Tube Screamer Hiss

[pgorey]

I've got a TS clone I built that did not have hiss in the past and now does when the effect is engaged and is amplified when the tone control is turned up.  It is harsh!  What would you guess is the best place to look?  I have an audio probe and this is the circuit I built:

http://www.tonepad.com/getFile.asp?id=81

I believe my version is the TS808 with asymmetrical clipping.  I can upload circuit pics once I'm home but was curious if there is an obvious place to look. 

[Mark Hammer]

When the tone control is dimed, the circuit applies a boost of roughly 5.5x to content over 3khz.  So it's no great surprise that any hiss becomes more obvious.  The question is where any hiss is coming from.  Several questions are prompted:

1) What op-amp are you using?
2) Are you using a transistor input buffer, and if so, what transistor are you using?

[pgorey]

Two transistors:
2n3904
and
Op Amp is a 4558 - I have it socketed so I could swap op amps.  Transistors are soldered in place though.  I don't know what an transistor input buffer is   Let me look that up and see if it fits into the circuit.

http://www.tonepad.com/getFile.asp?id=81

[Mark Hammer]

The two transistors in the TS are used as input and output buffers.

[Rob Strand]

There's a difference between it wasn't there before or you didn't notice it before.

If it wasn't there before I'd suspect Q1.  If it got zapped it could go noisy.

[BluffChill]

I can't see the schematic you're following, but I remember when I built my TS808 it was very picky about the tiny capacitor across the first op-amp (the 51pf in this picture http://tagboardeffects.blogspot.com/2012/02/ibanez-ts808.html) and I only had 22pF caps at the time - had to go and raid a bin in a local electronics shop and managed to get some 50pFs and a 47pF silver mica.

[antonis]

If you suspect Q1, try to entirely by-pass it.. (jumper from IN jack tip to Q1 Emitter)
(you might loose some treble but it's OK for testing purpose..)

[bool]

If you "zapped" the first bjt base, it will still work but will hiss and this is permanent - nonreversible. If this is the case, you will have to replace it.

[Mark Hammer]

I was experiencing nasty hiss in an old Heathkit guitar amp I bought.  Replaced the input transistor and the hiss went away.  It happens.

[pgorey]

Wow,  thank you for the replies!  I learned about transistor buffers and solved the issue.  Turned out I had the wrong op-amp in there.  It was still working but had the hiss.  Threw in a JVC4558D and the hiss was gone.  Slightly noisy on single coil but that always seemed to be the case with the original TS and other distortions and single coils.  Should I still replace Q2 for good measure?  How could I have popped Q2? 

[R.G.]

Reverse-breaking the base-emitter junction of a bipolar does increase hiss, and the hiss accumulates with each breakover, although the transistor does nominally still work in the circuit, mostly. Input transistors are subject to more abuse, coupled as they are to wires leading off into the static-electricity filled world. Over time, things go hissy that were never so in their infancy. There are schemes to "anneal" the junction by heating it, but it's not perfect, and "annealing" is not something pedal hackers are likely to be able to do successfully. So the cure is to replace the transistor.

It's not widely appreciated, but certain circuits have the ability to zap a base emitter at power on/off. Imagine an emitter with an emitter cap to ground charged up to 6-8V, and the base suddenly pulled to ground. Zap. It can also happen other ways. There was a whole chapter on this in a circuits book I once read.

The cure is to put a fast, low capacitance signal diode like the 1N914 or 1N4148 across the base emitter so it's reverse biased in normal operation. The device only appears as a small capacitance in its normally reverse biased condition, but wheneve the base-emitter is reverse biased, it conducts in the forward direction before the base-emitter can break over. No zaps, no accumulation of hiss.

If you ever really, really need to be sure a replaced transistor won't become hissy, use a diode.

[PRR]

> certain circuits have the ability to zap a base emitter at power on/off.

On another forum, audio-heads were building power amps of 40W static dissipation with not-cheap MOSFETs. Worked fine for many, but one guy was repeatedly killing his MOSFET, apparently at power-down.

It was a source follower with BIG (microwave oven) choke load. Only 20V supply. But when he pulled the plug in the 20V feed, ZAP!! (Other builders had been pulling the wall-plug so the 20V died slowly.) I simmed a 400+V gate-source transient. Advised a gate Zener as general hygiene, and hundreds of uFd on the board so voltage could not collapse instantly. No failures since.

[Rob Strand]

Input transistors are subject to more abuse, coupled as they are to wires leading off into the static-electricity filled world.

Some areas of the US are very prone to ESD problems.   I can't remember the areas we used to have problems but I suspect it was the lower humidity regions.   It's one reason why some people see problems and others don't. 

For individuals,  excessive ESD problems can be due to the carpet they have and the shoes they like to wear.

In my office I can definitely see an increase in general strength and frequency of ESD zaps when the humidity is low.

Once we told one of the young guys in the office that the antistatic foot-straps should be worn around your knee with the tail dangling behind you like a car static-discharge strap.   We got a photo of him wearing it like that 



Correct way to wear.

[PRR]

> For individuals,  excessive ESD problems can be due to the carpet they have and the shoes they like to wear.

Underwear is also an issue.

[Govmnt_Lacky]

Once we told one of the young guys in the office that the antistatic foot-straps should be worn around your knee with the tail dangling behind you like a car static-discharge strap.   We got a photo of him wearing it like that 

Lol. This has me thinking. I am ALWAYS getting zapped when I get out of my car. Turn off the ignition, open the driver door and ZAPPP!!!

[Rob Strand]

Underwear is also an issue.

If someone wore a foot strap for underwear it may raise some eyebrows (both the wearer and surrounding people).

Lol. This has me thinking. I am ALWAYS getting zapped when I get out of my car. Turn off the ignition, open the driver door and ZAPPP!!!

They were quite common in the 70's.  Somewhere along the way they have disappeared.

[amptramp]

> For individuals,  excessive ESD problems can be due to the carpet they have and the shoes they like to wear.

Underwear is also an issue.

Almost every anaesthetic used in hospitals is explosive and at one point, a hospital applied for insurance from Lloyd's of London.  They wrote a policy subject to the restriction that women not wear rayon or nylon underwear, only cotton.

[Rob Strand]

Almost every anaesthetic used in hospitals is explosive and at one point, a hospital applied for insurance from Lloyd's of London.  They wrote a policy subject to the restriction that women not wear rayon or nylon underwear, only cotton.

Very Interesting.  I have read a lot of stuff on risk in medical applications and I've never come across that before.  I did a project that came under the mining standards and the energy stored in capacitors must be limited to reduce risk of ignition.  So no big bypass caps.

[PRR]

The reference I saw was a transistor factory manager's memo, something about keeping a close eye on employee's underwear. He was told to re-word that.

Car straps: tires used to build static electricity just rolling. To the point that little springs were put in wheel bearing caps to bleed wheel-hub to spindle, to reduce discharge static in the radio. But also drag-straps, which became "cool" and were added to customs for no reason. I believe the change from Rayon to steel fabrics, and even higher doses of carbon in the rubber, made this moot as a rolling-shock problem.

I bet Lacky's pain is seat fabric, seat covers, or underwear. The instant diagnosis is to try Cling-Free spray for women's slips. A more economical treatment, which we use on the sofa in winter, is a small spoon of Fabric Softener in a spray jug of water. The basic fabric softener works by holding moisture in the fabric. The pre-made spray is great if you don't want to go to meeting with your slip soaking wet (alternatively clinging to your panties). The home-brew is fine if you spray the sofa at bedtime and get shocks from partner or doorknob, it will dry overnight, but not 100% dry so static dissipates.

[thermionix]

Almost every anaesthetic used in hospitals is explosive and at one point, a hospital applied for insurance from Lloyd's of London.  They wrote a policy subject to the restriction that women not wear rayon or nylon underwear, only cotton.

I was thinking earlier that I only see the grounding chains on ambulances.