WEM: Project V

Apart from sounding cool, the main claim to fame for the WEM Project V is that it was used by Brian Eno.  It's a little known but quite amazing sounding guitar effect from the late sixties / early seventies.

WEM PROJECT V DESIGN

It's really quite different to the usual suspects in terms of circuit design - it's a boost stage feeding a quasi-complementary symmetry amplifier (a power amp), followed by filtering switching.  

In some respects, the design is just a broken power amp (see GE handbook schematics below).  If C6 is removed, it's basically a clean boost - well almost clean, as the boost stage is a little dirty to start with.  Up the 100ohm resistor on the emitter in the boost stage to 1k, and it's a clean boost.

Before trying it with the switching / filter system - try it straight out of the volume control.   Is the switching worth it?  It's a pretty pure fuzz straight out the box.  The Ghost Effects Warmjet 5 has an interesting take on the tone control - worth a look.

FORUM DISCUSSIONS ON THE PROJECT V

There's a lengthy discussion on Freestompboxes on the Project V, and more on DIY stompboxes.   Both are well worth a read.  I've quoted some information below, and credited sources where appropriate.  

WEM PROJECT V - VERO LAYOUT

BIASING THE PROJECT V

Trimmer A (50K) sets the bias voltage of the junction of the two 4.7ohm resistors to half the supply voltage.   Set to 22k to begin.

Trimmer B (5K) sets the bias current to the output stage resistors.  Set to 2.2k to begin.

source:  FSB post.  Page 4, Grizzlytone.

In an amplifier 2 x diodes would be used, instead of a trimmer.

source:  DIY stompboxes.  Page 1, PRR.

PROJECT V VOLTAGES

Q3 base    1.45v

Q5 base    3.4v

Q7 base    5.15v

Junction point of 1.5k resistors    6.5v

Junction point 4.7ohm resistors    4v (with 8v supply)

source:  DIYstompboxes post.  Page 1, Digi2t.

 

PROJECT V SCHEMATIC

This is basically a re-draw of the schematic that was initially posted on FSB by grizllytone   

Just by way of comparison, this is a generic amplifier design from the 1964 GE Transistor handbook.   There are some very distinct similarities.  Diodes have replaced a trimmer for biasing, and of course, negative feedback paths are not the same on the Project V, otherwise there would be no fuzz.  Transistors in the output stage have been doubled up in some cases to help with power handling - it was 1964 after all.  

WEM PROJECT V - LTSPICE

This is the boost, and then before the switches (junction of 4.7 ohm resistors).  Everything below was done with gain at 100%

Now the output, switching as marked

Now the frequency response, switching as marked

CAPACITOR VALUES

Being a late sixties / early seventies circuit, it's scattered with capacitor values that are now hard to find.  

The 2.5u coupling capacitors as an example, the 320u output cap, and let's not forget the 1000u cap used for DC filtering.

Can common values be substituted, and where does it make a difference if you do?  Changing to common value caps will not matter, as there's definitely some room to move in this circuit.  I'd say the cap that makes the most difference is actually C6.  Cap values have to change quite a lot before really noticeable differences can be seen in this circuit.

2u5 COUPLING CAPS

Let's start with the 2.5u coupling caps.  This is taken at the output cap, just before the 1k volume pot.  2.2u vs 2.5u makes about zero difference on the coupling caps.  

I did read on one of the forums that digi2t found there was a slight difference - but up to you if it’s worth a try. 

There's not even that much difference between 1u, 2.2u and 10u

C6

There's a slight difference 2.2u vs 2.5u with C6, but nothing major.  Increasing C6 will increase the lower frequencies - so maybe it's worth playing with if you like some rumble.  

Note that if you remove this cap - no fuzz...  

Increasing from 2.2u, 4.7u, 10u, 22u, 47u and 100u, by the time you reach 22u it doesn't make much difference.  

C10

C10 is the bypass cap on the emitter of Q4.  There's zero difference between a 50u and a 47u. Big surprise... 

  

There's only a minor difference using a 10u or 22u

320U OUTPUT CAP

There's no difference between a 320u and a 330u. In fact there's very little difference at all until you get down to 10u.   So don't stress if you don't have a 330u cap.  

Despite being a small value, the 1k volume pot doesn't seem to have much bearing on the repose, so I have not included that here.

  

1000U POWER FILTER CAP

According to the post on FSB, the large cap would have been common on power amps of the day.   So it's there to feed a power-hungry amplifier, despite not having the power requirements of a power amp.

So in this case, dropping back to a regular size will not make a difference.  

Keep the 390 ohm resistor in, as it drops the voltage down to 8v.  This also reduces the chances of cooking transistors as a result of incorrect biasing.  

WEM PROJECT V ON VISUAL ANALYSER (BENCHTOP SCOPE)

Input signal:  440hz sine wave, approx 130mv TRMS.  Direct out from volume control, prior to filters (switches).

GAIN 100

 

 

GAIN 50

 

GAIN 25