Hiss

[DerStever]

I recently finished my Express using Ceriatone boards - Sprague Caps - Mojo Heybouers. Tubes are Mullards and Winged C on the Output. Allyn helped me with a hum issue - thanks Allyn!!!!

However now that the hum is gone - I'm noticing that this amp is really noisey - like lots of hiss - especially when I dial back the Hotplate to get a little louder at practice. I notice that most of you, when you post, don't mention Hiss so I'm assuming that you don't have the problem. Others do mention it, but has anyone cured it?

I've tried swapping tubes around and moving wires - eliminating any carbon resistors and shielding the input grid wire - I will be swapping the Mallory caps out next to SBE 6PS. But has anyone ever dealt with this and beat it? Please let me (us) know whats the best way to proceed.

Thanks - Steve

[Allynmey]

A good amount of hiss is inherant to the design. I've heard 3 real TW's and they all hiss. When I heard your amp, it sounded about the same hiss-wise as the real 'wrecks I've heard including one recentll serviced by Ken a few month ago.

Allyn

[lastwinj]

change out the cathode bypass caps, to low impedance variety. hiss should noticeably improve.


germ

[DerStever]

Germ, could you offer a brand and type suggestion - I'm using Spragues now?

Also I noticed last night a lot of the hiss - in fact most of it - was coming from various pedals I had in line for reverb, delay, chorus, and the tuner. Removing these brought the noise floor down substantially.

Any little noise in the input chain gets made into a BIG noiz by the time it gets to the speaker with these amps. I would suspect since the PI is a major gain stage that an effects loop wouldn't buy you too much either.

Does anyone else use effects and what do you use - or are there any really transparent ones which have little of their own noiz?

[jaysg]

change out the cathode bypass caps, to low impedance variety. hiss should noticeably improve.

Solens?

[Zippy]

"Better to remain silent and let others assume you a fool than to open your mouth and prove it" or something like that. But I cannot resist.

What is a low impedance capacitor? I don't get that. How would that - especially as a bypass capacitor - make a difference in this instance?

[jaysg]

Low effective series resistance (ESR) is good. Atoms are low ESR, however, plastic film caps like the Solens and Hovlands, etc. are even lower. On the downside, they're larger and more expensive at any capacitance. Plus you feel like a dork putting a 630V cap on a cathode. Then again, Germ may have some lower voltage rated caps in mind.
As to the second question, some of the noise in an amplifier is coming from first stage resistors. You'll get more noise from a 1M vs. a 100k vs. 10k and so on. It's a tradeoff vs. current. For example, the Sadowsky bass preamp that a lot of hot shot bassist stick in their boutique basses, is just the blackface fender tonestack driven by an op amp with a recovery stage. It's scaled down though -- 10k slope resistor not a 100K. In this case, every little bit of resistance hurts, especially where it can be reduced and not effect the actual gain. hth, and no flames please...

I've used CDE polyesters at 1.0uf. mouser 5989-100V1.0

[unklmickey]

change out the cathode bypass caps, to low impedance variety. hiss should noticeably improve.


germ

hi Germ,

this makes a ton of sense.

in industrial power supplies with humongeous capacitors, you often see a medium sized and a small capacitor paralleled with them. sometimes as many as 4 sizes total. (including the big one)

in addition to series resistance, the these monsters are coiled from such great lengths, that they also have considerable series inductance.

maybe it's heresy, but i wonder in the case of PS filtering and cathode bypass, using a small polyester or polystyrene cap in parallel with an electrolytic (that isn't low ESR) already in an amp, could provide a good result.

certainly it would look better to just have one "good" cap. but, if money is tight, or for proving where the problem exists, i think this might be a useful approach.

cheers,

unk

[lastwinj]

check out a mouser a catalog for low impedance types. just about any film cap will be lower ESR than an e-cap. but, there is a trade off. it may make the amp too smooth. e-caps do add a certain edge to the sound, from what i have found.

i tend to like Sanyo Oscons for getting rid of hiss. nichicon gen purpose are pretty good too, when compared to cheapy stuff like xicon, capxon, etc.

the reason for the noise, is the small resistance creates current flow. when current flows, noise can be imparted.

germ

[Zippy]

Thanks.

How low resistance are we talking here? The chatter is about series resistance of the capacitor but for this application the capacitor is bypassing (in parallel with) a resistor that is 1-3 kohms. No offense intended. I'm old and new (to me) ideas take a while to sink in.

I think the biggest take-home message in this thread is that the bulk of the noise is coming from the effects chain in front of the preamp.

[dynaman]

Sanyo Oscons? That sounds like sales pitch! Gee, where can I get those?

[lastwinj]

Sanyo Oscons? That sounds like sales pitch! Gee, where can I get those?

its a bit of a sales pitch, since i have about 2000 surplus ones in stock and ebay right now. but, i used them B4 i ever got sotck, and they did exactly what i expected them, too.

plus the purple shrink on em is sexy!!!
]
germ

[DerStever]

Yes that is the message and the question now is who uses effects, what kind, and what is your experience.

If you listen to the amp without effects its not bad at all - yeah I'd be interested in the Sanyo caps except your page doesn't work, Germ and I did a search for the Oscons and came up empty - If I can begin with no hiss with the amp, then adding certain quieter effects might work out really well.

Hey thanks guys for all of the help ! 'preciate it. Learn something new every day here.