KISS 111
KISS 111
THE OUTPUT TRANSFORMER AS AN IMPORTANT BLACK BOX
by Andre Jute
As we have seen, before we can design an amplifier, we must know for which speakers, to what level of output, and what the input will be. We have arbitrarily set the input to 2Vrms from a CD player or a pre-amp. We have calculated that, for the Lowther horns I have built to use with The KISS Amp, 3W will be more than adequate and 1.5W will be good enough. If you are working along in order to build this amp or a version of it, you have written down a number of watts that you think your speakers will require.
We have also already discussed quite a bit of metaphysics (a fancy name for sex, drugs and rocknroll). Amps and loudspeakers are like that. Good ones are very rarely designed by those who believe in the soundbite method. Amplifiers and particularly loudspeakers are art forms in metal and glass and wood, applied wu. I have never heard a truly pleasing audio component that was designed by a man who could not cook well. An audiophile is by definition a hedonist. The purse-lipped net-curtain twitchers even now gathering to warn you against my seductions are "audiophiles" only in Salem, puritan witch hunters.
The next component back from the loudspeaker is socketry and then the chassis and some wiring. They are worth some modest consideration which we will give them in a much later section.
The first important component back from the speaker is the output transformer (OPT TRX or just OPT). The OPT is next in importance to the loudspeaker as a key shaper of the sound your amp will produce, equal with the power tube. A bad or unsuitable OPT will ruin your sound, as will a bad application of even a good OPT. That doesn't mean that an inexpensive OPT will ruin your sound if it is applied right. It merely means that a better OPT (the ones I like best are relatively inexpensive) will give you better sound, though never quite as much as the extra cost might lead you to believe.
The OPT is usually made from copper wire wound around a steel core. There will be some form of internal isolation and possibly external protection such as a casing. In either case the apparent mechanical simplicity is deeply misleading.
The OPT in action comprises a whole welter of confusing compromises. You can read up about them and we shall have to return to some of them later. But for now you should, unless you want to make a lifetime of magnetics (the art and craft of transformers), treat the OPT as a black box which connects the power tube to the loudspeaker with reasonable precision.
We have to choose the tube and its operating conditions before we can specify the OPT. Single ended or SE OPTs differ from push pull or PP OPTs in having an airgap.
We want to choose our OPT black box by three main initial specifics. We shall derive the first two from the transfer curves (the operating conditions) of our chosen power tube and then use them to identify the available OPTs that match or approache our ideal on the third main specification.
a) Power handling, usually described as so much standing or quiescent current in milliamps which are thousandths of an Amp and abbreviated mA. This is half the total advised current capability, because it is for the zero signal condition or quiescence, a word you should remember. There is usually also a power rating in watts but usefully makers commonly describe transformers as suitable for a range of named tubes, so that you know the transformer under consideration is rated for the plate voltage of the tube you want to use.
b) Primary impedance, which is the load the OPT will reflect from the loudspeakers onto the plate (British English is anode) of the power tube. This is usually in thousands of ohms, so we talk of a 3.5Kohm or 5.6Kohm transformer when we mean one with a 3500 ohm or 5600 ohm primary impedance. (Impedance itself is a condition excited by the flow of electricity against the combination of resistance with the reactance of capacitance or inductance. You may think of it as a resistance which varies with frequency. We shall return to it repeatedly, I hope in small digestible doses.) All other things being equal, a low transformer primary impedance gives more output power which translates to deeper bass, a higher load gives smoother output with less distortion.
c) The bandwidth of the OPT. Audio bandwidth is ideally taken as 20Hz to 20kHz. Most likely no one on RAT can hear up 20kHz. Hardly anyone has speakers that go cleanly down to 20Hz. The organ can go down to 16Hz but hardly anyone bothers to record much under 40-50Hz, leaving the recorded harmonics to suggest the fundamental. The convention is that an allegedly "flat" 20Hz to umpteen megahertz component is actually 3dB down at 20Hz. Most high street speakers are exceptional if they are only 3dB down at 50z and inexpensive ones have nothing worthwhile below 80 or in some cases even 100Hz. At the high frequency end, stratospheric numbers are often bandied about as a marketing exercise but somewhere between 40kHz and 60kHz is ample high frequency extension. Thus the OPT bandwidth could be specced as -3dB at 10Hz and 60kHz or for a cheap OPT (or a very good one offering other desired compensations) as 30Hz to 30kHz.
As I said, the OPT is for now a black box whose characteristics will be determined by the power tube, the speakers and what the designer intends to achieve with his amplifier. We know the speakers and we have already chosen the 300B, in the case of some of us because of pleasing prior experience, in the case of the rest by its fine reputation. We have also chosen the topology, single ended or SE with zero negative feedback or ZNFB, partly to keep costs down by using only one power tube per channel, partly because the designer (in this case me) has much pleasing experience with this configuration. An SE tube by definition operates in Class A. But while that may seem a lot of knowledge, many important decisions about the power tube and its associated output transformer remain to be taken.
THE VOLTAGES IN THIS AMP WILL KILL YOU.
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All text and illustration is Copyright © Andre Jute 2004
and may not be reproduced except in the thread KISS xxx on rec.audio.tubes