Hi all,
Hoping for some guidance with applying the principles presented in the is web page http://www.valveradio.net/audio/smart-negative-feedback.html to a SE guitar amp. Its worth reading the whole article and the associated links.
While the author intends the application for small SE mantle radio, which may often have a grounded cathode / grid leak bias driver I would like to apply the approach to a cathode biased driver. The benefits described in the article would appear to be helpful for a typical single ended 6v6 or el84 guitar amplifier.
I have implemented the circuit ( with small changes based on parts on hand) both for a grounded cathode / grid leak and a cathode biased driver.
However want to understand if how it is best applied to a cathode biased driver stage.
My main concern is how to size a shunt resistor , or if it is preferable to apply the feedback to the top of the cathode resistor with out shunt resistor ?
This would involve the recalculation of the filters, which I'm struggling with.
One option is to setup a pair of dual gang 25k pots for both the nfb R8 / R9 and the R6 / R7 and tune it by ear. Then move the nfb point between the different application points and test again to see what works best. This may mean that the caps are sized correctly for one application point but not others.
I have added a 25k pot in parallel fixed resistor values to test this approach to the but find the the 25k pot is so sensitive at the low end that minor adjustments are difficult.
While this is not HiFi and does not need precise calculation or measurement I would like to ensure I'm heading in the right direction in a practical sense.
I do greatly appreciate the author's work and the many very experienced and expert contributors on this forum.
Hoping for your kind assistance.
Hoping for some guidance with applying the principles presented in the is web page http://www.valveradio.net/audio/smart-negative-feedback.html to a SE guitar amp. Its worth reading the whole article and the associated links.
While the author intends the application for small SE mantle radio, which may often have a grounded cathode / grid leak bias driver I would like to apply the approach to a cathode biased driver. The benefits described in the article would appear to be helpful for a typical single ended 6v6 or el84 guitar amplifier.
I have implemented the circuit ( with small changes based on parts on hand) both for a grounded cathode / grid leak and a cathode biased driver.
However want to understand if how it is best applied to a cathode biased driver stage.
My main concern is how to size a shunt resistor , or if it is preferable to apply the feedback to the top of the cathode resistor with out shunt resistor ?
This would involve the recalculation of the filters, which I'm struggling with.
One option is to setup a pair of dual gang 25k pots for both the nfb R8 / R9 and the R6 / R7 and tune it by ear. Then move the nfb point between the different application points and test again to see what works best. This may mean that the caps are sized correctly for one application point but not others.
I have added a 25k pot in parallel fixed resistor values to test this approach to the but find the the 25k pot is so sensitive at the low end that minor adjustments are difficult.
While this is not HiFi and does not need precise calculation or measurement I would like to ensure I'm heading in the right direction in a practical sense.
I do greatly appreciate the author's work and the many very experienced and expert contributors on this forum.
Hoping for your kind assistance.
Who's the "driver", V1 or V2? What's a grid leak bias? Example schematic please. In my ignorance, I dont know how one can ground a cathode and bias a tube without a negative voltage supply available. What's the "shunt resistor", where would it be in the schematic; R6, R7? Is "cathode resistor" R5? Did you mean shunt capacitor C4?
You should call out your nomenclature by associating it to the schematic reference designator. I'm lost...
By the looks of the low Z of the feedback circuit, perhaps a 1k dual gang pot would give better adjustment sensitivity.
Grid leak bias: grid traps electrons emitted by cathode, and thus becomes negative. A high value resistor (few megOhms) between grid and ground allows negative charge of the grid to "leak" slowly, but, as trapping of electrons by grid continues, the balance of trapping and leak creates steady negative potential of the grid. In this scheme cathode is grounded.
Actually in your scheme the first tube is already cathode-biased (two 470R resistors in series with transformer secondary, to ground). All you need to do is to replace the 10M resistor with something like 470K.
The posted schematic is from the article Ive referenced, yes and believe that as pictured it is a ‘combination bias’ of sorts partly from grid leak and partly from the NFB resistors to ground via the transformer secondary.
R6 and R7 are marked on the schematic.
Below is an example, where NFB has been returned to a ‘shunt’ resistor R2, though there is no second order low frequency roll-off.
Typical values seem to range between 47k and 150k
I do understand it will make a difference what the driver tube V1 is and how much open-loop gain there is.
I’m not suggesting to simply borrow the values from this circuit as they would not be applicable in other circuits.
Here is an example with NFB returned to the top of the cathode resistor
NFB can also be returned to the Grid of a driver tube….
I had considered that, however the bias voltage set with the 2x470R may not meet the desired bias.
Here is a partial redrawing of the linked schematic in the first post applied to 6GV8.
I did not determine the affect on the bias of the stage.
I had wondered if placing a large cap poly cap, between the transformer secondary and the NFB resistance acting as a source of cathode bias.
There are examples of NFB from secondary to to the output tube on SE amplifiers, but I have not seen this when applying the NFB to a preceding stage.
It worth giving it a try, though the filers may need to be recalculated to achieve the correct bias in the circuit it’s applied in.
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There is no real need for that "smart" negative feedback since with a simple filter an choosing a small input coupling cap , both high and low frequency can be attenuated as you wish and won't enter the amplifier in the first place ...
I have experimented a bit with attempting to put poles of filtering in the feedback path of a guitar amp. I even went so far as to put a complete tone stack where the presence control goes in a Marshall style push pull amp. Some of these created some new and unique tone / styles, but most if not all had instability or became full on oscillators at some positions of the controls. The tone stack in the Marshall circuit did have this neat "cranked guitar amp in a concrete tunnel" sound right before the point where it became a "police siren" (oscillator).
Simulation is worthless here as complete models of the OPT and speaker are not available, so you will need to experiment a lot. The more poles you put in the feedback loop, the more phase shift you will have. At some frequency somewhere, possibly outside the audio range the feedback may become positive and the circuit will oscillate. Often this oscillation will be below the audio range due to the other feedback path in your amp. You don't show the power supply, but if you go down this path you will need to decouple the two stages with at least a RC on the feed to the 6SQ7.
This circuit will be prone to changes in the speaker causing changes in its mood, as was the tone stack Marshall.
A simpler trick that I have employed in my HiFi and guitar amps is to return the feedback from the OPT secondary to the cathode of the output tube. There is less overall gain here, so it's easier to get a stable amp, and filtering in the path will only affect one stage. I leave the cathode bias resistor connected to ground, and simply return the negative end of the cathode bypass cap to the OPT secondary instead of ground. This does wonders on low buck OPT's to make them sound a bit "bigger." Filtering applied here can mildly shape the tone but rarely causes instability unless a high Gm output tube is used.
Simulation is worthless here as complete models of the OPT and speaker are not available, so you will need to experiment a lot. The more poles you put in the feedback loop, the more phase shift you will have. At some frequency somewhere, possibly outside the audio range the feedback may become positive and the circuit will oscillate. Often this oscillation will be below the audio range due to the other feedback path in your amp. You don't show the power supply, but if you go down this path you will need to decouple the two stages with at least a RC on the feed to the 6SQ7.
This circuit will be prone to changes in the speaker causing changes in its mood, as was the tone stack Marshall.
A simpler trick that I have employed in my HiFi and guitar amps is to return the feedback from the OPT secondary to the cathode of the output tube. There is less overall gain here, so it's easier to get a stable amp, and filtering in the path will only affect one stage. I leave the cathode bias resistor connected to ground, and simply return the negative end of the cathode bypass cap to the OPT secondary instead of ground. This does wonders on low buck OPT's to make them sound a bit "bigger." Filtering applied here can mildly shape the tone but rarely causes instability unless a high Gm output tube is used.
From the article the author is addressing a number of issues that input coupling cap size will not address.
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“An amplifier similar to that one can be found in majority of inexpensive radios -- from mid 30's to early 60's. Its performance is very poor by todays standards. High inductive output impedance provides no damping for the speaker, hum is not suppressed, bandwidth is narrow, frequency response is not flat, etc. The biggest problem is caused by an undersized (saving every cent) output transformer with insufficient magnetizing inductance (as low as 5H) and large leakage inductance (up to 300...500mH).
As a result, such amplifier efficiently reproduces from 150Hz to 7kHz. At low frequencies (50...100Hz), distortion reaches 25...35%. It is not surprising: the output tube V2 works into low and reactive load and easily generates 10...15% of plate current distortion. The transformer, due to its low inductance, suppresses the fundamental, but passes the harmonics, plus adds its own caused by magnetic non-linearity. Thus the circuit turns into a harmonics generator rather than amplifier.
Introduction of a simple resistive negative feedback (Fig. 2) does improve sound quality, but only at low listening levels.
Fig. 2. Resistive negative feedback
Resistive negative feedback lowers output impedance and attempts to broaden the bandwidth. For example, a typical 10dB feedback would stretch frequency response from 150Hz...7kHz to approximately 50Hz...20kHz -- beyond what the transformer can naturally reproduce. As a result, the amplifier struggles to reproduce low and high frequencies by driving/pushing the output tube hard. Overloading and clipping occurs, which sounds even worse than without feedback. In a sense, this phenomenon proves the point that negative feedback sounds worse than no feedback at all.
Frequency dependent negative feedback
A solution to the problem is making negative feedback frequency dependent so that its characteristics matches natural capabilities of the output transformer (and a speaker). In Fig. 3, by way of C5, negative feedback is designed to roll off the gain from 150Hz down. The amplifier no longer struggles to amplify frequencies it is not capable of doing well. Overdriving and overloading by low frequencies is reduced -- sound becomes much better.
Fig. 3. First order low frequency roll-off negative feedback
To deepen feedback even steeper at low frequencies, a second order shaping circuit can be used (Fig. 4).
Fig. 4. Second order low frequency roll-off negative feedback
Here two RC circuits are cascaded, creating together a 12dB/decade steep roll-off below 150Hz. At 50Hz the feedback reaches its full depth -- 100%. Sound becomes dry, but exceptionally crisp and transparent. The deep possible feedback down to 50Hz gives an additional benefit. In some radios the output transformer is located physically too close to the mains power transformer and picks up stray magnetic field of the later. It causes some hum, and the only way to reduce it (apart from relocating the transformer) -- is to provide maximum amount of feedback at hum frequency.
Two stages of RC circuits (R9C6 and R8C5) provide significant phase lag, which is good to compensate for phase advance created by the transformer, decoupling circuit R3C2 and self-biasing circuit R5C4. Do not select excessively large cathode capacitor C4. The best is to have R5C4 time constant of the same order as R9C6, so that its phase advance is hidden by the phase lag of the feedback circuitry. C2R3 should be set to just under 50Hz. It is better to select it empirically to avoid infra-low frequency peaking”
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The nomenclature, is to the original schematic linked to in the first post.
I clearly want to state i have no clam to this work, however have found it informative, and wish to understand how best to applying these principles.
The smart network is a pole-zero canceller. The zero of the network tries to cancel the dominant pole (often the OPT). The pole of the network is much lower in frequency so the dominate pole is moved down. This tends to make the NFB more stable. It often used to sort out LF instability when there are a few coupling stages as well as the OPT (often seen as speaker cone wobble). Its best either mathematically analyzed or simulated as a bode plot. For the OPT a small signal linear model is fine as that's where the oscillation builds up from. If the oscillation builds up the primary inductance drops and the dominate pole moves up usually improving stability and limiting the amplitude although that's a gross simplification. The main thing is to have the primary inductance measured at <50Hz.
Myself as well, thanks for sharing. Regarding the idea of an amplifier being a "harmonics generator", particularly for guitar, the concept is spot on. But it's the right harmonics and relative levels that makes for a more favorable sound. It would be interesting to see the harmonic spectra at, say, 500 Hz as their feedback design evolves. What spectra corresponds to "exceptionally crisp and transparent"?
Which corresponds to "the amplifier struggles to reproduce low and high frequencies" - and which corresponds to "no feedback at all"?
Of course, as an objective measure audio distortion spectra wont tell the whole tale, but they are certainly a glimpse into some of what's going on. If I were pursuing this, I'd certainly have my trusty USB sound card and REW setup to see the spectral effects of things like matching the time constants of the output tube cathode bias filter to that of just one of the two stages of feedback.
One part of Mastery in amplifier design would be something like "I want these harmonics in this ratio before clipping" and be able to get that out of most any pile 'o parts, knowing all the tricks, having the required depth of understanding of things like Magnetizing Inductance...Leakage Inductance. Put two OPTs in front of me, I couldnt say which has what, not like the Author can...all I know is Bigger Is Better.