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Build the Valve So Well, we know that there are lots of valve enthusiasts out there who believe that valves are just better; much more musical and pleasant to listen to than those sterile solid-state circuits with oodles of negative feedback and vanishingly small harmonic distortion. Of course, valve amplifiers do have drawbacks, like heat and fragility, but what if you could get ‘valve sound’ from a solid-state state amplifier? Well now you can, with our Nirvana Valve Sound Simulator. I T’S BEEN completely against the grain but we have now designed a solid-state circuit which deliberately distorts. Our Publisher, Leo Simpson, has had to be hand-cuffed, blind-folded, muzzled and otherwise restrained from doing what he normally does – driving us towards perfection. Now we have taken another ‘path’ to produce the ‘desirable and musical’ effects of valve amplifier circuitry. OK, OK, we know that if you want genuine, true ‘valve sound’, the only recourse is to use a valve amplifier. But we are presenting another way to musical nirvana which musicians 32  Silicon Chip commonly follow; using a solid-state amplifier with in-built valve circuitry simulation. This way, it’s the valve sound you have without using valves. Our Nirvana Valve Sound Simulator can be connected in series with any solid-state mono or stereo amplifier. It can be used by musicians or in the home for normal music listening. It lets you hear what valve sound is all about so you don’t have to go to the expense of replacing a perfectly good solidstate amplifier with a valve amplifier. What does it do? When a valve amplifier (sometimes called a tube amplifier) is compared objectively with a modern solid-state amplifier, the results can be somewhat uncomplimentary. The valve amplifier will typically have much higher distortion, more noise, more hum and certainly a less than straight-line frequency response when driving real loudspeakers. But the sum total of those effects is what valve amplifier enthusiasts want: a mellower, softer and (it’s claimed) more ‘musical’ sound. Our Nirvana Valve Sound Simulator does not add noise and hum but it will produce the same effects on the signal as a valve amplifier: softer symmetrical siliconchip.com.au Nirvana By JOHN CLARKE und Simulator or asymmetrical clipping at the point of overload, mainly even-order harmonic distortion similar to the effects of a single pentode valve stage and a frequency response similar to that of a good quality class-AB valve amplifier with transformer coupling to the loudspeaker. We simulate the pentode valve stage effect by using a FET source-follower in the signal path. The soft clipping effect is achieved in the same FET source-follower stage and it is fully adjustable for degree, asymmetry etc. We also need to simulate the effect of a valve amplifier’s output impedance on the frequency response of a typical loudspeaker. This is where solid-state amplifiers have a big advantage over valve amplifiers. Well-designed solidstate amplifiers usually have an output UPPER BASS RESONANCE MID-BAND IMPEDANCE HUMP HIGH FREQUENCY RISE IMPEDANCE LOWER BASS RESONANCE impedance which is less than onesixtieth of the nominal impedance of a loudspeaker, ie, something less than 150 milliohms. By contrast, a valve amplifier will typically have an output impedance of about 2Ω, depending on how much negative feedback is applied from the output terminals back to the earlier stages. The relatively high output impedance of the valve amplifier has two effects when driving loudspeakers. The first effect is a much lower ‘damping factor’ which is the ratio of nominal loudspeaker impedance to the amplifier’s output impedance. For a solid-state amplifier, we expect to see damping factors of 60 or more and that means that the amplifier has very tight control over the movement of the loudspeaker cone. This leads to less ‘boomy’ bass and lower distortion of bass frequencies. An equally important advantage of a very low output impedance is a much more linear frequency response from all loudspeakers. This is because loudspeakers do not have a constant impedance, but one that varies widely with frequency. So ideally, a loudspeaker should be driven from a voltage source and that means having a low output impedance amplifier. With the much higher output impedance of a valve amplifier, the considerable variations in a loudspeaker’s impedance over the frequency range means that the overall response will be much ‘bumpier’ or less smooth. Say, for example, a valve amplifier has an output impedance of 2 ohms and the loudspeaker has a nominal output impedance of 8 ohms. That means that 25% of the drive signal will be lost within the amplifier itself. But the effect is much worse because the loudspeaker’s impedance varies from less than 6 ohms to more than 30 ohms. Fig.1 shows a bass reflex loudspeaker impedance curve. Typically, these have a double hump at low frequencies, may rise to a second broad peak at the mid-frequencies (depending on the effect of the crossover network) and then rise again at the high end, due to the inductance of the tweeter. By contrast, loudspeakers in sealed cabinets have only one peak at the low frequency end. Any increase in impedance above the nominal value (eg, 8 ohms) at a particular frequency will result in a boost to the loudspeaker’s response, while a reduction will result in a drop in the response – see Fig.3. This diagram depicts the effect on the frequency response of four loudspeaker systems, 1kHz 10kHz FREQUENCY Fig.1: a typical bass reflex loudspeaker impedance curve. As shown, there’s a double hump at low frequencies, with the impedance then rising to a broad peak at the mid-frequencies (depending on the effect of the crossover network) and then rising again at the high end, due to the inductance of the tweeter. 10Hz siliconchip.com.au 100Hz August 2014  33 +15V 100 µF 100nF LEFT IN 47pF 22k 8 3 IC1a 2 4 22k VR1a 50k INPUT 10 µF Q1 2N5485 G S VR2 10k CLIPPING LEVEL 1M 1M 1.5k 1 µF 10k 2 MMC RING 3 IC1: LM833 TIP CON3 D 100nF 470pF 820Ω +9V A TP1 –15V –15V ZD3 9.1V 100 µF VR4 10k 10k +9V RIGHT IN 22k 8 IC2a 4 22k SLEEVE 47pF 6 7 VR5 10k λ K LED2 –PEAK IC2: LM833 10 µF Q2 2N5485 G 1M 1M 1.5k CLIPPING LEVEL 1 µF 10k 6 MMC 5 IC2b 620Ω 7 A +15V K K D1 1N4004 9–12VAC INPUT 10Ω CON1 S1 A A ZD1 15V 1W 470 µF 16V 4.7k R5* 10Ω K D2 1N4004 A λ A S VR3 10k VR1b 50k A TP2 470pF 820Ω –15V K D 100nF 10k 620Ω 1 LED1 +PEAK 5 IC1b 35V K –15V 1 100 µF 100nF 270Ω 35V K A ZD2 15V 1W 470 µF 16V λ LED5 λ K K LED4 –PEAK λ A R6* DC INPUT + 0V – A LED3 +PEAK * SEE TEXT CON2 R7* R8* K –15V SC 20 1 4 NIRVANA VALVE SOUND SIMULATOR previously published in SILICON CHIP, when driven by an amplifier with an output impedance of 4 ohms. As you can see, the main areas of boosting occur at the two bass resonances and at the mid-band impedance hump. For example, with the JV100 loudspeaker depicted at the top of Fig.3, the boost is as much as +3.9dB. Similarly, there is a broad boost to the response of more than +3dB from around 500Hz to 1.5kHz and a smaller boost to the tweeter at 34  Silicon Chip the high-frequency end. By contrast, if the same loudspeaker is driven by a solid-state amplifier with a typical output impedance of less than 150 milliohms, there is no boost or cut, as it should be! The Nirvana simulates these loudspeaker frequency deviations with a number of individually adjustable filters which are varied by the “Loudspeaker Response” control. The selection of a particular loudspeaker for simulation requires choosing a particular set of component values, to be discussed later in this article. The other control on the front panel of the Nirvana Valve Sound Simulator is for ‘Clipping Level’. If you want to delve more into valve sound, here are some interesting sites: (1) http://spectrum.ieee.org/consumer-electronics/audiovideo/the-coolsound-of-tubes (2) http://spectrum.ieee.org/consumer-electronics/audiovideo/the-coolsound-of-tubes/distortion siliconchip.com.au +15V 100 µF 35V –15V 47k LEFT OUT 4 6 7 IC3b 5 NP 11 RIGHT OUT 10 10 µF 150Ω 47k 8 IC3c 9 150Ω 10 µF SLEEVE VR6b 10k 100k LOUDSPEAKER RESPONSE 2.2pF C2L* R2L* C2R* 2 3 C1L* IC3a R1R* 1 R2R* 13 C3R* 12 C1R* 1M IC3d 14 1M LOWER BASS RESONANCE HIGH FREQUENCY RISE R3L* C5L* +15V 4 6 5 IC4b 7 100 µF 35V 11 C7L* C4R* R4R* 8 13 C7R* 100k IC4c UPPER BASS RESONANCE C6R* IC4a 10 IC4: TL074 1 9 1M 2 3 R3R* C5R* –15V UPPER BASS RESONANCE R4L* LOWER BASS RESONANCE HIGH FREQUENCY RISE 1M C6L* CON4 100k 2.2pF IC3: TL074 C3L* C4L* TIP NP –15V VR6a 10k R1L* OUTPUT RING 12 IC4d 14 100k MIDBAND HUMP * SEE TEXT MIDBAND HUMP 2N5485 LED1–5 D1, D2 A ZD1–3 K A S K K A G D Fig.2: the complete circuit of the Nirvana Valve Sound Simulator. The input signals from CON3 are amplified by IC1, then distorted and clipped by JFETs Q1 & Q2. IC2 provides an indication of clipping symmetry while IC3 & IC4 act as parametric equalisers to adjust the frequency response to match that of a typical valve amplifier driving loudspeakers. (3) http://en.wikipedia.org/wiki/Tube_ sound In use, the Nirvana Valve Sound Simulator connects between the preamplifier outputs and the power amplifier inputs of a solid-state amplifier. In amplifiers with a tape loop you can use this facility, while for a musician’s (eg, guitar) amplifier, it would be connected into the effects loop. As shown in the photos, the unit is housed in a compact case and can be powered from an AC plugpack. siliconchip.com.au Alternatively, balanced DC supply rails could be obtained from existing equipment. The socket for the AC supply is accessed from the rear, as are the 3.5mm stereo input and output sockets. Circuit details Refer now to Fig.2 for the circuit details. Each channel uses six op amps (all in four ICs) and a JFET, and both channels are identical. The input signal is applied via CON3, a stereo 3.5mm jack socket. If only a mono signal is required, then a mono jack plug can be used to apply signal to the left channel only. This will connect the ring terminal to ground and so prevent signal in the right channel. The following circuit description is for the left channel signal path. As shown, signal is applied via the tip connection of CON3 and is reduced by a factor of two, using two 22kΩ resistors, so that line-level signals will not necessarily cause clipping in the August 2014  35 4.2dB 3.9dB 3.6dB 3.3dB 3.0dB 2.7dB 2.4dB 2.1dB 1.8dB 1.5dB 1.2dB 0.9dB 0.6dB 10Hz Speaker Simulation JV100 24° 21° 18° 15° 12° 9° 6° 3° 0° -3° -6° -9° 100Hz 1kHz 10kHz 4.4dB 24° 4.0dB 21° 3.6dB 18° JV80 3.2dB 15° 2.8dB 12° 2.4dB 9° 2.0dB 6° 1.6dB 3° 1.2dB 0° 0.8dB -3° 0.4dB -6° 0.0dB 10Hz -9° 100Hz 1kHz 10kHz 3.0dB 16° 14° 12° 10° 8° 6° 4° 2° 0° -2° -4° -6° -8° 2.7dB 2.4dB JV60 2.1dB 1.8dB 1.5dB 1.2dB 0.9dB 0.6dB 0.3dB 0.0dB -0.3dB 10Hz 3.6dB 3.3dB 3.0dB 2.7dB 2.4dB 2.1dB 1.8dB 1.5dB 1.2dB 0.9dB 0.6dB 0.3dB 0.0dB 10Hz 100Hz 1kHz 10kHz 21° 18° 8-Inch Woofer & Piezo Horn 15° 12° 9° 6° 3° 0° -3° -6° -9° 100Hz 1kHz 10kHz Fig.3: these curves simulate the wide deviations from a flat frequency response for four loudspeakers previously published in SILICON CHIP, caused by the interaction of the varying loudspeaker impedance with the typical 4-ohm output impedance of a valve amplifier. The amount of boost can be seen on the left-hand vertical axis (in dB) while the deviation in phase is shown in the dotted curves and the corresponding right-hand vertical axis (in degrees). These same effects can simulated with the Loudspeaker Response control of the Nirvana Valve Sound Simulator. following JFET stage if op amp IC1a is set for minimum gain. IC1a’s gain can be varied between 1.2 and 13 by potentiometer VR1a which sets the signal clipping level in the JFET stage. When VR1a is set for minimum gain, the input signal needs to reach 1.66V RMS before clipping Main Features • • • • • • • • Simulates the frequency response of a valve amplifier when driving loudspeakers Provides mainly even-ordered harmonic distortion, ie, second, fourth, sixth etc Input level control sets distortion threshold and clipping Soft clipping on overload Clipping indicators for positive and negative signal excursions Clipping symmetry can be adjusted One of four different loudspeaker responses can be used or design your own Can run from a 9-12VAC supply (eg, a plugpack) or a ±12VDC to ±45V DC dual supply (eg, from existing equipment) 36  Silicon Chip occurs and when VR1a is set for maximum gain, the input signal only needs to reach 109mV RMS before clipping. Following IC1a is the JFET amplifier stage, Q1. This is configured as a source follower (similar to a bipolar transistor emitter-follower or a valve cathode-follower). The JFET produces harmonic distortion similar to that in pentode valve stages (predominantly even harmonics) and it also produces soft signal clipping when overloaded. The signal from IC1a is fed to the Q1’s gate via a 100nF capacitor, while the signal output is taken from Q1’s source. Trimpot VR4 adjusts Q1’s operating current and this varies the symmetry of clipping, ie, whether the siliconchip.com.au signal clips symmetrically or whether it clips the positive or negative signal swings more severely. IC2a drives the positive and negative clipping indicators. It compares the input and output signals of Q1. When the signals differ, such as when Q1 is clipping, the output of IC2a swings high or low to drive LED1 (positive clipping) or LED2 (negative clipping). For this indication to be accurate, IC2a’s gain needs to be carefully adjusted to be equal to the gain of Q1, using trimpot VR2 (or VR3 in the right channel). Loudspeaker simulation The output signal from Q1 is then fed to the loudspeaker simulator section which comprises op amps IC3b, IC3a, IC4b & IC4a (the equivalent functions in the right channel are provided by IC3c, IC3d, IC4c & IC4d). IC3b can be regarded as the main op amp and its feedback network is modified by op amps IC3a, IC4a & IC4b which can each be regarded as singlefrequency equalisers, much like those used in gyrator-based graphic equalisers. The difference is that we have no slider controls to vary the individual equalisers. The maximum gain at high frequencies is set by ‘high-frequency rise’ components R1L and C1L and the overall gain is set by VR6a, the Loudspeaker Response control. IC3a is the equaliser providing the simulated lower frequency impedance peak in a bass-reflex loudspeaker system. IC4b adds the upper bass peak for bass-reflex systems and the main peak in sealed systems. In the latter case, IC3a is effectively disabled and has no effect on the overall frequency response. Finally, IC4a provides a mid-band impedance hump that may be present with some speaker systems. So each of the three equalisers boosts a defined frequency band about a certain centre frequency. By selecting the values of the capacitors and resistors, we can set the required tuning frequency and shape of the boost. We have designed the speaker impedance simulation circuitry using LTSpice (see www. linear.com/designtools/software/). This SPICE simulation program from Linear Technology can be used with Windows or Mac operating systems. The circuit file for this loudspeaker simulation (Valve Simulator.asc) is available on our website. You can siliconchip.com.au Parts List 1 double-sided PCB, code 01106141, 129.5 x 100mm 1 front-panel artwork, 132 x 27mm OR 1 front-panel PCB, code 01106142 1 rear panel artwork, 132 x 27mm 1 ABS instrument case, 140 x 110 x 35mm (Jaycar HB-5970, Altronics H 0472) 1 9-12V 50mA AC plugpack (optional, see text) 1 PCB-mount DC socket (CON1) 1 3-way PCB-mount screw terminal block, 5.08mm pitch (CON2) 2 3.5mm PCB-mount stereo jack sockets (CON3,CON4) 1 SPDT PCB-mount toggle switch (S1) (Altronics S 1421) 1 16mm dual-gang 50kΩ linear potentiometer (VR1) 1 16mm dual-gang 10kΩ linear potentiometer (VR6) 4 10kΩ horizontal trimpots (VR2VR5) 2 knobs to suit potentiometers 2 DIL8 IC sockets (optional) 2 DIL14 IC sockets (optional) 4 No.4 x 6mm self-tapping screws 4 PC stakes (GND,GND,TP1,TP2) 1 100mm length of 0.7mm tinned copper wire Semiconductors 2 LM833 op amps (IC1,IC2) 2 TL074 quad op amps (IC3,IC4) 2 2N5485 JFETs (Q1,Q2) 2 3mm high-intensity red LEDs (LED1,LED3) 2 3mm high-intensity blue LEDs (LED2,LED4) 1 3mm high-intensity green LED (LED5) 2 15V 1W zener diodes (ZD1,ZD2) 1 9.1V 1W zener diode (ZD3) 2 1N4004 1A diodes (D1,D2) change the values and set the loudspeaker simulation curve yourself if you wish. Otherwise, we have a table that produces impedance curves for some typical loudspeakers. Power supply Power for the circuit can come from an AC plugpack (9-12V) rated at 50mA or more. Alternatively, positive and negative DC supply rails from existing equipment can be used. In the latter case, power is applied via CON2. Resistors R5, R6, R7 & R8 are used Capacitors 2 470µF 16V PC electrolytic 5 100µF 35-63V PC electrolytic 2 10µF 16V PC electrolytic 2 10µF 16V NP PC electrolytic 2 1µF monolithic ceramic 4 100nF MKT 2 470pF ceramic 2 47pF ceramic 2 2.2pF ceramic Selected capacitors JV100 simulation: 2 x 330nF, 2 x 150nF, 2 x 47nF, 2 x 22nF, 2 x 6.8nF & 2 x 1nF MKT, plus 2 x 470pF ceramic JV80 simulation: 2 x 270nF, 2 x 100nF, 2 x 56nF, 2 x 22nF, 2 x 6.8nF & 2 x 1nF MKT JV60 simulation: 2 x 120nF, 2 x 82nF, 2 x 22nF, 2 x 12nF, 2 x 6.8nF & 2 x 1nF MKT, plus 2 x 470pF ceramic 8-inch woofer with piezo horn simulation: 2 x 270nF, 2 x 100nF, 2 x 33nF & 4 x 4.7nF MKT Resistors (0.25W, 1%) 8 1MΩ 2 1.5kΩ 4 100kΩ 2 820Ω 2 47kΩ 2 620Ω 4 22kΩ 1 270Ω 4 10kΩ 2 150Ω 1 4.7kΩ 2 10Ω Selected resistors JV100 simulation: 2 x 22kΩ, 4 x 12kΩ, 2 x 10kΩ JV80 simulation: 2 x 33kΩ, 4 x 10kΩ JV60 simulation: 2 x 22kΩ, 4 x 12kΩ, 2 x 10kΩ 8-inch woofer with piezo horn simulation: 2 x 10kΩ, 4 x 8.2kΩ Power supply resistors R5-R8: see text & Table 1 when the external supply is 15V or more. They provide the voltage drop for 15V zener diodes ZD1 and ZD2. Table 1 on the following page shows the resistor values required for various supply voltages. Construction The construction is straightforward with all the parts mounted on a PCB coded 01106141 and measuring 129.5 x 100mm. This is housed in a small instrument case measuring 140 x 110 x 35mm (W x D x H). August 2014  37 47pF A VR1 50kΩ LED5 LED1 LED2 A 10 µF NP 10 µF NP R4R C6R C5R C4R R3R IC3 R1R TL074 C3R 1M 2.2pF A C7R IC4 TL074 1M C3L A 150Ω 47k 47k R4L C7L C6L C5L 1M R3L C4L 1M R2L C2L R1L 100 µF A GND 620Ω 620Ω 1M 820Ω 22k 47pF 100 µF 10k IC2 LM833 10k 470pF 100nF 22k S1 10k IC1 LM833 10k 4004 470pF 820Ω 4004 4.7k 10Ω VALVE SIMULATOR 14160110 01106141 C 2014 D2 100nF 22k R6 100nF VR3 10k VR2 10k D1 100k 100 µF 1 µF 100nF 22k 15V 1W 15V 1W R8 1 µF ZD1 ZD2 100k TP2 2N5485 Q2 1.5k 1M 1.5k R7 R5 + 2N5485 Q1 150Ω 100k 100 µF TP1 470 µF 470 µF 100k 1M 1W R 10 µF 100 µF 9.1V 10Ω 10 µF VR5 10k 270Ω + R VR4 10k The PCB is fastened into the case using four selftapping screws which go into integral corner pillars. CON4 L R2R ZD3 OUTPUT CON3 L C2R INPUT GND CON2 +V 0V –V 1M CON1 9V to 12V AC in 2.2pF VR6 10kΩ LED3 LED4 C1L C1R Fig.4: follow this parts layout diagram to build the PCB. Resistors R1-R4 and capacitors C1-C7 in the filter networks are selected from Table 2, while the power supply resistors (R5-R8) are selected from Table 1 (see text). Table 1. Dropping Resistors For External Dual Supply Rails Supply Voltage R5 R6 R7 R8 ±45VDC 2.7kΩ 1W 2.7kΩ 1W 2.7kΩ 1W 2.7kΩ 1W ±40VDC 2.2kΩ 1W 2.2kΩ 1W 2.2kΩ 1W 2.2kΩ 1W ±35VDC 1.5kΩ 1W 1.5kΩ 1W 1.5kΩ 1W 1.5kΩ 1W ±30VDC 620Ω 1W – 620Ω 1W – ±25VDC 390Ω 1W – 390Ω 1W – ±20VDC 220Ω 1/2W – 220Ω 1/2W – ±15VDC 10Ω 1/2W – 10Ω 1/2W – ±12VDC 10Ω 1/2W – 10Ω 1/2W – Note: a dash (–) means that no component is installed. Before installing any of the parts, you need to use Table 2 to select the required values for resistors R1-R4 and capacitors C1-C7 to simulate a particular speaker. These values depend on the speaker load that is being simulated, as explained earlier. Basically, Table 2 shows the values required to simulate various loudspeaker loads. In other words, you can simulate the sound of a valve amplifier driving one of these types of speakers. If you don’t have a preference, we suggest using the JV80 values. Alternatively, you can determine your own component values based on LTSpice Table 2: R & C Values For Vented, Sealed & Piezo Horn Loudspeakers HF Rise First Impedance Peak Second Impedance Peak Midband Hump Loudspeaker VR6 Setting C1 R1 C2* C3* R2* C4 C5 R3 C6 C7 R4 JV100 (8-ohm) 5.6kΩ 470pF 22kΩ 330nF 22nF 12kΩ 150nF 6.8nF 10kΩ 47nF 1nF 12kΩ JV80 (8-ohm) 5.6kΩ – – 270nF 22nF 10kΩ 100nF 6.8nF 10kΩ 56nF 1nF 33kΩ JV60 (4-ohm) 3.9kΩ 470pF 22kΩ 120nF 22nF 12kΩ 82nF 6.8nF 10kΩ 12nF 1nF 12kΩ 8-inch speakers, with piezo horn (8-ohm) 3.9kΩ 4.7nF 8.2kΩ 270nF 33nF 8.2kΩ 100nF 4.7nF 10kΩ – – – Note 1: R & C numbers show an ‘L’ suffix for the left channel components and an ‘R’ suffix for the right channel components on the circuit and PCB layout. Note 2: * denotes no component for a sealed enclosure. Note 3: VR6 setting shown is for 4Ω output impedance amplifiers. VR6 is set to a lower resistance for lower output impedance. Note 4: a dash (–) means that no component is installed. 38  Silicon Chip siliconchip.com.au 3-way screw terminal block CON2 is necessary only if you are using an external split DC supply. Now for the two potentiometers (VR1 & VR6). Before fitting them, cut their shafts to suit the knobs using a hacksaw and clean up the ends with a file. It’s also necessary to file away a small area of the passivation layer at the top of each pot body, to allow an earth wire to be later soldered in place (see Fig.4). The pots are then fitted to the PCB, noting that VR1 is 50kΩ and VR6 is 10kΩ. Push them all the way down onto the PCB before soldering their pins. The two 3.5mm jack sockets (CON3 & CON4) can go in next, followed by PC stakes for TP1 & TP2 and at the two GND positions (one to the right of VR1 and one to the left of CON3). Installing the LEDs simulation, as explained earlier. You also need to decide on the power supply that you will be using and select resistors R5-R8 from Table 1 if using an external split DC supply (ie, one with positive and negative supply rails). This could come from a power amplifier or preamplifier, for example. Alternatively, resistors R5-R8 are not required if using an external 9-12VAC plugpack supply. Fig.4 shows the parts layout on the PCB. Begin the assembly by installing the resistors. Table 3 shows the resistor colour codes but you should also check each one using a DMM before mounting it in place. Follow with the IC sockets, diodes D1 & D2, zener diodes ZD1-ZD3 and trimpots VR2-VR5. Take care to ensure that the diodes and zener diodes are orientated correctly and note that the IC sockets all face in the same direction (ie, pin 1 at top left). The capacitors are next on the list. Table 4 shows the codes used on the smaller ceramic and MKT types. Be sure to orientate the polarised electrolytic types correctly and note that the two 10µF electrolytics at top right are non-polarised (NP). Switch S1 and power socket CON1 are necessary only if using the AC plugpack for the supply. Conversely, Table 3: Resistor Colour Codes o o o o o o o o o o o siliconchip.com.au o o No.   8   4   2   4   4   1   2   2   2   1   2   2 Value 1MΩ 100kΩ 47kΩ 22kΩ 10kΩ 4.7kΩ 1.5kΩ 820Ω 620Ω 270Ω 150Ω 10Ω 4-Band Code (1%) brown black green brown brown black yellow brown yellow violet orange brown red red orange brown brown black orange brown yellow violet red brown brown green red brown grey red brown brown blue red brown brown red violet brown brown brown green brown brown brown black black brown The five LEDs are installed with their leads bent down through 90°, so that they later protrude through matching holes in the front panel. First, check that the anode (longer) lead is to the left (lens facing towards you), then bend both leads down through 90° exactly 8mm from the rear of the plastic lens. This is best done by folding them over a cardboard strip cut to 8mm wide. Once that’s done, install each LED so that its horizontal leads are exactly 4mm above the PCB. In practice, it’s just a matter of pushing each LED down onto a 4mm-thick spacer (eg, a cardboard strip) before soldering its leads. Use a green LED for LED5, red Table 4: Capacitor Codes Value 1µF 100nF 470pF 47pF 2.2pF µF Value IEC Code EIA Code 1µF 1u0 105 0.1µF 100n 104   NA 470p 471   NA   47p   47   NA   2p2    2.2 5-Band Code (1%) brown black black yellow brown brown black black orange brown yellow violet black red brown red red black red brown brown black black red brown yellow violet black brown brown brown green black brown brown grey red black black brown blue red black black brown red violet black black brown August 2014  39 brown green black black brown brown black black gold brown 06/24/14 11:19:31 Valve Sound Simulator Spectral Response +9 -10 +8 -20 +7 -30 +6 -40 +5 -50 +4 Amplitude Variation (dBr) Spectral Power (dBV) 0 -60 -70 -80 -90 -100 +1 -3 -4 -140 -5 1k 2k Frequency (Hz) 5k 10k 20k Fig.5: spectrum analysis of the output signal (1kHz input), showing strong second harmonic distortion along with third, fourth, fifth and sixth harmonics at lower levels. LEDs for LEDs1&3 and blue LEDs for LEDs2&4. The PCB assembly can now be completed by earthing the pot bodies to the GND PC stake next to VR1. That’s done using a length of 0.7mm-diameter tinned copper wire (see Fig.4 and photos). You can straighten the tinned copper wire by clamping one end in a vice and then stretching it slightly by pulling on the other end with pliers. It can then be bent to shape so that it contacts the GND stake and soldered. Minimum Loudspeaker Response -1 -130 500 Intermediate Loudspeaker Response 0 -2 200 Maximum Loudspeaker Response +2 -120 100 -6 20 50 100 Before installing the PCB assembly in the case, you have to drill a number of holes for the front and rear panels. The accompanying panel artworks (Fig.7) can be copied and used as drilling templates or you can download them (in PDF format) from the SILICON CHIP website and print them out. alve NirvanalaVtor Simu SILICON CHIP Power Clipping Level - + L Peak + R 200 500 1k Frequency (Hz) 2k 5k 10k 20k Fig.6: this graph shows the frequency response of the unit when set to simulate driving JV60s, with the Loudspeaker Response knob in three different positions. On the front panel, you will need to drill (and ream) a 5mm hole for switch S1, 3mm holes for LEDs1-5 and 7mm holes for the pot shafts. The two stereo jack sockets on the rear panel require 6mm holes, while the DC power socket requires a 6.5mm access hole. Once that’s done, print the artworks from the website onto photo paper and attach them to the panels using silicone sealant. The holes can then be cut out with a sharp hobby knife. Alternatively, you can purchase a PCB-based front panel (blue with white labels) with pre-drilled holes from the SILICON CHIP Partshop. After that, it’s just a matter of fitting the panels to the PCB, sliding the assembly into the case and securing the PCB to the four corner mounting pillars using No.4 self-tapping screws. The assembly can then be completed by pushing the knobs onto the pot Final assembly 06/24/14 11:04:52 +3 -110 -150 Valve Sound Simulator Frequency Response Loudspeaker Response shafts. Reposition the end pointers of the knobs if necessary, so that they correctly point to the fully anti-clockwise and fully clockwise positions. Testing If you haven’t already done so, insert the four ICs into their sockets, taking care to orientate them correctly. Next, apply power and check that the power LED lights. If that checks out, check the supply voltage between pins 8 & 4 of both IC1 and IC2 and between pins 4 & 11 of IC3 and IC4. This should be around 30V DC if you are applying 12VAC via CON1. Alternatively, you can apply ±12V DC or more via 3-way screw terminal block CON2. Note that you will only get around 25V (ie, ±12.5V) if using a 9VAC supply. Regardless, there should be about 9.1V across ZD3. Assuming these supply voltages are Fig.7: these two artworks can be copied and used as drilling templates for the front & rear panels. They can also be downloaded as a PDF file from the SILICON CHIP website. Power 9-12VAC Output 40  Silicon Chip Input siliconchip.com.au all correct, follow this step-by-step procedure to adjust the unit: Step 1: connect a DMM set to volts between TP1 and a GND stake and adjust VR4 for a reading of 5.8V. Similarly, adjust VR5 for a reading of 5.8V at TP2. This gives more or less symmetrical clipping for both Q1 and Q2. Step 2: apply a low-level 1kHz signal to both the left and right inputs and adjust VR2 & VR3 so that the positive and negative peak LEDs in both channels are off. You will find that there’s a ‘dead spot’ in each trimpot’s setting range where both LEDs are off. Set each trimpot to the middle of its dead spot. If the LEDs do not extinguish with this adjustment, try reducing the signal level using VR1 or at the signal generator (note: if you don’t have a signal generator, it’s easy to find a virtual instrument online). Step 3: increase the signal level so that the clipping LEDs begin to light. When that happens, readjust trimpots VR4 & VR5 to give symmetrical clipping, so that both the red and blue clipping LEDs light at the same time (ie, for the positive and negative signal excursions). Finally, note that the input and output sockets can be linked to RCA connectors via adaptor cables (ie, 3.5mm stereo jack plug to RCA). For mono use, a mono 3.5mm jack plug can be used in which case only the left channel will be supplied with signal and the right channel input will be grounded. A mono plug could then also be used for the output since the right channel SC will not have any output. The rear panel carries access holes for the input and output sockets and for the power socket. Note how the metal bodies of the two pots are earthed to the GND stake using a length of tinned copper wire. Fig.8: the output of the unit (green) compared to the input (yellow) at 1kHz. The signal level is set below clipping and the distortion residual (blue) is primarily second harmonic. This can be clearly seen as the residual is at twice the fundamental frequency, ie, 2kHz. Fig.9: the same traces as in Fig.8 but with more input signal, causing clipping. The effects of soft clipping and the frequency response shaping filter are evident. siliconchip.com.au Fig.10: the input signal is still being clipped here but now we have adjusted VR4 & VR5 to give asymmetrical clipping, resulting in a different type of distortion. August 2014  41