Project Q & A with Theo and Thom
Thom, what were your requirements for the project?
Overall, I was after what I identify as a specifically valve sound quality. Some vintage valve gear I've used has it, and that’s what I wanted.
In the studio I work mainly with electronic sound sources, such as synths, samplers, drum machines, DAW etc.
Valves amplifiers, eqs and mechanical reverbs offer unique ways of processing audio that can imbue it with what I find to be very desirable sonic characteristics.
These characteristics were clearly recognisable in examples of AAE's hardware I listened to online.
Next, I was intrigued by the control AAE units gave over the sound being produced, such as in the eq design and spring reverb driver. Its like they have qualities of being instruments in themselves, and that quality lends itself to a way of working in the studio which has always excited me.
Preparing a new case - ensuring plenty of air flow keeps internal components cool for longevity and reliability.
My basic requirements when I approached AAE were for a two channel unit I could use at any step - recording, mixing or pre-mastering. It needed to be flexible, dynamic and low noise enough to support subtle through to extreme application. For example it might be used for severe distortion and filtering when recording; but then later across a final mix for subtle introduction of warmth, tone shaping and ambience.
During our initial design discussions Theo's deep knowledge and practical experience in valve audio design was both fascinating and inspiring. I got more and more interested in the wide range of effect valves are capable of having on an audio signal. We talked alot about features that would exploit this characteristic, such as triode, pentode, on and off load operation, a combination of active and passive eq components and so on. The final design added a continuous bias control and feedback circuit for the same reason.
In addition I wanted the order of the Distortion and EQ modules to be swappable because in the past I've found this can make a big difference to the achievable range of tones. Finally, I wanted a filter section to sculpt the distortion output, and make it more flexible for mixing with the non-distorted signal.
As the project developed and Theo planned out the signal flow I realised send and return points at stages along the audio path would be useful for integration of the various components into a studio environment. This enables external processing and parallel mixing of signal feeds through or from the unit as necessary.
Overall the design has become quite modular. Theo described it as akin to an analogue audio computer, which I really like the idea of. After a couple of sessions testing and tuning the first channel it's clear there's huge scope for sound shaping in there, all built on a foundation of fantastic valve sound quality.
Exactly what we were after!
Theo, can you describe the project from a technical perspective?
This is a dual mono project that combines sections and circuits that I've designed and built in the past. It's effectively a valve based analogue sound processor - a combination of a distortion unit (DU), various frequency boosters/equalizers (EQ's), valve spring Reverb (REV) & Return (RET) system plus various inserts (send/return sections) per channel.
Bespoke analogue - completed wiring on the underside of one channel of the EQ/DU module.
The inputs accept a wide range of line level signal from 0.316Vrms (-10dBV old unbalanced standard) to 1.23Vrms (+4dBm balanced standard) or even higher depending upon how the input stage gain switches and level controls are set. The main inputs and outputs are balanced using XLR type sockets, along with optional unbalanced jack sockets.
The unit is able to be adjusted to produce clean, mild or even heavily distorted sound.
The type of harmonics generated can be even and low in order, or odd if so desired for a harsher sound. This harmonic distortion can be combined with adjustment and boosting of certain frequencies through the various EQ's, and it can be very effectively used to shape and animate the sound qualities captured in digital recordings.
On the other hand, the unit can also be used as an extreme sound effect; for instance by introducing the Positive Feedback and OFF LOAD mode features in the DU section.
Next are the various modes that the valve Spring driver operates in. These can produce anything from traditional mild reverb to serious overdrive spring distortion.
Finally, the various frequency shaped signals - dry, reverberated, with or without overdrive distortion (it all depends on how each section is set) plus the original signal are mixed at the output stage and a variety of different sounds can be created.
It's important to emphasise that just like with any analogue processor, experimentation by the user is necessary!
To achieve the required sound quality and sonic flexibility within a valve based design, can you give some more detail on the design approach you took?
Let's start with the ' DU post EQ mode' which is when the input stage drives the EQ driver:
Both of these stages are triode valve designs - 12ay7 and 12au7 respectively.
The 12ay7 is a very versatile input stage valve, because of the range of signals it can accept. It has a slightly lower gain and plate resistance than the commonly used 12ax7 (UK ECC83), so it can accept higher level signals and can drive a lower resistance potentiometer (the DRIVE) control. Its input gain can also be adjusted through the gain switch by introducing local negative feedback, if the desire is to reduce the distortion on high input signals. This tube was widely used in some of the early Fender guitar amps of the "Tweed" era.
The 12au7 (UK ECC82) is the best choice to follow, due to its even lower amplification and plate resistance, accepting the high level signals coming out of the 12ay7 stage through the DRIVE control. Its low plate resistance (especially when it works in parallel mode) makes it possible to drive (and interact with) the passive EQ.
The way this tube is biased is important for creating the "second harmonic sound", and for being able to benefit from its grid's ability to accept high signals before it runs into the grid current region, thus avoiding the blocking distortion effect. However, if the DRIVE control is set high enough this valve will produce grid current distortion... it's all a matter of choice.
Totally tubular - 12ay7 and 12au7 valves installed in one channel of the EQ/DU module.
Alternatively, on "low gain " mode it becomes very clean through local negative feedback again. The 12au7 is can be found in both pre-amps and phase inverters in Hi-Fi power amps, making it a very versatile device too.
The gain of the 12ay7 can be adjusted by a 3-way switch, and the 12au7 stage input is controlled by the EQ DRIVE control, as well as having its own 2-way gain switch.
These control and gain switch settings are very important for the initial sound processing, For example, with a 0 dB (0.775vrms) input signal, even if the gain switch is set high, as long as the EQ DRIVE control is set somewhere around half way, the distortion will be low, mainly even and second harmonic type.
As a result the sound is clean but with a colouration which may also be expressed as "warmth". This is because the triodes are operating under conditions which aim to achieve a high level clean signal at the plate of the second (12au7) valve.
One of these operating conditions is the relatively high power supply (400V) voltage, another is the choice of the load that each one of these valves "see". Carefully selected for second harmonic distortion, the amount of load is proportional to the amplitude (strength) of the input signal and choice of control settings. For these reasons the two stages cannot produce very high levels of distortion.
The passive EQ loads the EQ driver and plays a serious role in the sound here - firstly due to the loading effect and secondly it emphasises certain harmonics, reduces other harmonics and gives a certain "character" to the sound.
The EQ interacts with the stages before and after it. If you turn up the EQ DRIVE control and set both switches high to overdrive the EQ DRIVER, you can use the EQ to boost or attenuate harmonics that have been generated by this stage.
Similarly you can make those two initial stages clean by the use of the control switches, and then use the EQ to shape the signal before it enters the following stages (ie DU, REV DRIVER etc), setting the gain(s) on these stages to overdrive one of more of them.
The high signal solid state buffers that pick up the signal from the EQ also work at 400V. In order not to affect the signal, these circuits must never reach clipping point and are very transparent.
The Distortion Unit features two valve operating modes - Triode and Pentode - what are the differences between them?
The purpose of all the options and controls is to access the wide range of wave shaping, or distortion effects which valves have a unique ability to produce with audio signals.
At low input signals TRIODE mode produces predominantly second harmonic distortion which is more musical. Drive and gain controls must be set low to drive the DU softly if this kind of sound colouration is desired; and to avoid clipping. As the drive level is turned up, more distortion will be generated, first low even, then low odd (like third) and a smaller amount of higher orders both even and odd, until eventually soft clipping occurs.
In PENTODE mode distortion starts earlier, as far as the drive level is concerned. PENTODE output will produce more third and higher order odd distortion products, in addition to second and higher even products. By overdriving the power stage in higher drive settings and with gain switched high, harsh edgy sound and a boost in high mid and treble occurs.
Triode and Pentode modes can also both be switched between Low and High loading operation. What effect does this have?
The loading affects the behaviour of the valves and associated circuitry, giving access to a greater variety of tonal colouration through shaping of the audio signal.
Its important to note that the 'load - mode' rotary switch is simultaneously switching parts of circuits on both primary and secondary sides of the DU, so it affects them both.
The terms Low and High relate to the load that the DU output "sees", or is subject to.
In TRIODE mode, 'low load' is essential for a mild and low distortion tone, especially if the DRIVE control is also set low.
In PENTODE 'low load' mode, the valves' screen voltage is also switched to a lower value and as a result greater distortion, and more odd harmonics are produced.
'High load' in TRIODE mode creates greater even harmonic distortion.
'High load' in PENTODE mode generates less distortion (compared to 'low load') in this particular circuit. For low level drive signals the second order dominates, followed by the third. As the drive signal increases, so does the output (plate) signal, and it will contain higher orders of odd distortion and some even.
In this DU design I've chosen the high loading to be near the 'critical load' value.
This was the norm that audio designers tried to achieve in the 1950s and 60s when they were designing valve circuits for anything from table radios, to low power guitar amps and Hi-Fi's. The term 'critical load' refers to the load value that audio engineers derived from testing and extensive mathematical and graphic analysis.
Pentodes are more complicated than Triodes due to their extra electrode, known as the 'screen', and the voltage of this screen is also related to the critical load value.
So why use the critical load value? Pentodes are far more power efficient at critical load, so this choice results in a good compromise between low distortion and power. When this approach is combined with NFB (negative feedback) it reduces the residual distortion which increases at higher output signal levels, and it reduces output transformer core saturation distortion. However, low distortion was not of primary concern in guitar amps (and small PA's) so these employed very little NFB, like the Fender Champ with its classic sound.
In this DU design, power is not an issue because it's not being used to drive a speaker. But, if you want to emulate this sound in one of the DU modes then you go down the 'critical load' route. NFB is not applied because this results in a more characterful sound, and if you want lower distortion you can simply reduce the DRIVE and increase the OUTPUT LEVEL.
So to summarise
TRIODE LOW = low distortion
TRIODE HIGH = more distortion primarily second on low signals
PENTODE LOW = high distortion, plenty of third and odd products at higher signals
PENTODE HIGH = less distortion mainly second with a bit of third, products increase at higher signal levels due to the absence of NFB
To be continued...