Hey all!
I know that this forum is a bit of a mix, probably mostly leaning towards the HiFi community, but I have been reading a lot about Current Feedback (CFB) amplifiers being used in amplifiers designed for guitar.
From what I understand, the current feedback comes from the speaker's negative terminal and raised the output impedance of the amplifier and lowers damping which allows the guitar speaker to react more with the guitar/preamp.
I was wondering if anyone knows of any modules or amplifiers that are built this way?
For reference, I first heard about this reading Elliot Sound website
I have also read that this is how Pat Quilter (Quilter/QSC) designs his guitar amps.
Thanks!
I know that this forum is a bit of a mix, probably mostly leaning towards the HiFi community, but I have been reading a lot about Current Feedback (CFB) amplifiers being used in amplifiers designed for guitar.
From what I understand, the current feedback comes from the speaker's negative terminal and raised the output impedance of the amplifier and lowers damping which allows the guitar speaker to react more with the guitar/preamp.
I was wondering if anyone knows of any modules or amplifiers that are built this way?
For reference, I first heard about this reading Elliot Sound website
I have also read that this is how Pat Quilter (Quilter/QSC) designs his guitar amps.
Thanks!
There are different definitions of current feedback. Looks like you are referring to taking a feedback signal from a current sensing resistor in series with the load? If so, sometimes that can be combined with voltage feedback to good effect. That could be more common for a guitar amp type of thing. Its that some speakers work better (at least at some frequencies) if driven with a current source, and other speakers work better if driven with a voltage source. The mixed voltage and current feedback can give some of both.
The other kind of current feedback is when the output voltage of an amplifier is returned to the input stage (or close to it) in the form of a current rather than a voltage. In that case a pole in the global feedback loop can be more or less be eliminated. This type of current feedback is sometimes used in high performance hi-fi amplifiers.
The other kind of current feedback is when the output voltage of an amplifier is returned to the input stage (or close to it) in the form of a current rather than a voltage. In that case a pole in the global feedback loop can be more or less be eliminated. This type of current feedback is sometimes used in high performance hi-fi amplifiers.
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Some info on current drive at: https://www.current-drive.info/
Its also possible, as I was trying to say before, to use some mix of voltage and current drive by using feedback from both speaker voltage and speaker current.
Its also possible, as I was trying to say before, to use some mix of voltage and current drive by using feedback from both speaker voltage and speaker current.
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Thank you for sharing.
Do you have any examples of to implement feedback from speaker voltage and speaker current?
Do you have any examples of to implement feedback from speaker voltage and speaker current?
I made a subwoofer which had voltage feedback from the dual-coil woofers, and current feedback from a resistor, which was later replaced by a LEM current transducer.
I replaced the Maplin MOSFET amplifiers with Hypex UCD amplifiers.
I had weird oscillations, concluding that it would be better to stick with the usual Class AB amplifier, than struggle with trying to compensate the weird phase-shifts introduced by Class D.
That was with the UCD which has post-filter feedback. The difficulty of including any class D amplifier within a feedback loop was discussed by EVA, on this forum, switching waveforms considerably complicate the issue.
I replaced the Maplin MOSFET amplifiers with Hypex UCD amplifiers.
I had weird oscillations, concluding that it would be better to stick with the usual Class AB amplifier, than struggle with trying to compensate the weird phase-shifts introduced by Class D.
That was with the UCD which has post-filter feedback. The difficulty of including any class D amplifier within a feedback loop was discussed by EVA, on this forum, switching waveforms considerably complicate the issue.
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@johnnyx that makes sense.
Class D itself seems pretty complex.
Class AB is great but not as small/portable which is ideal for guitar and bass amps.
With that said, I’m sure it’s possible to use Hypex or ICE for the power amp but I’m really interested in current feedback as a means to raise output impedance/lower damping and create more responsiveness when playing
Class D itself seems pretty complex.
Class AB is great but not as small/portable which is ideal for guitar and bass amps.
With that said, I’m sure it’s possible to use Hypex or ICE for the power amp but I’m really interested in current feedback as a means to raise output impedance/lower damping and create more responsiveness when playing
Yes it is. I looked again at your first post, but I totally missed where you asked for Class D in the title.
The Peavey (and I presume similar solid-state "current feedback" guitar circuits) do this to raise the effective output impedance that the speaker sees to attempt to replicate the higher impedance of tube amps, which of course give the more traditional electric guitar sound.
But yes, and johnnyx was alluding to, Class D is a sampled signal, and this gives a delay that adds increasing phase shift to higher frequencies, and virtually guaranteeing oscillation. The alternative would be to add some substantial resistance (more than in the Peavey circuit) in series with the speaker to approximate the lower tube circuit damping factor. This would lower efficiency, but it may be a wash with Class D being higher efficiency than Class AB.
Current Feedback seemed fascinating at first.
Then it is found to not be very useful unless a wideband speaker application is used.
Benefits to some to none at all have been shown depending on specific driver types and designs.
Motional Feedback has been used to lower distortion for HiFi sub woofers or woofers.
Non existent frequency in guitar. And distortion is used, so pointless
Then thinking it must be great then for " Guitar" or MI ( Musical Instrument ) Use
Since most use a Open Back / Open Baffle 10 or 12" speaker wideband.
Then in reality the distortion of even guitar " Clean" tone is so high
And 80% of the time a light to heavy overdrive is used for Guitar
From 10 to 30% distortion.
There is absolutely no benefit.
The typical baffle design, and off axis responses are so horrible.
Aka typical guitar tone and all the industry magical descriptions.
What exactly would current feedback help?
Current drive for a MI application would be the longest biggest waste of time rabbit hole to fall into.
Not sounding like a " traditionalist" Class D amplifiers don't belong in guitar amps.
As a engineer they cant go into clipping so your whole design is all preamp and limiters
that loose a lot of voltage swing to make a class D amp work.
Portability is not a problem for a 30 to 60 watt class AB amplifier. A board and 2 power transistors is small.
A linear power supply is not heavy. And the amp will be as large as a 1x12 or 2x12 anyways.
So portability is complete nonsense. Because speaker weight and the cabinet dont change.
You can waste time with a switch mode power supply. Woo Hoo. It will die after 3 or 4 years
And a little 60 to 150 VA transformer is nothing weight wise for a typical guitar amp.
Mesa Poopie and Genz Benz used tube emulation resistors for class D amps.
Thay made poor amplfiers worse by trying to emulate the dampening of a spongy transformer coupled amplifier.
Aka makes more distortion, and already over dampened small speakers cabinets sound more bloomy and slow.
AKA " tube distortion" marketing
Then it is found to not be very useful unless a wideband speaker application is used.
Benefits to some to none at all have been shown depending on specific driver types and designs.
Motional Feedback has been used to lower distortion for HiFi sub woofers or woofers.
Non existent frequency in guitar. And distortion is used, so pointless
Then thinking it must be great then for " Guitar" or MI ( Musical Instrument ) Use
Since most use a Open Back / Open Baffle 10 or 12" speaker wideband.
Then in reality the distortion of even guitar " Clean" tone is so high
And 80% of the time a light to heavy overdrive is used for Guitar
From 10 to 30% distortion.
There is absolutely no benefit.
The typical baffle design, and off axis responses are so horrible.
Aka typical guitar tone and all the industry magical descriptions.
What exactly would current feedback help?
Current drive for a MI application would be the longest biggest waste of time rabbit hole to fall into.
Not sounding like a " traditionalist" Class D amplifiers don't belong in guitar amps.
As a engineer they cant go into clipping so your whole design is all preamp and limiters
that loose a lot of voltage swing to make a class D amp work.
Portability is not a problem for a 30 to 60 watt class AB amplifier. A board and 2 power transistors is small.
A linear power supply is not heavy. And the amp will be as large as a 1x12 or 2x12 anyways.
So portability is complete nonsense. Because speaker weight and the cabinet dont change.
You can waste time with a switch mode power supply. Woo Hoo. It will die after 3 or 4 years
And a little 60 to 150 VA transformer is nothing weight wise for a typical guitar amp.
Mesa Poopie and Genz Benz used tube emulation resistors for class D amps.
Thay made poor amplfiers worse by trying to emulate the dampening of a spongy transformer coupled amplifier.
Aka makes more distortion, and already over dampened small speakers cabinets sound more bloomy and slow.
AKA " tube distortion" marketing
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Would agree that good sounding guitar amp systems are complex analog devices (although the emulator guys keep trying).
However, don't know if the OP is looking for killer guitar sound for some particular genre, or for general studio use, or what.
It can get to be a very different subject of discussion from the more narrow question the OP asked about in the 1st post.
However, don't know if the OP is looking for killer guitar sound for some particular genre, or for general studio use, or what.
It can get to be a very different subject of discussion from the more narrow question the OP asked about in the 1st post.
Do you play guitar?
I’ve been playing over 20 years. In that time I’ve owned probably 10-15 different amplifiers and played countless others.
I’m not saying that you’re wrong, but I will say that quite a few engineers who were also musicians (or worked closely with musicians) chose to implement current feedback into their guitar amp designs:
Peavey, Yamaha (Rivera), and Quilter. I’m sure there are more but those are examples that guitar players who like tube amps use as reference for “good solid state amps”
I’m not above Class AB and of those examples I listed Quilter is the only one that used Class D. And his designs include a limiting circuit, so maybe it’s over my head.
As for weight, I was thinking about heat sinking and that smps is probably lighter than a transformer. I think there are engineers that would argue against your statements about smps vs transformers longevity and the fact that transformers are inherently going to create more noise.
All that said, I appreciate your input and opinion on the matter. I only know enough to be dangerous.
@Markw4 I should have been more specific about what I want to do which is eventually make a hybrid amp (tube preamp, ss power amp).
Based on what I have read, current feedback in power amps helps a solid state power amp behave more closely to how a tube power amp and output transformer react with guitar speakers.
Based on what I have read, current feedback in power amps helps a solid state power amp behave more closely to how a tube power amp and output transformer react with guitar speakers.
Okay. If you have a tube amp with an output impedance switch on the back, you can try each setting and see how it affects the sound. When I have tried that its like it can be interesting for awhile to change the sound a little that way, but I can get tired of it too. That said, guitar amps, if they use output feedback at all, its usually with pretty low loop gain. So it can be possible to try variable blending of voltage and current-drive feedback to tune the sound to where you like it best. IIRC, Richard Marsh published a simple circuit to do that a long time ago, but I don't know where to find it now.
However, my particular approach to guitar sound probably starts at the overall mixability of the band. That may start with considering the vocal range of the singer and the genre. Each instrument will have to fill the frequency space in a way complementary to overall good sound in a workable mix.
The other end of the problem has to do with the guitarist's playing style, technique, physiology, etc. They need an instrument they can make sing with their fingers. For electric guitar that includes the amp, speaker, and speaker cab.
This is all for a guitar used in a band that wants overall great sound.
OTOH, guitarists often want to start out with a sound they enjoy for noodling alone. IMHO, that's a different application. One maybe good for stimulating creativity and motivating staying in practice (like with a metronome).
Then there are people that just want to build an amp because it would be a fun project (and maybe it would even sound good). Maybe other people want to see if they have what it takes to be a commercial amp designer.
You know, different strokes for different folks.
So, would it be correct to assume this is just for a fun project?
However, my particular approach to guitar sound probably starts at the overall mixability of the band. That may start with considering the vocal range of the singer and the genre. Each instrument will have to fill the frequency space in a way complementary to overall good sound in a workable mix.
The other end of the problem has to do with the guitarist's playing style, technique, physiology, etc. They need an instrument they can make sing with their fingers. For electric guitar that includes the amp, speaker, and speaker cab.
This is all for a guitar used in a band that wants overall great sound.
OTOH, guitarists often want to start out with a sound they enjoy for noodling alone. IMHO, that's a different application. One maybe good for stimulating creativity and motivating staying in practice (like with a metronome).
Then there are people that just want to build an amp because it would be a fun project (and maybe it would even sound good). Maybe other people want to see if they have what it takes to be a commercial amp designer.
You know, different strokes for different folks.
So, would it be correct to assume this is just for a fun project?
Here I asked for similar idea and got an answer how it can be done theoretically to an existing amplifier design which was produced in masses.
Maybe someone checks out if it can be done like proposed
Maybe someone checks out if it can be done like proposed
Current feedback and current output are two different things. IIUC the amplifier you are describing in your post is current controlled output amplifier because the speaker current is the controlled amplifier output quantity vs the other (more common) option of voltage output amplifier. Either of these two output types can be built with voltage feedback or current feedback topologies.
It’s definitely for fun, but it’s also for “science”.
I would like to test a lot of separates (preamps, power amps, and speaker/cabs) and if some things work well together I might be interested in building them for people, but that’s not an immediate goal or anything.
I play a little guitar....too.
The world of guitar amps is totally far removed from HIFI amps (you know it)...
The tube transforms the sound, to their advantage for the guitarist, harmonic even in distortion and clear and airy sound..
No emulation will replace my tube guitar amp...
When the output transformers and tubes saturate..... I am on the planet HENDRIX. 🙂 🙂 🙂 🙂
What some folks (including me) have found is that speaker cabs are one of the biggest problem areas for getting good sound. Usually they are badly underdamped, boomy, muddy, etc. That's not necessarily the most musical. Guitars, amps, and speakers themselves, don't seem to be as much of a problem on average as cabs. Now, its also possible to over-dampen speaker cabs, and a well damped cab which has controlled damping from added tone-wood bracing (such as maple strips) can get kind of heavy to lug around without a small fold-up hand truck. Anyway, its something to consider. Regarding amps, its possible to dial in some great sounding guitar tones from a cheap Fender practice amp. Its also possible to get worse tones out of a more expensive tube amp. It really depends a lot on the specifics.
Even one of these can make a lot of good tones if you know how to dial in an amp and a guitar using the knobs and switches: https://www.sweetwater.com/store/de...tE6xN17Lnu7FnRD3ohkaAlzSEALw_wcB&gclsrc=aw.ds