Serendipity Voice knob

[insomniac2295]

Just built this fella and I really can't tell what the voice knob does. I've heard it's fairly interactive, so I've spent a lot of time with different settings. Still nothing. I've also heard that its change is subtle, but I almost feel as if I'm getting no change out of it. Can someone learn me?

[mgwhit]

Clockwise increases gain and limits bass, counter-clockwise limits gain and increases bass.  It's definitely interactive with the Drive knob.

[sgmezei]

Hmmm I thought I heard it was a mods control but I guess I need to go tweak mine and see what happens.

[sgmezei]

I just meant play it and try out different settings. I like the sound and just kind of set it and forgot it. On my board right now.

[insomniac2295]

Right now I'm playing it with my bass through a crappy peavey combo at low-ish volumes. Perhaps I'm just not hearing it.

[frankie5fingers]

I'll try it with a bass and see if I get the same result & let you know. Scott, yeah I like to set and forget but I can't stop messing with it. I've found so many great settings, but then the next time I plug in it doesn't sound "quite right" so I try another.  I love the pedal but still can't dial in one "this is the one" setting. Currently I have the OPA2134 in it - I ran the OPA2604 for a couple of weeks, thought it was the perfect tone, then the other day, plugged in the 2134 and... well you see the pattern.  I found the 4558JRC a little "ratty" (nice though).  I have an AD712 en route. So far anyway, the ICs haven't made the definitive difference, but I can hear it and each one creates different parameters for the controls so the process starts all over again each time. I've found the voice and gain controls are SO sensitive that the slightest turn makes a big difference tone-wise.  IME anyway.

[insomniac2295]

Thanks! This I starting to sound like a tech help thread, but I checked all caps and resistors. All are the right values. Can't see any bad or questionable solder joints, and I just tested and found the pot to be good. Been using a JRC4558D and a TL062

[madbean]

Not much to add to what has already been stated here. Here's how I break this stuff down in my own way:

The Voice pot is a variable resistor, i.e. it changes resistance as it is turned. Since lugs 1 and 2 are connected, this means that when it is fully counter-clockwise it produces the full resistance of the pot, 10k. When fully clockwise it is 0k. Since it is linear, then you know the "in-between" points more or less fall in even steps. IOW, turning the pot 10 degrees in the bottom part of the sweep produces the same rate of change in resistance as it does in the top part of the sweep, or any other are for that matter.

There are two "states" to consider with the variable pot:

One state is the amount of gain produced by the negative feedback loop which is all the stuff between pins 1 and 2 of IC1. It's helpful here if you just forget the amount of feedback there is variable by the Drive pot and consider it fixed. So, think of Drive as just a 500k resistor for the moment and ignore R9.

When the Voice pot is all the way up, it produces 0 resistance and therefore we can consider it removed from the circuit. Using the formula for calculating gain in a non-inverted feedback loop, we see

Gain = 1+ (Drive/R3) = 1+(500/1) = 501

When the Voice pot is fully counter-clockwise, we get the full resistance of 10k, so the formula for gain becomes

Gain = 1+ (Drive/(R3+Voice)) = 1+(500/11) = 46

So, just by turning the Voice pot we've varied the gain output by a factor of about 11. IOW, our gain goes DOWN when the Voice pot is all the way down and vice-verse and it will be quite noticeable since it the difference between the two is an order of magnitude.

Now consider the second "state" which is the filter created by connecting R3, the Voice pot and C3 to Vb. Since Vb is virtual ground, just consider it as a simple low pass filter. The cut-off point of this filter depends on two values: the total resistance and total capacitance connected to ground. You should also consider impedance, but since C3 is fixed I don't think you have to worry about it (maybe someone knows better). Anyway, we already know that Voice is a variable resistor, so that means this filter is also variable.

When the Voice pot is up, we have a 1k resistor and 100n cap. Using the formula for single pole HP/LP type filters we have

f(cutoff) = 1/(2*pi*R*C) = 1/(6.28*.001*.1) = 1592 Hz

When the Voice pot is down, the formula becomes

f(cutoff) = 1/(6.28*.011*.1) = 145 Hz

Keep in mind that the "cutoff" here is not a sharp knee. Rather, it is -6dB per octave. So, the further away from the cutoff the more drastically the frequency response is reduced. This means when our cutoff is 1592 Hz, we have more mid-frequency content with the high frequencies being steadily reduced by the filter. When the cutoff is at 145Hz, we reduce both mid and high frequency content.

The two states here are what make the Voice pot what it is. It simultaneously changes the mid-frequency content and overall gain. Remember this example kept the Drive pot as a fixed resistor. When you remove this restriction, you can see now that the Drive pot also influences the total amount of gain produced it also works with C2 to produce another variable filter but that is for high frequency content only) . IOW, the two controls are interactive, as mentioned in earlier posts.


Here's a much better (and probably more correct) technical explanation of how this works: http://www.geofex.com/Article_Folders/TStech/tsxtech.htm

Look at the "clipping" section.

[sgmezei]

Wow thanks for the insight Brian. This makes alot more sense now.

So you're saying they're interactive then? 

[insomniac2295]

wait, so the knobs are interactive? 




Thanks Brian! That was very insightful!

[madbean]

wait, so the knobs are interactive? 

The Drive and Voice controls together set the total possible gain of IC1A and therefore the amount of clipping that is applied to the filter cut-off point set by the Voice control. The important thing to remember is when you lower the Voice control into more bass-heavy settings, you need to increase the Drive control to maintain the SAME amount of clipping as you had before.

You can alter the overall feel of the two controls by varying the 100n cap and the value of the Drive pot. So, 150n will give you a bit more bass and 1M will give you more gain, and so on.

[sgmezei]

sorry if I added to the confusion, was unintentional......

[insomniac2295]

Sorry, internet sarcasm does not work well 

[madbean]

Well, I was using super meta anti-sarcasm.


(facepalm)