Oscilloscopes and Audio Testing, Plus How to Win a Scope!

Question: What is needed on the scope to show that a tube or valve preamp stage is used in a pedal direct into the sound board:

The tube sound is often subjectively described by uncritical listeners (that is, not audio professionals) as having a “warmth” and “richness”, but the source of this is by no means agreed on. It may be due to the non-linear clipping that occurs with tube amps, or due to the higher levels of second-order harmonic distortion, common in single-ended designs resulting from the characteristics of the tube interacting with the inductance of the output transformer.

http://en.wikipedia.org/wiki/Tube_sound

I wish to show that a digital audio signal run through a tube post gain stage produces this “warmth”.   I can detail the exact 2nd harmonic characteristics of this sound at least in part mathematically.    ‘With that information, how would I go about illustrating the difference as scope traces?  The tube post gain vs straight digital.

Question 2:  What do I need to do to win a scope?  Used, cosmetic defect, I  don’t care, I’ve used them since 2nd year engineering school and lately have been wanting to work with them again.    

3rd and last question:  In the procedure taking a line level stereo audio signal and modulating it for FM transmission, how can a scope be used to optimize design for the highest quality audio reproduction possible?

Answer: Thanks for the excellent questions. I’ve divided the answer into three parts.

1. Vacuum Tube versus Solid State Sound

The discussion here will be about consumer audio products, but there are many similarities to professional audio products, too.

As a class, not all vacuum tube amplifiers sound the same.  And as a class, not all solid state amplifiers sound the same.  However, it has been generally found that there are differences in ‘tube’ sound versus ‘solid state’ sound.  There are differences in amplifier topologies; and differences in interfaces between preamp, power amp, and loudspeakers (i.e. load impedance and resonances).

 The following amplifier characteristics are thought to be some of the causes of audible differences:

Clipping characteristics, the proportions of low order harmonic distortion products and high order harmonic distortion products, frequency response, and damping factor (output impedance).

Clipping: The effects of clipping can be eliminated, as long as the amplifier, loudspeaker, and playback sound levels are such that the amplifier never clips.  Any amplifier that is driven into hard clipping will cause most music to sound terrible (although the intent of some electric guitar playing intentionally makes use of this highly distorted sound). 

If the intent is to play music very loudly at very low distortion, then either a very efficient loudspeaker is required, or a very powerful amplifier is required. 

For clipping testing, a function generator, scope, and load resistor are required. The shape of the clipped signal will vary from amp to amp, and according to the amount of overdrive to the amp.

Harmonic Distortion at moderate levels: For most amplifiers that are played at levels well below clipping, the distortion is very low. 

For harmonic testing, a low distortion function generator, scope that has an FFT function, and a load resistor are required.  The distortion may be low enough so that a scope cannot see any harmonics other than the 2nd and 3rd (higher order harmonics may not be visible).

In order to see low amplitude high order harmonics of amplifiers, a spectrum analyzer may be required.

Check the level of the harmonics, the number of harmonics, and the rate that their amplitude is reduced as the harmonic number increases.

Frequency Response: Most music fundamentals and harmonics are within the frequency response of the amplifier, but the amplifier may attenuate the signal at the frequency extremes.

For frequency response testing, a function generator, scope, and load resistor are required. 

The gain of the amplifier should be tested over its rated frequency range, both at low power, and at rated power.

Damping Factor: Different damping factors react with loudspeakers to cause different frequency responses, and different transient responses.

A function generator, scope, and multiple load resistors are required to test damping factor.

The output voltage is tested, as the load resistance is changed.  The damping factor (output impedance) is calculated from the output voltage changes, versus the load resistance changes.

2. Winning a Free Scope

Winning a free scope can be difficult but the more entries you have the better the chance. Tektronix participates in many events and shows throughout the year where we give out scopes as well as other prizes. I’d encourage you to check the ‘events’ page on the Tektronix site regularly for update-to-date details (www.tektronix.com/events) . In fact we will be giving away 5 Tektronix MSO2024 Digital Oscilloscopes during Embedded Systems Conference (ESC) Silicon Valley next month. And, we give away a scope each month at our Scope Central online community (www.tektronix.com/scopecentral). You can enter daily–thus increasing your odds of winning!

3. FM Stereo Transmission Quality

The quality of sound from an FM transmission depends on many factors.

The music source material (i.e. CD), playback source (i.e. CD player), sound boards, compressor (presence or lack thereof), and FM Transmitter are all responsible for the quality of sound.  Also, the correct setting of signal amplitudes at each point along the way is paramount to ensure good sound quality.

If the desire is to retain the original dynamic range of the music as it is recorded on the CD, then there should be no compressor in the station audio chain.  Some classical and Jazz stations follow this model.

The maximum input of each stage, including the transmitter modulator input, should be rated by the manufacturer.  These ratings will be used in setting the proper levels in the audio chain. A test CD with full amplitude sine wave (0 dB) should be put in the CD player, and the output sent down the audio chain. 

Starting from the CD player output, to the first input stage, a scope should be used to measure the signal level.  Set the gain levels of the audio at each point along the path so that the signal amplitude is near to, but not at the maximum input rating of each stage along the way.  Too much signal amplitude will cause distortion (or if the signal is way to large it will clip); and too small of a signal amplitude will have a poor signal to noise ratio.

Music CDs have wide variations in their recorded level output.  And without pre-playing and testing each CD for the loudest portion of a recording, it will be an unknown. 

A station philosophy decision must be made as to whether or not to change the gain on the sound board for each CD to maximize the signal to noise ratio.  The danger of adjusting the level individually for each CD, is that a loud portion will come along and cause clipping.

If it is decided to do gain adjustments, then for low level CDs, the gain of the sound board will have to be raised.  Again, the scope can help to set the level.  Then, it will have to be set again according to the next CD recording level.

A compressor can assist in reducing the severity of incorrect level setting, but the tradeoff is a reduction of dynamic range of the music. If there is a compressor in the audio path, it will be a compromise between dynamic range, versus the level you set into the compressor.  If the level is set so that the full scale CD does not cause the compressor to compress at all (might as well not use a compressor).  If the level is set too high, the compressor will be working all the time (the dynamic range will be severely limited).  The setting between those two levels is a compromise, and the best setting is up to the taste and goal of the station.

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Explore posts in the same categories: Ask Scope Guru Q/A, Oscilloscope Fundamentals, Uncategorized

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