Blend control on a boost

[dbp512]

So I just got into building, and recently breadboarded a few things. Tonight as I was playing around with a treble booster, I wired the whole circuit in parallel to lugs 1 and 3 of a pot, and wired the input to lug 2. This allows me to sweep between the original signal and the boosted one, something I haven't seen in many schematics. I played around with it a bit, and a high gain with a low mix sounds different than turning the gain down, but a migraine kept me from testing more. Do the blended signals average out to the same as a reduced gain, or is something different going on? Why isn't this utilized more? I saw it on some fuzz schematic on google, I'm not even sure where. 
 
One potential thing I can think of is phasing issues, if semiconductors induce phase shift like opamps. I don't really know what I'm talking about, since I'm learning things as I'm doing them and never had any formal experience with circuits outside of an introductory college physics class. I accidentally studied biology instead

[Ben Lyman]

Check out the pedal I just made with a lot of help from all the generous people on this forum.
It has a clean boost circuit with a diode distortion blend circuit.
Toggle switch turns it into a fuzz pedal with two different sounding fuzzes that can be blended.
http://www.diystompboxes.com/smfforum/index.php?topic=112014.0

[GibsonGM]

Do the blended signals average out to the same as a reduced gain, or is something different going on? Why isn't this utilized more?
 
One potential thing I can think of is phasing issues, if semiconductors induce phase shift like opamps.

Don't knock mitosis & meiosi, dbp   I majored in bio too, LOL.

OK - in a perfect world, the principle of superposition would suggest that 2 of the SAME frequency, blended, would double in amplitude if both were of equal level and if both are IN PHASE (you answered this at the bottom of your post).  For those in-between levels and freq's, the math would be complex to get a *real* number (doubling? 2.35x? and so on).   

I think that SOME frequencies aid, and some frequencies "fight" a little if the phase isn't totally right, as it would not be if you you pass thru any RC networks; the impedance changes for specific frequencies, favoring some over others.   This may result in what you describe as 'sounding different', and can often be desirable, sure.

I suspect that this stuff isn't used more to keep the parts count down (cost), and due to the trade-off between some desired end and the easiest way to get there.   Probably a little unpredictable, and not as useful over a range as maybe an overdrive control...but yup, some boutique stuff does this, often with good results!   

[dbp512]

Ben, thank you for your link, there were a lot of useful comments and tips in there, and the schematic is quite helpful. I haven't seen the 9V line running through the carrier resistor before running into the base resistor, I usually saw them separately. Do you know if there is there any difference? 
 
Gibson, oh I ain't knocking biology at all! I'm actually in the process of applying to pharmacy school, and I don't regret it at all. I just wish my high school and college physics classes covered transistors in their circuits lectures. In college I relearned the same things about resistors and capacitors as I already did. If I only knew how cool these were, how useful they can be in a variety of ways, and most importantly how simple they are. 
 
I figured the waveform would get complex if things did get slightly out of phase. Not enough to make a phasing sound, but enough to get a cool sound I guess. I'm not sure if I'll keep it or not, but until my veroboard arrives I'll probably keep tearing things apart and trying to build new ones. Its like legos, for adults. 
 
One general question, for a circuit I built I needed an NPN and a PNP, so I got them but I don't really like the circuit. I was wondering if I could connect them in parallel and bias them differently to get another weird, mixed sound. I read another thread here asking about that (which gave me the idea), as well as a few results from google, and it looks like it could work, but I don't want to burn anything out. I'd rather not have to place another parts order already!

[GibsonGM]

Well, the short answer would be 'not really', but there MAY be some tricks one could do to crowbar them into the same audio circuit.  Asking for the trouble of complexity... I'd just use one or the other.  Did you buy only ONE of each?  Next time, get 50


OT:   I can totally relate...a biologist (or other "..ist") could easily have a use for electronics knowledge in research!  If the two had been connected in my education, I probably would have gone farther later in the field.   Even the Arduino has possibilities for data collection. 

[midwayfair]

It's not a blend exactly -- it's a negative feedback control, which is what you get when some amount of out-of-phase signal is connected to the signal source. A transistor when used for voltage gain as with your treble booster inverts the phase of the signal at the base.

There are other places you could put a negative feedback control. It could go from the output to the emitter of the transistor (in which case it doesn't load the guitar), or from the collector to the base (this is, in fact, one way a lot of transistors are biased) for instance. Note that if you had two transistors set up as gain stages, you'd end up with positive feedback -- a howling sound. Some pedals do that intentionally, like the Fuzz Factory.

One thing that's happening here, though, is that the input cap for the transistor is a small value, which means that the out-of-phase signal is missing the lower frequencies. (If you're curious, the presence control on Fender amps works similarly.) Your guitar pickups are also heavily loaded even when the blend control shorts the input and the output, so it's not exactly a "clean" way to do a clean blend. But if you like it go for it.

[dbp512]

Midway, thank you that was a very informative response. Now that you mention it its obvious that its functioning as feedback, I can't believe I didn't think of that because I attached the input to the same pot. Although I thought fender presence control connected the feedback loop to the cathode of the tube (thats the side that goes to ground, right?) with a cap wired to ground to eliminate negative feedback of higher frequencies, which in turn boosts them. Wouldn't connecting the feedback line to the base of the FET cause the frequencies that pass to get amplified (as opposed to feeding back from the collector to emitter, akin to fender if I'm not mistaken, boosting frequencies that don't make it). Or do I have things backwards because of the phase inversion? 
 
The reason I had the two types of BJTs is because the Escobedo Harmonic Jercolator schematic called for both of them. The veroboard layout is online, but I did my best to convert that to a simple schematic which I can post when I get back home. In short, the emitters from both are connected together, then the carrier of the PNP leads into the base of the NPN with a cap in series. I have no idea what the signal is doing, but it did make a cool distorted sound. I'll see if theres some literature on it. 
 
Gibson, yup, I only bought the minimum parts I needed for 3 circuits. I thought that would be the cheapest way to get what I needed to start building, but I quickly found it doesn't allow much experimentation. I need to get some veroboards (amazon seems cheapest), but soon I am going to order a bunch of parts. I've been refraining from doing so because I didn't want to order to many parts that I wouldn't end up using. Resistors are cheap and easy to stock up on, but I didn't know what value caps and what sort of semiconductors I should get. Now that I have a functioning gain circuit, I can sell my OD pedal and most likely use that money to place a bigger order. Its a fun slippery slope down this addiction, eh? Its like a slip n slide down a mountain. Any suggestions on parts would be more than welcome. 
 
And to continue our other conversation, I actually majored in neuroscience which is all circuits anyway. This overlaps a bit (although there aren't phase issues or the need to amplify the signal). I guess better late than never though.