Fullscreen HTML5Med Res (??MB)HTML4

AUDIO OUT AUDIO OUT L R By Jake Rothman A phantom-powered DI box using a JFET/BJT cascode F ollowing on from last month’s column with the theme of unusual experimental audio circuits, I’ve dug another one out from my notebooks. This direct injection (DI) box circuit is unusual in using a JFET and BJT in cascode to handle the relatively high phantom power voltage. Electric guitars need to be loaded with a high impedance, over 1MΩ. Otherwise, they sound dull and the high-resistance volume and tone controls don’t work properly. This is at least partly because they evolved during the valve era, and guitarists are conservative musicians. I’ve tried to persuade some musicians that a low-impedance balanced output is the way to go for the best sound quality, but they are wedded to their legacy hardware. Also, the idea of drilling a 22mm hole in their beloved instrument for an XLR socket is anathema to them. The solution to this dilemma is a direct inject (DI) box, which provides a high-impedance input for the guitar and a balanced, low-impedance output to feed a low-to-medium-impedance input, like a mixing desk input. Mixing desk inputs typically have an input impedance of 5–10kΩ. Recording guitar An important factor in getting a good electric guitar sound in the studio is to mic up a valve guitar amp and mix it with the DI input to the desk, as illustrated in Fig.1. Guitars do not sound good recorded directly. They need the overdrive distortion of a valve amplifier and the multiple breakup modes of a light-weight paper speaker cone. Multiple effects are also used on both channels to give a massive sound with stereo spread. This combination of two mains Earthed inputs would normally cause a hum loop between the amp and Room acoustics Fig.1: a typical studio setup for recording electric guitar. Guitar amplifier (preferably valve design) Guitar plus FX pedals Speaker Microphone Mixer channels Reverb and other effects added the mixing desk, so all useful DI boxes have a transformer-isolated balanced output. A direct feed from the guitar to the amp has to be provided, as shown in Fig.2. Going active Attempts were made in the 1980s to make active, transformerless DI boxes but they never quite eliminated the hum because the output has to be referenced to the ground of the guitar amp. Most DI boxes are passive devices, using a 10:1 step-down transformer (Fig.3), which needs expensive Mu-metal shielding and gives a poor frequency response. By using an amplifier to drive a lower-ratio transformer, as done here, these problems are eliminated. The downside is that a power supply is needed. This is normally a 9V block (PP3) battery, which is expensive and has a short life. The solution used here is to take advantage of the 48V phantom power available on most mixing desks and computer studio interfaces. Amplifier design Since phantom power is a relatively high-voltage, low-current supply, a discrete amplifier design is preferable to an op amp, as well as being more fun to design. A JFET input is a good idea for a high input impedance, since it gives lower noise with the high source impedance of the guitar. Most low-cost JFETs are limited to 30V, which gives a good excuse to employ a cascode. A cascode is where two transistors Guitar jack output to amplifier Fig.2: a basic direct injection (DI) box circuit. Amplifier DI Box Isolation transformer Stereo output to recorder Input Output XLR balanced 3 – output Guitar jack input 2 + Balanced output 0V 1 Lift Amplifier 50 DI (direct inject) Ground lift switch sometimes fitted here Practical Electronics | June | 2025 Special 1:1 to 25:1 step-up audio transformer (Difficult to achieve more than 100Ω input impedance at high frequency so guitar sound is compromised.) 600Ω output impedance Guitar input 3 2 Mumetal shield Balanced output Male XLR socket 1 1kΩ ground lift 1nF RF bypass Fig.4: four examples of the basic cascode configuration. Both active devices can be of any type (bipolar, JFET, MOSFET or valve). Load resistor Practical Electronics | June | 2025 RL 22kΩ RL 100kΩ +114V Vb Fixed bias Vb typically 2-5V Output +5.2V 2N5459 JFET D 0V Input Input Standard bipolar High-voltage device MP5A42 Fixed low-voltage 6.8kΩ stops VCE modulation 0.6V G S 1MΩ 0V 0V Gain = 160x V+ 240V V+ 12V RL 82kΩ V+ RL (Inductor eg, IF coil) +120V A 68kΩ Output 470kΩ Output 7.4V 15kΩ G Valve ½ ECC82 D K 0V 4.9V G2 10nF 3N200 BF528 Dual-gate MOSFET 0V BFW10 G1 Input S Input Fig.5: Douglas Self’s cascode amplifier circuit from Wireless World, February 1979. 1MΩ 1MΩ 0V 0V R5 6.8kΩ 47kΩ 220Ω 47nF V+ +38V 330µA 14.6V R10 22kΩ C5 22µF 35V 11mA + The cascode was popular in topnotch Hi-Fi amplifiers from the 1970s. The PE Gemini designed by Ferranti engineers was the first in a magazine, and it still sounds and measures very well today. A cascode was necessary to avoid an odd distortion arising in the main voltage amplification stage (VAS) transistor when the emitter-collector voltage (VCE) swing was almost railto-rail. This large swing caused the transistor to modulate its own gain and junction capacitance by a mechanism called the Early effect (named after James M. Early), where the thickness of the active region of the semiconductor varies with applied voltage. By using a cascode, the voltage across the lower transistor is effectively fixed, at a few volts, preventing this modulation. The effective capacitance is also reduced, improving high-frequency performance. This configuration is still popular in RF amplifiers and deflection amplifiers in cathode ray tube (CRT) oscilloscopes. The Early effect is less of a problem with modern VAS transistors, many of which were originally developed for deflection transistors in CRT monitors in the 1980s. The data sheets of such transistors will often display a very flat hFE vs VCE curve. Those made by Toshiba, Sanyo and V+ 238V R9 22kΩ C3 + 47µF 10V R8 2.7kΩ 0V R8 2.2kΩ TR3 BC182 23.4V R12 1kΩ C4 15pF 1.4V R7 680Ω 3mA TR2 BC182 0V Input R1 10kΩ Zin = 10kΩ Max gain = 7x inverting Output R11 2.2kΩ 0.5W C6 + 22µF 35V 11mA sink C1 22µF 15V TR1 BC182 0.6V R2 1.5kΩ 0V 0V AC feedback DC feedback 0V R3 22kΩ C7 22µF 35V 24V 0.8V + The Early effect 220V Output Fig.3: a passive DI box circuit. are placed one on top of the other, in series, as shown in Fig.4. The supply voltage drop is shared by both devices, allowing a low-voltage device, in this case the JFET, to be used for the lower input device. There’s nothing stopping you from cascoding bipolar junction transistors (BJTs), or even a combination of BJTs, JFETs, Mosfets or even valves. For guitar practice amps, I like to use a JFET at the bottom with a valve on top. A dual-gate Mosfet can be considered a ‘cascode in a can’! There are other advantages to the cascode configuration that are not of great importance here, but are worth knowing. This simple circuit gives an opportunity for experimentation. V+ + Guitar output R4 22kΩ C2 + 22µF 15V 0V Volume VR1/Rf 50kΩ Log CW Hitachi are considered the best. In the quest for maximum power, the cascode is used less in discrete power amplifiers because it loses a couple of volts of headroom for a given supply voltage. It is still used frequently in high-quality JFET audio op amps, where there is virtually no limit on the number of transistors that can be used. The first time I saw a cascode was in Douglas Self’s 1979 Wireless World pre-amp, where it was employed in the main active gain stage (Fig.5). This three-transistor circuit gave the same performance as the NE5534 op amp at a tenth the price (at the time). I upped the power rail on mine to the standard telecom/phantom 48V and used a Bourns 91 conductive plastic volume control. An interesting tweak is the extra resistor to boost the current to the lower transistor (R8), since this stage usually saturates first. Sadly, this lovely preamp was stolen one jazz night at the Hebden Bridge Trades Club in 1993. I’ve been meaning to build another ever since. 51 “Root canal” fuzz One overlooked problem with the cascode is its horrid overload waveform; modern power amps are expected to clip cleanly. I use it here to give an unusual fuzz effect, where the top of the square wave starts collapsing in on itself (Fig.6), generating a rash of extra ‘harsh’ high-frequency harmonics. A practical circuit Fig.8: the prototype circuit on stripboard. Note that the Vigortronix output transformer has a 2:1 step-down ratio, reducing the minimum gain to unity, while doubling the output current available. The fuzz switch, SW2, just removes all NFB, giving full gain. A treble boost function is obtained by bypassing some of the NFB to ground via capacitor C3. This function is switched in with SW1. Interestingly, jazz guitarists like the effect, while rock guitarists hate it. The whole circuit is non-inverting and the output transformer should be wired to ensure the phasing is correct. Most mixers have a phase invert button. If yours doesn’t, it may be worth incorporating one on the output transformer, since some guitar amps are inverting. +36V 3.2mA R3 22kΩ R5 15kΩ 52 C6 100µF 35V +14.7V 6.8kΩ 13V 1.2mA TR1 2N5457 *T1 VR2 1kΩ R6 6.8kΩ *Output transformer Vigortronix 600Ω 2+2:1+1 Repanco/OEP TT3 3.6:1+1 LT44 Economy Gain CW + Max Treble boost NFB C4 + 22µF 15V +1.7V VR2 1kΩ Trim sym clip ON SW1 6.8kΩ R9 100Ω R7 750Ω 2mA +10.7V 0V 0V Feed phantom power via mixer +48V Integral 6.8kΩ resistors) 13V +11.4V R4 10kΩ Output to guitar amplifier TR3 BC559 TR2 BC549 Cascode stage VR1 1kΩ Fig.6: the strange clipping effect of this cascode circuit. There is not much to say here since the circuit is experimental, but it lends itself to stripboard construction, as For the power supply, a centre tap on C3 330nF C2 470pF Construction Power supply C1 10nF R2 2.2MΩ the output transformer is needed. The phantom power is fed through 6.8kΩ resistors mounted in the mixer (shown faded on the diagram). This supply is decoupled by capacitor C7. Pin 1 (0V) on the XLR connector is fed through ground-lift resistor R11 and bypassed for RF by capacitor C8, then connected to signal ground. The internal connection between the phantom power supply 0V and op amp power supply 0V in the mixer could cause an Earth loop. It might be worth fitting a ground-lift resistor here as well. Originally, phantom power was designed for microphones, which have no mains Earth connection. This is an area that needs further investigation. + Finally, we get to a buildable circuit, shown in Fig.7. The input is AC-coupled by capacitor C1, while the combination of resistor R1 and capacitor C2 act as a low-pass filter to provide a degree of RF rejection. The input impedance is defined by resistor R2; I find that 2.2MΩ gives the best results with most guitars. The input signal is applied to the gate of N-channel JFET TR1. This can be any low-current, low-IDSS type, such as the 2N5457 shown here. The J202, J305 or BF245 are also suitable. The popular 2N3819 and similar types need too much current, which would overload the phantom supply. Since the gate source voltage (VGS) for a given channel current (ID) varies so much between individual JFETs, preset VR2 is needed to trim for symmetrical clipping. Trimmers are bad for mass production, but interesting for tweaking. Some guitarists like asymmetrical clipping for the even harmonics it produces, similar to how a 12-string guitar has extra strings tuned an octave higher. The upper transistor of the cascode is a high hFE device because the partition noise is less. Since the voltage is shared, a low-voltage 30V transistor can be used, such as a BC549C. The cascode stage gives a gain of around 15×. The signal is then directly coupled to a second PNP BJT transistor, boosting the open-loop gain Fig.7: the to around 400×. complete circuit The final gain for the JFET is controlled by cascode DI box. negative feedback (NFB) through resistor R6. The minimum gain is around 2× with the gain control High impedance pot, VR2, fully R1 guitar input 1kΩ anticlockwise. XLR output to mixer 3 – C5 22µF 15V + 2 Fuzz (open loop) C7 + 100µF 50V 1 R10 100Ω C8 1nF ON R8 100kΩ SW2 Ground lift Practical Electronics | June | 2025 NEW! 5-year collections 2019-2023 Fig.9: the wired assembly, tested but not yet in a box. shown in Fig.8. The output transformer is quite heavy, so it is a good idea to use a separate mounting PCB for it, such as the ones described in Audio Out, November 2022. Fig.9 shows the PCB wired up to the transformer, connectors and pots before they were installed in a case. A 6.35mm TRS jack could be substituted for the XLR connector. I was unable to create a proper PCB or detailed construction plans since I just moved and haven’t completed my horrendous house/workshop repairs (the old wiring needs a lot of attention!). While on the subject, AOShop is now located at Manchester House, Market Street, Craven Arms, Shropshire SY7 9NN. The phone number and email are still the same as before: 01597 829 102 PE & jrothman1962<at>gmail.com. Parts List – Phantom-powered DI box 1 2×2-inch (~50×50mm) piece of Veroboard 1 Vigortronix VTX-101-002 2.4kΩ to 600Ω transformer, 2+2:1+1 (T1)* 1 male XLR 3-pin chassis-mount connector 2 6.3mm mono jack sockets Qty Value 4-band code 5-band code 2 miniature SPST toggle switches 1 2 .2 M W 1 10kΩ logarithmic potentiometer (VR1) 1 100k W 1 1kΩ trimpot (VR2) 1 22k W Semiconductors 1 2N5457 N-channel JFET, TO-92 (TR1) 1 BC549C NPN transistor, TO-92 (TR2) 1 BC559 PNP transistor, TO-92 (TR3) 1 1 1 1 1 2 15k W 10k W 6 . 8 kW 1 . 0 kW 750 W 100W 1 15kΩ metal film (R5) 1 10kΩ metal film (R4) 1 6.8kΩ metal film (R6) * Alternatives: Repanco TT3/OEP E187B 3.6:1+1 Jaycar MM-2532 1:1 (cheap) Practical Electronics | June | 2025 PDF files ready for immediate download 2018-2022 All 60 issues from Jan 2018 to Dec 2022 for just £49.95 PDF files ready for immediate download 2017-2021 All 60 issues from Jan 2017 to Dec 2021 for just £49.95 PDF files ready for immediate download 2016-2020 Capacitors 1 100µF 50V electrolytic (C7) 1 100µF 35V electrolytic (C6) 2 22µF 16V electrolytic, aluminium or tantalum (C4, C5) 1 390nF (preferred) or 330nF polyester (C3) 1 10nF polyester (C1) 1 1nF ceramic (C8) 1 470pF NP0/C0G ceramic (C2) Resistors (all ±1% ¼W) 1 2.2MΩ (R2) 1 100kΩ (R8) 1 22kΩ metal film (R3) All 60 issues from Jan 2019 to Dec 2023 for just £49.95 All 60 issues from Jan 2016 to Dec 2020 for just £44.95 PDF files ready for immediate download 1 1kΩ (R1) 1 750Ω (R7) 2 100Ω (R9–R10) Eagle LT722 1.4:1 (cheap) 600Ω 1:1 modem transformer (cheapest) See page 41 for further details and other great back-issue offers. Purchase and download at: www.electronpublishing.com 53