trouble with the NPN boost - distorted signal :(

[BramcoteLorne]

hi guys,

been lurking on here for a while, and now I'm after a bit of advice:

I've built the NPN boost (and jolly good it is too) and been fidling with a few other things, based on what I have been reading on here.

My problem is that I'm getting a weird sort of fuzz/distortion noise when I play the strings hard through the effect

It gets worse the more the gain is turned up - below half it's not too bad - on full gain it's really breaking up badly on heavy strokes.


it's not a musical sort of breakup. I can hear the transitor gaining up the further the pot is turned and can hear how it's meant to break up a bit.

The breakup is below it, it's the same sort of sound you get when a speaker is on the way out and it can't handle the low freq's. Not nice!

I built a new one last night and they both suffer from the same symptoms. I have tried 3 differant amps and 2 guitars and it's still the same sort of breakdown of signal on the bass.

I've even changed the transistors (Germ 2222 and Si 547 and a few others) and the problem is still there.  

Sometime it seems worse and sometimes it seems not so bad, but I guess that might just be me.

any help would be great - I'm running out of options, I think something is getting overloaded somewhere along the line, but can't understand why I can't find any info on here if it has happened to other people.

:cry:


Bram.

[petemoore]

I believe this is the circuit [Gus's NPN boost] that is covered in the beginners section. Have you read through there? Also check FAQ and debugging.
 If you have a DMM, read each transistor pin's voltage difference to ground, and the battery voltage used, and post the Q's E/B/C pin and PS  voltages. If you don't have a DMM, getting one makes debugging more gratifying.
 The symptoms...generally if you have these, you're building the wrong circuit [not the case here] or the circuit is not set up or wired correctly.

[BramcoteLorne]

hi,

thanks for your reply

I can get hold of a multimeter, but I don't really know how to use one - Could you explain to me exactly how to measure the voltages on the transitor and the battery?

I'm guessing from what you have said I need to connect in turn the 3 legs of the transistor with one prong of the meter to the ground point on the curcuit with the other, and note the values down.

With the battery do I just need to connect the + to the ground point on the curcuit.

what does the meter need to be set to?


thanks again for all your help,

Bram.

[BramcoteLorne]

ok I think I have done that I set the meter on DCV 200 (if I remember right)  as it gave the most readable results, these were:

C = -04.0
B = -02.1
E = +01.6

the battery was -0.75

I also took readings from:

the emmiter to the collector  - 0.24
the collector to the base      +0.15

can anything be told by these results?


Bram.

[petemoore]

hi,

thanks for your reply

I can get hold of a multimeter, but I don't really know how to use one - Could you explain to me exactly how to measure the voltages on the transitor and the battery?
  >>Probably you'll be setting it to DCV  something like  20VDC Range. test a tasty battery and see if you get something 'normal' like 8.2VDC or 9.1VDC from the battery...if so you're probably getting an accurate DC read on the meter.
I'm guessing from what you have said I need to connect in turn the 3 legs of the transistor with one prong of the meter to the ground point on the curcuit with the other, and note the values down.
  >>>I clip the black lead to ground, and check grounds with the 'beep' mode..if you have diode checker set DMM dial to this symbol: ---->l, connect the leads you should hear a beep. check that circuit grounds and battery clip ground all are made. Check that battery clip + and - are not connected [that causes battery shorts].
  Leave black lead on ground, touch red probe to Q1 collector and write down the DC voltage, reapeat for Q1 base and Q1 emitter.
With the battery do I just need to connect the + to the ground point on the curcuit.

what does the meter need to be set to?


thanks again for all your help,

Bram.

[petemoore]

ok I think I have done that I set the meter on DCV 200 (if I remember right)  as it gave the most readable results, these were:

C = -04.0
B = -02.1
E = +01.6

the battery was -0.75
 Does the DMM have a 20v dc range? It is possible the battery in the meter needs replaced, it is used as a comparator. these readings are...well the collector is more than ten times the battery, which can't be, possibly the decimal point got typo'd. Also I see negative and positive readings on the transistor. Try the readings again.
  If I got a .75 Voltage potential reading from a 9V battery, I'd taste the battery. A quick test I like to do on the meter is take a fresh tasting battery and see if I get something near 9V dc Reading.

I also took readings from:

the emmiter to the collector  - 0.24
the collector to the base      +0.15

can anything be told by these results?


Bram.

[BramcoteLorne]

hey thanks for all your help so far pete

I'm getting way deeper into this side of things than I ever thought I would when I decided to build myself a boost!

still I'm really enjoying it - I guess it's just a bit harder for me seeing as I don't have any previous experience of electronics.



ok:

tested across the battery at DCV 20 and got 8.02

With the meter still on DCV 20 I clipped the black lead from the meter to the ground and touched the red lead on the 3 solder points below the transformer and I got these

C = 4.29
B = 2.34
E = 1.82


I checked a couple of times over about 10mins to make sure I was doing it right and the values decreased a tiny little bit over the time.

I have been reading that the sound I am hearing is a transistor misbiased, but I have no idea what a misbias is or how to bias correctly.

Again and I'm guessing here - there should be some sort of ratio between the 3 readings? and if the reading are not within that ratio then you get the sound that I'm hearing.


anyway,

thanks again for all your help.


Bram.

[petemoore]

R.G Has typed up a beautiful essay on transistor biasing, reads at GEO are all good, and will help increase your understanding of all that goes 'stomp'.
 Those voltages look like you could get some amplification possibly from the transistor, I haven't read the begginer project files, but would think there are reference to 'good' transistor voltages [what voltages to look for in this circuit].
 But at first glance of these new readings, the battery for one looks 'acceptable' in that the DMM is showing something in the pocket of what a 9V battery would read. We'll assume your readings are now usable.
 The base is showing a difference of more than .6v more than the emitter, and the collector is very close to 1/2v [1/2 battery supply].
 The emitter voltage seems high, I'lll look to see how it's connected, if the emitter voltage is lower, there's more room for voltage swing between wherever it sits and 9V. Check around the emitter, the resistance values.
 Also if you have a socketted transistor, pull the transistor, and measure the resistance from ground to the emitters socket lug. Post the reading, it may be 'off' since there may be a way for current to bypass those two points another way through the circuit. Testing in circuit resistances takes mapping and calculating, I do it sometimes though, even tho it's redings may be 'dubious'...it depends on 'where in the', and 'what' circuit.

[petemoore]

Did you socket the transistor? If you did, pulling the transistor on this circuit will let you read resistances I could look over and find if there is a misvalue, or if the readings are 'dubious'...another path knocking the readings off accuracy in the circuit.
 With tranny out, if you clip a lead to where the 100k, 47k, 5k pot and 10k meet, red probe reading resistances should see:
 10k at transistor base socket lug
 100k to where 9v connects
 The 47k and 5k pot share series connection to ground, attempted measuring of the 47k will measure the parallel 47k and 5k pots setting to ground...dubious...unless you lift the 5k pot's ground connection, then measure the 47k.
  10k Brown Black Orange
  100k Brown Black yellow
  47k yellow green orange
  Orange sometimes looks red to me when I read color codes, I used to always, and often still, measure actual resistance of each R just before installation.

[captntasty]

Bram & Pete,  
Just stumbled on this post...  I too have built a few boosters that have the same problem.  I call it a secondary distortion - a little fuzz underneath the primary signal - almost like a blown speaker - does that sound familiar?  I also have a small 10W tube amp that has the same symptom - but it is not the speaker - drives me nuts.  I guess I'll be checking the bias on all these.

Many thanks, Patrick

[BramcoteLorne]

pete,

thanks again for all your help, I'm going to try out these next few steps tommorow and let you know the readings I get.

I think it's excellent you've got time to help people like me  

patrick: that is exactly the right description of the sound - looks like we have the same problem!  :wink:

anyways thanks again guys, and I'll let you know more when I try the readings tommorow.



Bram.

[BramcoteLorne]

hi,

with the transistor out the measurements I got are as follows:


C :  7.83
B :  2.42
E :  0.00


I have built up the curcuit again   :?  and I got the exact same problem with the new one.

The reading above are still from the original one tho.

You mentioned that the base should be 0.6v higher than the emmitter and in the previous readings I took it is only 0.52 higher. could this be the cause of my problems?

If so how do I go about changing this? do I need and extra resistor?


thanks again for all your time.

[petemoore]

Are you working from the hand drawn scheamtic? It can be a little hard to read...
  Near the bottom left on the schem is 'another' 9V connection.
 I believe all the crossing lines 'are to be connected.

[BramcoteLorne]

hi pete,

this is what I am using:



I have compared it to the hand drawn on tho.

I just can't see what I have done wrong,  I have built it 4 times now, and the effect works fine - but still the bias problem.

is it anything to do with my transitor being a a germanium instead of a silicon?


thanks,


dave.