Part the Third
“You certainly usually find something, if you look, but it is not always quite the something you were after.”
― J.R.R. Tolkien,
In this the penultimate part of the journey the path forks and I take a look at some experiments with using a valve preamp and improvements to the original FET preamp.
Opening the Valves
Eric Thacker, better known as Ecca, had demonstrated on the Shadows Music forum that Piet’s FET and op-amp preamp design could be replaced with a valve (tube) preamp that was available from a number of eBay sellers in Hong Kong and China. (Here, for example).
The circuit was originally designed to be used as a Hi-Fi stereo preamp using two 6N3 dual triode valves, one valve for each stereo channel. For the eTap2HW module we only require a mono audio pathway so the left channel of the stereo pair was used as the input preamp feeding the echo module and the right channel became the recovery amp, taking the echo module output and acting as a buffer and output level control.
Eric had produced a PCB for his ‘Eccamatic’ design which allowed the echo module to be plugged in, provided a step-up power supply to generate the HT voltage and the LT heater supply for the valves and had connections for the three pots that adjust the echo settings.
Having already designed the automation circuitry it was easy to adapt the design so that the Arduino Uno would feed the pot inputs on the Eccamatic board, reading the required settings from pots mounted on the front panel.
The front panel also had input and output pots, controlling the signal levels.
I have described this valve EchoTapper build in a series of earlier posts and if you’d like more details just click on the Valve entry in the Categories list on the right of this page.
When the new unit had been completed, the sound was quite pleasing with less hiss and noise than the FET preamp but rather ‘sterile’ as might have been expected from it’s origins as a Hi-Fi design. As I am sure you will know, guitar amplifiers and the early Meazzi echo units have circuits that are far from Hi-Fi for a very good reason (at least in the case of the typical valve-based guitar amp) – the almost perfectly flat frequency response of a Hi-Fi design and it’s deliberate lack of harmonic distortion (achieved largely through the use of negative feedback) is really not what guitarists want or indeed need to get the sound they desire. The original Meazzi preamp is hardly a classic of valve design but somehow has that magic sound that we have come to love.
Piet’s FET preamp design had made use of the ‘Fetzer Valve’ circuit which uses an FET to emulate the sound of a triode valve so it does create harmonically rich sound when the input level control is set correctly to match the guitar pickup level to ‘push’ the Fetzer ‘valve’ into generating the pleasant sounding harmonics we all like.
Steve Mitchell soon had his thinking cap on and he came up with a number of modifications to the valve circuitry that, according to his simulations of the circuit which he had made using the TINA SPICE modeller, should tailor the frequency response better to the typical guitar pickup signal and also generate those desirable harmonics to give that rich sound we all love.
Steve and I came up with a three stage process that culminated in a final design that sounded great.
Stage 1: Remove the built-in stereo volume control pot and increase the input impedance
Stage 1 was essential to allow the input and output level controls to be separated and to provide a better match to the typical guitar pickup’s high impedance.
Stage 2: Remove the negative feedback from the preamp and add Meazzi-like tone shaping
Stage 2 was regarded as highly recommended to obtain a better tone and more gain which would encourage the desirable valve harmonics and tailor the frequency response which was very flat and resulted in a ‘lifeless’ sound, thus improving the sound for guitar purposes.
Stage 3: Add a separate Gain pot and three-way tone switching
The Stage 1 and Stage 2 modifications had resulted in a really good sounding preamp but Steve came up with Stage 3 which would take the whole thing to another level. Whilst not regarded as essential, these mods did make the unit even more flexible. There would now be three ‘level’ controls, the new Gain pot controlling the drive to the second valve stage in the left channel of the amp. The Volume pot controls the signal to the echo module and the Master pot controls the level from the output socket.
The 3-way tone switch provided ‘normal’, ‘vintage’ and ‘warm’ options by tailoring the bass roll-off. On the unit in the picture (built for Mario Voltolini in Italy) I also added, at his request, a Cutting Edge Filter (CEF) which could be switched in and out if required.
Biting the Bulletins
In parallel with the improvements to the valve preamp, Steve was also working on a series of modifications to Piet’s FET preamp design.
One of the problems with any FET is that individual devices, even from the same batch, can have parameters that vary over quite a wide range and this makes it difficult to design a circuit that will allow the FET to operate in the optimum part of its characteristics.
Steve and Rolf Holmberg collaborated and came up with a spreadsheet that would calculate the best resistor values to bias the FET into the optimum operating point. This would be a bit complex for the average user to get to grips with so Steve and I came up with a simpler ‘flowchart’ approach. This would allow the best operating point for the FET to be found using a simple voltage measurement and a little trial an error with a few preferred resistor values.
Steve put together a series of ‘bulletins’ that were published on Piet’s EchoTapper blog which also included simple modifications to improve positive signal headroom, improving the FET gain and reducing white noise (hiss). These mostly involved removing or changing some resistor values and adding some extra capacitors.
Piet has since modified his original preamp PCB design to accommodate these improvements and this is documented on his blog.
Now I had two excellent sounding echo units but I had an urge to see if it would be possible to squeeze all this technology into a smaller package, one that would fit on a pedal board or sit neatly on the floor at my feet like a typical guitar effects pedal.
Thus began the germ of an idea that would result in the Blue Nebula. It had to be compact, so that ruled out the use of valves and it needed to be much easier to build, with minimal off-board wiring so that, hopefully, anyone with sufficient experience would be able to put one together from a kit or buy a completed built-to-order unit at reasonable cost.
We had to go from this …
to this …
To be continued.