The VCF or Voltage Controlled Filter

VCF - the Doepfer A-100

 The Voltage Controlled Filter,  High or Low?

The voltage controlled filter is another very important block in the design of the analogue synthesiser. (Digital control and processing of most blocks within the synth has become more popular because of lower pricing, mass production, comm etc).

The video below covers the history of the Minimoog with some of the artists whom shaped our musical destiny in electronic music.

Passive filtering using capacitors and resistors

Below, figures 2 and 3 show how a capacitor passes high frequencies more readily than the lower spectrum using the Xc or capacitive reactance formula – thus creating high pass and low pass filters in a potential divider network. VR1 would simulate the capacitor C1 in Fig. 2. Note that this is a passive bypass filter.

VCF - high and low pass filters
High Pass and Low Pass Filters – corner frequencies of fig 2 and 3 = 1 / 2.π .R.C

Active Filters

VCFs are controlled bandwidth frequency amplifiers allowing for high and low frequency cut-off, throughput of a specific bandwidth, notch-filtering and a Q-factor slope control for attentuation.

VCF - the Doepfer A-100
Doepfer A100  –  Nina Richards Creative Common Licence ZoeB

Notch Filtering

(Q-factor, or quality factor determines bandwidth of a filter).

Notch filtering is a great way to reduce a frequency which may be causing interference, a deliberate action to cause a specific sound and of course is sometimes used to reduce acoustic feedback.  (this is another subject though).

VCF voltage controlled oscillator
Voltage Controlled Filter – VCF

The circuit on the LHS, the High Pass Filter explanation is given below.

All filters will have some Resistance, Inductance or Capacitance component incorporated. A capacitance will pass more current as the frequency rises and in an inductance the opposite is true.

The NPN or BC549C acts as a buffer amplifier with very high input impedance and low output impedance. The PNP BC559C is where the action happens.  The PNP transistor acts as an amplifier where the collector and emitter is coupled to a capacitor which varies in resistance according to the control voltage i.e. the more negative Cv the less resistance.  As the resistance becomes less the circuit starts passing only the higher frequency spectrum.

VCF - Band pass and Band stop Filters
Band pass and band stop filters
Filters – Notch, Band-pass, High-pass, Low-Pass

The Q-factor or Quality factor of a circuit determines how tight the bandwidth is controlled. Whereas we add resistances into an LC circuit to reduce the quality factor (Q-factor) in radio circuits we need the LC (inductance and capacitance) Q-factor often to be as high as possible to improve sensitivity.


Equalisers used in audio allow for any part of the audio bandwidth to be cut or boosted, usually +/- 6dB or +/-12dB. Parametric equalisers control the center frequency and bandwidth range as well making for finer and more detailed sound.


There is a lot of confusion over dB and the measurement thereof. The Sengpiel Audio website explains the difference between dBu and dBV very clearly and has an online converter.

Filters, VCOs, PLL, Ring Mod, AM, FM all play an integral role in the audio synthesiser. Modern synthesisers have become extremely sophisticated with the use of microprocessing and the use of digital electronics. Indeed, the circuit board may look so much more sparse than the old Minimoog but such is the way of technology.

Next:  Envelope Generation and ADSR (attack, delay, sustain, release)

Further Reading:  Active Filters – Rod Elliott.

ADSR – Attack, Decay, Sustain, Release

The VCA or Voltage Controlled Amplifier

The VCO or Voltage Controlled Oscillator

Vintage Synthesisers using Ring Bridge Modulation

Total Harmonic Distortion meter

The VCA or Voltage Controlled Amplifier

The VCA  – a Voltage Controlled Amplifier which doesn’t amplify.

The VCA, in this case used in an audio instrument such as an audio synthesiser is usually the last block in the chain.  VCAs have a control voltage, on peak control voltage the entire signal is allowed through, on zero volts or lower the signal is blocked.  VCAs can also be termed voltage controlled attentuators.

The VCA is usually of 2 quadrant or 4 quadrant type, the output is blocked when the input control voltage is at zero. Although a VCA can be made to amplify an input signal, here the VCA is used and predominantly so as an attentuator. 2 quadrant designs have no output when the CV is at or just below 0V, 4 quadrant designs invert the output signal, and with gain set to an absolute value of CV below 0V the gain then rises. Four quadrant designs are used in amplitude and ring modulation effects. VCAs are therefore ideal for envelope shaping. The  MOTM-190 can be switched to either 2 or 4 quadrant mode.

  • VCAs are often used in circuits for compressing, companding (compressing/expanding to improve S/N) and limiting.
  • DCAs or digital controlled amplifiers are another variant, describing the controller type which varies the gain.

Although years back VCAs were usually of the discrete type; getting to grips now with most VCAs one would be looking at a high quality, multi-bit controlled unit in an IC package.

VCA using an LM101
VCA using an LM101

The VCA above is gain controlled by FET 2N5457 which acts as a variable voltage resistor. The circuit is not unique, many opamps may and will have better performance but this is a typical design where the linear voltage change across the FET ultimately results in well controlled gain of the LM101 (or other).

Another circuit showing similar principles below.

vca - linear voltage control
Linear Voltage Control VCA

Although many circuit designers are using operational amplifiers in their circuits there are some that feel with a discrete component approach the output quality can be greatly improved. See below, under further reading.

As discussed before, the VCA is used in limiting, companding and compressing resulting in the use of additional controls which vary attack, delay and sustain, release times. (In synth language ADSR) or AGC (automatic Gain Control).

VCA - 2 and 4 quadrant multipliers
2 and 4 Quadrant Multipliers (Courtesy Texas Instruments LM13700)

Above we have the 2 quadrant multiplier or amplitude modulator and the ring modulator both using dual operational transconductance amplifiers, the LM13700 with linearizing diodes and buffers.

For the AM or 2-quadrant multiplier refer to chapters 20 and 21 of the application notes which explains the VCA.

Note the additional path in the RM where a resistor (RM) is installed between out and invert input. RC for Vin2 is a potentiometer.

In our previous article on RM we cover the 4 diode bridge circuit used in SSB (RF).

In our next article we look at how VCF or voltage controlled filters work.

Further Reading: 

VCA Techniques by the honourable guru Rod Elliott. Good solid write-up, as always.

Analog Devices – AN213 / AD539

The Ring-Bridge Modulator


Oscillators for the Newbie – Positive Feedback

Oscillators - Hartley and Colpitts

Some of the best known oscillators – for the beginner

For those interested in electronic circuits it’s almost impossible to stay clear of the humble oscillator. In it’s simplest form the oscillator generates an alternating current from direct current. It is used most often in radio receivers and transmitters, digital circuits (possibly timing, clock), audio effects and of course in function generators where the output is almost universally sine, sawtooth and square waves.

Relaxation with an Oscillator

In my opinion the easiest oscillator to make would be the free-running astable multivibrator using two transistors or using the 555 IC. The output is square wave and is usually running in the audio spectrum (but not always).

Astable Multivibrator - 2 transistor
Astable Multivibrator – 2 transistor – also known as a Relaxation Oscillator. Also 555 osc. on the LHS

Astable (running free)

In the interests of this page let’s look at the 2 transistor Astable multivibrator. Why astable?  There are 3 basic configurations used in multivibrators: astable, bistable (which has two stable states duh!) and the monostable. Guess which one only has one stable state?  The astable multivibrator is free running with a square wave output. It has two transistors with a positive feedback path made up of the cross connected capacitors, Q1 collector to Q2 base and Q2 collector to Q1 base. If Q1 conducts first due to possibly having a higher gain than Q2, capacitor C1 is charged through R2 which forces the Vbe of Q2 to rise until conduction which in turn charges C2. This process makes up the basics of an oscillator. The positive feedback is what enables these transistors to switch on and off, components R2, C1 form the time constant for Q1 low state (in this case) and R3, C2 form the time constant for Q2 high state (in this case).

Although oscillators come in many shapes and forms audio oscillators use predominantly resistances and capacitances (RC) in the tuning circuit compared to radio frequency oscillators which use inductances and capacitances (LC).

Positive versus negative feedback

Now in the real world when designing an audio amplifier the last thing we want is positive feedback which can cause the amplifier to go into oscillation. Positive feedback is when the output is fed back into the input but in phase. Negative feedback is out of phase.

RF Oscillators

In radio telecommunications it is guaranteed that the student will have the Colpitts, Hartley and Crystal Oscillator thrown into their studies. These are extremely popular due to their high performance, the Xtal (Crystal) being the most superior but locked to resonance of the Xtal.

Oscillators - Hartley and Colpitts
Hartley and Colpitts Oscillators – Wiki / Author Krishnavedala

Note the Hartley oscillator split inductance / capacitor configuration as opposed to Colpitts, split capacitance / inductance.

The crystal oscillator is a high performance oscillator, highly stable and very accurate.  Although Xtal oscillators boast temp stability, this is not entirely true. Early Xtal oscillators were mounted in temperature controlled ovens to reduce drift. These oscillators were often used in PLL circuits as the reference oscillator.

Next:  Wien Bridge sine wave oscillator.

The 741 Operational Amplifier

Operational amplifier circuit GAP/R- P65

Operational Amplifiers demystified – starting with the uA741

As a writer of some of the articles here we do try to keep the topic off common ground, throw in a bit of humour but give the reader some insight into components and circuit design. Op-amps we treat no differently.

Today we look at op-amps in general and specifically look at the brain child in 1968 of British Fairchild engineer, Dave Fullagar, the incredible 741 op-amp.

(note the charity work done by Mr. and Mrs. Fullager in South Africa)

In simple terms an operational amplifier is a DC coupled voltage amplifier which has a differential input and a single ended output.

Operational Amplifier LM741
The world’s most famous Op-amp, the LM741 (known as the 741)
741 Schematic - opamps
741 Schematic

GAP/R George A Philbrick Researches – currently Microchip

The world’s first commercial op-amp was known as the K2-W, manufactured from 1952 until 1971.

The first tube op-amps were known as general purpose, high gain, DC coupled, inverting feedback amplifiers.  Although many experimenters see these devices as being rather simple it’s only when one looks at the GAP/R P65, an early discrete component solid state op-amp designed and built by Alan Pearlman, co founder Nexus,  that we note the complexity of the circuit. Pricing for the later P45 was in the region of U$120.00 each.

The company veered clear of Germanium transistors and only started pushing solid state once silicon transistors proved reliable.

Operational amplifier circuit GAP/R- P65
Alan Pearman’s GAP/R P65

About the time of GAP/R startup Robert Page Burr and Thomas Brown started a company, Burr-Brown (of great audio fame) which was purchased out by Texas Instruments in 2000. Burr-Brown is known to manufacture some of the best quality operational amplifiers used in analog circuitry and DSP / electronic signal processing.

So what’s the interest in Op-Amps?

Well, recently we ran an article on the not-so-dreaded 4558.  I mean what’s the point of shooting down a 1970s chip in 2017 for poor performance? An Op-Amp has made out life easier, it’s made designing cheaper, it’s made audio simpler and there’s very little one cannot do with it. And it’s analogue. Well mostly, of course.

We can multi-vibrate, add, subtract, differentiate (or), compare (comparator), integrate (or), filter, chop, amplify, invert and …. the list goes on.  Op-amps are used in digital circuits as well as analogue. Op amps are temp stable. Gain calculations are simple, in the op-amp (741) image above the gain is equal to R2/R1. The output is in phase. Input to the (-) negative terminal inverts the phase.

What more can one want from a device which in open loop mode (without feedback resistors, gain can be anything from 10 000 to 100 000 (or higher).

Many manufacturers now give open loop gain for an op-amp as V/mV.  E.g. 10V/mV = 10 * 1000 or 10 000.   The 741 shows a typical open loop gain of 200 000.

LM741 Electrical
Texas Instruments LM741

Some historical and other facts

Tube negative feedback devices were in use pre 1940s.

The op-amp 709 was plagued by issues, instability, latching and self-destruction was not uncommon. The 741 was the fix.

The uA741 was commercialised in 1968, possibly the largest selling op-amp. They are also amongst the the cheapest – about R8.00 at Yebo.

The opamp cannot live in a world without feedback.

The input (voltages) will try to remain the same through the feedback path, if not, the output will swing towards the polarity of the most positive input e.g. if invert is high, then the output goes negative and vice versa. This forms the basis of many configurations.

Further Reading:

Walt Jung – Chapter H, Op Amp History

Philbrick Archives – GAP/R and the commercialisation of opamps

That awful JRC4558 audio IC

That nasty 4558 beater the Texas Instruments INA217

The JRC4558 – tried and trusted workhorse

So there you have it, out of the horses mouth, the op amp we covered in the Dixon PM121USB is awful.  Reading about this chip on nearly all the forums I am surprised that we still use it. It’s cheap, nasty, has piss-poor bandwidth and lacks “quality” sound.


That nasty 4558 beater the Texas Instruments INA217
That nasty 4558 beater the Texas Instruments INA217 instrumentation amp.


The first time I came across this chip was when the Titanic was still afloat, perhaps in a preamplifier but more like a graphics equaliser.  The equaliser is about late 70s vintage early 80s. It was a repair job and once completed sounded damned good. Ahem, does that mean that the modern equalisers sound better? Does this chip do any circuit justice?

Let’s backtrack a bit and look at some of the older, so called mediocre ICs of our time. The 741 series was a masterpiece. We built all sorts of circuits with these chips and even for “mediocre” use, the disco mixers used them in droves in preamplification. Now in the 70s maybe this was like magic, this little chip which sounded pretty good. I recall the JRC (Japanese Radio Company) getting into the act – bringing out a dual op-amp, the JRC4558.  Now I did a lot of work on JRC equipment in the 70s and 80s, all marine receivers and transmitters and let me tell you, they were anything but junk. In fact when I hear anyone on the forums talking about junk gear, I, like many, would want to get into the persons head and discover for ourselves where they get their information from.

No, the 4558 is not junk, at the time it was a very able little dual op-amp and was well justified to be used in almost all audio (and other) gear. The TDA741 still enjoys use today and no, it is neither junk nor ill suited for any application.  The bigger issue is when it starts being put into high quality audio gear – the 4558 has a bandwidth of about 3MHz under optimum configuration, the 5534 10MHz, slew rate 1V/uS compared to 13V/uS and near immeasurable noise figures on the 5534. (real engineers don’t worry about all this especially when our hearing maybe caps at 16kHz and not 16MHz). So it’s no wonder that people complain about these units been used in a preamplifier. But when mass feeding the peoples we suddenly have a problem – the 5543 costs R15.00 each and the 4558 R1.00 to R5.00.

There is an advantage however, DIY.  Sticking expensive chips into breadboard is never a solution. The 4558 lends itself perfectly to the cause. Once all is good we can hopefully transpose with a better chip, like the 5532 or 5534.

The 4558 is still useful as a comparator, DC work or anything which doesn’t require very fast switching.  Sonically the more expensive chips do sound cleaner but then we need low noise resistors and high tolerance circuits.

For general purpose work I find nothing wrong with this old workhorse. If you really want good quality sound then I do suggest looking at the INA217 or the THAT1510P. Definitely NOT for HOT exchanges but then if you want to rip off the JRC4558 then you need to start afresh.

So no, sorry to retract – the 4558 is not crap. Different strokes for different folks. This article may be slightly tongue in cheek but be wary of why we don’t like something, it could be because we were told so.


South African mains power – changes to receptacles

The South African SANS 164-1 and 164-2

RSA Plugs and Sockets – the SANS 164-2 requirement

Most people in South Africa may not be aware of the new 220~240V mains receptacles which came onto the market in 2015. With so many different standards used world-wide it’s interesting to note that this specific “style” or “form” is supposedly not only the safest in the world but is presumably the newest. “We don’t know because we don’t care” is a motto that seems to be that of most manufacturers opposed to the new IEC standard which means  (for them) more costs involved in shipping different plug types world wide.

The South African SANS 164-1 and 164-2
The South African SANS 164-1 and 164-2 (new) configurations

The new standard, known as the SANS 164-2 is nothing really unique. Indeed it’s not unique either that the IEC could never adapt every country in Europe to a common standard either, like a nationality’s flag.

As I see it, a common problem with our 240V mains plug system is that in the use of the “male” the top can come off exposing the live connection. The other is of course more common, the exposed trio of contacts, Live, Neutral and Earth just waiting for little fingers or in a tight fit connection, an uninsulated knife or screwdriver to pry the plug from the socket.

SANS protocol is that all electrical machinery is required by law to have our mains connector (plug) shipped with the device.  If not they do give the store or supplier a conformance notice which prohibits the sale of such goods until the devices are supplied with the correct mains plug, either three pin or two pin.

It’s disappointing that there are so many different standards because ironically this should be and is a government conformance measure – one would think that with either 110V or 220V there would be a) two different plug types to prevent incorrect voltage connection b) internally fused to be in line with wattage of the appliance c) correct pin offset to prevent Neutral and Live from being swapped c) waterproof and sealed d) loose plugs carry waterproof gasket e) the Live has a male/female part to it (meaning the live outer casing does not make contact) and of course f), the favourite – the socket having a safety cover.

Also, remember that if the socket is fused that is fine but it does not prevent a fire. A 15A fused socket powering a 100W device (say 0.5 A ~ 1A) which is not fused is inviting trouble. Homeowner’s should be aware of the risk here!

Dangerous appliances – does your toaster conform (or electrical blow heater)?

There are two very dangerous appliances in every home, one being the toaster which has mains live wiring accessible to any probing hand and the other, the washing machine. Yes, there are others but let’s stick to the toaster.  There are ways to make the domestic toaster much safer than what it is – starting with preventing the element wire from being immediately accessible by tiny hands. The Panasonic NB-G110P is a case in point. Nothing as glaringly sophisticated but still using infra-red.  Possibly a bit cheaper then, yes?

Cell phones and Downlighting

I believe the common and garden cell phone charger is a huge fire hazard, especially when plugged into bedroom sockets which are usually well hidden behind a fire waiting to happen curtain. I’d go the route of never putting a socket in a place where there could be a fire. Ditto downlighting, high wattage transformers, lamps and how is the cable? Down lighting should never be fitted by a carpenter, likewise don’t ask your electrician to build you a rocking chair. (actually I know of one that can).

I’m all for technology changes but let’s keep the safety standards consistent and the rules that govern this consistency be of an international standard. Every man for himself is sometimes just plain stupid.

Some reading:

SA to get new electrical sockets, plugs


Chinese electronic gear – hit or miss!

Chinese products hit ot miss

Steering clear of Chinese manufacturers – with or without merit

It’s a well known fact that most musos and audiophiles steer clear from the cheaper Asian imports like the plague. There is a huge amount of misconception about this, most probably stemming from the forums and ignorance.

Chinese electronic gear hit or miss
Spares Box

I am a huge believer of home brewed or “tuis gebakte” consumer goods and if it wasn’t for our rather feisty unions, inefficiency and poor education I do believe South Africa could be a manufacturing giant. We are in the motor industry but why not in the electronics sector? Many years back there was boastful talk about Stellenbosch being the second Silicon Valley. So what happened?  Price, efficiency, education.

Education is key for design – work ethic for manufacture

China has a mammoth manufacturing industry. They have a very high score rating in maths and science (here I should point out that Taiwan is or was the world leader). The people are bright, industrious and competitive. Anyone working in the electronics sector and specifically mixing with Asian engineers, marketers and the general sales force will understand the dynamics at play. Especially the enthusiasm. Mail a Chinese (or South Korean or Japanese) company today and you’ll have a reply within a few hours, not matter what time zone you are in. This kind of attitude makes China an attractive place to manufacture.

jack socket - stereo 6.3mm
Jack Socket – stereo (the nut is for illustration purposes only)

The argument is that as soon as a product goes to China for production the resulting products are always of poor construction.   To be clear, this was a problem with many brand names. We had the golden product which for some strange reason never shared the same quality of the resulting mass production. Shortcuts, poor quality materials and components and often rather unfinished goods were the order of the day. Quality workmanship did appear only once there was proper QC in place and there was rock solid supervision. I work in the sector so ethically I cannot mention the brand names (many) which succumbed to poor control and components. But at some stage the QC changed, control was tighter, things got more expensive and the complaints stopped. Ditto the professional audio market.

Our DIY sector 

Here is a standard dilemma for anyone building their own projects in South Africa:  Purchase a mono or stereo jack socket. They vary in price from about R10.00 to R75.00 ex VAT.  The cheaper variety is almost always of Asian build and the nut/threads are of exceptionally poor quality.  Why do we even bother? As soon as we start going up in price the quality improves until we get what we really want – a lifetime laster. At between R40.00 and R75.00 a pop. And they are also made in Asia i.e. China. (maybe I exaggerate a little but if a nut cannot be tightened beyond finger torque there may be a problem. How good are the contacts then?).

Doing the maths

The well known Dixon mixer PM121USB is a conundrum.  It features 12  audio channels and a stereo bus plus 200W per channel amplifier. This used to go for R 2 999.00 and now with ROE sits at R 4 299.00. This is very cheap for what you get. And the sockets are of very good quality (in my mind). So in mass production the manufacturers do get better quality products for less.  A rough estimation puts the total price on all the mono sockets at between R600.00 and R700.00 at current pricing.  No manufacturer would be buying in these products at that pricing (I give RRP not cost) even with 33% to 40% markup.  So in fact we are being ripped off – and not by the suppliers in Asia.

I mention the PM121USB mixer as an example purely because there has to be a break-even point – to self build a unit at RRP (recommended retail prices, if there is such a thing) it would cost double or more. The transformer alone sits at about R 1 200.00 to R 1 500.00 RRP in the RSA. But in mass production the pricing becomes attractive and I dare say, the quality is pretty damned good. Remember it gets used by DJs of all types and it does get bashed around, gauging by the images on Gumtree.

Focusing on the hardware

So where does this leave us?  Better quality mono and stereo jacks, fuseholders, switches and potentiometers?  This all falls within the spectrum of the DIYer.  We just cannot afford to pay 4 times the price to get a quality product, especially if we need this to last a lifetime.  (I purchased some sockets from a well known supplier where not one of the nuts could be tightened properly, the switches all had huge lateral movement, the fuse-holders had thin tin lugs, and the list goes on). Resellers should offer alternatives and not just online because often there is a stock shortage.

Margins are tight

Running an electronics store is not easy, especially those that survive on selling electronic components. Much of this is driven by a passion, margins can be tight as each store needs to be highly competitive.  But why the poor quality (of some components) in South Africa? I am not singling out any particular store, I do believe that in huge quantities components from companies like Farnell pricing to the consumer would be drastically cut and we’d end paying R20.00 for a good product as compared to R10.00 for a dud. And yes, most probably Farnell source from Asia as well. Just a point to note.

To close off, manufacturers in China have to be carefully selected, QC is vital throughout the manufacturing cycle, not just the Golden Product. This is a gap in our market – quality hardware at reasonable pricing.

The same applies to the chassis components (which includes rack mount)  and mains transformers. Compare the price of a computer rack mount chassis to that purchased through an electronics store.

Editor’s Note:  When it comes to to 200~240V a.c. plugs and sockets the mind boggles at the various qualities of build one has come to expect in South Africa.  Do they get checked by the SABS? Plugtops which can easily be pulled off the base leaving children access to the live terminal are frighteningly popular.  Multiplug sockets with a maximum of 5A wire rating.  And the ever popular mains two pin plug, with or without the earth strap. SABS/SANS is strict on this but we still see them.

And in the beginning…. we had Ohm’s Law

Ohms Law - carbon resistors

Ohm’s Law – the Genesis of Electronics and Electrical Engineering

We all marvel at the intricacies of electronic circuits, electron current flow and Fleming’s Right Rule but to get right down to basics we need to know only one thing, really… Ohm’s Law, named after the German physicist Georg Simon Ohm.

Ohms Law - carbon resistors
Carbon Resistors

To know what current flows through a circuit we need to know the potential difference across any given known resistance, to get the voltage across a resistance we need to know the resistance value and current flow and of course, resistance value can be calculated by the known current flow and measured potential across the resistance.   In Figure 2 we have the Ohm’s Law triangle just to help learners get unstuck – remember V = IR or R = V/R or I = V/R. Get it?

If Ohm’s Law is just the beginning then where does it all end?

Most teenagers (or younger) start their hobby or later career in electronics by building circuits from magazines, the internet or many top class books available.  Electronics is just a term – from Wiki: “Electronics is the science of controlling electrical energy electrically, in which the electrons have a fundamental role”.  Yes, that is about it, Electronics is all about the flow of electrons. Electronics has many branches, industrial, audio, automation, light, RF, computers, etc. Ohm’s Law is used in every single circuit design or fault diagnosis.

Ohms Law Triangle
Ohms Law Triangle

We may learn about Kirchoff, Joule and Coulomb’s  Laws, Thévenin’s theorem but Georg Ohm’s Law always holds precedence.  Our analogue friends all talk about power to the loudspeaker measured in Watts but did you know that in simple terms one Watt is equal to one Volt times one Ampere. There again, back to Ohm.

AC has it’s own set of rules

Unfortunately nothing is ever as simple as ABC.  In the real world we have alternating current where suddenly we have terms like amplitude, frequency, period etc.  Now suddenly we have the effects of AC on an inductance and / or capacitance, current leading voltage or lagging. We hear about impedance, reactance, reluctance and resonance. We find pi or π now showing it’s face around every corner, most often in the format 2π times something or other…

The beauty about building circuits …

The reality is of course when building circuits you don’t need to know jack about π or reactance or impedance.   What you do need to know is always Ohm.

The truth is…

If you are like me, the article writer, you will be interested in RF, you will be interested in audio and then possibly sonar, radar and every other wee thing affiliate to this lovely hobby.  To me resonance is one of the most important aspects to understanding electronics in it’s analogue form – it covers so many different parameters used in circuit design that it is not surprising therefore that it’s one of the most written about subjects in radio technology. And just when you thought you knew everything, along came the NE555.

(author’s comment – this article is not meant to be simplistic but rather to give the young and interested an injection into the world of Ohm which I do believe is the stepping stone into the electronics industry).



Sound quality, tips and tricks

Tips and Tricks for Sound

Home user audio setup without the hassle.

Sound in this case refers to the non-noise variety.

In many instances headphones are man’s ultimate friend. Unfortunately in the real world we also like to feel the sound, have the luxury of depth, colour, bass and all the magic that goes with it. Whilst a good set of headphones does the trick often just two tools in your briefcase make a huge difference:

i) A scientifically calibrated microphone and (ii) REW, the free but powerful software program, Room Equalisation Wizard. REW helps you analyse your living or sound room acoustic performance for optimal placement of speakers.

Tips and Tricks for Sound
Using REW – Room Equaliser Wizard

The part we won’t like is always in the final analysis – too many times your room is just crappy. I am in such a position, the listening or home theatre room is in the shape of a polygon. (concave, where one wall angle is more than 180 deg).  For home theatre use it is extremely difficult to set up and really needs acoustic treatment. Ditto for home studio use as well, just too many reflections and of course, there’s a road at the back.

There are many articles written about the treatment of a room to get the best in acoustics and reproduction which mostly applies to studio use – here we will apply some common sense to prevent things from going wrong from the outset.

Notes for those techno gurus:

The USB microphone UMIK-1 from miniDSP is not available in South Africa as far as I know. You will pick them up on Gumtree from time to time otherwise look at the resellers overseas. 2nd hand they go for about R1 000.00. This piece of kit plus REW on your notebook make for easy setup. I used a Behringer ECM8000, Focusrite Scarlett 18i20 (overkill but I had one on hand) and REW on a notebook. Note that condenser microphones need phantom power.

Noise, buzz and hum

Buzzing is a serious irritation. Often swapping the live and neutral leads on the amplifier can make a difference. Is it advisable though from a safety perspective? No.  Your live is always switched. Check your screened cables. Don’t you just hate those cables which have about a millimeter of insulation thickness with a micron of copper inside. Sold to look expensive. Make up your own cables using proper microphone cable – best to get the balanced type which can be used for RCA, XLR and 6.3mm stereo jack plugs or sockets. (RTS), this is twin core with a good quality screen.

Balanced over non-balanced

Big advantage of professional audio is balancing the signal but for shorter 2m lengths this should not pose aproblem. Most users complain of turntable hum or buzzing. Hum is often caused by noise from the mains transformer – this can be 50Hz or 100Hz, the 100Hz coming from the output of the rectifier (South African mains frequancy x 2).  More often than not the hum/noise comes from either too many ground points (use a central grounding point), broken ground point (screen) and never forget the obvious, the HOT part of the signal is missing due to a broken wire. This one is so obvious that we overlook it – cheap and nasty cables are always the problem here.

Hum and noise is a big problem and even seasoned audio engineers battle to get the ultimate signal hence the ground lift on injection boxes. For your home system try switching off the amplifier and listen for immediate changes.  It could even be a leaky reservoir capacitor but this will be heard with no signal input.


My own nightmare – hum from one cartridge (Audio Technik) and nothing from a Stanton.  Grounding wire inside the turntable tone arm making no contact.  Using a jumper with croc clips from tone arm to signal shield clears the noise. Both cartridges brand new. Same problem with both turntables, SL-1200 Technics. Ah-ha, different amplifier and the hum disappears.

Here’s the problem.  The Stanton did indeed ground to shield. Reversing the L-N mains connectors of the first amplifier made a difference.  The on off switch also switches both N and L.  Make sure the metal casing (if it has one) is earthed.

Turntables generate only a few mV p-p and noise is often a problem caused by poor grounding, broken cables etc.  For a moderately expensive turntable the LT-1200s failed to impress in the cable area. Whether you are a DJ or not always pay attention to the condition and quality of this cable.

Likewise microphones also run at only a few mV and setting up a mixer to have little or no noise at it’s output can be a tedious exercise. For live shows and studio recording this is a strictly no-go area.

Most complaints come from users noting that touching the casing of the turntable or amplifier reduces the noise.  To troubleshoot you will need to think methodically – was this problem there before,  are you using the same mains outlet, are you using a multi-plug, etc? Even pulling an input signal RCA plug out half way to break the earthing can make a difference. Sometimes in very bad cases the amplifier has poor grounding inside the chassis.  This can heard in the form of clicks and pops, speaker protection kicking in and worse case scenario, burning of the speaker voice coil.  (Akai amplifier, multiple grounding points, chassis corrosion causing output voltages to swing to +40 or -40V without input signal. Believe it or not).


Keep It Straight and Simple.  In every case where I have had a myriad input connections it becomes difficult for family to switch the inputs and/or we have a noise problem with low level signals. Home theatre amplifiers don’t have a MC or MM input. When playing around with signal matching always have a low impedance plugged into a higher impedance and not the other way round.

A turntable cannot be played through a microphone input because the signal needs to be RIAA (Recording Industry Association of America) equalised known as de-emphasising.  During the recording process it is emphasised, emphasising the higher frequency notes over the lower.  The reverse is applied on playback.  Your vinyl will sound absolutely horrible playing back without the de-emphasis and if I recall, very tinny.

Speaker setup

Ummm. Not very evident but placement can be a nightmare.  As a rule of thumb, main speakers 6 foot from amplifier or 12 foot apart, sub to the side. Latency to rear speakers setup according to amplifier manual. Volume control on each channel set up according to manual. If all else fails use stereo.  No jest here.  Many purists don’t watch their movies in 5.1 or 7.2 or whatever.  Magical speaker wire?  A test in the 70s states that telephone cable (you know the solid wire type) had the best results). Move speakers away from the wall.  There’s a lot of trickery in placement and a lot comes down to the speaker design. Fiddle. Don’t forget spikes, metal or rubber.

Noisy speakers?  Damaged voice coil or loose wire. The worst one is shorting cables because the amplifier is not going to like this one bit.

Amplifier showing protection mode

Use a hair dryer before switching on.  Sony amplifiers just love to be over sensitive. They are not the only ones.

As an aside…

Whilst some of the tips mentioned above won’t necessarily eradicate the hum, buzzes, clicks and grunts on your system it may prove to be helpful – a noisy background no matter how small is irritating. Shorted speaker cables are a very common occurrence which usually happens when the phantom does house cleaning.  Although amplifiers have current protection don’t hedge your bets that it’s totally fail-safe or foolproof.

As a DJ in the 80s it was seriously stressful to start any show which was perfect the previous evening with a background hum of sorts, even clicks from a fridge thermostat. In most cases it was caused by broken cables – retailers should be banned from selling poor quality cables with so called gold plating and crystal coated silver wire. Time and time again making up ones own cable sets were the best solution. In those days professional audio was only for the deep pocketed.  It’s really time that a standard be reached where your audio pre and power amplifiers start using XLR above a certain price bracket.

Next edition:  Setting up REW

The weird art of Interfacing

Denon DN-500AV

The weird art of Interfacing

Years back we had the DIN plug and socket, often for interconnection between tape deck REC/PB or turntable.

DIN Socket

These were dreaded in that users would pull the plugs out by the leads (and had cheap-arsed plastic back ends) and were tricky to re-solder usually ending up like mosquito shit at the joints.  Although these were very popular professional audio had long time been using TRS and XLR plugs and sockets. But DIN still is used for MIDI.

XLR Socket
TRS Jack
TRS Jack

The TRS (Tip Ring Sleeve) could be used for inserts as well as stereo.   By inserts we mean that once the jack had been inserted completely it would break the circuit inside a mixer, take the output side (hot) into an effects unit and then be returned via the ring – or one could partially insert the jack to only act as an insert without breaking the circuit.

The XLR was usually used in balanced line topology for microphone (or line) to reduce noise.

The weird art of Interfacing – loudspeakers

Speakon for loudspeakers

Modern amplifiers can be extremely powerful, such that the voltage across the speaker cable easily surpasses 100V peak-to-peak.  This can be dangerous for installation technicians when troubleshooting live.  The speakon isolates any uninsulated wiring.

Now just assume that you aren’t an avid follower of diamond crusted or gold braided speaker wire and all amplifiers and loudspeakers had a speakon as a terminal connector.  Wouldn’t this just be easier than connecting wires to terminal posts, soldering banana plugs onto wires which plugged into the rear of speakers or having to use 6.3mm jacks into the rear of speakers. The live element is not exposed and cannot short against anything. In most cases of premature amplifier failure it’s due to this very reason and of course transistors self destructing into a 2 Ohm load.

RCA Socket
RCA Socket

The dreaded RCA socket – how many times have you fiddled around with a stereo or home theater system to get one of the channels working only to find that the damned RCA patch cord is open circuit. They serve a purpose but in all honesty its time we moved on.

But the reality is…

Limited space, trying to make things smaller and of course reduce shipping charges. All this results in a cheaper commodity for the end user.

Have you noticed that there are home theater systems with only preamplifier outputs designed to drive a balanced line to audio power amplifiers of your choice. These cost more than a fully blown 7.2 channel pre- and power amplified system.

Denon DN-500AV
Denon DN-500AV

This one is a beaut – for the home theater specialist who can now select his own power amplifiers which will never go out of fashion and will have speakon connectors. Yes, strangely enough many modern professional audio amplifiers, even rated upwards of 400W RMS per channel will set you back less than a moderately priced home theater system with power amplifiers.

And that concludes this part of the article and also a good reason to determine just exactly you really want. I go with XLR, TRS and speakons every time. If it works for the recording industry and live bands it should work for you too.