The main sources of noise in boosters?

[Alpha579]

Hi all,
What are the main sources of noise in booster pedals? And what can you do to reduce hum and hiss? Im guessing these would help:
Quality Components
Good Shielding
Layout of pedal in enclosure???
Less resistance in signal path
High input Z???
Low output Z???

Thanx for all help and advice,
Alex

[petemoore]

Probably most critical early in the chain, noise in boosters seem to be a pondered subject.
 Guy wrote he used low tolerance fancy caps and resistors in LPB ones and they worked great, try a 2n5089 or MPSA18 high gain/low noise transistor or other type/ choice. Silver Mica IIRC.
 Make sure all connections are 'mechanically made' as opposed to solder spans.
 ..any noise occuring beofre the booster gets boosted.

[RDV]

The amount of gain/boost and the guitar are the main sources of hum in boosters or fuzztortions, though I know what you're asking.

RDV

[Paul Perry (Frostwave)]

When people say "where is the noise coming from" I always say "what kind of noise is it?".
IF it is hum, then sheilding & power supply caps can help.
If it is a Mexican AM station then ferrite beads.
If it is frying eggs it might be a cap.
But if it is "white noise" then... maybe a semi is doing it.
In my experience though, as RDV says, it is just shit that was there but you coulden't hear it before, since you blew out your ears at a concert ten years ago.

[R.G.]

What are the main sources of noise in booster pedals?

For Hiss:
(1) circuit design, particularly biasing technique
(2) resistor technology
(3) choice of active devices - use low noise transistors and ICs
(4) silicon abuse - you have to protect the transistors
(5) internal oscillation, which can be heterodyned down into audion as something like nasty hiss.

For Hum:
(1) poor grounding techniques
(2) poor shielding
(3) poor physical location (i.e. - don't put them near power transformers)
(4) poor AC power supply design
(5) poor room electrical environment
(6) non-shielded and poorly wired guitars

And what can you do to reduce hum and hiss? Im guessing these would help:
Quality Components

Yep. Use low noise transistors like the 2N5088/2N5089/MPSA18. Use metal film resistors. Capacitors do not create noise at all unless they are leaking.

Good Shielding

Yep.

Layout of pedal in enclosure???

Only to the extent that it prevents internal oscillation that sometimes appears as noise.

Less resistance in signal path

Marginally. Lower impedance signal path is more important.

High input Z???
Low output Z???

Kind of. Low impedance signal sources are much less prone to noise than high impedance ones. So if you convert the high-Z of the guitar to a low-Z signal quickly, you're better off. The best way for the limited purposes of keeping signals clean is to put a signal buffer out on the guitar. That may be at odds with getting a good distortion sound from a few fuzzes like the fuzz face, but it's sure a quieter, sparklier tone for everything else.

Most important is good circuit design.
(1) Low noise parts - metal film and noise rated transistors
(2) Low noise biasing techniques like "noiseless biasing"
(3) transistor input protection - reverse-breaking a bipolar transistors base-emitter junction ...even one time... will permanently increase its internally generate noise. Use diodes, caps, and biasing to ensure that this cannot happen.

[RDV]

In my experience though, as RDV says, it is just shit that was there but you coulden't hear it before, since you blew out your ears at a concert ten years ago.

It was a Foreigner, Frank Marino & Mahogany Rush, & Ted Nugent Concert on July 4 1977 that started me on the road to serious hearing disability! It's all been downhill from there!



RDV

[petemoore]

For me it was the third row at the Allen theater [Cleve.] in 1976....ZZ TOP...4 Gold colored signature custom Marshall STACKS ....Per Side...Damm Demm Boys Played L O U D...I had one serious BUZZHEADED/Noise Hangover from that one. One Heck of A SHOW tho...wheshew...

[Hal]

For me it was the third row at the Allen theater [Cleve.] in 1976....ZZ TOP...4 Gold colored signature custom Marshall STACKS ....Per Side...Damm Demm Boys Played L O U D...I had one serious BUZZHEADED/Noise Hangover from that one. One Heck of A SHOW tho...wheshew...

311 at Hammerstein ballroom last year.  My ears literally rang for a week after that...

I think the discussions about noise are pretty comprehensive so far.  HOwever, make sure you're using a quility cable, when you're testing.  I have some crappy cables and dont really mind them so much, but they _do_ introduce noise.

[Alpha579]

(2) Low noise biasing techniques like "noiseless biasing"

How do you bias 'noiselessly'?  :?  :shock:

[R.G.]

How do you bias 'noiselessly'?

Perhaps I should have said "so-called "noiseless biasing"".

In the most common biasing setup for a bipolar transistor, the base is connected to the center of a two-resistor voltage divider which divides the power supply to the proper voltage for the base bias. The biasing voltage is Vcc* R2/(R1+R2) where R1 and R2 are the resistors. The impedance of the biasing source is R1 parallel R2. The noise contributed to the transistor by these two resistors is the RMS of the thermal, voltage, current, and excess noise of the two resistors.  You can control the excess noise by using metal film resistors, but the voltage and current noise contributions are what they are, and flow directly into the transistor base. Thermal noise is with us always.

You're usually going for high input impedance in a setup like this, so you tend to use high gain transistors and also high resistance value R1 and R2. This maximizes the thermal noise of the resistors (by usng the biggest resistors) and the current noise because you have to have about ten times the base bias current flowing in the bias string to get the base bias current for the transistor to be stable.

So-called noiseless biasing changes things with one resistor and one cap. You break the connection between the base and the connection of R1 and R2. You bypass the connection of R1 and R2 to ground with a cap, thereby attenuating any noise they make. Then you introduce R3, in series from the junction of R1, R2, and the cap, to the base of the transistor. Several things happen. (1) There is only the base current flowing in R3, so current noise is diminished. (2) The voltage across R3 is minimal, so excess noise and voltage noise is minimized. (3) Bias impedance is high (well, OK, equal to R3, but that can be high because it doesn't have to establish a stable bias voltage).

Thermal noise is only the thermal noise of R3.

This is about as good as it gets with resistor-string biasing. It has the same effects on opamps and FETS, although they have such small input currents that the advantages for current noise only is minimal. It still reduces the others, except thermal.

Does that help?

[Alpha579]

thanx Rg, great explanation! for the filter cap at the junction of R1 and R2, would it be quite large (eg 47uf), and if so, would you be better off using electrolitc(i cant spell :oops: ) or Tantalum? Im guessing tantalums would last longer...

[R.G.]

Oddly enough, this is one of those places where the cap size is not needed to be huge.

Normally you'd try to get the cap impedance to be less than one tenth the impedance of the parallel combination of R1 and R2, which usually leads to electrolytic. (aluminum and tantalum are both electrolytics) Aluminum is cheaper, tantalum has lower ESR per capacitance. Tantalum might last longer, but in this application there's substantially no stress on the device, so it will last at least its shelf life, probably longer since the applied voltage keeps the oxide well formed.

However, the nature of noise is that thermal/white noise typically has equal power per Hertz of bandwidth, so the apparent loudness of noise goes up with frequency - that's why white noise sounds so shrill. The power is concentrated in the higher frequency where there are more hertzes to have power. That is what makes hiss more prominent with treble boost.

So you can get away with a smaller cap if you feel like economizing. I wouldn't, though. I'd just flip in a 10uF, and leave room for a 0.1uF ceramic. This is because ceramic has a low ESR at high frequencies, and when the electro's ESL and ESR dominate, the ceramic is still a low impedance bypass. But you usually don't need it.

[b_rogers]

RG are there any well known circuits that use the "noiseless biasing"?

[Alpha579]

RG, so if you have a large resistor from the base of the transistor to the potential divider/cap to ground, you have a higher input Z, but does the larger resistance then mean more noise???

[onboard]

Seems to me that a high value R3 wouldn't equal more noise, since both applied voltage and current are going to be low. Although I could be reading R.G.'s explanation wrong.

What value range are we talking about, 1M? At 4.5V (assumed Vr) that would be .0045mA right? Or is my decimal misplaced...and my understanding wrong :oops:

[petemoore]

I think that's going to be a job for "VR ClipMeg} a 1 meg pot to see whats up with that...I gather tweaking the other resistors might get some mileage ? IIUC...because the added resistor would be the only noise maker, one could get a
  (1) There is only the base current flowing in R3, so current noise is diminished.
  (2) The voltage across R3 is minimal, so excess noise and voltage noise is minimized.
  (3) Bias impedance is high (well, OK, equal to R3, but that can be high because it doesn't have to establish a stable bias voltage).
   Thermal noise is only the thermal noise of R3.
 ...1 and 2 have the potential right there to reduce noise.
 Since R3 is the only potential noisemaker left of the group, possibly a few more yards of mileage could be skqeezed by $pending on the fancy fixed R there. At least this way you could find out the difference without having to order all metal films for the project.
 I suppose there's enough current happening from the emitter/collector that resistor noise there is such a small %age of signal it doesn't bear typeing or tweeking on.
 Using HG/LN transistor like NTE47 or 2n5089 , Zach says he likes the MPSA18..I don't happen to have a booster with one in it right now, but using these transistors...the noise floor is PD low with them, could be interesting to try on a compressor. As far as noise, certain pickup settings of mine have the boosters beat by a mile...that's where noise attention is needed for me at least.

[Alpha579]

Thanx Pete!
The reason im asking all these questions is that im trying to build the quietest Treble booster i can, because i have a crispy cream treble booster, and it can get quite noisy with my single coils...

would having small value emitter and collecter resistors lower noise?
Alex

[R.G.]

Peter's amplification on R3 is correct. R3 should only have less than a volt of DC across it, since it likely feeds a tiny base current to the active device. The low voltage and current put it into a similar position to those gate resistors in JFET circuits - although the resistor is large, it contributes only thermal noise because of the low voltage and current.

The collector and emitter resistors have a lesser effect. Any noise on the emitter resistor is amplified by the device (the emitter is a "noninverting input" for a bipolar transistor in common-base mode) so it should have low excess noise, and the signal voltage is already amplified by the time it winds up on the collector resistor, so the collector resistor's noise is fighting a much larger signal, and the signal-to-noise ratio is much better already. The emitter and collector resistors are much less critical. And the emitter resistor is likely to be bypassed for higher gain anyway, so its internall generated noise is attenuated by the bypass cap.

This points up a truism in low-noise design: Get a bag of low noise gain in your first stage. That boosts the signal to where the noise contributions of succeeding stages are less critical. Noise problems in new-parts construction are usually first-stage problems.

As for small-value collector/emitter resistors lowering noise, it's usually not worth the effort.

If you're really after low noise boost, give it up on bipolar transistors and go get yourself an LM381. This IC was designed for lowest noise amplification of cassette tape heads. I had a friend in college that did a phono preamp from one of these. As a demo, he turned on his stereo, turned the gain up full. What we heard was almost silence. We could hear some hiss by putting our ears next to the speaker. He then put on a disc and used the liquid damped arm lowering device on the turntable to lower the needle onto the record. The little <clik> when the needle dropped into the groove almost deafened us and did damage one of his speakers.

It is possible to do better than that with discrete designs, but it takes a concerted design effort and high building skills. The rate of return on low noise effort is huge with the LM381 and gets lower as you try to do better. There are some moving-coil phono preamps that might do better by doing things like using 24 low-noise transistors in parallel, stuff like that.

[Alpha579]

So, am i right in saying, the more current through R3, the more noise? If so, by keeping R1 and R2 high values, the current and thus voltage drop across R3 is minimal?

[Alpha579]

bump...