electro harmonix tube compressor noise

[nag hammadi]

anyone else experience a bunch of noise with the black finger?

if so, anyone know how to reduce it?

[TELEFUNKON]

wiggle the tubes around a little in their sockets

[nag hammadi]

i have wiggled myself silly,  changed tubes, even tried smiling at it     

[R.G.]

i have wiggled myself silly,  changed tubes, even tried smiling at it

Did you promise to respect it and call it in the morning?

[nag hammadi]

well, the last time i did that i ended up with this ring on my finger...

and that did absolutely NOTHING to reduce the noise in my life!

[Mark Hammer]

Yeesh!  I don't wish to be critical, and I appreciate your current misery, but I hate when people say they have a "noise" problem with something.  What counts as something describable under the category "noise" can include a lot of different things whose origins can also be very different things.  The first step is to indicate specifically what you mean by "noise".  I mean, is it crackling?  hiss?  hum?  motorboating? lactose intolerant stomach gurglings?

[nag hammadi]

i am sorry for the *&^*&^ical description...   

i am refering to hiss.

[The Tone God]

Did you promise to respect it and call it in the morning?

Thanks for giving me my daily belly laugh R.G.!

Andrew

[nag hammadi]

a belly laugh is always better than a gut punch!

[Mark Hammer]

Is this the same hiss complaint that one often hears about compressors, or is this unusual hiss in your estimation?  I ask because the automatic gain increase that compressors often impose under low signal conditions raises the level of whatever the compressor sees at its inputs.  If what it sees has some hiss, then that hiss is boosted.

[Eb7+9]

anyone else experience a bunch of noise with the black finger?

if so, anyone know how to reduce it?

you might have a noisy tube, but still that was my question when we reversed it ... the sigal path seems to develope an unreasonably large amount of gain only to throw it out again through attenuation, at least it's way larger than what's required to counteract losses in the LDR divider network ... the intent of the over-gain could be to bring out some "tubeyness" from the signal path but it's also a strong source of hiss ...

aside from loosing the semi-saturated feel of the signal path you could halve the number of gain stages (they're in two groups) and compensate accordingly in the attenuators to maintain overall transfer gain levels basically leaving the sensing and LDR network unchanged and getting the unit to perform the same functionally but with less noise ...

as I recall two of the controls were mis-labelled ... the Squash and LED/Bulb selectors or something

JC

[moosapotamus]

What about just reducing the gain (a couple of resistors) in the two output stages... Might that bring the hiss down a bit?

~ Charlie

[Eb7+9]

great point Charlie,

that's a simpler way to mod and it helps retain the circuit's flavour more ... I guess what you're suggesting is dropping the gain on each pair by exactly the amount dropped by the attenuation and then removing the attenuators where they occur ...

why I suggested the possibility of going through two stages only is because you'd get less total noise going through two stages with OVERALL gain products (stages and attenuators) set to stock level rather than going through fours stages set to same ... the self-noise of each device is being amplified by the stage it's in and then again by the next stage, and so on ... it's better from a noise accumulation point of view to go through a lesser number of stages ... but then there's a bunch more of trace cutting and bridging involved ...

definitely easier to tweak plate loads and cancel the attenuators ...

[hairyandy]

How recently did you buy the Black Finger?  The reason that I ask is that the band that I work for received three of them from EH in 2004 and all three failed shortly thereafter.  The reason given was that there was a problem with the machine that did the soldering at the plant for that batch and that there was a number of cold solder joints that manifested in lots of noise and then total failure.  I had/have one of them and in mine, an electrolytic cap in the power supply section literally just fell out one day, I noticed it by hearing some rattling in the pedal when I picked it up.  I figured out the orientation and re-soldered it and it's been fine since.  All three of them were VERY sensitive to bad/dirty power.  Even after we got new ones from EH, they were still extremely noisy in bad power situations (ie: gigs in Europe and Puerto Rico).  We put a Variac on one of the guitar rigs to assure 120v and ever since the Black Finger has been really nice and quiet and sounds great.

I'm not guaranteeing any solution, but from my experience I would suggest to check your power source.  Make sure that you are feeding it adequate power and start there...

Good luck,

Andy

[moosapotamus]

Hey Andy, I had to resolder a psu cap in mine, too! Nice idea using a variac, btw.

... I guess what you're suggesting is dropping the gain on each pair by exactly the amount dropped by the attenuation and then removing the attenuators where they occur ...

Right. I'm just not exactly positive about which resistors to change to what values, tho.

... it's better from a noise accumulation point of view to go through a lesser number of stages ...

Is there another side to that coin? Not sure if this applies here, but I'm thinking of Mark Hammer's Crank circuit, as an example... Since gain is multiplicative, you can use lower gain in multiple stages to get a much larger overall gain. But, maybe noise is multiplicative, too. not sure about that.

~ Charlie

[nag hammadi]

actually, it belongs to a friend that comes into the shop quite a bit.  he bought it a couple of years ago, so that seems interesting.

maybe i will just go back and touch up all of the points and see where that takes me. 

if that doesn't do it, i'll dive in deeper as recommended.

as always, thanks for beingm y teachers!

[Eb7+9]

There's a strong sign that JC Morrison was aware of the hiss problem and tried to fix it through passive means ... namely, the plate-grid high frequency shunts in the last two stages (that's C4-R9 and C7-R12 in the diagrams below), there for no other reason I can imagine ... one owner I know says he notices the hiss a lot more when recording or playing with lots of gain - makes sense ... anyhow, here's a few suggestions for balancing the signal path gain picture so the hiss can be lowered while the compression response of the circuit left relatively unaltered ...

diagram 1 : Sources of hiss in front-end



(i) pre-gain control should always be set to max for best S/N
(ii) RCA suggests using 100k plate loads for best S/N in 12ax7 gain stages, about 5~6db difference in gain from 180k/220k (maybe 11db total for both)
(iii) bypass cap in second stage strong source of hiss and fidelity loss, about 10db difference in gain
(iV) attenuator (voltage divider) set by R3 and R20 throws away signal, shorting R3 gives back 6db of signal

so reversing all hiss enhancing mechanism in the first two gain stages yields a rough total gain change of :

-11dB -10dB +6db = -15dB of gain loss, or a factor of 1/5.55 ...

which means the side chain amp needs to have it's gain boosted by a factor of 5.55 ... that 33k in the first side-chain gain stage should then be 33k/5.55 = 6k ohm for the sensitivity to remain the same, with room to make the gain higher if need be ...

diagram 2 : Sources of hiss in recovery amp




(i) same thing as above with 220k plate loads, 12 db difference in gain if both replaced by 100k (or paralleled by another 220k)
(ii) 100k grid blockers each steal about 25% of signal, together that's a 5db difference
(iii) the resistor in series with the volume pot throws away about 3db

so reversing all hiss enhancing and signal loss mechanisms (shorting the grid blockers) in the last two gain stages yields a rough total gain change of :

-12dB +5db +3db = -4dB of gain loss, or a factor of 1/1.63 ... not a huge change there

the thing to remember is both front and recovery stages have quite a bit of gain already, total signal path gain change is -15db -4db = about -20dB or a factor of 1/10 ... I'm told the volume control is typically set low on this pedal in which case this difference in signal path gain could be acceptable ... I leave it to you researchers to determine this ... another thing would be to use 12au7 tubes instead of 12ax7's for less noise - good DI/HiFi pre circuits often operate on 12au7's ... again, same thing, side-chain gain would have to be upped accordingly ...

in fact, you might be satisfied with simply swapping tube types, removing the bypass cap in the second stage and tweaking side-chain gain ... depends on how far you wanna go spec and work wise because these combinations all work in tandem ... of course, in the process of doing all these mods you'd be losing some of the tubey MOJO that's part of the intent in the design ...


JC

[moosapotamus]

This is cool, JC!
Soon as I get a chance, I'm going to have to give some of it a try.

Thanks!
~ Charlie