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 Post subject: Hiss: Are certain resistor values more likely to cause it?
PostPosted: Sep Tue 15, 2020 7:47 pm 
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Are certain resistor value ranges more likely to cause inordinately high hiss levels in a vintage preamp if the resistors are out of spec?


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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Tue 15, 2020 11:16 pm 
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No, a resistor value all by itself is not going to cause hiss. It IS possible for a resistor to be "noisy", but not very likely.

Is this vintage preamp solid state, or tubes? Many things can cause hiss. Sometimes it's just an open input... with no input termination, the circuit may just be noisy. Try terminating said input and see what happens. No termination, the input probably floats at very high impedance and can pick up any stray signal of any kind sometimes.

That's all general commentary. For specifics, we'll need the specific preamp you are working on.

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Tue 15, 2020 11:28 pm 
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Barry H Bennett wrote:
No, a resistor value all by itself is not going to cause hiss. It IS possible for a resistor to be "noisy", but not very likely.

Is this vintage preamp solid state, or tubes? Many things can cause hiss. Sometimes it's just an open input... with no input termination, the circuit may just be noisy. Try terminating said input and see what happens. No termination, the input probably floats at very high impedance and can pick up any stray signal of any kind sometimes.

That's all general commentary. For specifics, we'll need the specific preamp you are working on.


Hi Barry,

Many thanks. It's a Zenith Y960. It has both hiss and hum. This noise existed both before and after the recap.

Please see this thread for photos: viewtopic.php?f=6&t=384016


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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Tue 15, 2020 11:33 pm 
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It's not a good idea to start a second thread on the same topic. Just gets everyone confused, and it may violate forum rules also.

But as a separate thread on resistors causing hiss, perhaps leave it at just that. A signal tracer should quickly tell you approximately where in the signal flow the hiss is originating.

Hiss and Hum are two entirely different things also... Hum is generally limited to 60 or 120 Hz. Hiss is broadband noise.

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 12:04 am 
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Resistors being out of spec does not make them noisier in itself. They can go bad and be noisier.
Different resistor construction also make s difference.
Higher values of resistors can cause more noise and well as higher DC current through them, but you would have to redesign the circuit to improve noise from those effects.
https://eepower.com/resistor-guide/resi ... or-noise/#

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 6:37 am 
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Location: Cedar Rapids, Iowa, USA
I have seen single noisy resistors in radios where all the others did not cause a problem. I don't know if the value has much to do with it. As an aside, I think a poor connection, as a cold solder joint or ground return, or tube/connector pin contact could also cause noise.
John


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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 1:01 pm 
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The basic "noise floor" is set by the resistance. The "mean squared" noise voltage is given by 4kTR*DF, where k is a constant, R is the resistance, and DF is the bandwidth (delta F):
https://en.wikipedia.org/wiki/Johnson%E ... stribution.

Other types of noise---eg the so-called "1/f noise" can be due to mechanical issues in the actual component

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 1:05 pm 
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I suppose the first question that I should have asked is, at what volume level is this hiss being noticed? As mentioned, every resistor has some noise floor, so if you amplify it enough, you'll hear some hiss (and not just resistors btw).

If you're turning the volume all the way up with no audio signal dialed in, you're going to hear hiss, noise, and hum. The question is... what is the level of said garbage in relation to the volume of the desired signal. i.e. what's the ... umm ... signal to noise ratio. :). If it's 60dB, I think you're just fine. Impossibly fine in fact lol.

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 1:18 pm 
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Barry H Bennett wrote:
I suppose the first question that I should have asked is, at what volume level is this hiss being noticed? As mentioned, every resistor has some noise floor, so if you amplify it enough, you'll hear some hiss (and not just resistors btw).

The resistance in the equation is the net resistance of at the relevant location in the circuit being analyzed. (I call it the "node impedance")

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 1:22 pm 
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I think Nyquist was when I decided to stop auditing courses, and just concentrate on what they'd hired me for... make stuff work ;-)

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 4:06 pm 
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An old timer very knowledgeable professional radio technician, now deceased for many years, once told me that his experience with ribbed body dog bone resistors was that they were much quieter than dog bone resistors of the same resistance value having smooth bodies. I have no technical data to support his claim but thought I would throw his comment in the mix since resistor noise is being discussed

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 6:51 pm 
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There are two sources of noise in resistors and other electrical components; thermal noise and excess noise. The total noise of a resistor is their sum. Thermal noise is that caused by thermal agitation of the atoms in the resistor alone. Noise voltage is a related term; if you consider a practical resistor to be a noise generator in series with an idealized resistor that is free of noise, then by Ohm's Law we see that the noise voltage at the terminals of a resistor is the noise generator current times the resistance. Therefore the higher the resistance value, the more thermal noise voltage a resistor will produce.

Excess noise is that due to all other sources besides thermal. It can be due to the construction of the resistor. For example, carbon comps have more excess noise than metal films simply due to the way they are made. Metal films with metal end caps that are crimped on have more excess noise than metal films with ends that are soldered or welded. Application of DC voltages can cause excess noise in certain kinds of resistors, and it tends to increase in most resistors as they age. Also, shoddy materials, or damage caused by extreme humidity, overheating, or over-voltage can make a resistor develop excess noise after it has been in use a while. One is not directly related to the other, but if you have a carbon resistor that has aged and drifted out of spec, it could well have gotten noisy too.

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 Post subject: Re: Hiss: Are certain resistor values more likely to cause i
PostPosted: Sep Wed 16, 2020 9:01 pm 
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A common name for "excess noise" is "1/f noise", reflecting the fact that the spectral density varies inversely with frequency

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"Voltage is fun to watch, but it's the CURRENT that does the work."


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