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A SUBSONIC FILTER IS HIGHLY RECOMMENDED IF YOU SPIN VINYL, USE PORTED SPEAKERS, AND WANT THE VERY BEST PERFORMANCE FROM YOUR ANALOG FRONT-END

1/27/2014

By Michael Miguest

The majority of today's Pre-Amps don't have a phono input, tone controls, a balance control, and let's not even talk about turnover and roll-off settings for older recordings, contour, stereo reverse, a phasing switch, mono a+b, mono a, and mono b modes, and a low frequency filter. My restored Harman Kardon Citation 1 preamplifier has all of these features and not one but two phono inputs. I embrace and use all of these features with one exception. As an audio purist/minimalist, I do not engage the tone controls because in my opinion, they compromise the audio presentation. I set these to zero, which takes them completely out of the circuit. I set my turnover and roll-off switches to RIAA. The following are the Citation 1's Specifications :

Frequency Response at 1W...............(+0dB, –3dB) : 5Hz - 80kHz
Distortion (Stereo).............................<.05% at 2 volts
Hum and Noise.................................High Level Input: 85dB below rated output /
..................................Low Level Input: less than 1.5 microvolts referred to input terminals
Output Level.....................................Main Outputs: 2 volts / Tape Outputs: 0.3 volts
Input Sensitivity Line (high)................0.2 volts
Input Sensitivity Line (low)..................Phono RIAA: 2.5mV / Tape: 2mV / Ceramic Phono: 0.1 volts
Filter................................................Low Frequency: FLAT; 15Hz subsonic filter; 75Hz rumble filter

I initially set my Low Frequency Filter switch to flat until I discovered some very amazing facts.There exists a phenomenon called woofer pumping aka woofer excursion. Frequencies below 20Hz are usually not able to be reproduced, and with the exception of synthesizers and pipe organs, are not a wanted part of the audio spectrum. This is especially troublesome with phono systems, since many of the vinyl discs you treasure (or wish to transcribe to CD) will be warped to some degree. Any warp in a vinyl disc will cause large outputs in the subsonic region, typically well below 20Hz. For example, a 33 1/3 RPM album with a single warped section will create a signal in the pickup at 0.55 Hz (33.3 RPM / 60 = 0.555 Hz). This is a signal that will cause significant cone movement, but is undesirable in the extreme. Not only will vented subs be completely unable to handle such a signal linearly, but sealed subs will also be stressed. Large amounts of available power will be wasted trying to reproduce a signal that was never intended to be there in the first place. To be effective, a subsonic filter has to be very steep - this allows all wanted frequencies to get through, and rejects those that will only cause problems. Note that my Citation 1 Pre-Amp's subsonic filter is set at 15Hz. http://sound.westhost.com/project99.htm

1) LP pressings, even high quality ones, have surface ripple many times larger than the width of a groove, take a look sometime at the surface of an LP against some backlight. The frequency of ripple is around 0.5Hz to 10 or 20Hz, that is, subsonic.

2) the arm and cart have a resonance at very low frequencies and this resonance band contains the frequencies of the disc surface ripple.

3) the RIAA curve puts 20dB of gain, the maximum, right where this same band of subsonic ripple exists. The result is a big subsonic ripple waveform sent to the input of the preamp and power amps. Most modern amps can reproduce this stuff so it arrives at the speaker woofer and if the combination of three factors is unfavorable you get massive woofer cone flutter.

There is no sonic information below 15 Hz on an LP. It's just noise, and it can lead to IM distortion, and it over-works the amp. As a purist, I was astounded at the results of the subsonic filter setting. I just knew it would compromise my sound quality. What I experience however, with the subsonic filter setting is a greater sense of power from my amp, more PRAT, an enhanced mid-range, greater bass articulation, greater decay, greater 3 dimensionality, lower noise floor, and a more liquid, musical presentation with a noticeable increase of detail and clarity. I attribute my system's enhanced verisimilitude to power not being wasted on trying to reproduce the inaudible, low frequency ( subsonic) noise containing no sonic information but instead being used to reproduce the audible sonic range. For the record, in order to experience the maximum performance from your analog front-end, your turntable must be properly isolated and your tonearm and cartridge must be properly matched. With that said, the sad reality for many (there might be exceptions) users of ported speakers that spin vinyl is that a good subsonic filter is not an option but simply a necessity.

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TUBES VERSUS TRANSISTORS

1/16/2014

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By Michael Miguest

It's 2014 and the debate still rages on. My present system includes the following components: a restored Harman Kardon Citation 1 Preamp, a VTL Stereo 90 Deluxe Ultra-Linear Power Amp, a Modified Monarchy NM 24 Tube DAC, a Monarchy AC-Regenerator, a Proceed CD Transport, a Thorens TD 126 MKIII Turntable using a Denon 103r, a KAB Modified Technics SL-1210 MKII Turntable using a Denon 103, an Allnic AUT 2000 Step-Up-Transformer, cabling: Soundstring Gen II Interconnects, Speaker Cables and Power Cords, Dynamic Design Interconnect, and an Audience AU 24e Interconnect, Creative Cable-Silver Bullet Digital cable, Black Diamond Racing MKIII isolation cones, Maple Blocks, and Focal Utopia Diva Be Speakers. Tube complement of the Harman Kardon Citation 1 Pre-Amp are 4 NOS 12AX7 Telefunken Ribbed plate and 4 NOS 12AT7 Mullard 10M series. Tube complement of the VTL Ultra-Linear Power Amp are 4 New Sensor Corporation Genalex KT-88 Power Tubes from Jim Mcshane, 2 NOS 12AT7 Siemens 801S's and 2 NOS 12AT7 triple mica black plate 5 Star GE. Tube complement of the Modified Monarchy NM 24 are 4 NOS 6922 Siemens Halske. In my opinion, tubes, especially NOS tubes, sound better because their distortion products are more musical. Tubes provide a more appropriate load to transducers. Those are the fundamental reasons why tubes sound better, but is there more?

Tube amplifiers produce more (total) harmonic distortion, but the type they produce is referred to as even order distortion, and is not as harsh-sounding as the odd order distortion transistors produce. Large amounts of even-order distortion (as high as 1-2%) produces little listening fatigue, and can even be relatively pleasant. Electric guitar players favor tube amps for this very reason. On the other hand, small amounts of odd-order distortion (less than .5%) are audible, even by untrained ears, and make music harder to enjoy. Some people say that tube amps color the sound. Perhaps that is true, but maybe tubes, from a psycho-acoustic perspective, just do something in a way that matches how the ear functions, or the brain processes sound. I have had both all solid-state and all tubes systems and tube/solid-state combination systems and with an all tube system, subtle artist intonations are clearer and more pronounced, fostering a greater sense of realism and emotional connection to the recorded event.

Let's take a look at the advantages and disadvantages of both taken from the Audio Archive:

http://www.theaudioarchive.com/TAA_Resources_Tubes_versus_Solid_State.htm

Vacuum tubes – Advantages

Highly linear without negative feedback, especially some small-signal types
Clipping is smooth, which is widely considered more musical than transistors
Tolerant of overloads and voltage spikes
Characteristics highly independent of temperature, greatly simplifies biasing
Wider dynamic range than typical transistor circuits, thanks to higher operating voltages
Device capacitances vary only slightly with signal voltages
Capacitive coupling can be done with low-value, high-quality film capacitors
Circuit designs tend to be simpler than semiconductor equivalents
Operation is usually in Class A or AB, which minimizes crossover distortion
Output transformer in power amp protects speaker from tube failure
Maintenance tends to be easier because user can replace tubes

Vacuum tubes – Disadvantages

Bulky, hence less suitable for portable products
High operating voltages required
High power consumption, needs heater supply
Generate lots of waste heat
Lower power efficiency than transistors in small-signal circuits
Low-cost glass tubes are physically fragile
More prone to micro-phonics than semiconductors, especially in low-level stages
Cathode electron-emitting materials are used up in operation, resulting in shorter lifetimes (typically 1-5 years for power tubes)
High-impedance devices that usually need a matching transformer for low impedance loads, like speakers
Usually higher cost than equivalent transistors

Transistors – Advantages

Usually lower cost than tubes, especially in small-signal circuits
Smaller than equivalent tubes
Can be combined in one die to make integrated circuit
Lower power consumption than equivalent tubes, especially in small-signal circuits
Less waste heat than equivalent tubes
Can operate on low-voltage supplies, greater safety, lower component costs, smaller clearances
Matching transformers not required for low-impedance loads
Usually more physical ruggedness than tubes (depends on chassis construction)

Transistors – Disadvantages

Tendency toward higher distortion than equivalent tubes
Complex circuits and considerable negative feedback required for low distortion
Sharp clipping, in a manner widely considered non-musical, due to considerable negative feedback commonly used
Device capacitances tend to vary with applied voltages
Large unit-to-unit variations in key parameters, such as gain and threshold voltage
Stored-charge effects add signal delay, which complicates high-frequency and feedback amplifier design
Device parameters vary considerably with temperature, complicating biasing and raising the possibility of thermal runaway
Cooling is less efficient than with tubes, because lower operating temperature is required for reliability
Power MOSFETs have high input capacitances that vary with voltage
Class B totem-pole circuits are common, which can result in crossover distortion
Less tolerant of overloads and voltage spikes than tubes
Nearly all transistor power amplifiers have directly-coupled outputs and can damage speakers, even with active protection
Capacitive coupling usually requires high-value electrolytic capacitors, which give inferior performance at audio-frequency extremes
Greater tendency to pick up radio frequency interference, due to rectification by low-voltage diode junctions or slew-rate effects
Maintenance more difficult; devices are not easily replaced by user
Older transistors and ICs often unavailable after 20 years, making replacement difficult or impossible

Here is Ayon Audio's perspective on tubes:

Simpler, purer circuits
At Ayon Audio, we feel that tubes are the sonically superior technology for audio, as their generally simpler circuits and smaller number of components provide for a purer signal path and consequently more truthful signal handling. This is because fewer components provide for fewer elements in a circuit to degrade the signal, distort and muddle the sound. Simple circuits also provide for inherently higher reliability, since there are fewer parts which can fail. Tubes are also more tolerant of circuit drifts and deviations in component specifications, and thus can be used in simpler, purer circuits.

More benign overload and distortion behaviour
A lot of music features great dynamic signal swings, and it has been well established that in tube amplifiers the onset of clip/overload as maximum power is reached is gradual and rising distortion is of predominately low even-order harmonic nature. In comprehensive listening tests, even high levels of even-order harmonic distortion has been found to be significantly less offensive to the ear than even small levels of the harsh, odd order harmonic distortion produced by solid-state circuits when they reach their power limit and enter clipping. In transistor amplifiers, the distortion rises very quickly as the maximum power level is reached, showing almost square wave characteristics, and a high DC component, - which can easily destroy loudspeaker drivers if not stopped to do so.

Vacuum tubes and “Tone”
The difference in the distortion characteristics between the two technologies can be well illustrated by looking at their effects in guitar amplifier design.Tube guitar amplifier manufacturers have traditionally designed their circuits to drive the output stages into overload distortion, using the resultant distortion to achieve their trademark “tone”. In a tube amplifier, this tone contributes to the amplifier's sound, but in a solid-state amplifier this distortion is audibly intolerable and easily destroys the speakers. When transistors overload (in a discrete circuit or in an OP amp), the dominant distortion product is the third harmonic. The third harmonic "produces a sound many musicians refer to as blanketed”. Instead of making the tone fuller, a strong third actually makes the tone thin and hard. On the other hand, with tubes (particularly triodes) the dominant distortion product is the second harmonic: “Musically the second is an octave above the fundamental and is almost inaudible, yet it adds body to the sound, making it fuller”. Tubes sound better because their distortion products are more musical. Tubes provide a more appropriate load to transducers. Those are the fundamental reasons why tubes simply sound better.

Vacuum tubes are the more linear and require less feedback
Tubes are voltage amplifiers as opposed to transistors which are current amplification devices. As a consequence, tubes are a more linear amplification technology, requiring less overall negative feedback to make the circuit linear. Negative feedback re-injects a sample of the amplifier’s output signal back into the input, 180 degrees out of phase, in an attempt to reduce amplifier non-linearity and distortion. In practice, negative feedback tends to slow the amplifier down and sucks the emotion and life out of the music. High feedback designs usually sound sterile, boring and lifeless, while low or zero feedback designs provide for a more immediate and natural sound. Depending on technology and type of the used output device, transistor amplifiers generally require the use of over 40dB of local loop or global negative feedback.

Superior dynamic capabilities
The higher working voltages present in tube amplifiers generally allow for wider voltage swings and better signal headroom before entering into overload territory. Higher working voltages yield higher audible energy storage* with lower value capacitors. ~ 500 volts working voltage in a tube amplifier approximate about ~ 80 volts in a transistor circuit. This is most likely why many listeners feel that tubes sound more powerful.

*Audible energy storage is voltage squared divided by 2 multiplied by capacitance.

The two professional societies that have the most to say on this subject are the IEEE (Institute of Electrical and Electronics Engineers) and the AES (Audio Engineering Society). Both of these professional societies publish peer-reviewed journals, with articles written by engineers and scientists who work in the professional and consumer audio industry, as well as in cutting-edge academic research. If you are seeking a balanced view on this debate, direct yourself to either or both of these societies.
The IEEE published “The Cool Sound of Tubes” in their August 1998 issue of IEEE Spectrum. In the same article, there is also a useful sidebar on tube versus transistor distortion.

The AES (Audio Engineering Society) published a journal article in May 1973 titled "Tubes versus Transistors: Is There An Audible Difference" that focuses primarily on the distortion aspects of tubes versus transistors.

One of the more interesting quotes from the AES article:“Our extensive checking has indicated only two areas where vacuum-tube circuitry makes a definite audible difference in the sound quality: microphone preamplifiers and power amplifiers driving speakers or disc cutters. Both are applications where there is a mechanical-electrical interface.”In addition to speakers, disc cutters and microphones we can include phono cartridges and musical instrument pick-ups (ie. guitars) in the world of mechanical-electrical interfaces where tubes have an advantage.

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A SUBSONIC FILTER IS HIGHLY RECOMMENDED IF YOU SPIN VINYL, USE PORTED SPEAKERS, AND WANT THE VERY BEST PERFORMANCE FROM YOUR ANALOG FRONT-END

TUBES VERSUS TRANSISTORS

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