Saturday, May 5, 2012

Crescendo Millennium Edition

Remake of A Famous Amplifier

Design by T. Giesberts
Source: Elektor Electronics 4/2001

In early 1984, Elektor magazine beat the competition hands-down by publishing the design of a then-revolutionary MOSFET amplifier. Even now, this amplifier enjoys an enthusiastic following. In response to many requests, we have brought the design up to date and given some attention to improved reliability and operating safety. The output power is 90 watts into 8 ohms or 135 watts into 4 ohms, which should leave little to be desired for most users.

Thursday, May 3, 2012

The Basics of Designing a Parafeed Amplifier for Headphones

Part 1: How a tube works

A tube has 3 electrodes (hence triode, pentodes are for another day) – the anode, also called the plate, the cathode, and the grid, also called the control grid. When there is a positive voltage applied to the anode, current flows from the cathode to the anode (Why? Click here

What the heck is parallel feed?

Suppose you’re the best transformer manufacturer in the world. You’ve designed and built great Single Ended transformers for all the coolest tubes, 300B, 2A3, 45, 211, etc.,etc. But the urge to create has you wanting more and better alternatives.

This was the dilemma faced by Mike LaFevre of MagneQuest a few years back. After several months of study, Mike found a unique solution which had been only lightly addressed in obscure passages of the great electronics texts of the past - parallel feed, also known as shunt feed.

A conventional airgapped single ended output transformer must deal with two types of current flux, DC, which is the direct current from the power supply feeding the plate of the tube, and AC flux, which is the music signal passing from the plate to the primary winding, through the secondary and out to the loudspeaker. These two types of current flux create divergent requirements in the construction of the transformer. It must be capable of handling high direct currents, which requires use of an air gap in the lamination stack and a relatively large core of lamination material to avoid magnetic saturation. This works against the requirements necessary for maximum AC bandwidth, less windings (smaller core means less wire means lower capacitance), high inductance (more easily achieved with interleaved laminations) and high permeability lamination material (for faster, more dynamic response to the AC signal).

Single Ended vs. Push Pull (Continued): The Deep, Dark Secrets of Output Transformers

by Eddie Vaughn

A Burning Question

Here's a question I've been asked by several folks, including a gentleman who has a hearing condition that makes tizzy, grainy systems and loud listening levels painful. "Does the Carina SE EL84 amplifier have good detail and dynamics at low listening volumes, or must it be cranked up so that you can hear everything clearly?" I explained that it sounds very detailed and articulate at sub-watt output levels, and as a matter of fact it's inside the first watt where it truly excels. I immediately followed my statement with a question, "What amplifier are you using now?" It turns out that this particular gentleman has a push pull amp. Ah-ha, just as I'd suspected! While his PP amplifier is different (I'd say better) from most in that it operates it's four 6BM8s in strapped-triode and uses very minimal negative feedback, it still suffers from one of the drawbacks inherit to PP. He says that while it sounds positively wonderful at higher levels, he finds that it lacks detail and dynamics when played at very low volume. His main reservation about buying a Carina was the fear that it will exhibit this same trait.

It won't. Definitely not.

Single Ended vs. Push Pull: The Fight of the Century

by Eddie Vaughn

If you're familiar with tube amplifiers, you know that the two major methods of power stage operation are single ended (SE) and push pull (PP). As with so many things in life, most people are highly opinionated when it comes to these two choices. While any confrontations between the two camps are less likely to end with blood spilled on the floor than say, vinyl versus digital or tube versus solid state (proponents of SE and PP are, after all, still "brothers in tubes"), their clashes can nevertheless become a bit heated at times. The real danger here exists to that poor, well-meaning soul who enters into the discussion and tries to play the peacemaker by extolling the virtues of each method and proclaiming them equals, which is the "raw nerve" for both camps. When this happens, you'll usually see Camp SE and Camp PP rise up in unison, beat him like a rented mule and toss his limp, lifeless body aside as fodder for the vultures, and then resume hostilities as usual betwixt themselves.

Wednesday, May 2, 2012

The PLH Amplifier

Copyright 2005 Nelson Pass

Introduction: The JLH Amplifier

In 1969 John Linsley-Hood wrote in Wireless World:
During the past few years a number of excellent designs have been published for domestic audio amplifiers. However, some of these designs are now rendered obsolescent by changes in the availability of components, and others are intended to provide levels of power output, which are in excess of the requirements of a normal living room.  Also, most designs have tended to be rather complex.

In the circumstances it seemed worthwhile to consider just how simple a design could be made which would give adequate output power together with a standard of performance, which was beyond reproach, and this study has resulted in the present design.

He then described a Class A power amplifier using three gain stages of bipolar transistors in a topology, which continues to be admired for its elegant simplicity and sound quality.

The centerpiece of this design is the middle stage, an NPN transistor used as a phase splitter, simultaneously driving the positive half of the output stage and the negative half with symmetric signals of opposite phase.

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JLH Class A Amplifier (1996)

Class-A Power

After  two  and  a  half  decades,  John Linsley-Hood’s  Class-A  power  amp  is still  rated among the  best.  Here,  John  explains  how  to bring  the  design up  to date,  adding enhancements such as dc-coupled output.

Electronics World, September 1996 

The  current  debate,  among  the  more  reactionary  of  the hi-fi  devotees,  about the  relative  merits  of thermionic valve operated audio amplifiers makes intriguing reading, if only because, in a sense, this is ‘where I came in’. I will explain.

JLH Class A Amplifier (1969)

Simple Class A Amplifier

A  10-W  design giving  subjectively  better results  than  class  B
transistor amplifiers

by J. L. Linsley Hood, M.I.E.E.

Wireless World, April 1969

During  the past  few  years  a number  of  excellent  designs  have been published  for  domestic  audio amplifiers. However, some of these designs are now rendered obsolescent by changes in the availability of components, and others are  intended  to provide  levels of power output which are  in excess of  the requirements of a normal living room. Also, most designs have tended to be rather complex. In  the circumstances  it seemed worth while  to consider  just how simple a design could be made which would give adequate output power together with a standard of performance which was beyond reproach, and this study has resulted in the present design.

Tuesday, May 1, 2012

Valves at Low Plate Voltages (2)

Part 2: more power!
by B. Kainka

Source: Elektor Electronics 10/2003
Kainka's site:

Genuine power valves such as the EL84, EL95, ECL80 and ECL86, and in particular the PL504, do of course offer more power output at low anode voltages than the ECC81 and ECC82 types we discussed in the first part of this series. With the PL504 it is even possible to drive an ordinary low-impedance loudspeaker, using an anode voltage of only 27 V. In this second article we will also be looking at some miniature Russian ‘battery valves’, which not only operate at low anode voltages, but also demand considerably less heater power. If, after our initial experiments with ECC81s and ECC82s (or their US equivalents 12AT7 and 12AU7), you have come to the conclusion that our valve headphone amplifier could do with a little more power, then it is time to take a look at some valves which are designed for higher power.  Suitable candidates would be output-stage valves such as the EL84 (6BQ5), EL95 (6DL5), ECL80 (6AB8), ECL86 and similar types.

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CD Musik Anda Asli Atau Bodong?

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Valves at Low Plate Voltages

Interesting and surprising experiments with valves
by B. Kainka 

Source: Elektor Electronics 9/2003
Kainka's site:

Is it just nostalgia, or are valves really somehow better than transistors? Recently valves have been making something of a comeback in many areas. Using valves seems to involve a lot of effort, and in particular the high voltages frighten many people off. But there are dozens of valves lying about in many a cellar — so why not try something new with those old valves? Valves are usually driven using an anode voltage of 250 V or more; practically never with an anode voltage below 100 V. For power amplifiers, in particular for radio transmitters, several kilovolts can be used. Such inconveniently high voltages naturally put many people off, as do the special transformers and high-voltage electrolytic capacitors that are needed. But things need not be like this. A series of experiments has shown that most of those valves nostalgically kept at the back of the cupboard will work at very low voltages. Of course, we are not talking about achieving the ultimate in power or amplification, but for simple applications — and a bit of fun — it fills the bill. We will describe in this article how to build simple circuits using valves with a minimum of fuss. Operating at anode voltages of, for example, 12 V is not recommended by manufacturers and is not covered in any data sheet. So, if we are going to learn anything, we will need to experiment and make some measurements. In order to forestall criticism from committed valve-lovers, we should say that the aim here is not to build the last word in hi-fi amplifiers or find the optimal operating point for some particular valve. We are more interested in gaining some experience with valves in a simple and safe way. There is a special satisfaction in building a small circuit and making a simple working device. And it is not just about feeling the warm glow from the cathode: it is like going back in time to the early days of electronics, when the (relatively simple) technology was dominated by amateurs and everything was visible. Valves in their glass envelopes are certainly more ‘transparent’ than ICs in plastic packages. Of course, we could do things ‘properly’, and use anode voltages of 250 V; but that would not exactly make for simple and relaxing experimentation on the bench. A chassis would be required and everything would have to be built carefully into a case. And we would always have to watch out for those dangerous voltages. None of this is a problem if we stick to low voltages.