What’s in a Vocoder – voice synthesisers and Robots

Kraftwerk, Styx and My Vocoder – Shaping human voices

Generating and shaping audio and RF wave patterns has long been an interest for many and is seen in the medical, military and commercial industries.

Not wanting to re-invent the wheel but rather looking at another spin, what is the appeal?

Although there are many makes and models, dedicated or otherwise two entry level but well rated models are the Roland VT-3 and the Paia Vocoder designed by Craig Anderton. Check out an upcoming series as we attempt to build our own.

An off-tuned SSB received signal through the BFO will create vocals which sound like monkey chatter or off pitch. It doesn’t take radio students long to see the multiple effects ring modulators and different tones have on the output.

Circuits and Kits

Instructables – Build and Analog Vocoder by Planetone

MFOS – Music from Outer Space / Article / Makezine

Ghetto Talk Box

Muffwiggler / Elektor 10 channel vocoder

And of course….

Crank That Soulja Boy Talk Box (from Ghetto Talk Box)

Audio Tools

Two of my favourite audio tools would be Audacity (free) -multitrack recording which allows effects and of course, Reaper, a fully equipped DAW.  Free for 60 days then although not crippled, $60.00. You will not find this kind of software anywhere for this price.

Both the above allows vocoder inserts and with proper manipulation are exceptionally good.


Further Reading

How Vocoders work – Craig Anderton (Paia)

Transistor Equivalents – old equipment restoration

Where to find transistor equivalents?

Restoring old gear can be a nightmare when it comes to some of the more obscure transistor types, and a pretty expensive exercise as well. Transistor equivalents for substitution is often a necessity when repairing vintage gear. This applies equally well to silicon devices and not just Germanium.

Years back we were confined to telephonic conversations or writing letters to the supplier, the snail mail variety.  In South Africa we are fortunate enough to have proactive suppliers in the electronics industry, unlike in the motor or metal industries where staff are still learning how to send emails, except if they see a big sale pending. Continue reading “Transistor Equivalents – old equipment restoration”

Multiplexer and Demultiplexer switching (follow up)

Using Electronic switching to route signals

As a follow up to our article on multiplexing (and demultiplexing) or MUX/DEMUX and running my own experimentation with a CD4051 it comes as no surprise that this chip works well but there are limitations.

As I am no expert in this field I must confess that the biggest obstacle IMO when switching or routing signals is not the degrading of signal quality necessarily but the dangers of phantom power.  As most microphone preamplifiers use an XLR plug or TRS combo, the TRS does not pass the +48V but the XLR does.  Now if someone unwittingly connected his expensive tape deck to XLR outputs and then into an XLR mic preamp there is a chance that they would end up with a toasted front end to the deck. Yes, we all like XLR plugs but this can be a problem. How to eliminate this problem? Continue reading “Multiplexer and Demultiplexer switching (follow up)”

Hexfets Vs Laterals Vs BJTs

Mosfet Devices Versus Bipolar Junction Transistors

I will try not to be biased in this article. Pun intended.  Again, no re-invention of the wheel so we will guide the newbie in the right direction.

General circuit - input choked

There’s different trains of thought when it comes to the different transistor types out there and which is suitable for what. Continue reading “Hexfets Vs Laterals Vs BJTs”

Winding your own OPT

Local suppliers – South Africa

As an addendum to our previous article “Alternatives to Tube  Output Transformers” we thought it pertinent to add a bit of local flavour,  Mars Amps in Paarl to the mix.  No, I have not met the gent but Karel Mars is a well known tube guru in the country and lends his expertise to the DIY community either through workshops or on the local avforums website, found here.  (OPT winding).

Mars Amps has a variety of tube pre- and power amplifiers available in kit form, shipping both locally and internationally.

See http://www.marsamps.co.za/product/output-transformer-opt-jtm45/



National Semiconductor – LME49811

LM4702 - Mouser

The LME49811 from National Semiconductor / TI


Class D amplifiers and drivers are becoming so popular that some of the largest IC manufacturers are no longer focusing on class AB or A driver chips.  This is certainly the case for TI and sub group National Semiconductor.  Of course some of us don’t like class D.  What about our firm favourites: LME49xxx series or the LM4702? Continue reading “National Semiconductor – LME49811”

Protecting your Amplifier

Transistor Junction Temp and Derating

Protection devices for Audio Amplifiers

There is a lot of theory put into amplifier design and when one looks at these complicated circuits often the protection circuits are more sophisticated than the gain stages.  Purists believe the only true amplifier is a piece of wire with gain.  Engineers often criticise the protect circuits of creating their own problems, along with causing inferior quality reproduction.

These are the major problems come across in the design of any amplifier:

  • Heat – is there sufficient cooling for heavy duty work?
  • Can the output transistors go out of their Safe Operating Area?  (known as SOA).
  • Can the amplifier drive a 2 Ohm load when minimum load spec is 4 Ohm?
  • The dangers of mains voltage boosting and sagging?
  • Oscillations and instability.
  • Over driving and distortion.
  • Loudspeaker protection – expensive lessons to be learnt.

These are just a few of the problem areas in the design of amplifiers. Continue reading “Protecting your Amplifier”

Current Mirrors and Long Tailed Pairs

Differential Amplifiers – what they do  (Part One)

Long tail pairs or differential amplifiers have one important purpose:  To amplify the difference between two inputs or act as a switch.

Differential or Long Tailed Pair amplifier
Differential Amplifier or Long Tailed Pair amplifier

In a perfect case the voltage gain of the differential amplifier is the product of the differential gain or Ad and (Vin+  subtract Vin-). Continue reading “Current Mirrors and Long Tailed Pairs”

Sziklai Pair and Quasi Complementary output stages

Who made this well-known Quasi Complimentary Amplifier

Sziklai versus Darlington

The Sziklai, like the Darlington is a very high current gain device.

After dabbling on a few pages about Darlington transistors, what about the pseudo Darlington, the compound feedback pair, the Super Transistor, the Sziklai stage.  (in some of the images Sziklai is spelt Ziklai – apologies for this)

One needs to go back a few steps to understand the significance of the Sziklai design (after George Sziklai).  Many years back the only transistors used in power amplifiers were Germanium types which had a few quirks, the most dodgy being thermal runaway. Continue reading “Sziklai Pair and Quasi Complementary output stages”

National LM4702 Vs Sanken STK350-xxx

A look into National Semiconductor’s LM4702 VAS

First thing here for those that aren’t aware:  “SANTA CLARA, Calif., Sept. 23, 2011 /PRNewswire/ — Texas Instruments Incorporated (TI) (NYSE: TXN) today announced the acquisition of National Semiconductor (NYSE: NSM) is complete.”

I thought the first paragraph is relevant to our subject based on the confusion come across in forums on who owns what.

The LM4702 driver Continue reading “National LM4702 Vs Sanken STK350-xxx”