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previous: Michael Hi, thanks for responding to my que... -- 5/22/2004 6:30 AM View Thread

Re: Power supply for combo effect

5/25/2004 6:17 PM
Mark Hammer
Re: Power supply for combo effect
You learn fast, grasshopper!  
Yes, the Guvnor has some additional supply conditioning, though not as much as you'd think. The pair of 47k resistors and 10uf cap really only provide a decoupled "reference" voltage, which is half the supply voltage (4.5v). Improved power smoothing would involve a larger-value cap (e.g., 47-220uf) between V+ and ground, which can be implemented on both the Guvnor and OS, incidentally.  
The reference voltage provides the fulcrum of the signal "see-saw" to go positive and negative, relative to. Since that reference voltage (also called Vb, Vref, bias, floating ground, and probably some other things too) is essentially like another mini power-supply, it has its own little "smoothing cap" (the 10uf cap) so that variations elsewhere in the circuit don't affect it.  
The best thing you can do for noise is to start with a clean hot signal. An onboard buffer/preamp on the instrument and hum-rejecting pickups is always a good start. Doesn't have to be much of a boost to be effective.  
A decent low capacitance cable to the first pedal in line is always helpful. Similarly, if you can score some 1% metal film resistors for the initial input components on your pedals, that can help. Some op-amps are quieter than others, as well. Something like an OP0275 or an NE5532 can work nicely for the OS. As well, although most guitar pedals are designed without any restrictions on high end, the fact of the matter is that most amps have speakers that don't deliver a whole heckuva lot above 10khz. As such, rolling off treble above 10khz can help alleviate some hiss without sacrificing much useful treble.  
Active noise suppression is always a touchy issue. The classic solution is a noise gate, but these need to be adapted to each situation/instrument/style and can often charge disruption of the attack and decay of notes as the price of happy noise-free performance in the quite parts if not a perfect mariage with the music/signal properties.  
Noise filters and downward expanders (and especially units that combine them, like the Hush) can work a lot better. These turn the treble down in the quiet parts, and exaggerate the vloume differences in quiet parts as well. This produces a more natural decay.  
Finally, judicious use of compression is also another place to keep noise under control. The OS is set for a fixed amount of compression by having a fixed amount of gain in the op-amp. The 220k and 10k resistors in the op-amps feedback loop set the gain for 23. Part of the reason for this is to deliver enough of an envelope signal to drive the FET appropriately. If the input signal is a little hotter, and the OS gain turned down a little, the amount of hiss-boosting it does in the quiet parts will be reduced. You can also consider putting a cap in the feedback path of the op-amp for more treble cut. For example, if you upped the 10k resistor to 12k, and stuck a 68pf cap in parallel with the 220k resistor, you'd have a gain of just over 19 and a rolloff at around 10.6khz. So, slightly subtler compression and a bit of hiss-taming.  
Make sense?