Hey hey,
I try a lot of amplifiers, and I always use my computer as a source. It has a dedicated PCI sound card that I use.
On most newer-ish amps (80s and up), this works flawlessly. Great sound.
On most older amps (70s), there is horrible noise when it gets plugged into the computer. The noise is dependent on what I do, for example moving the mouse, opening windows, etc. It's just a garbled sound that is ever changing. The amount varies from amplifier to amplifier, but in the amp I tried today the result is extreme - loud noise on all inputs, despite the computer only being connected to one, and completely independent of volume control position. Even if I choose phono - which has shorting plugs on the inputs - the noise is the same.
What would cause all these 70s amps to pick up noise from my computer? I just don't get it.
I try a lot of amplifiers, and I always use my computer as a source. It has a dedicated PCI sound card that I use.
On most newer-ish amps (80s and up), this works flawlessly. Great sound.
On most older amps (70s), there is horrible noise when it gets plugged into the computer. The noise is dependent on what I do, for example moving the mouse, opening windows, etc. It's just a garbled sound that is ever changing. The amount varies from amplifier to amplifier, but in the amp I tried today the result is extreme - loud noise on all inputs, despite the computer only being connected to one, and completely independent of volume control position. Even if I choose phono - which has shorting plugs on the inputs - the noise is the same.
What would cause all these 70s amps to pick up noise from my computer? I just don't get it.
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It is probably a grounding issue, I experienced the same thing with certain (not necessarily old) amplifiers. If your PC is not connected to a REAL grounded outlet, then the half of the line voltage is present at the metal case of the PC, you can mesure it. This is caused by the line filtering circuit used in the switching PSU of the computer (the common point of two small filter capacitors is connected to the metal case, which is supposed to be grounded, if not, the voltage sits there on the case). That voltage -if it gets connected to your amp's case through the interconnect cable- is enough to cause all kinds of weird noises.
I know the noise you mean from the pc, I changed the psu in our win7 desktop machine and got the same issue.
It took ages to work out what was going on, as you say mouse movement or scroll wheel use brings on a sound like distant wall drilling. Nothing else had changed in the system at the time and it had been playing nicely for a couple of years prior to this.
Luckily I was given a dac with an optical in shortly afterwards which cured it. For any future psu shopping I will be avoiding the budget ones.
John
It took ages to work out what was going on, as you say mouse movement or scroll wheel use brings on a sound like distant wall drilling. Nothing else had changed in the system at the time and it had been playing nicely for a couple of years prior to this.
Luckily I was given a dac with an optical in shortly afterwards which cured it. For any future psu shopping I will be avoiding the budget ones.
John
The same thing happens with Class D and switching power supplies . Even though they are grounded at the supply if you touch the case you can feel a voltage tingle and at times a change in noise dependant on the design . I don't know enough to explain why just that this is common and i guess older amps are less able to reject noise many i have owned have a hiss through speakers , things have moved on i guess.
A good well shielded interconnect helps and maybe some snap on ferrite beads on the pc power lead ?
A good well shielded interconnect helps and maybe some snap on ferrite beads on the pc power lead ?
I already have a very good PSU in the computer, it's one of the top models from Seasonic (the most respected manufacturer out there), so I don't think that is at fault.
How does a ground loop work? Can it be solved? It doesn't matter if I connect the amp/pc to another outlet, or use individual outlets. Both PC and amp is grounded, but the same thing happens with not grounded amplifiers from the 70s. It just baffles me that this can happen.
I am using a normal shielded RCA cable, but it also has a ferrite bead built in. Doesn't matter though, same thing happens regardless of the cable I use. Literally nothing can stop the noise from happening, is it a bad design of old amplifiers? I mean, it does work flawlessly with slightly newer amps.
It also happens with laptops, but not to the same degree. Same type of noise but not as much.
How does a ground loop work? Can it be solved? It doesn't matter if I connect the amp/pc to another outlet, or use individual outlets. Both PC and amp is grounded, but the same thing happens with not grounded amplifiers from the 70s. It just baffles me that this can happen.
I am using a normal shielded RCA cable, but it also has a ferrite bead built in. Doesn't matter though, same thing happens regardless of the cable I use. Literally nothing can stop the noise from happening, is it a bad design of old amplifiers? I mean, it does work flawlessly with slightly newer amps.
It also happens with laptops, but not to the same degree. Same type of noise but not as much.
You may find that the 70s amps are simply picking up RF noise on their outputs, coming from the computers SMPS. This is because those amps typically dont have any kind of lowpass filtering on their inputs to prevent this, whereas amps from the 80s onward typically do (for use with CD players).
A quick test - add 1K resistor in series with the input, with a 100pF capacitor to ground on the amp side of this resistor.
A quick test - add 1K resistor in series with the input, with a 100pF capacitor to ground on the amp side of this resistor.
Sounds like you've solved my problem!
I took a look at the schematic of some known working amps, and I see this:
Looks a lot like what you've suggested. I will try it! Obnoxiously, I have no pF range capacitors, so I'll have to order some... I'll be back sometime. Another motivator to sort that big order I've been postponing for weeks now. Ordering parts is so boring...
I took a look at the schematic of some known working amps, and I see this:
Looks a lot like what you've suggested. I will try it! Obnoxiously, I have no pF range capacitors, so I'll have to order some... I'll be back sometime. Another motivator to sort that big order I've been postponing for weeks now. Ordering parts is so boring...
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I had exactly the same issue.
I would recommend you buy a cheap external DAC, these can be a cheap as 15 Euros.
Some thing like a FiiO D3 (D03K) Digital to Analog Audio Converter is bound to be better than the internal PCi card.
You can also get SPDIF switches that allow you to use many digital sources and switch a lot quieter than in most amplifiers.
I would recommend you buy a cheap external DAC, these can be a cheap as 15 Euros.
Some thing like a FiiO D3 (D03K) Digital to Analog Audio Converter is bound to be better than the internal PCi card.
You can also get SPDIF switches that allow you to use many digital sources and switch a lot quieter than in most amplifiers.
So jaycee saved me from a very long running problem!
I took a trouble amp and installed a 470R resistor in between the volume pot output and the input capacitor and put a 100 pF capacitor between the amp side of the resistor and ground.
Noise gone! Absolutely dead quiet.
And that was it. So simple. I'll do it routinely to all amps I come across from now on.
Thanks so, so much!
I took a trouble amp and installed a 470R resistor in between the volume pot output and the input capacitor and put a 100 pF capacitor between the amp side of the resistor and ground.
Noise gone! Absolutely dead quiet.
And that was it. So simple. I'll do it routinely to all amps I come across from now on.
Thanks so, so much!
Yeah, no cell phones before 1980. No protection for them, in or out. You've just figured out the in. I'm using 33 pf.
I have amps so primitive (1966 design) that they don't have a zobel on the output either. This keeps RF from leaking in the speaker terminals. Make sure any old amps have that, too. 11 turns wire wound around a AA battery parallel a 10 ohm resistor between output transistor and speaker terminal.
And keep the steel case closed.
I have amps so primitive (1966 design) that they don't have a zobel on the output either. This keeps RF from leaking in the speaker terminals. Make sure any old amps have that, too. 11 turns wire wound around a AA battery parallel a 10 ohm resistor between output transistor and speaker terminal.
And keep the steel case closed.
I've looked over the schematics of some known good amps, specifically from Pioneer, and it seems they favor using this arrangement in all of their amps. I think I am going to follow their footsteps whenever I encounter this issue from now on.
Got any objections against this? And what's the purpose of the 220K resistor?
Got any objections against this? And what's the purpose of the 220K resistor?
If there is an electrolytic cap ahead of the 220 resistor, the 220k resistor to ground is to keep the capacitor polarized the right way. Electrolytic caps sound stupid when the junction passes through zero and the chemical forming or unforming of the oxide layer occurs. Having that point near zero also makes the jfet input work right with the input line down near zero v when idle.
I simplified the above schematic but there's no capacitor anywhere in the signal.
By the way, I celebrated too soon. The amp that initially made me start this thread, because its noise was way above what I've encountered before - has now gotten the same treatment. 470R resistor and 100 pF capacitor. No difference. Same noise still. This time I put it before the pre-amp section though, rather than before the power amp. Maybe that's different, but I doubt it. Even putting the filter right at the RCA connectors won't stop the noise.
In fact, even with the signal path to the power amp section completely cut off, I still get the noise out of the speakers.
How is this even supposed to work? I calculated the filter to cut off at 3,4 MHz. Wouldn't it be better to be somewhere in the KHz range?
By the way, I celebrated too soon. The amp that initially made me start this thread, because its noise was way above what I've encountered before - has now gotten the same treatment. 470R resistor and 100 pF capacitor. No difference. Same noise still.
This time I put it before the pre-amp section though, rather than before the power amp. Maybe that's different, but I doubt it. Even putting the filter right at the RCA connectors won't stop the noise.In fact, even with the signal path to the power amp section completely cut off, I still get the noise out of the speakers.
How is this even supposed to work? I calculated the filter to cut off at 3,4 MHz. Wouldn't it be better to be somewhere in the KHz range?
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There has to be an RF trap on the speaker output. Zobel network.
There has to be an RF trap on the AC power input. toroid chokes in series with the AC is standard these days, saw some in a LED TV I was scrapping the heat sinks out of today. Also X2 cap across the power line, usually integrated into the IEC socket. Saw that in that Samsung TV.
There has to be a grounded steel box. Real killer, some 70's amps were all plastic. No cell phones or home computers in those days.
There has to be an RF trap on the AC power input. toroid chokes in series with the AC is standard these days, saw some in a LED TV I was scrapping the heat sinks out of today. Also X2 cap across the power line, usually integrated into the IEC socket. Saw that in that Samsung TV.
There has to be a grounded steel box. Real killer, some 70's amps were all plastic. No cell phones or home computers in those days.
The main reason for the input filter is to snuff out the chance of any RF picked up by the signal cable being rectified by the diode junctions in the input stage. I have always used 1K/100p as a "rule of thumb". 2K2/100p has -3dB at around 720KHz which generally works well.
Output zobel/boucherot cell can also be important as those speaker cables also make lovely antennas!
Output zobel/boucherot cell can also be important as those speaker cables also make lovely antennas!
Elliott Sound Products - Audio Power Amplifier Design Guidelines Nice little article about it
It's not so much about the low pass filter effect on audio, more to do with the effect on input impedance it has at megahertz frequency. Attached shows a typical input network simulated in LTSpice.
Bottom trace shows the filter rolloff, but the top trace shows the input impedance seen. Notice how when we're up in the megahertz region, there is a very low impedance to ground. The RF is shunted to ground instead of being rectified by the input stage, which leads to the noise.
Bottom trace shows the filter rolloff, but the top trace shows the input impedance seen. Notice how when we're up in the megahertz region, there is a very low impedance to ground. The RF is shunted to ground instead of being rectified by the input stage, which leads to the noise.
Attachments
I read all that ESP article, but it didn't help much with this strange issue. I've done some further experiements, and now the noise is much worse than originally.
- I installed an X2 capacitor across the power transformer primary (didn't think it'd help, but just for good measure)
- Rerouted the cables (noticed that the main PE connection crossed all the speaker output wires, so I changed that.
- If I connect a wire between the computer case and the amplifier case, the noise gets slightly lower but with the addition of some hum.
- Unplugged every single thing in my home, but the computer and amp, no difference.
So yeah, that made it much worse somehow. But I also discovered that I don't need to connect the computer RCA terminals at all, I just need to touch the RCA shield coming from the computer, to any GND part of the amp, and the noise comes in, on both channels equally loud, independent of volume control and source. It's LOUD, you can hear it through the headphones without having them on. It basically just screams at you. EECHEEEECHCEEEECHCH ...
- I installed an X2 capacitor across the power transformer primary (didn't think it'd help, but just for good measure)
- Rerouted the cables (noticed that the main PE connection crossed all the speaker output wires, so I changed that.
- If I connect a wire between the computer case and the amplifier case, the noise gets slightly lower but with the addition of some hum.
- Unplugged every single thing in my home, but the computer and amp, no difference.
So yeah, that made it much worse somehow. But I also discovered that I don't need to connect the computer RCA terminals at all, I just need to touch the RCA shield coming from the computer, to any GND part of the amp, and the noise comes in, on both channels equally loud, independent of volume control and source. It's LOUD, you can hear it through the headphones without having them on. It basically just screams at you. EECHEEEECHCEEEECHCH ...
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