Another one of those Mosfet Booster problems

[iwantmypie]

So I did a pretty extensive search on here trying to solve my problem but although I found a lot of useful information I couldn't exactly pinpoint it to my situation.

I was originally having a problem with a background hiss that I get when I engage the pedal, it's almost like the floor level is raised but it does this at all levels of the gain control. Before I get into details I will state that I do not get this noise when I use a 9v battery, only when I use my external Boss power adapter. I also do not have the reverse log 5k pot that is recommended as well as a LED in place of the zener.

So here is my other problem, the one I am more concerned about. I've read a lot about biasing this pedal and installing a 100k trim pot in place of R2 on Jack's schematic. I do not have one at the moment so I have just been swapping different resistors to get results. When I first built this I used the 62k that was in the schematic. I am using the BS170. With the 62k in place I get these readings off the BS170

D: 5.86
G: 0.5
S: 3.29

The voltage of my source comes in at 9.17

With this set the way it is the pedal works. When first reading this I must have skipped over the part where Jack states having the drain voltage around 4.5 so I decided to swap some resistors. With the bias resistor changed to 27k I get the following reading

D: 4.58
G: 0.66
S: 4.57

With the drain now lower and what I believed to be more of a proper setting, I hardly get any audible sound coming out of the pedal. The volume is extremely low and changing the gain does not help.

Sorry if a problem like this was already discussed in the past but I could not find what I was looking for. If anyone needs more DMM readings I can post them. Any help to either of my problems would be of much thanks.

-MD

[PerroGrande]

The noise issue is one you essentially answered in your question.   The pedal is quiet when running from battery, but gets added noise when run from an adapter.

The noise is arriving through your adapter.  This happens.

In many cases, this can be corrected by either changing to a different adapter, or adding additional filtering/shielding to the pedal.  A common technique worth trying is to place a 100 Ohm resistor in series with the incoming V+ line, and two capacitors (one large, 10uF or more and one smaller, 10nF or so) to ground.

Now -- on to the voltages...

If the pedal works to your satisfaction with the 62K in place, then leave it like that!  Besides, Jack recommends a range from 4.5 to 5.5, and you're close to that at 5.86.

However, I would actually expect to see the gate voltage a little higher -- like around 5.5 volts with the default 62K resistor.  The current through the 10M resistor is very, very small, so it is possible that your meter is pulling it down.  Again, if it works, I'd leave it alone. 

When you changed the resistor to get 4.5 volts on the drain, the source rose to 4.57.  With the drain and source this close together, the MOSFET is not in its saturation region, so this is why you didn't get satisfactory results. 

[iwantmypie]

Thank you for the quick response Perro! I'm about to perform the filtering mod you suggested and I will probably to to lower the bias a little but otherwise just keep it where it was. I will post back with the results. Thanks!

[iwantmypie]

i tried the filtering technique like you said and it didn't work. I saw somewhere else on here someone recommended using a 100uf capacitor with the 100ohm resistor but I do not have one that big at the moment, only a 470uf  I don't know if I should try that or if it would be too big.

[mdh]

Bigger is better for psu filtering... as long as the voltage rating is high enough, go for it.

[iwantmypie]

yeah it's 16v so I'll have to try it out tomorrow when I get home for work.

Time for some shut eye, thanks for the help so far guys!

[Barcode80]

by the way, there is no need to put two caps for the filtering. All you do when you put two caps from supply to ground is add their value, so 100uF and 100nF would give you 100.1 uF. anything 100uF or better will be fine, and 470uF is certainly bigger

[iwantmypie]

So I finally got my C5K pot and that problem is fixed and I installed that 470uf capacitor in for filtering. The cap definitely helps but there is still some noise. I say some because it is still there but its very little and a lot less than it use to be. Im still considering just buying a few more BS170 s and just experiment to see if one is better than the other. Thanks for all the help again!

[cpm]

by the way, there is no need to put two caps for the filtering. All you do when you put two caps from supply to ground is add their value, so 100uF and 100nF would give you 100.1 uF. anything 100uF or better will be fine, and 470uF is certainly bigger

here is an interesting reading and facts:
http://www.cliftonlaboratories.com/bypassing.htm

[Unbeliever]

here is an interesting reading and facts:
http://www.cliftonlaboratories.com/bypassing.htm

Great link, complete with frequency response graphs. People should go and read this before handing out 'truth' as advice. 

[PerroGrande]

Another thing to consider, of course, is how well shielded the unit is.  Poor shielding is another potential source of noise like the type you're describing (in addition to our old friend hum, of course).

Of course, there is always the option to operate the unit from a battery or look into a cleaner power supply. 

[Barcode80]

here is an interesting reading and facts:
http://www.cliftonlaboratories.com/bypassing.htm

Great link, complete with frequency response graphs. People should go and read this before handing out 'truth' as advice. 

those experiments deal with bypass caps in path of a signal line, not a power line. for filtering purposes, there is no need for the extra cap. and the snideness is not appreciated.

[cpm]

here is an interesting reading and facts:
http://www.cliftonlaboratories.com/bypassing.htm

Great link, complete with frequency response graphs. People should go and read this before handing out 'truth' as advice. 

those experiments deal with bypass caps in path of a signal line, not a power line. for filtering purposes, there is no need for the extra cap. and the snideness is not appreciated.

sure, not very useful for us, and is focused on HF signals, but is still an interesting reading on how capacitors behave.

[PerroGrande]

Well, geez... I didn't mean to start anything by suggesting a second capacitor 

I suppose I should toss in the reason that I brought it up...

In this case, the complaint was related to noise floor more than hum.  I interpreted this, rightly or wrongly, as the need to remove some higher frequency components that might are nearly certainly arriving c/o the power supply.  I suggested the bigger cap because the MOSFET boost de-couples the Vref voltage, but does not include a filter cap (or, for that matter, polarity protection on the 9v line).  No biggie, but I thought that it might be wise to include, especially since the original complaint was obviously power-supply related.

Then I remember back to a previous thread where we were discussing power supply filtering and so forth and that how larger electrolytic caps have a tendency to act like an inductor at higher frequencies.  Wanting to prevent that risk from entering the equation, I included the smaller cap in my suggestion.  It isn't uncommon to include a small filter cap on power supply lines physically near a component to knock down any "stuff" that might show up from circuit proximity, etc.  That ran through my brain whist I was typing as well. 

[Barcode80]

Well, geez... I didn't mean to start anything by suggesting a second capacitor 

I suppose I should toss in the reason that I brought it up...

In this case, the complaint was related to noise floor more than hum.  I interpreted this, rightly or wrongly, as the need to remove some higher frequency components that might are nearly certainly arriving c/o the power supply.  I suggested the bigger cap because the MOSFET boost de-couples the Vref voltage, but does not include a filter cap (or, for that matter, polarity protection on the 9v line).  No biggie, but I thought that it might be wise to include, especially since the original complaint was obviously power-supply related.

Then I remember back to a previous thread where we were discussing power supply filtering and so forth and that how larger electrolytic caps have a tendency to act like an inductor at higher frequencies.  Wanting to prevent that risk from entering the equation, I included the smaller cap in my suggestion.  It isn't uncommon to include a small filter cap on power supply lines physically near a component to knock down any "stuff" that might show up from circuit proximity, etc.  That ran through my brain whist I was typing as well. 

no harm done, my frustration was directed more at the remark on "handing out truth" that Unbeliever posted.