

Since it is a differential pair, the enter feeds just one aspect as a result of the sign is the voltage distinction between the transistor bases. However to protect the important circuit stability it’s necessary to make use of a twin transistor for Q1 and Q2, the place each transistors are on the identical die. There’s no want for one more determine exhibiting this VCA’s output as a result of it’s almost an identical to Determine 2.

After all, a normal common-emitter single transistor amplifier stage additionally multiplies present, however this circuit’s balanced design has a number of benefits. First, not like the FET VCA, the acquire change is completely linear, precisely monitoring the quantity of change within the management voltage. Extra necessary, because the management voltage varies the acquire, the DC voltage on the transistor collectors shifts up and down in sync. By balancing the circuit with two equal collector currents feeding a differential op-amp enter, the DC shift cancels out. So once more the voltage distinction on the collectors is the audio sign. With out this balancing act you’d get a thump when the management voltage adjustments, or a loud click on if it adjustments shortly with a quick assault or decay from the ADSR.

This circuit is just like the VCA that Robert Moog designed for his synthesizers, however was improved by Johan Verhoeven at Cranium & Circuits to make use of a differential op-amp as an alternative of a extra advanced association of discrete transistors. I simplified the circuit additional by eradicating some pointless elements, and specifying 0.1% resistors for R3 and R4 to permit eradicating a trim pot. Readers fascinated with synthesizer circuits will benefit from the Cranium & Circuits web site. It’s an ideal supply of schematics, and likewise absolutely assembled modules. The hyperlink is listed in Sources.

VCF

The circuit proven in Determine 5 known as a ladder filter due to the vertical association of filter sections. Determine 6 exhibits how the filter’s response adjustments with completely different management voltages. The core circuit was designed by Robert Moog for his MiniMoog synthesizer, so I declare no credit score for its elegant design! My solely change was to change the management voltage summing on the decrease left to accommodate the assorted modules in my very own guitar-controlled synthesizer. The patent for this filter expired many years in the past, so it’s now public area for anybody to make use of for any goal.

 

As with VCA #2, this ladder filter pulls down on the underside set of transistor emitters to alter the impedance of the transistors above. Every higher transistor pair serves as one filter pole, so all 4 levels set the filter roll-off to 24dB per octave. Optimistic suggestions by potentiometer R7 boosts the output of the filter at its cut-off frequency, which additionally will increase the Q of that increase. On an actual MiniMoog, the resonance could be elevated sufficient to trigger the filter to oscillate. The utmost quantity of suggestions out there is about by R8, so that you would possibly want to cut back (or improve) its worth barely to resonate solely on the very highest setting of R7. Notice that the unique Moog schematic specs R7 with a reverse audio taper, however a linear taper pot can be a lot simpler to seek out.

This circuit can be balanced to keep away from thumps and clicks on the audio output when the management voltage adjustments the frequency. As with VCA #2, matched transistor pairs are used to protect the important stability, however they’re wanted just for the underside pair beneath the ladder. Once more, the BCM56DS is a superb and reasonably priced alternative. It in all probability wouldn’t take a lot effort to interchange the discrete transistor-based differential amplifier with an op-amp to avoid wasting a couple of dozen elements, as Johan Verhoeven did for VCA #2. However I left the schematic as is for its historic worth, and to keep away from inadvertently altering the filter’s extremely esteemed sonic character.

The final merchandise value explaining on this filter circuit is Keyboard Monitoring, whose voltage enter is on the backside left of Determine 5. Usually, a filter is swept robotically by its personal ADSR as every word is performed, or left at a static place, or various manually by the musician for expression. Most analog synthesizers even have an choice for the filter to trace the keyboard, so taking part in increased notes shifts the filter frequency increased along with any sweeping or LFO results. For instance, in case you have a resonant filter increase that brings out the third harmonic of a word, you’ll need the filter to trace these notes as you play up and down the keyboard. Keyboard monitoring additionally makes doable sound results similar to wind noise that may be various in pitch and even performed as a melody.

To ensure that the filter to exactly monitor the pitch of the present word, a potentiometer is required. This could possibly be an inner trim-pot on the filter’s circuit board, or a entrance panel management that lets the consumer scale the quantity of monitoring. However typically, when a word rises or lowers one octave, the filter’s cut-off frequency ought to go up or down the very same quantity. As famous on the schematic, R22 is definitely a set resistor in sequence with a trimmer to exactly set a 1-to-1 ratio of word pitch versus filter frequency.

Lastly, when you’d prefer to dig into the circuit’s inner operation you may swap the show from a stepped frequency response to the oscilloscope view. First, right-click the command line that at the moment specifies AC evaluation:

.ac oct 50 10 20000

Then Cancel, then choose Remark to disable it. Subsequent, right-click this command which is now a remark, to allow Transient Evaluation:

.tran 0 1 0

Then choose “SPICE directive” to allow it. As with VCA #2, you also needs to disable the .step command, and alter {X} to a quantity between 0 and 10 to set the filter’s cut-off to a single frequency manually.

Al Effective

Even when you don’t intend to construct a complete analog synthesizer, the circuits proven all through this sequence can function “cookbook” components to make use of in different tasks. After I began studying electronics within the Nineteen Sixties there was no Web. By the Nineteen Seventies, I used to be in my 20s working as a technician at digital corporations, so I had entry to actual engineers who had been variety sufficient to reply my newbie questions. I additionally bought to learn each subject of Electronics and EDN, glorious magazines provided without spending a dime to electronics engineers. In each subject each magazines provided intelligent circuit designs and fascinating hacks, in addition to business information and product opinions. The Nationwide Semiconductor books talked about earlier are one other fabulous useful resource for “cookbook” model constructing blocks. As we speak, you may Google the identify of a circuit sort and give you myriad circuits starting from elegant to incompetent.

My level is that circuit components like these offered on this sequence can be utilized and reused time and again. The Interval-to-Voltage-to-Interval article confirmed two varieties of Log converter circuits, a Pattern & Maintain, and easy methods to create octaves utilizing primary digital logic chips. The LFO/ADSR article defined hysteresis and confirmed find out how to add that to a voltage comparator. The Enter Part article exhibits find out how to make a noise gate, in addition to a “no-drop” full-wave rectifier that avoids dropping 0.6V throughout the diodes. That article additionally included a intelligent “cheat” exhibiting find out how to simply choose capacitor values for the 555 timer IC. This text exhibits find out how to make two varieties of voltage managed amplifiers plus the basic Moog low-pass filter.

That is how I realized find out how to design digital circuits, by piecing collectively smaller constructing blocks from magazines, and the applying notes from semiconductor makers. I don’t have a level in electronics, and I’m no math knowledgeable! However I can assemble circuit components and ultimately give you one thing that works. On-line calculators for each sort of filter are available, saving you effort and time to seek out appropriate resistor and capacitor values. Within the previous days you’d have to unravel formulation similar to:

ƒc = 1 / (2 × π × R × C)

time and again as you experiment with completely different frequencies in a high-pass filter. The LTspice software program I utilized in all of those articles is vastly simpler and quicker—and far cheaper! — than really constructing and modifying circuits and proudly owning a bench stuffed with check gear. Certainly, life at the moment is sweet for electronics hobbyists. aX

This text was initially printed in audioXpress, August 2022.

Mission Information

To obtain the hyperlinks for LTspice VCA circuit file: [VCA-FET.asc]; the LTspice VCA circuit file: [VCA.asc]; and the LTspice VCF circuit file: [VCF.asc], go to:

http://audioxpress.com/web page/audioXpress-Supplementary-Materials.html

Recources

LTspice, www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

LTspice on-line assist discussion board, https://teams.io/g/LTspice

Nationwide Semiconductor’s Functions Notes handbooks, https://archive.org/particulars/manuals-nationalsemiconductor

Cranium & Circuits, www.skullandcircuits.com

E. Winer, “Constructing a Guitar-Managed Synthesizer: The Pattern & Maintain Time Machine,” audioXpress, April 2022.

E. Winer, “Constructing a Guitar-Managed Synthesizer: Frequency to Volts and Again,” audioXpress, Might 2022.

E. Winer, “Constructing a Guitar-Managed Synthesizer: LFO and ADSR,” audioXpress, June 2022.

E. Winer, “Constructing a Guitar-Managed Synthesizer: Enter Part & Choose Detector,” audioXpress, July 2022.

Learn the opposite articles on this Constructing a Guitar-Managed Synthesizer sequence

Half 1 – The Pattern and Maintain Time Machine

Half 2 – Frequency to Volts and Again

Half 3 – LFO and ADSR

Half 4 – Enter Part

In regards to the Creator

Ethan Winer has been an audio engineer {and professional} musician for greater than 45 years. His Cello Rondo music video has acquired almost 2 million views on YouTube and different web sites, and his guide The Audio Professional printed by Focal Press, now in its second version, is on the market at amazon.com and Ethan’s personal web site ethanwiner.com. Ethan can be a principal at RealTraps, a US producer of high-quality acoustic therapy.