Broadly speaking, global NFB reduces gain, distortion and output impedance. In a sound reproducer these are often seen as good things. For an instrument it all depends on what sound you want. NFB in a guitar amp can be used to turn low-order distortion into smaller amounts of high-order distortion, or to introduce frequency reponse anomalies. NFB will harden the effect of clipping, which may or may not be what you want.
Thanks for the reply. To tell you the truth, i am not too sure what i want...I want an amp with character. Basically i would like to understand what i should expect from an amp with NFB and without (sonicwise) and then base my choice. that's mroe or less what you wrote. Would you have some references on the web i could look up about this matter? Specifically what NFB does to sound?
Also, could explain a little further what you mean harden the effect of clipping?
Thanks a lot
Also, could explain a little further what you mean harden the effect of clipping?
Thanks a lot
There are others on here far more knowledgeable than I when it comes to guitars and their amps. I just listen to them; they play them.
When an amp with NFB begins to clip the NFB will push it further into clipping, so if you look at the waveform it has sharper edges and a flatter top. This means more high-order components, which sounds harder or harsher. However, below the clipping threshold the amp may sound cleaner.
When an amp with NFB begins to clip the NFB will push it further into clipping, so if you look at the waveform it has sharper edges and a flatter top. This means more high-order components, which sounds harder or harsher. However, below the clipping threshold the amp may sound cleaner.
+1 what DF96 said.
More reading follow the link in the 2nd post:
The Amp Garage :: View topic - Negative feedback - How much?
If you want character, forget the NFB IMO, guitar amps have no need of it, don't know why they ever used it ever.
Like a clean guitar amp cannot be designed "clean enough" without NFB? What were they thinking????
More reading follow the link in the 2nd post:
The Amp Garage :: View topic - Negative feedback - How much?
If you want character, forget the NFB IMO, guitar amps have no need of it, don't know why they ever used it ever.
Like a clean guitar amp cannot be designed "clean enough" without NFB? What were they thinking????
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If you want an amp known for its character you might consider something like the 50's tweed Deluxe 5E3 circuit.
It has no NFB as well as being one of the simplest designs ever. Many variations have emerged over the years, but the original is one of the most loved amps to ever come along.
YMMV
Edit,
A lot has to do with your target tone. You will not get Marshall tones out of a tweed deluxe easily, but if you are wanting some very raw bluesy tone, its tough to top a well built 5E3.
It has no NFB as well as being one of the simplest designs ever. Many variations have emerged over the years, but the original is one of the most loved amps to ever come along.
YMMV
Edit,
A lot has to do with your target tone. You will not get Marshall tones out of a tweed deluxe easily, but if you are wanting some very raw bluesy tone, its tough to top a well built 5E3.
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-cough- Fender Bassman -cough- all classic Marshall amps -cough- all classic Hiwatt amps -cough - Soldano SLO100 and the countless derivates of the design -cough, cough...
Negative feedback is a "tool" just like any other circuit topology. You can use it to increase headroom and linearity of both amplification and frequency response, naturally at the cost of overall "smoothness" of clipping. Some find those characteristics favourable and many, many classic designs are based on them. Not to mention you can introduce feedback -controlled tone shaping like resonance and presence controls, or in rarer cases a "damping" control that alters the magnitude of negative feedback.
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High Gain Overdrive guitar amps use GNFB (in moderate amounts) to achieve rapid transition from clean to mean.
Combined with the feedback you get when the strings pick up the output of the amp, you get a low level trem effect to a greater extent with GNFB than without it as well as slightly more sustain.
Combined with the feedback you get when the strings pick up the output of the amp, you get a low level trem effect to a greater extent with GNFB than without it as well as slightly more sustain.
Once built a guitar amp for a guy who wanted a specific "sound"... so I gave him a knob for everything so he could adjust (within safe limits) to get the sound he wanted ... the plan was to measure the pots, and fit resistors... he said "Just leave it as it is , it's fantastic" so it was left... my one and only claim to fame, is that amp played the Albert Hall in London
I think you're rather chasing your tail with your question, "Could someone give me a run down of the advantages and disadvantages or having negative feedback on a push pull guitar power amp? What about sonic performance?" Looking to NFB as a way to provide "character" is like searching for an undefined stone on the face of a mountain of more important data.
"Basically i would like to understand what i should expect from an amp with NFB and without (sonicwise) and then base my choice."
I would begin searching for "character" or "tone" not with NFB but with what tube types are being utilitized in the circuitry. Triodes as output tubes can run without NFB or relatively little localized feedback as the tube has a sort of self NFB effect. Pentodes and beam power tubes will generally measure cleaner with small amounts of localized feedback applied. High amounts of global NFB are frowned upon by both the instrument and the audiophile market as they lead to a more unstable amplifier which will exhibit distortion components (significant high order vs minimal low order) and phase shift problems you can hear but which will not show up on a typical THD test bench measurement. I know of no instrument amplifier with specs for whether the NFB is local or global and how much of each has been utilized. Each of these shifts in characteristic sound achieved solely through tube selection will be superceded by the quality of the design and the monies put into construction. Cheap transformers and a low quality power supply will swamp any change made by NFB in any amplifier. So your question has no definitive answer since the tube type will open up the possibility of not using feedback at all or demanding a small amount to stabilze the amp and the quality of design/parts/construction will significantly determine the character of any particular amp.
Further, most anyone familiar with tubes will tell you the sonic differences between a 6L6 tube type, a KT88 and an EL34 are far greater than the effect of NFB combined with each tube type. Those pentode/beam power tubed amps would be units with a decent amount of wattage output. There is a trend in guitar amps right now which favors the "retro" low powered (five watts or less) amplifiers which are not typically constructed with the aforementioned tubes. Stepping down to an EL84 output tube would push the sonic envelope in another direction all together while paralled 12AX7's would provide yet another sonic character. Driving an EL84 output tube with a particular triode pre amp tube would also significantly alter the sound of the amp. And, should you not prefer the sonics of, say, the JJ 12AX7 which came OEM in the amp, you can swap it for another brand or vintage of tube or even go to a 12AU7 to further alter the character of the amp's output. Or you could even rewire the pre amp tube socket to accept, say, a 6DJ8 if you like the sound of that tube. Tube rolling has hit the guitar amp market full force and shows no more signs of abating there than it does in the audiophile community. The tube type, parts employed and the topology of the amplifier circuitry will have a profound effect on; 1) the sonic character of the amp and, 2) whether the amp requires NFB or not. Another truism with tubes is the generic character of any specific tube is fungible within the circuitry that surrounds it. Push/pull amps will, if you are familiar enough with the basic character of each topology, sound distinctly different than would a single ended amp. A little search engine work should provide you with sufficient clues as to what you can expect in a comparison of SE vs. PP. However, most of those differences are more noticeable on a test bench rather than in actual use with real world music. Hearing one PP amp and one SE amp wouldn't even begin to scratch the surface of whether you preferred one over the other. Hearing a pentode tube run as a triode isn't the same thing as hearing a triode based amplifier.
What most - not all, but most - electric players are interested in is "crunch". How do the tubes distort and at what level of input voltage? Is the distortion controllable and somewhat gradual in its onset or is it full out by the time the gain control hits nine O'Clock? Is the overdrive achieved with a master gain control on the head amp or with the instrument's output controls? While NFB can be used to manipulate the gain at the front (input) end of the pre amp and power amplifier, there are multiple other methods of acquiring gain and overdive with an electrified instrument. You might decide it's more effective to buy a " clean" amp and then have a "tube screamer" or a digital modelling pedal which you can insert and delete at will. You can always manipulate away from clean but you can never get away from an amp that has an overriding character of its own. While you might not yet know what "character" you want, you'll have to decide what it is you think you want before you can find it in an amp. Otherwise, what you'll do is fall into the constantly changing gear trap where you have no idea what you want but you keep buying gear hoping to find the one amp that does what you want. Well, in guitars, "tone" is ever elusive and too many players spend their entire life constantly swapping gear just to get the sound they heard on so and so's new album. IMO that's a very wasteful way to go about buying equipment. You should understand above all else that "tone" is first and foremost in the player and not in the gear and that BB King would still sound like BB King if he were given a $99 Squire Strat and a $69 Vox practice amp.
You should know whether you want to constantly overdrive the amp to achieve a break up which provides, first, extended sustain and then quicly falls into crunch. Or would you prefer a more "acoustic amp" sound which has little in the way of crunch but which can be manipulated to provide improved sustain alone. Are you looking for a blues sound, a jazz sound, a rock or metal sound? These are far the more distinct personality traits of any instrument amplifier than would be just how many dB's of NFB have been used in the circuitry of the amp. Looking stirctly at NFB within a particular amp, any qualified tech can swap an input cap to alter both the sonic character of the amplifier and the amount of NFB present. Swap a part here or clip a jumper there and suddenly you can have a very different amp to play with. The guitar amp forums are awash in mods to existing amps in order to provide some sonic character other than what the designer intended and they purchased. Therefore, any amplifier's tone can be altered to some extent just by a simple parts swap or less. But a simple parts swap isn't going to change a blues head amp into a metal head amp when it comes to the basic, characteristic tone of the product.
That said, the amplifier is still the most neutral or "characterless" component in the chain of electronics. A P90 pick up vs a single coil vs a mini humbucker will far outweigh the sonic differences of a few DB of NFB change. A solid body instrument vs an archtop will far outweigh the NFB issues. Ash vs mahogany will noticeably alter the sound. A short scale vs a long scale instrument determines the most basic sound you will achieve. Your choice of strings and their gauge will be a more significant alteration to sonics than would the NFB within a circuit. Pick, fingers or hybrid? Overdriving the front of the pre amp with lots of gain from the instrument will have a very distinctive character as will choosing a head amp with a master gain and a volume control. While NFB can play a very small part in the total result, it is in reality only influenced by these and many more determinants all of which are far more significant to the final product.
Very importantly, are you a player who values the "sense of touch" which certain genres and styles of play require? Playing palm muted power chords and metal shred doesn't require much in the way of "touch" from the amp. Playing Chet Atkins style Country/Jazz/Swing willl place a premium on "touch". If you value the sort of playing which can benefit from a highly sensitive chain of electronics, the virtue to be found in an amp with little to no NFB would be the more straight forward, low parts count of the amplifier's signal chain. Read the ads for the various "boutique" amps and the famous tube amps in history and you'll often hear this "straight wire" aspect of the amp's performance being touted. One difficult aspect of judging "audiophile" sound quality is the very fact the charcteristic sound of an elecrtrified instrument can be so utterly quixotic and, by the turn of a knob, flick of a selector switch or the insertion of a single effects pedal, that character can be altered even within the confines of a basic twelve bar progression. Of all the things that can easily manipluate the character of the chain, NFB would be very far down on the list of important elements.
Finally, with any amplified instrument's chain of sonics will be the speaker/driver it plays into. Once again the buyer must decide whether they want a system that plays overdriven, crunchy, swampy, in your face tones or whether a more laid back and clean sound is where your playing resides. Are you more into metal, Chicago electrified blues or highly influenced by James Taylor? Here reading about a few of the classic amps used by well known performers who are playing in the genre which you prefer will start your education towards what sort of "character" you might want to explore in an amplifier/speaker combination. Clapton (and hundreds, if not thousands, of other players) ran with high wattage Marshall amps set to "12" when playing their style of blues rock. The three famous Kings; BB, Freddie and Albert wouldn't have used anything more than a smallish Fender or Gibson system with or without reverb. By far the driver and cabinet provide significantly more control over the final character of the sound than will NFB. Finally, how will the amp be used? Is this a bedroom system for your personal enjoyment or is this a set which will be used to play in a more public performance? If the former, then NFB remains a very minute and potentially highly adjustable component in the final sound. If the latter, then probably the choice of playing straight through the amp combo or playing into a mic placed a few inches in front of the driver and then run through a public address system will negate any and all characteristics of NFB as they relate to the character of your playing. All in all, assessing the effect of NFB on the final character of your sound is similar to figuring out just how much the shortstop's shoes play into a World Serie's winning team's total performance. You're trying to catch gnats in a tornado with the assumption you can look at a spec for NFB and therefore determine the character of any chain of electronics.
"Basically i would like to understand what i should expect from an amp with NFB and without (sonicwise) and then base my choice."
I would begin searching for "character" or "tone" not with NFB but with what tube types are being utilitized in the circuitry. Triodes as output tubes can run without NFB or relatively little localized feedback as the tube has a sort of self NFB effect. Pentodes and beam power tubes will generally measure cleaner with small amounts of localized feedback applied. High amounts of global NFB are frowned upon by both the instrument and the audiophile market as they lead to a more unstable amplifier which will exhibit distortion components (significant high order vs minimal low order) and phase shift problems you can hear but which will not show up on a typical THD test bench measurement. I know of no instrument amplifier with specs for whether the NFB is local or global and how much of each has been utilized. Each of these shifts in characteristic sound achieved solely through tube selection will be superceded by the quality of the design and the monies put into construction. Cheap transformers and a low quality power supply will swamp any change made by NFB in any amplifier. So your question has no definitive answer since the tube type will open up the possibility of not using feedback at all or demanding a small amount to stabilze the amp and the quality of design/parts/construction will significantly determine the character of any particular amp.
Further, most anyone familiar with tubes will tell you the sonic differences between a 6L6 tube type, a KT88 and an EL34 are far greater than the effect of NFB combined with each tube type. Those pentode/beam power tubed amps would be units with a decent amount of wattage output. There is a trend in guitar amps right now which favors the "retro" low powered (five watts or less) amplifiers which are not typically constructed with the aforementioned tubes. Stepping down to an EL84 output tube would push the sonic envelope in another direction all together while paralled 12AX7's would provide yet another sonic character. Driving an EL84 output tube with a particular triode pre amp tube would also significantly alter the sound of the amp. And, should you not prefer the sonics of, say, the JJ 12AX7 which came OEM in the amp, you can swap it for another brand or vintage of tube or even go to a 12AU7 to further alter the character of the amp's output. Or you could even rewire the pre amp tube socket to accept, say, a 6DJ8 if you like the sound of that tube. Tube rolling has hit the guitar amp market full force and shows no more signs of abating there than it does in the audiophile community. The tube type, parts employed and the topology of the amplifier circuitry will have a profound effect on; 1) the sonic character of the amp and, 2) whether the amp requires NFB or not. Another truism with tubes is the generic character of any specific tube is fungible within the circuitry that surrounds it. Push/pull amps will, if you are familiar enough with the basic character of each topology, sound distinctly different than would a single ended amp. A little search engine work should provide you with sufficient clues as to what you can expect in a comparison of SE vs. PP. However, most of those differences are more noticeable on a test bench rather than in actual use with real world music. Hearing one PP amp and one SE amp wouldn't even begin to scratch the surface of whether you preferred one over the other. Hearing a pentode tube run as a triode isn't the same thing as hearing a triode based amplifier.
What most - not all, but most - electric players are interested in is "crunch". How do the tubes distort and at what level of input voltage? Is the distortion controllable and somewhat gradual in its onset or is it full out by the time the gain control hits nine O'Clock? Is the overdrive achieved with a master gain control on the head amp or with the instrument's output controls? While NFB can be used to manipulate the gain at the front (input) end of the pre amp and power amplifier, there are multiple other methods of acquiring gain and overdive with an electrified instrument. You might decide it's more effective to buy a " clean" amp and then have a "tube screamer" or a digital modelling pedal which you can insert and delete at will. You can always manipulate away from clean but you can never get away from an amp that has an overriding character of its own. While you might not yet know what "character" you want, you'll have to decide what it is you think you want before you can find it in an amp. Otherwise, what you'll do is fall into the constantly changing gear trap where you have no idea what you want but you keep buying gear hoping to find the one amp that does what you want. Well, in guitars, "tone" is ever elusive and too many players spend their entire life constantly swapping gear just to get the sound they heard on so and so's new album. IMO that's a very wasteful way to go about buying equipment. You should understand above all else that "tone" is first and foremost in the player and not in the gear and that BB King would still sound like BB King if he were given a $99 Squire Strat and a $69 Vox practice amp.
You should know whether you want to constantly overdrive the amp to achieve a break up which provides, first, extended sustain and then quicly falls into crunch. Or would you prefer a more "acoustic amp" sound which has little in the way of crunch but which can be manipulated to provide improved sustain alone. Are you looking for a blues sound, a jazz sound, a rock or metal sound? These are far the more distinct personality traits of any instrument amplifier than would be just how many dB's of NFB have been used in the circuitry of the amp. Looking stirctly at NFB within a particular amp, any qualified tech can swap an input cap to alter both the sonic character of the amplifier and the amount of NFB present. Swap a part here or clip a jumper there and suddenly you can have a very different amp to play with. The guitar amp forums are awash in mods to existing amps in order to provide some sonic character other than what the designer intended and they purchased. Therefore, any amplifier's tone can be altered to some extent just by a simple parts swap or less. But a simple parts swap isn't going to change a blues head amp into a metal head amp when it comes to the basic, characteristic tone of the product.
That said, the amplifier is still the most neutral or "characterless" component in the chain of electronics. A P90 pick up vs a single coil vs a mini humbucker will far outweigh the sonic differences of a few DB of NFB change. A solid body instrument vs an archtop will far outweigh the NFB issues. Ash vs mahogany will noticeably alter the sound. A short scale vs a long scale instrument determines the most basic sound you will achieve. Your choice of strings and their gauge will be a more significant alteration to sonics than would the NFB within a circuit. Pick, fingers or hybrid? Overdriving the front of the pre amp with lots of gain from the instrument will have a very distinctive character as will choosing a head amp with a master gain and a volume control. While NFB can play a very small part in the total result, it is in reality only influenced by these and many more determinants all of which are far more significant to the final product.
Very importantly, are you a player who values the "sense of touch" which certain genres and styles of play require? Playing palm muted power chords and metal shred doesn't require much in the way of "touch" from the amp. Playing Chet Atkins style Country/Jazz/Swing willl place a premium on "touch". If you value the sort of playing which can benefit from a highly sensitive chain of electronics, the virtue to be found in an amp with little to no NFB would be the more straight forward, low parts count of the amplifier's signal chain. Read the ads for the various "boutique" amps and the famous tube amps in history and you'll often hear this "straight wire" aspect of the amp's performance being touted. One difficult aspect of judging "audiophile" sound quality is the very fact the charcteristic sound of an elecrtrified instrument can be so utterly quixotic and, by the turn of a knob, flick of a selector switch or the insertion of a single effects pedal, that character can be altered even within the confines of a basic twelve bar progression. Of all the things that can easily manipluate the character of the chain, NFB would be very far down on the list of important elements.
Finally, with any amplified instrument's chain of sonics will be the speaker/driver it plays into. Once again the buyer must decide whether they want a system that plays overdriven, crunchy, swampy, in your face tones or whether a more laid back and clean sound is where your playing resides. Are you more into metal, Chicago electrified blues or highly influenced by James Taylor? Here reading about a few of the classic amps used by well known performers who are playing in the genre which you prefer will start your education towards what sort of "character" you might want to explore in an amplifier/speaker combination. Clapton (and hundreds, if not thousands, of other players) ran with high wattage Marshall amps set to "12" when playing their style of blues rock. The three famous Kings; BB, Freddie and Albert wouldn't have used anything more than a smallish Fender or Gibson system with or without reverb. By far the driver and cabinet provide significantly more control over the final character of the sound than will NFB. Finally, how will the amp be used? Is this a bedroom system for your personal enjoyment or is this a set which will be used to play in a more public performance? If the former, then NFB remains a very minute and potentially highly adjustable component in the final sound. If the latter, then probably the choice of playing straight through the amp combo or playing into a mic placed a few inches in front of the driver and then run through a public address system will negate any and all characteristics of NFB as they relate to the character of your playing. All in all, assessing the effect of NFB on the final character of your sound is similar to figuring out just how much the shortstop's shoes play into a World Serie's winning team's total performance. You're trying to catch gnats in a tornado with the assumption you can look at a spec for NFB and therefore determine the character of any chain of electronics.
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Tube Amp Buying Basics - Premier Guitar
http://www.premierguitar.com/Magazi...Wanted_to_Know_About_Output_Tubes.aspx?Page=1
http://www.premierguitar.com/Magazi...u_Ever_Wanted_to_Know_About_Preamp_Tubes.aspx
http://www.premierguitar.com/Magazine/Issue/2009/Feb/Which_Tubes_Are_For_You.aspx
http://www.premierguitar.com/Magazi...t_Different_Approaches_to_Guitar_Effects.aspx
http://www.premierguitar.com/Magazine/Issue/2009/Dec/Acoustic_EQ_for_Stage_Part_1.aspx
http://www.premierguitar.com/Magazine/Issue/2009/Nov/Front_to_Back_with_Brad_Paisleys_Rig.aspx
http://www.premierguitar.com/Magazi...Wanted_to_Know_About_Output_Tubes.aspx?Page=1
http://www.premierguitar.com/Magazi...u_Ever_Wanted_to_Know_About_Preamp_Tubes.aspx
http://www.premierguitar.com/Magazine/Issue/2009/Feb/Which_Tubes_Are_For_You.aspx
http://www.premierguitar.com/Magazi...t_Different_Approaches_to_Guitar_Effects.aspx
http://www.premierguitar.com/Magazine/Issue/2009/Dec/Acoustic_EQ_for_Stage_Part_1.aspx
http://www.premierguitar.com/Magazine/Issue/2009/Nov/Front_to_Back_with_Brad_Paisleys_Rig.aspx
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Actually, B.B. King's likely most important mainstay amp is a solid-state Lab Series L5 and Albert King's (at least during his hayday in late 1960's) was a solid-state Acoustic Control 260 or 270. Clapton also recorded a lot with a single-ended Fender Champ tube amp as well as with a solid-state battery-powered Pignose 7-100, which today would be pretty much considered a toy.
Amps aren't everything and very often tone associations concerning musicians can be flat out incorrect. What we think that sounds like a dimed tube Marshall stack could as well be a half-watt solid-state amp pushing a 5-inch speaker. AND tone is 99% in the fingers.
Anyway, soufiej, thank you for a very informative post.
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NFB in guitar amp
Presence control is usually achived as part of NFB. So if you need Presence control....
NFB will stiff up response of low notes. i.e. 6th string. Modern Resonance control is also part of NFB.
A few apms (considered the best one) that use NFB (even if some of them doesn't have presence control):
Fender Deluxe Reverb (2 x 6V6, 1 x 12" speaker, great for lead)
Fender Super Reverb (2 x 6L6, 4 x 10" speakers, great for clean)
Fender Vibroverb (2 x 6L6, 1 x 15" speaker, as above)
Marshall JCM800 (50W 2204 with half stack cabinet and you are in rock heaven)
What else do you need?? The limit is just your or mine playing ability!
Enough said.
NFB will be soon blaimed for ozone holes, too!!
Presence control is usually achived as part of NFB. So if you need Presence control....
NFB will stiff up response of low notes. i.e. 6th string. Modern Resonance control is also part of NFB.
A few apms (considered the best one) that use NFB (even if some of them doesn't have presence control):
Fender Deluxe Reverb (2 x 6V6, 1 x 12" speaker, great for lead)
Fender Super Reverb (2 x 6L6, 4 x 10" speakers, great for clean)
Fender Vibroverb (2 x 6L6, 1 x 15" speaker, as above)
Marshall JCM800 (50W 2204 with half stack cabinet and you are in rock heaven)
What else do you need?? The limit is just your or mine playing ability!
Enough said.
NFB will be soon blaimed for ozone holes, too!!
A resonance control was not common in a classic tube amps like Fender black/silverface, Marshall Plexies etc. Not to mention earlier amps. If I'm not wrong, Peavey 5150 from the beggining of 90s was one of the first amps that used that control. So, it is used in modern amps, VHT and so on...
Modern Reseonance control = control found on modern guitar amps (not all, obviously)
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