Analog Amplifiers. Are You Sure?

Radford ZD22 All Analog Pre-Amplifier

Quad 405 All Analog Solid State Power Amplifier

Leak TL50 Plus All Analog Valve Power Amplifier

A Pause for Thought

You wanted an all analog signal chain for stereo or even multi-channel sound……well think again!

When you buy your new ‘analog’ amplifier or home theater receiver, ask yourself, how analog is it? In order to build cost-effective amplifiers and receivers manufacturers have had to turn to modern semiconductor technology and ‘digital’ techniques in order to support, in particular, multi-channel sound, and to keep themselves cost competitive. Clearly there are many areas of signal processing, such as CD, DVD, streaming etc. and particularly multi-channel audio, where all analog hardware is just not possible or appropriate, but are the remaining signal paths truly all analog when you want them to be so? Unfortunately in the majority of cases they are not, even for the simplest of signal paths, like good old stereo.

The images that you see above are of truly ALL ANALOG amplifiers every part of the signal chain and control, including the power supplies are analog. That doesn’t mean that there is some fundamental problem with having small, medium, large and very large scale integrated circuits (SSI, MSI, LSI, VLSI), and digital signal processing (DSP) integrated circuits (IC) in an amplifier, it is often required and there is not, especially when engineered correctly. Even high end receivers like my beloved and discontinued Denon AVP-A1 that is supposed to have a fully balanced all analog signal path, including an analog THX 80Hz sub crossover, are not pure analog signal paths. Why? Because the signal has to pass through several LSI chips, like digitally controlled switches and all its level controls are digitally controlled stepped attenuators. So what has happened to modern designs?

Ignoring the obvious requirements of DSP for dealing with digital sources, digital techniques and technology are now pervasive throughout most designs. Why? Because the associated semiconductor integrated circuits are fundamentally inexpensive and easy to mass produce in a cost effective and reliable manner. They are found in many areas that traditionally would be a mechanical or analog designed system like:

1. A switch or selector
2. A level control (volume/tone/EQ)
3. Power supplies
4. Power amplifiers

Clearly it is far less expensive to surface mount a digitally controlled attenuator or switching chip than mount a motorised mechanical volume/level potentiometer or mechanical switch and wire it to the printed circuit board. But there is the ‘rub’. These so called analog passive semiconductor devices are NOT passive in the same manner as a mechanical switch or potentiometer. Yes they are probably more reliable, may last longer and are far easier to remotely control. However, they produce distortion, noise, increase crosstalk and can be overloaded; equivalent mechanical devices DO NOT do any of that.

Here are two very popular pieces of MSI semiconductor that are used in some high end receivers and pre-amps:

While these semiconductors provide excellent technical performance it can clearly be seen that they introduce very low levels of distortion, crosstalk and can be overloaded. None of these issues would exist in the purely analog world when compared to the performance of passive modern plastic film potentiometer or gold plated switches that add no measurable noise, distortion or crosstalk.  (In a well executed design and placed at the correct signal levels these imperfections while measurable are probably not audible). But my point is made.

Image Credit: Marantz. AV8805A Receiver

Take a look at all the large and medium scale integrated circuits and DSP’s inside the new Marantz AV8805A (its all the gray square thingies). Also of note are the fifteen discrete analog line amplifiers, the ubiquitous switched mode supply in the upper LH corner and the screened analog toroid power supply in the lower LH corner.

So what other areas of design do digital techniques creep into and replace pure analog technologies?

Power Supplies

With analog power supplies the worst issue was screening the transformers magnetic field from entering the electronics -an easy fix, the toroid and/or a Mu-metal screen. With digital switched mode supplies there is significant effort expended into stopping the RF levels created by the high speed switching from being radiated and entering the electronics. In low level or even high level circuits this can become an onerous task that is never fully rectified as can be seen by the inclusion of analog power supplies for high end amplifiers (and DVD players) even when a switched mode supply is part of the base design. No manufacturer adds expensive hardware and doubles their work if there isn’t a tangable performance benefit. Unless of course they are ‘pandering’ to those audiophiles who will pay a lot more just because it is supposed to sound better!

Just look around you at the number of wall wart power supplies; you are surrounded by them these days. Usually an inexpensive single sided switched mode power supply that is often used to drive quite high end equipment. (see my review of the MX-VYNL pre amp here). Why are the used? Several reasons none of which are to improve the audio performance of the product:

1. They can be easily approved by Underwriters Laboratory (UL) for safety and so the piece of equipment that they drive does not need to be submitted for UL approval. A huge cost and time saving.
2. They are VERY inexpensive, but fortunately quite reliable.
3. They theoretically remove some of the issues of RF interference to outside and away from the device they power.

Unfortunately these mostly single rail supplies often feed equipment that requires two supply rails. This necessitates a switching DC to DC converter in order to generate a second supply rail as many designs use operational amplifiers that require two supply rails, one positive and one negative relative to ground. So back into your equipment goes a switched mode supply!

Power Amplifiers

The latest high efficiency Class D type power amplifiers that seem to be sweeping the amplifier world are basically switching power supplies. They have been used for years in sub-woofers due to their high efficiency and low heat output, but with there vastly improved designs have found themselves in many high end receivers, amplifiers and active speaker systems, especially subs. All my Genelec speakers use Class D type power amplifiers. They have all the same RF interference radiation issues as a switched mode supply, require complex control and protection circuits and require very carefully designed low pass filters to remove the switching square waves and filter them into the analog waveform. Why do all this? Again, they can be mass produced in a less expensive manner than analog power amplifiers and require virtually no setup, no expensive heat sinks and associated space and no fans. They absolutely do not sound better or perform better than a conventional Class A or AB amplifier but can provide lots more power from a much smaller space in a far most cost effective manner.

NOTE: Class D was the original classification for this design of amplifier. See here for a description of other switching class amplifiers. 

So what are there benefits of using all this digital technology in place of analog circuitry and components? Two. One to keep the manufacturing cost of the amplifier/device as low as possible as the vendor tries to compete with other manufactures doing the same, and two, to provide all those amazing processing options to support the burgeoning digital revolution. It’s not always about providing the user with a better sounding product. Its also about making the product more cost competitive and staying in business. Clearly it has become impractical and too costly to control and switch 15.1 channels of audio using purely passive analog devices.

Unfortunately even some high end audiophile two channel amplifiers have gone over to the ‘dark side’ and those passive motorized volume controls, plastic film potentiometers and gold plated switches are going the way of the Dodo. Rapidly followed by the analog power supplies and power amplifiers.

No, I am not a ‘stick in the mud’, nor am I against technological progress but in my book, either something is analog or its not. As a purist, there is no halfway house and even my Denon AVP-A1 barely scrapes into the analog livery. I have worked with audio for over 50 years and to be honest my STEREO system (see opening pictures) from 50 years ago sounded just as good if not better than what I own today. I am sure the time is fast approaching when pure analog will truly be a thing of the past except for the affluent few….Why? Digital technology and techniques are very flexible and adaptable to new developments, and when mass produced very inexpensive, opening up markets that would otherwise not be available to the manufacturer. These techniques also provide a bewildering range of functionality to the end user, that could never be made available through the use of all analog techniques no matter how much you spent on them, just as in the case of room digital equalization.

As a simple closing example take a look at the Allegri reference pre-amplifier, list £6,667 + VAT. (Approx. $8,500.00) Its not really a pre-amplifier at all but a stereo pure analog volume control, it contains no gain, only attenuation, no EQ and no processing. Its control is via a microprocessor driving reed relays that select the appropriate taps on an auto-transformer signal path in order to control its level; that’s it! However, being based upon transformer techniques it too creates some distortion and can be overloaded. Notably it uses an external analog power supply and its microprocessor digital clock is only enabled during level changes. Quite an extreme solution to a simple problem!

So buyer beware!

My original all analog system.