More Stability

High Frequency Stability

All our teach-ins are supported by references you can find at the end

Above: frequency and phase plot for the Era Gold V phono stage preamp operating into zero to 5,000pf loads.

You've probably heard the phrase "high frequency stability" or, in its negative tense, "high frequency instability" mentioned in passing but may not be aware what it refers to and its importance in you enjoying listening to music.

If an amplifier's output goes 180 degrees out of phase or even close while it still has gain, it will oscillate and therefore become highly unstable. Most power amplifiers will become very hot or will self-destruct. Preamplifiers, if they do not get hot or self destruct, will at worst make odd noises, sound piercing bright, exhibit reduced bass output and rectify radio frequencies (they will "tune" into the nearest station to the frequency of instability).

It is therefore very important for us to know our products are stable at high frequencies. So we now run up every design on our books using a powerful simulator running on both "SPICE" and "XSPICE" platforms. As you can see on the x-axis, it runs right up to 1THz which is in the infra-red light region. Few items of real-world test gear can reliably perform such measurements, and few organisations other than say NASA, can afford those that do.

Stability is ensured if the phase of the output signal is close to -90 degrees or less at all points where the amplifier has gain - that's all points 0dB and higher on the above graph indicated by the blue curve. This is at the output of the amplifying device (op-amp, valve or transistor) before any output filter and with any input filters removed. The input reference curve being the straight red line indicating that no input filters have been used in the test.

The above graph is from the Era Gold V and also shows the RIAA phono stage replay filter response from 20Hz to 20kHz, and also the Era Gold V's 50kHz "cutter knee".

The "roll-off" which starts around 4MHz is what we are interested in here. Notice that it falls at a rate of 20dB/decade (6dB per octave) all the way through and past the 0dB (unity gain) closure. At the point the blue curve intersects the 0dB line follow the dotted line upwards to where it intersects the green phase curve. You will see the phase here is approximately -93 degrees demonstrating that the Era Gold V doesn't suffer from high frequency instability - in fact it is highly stable!

You will also notice the output level keeps on falling - this is important. If the output level were to climb back up to 0dB or more, it would clearly show the amplifier oscillating. It may seem a little far fetched that an amplifier could still have the ability to work at such incredibly high frequencies, but we use video frequency techniques and the graph clearly shows no tendency to creep back right the way up into the infra-red spectrum.

It is possible to cheat SPICE and XSPICE results by leaving out parasitic behaviour characteristics of components, however, we model-in all parasitics and use only manufacturers approved semiconductor models.

We will follow up this article with links to the full-size stability graphs of all our products as they become available.

Online References:

http://analogzone.com/acqt0131.pdf

http://analogzone.com/acqt0214.pdf

http://analogzone.com/acqt0307.pdf

http://analogzone.com/acqt0418.pdf

http://analogzone.com/acqt0530.pdf

http://analogzone.com/acqt0704.pdf

http://analogzone.com/acqt0529.pdf

http://analogzone.com/acqt0814.pdf

http://analogzone.com/acqt1211.pdf

http://www.en-genius.net/includes/files/acqt_092407.pdf