crackle NOT okay

[Stratomaster]

I think that's just your guitar, and now it's amplified.  Mine doesn't do it, but I have played other people's guitars that have had a similar phenomenon.

[markm]

Mine's pretty darn quiet.

[blanik]

I think that's just your guitar, and now it's amplified.  Mine doesn't do it, but I have played other people's guitars that have had a similar phenomenon.

how dare you implicate my beloved Crap-O-Matic guitar! 

anyway, it's not boxed yet and all the jacks and power are held with alligator clips... and the hum is very subtle so it "might" diseapear when boxed.... 

[R.G.]

Did you ever stop to think that an LED/LDR can be driven from a pot so that the LDR makes a kind of remotely controlled resistor that *can't* crackle? It might take some dinking to get one with a low enough resistance, but it solves crackling directly.

[mountainking]

Did you ever stop to think that an LED/LDR can be driven from a pot so that the LDR makes a kind of remotely controlled resistor that *can't* crackle? It might take some dinking to get one with a low enough resistance, but it solves crackling directly.

You'll probably want to put a resistor in parallel with the LDR, right?  What the resistance range of the average LDR?

[stm]

Did you ever stop to think that an LED/LDR can be driven from a pot so that the LDR makes a kind of remotely controlled resistor that *can't* crackle? It might take some dinking to get one with a low enough resistance, but it solves crackling directly.

An LDR sounds like a clever solution--I already thought of this some time ago, and so did ZV way before when working on the SHO...  Apart from the extra power consumption to have an LED hitting hard on the LDR to have low resistance, there is an important gain loss to be paid.

In an old thread, when questioned about using an LDR to avoid the crackle, Z himself replied that he had already tried the LDR approach, however it took away a big deal of gain--a statement which I have already verified: at minimum gain the circuit has almost 0dB (unity gain); with the gain pot at noon you get almost 20dB (10x), and with the gain pot dimed you get almost 40dB (100x).  So far so good, however as little as 100 ohms will drop the gain down to around 30dB (30x), so you are actually sacrificing 10dB of gain (3x), assuming you can actually have an LDR with as little as 100 ohms.

Using a JFET or MOSFET as a voltage-controlled resistance is not going to make it either.  You need very little voltage across such a device (less than 100mV) to avoid distortion or at best to avoid coloring the signal. Probably the best you can do is get a good quality pot (to reduce crackle as much as possible) and maybe adding a mute pushbutton should you want to avoid crackle while adjusting the gain in a live situation.

[mountainking]

I was going to mention that thread, but I couldn't remember if anyone had absolutely confirmed that this setup wouldn't work without taking away anything from performance of the circuit.

[markm]

Isn't the crackle something that is unique about this circuit and makes it kind of desirable?
It's a "novelty" of the SHO.
Or, how about just a cap to ground......that would eliminate the crackle.

[stm]

Or, how about just a cap to ground......that would eliminate the crackle.

If the cap is large enough it would kick the gain to nearly 40dB...  If it is not large enough, it will change the frequency response (highs to max, bass with a 6dB/oct rolloff).

[mountainking]

One of my own designs has a similar type of gain control and I could not find any cap value that did not alter the tone. It also did not completely eliminate the scratching sound and it reduced the gain abit. All this stuff we're talking about has already been discussed in a lenghty thread that will be easy to find by using the search function.

[Gus]

A cap off the wiper does not work the same

Hints

  Look at whats changing

look close at the resistor from drain to gate then think about what happens as the gain is turned up.

The source R is changing

[blanik]

anyway, it's not boxed yet and all the jacks and power are held with alligator clips... and the hum is very subtle so it "might" diseapear when boxed.... 

now boxed and indeed the hum isn't there anymore... very cool booster... nice headroom boost!

i thought it would be much louder when cranked (to the point of being dangerous on the amp) but it's quite manageable..! 


Happy Pirate 

[stm]

Hints
Look at whats changing
look close at the resistor from drain to gate then think about what happens as the gain is turned up.
The source R is changing

Speaking of bias, there are several aspects of the circuit that change as the Crackle OK pot is adjusted: gain, output dynamic range, drain bias point and clean headroom. Let's see:

1) With gain at minimum you get almost unity gain and output dynamic range covers from 4.5 to 9V (so you can have a peak-to-peak output of 4.5V).  In this condition drain bias voltage is set around 7.2V, so you have 1.8V towards 9V and 2.7V towards 4.5V, which indicates you can have a maximum clean signal of 1.8*2=3.6V peak-to-peak only.  If drain bias voltage were centered exactly between 4.5V and 9V, i.e. at 6.75V, then maximum dynamic range with clean output would be obtained.

2) On the other hand, at max gain you have almost 40dB and output dynamic range extends to almost 0V up to 9V!, however drain bias voltage centers around 3.6V instead of Vcc/2 or 4.5V.  This extends overall headroom but clean output dynamic range is not optimum either.

Notice that we have a lower than optimal drain bias voltage, where in the minimum gain we have a higher than optimal bias voltage. Somewhere around mid gain the drain bias voltage actually corresponds to the optimal bias voltage for maximum clean dynamic range.

Where I am heading with this?  I found a way of making the bias to self adjust to half the dynamic range by changing down to 75% the value of the lower bias resistor and changing its lower end from GND to the source pin of the FET.  In this case clean dynamic range is optimized.  Someone might argue having asymmetrical clipping is preferrable.  Well, it is a matter of personal taste and it depends on what are you after.  If you want clean booster operation then max dynamic range is preferred.

[Gus]

look at input R with gain changes

[WGTP]

The obvious solution is to work the Crackle into the song your playing.  Either as a prelude or in rhythm or anti-rhythm during the song.  Be creative.   Flange or repeat it. 

[blanik]

anyone was thinking about putting the circuit in the guitar? maybe removing a tone control and putting the sho volume there instead, as a "master vol"... 

i'm sure we all did... lol

[blanik]

yesterday i tried the SHO (at my rehearsal) on my old peavey Classic 74 wich has a 2 tubes power amp with a solid state preamp... the sho didn't do anything good on this amp as opposed to my twin...

it just muddied up the sound without really raising the volume...

it there something in the input of a SS preamp that would void the usefullness of a SHO??

R.

[A.S.P.]

Yes: solid state circuit path.

[stm]

Most solid state preamplifiers don't overload gracefuly when overdriven, hence no benefit in using a booster.  JFET preamps like the Fetzer Valve do tend to behave better when overdriven, like in Thor's first input stage.

The only commercial amp I know that uses a Jfet preamp stage are Carvin Amps, which are said to be quite good sounding amps, considering they are SS.  In addition, the overdrive channels are implemented with CD4049 gain stages.

[ulysses]

replace the output to ground 100k resistor with a 50k volume pot - replace the "crackle ok" pot with a 5k internal trimpot to set gain

this would completly eradicate the crackle.

im not sure why zvex put the crackle in there at all? then again, some people seem to think the crackle is a good thing..

cheers
ulysses