thebugger

modified 10 years ago

Question

116

01:14:53

Okay this has been bugging me for a while. So here's the deal. Every amplifier I've made (transistor, tubes), has always had a large hum at the output if i don't ground the input with a small value resistor. I've often thought it was a miller effect, but just to make sure what's your opinion. 

published 10 years ago

hurz

10 years ago

Are you stoned? You forgot the question? And mark???

faceblast

10 years ago

they're signals from space

Karlberg

10 years ago

Sounds like good old ground hum. AC from a wall out is 60hz and even with the best filtering some always gets through from the power supply. Not sure about the physics but connecting chassis or negative to ground, like a radiator or kitchen sing fixes it. I've heard rumours that HAM magicians can make fake ground and other voodoo but as far as i know it's just part of electricity and life.

thebugger

10 years ago

No it's not hum. I've ruled out hum. Hurz i stopped getting stoned a few months ago, but i used to do it daily hahaha. The question is in the description. What may be the cause of that?

thebugger

10 years ago

I've ruled out power supply hum and ground loops. It's something else.

faceblast

10 years ago

possibly earth is noisy; try moving somewhere where the dirt is quiet

thebugger

10 years ago

No I've isolated all circuits from ground earth. I use an isolation transformer. As i said i ruled out ground problems and power supply hum. It must be something else. The funny thing is that it occurs in both transistors and tubes. That's why I'm swayed towards miller effect, because parasitic capacitance is always present in every circuit. Anyway i need a different opinion, that's why I'm i posted this topic

faceblast

10 years ago

I built a subwoofer preamp and installed it and there's no hum at all so I dunno what you're doing wrong. input signal lands on an RC of 10μf and 22kΩ. input ground goes directly to circuit ground. no hum. it's silent. circuit ground is not connected to chassis ground. chassis ground is connected to dirt for safety, it is not part of the circuit in any way.

faceblast

10 years ago

what frequency is the hum

faceblast

10 years ago

have you traced it back to the inputs with your cro

thebugger

10 years ago

The hum is at low frequency, but there's no hum when i add a grounding resistor like the 500ohm resistor in the circuit. Preamps typically use a very well regulated power supply which I've taken into account. I don't think it's ground effect, nor ripple problem. It occurs only if the output is left ungrounded, and completely goes away after i ground it

faceblast

10 years ago

are these on different supplies

thebugger

10 years ago

No, they're on a single supply, but with every precaution taken. On my tube circuits, i use double RC filtering for the preamp and phase inverter.

wyoelk

10 years ago

If you live by a large industrial plant, they can feed crap signal into the earth. Is your mains for your place from a 3 phase bank that can give hum, delta more than wye. Unplug all appliances in the place you live. Get stoned??????

wyoelk

10 years ago

I forgot you live in Europe with numerous voltage residential issues. Drive your own ground rod outside your place. Change to, yeah, only fluorescent lighting as incandescent cause hum also.

thebugger

10 years ago

People listen to me, I don't use earth ground for my circuits. I always separate them via an isolation transformer. Our grounds are very noisy, I know that, and I know how to avoid that too.

hurz

10 years ago

Ground isn't noisy at all, cuz its just a reference. This is a single bjt amp and the reference is 0V ; Zero Volt! Doesn't matter how its going up or down. ITS YOUR REFERENCE! Its as I told you many times this isolated single transistor amp "systen" were it doesn't matter were "ground" reference is. This World is relative and the PROBLEM starts if you put a SECOND device to this one! Then the trouble starts. If ground "reference" is the same at all of your connected systems, the there is no HUM.

faceblast

10 years ago

it's signals from space

faceblast

10 years ago

hum is caused by small men jumping up and down

thebugger

10 years ago

I get how ground noise works hurz. And i believe I've eliminated this problem

zorgrian

10 years ago

We all bow down in homage to you!

zorgrian

10 years ago

An isolation transformer will do nothing to stop hum if you have what's called an earth loop (difference in relative earthing or resistance between the earth / ground / zero volt points within your circuit stages). This can quite easily create a nasty hum even within ONE stage if the circuit is built badly.

abobaker

10 years ago

My modest opinion is that you should try putting chassis and connecting it to dirt as faceblast suggested and of course not using the dirt for grounding your amp.

thebugger

10 years ago

I can't have an earth loop if the isolation tranaformer has separated the earth ground from the circuit ground. I can have a circuit ground loop, but I've taken precautions about it. I often use the star topology to minimise hum. And you people are not listening me. Once i ground the input through a resistor, all noise stops. That doesn't fit in the ground loop scenario. The problem with that is that the grounding resistor should be below 100k, to completely minimize hum, but them I'm having trouble coupling it to the previous stage, because tube amps are typically of high impedance.

abobaker

10 years ago

Ah ok my bad .. So before grounding input with this 100k .. is it just floating ??!

faceblast

10 years ago

tried this again. I put the input in my mouth and got a loud 50Hz hum straight away. load the input to ground and it goes away. took the wire out of my mouth and it goes away.

thebugger

10 years ago

Exactly! It's like it's picking up RF noise and demodulating it to produce audible noise. Grounding it through a resistor, loads the input too much and limits this effect. That's the best way i could explain it.

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