Whisper compressor schem via web.archive.org

[timo]

Hi all,

This is my first post here even though I've been reading and searching this forum religiously for months. As a pedal builder I'm an utter newbie (I've only breadboarded some simple things) but I'm kinda trying to compensate that by reading a LOT

Anyway, As a bass player I got interested in the Craig Anderton EPFM Compressor and found references to the (supposedly great) Orman/Henry Whisper Compressor. I managed to find the schematic with web.archive.org:s wayback machine at http://web.archive.org/web/*/http://www.mall.lakes.com/~map/guitar.htm. I thought that some of you would also like to see it  (click on either date, scroll down for schematic link)

Speaking of the schematic, if I would want to omit the tone stack would the 110k resistor R7 still be required for the circuit? Also, in the power supply, will the diodes D1 and D2 provide rectification for an AC power supply or are they for reverse polarity protection?

[Ge_Whiz]

Speaking of the schematic, if I would want to omit the tone stack would the 110k resistor R7 still be required for the circuit? Also, in the power supply, will the diodes D1 and D2 provide rectification for an AC power supply or are they for reverse polarity protection?

You'd still need R7. In principle, D1 and D2 would provide half-wave rectification of AC which would charge the two 10uF capacitors with DC. If the circuit pulls only a small current it would be okay - but it doesn't look very elegant. It could be rather noisy.

Incidentally, continuing down the page leads to "The Swiss Army Preamp", which includes an unbalanced-to-balanced op-amp circuit for DI. I know somebody around here was asking about producing balanced DI outputs recently, but I can't find the thread.

[jimbeaux]

Also, in the power supply, will the diodes D1 and D2 provide rectification for an AC power supply or are they for reverse polarity protection?

D1 & D2 are for reverse polarity protection.

[Mark Hammer]

The tone control is in there because, like most compressors, the Whisper WILL appear to lose some treble.  (I've never truly understood how that happens, but it has nothing to do with bandwidth loss).  Although you may not want the tone control as shown, you may find yourself wanting something to restore the treble in some instances.  So here's what you do.

1) Point E goes directly to the output of IC1a, with the cap and resistor linking the output of that op-amp stage to the output stage.  Quite frankly, I doubt you need 110k.  A 100k unit will suffice, although compressors benefit from low-noise metal-film resistors in noise-sensitive locations such as this.  Whether the cap needs to be an NP type is also a matter of taste.  A plain vanilla 1uf electrolytic will certainly work there, though perhaps noise levels may be different.  If it was being used for wide-bandwidth acoustic ensembles, I'd fuss about cap type, but for guitar a normal electrolytic meets 98% of our needs.  I'm assuming that when he has a chance, Jack will chime in here and offer some better-informed insights.

2) Because IC1b is being used in inverting mode, this offers up some useful possibilities.  Note that the gain of this stage is given by the feedback resistance, divided by the feedback resistance.  In this case, max gain is 10x using a 100k input resistor and 1M pot.  If you place an RC network in parallel with R7, you can increase the gain for content above a designated frequency.  So, if you paralleled R7 with a 3300pf cap in series with a 47k resistor, that would provide a rolloff starting a little above 1khz.  A 2200pf cap would bump that up to a little above 1.5khz.  The gain for that part of the signal would be the feedback resistance divided by 47k, rather than 100k.  So, at max gain, you would have a gain of 10x for stuff below a 1000hz or so (depending on rolloff selected), and a gain of 21x for stuff above 1khz. 

Note that the rolloff is gentle, rather than abrupt, so a nominal cutoff frequency of 1khz will not mean double the gain starting right there.  People have used this suggested mod with the Anderton EPFM-II compressor with good outcome (though different component values).

[Noplasticrobots]

I just came across this thread and was wondering about the TL061. Could I replace it with one side of a TL072/082?