Thursday 3 January 2013

Runoffgroove Omega

Continuing with the amazing Runoffgroove library, this time with Omega treble booster. The description below points out that this may be the last treble booster you'll need to build. ROG sound clip is here.

From ROG website:
This project began as a search for a FET treble booster similar to the well-known Dallas Rangemaster. We wanted to overcome several problems associated with building a Rangemaster clone, such as:

Getting a PNP or NPN Ge transistor with low leakage and proper hfe (50-100)
Bias variation with temperature
Poor noise performance (mainly hiss)
In addition, we wanted to retain several features of the original circuit, such as:

Guitar loading
Frequency response
Overall gain
Low output impedance
Non-linear behavior and soft clipping near collector cutoff
A variation of the Fetzer Valve, which we named the High Gain Fetzer Valve or HGFV, allows the accomplishment of all these goals. The HGFV uses a fixed high-valued drain resistor to increase the gain up to 30-35 dB (30-50 times). The drain bias voltage is adjusted by varying the source resistor, which is bypassed by a large capacitor.

During the development we found that varying the input impedance greatly affected the range of boosted frequencies, hence we named this control Range. For best results, the Omega should be driven directly by your guitar.

The output impedance of the HGFV is on the high side, so a MOSFET buffer takes care of driving hard whatever it follows. The gate of this MOSFET is fed from a voltage divider built on the drain resistor of the HGFV stage. This avoids clipping during the lower signal cycle in the output buffer.

The output control could have been named Level or Volume, but we preferred to call it Master to increase the mojo factor.

The resulting circuit is so flexible that it ranges from a treble boost with a slight amount of grit (in fact pretty faithful to a Rangemaster but with less hiss) to a fat overdriven boost (comparable to a gainy LPB-2). In between there are many sweet spots to be found that will correspond to your personal tastes and gear.

The drain bias voltage of the first stage plays a role in determining the character and grit added by the circuit. We encourage you to experiment and find the bias voltage that works best for you. A bias voltage of 7V proved to be faithful to the Rangemaster tones, while a lower drain voltage in the range of 4.5 to 5V was somewhat better for the fat sounds.

We thank Michael Klein who provided the name for this circuit. Omega is the last letter of the Greek alphabet, and we believe this is the ultimate booster. It is also the symbol used to represent ohms, the unit used to measure impedance, and it fits nicely into our circuit since the input impedance is strategically varied to alter the frequency response.



10 comments:

  1. Mark. you can mark this down as verified.loads of output and gives the naga viper a good run for its money tonally.

    Graham

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  2. I just built this one, awesome boost, definitely a one to box, thanks for the layout !

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  3. And I've made 3 fetzer , the 2 of thiw for preamp for my tda2030 homemade guitar amp and one stompbox edition..
    my questions is..
    1)the 100k trim i thing is too much ohm.. Because if I want to set it should do little and slight movement..It is a small point that works(if i put 10k or 22k is better? why not?)
    2) power supply with clear sound?

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  4. Great layout and circuit. It rivals the real rangemaster

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  5. Will this fit in a 1590A box?

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  6. hi guys im thinking of boxing this infront of a sun fuzz face... but the omega has a buffer on the output. would it be best to leave the buffer out?

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  7. Ok, need help with this one. I have built many pedals but this one has me perplexed. I am passing signal but with everything full up I am right at unity in volume. The controls work but the "boost"in signal is not there? Changing bias voltage makes it clip a bit but volume stays the same
    I have checked cuts and links and solderbridges and wiring. I even changed the Transistors with several diff ones to rule out bad components there. I get some scratchyness on one of the pots but I cant measure any DC leaking from the caps.

    Voltages Q1 G 0v
    S .733v
    D 5.48 v tried everything from 4.5 to 7v

    Q2 D 9.84v
    G 9.59v
    S 7.23v

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  8. That "scratchy" pot seems to be in the same league as the Super Hard-On "crackle ok" It is the way transistors react when you are tickling them ;o)

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