The Phædrus Audio PHAB, PHAME & PHI

....... and PHILTER, a passive EQ and tube recovery amplifier

......the tube mic-pre, wow it is so rich, that you can almost hear the vocal string vibrating, its so awesome......It is so clean....... After recording vocal with PHAB, we all agree that the whole music has to be redone again, and all signals have to go through PHAB, and PHI.

The Phædrus Audio PHAB, PHAME and PHI products were developed from our work on the LONDON Console - a recreation of the famous, all valve (vacuum tube) Stereosonic mixer used to record The Beatles.

Go here for demo tracks using the PHAB, PHAME and PHI, and the London Console.

Although the LONDON Console has provoked lots of interest from all over the world, only a very few could afford a full console. Moreover, given the highly "tracked" nature of most modern production, often all that is required to obtain the magic "Beatles sound" on a track-by-track basis, is one, or a few, components from within the full console. We therefore felt there was a roll for a range of small, modular products; each with a specific role within the modern DAW-based studio.

The first three modules that we released were the PHAB microphone pre-amp', the PHAME instrument pre-amp' and the PHI DI-box.

To these three, we have now introduced, the PHILTER, which re-creates the famous passive-EQ curves which so well served four lads from Liverpool.



The Phædrus audio PHAB, PHAME, PHI and PHILTER are available...... NOW! Contact us at sales@phaedrus-audio.com or go to our How to buy page.

The PHAB, PHAME and the PHI (and the new PHILTER) are each based around the fundamental building-brick of the Stereosonic, consoles: the modular amplifiers. In these famous mixers, these amplifiers were either the German manufactured V72(S) amplifiers

or the, very rare British variant.

The modular amplifiers

Ironically, despite the fact that many more V72 amplifiers exist than do the British (.47) replacements, the EF804 valves which the German amplifiers use are no longer in current production and are becoming very expensive: whereas the EF86 and ECC88 (E88CC) valves employed in the British amplifiers remain in production and are widely available. So, a new amplifier, designed from the common source of Mullard's reference audio designs, and thereby sharing a common heritage with the Britiah modular amplifiers, was developed.

We called this amplifier the Phaedrus "PHILHARMONIC" (literally, music loving). A dozen of these PHILHARMONIC amplifier form the backbone of the Phædrus Audio LONDON Mark II console and the amplifier is the heart of the PHAB, PHAME and PHI products. (The PHILHARMONIC amplifier circuit is described in detail below.)

Mullard published many reference designs which were used in developing the modular amplifiers. Phædrus Audio went back to these same references to develop the PHAB, PHAME and PHI products.

PHAB

The Phædrus Audio PHAB microphone preamplifier is essentially the front-end of a single-channel of the LONDON console, in which PHILHARMONIC modular-amplifier is combined with a rotary attenuator, switchable pad, high-pass filter and gain-switching circuitry similar to that employed in the famous sixties consoles.

PHAME

The Phædrus Audio PHAME is a development of the PHAB in which the very high input impedance of the input pentode valve is exploited to provide a very high quality instrumental preamplifier.

PHI

The Phædrus Audio PHI DI-Box is a product designed for a wide range of applications; from live use to the recording studio. Partnered with the PHAB it provides a solution for almost any instrumental or vocal preamp' application.

The Phædrus audio PHAB, PHAME and PHI are available...... NOW! Contact us at sales@phaedrus-audio.com or go to our How to buy page.


Construction - or star earths (grounds) and curly tracks

Given their vintage, the V72 amplifiers and the British modular amplifiers were built on a metal chassis, with the components hardwired onto tag-strips. Although Phædrus Audio equipment uses printed circuit boards to ensure consistent performance and reliability, the equipment follows "classic" practices such as star earths and "natural contour" tracking, just like hook-up wire.


Every Phædrus Audio PHAB, PHAME and PHI product is hand assembled and individually tested. A test pro-forma is provided with every unit. Phædrus Audio offer comprehensive service for products both inside and outside of their warranty period.



The Phædrus audio PHAB, PHAME and PHI are available...... NOW! Contact us at sales@phaedrus-audio.com or go to our How to buy page.


About Valves (Vacuum Tubes)

Valve (tube) types

The Phædrus Audio PHAB and PHAME preamplifiers employ an EF86 pentode input valve (tube) and an ECC88 valve (tube) as the output device. Positions for the valves are very clearly marked on the PCB silkscreen as indicated in the illustration.

The valves (tubes) that are installed in Phædrus Audio?s products during assembly are selected to give the best possible performance. We offer versions of all products with NOS (rather than current production) valves. But these must be chosen carefully, and be pre-screened. Replacing the supplied valves (tubes) with different, collectible, or "high-end" valves (tubes) types may not affect any improvement and might cause deterioration of performance.

EF86

The EF86 is a low noise pentode, originally manufactured by many different factories and branded by Philips, Mullard, Telefunken, Valvo, and GEC, TESLA and Siemens/RFT. Many NOS valves are still available. Equivalents include the CV2901 (the UK service designation for the EF86) and the CV4085; a special quality version for military use. The EF86 tube is still being produced in Russia and in the Slovak Republic under JJ Electronics brand name (formerly Tesla). Chinese close equivalents are also still being manufactured.

ECC88

The ECC88 is a frame-grid, sharp cut-off, twin (dual) triode with an internal, separating screen. The valve was originally designed for service in the front ends of VHF radio receivers. The ECC88 (E88CC) tube type is equivalent to the North American 6DJ8 tubes and the (rare) military branded CV2492. The transposition of the figures from ECC88 to E88CC was Mullard's way of denoting a special quality build and is equivalent to the US 6922. Russian 6N1P tubes and the Chinese 6N1 tube are NOT equivalent. NEVER install a 6N1(P) in the Phædrus Audio PHAB or PHAME product. The ECC88 valve is sill being manufactured in China and in the Slovak Republic under JJ Electronics brand name (formerly Tesla).

12AU7 or ECC82

The 12AU7 is a popular, miniature 9-pin medium-gain dual triode valve (tube) and is used in many instrument and hi-fi amplifiers. Both triodes are used, in cascade in the Phædrus Audio PHI product. The 12AU7 is also known in Europe under its Mullard-Philips tube designation ECC82. This tube is widely available both new and NOS and a number of special quality equivalents are available. Current production of 12AU7 takes place in Russia, Slovakia, and China. Phædrus Audio selects and recommends the long anode (plate) version of the ECC82 (ECC802S) or 12AU7 valve for application in the PHI DI-Box.

Valve (tube) lifetime

You should replace the valves in the tubes in the Phædrus Audio products only when you start to notice changes in the sound quality. If the gain of the preamplifier decreases noticeably, then this is certainly evidence of the onset of valve (vacuum tube) failure. Before this, the tone may become "dull" and transients may be become "blunted".

That said, the lifetime of a valve (tube) is largely determined by the lifetime of its cathode emission and the small-signal valves (tubes) used in the PHAB, PHAME and PHI use oxide cathodes, which can provide adequate cathode emission for 100,000 hours or more. That's over eleven year?s continuous use. So do not replace valves (tubes) just because they have seen a few years service.

Phædrus Audio Ltd. can provide suitable valves (tubes) as spares which, after a burn-in period, are screened for best performance in your Phædrus Audio product. These are available as line items:

Please contact your dealer or Phædrus Audio Ltd. for current prices.


Instructions for use

PHAB - Application and connections

Mixer and computer interface manufacturers operate in a very competitive environment. Even a small mixer must (by definition) contain 8, 16 or perhaps 32 microphone amplifiers. Similarly, a computer interface designer has to "shoe-horn" a microphone amplifier into a box with a host of other features and a noisy, semi-digital environment. Unsurprisingly, these amplifiers are designed on a careful budget and contain some compromises. Hence, the use of a few dedicated, stand-alone microphone preamplifiers, solely designed with sound-quality in mind, can transform the quality of your recorded signals. That is the aim of the Phædrus Audio PHAB tube microphone preamplifier: to offer just such a no compromise microphone amplifier in a product which incorporates the standards and qualities adopted in classic recording gear. A block diagram of the PHAB is given below. The unit is connected into a typical system as shown.

Block diagram of PHAB

Controls

Input rotary attenuator, Pad and Gain switch

The PHAB microphone preamplifier operates with a constant-gain amplifier and attenuates the signal reaching the amplifier for level control. This is NOT common with modern equipment; in which the gain is adjusted by modifications to the feedback network. Yet this approach was completely standard in the 1950s and 1960s.

The Phædrus Audio PHAB tube preamplifier is built around a modular amplifier with a fixed gain of +40dB (with a switch available to raise this to approximately +46dB). This amplifier is preceded by a rotary, switched attenuator control, offering attenuation of 0 to -30dB in steps of 6dB. A further pad (operated by a front panel switch) is available, to increase the attenuation to -40dB; thereby matching the gain of the modular amplifier and reducing the overall unit gain of the equipment to unity. These controls (the input attenuator, pad and the gain switch) are used to set the overall preamplification level of the unit and feed a suitable electrical level to your downstream equipment.

Normal operation should be to operate WITHOUT the -10dB pad engaged and with the preamplifier gain set to 40dB. Only operate the pad switch when the signal from the microphone is too great for attenuation via the rotary attenuator control: and only operate the +46dB gain switch when the signal from the microphone is too low, even when the rotary attenuator is set to 0dB.

HPF (rumble filter)

By virtue of its very high quality input and output transformers and its minimalist, wideband, valve circuitry, the pass-band of the Phædrus Audio PHAB preamplifier is extended in both the bass and extra-high frequency ranges. Due to this, unwanted, very low frequencies, due to traffic or air-conditioning "rumble" may be picked up by the microphones and amplified. The high-pass filter (HPF) filters out these frequencies and prevents them from either, intermodulating with the wanted signals with in the microphone preamp' itself, or, being fed to downstream equipment. This filter section is engaged by depressing the HPF switch.

+48V "Phantom" supply

An internal phantom supply is provided to power the microphone connected to the input. This supply is engaged by depressing the "+48V" switch on the front panel of the unit. Although this supply is designed to "ramp-up" slowly, it is NEVER a good idea to switch phantom power onto a microphone on a channel with an open fader because it can create "pops and bangs" which can damage electronic equipment, speakers and fray nerves! Always mute any following circuit before switching on the phantom (+48V) supply. To avoid loud transients, always make sure phantom power is off when connecting or disconnecting microphones.This supply is designed to support the hungriest of microphones.

PHAME - Application and connections

The architecture of the PHAME preamp'unit and how it is connected is shown in the accompanying diagrams.

Direct injection (DI) provides a clean way of applying instruments to your mixer or computer interface. This is especially interesting today, now that there are so many plug-in effects and amplifier emulations which run as software on the computer host. All that is required, when working this way, is to get a natural signal into the computer's analogue to digital conversion circuits. The software will then "take it from there".

But if only things were that simple! The problem is that a normal passive DI-box, or a solid-state input stage can rarely offer the benefits of a valve preamplifier. Here's why...........

Voicing and the valve (tube) amplifier

Guitar pickups are made by winding about 8000 turns of wire on a former or bobbin which surrounds several permanent magnets. The resistance of the wire is relatively low, but the inductance is high and the device therefore has a rising output impedance. The guitar connects to the amplifier by means of a coaxial cable with the inner, signal wire surrounded by the earthy "screen". The construction of this type of cable is not a million miles away from the construction of a capacitor and, unsurprisingly, the coaxial cable displays a significant capacitance at audio frequencies. When the inductive pickup is connected via a capacitive cable, a resonant circuit is formed and we can model this and confirm that this creates a musically-significant resonant-circuit well within the audio band. (Trace below.)

This resonant mechanism is one of the principal voicings for the electric guitarist. Three factors affect the resonant circuit: the inductance of the pickup; the capacitance of the cable; and the input impedance of the amplifier: essentially the greater the capacitance and the greater the inductance, the lower the formant frequency. This is why expensive "Custom Wound" pickups and Humbuckers (which have more turns and therefore greater inductance) sound considerably more "chunky" or "fatter" than standard pick-ups. It's the major reason why cables really do sound different; fatter cables (because of their lower capacitance) "singing" better than cheap, thin cables.

The effect of the amplifier input resistance is to damp the resonant circuit. Valve preamplifiers do not do this because of the common value of 1M? for the bias resistor of the first valve. Solid state circuits on the other hand ? due to their higher bias currents and lower circuit impedances ? can have a major damping effect on the input signal, destroying so much of the character imparted by the resonant circuit (see trace below).

In short, the formant frequency is absolutely crushed and the character of the voice destroyed. Amongst many other reasons, this, so many solid-state guitar preamplifier designs fail to have the "spring" and "life" of a good valve amplifier: the precious signal is choked before it reaches (literally and metaphorically) first base!

Controls

Input attenuator

The input attenuator is a good quality, high-value potentiometer; just like the volume control on an amp?. Set this control so as to drive the appropriate level to the downstream equipment.

Gain select (low/ high)

The gain select switch should normally be set to the low position. In the high position it is possible to overdrive the tube preamplifier circuitry from a guitar. This setting is for low output instruments and/ or creative effects only.

Ground (earth) lift

Hums and buzzes are the bane of an audio engineer?s life. Unfortunately, it is the nature of a high-impedance, unbalanced instruments like an electric guitar to be highly susceptible to the AC electromagnetic fields thrown off by all mains-powered equipment. Often all that is required to solve the problem and reduce the hum (and/or buzz) to a manageable level is to orient the instrument differently so that the pickups no longer intercept so much changing flux. But sometimes hums and buzzes prove to be particularly stubborn and intractable, regardless of the guitarist?s pirouetting! When this happens, it is usually because of a ground-loop (or earth-loop).

A ground-loop forms when an unbalanced, audio signal flows in a circuit in which the signal-return (usually the cable braid), provides a current path which is in parallel with the installation earth. This is a problem because "all earths are not equal". That's to say, the installation earth will be carrying certain leakage currents. These currents set up a potential across the installation wiring which drives a circulating current in the braid of the audio cable. The result is a ?hum? voltage superimposed upon the audio signal.

The adoption of balanced signals, and especially audio transformers should ensure that ground-loops (or earth-loops) should NEVER occur. Certainly the Phædrus Audio PHAME, by being both double-insulated and incorporating an earth-free, transformer-balanced output stage can never create a ground-loop in a correctly wired studio. However, connection to an earthed (or grounded) guitar amplifier can cause a circulating current should pin 1 of the output XLR be earthed (grounded) elsewhere in the installation.

This is the role of the GROUND-LIFT switch. It interrupts the connection of pin 1 of the output XLR to the chassis of the PHAME and to the sleeve and screen of the guitar wiring. Adjust the position of this switch to eliminate any ground-loop ?hums?.


PHI

Application and connections

Simple DI units utilise a transformer to balance the unbalanced signal and feed it to the microphone input of your mixer or microphone preamplifier. This technique provides a reasonably high-impedance as presented to the instrument; typically a few tens of thousands of ohms. Adequate for a keyboard, for example: at a push, sufficient for a bass guitar. But the tone of an electric guitar is completely destroyed, unless it drives an impedance hundreds of times greater than can ever be provided by a transformer. The only answer is to present the instrument with the very high input impedance, such as it "sees" when plugged into a valve amp' (see Voicing and the valve amplifier section above). This is the role of the Phædrus Audio PHI DI-Box. The dual valve (tube) stage acts as an impedance-converter, presenting the instrument input with an impedance of approximately 1Megohm. The output of the unit then feeds the microphone input of your mixer, computer interface, or your PHAB preamplifier.

The Phædrus Audio PHI DI-Box is connected as shown in the diagrams above. Ensure that the "LOUDSPEAKER/INSTRUMENT" switch is in the "INSTRUMENT" position. However, note that feeding the output of the PHI into the studio console will negate some of the benefits of the PHI?s unique character. Use a premium-quality microphone preamplifier (such as Phædrus Audio?s PHAB) for the very best results.

Controls

Instrument/ Loudspeaker switch ? LOUDSPEAKER MODE

As a general rule, guitar and bass guitar and certain electric pianos and organs rely on the distortions in their amplifiers and loudspeaker-cabinets for the character of their sound. Ideally of course, these instruments need to be captured with microphones, rather than applying a line-level signal to the mixer. However, this is not always possible for a host of reasons including: unacceptable "spill" from other sources; feedback problems; room resonance issues; cabinet "buzz", inadequate microphone response and so on.

In this situation, it is useful to be able to DI, not the output from the instrument, but the signal which drives the loudspeaker. This signal is not modified by the cabinet, but it does have - at least - the character of the amplifier, its tonal modifications and distortion characteristics, imprinted upon it. The Phædrus Audio PHI is able to provide this functionality. However, this loudspeaker signal is of a very much larger amplitude than this signal which leaves the instrument - it has been amplified after all! It is therefore necessary to attenuate this signal before presenting to the DI circuitry and this is exactly what is done when the "LOUDSPEAKER/INSTRUMENT" switch is moved to the "LOUDSPEAKER" position. The attenuation is -32dB.

When the Phædrus Audio PHI is used in LOUDSPEAKER mode, it is important to observe the correct polarity of the signal fed from the amplifier to the PHI. To avoid short-circuiting the output of the amplifier, the earthy side of the loudspeaker signal MUST be connected to the sleeve of the input jack. In addition, as a precaution, ALWAYS operate with the Ground lift switch in the ?LIFT? position when operating the PHI in this way.

Flat/ Cabinet switch

The problem with the signal from the amplifier is that it often contains many high frequencies which are not reproduced by instrumental loudspeaker cabinet. The result is a ?fizzy?, over-bright signal; very different from that heard from the loudspeaker. The reason for this is that the loudspeaker and cabinet act together to form an asymmetrical band-pass filter; steeply attenuating the treble above about 4kHz, and gently rolling off the bass.

The signal from the amplifier may therefore be very greatly improved by the incorporation of an electrical filter which reproduces the electro acoustic band-pass of the loudspeaker cabinet. To this end, the Phædrus Audio PHI - DI unit, is enhanced by the inclusion within the unit of a loudspeaker-emulation filter which imprints the response of a typical, closed-back, guitar cab' onto the signal. The general response of this filter is illustrated above. Essentially the response falls away gently in the bass, peaks in the mid-treble and falls away rapidly above about 4kHz as shown in the figure.

Ground (earth) lift

A ground-loop (earth-loop) forms when an unbalanced, audio signal flows in a circuit in which the signal-return (usually the cable braid), provides a current path in parallel with the installation earth (ground). Small potentials in the installation earth (ground) wiring, drive a circulating current in the braid of the audio cable. The result is a ?hum? voltage superimposed upon the audio signal.

The adoption of balanced signals, and especially audio transformers should ensure that ground-loops (or earth-loops) should NEVER occur. Certainly the Phædrus Audio PHI, by being both double-insulated and incorporating an earth-free, transformer-balanced output stage can never create a ground-loop in a correctly wired studio. However, connection to an earthed (or grounded) guitar amplifier can cause a circulating current should pin 1 of the output XLR be earthed (grounded) elsewhere in the installation.

This is the role of the GROUND-LIFT switch. It interrupts the connection of pin 1 of the output XLR to the chassis of the PHI and to the sleeve and screen of the guitar wiring. Adjust the position of this switch to eliminate any ground-loop ?hums?.

The Phædrus audio PHAB, PHAME and PHI are available...... NOW! Contact us at sales@phaedrus-audio.com or go to our How to buy page.


Links

Back to home page

Address all mail to sales@phaedrus-audio.com


Maidstone . Kent . UK .


© Phædrus audio 2011. All rights reserved.