Audio Technologies

Rounded Rectangle: Text Box: Copyright 2012 Audio Technologies, Miltiadis Logothetis Laboratory, All Rights Reserved.


Χαμηλή παραμόρφωση και υπολογισμένη εφαρμογή αρνητικής ανάδρασης

Τα τελευταία 10 χρόνια στο εργαστηριό μας, έχουμε αναπτύξει μια δική μας φιλοσοφία για την εφαρμογή της αρνητικής ανάδρασης και τις τοπολογίες ενίσχυσης, σχεδιάζοντας κυκλώματα με στόχο THD μικρότερη από 0.001%.

Αυτό σημαίνει ότι οι αρμονικές θα είναι 100db χαμηλότερα της θεμελιώδους με αποτέλεσμα η απόδοση του συστήματος να εξαρτάται από τις πηγές και όχι από τον προενισχυτή και τον τελικό ενισχυτή.


Ζυγισμένες είσοδοι

Οι κυκλωματικές τοπολογίες της Audio Technologies εγγυώνται ότι όλοι οι εξωτερικοί θόρυβοι και επηρεασμοί  είναι αποκλεισμένοι, χωρίς τις ζυγισμένες μεθόδους που με ελάχιστη απόκλιση θα προκαλέσουν φασική παραμόρφωση. Οι XLR ακροδέκτες που υπάρχουν σε κάποια προϊόντα μας έχουν συνδεσμολογία single ended.


Low distortion and calculated applied negative feedback

Over the past 10 years in our laboratory, we developed our own philosophy on the application of negative feedback in amplifier circuits, designing systems with THD less than 0.001%. 

This means that harmonics will be 100db below the fundamental so the system performance will be determined by the sources and not the preamplifier and power amplifier. 


Balanced inputs

Audio Technologies circuit topologies ensures that all external noises and influences are shut out, without the balanced principles that with any minimum deviation can cause phase distortion. The XLR connectors found in some of our products are single ended connected.


The distortion and db relation on our measurements

The code to deciphering db into percentages is percent and equals 100x10^(dB/20), where the db retain negative sign.

0dB = 100%

-10dB = 31.0%

-20dB = 10%

-30dB = 3.1%

-40dB = 1%

-50dB = 0.3%

-60dB = 0.1%

-70dB = 0.03%

-80dB = 0.01%

-90dB = 0.003%

-100dB = 0.001%

-110dB = 0.0003%

-120dB = 0.0001%


Other distortion measurements we use

PIM: or Phase Inter-modulation Distortion results from the change in signal phase as a function of signal amplitude. 

PIM is simply related to the change in the amplifier closed loop roll-off pole with signal, regardless of the underlying cause.


AIM: or Amplitude Inter-modulation Distortion compression, If there is, open loop gain decreases, the closed loop pole comes in and there is more in-band phase shift due to that pole and PIM will thus result. 


IIM: or Interface Inter-modulation Distortion is created when the output impedance of the power amplifier changes as a function of signal current.

IIM was originally blamed on amplifiers that relied on negative feedback to establish low closed-loop output impedance, but even amplifiers without negative feedback exhibit IIM, since their output impedance can change as a function of signal due to many other causes. 

For a given amount of negative feedback, lower open loop output impedance will produce better results.


TIM: or Transient Inter-modulation Distortion is a high frequency distortion brought on by the rate of change of the signal.


Hard TIM: is synonymous with slew rate limiting.


Soft TIM: is associated with the onset of slew rate limiting and is indistinguishable from other forms of high-frequency nonlinearity.

Designs with no NFB force the design of an input stage with high dynamic range to create TIM due to transient error signal overload.




Phase noise measurement

Jitter measurement

For a signal with idealized square edges the jitter is how much variation there is in the period.

In theory time and frequency domain measurements are complements, but in practice the way to measure them differs.

We can use real time analyzers with enough bins to sweep slowly and average over cycles from the lowest frequency modulation.

We can also use digital oscilloscopes that have adequate memory to sample enough cycles to obtain the real average while maintaining enough temporal resolution to accurately measure the period of each cycle.

Measurement setup computes jitter by comparing the incoming signal against a higher rate reference clock and the nominal clock signal is recovered from the incoming data stream.

Spread spectrum refers to operations in frequency domain.

Jitter and skew refer to time measurements.


Period jitter

Period jitter or Jper is the time difference between a measured cycle period and the ideal cycle period.

Due to its random nature, this jitter can be measured peak-to-peak or by root of mean square.

Mathematically, Jper is described as:
Jper = Tper(1)-To

Where To is the period of the ideal clock cycle.

Since the clock frequency is constant, the random quantity Jper  must have a zero mean.

Thus the RMS of Jper is calculated by:

RMS Jper=√(J²)


Phase-noise spectrum

The definition of the phase-noise spectrum L(f), is defined by  the power spectrum density of a clock signal as SC(f).

The SC(f) curve results when we connect the clock signal to a spectrum analyzer.

The phase-noise spectrum L(f) is then defined as the attenuation in dB from the peak value of SC(f) at the clock frequency fC, to a value of SC(f) at f.

Thus the phase-noise spectrum L(f) is written as:

L(f-fc)=10 log{Sc(f)/Sc(fc)} in dbc


Period jitter (JPER) measurement

We commonly use a high-precision digital oscilloscope to conduct the measurement.

When the clock jitter is more than 5 times larger than the oscilloscope's triggering jitter, can be acquired by triggering at a clock rising edge and measuring it at the next rising edge.

If the duration of the scope trigger-delay is longer than the period of a high-frequency clock, we insert a delay unit in the setup that delays the first rising edge after triggering so that it can be seen on the screen.

When high-precision results needed we use a post-sampling process of the data sampled from high-speed digital oscilloscopes to estimate the jitter.


Phase-noise spectrum L(f) measurement

L(f) can be measured with a spectrum analyzer directly from the spectrum Sc(f) of the clock signal.

But the practical way to measure the phase noise uses a setup that eliminates the spectrum energy at fc.

Relation between RMS period jitter and phase noise

Using the Fourier series expansion, it shown that a square-wave clock signal has the same jitter behavior as its base harmonic sinusoid signal so jitter analysis of a clock signal is easier.

The sinusoid signal of a clock signal, with phase noise, is written as:


Period jitter is written as:

Jper = θ(t)/2πfc

The relationship between the period jitter Jper, and the phase noise spectrum L(f) is written as:

∞     L(f)/10

RMS Jper=1/2πfc√2∫10        df



In our lab we measure Phase noise using the following setup:

1. Reference rubidium frequency standard generator

2. Spectrum analyzer

4. Oscilloscope

5. Phase variations into voltage variations converter

6. Ultra low noise power supplies

The output resistance

How we calculate the output resistance of our tube amplifiers

(With negative feedback applied).


Ro=  (Raa + Rpw + Rsw . Oir)/ Oir ( 1 + β . A . μ/Tr )   where


Ro= output resistance (closed loop output resistance at the secondary output terminals).

Raa= twice the Ra of one output tube.

Rpw= output primary winding wire resistance.

Rsw=  secondary winding wire resistance.

Oir= output transformer impedance ratio (the Tr squared).

β= fraction of output secondary voltage fed back to be in series with the input voltage.

A= gain of the stages (Vgrid to grid of the output tubes / Vgk of the input tube).

΅= amplification factor of one output tube.

Tr= turn ratio of the output.



The output transformers used in our power amplifiers were the most difficult and expensive parts to design and construct because of the following conflicting demands.

Low leakage reactance combining both leakage inductance and inter winding capacitance from the primary to the secondary winding to avoid loss of high frequency signals.

Low level of leakage inductance from one half of the primary to the other to reduce the discontinuities due to push-pull operation and the odd order harmonic distortion resulting.

High primary inductance for good low frequency response and low winding resistance to avoid power losses.

High quality laminations to ensure a low level of core induced distortion due to magnetic hysteresis.

We successfully attempted on a computer the actual real performance outcome of our output transformers based on dimensioning the known winding details and use the data with SPICE. 

We made all the prototype transformers in our laboratory.


Our output transformers

Vacuum tubes test

All valves are tested in our laboratory for a number of characteristics.

These include heater current, plate current, screen current, negative grid current, mutual characteristics*, transconductance**, noise, microphony and amplification factor.

All signal tubes heater voltages are stabilized.

Any variation of the heater voltage will involve decreased maximum grid circuit resistance for a higher heater voltage and decreased plate current for lower heater voltage.

Power tubes and signal tubes are separated in pairs.

*The mutual characteristics is a very important test.

We drawn them by maintaining the plate voltage constant and varying the grid from the extreme negative to the extreme positive voltage.

For any particular plate voltage there is a negative grid voltage at which the plate current becomes zero.

This is the point of the plate current cut-off and any increase of grid voltage in the negative direction has no effect on the plate current which remains zero.

**The transconductance is another important test.

A special setup measure the incremental change in current to any electrode divided by the incremental change in voltage to another electrode under the condition that all other voltages remain unchanged.

The final test is listening the tubes to our production and research prototypes.

We can also test electron tubes, according to the USA department of defence 19-04-2011 method, with applicable specifications MIL-STD-1311D.

To learn more about the above testing method contact


We design and build multi-layers, very low leakage inductance transformers for electrostatic loudspeakers that operate over an extremely wide frequency range at very high voltages.

For the above transformers SPICE is used.

The below basic formula among others is used

Lpr= (μo.μr.Npr.A)/(L+μr.D)   where

Lpr= primary magnetizing inductance

Μo= magnetic permeability of vacuum, 4π×10^−7 (H/m)

Μr= relative magnetic permeability of the core material

Npr= number of primary turns

L= length of the magnetic circuit (m)

A= cross-sectional area of the magnetic circuit (m^2)

D= length of the air gap (m)

Our ESL input transformers