If you would apply negative feedback from the power amplifier output to the preamplifier input with a fixed feedback network, and if you would manage to get it to work somehow, the volume control wouldn't control volume but loop gain.
I agree with Jan, though. Reading the first and sixth posts, this is clearly not what BKr0n had in mind.
I think there is just some confusion about the term preamplifier, and that BKr0n simply means the input stage of the power amplifier.
I agree with Jan, though. Reading the first and sixth posts, this is clearly not what BKr0n had in mind.
I think there is just some confusion about the term preamplifier, and that BKr0n simply means the input stage of the power amplifier.
You're right. I thought that's what the input stage was colloquially called. Sorry for the confusion.
Is it possible for a thread starter to edit the thread title or can one only send a request to the moderators using the report button?
In that case you could report your own post using the report button and ask for a change of the title.
Anyway, to answer the question you meant to ask: for an ordinary amplifier with a high-impedance input, low-impedance output and accurate voltage-to-voltage transfer, the feedback network is almost always a voltage divider consisting of two resistors. Thermal noise can be kept under control by using sufficiently small values for the resistors.
In principle you could use some other type of voltage attenuator as feedback network, for example a step-down transformer or a capacitive voltage divider, but it is almost never done. It usually turns out to be impractical compared to just using two resistors.
For an amplifier with high-impedance input and high-impedance output and an accurate voltage-to-current transfer, as is used for current-driving loudspeakers, the feedback network is usually a small resistor in series with the load.
Anyway, to answer the question you meant to ask: for an ordinary amplifier with a high-impedance input, low-impedance output and accurate voltage-to-voltage transfer, the feedback network is almost always a voltage divider consisting of two resistors. Thermal noise can be kept under control by using sufficiently small values for the resistors.
In principle you could use some other type of voltage attenuator as feedback network, for example a step-down transformer or a capacitive voltage divider, but it is almost never done. It usually turns out to be impractical compared to just using two resistors.
For an amplifier with high-impedance input and high-impedance output and an accurate voltage-to-current transfer, as is used for current-driving loudspeakers, the feedback network is usually a small resistor in series with the load.
One addition: what I just wrote about thermal noise and feedback networks may give the impression that resistors with a low value produce less thermal noise than those with a high value. That's correct when it is the noise voltage that matters, it is the other way around when noise current matters. Anyway, that's a whole different subject.
Well that is because its a signal carrying resistor and you have to select it correctly and I use the same type in my grid stoppers.
low ppm resistors= low self noise. Keeping below 70% of the resistor's voltage rating will also keep self noise down.
The resistors I use for these applications (for feedback, grid, cathode, and sometimes plate) is the Vishay/Mills MRA series.
Resistance value actually means nothing to self noise other than all low value resistors don't develop a big voltage potential across them that causes great amounts of self noise. You choose the correct resistor that would be low self noise, not effected by magnetism, and doesn't radiate it either.
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This actually makes a lot of sense then. One of the parts I'm using in my design, the INA851, actually asks for an input bias current. I may be able to kill two birds with one stone by making something that takes both of these things into account.
Is that why 10k ohm resistors are used so often?
So really it does come down to heat noise then. Are there other ways to increase this stability? Or will high tolerance resistors be enough to maintain signal integrity?
Most noise is thermal noise. But some devices like tubes with their hi-z pathways and wide bandwidth gain are sensitive to interference and one must build with components that will help and reject all unwanted signals and not turn itself into an antenna for interference. Special low signal Insertion loss resistors such as non-inductive wire wound resistors is what I use in those sensitive areas.
I know a guy who I turned him on to these resistors for his turntable preamp build. After some time, he told me loves the results and uses them almost entirely for his builds.
Another thing people didn't understand how the old guys build and why they used big wattage resistors was since the resistor is dissipating in a circuit 1/8 of the wattage rating, the thermal noise was mostly non-existent. But since the non-inductive wirewound was not mass produced back then, the carbon comp resistor was used a lot. But it had issues of attenuating high frequencies due to its construction, and it would caused hiss in the circuit.
The other reason why they oversized their resistor is so that there is mechanical strength when assembling the circuit in point to point hand wiring.
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Thermal noise depends only on the value of the resistor and on the temperature.
Besides thermal noise, there is also excess noise or 1/f noise (also known under various other names). It manifests itself as random resistance fluctuations, so it only affects the noise floor when there is a DC current flowing through the resistor. It depends a lot on the type of resistor; wirewound resistors have very low excess noise, metal film and thin film resistors are also pretty good, carbon film is worse and carbon composition is awful.
Yes,
However, in working circuit its how its heated up and how sensitive the circuit is to it. Anytime when power dissipation gets close to the maximum limits, is when the part's thermal noise is the highest.
True, especially for wirewound resistors. When used close to their power rating, some of them can get so hot they desolder themselves if their connecting wires are cut off too close to their bodies.
So then in this scenario, the vishays are a much better choice as opposed to smd resistors altogether?
While we're on this subject, I guess another question would be what about for things like inductors or transformers? I've been reading up on iron loss and the use of FDNRs as a replacement. Would using this type of resistor also be good for a circuit like that?
@BKr0n I have the impression you picked up some pieces of information but have way too little general knowledge of electronics to connect the dots. I think you should read some electronics textbooks, starting with very basic ones. It will take some time, but after that things will make a lot more sense.
Simple approaches may fail due to too many poles in the overall response, leading to instability - the feedback network would probably have to be tailored specific to each preamp/poweramp combination to make this viable - like a composite opamp.
Many preamps have a low pass filter frontend to limit slew rate and reject RFI, which is not going to help - that's best pulled outside any loop.
But yes, nesting feedback loops like this can definitely increase performance.
Yes through hole parts are better performing, and there are few SMD that are good, but are hard to source. There is a few that do work ok, but almost three times expensive as the premium thru hole resistors but most of the time the parts houses usually don't carry them unless some OEM ordered some for their builds. So resistance values becomes an availability issue.
Vishay/Mills resistors. The Vishay industrial package are black coated. The Mills are brown enamel. Mills resistors would be more known to you as they were very popular for Zobel networks in audiophile crossovers.
Vishay labelled resistor (MRA12):
Mills labelled MRA5 and MRA12:
I know there are other manufacturers for them. But these are the best ones I found several sources that sell them at reasonable prices. Granted I could go to an audiophile parts site, but you always pay a premium price there.
Ah I recognize those, but not from audio. Saw them before in RF equipment. Makes a little more sense now.Vishay labelled resistor (MRA12):
Mills labelled MRA5 and MRA12:
So then in the case of going from a high power output stage and feeding back to a low power input stage, wouldn't there be some sort of loss while you are attenuating the signal like that? The energy has to go somewhere right?