This particular equipment is Klippel Nearfield Scanner:
https://www.klippel.de/products/rd-system/modules/nfs-near-field-scanner.html
but there are other software alternatives, including free ARTA and REW.
https://www.klippel.de/products/rd-system/modules/nfs-near-field-scanner.html
but there are other software alternatives, including free ARTA and REW.
More than that, typically 70k.
Importance of magnetic shielding in a SEP amplifier cannot be overstated. I remember, while working on amplifier, powered off, when soldering iron was close to open-frame OPT, I could clearly hear hum in the speaker! I always use power supply on a separate chassis.
Another overlooked problem is mechanical feedback between OPT and tubes. Transformer "sings", pretty loud, due to magnetostriction and electromagnetic-induced vibration. If tubes and transformer are rigidly attached to the same chassis, tubes will pick up transformer noise.
A side note on the strength of Magnavox paper cone
When measuring Bl, I loaded the cone with two lead donuts totaling 907 grams. The cone had no problem lifting this mass without sustaining any damage.
When measuring Bl, I loaded the cone with two lead donuts totaling 907 grams. The cone had no problem lifting this mass without sustaining any damage.
3. Magnavox Belvedere
I got into tubes in 2002 after bringing home a 1947 Magnavox Belvedere console that someone put on the curb. This seemingly inconsequential event set me on a journey, mostly unsettling, but sometimes rewarding, towards a good-sounding tube amplifier.
On completion of restoration, the Belvedere surprised me with its sound. That time I used Kenwood KR-420 (with upgraded output transistors and biased to Class A), and Sony bookshelf speakers, after trying and rejecting many other components. The Belvedere sounded in a way that Kenwood-Sony combo did not. The Belvedere had straight-in-your-face immediacy, presence, and impact. It was easy- going and enjoyable, like listening to a brass band in a park standing right next to it, when your kid pulls you by your hand, but you want to have more. By comparison, the Kenwood/Sony did everything correct, maybe more correct than the Belvedere, but it wasn't the same fun. Unfortunately, Belvedere's WAF was zero, so, after a couple of weeks of fun, back to the curb it went. It didnt stay long there. Some lucky soul picked it up the same day.
I got into tubes in 2002 after bringing home a 1947 Magnavox Belvedere console that someone put on the curb. This seemingly inconsequential event set me on a journey, mostly unsettling, but sometimes rewarding, towards a good-sounding tube amplifier.
On completion of restoration, the Belvedere surprised me with its sound. That time I used Kenwood KR-420 (with upgraded output transistors and biased to Class A), and Sony bookshelf speakers, after trying and rejecting many other components. The Belvedere sounded in a way that Kenwood-Sony combo did not. The Belvedere had straight-in-your-face immediacy, presence, and impact. It was easy- going and enjoyable, like listening to a brass band in a park standing right next to it, when your kid pulls you by your hand, but you want to have more. By comparison, the Kenwood/Sony did everything correct, maybe more correct than the Belvedere, but it wasn't the same fun. Unfortunately, Belvedere's WAF was zero, so, after a couple of weeks of fun, back to the curb it went. It didnt stay long there. Some lucky soul picked it up the same day.
So what was inside the Belvedere?
The 20W-rated amplifier was nothing special, 6J5 voltage stage, 6SN7 paraphrase inverter, and PP 6L6G output, all RC-coupled. However, there was no NFB of any kind, global or local. The output transformer was smallish; it was mounted on the frame of one of the speakers. There were two Magnavox "light FC" main speakers, parallel VCs, each with a concentrically placed PM cone tweeter. The crossover was just a single capacitor. 250 Ohm FCs, connected in series, were part of CRC power supply filter. FC power was relatively low, about 6 W per unit, so electromagnets were running well below magnetic saturation.
The 20W-rated amplifier was nothing special, 6J5 voltage stage, 6SN7 paraphrase inverter, and PP 6L6G output, all RC-coupled. However, there was no NFB of any kind, global or local. The output transformer was smallish; it was mounted on the frame of one of the speakers. There were two Magnavox "light FC" main speakers, parallel VCs, each with a concentrically placed PM cone tweeter. The crossover was just a single capacitor. 250 Ohm FCs, connected in series, were part of CRC power supply filter. FC power was relatively low, about 6 W per unit, so electromagnets were running well below magnetic saturation.
I let the Belvedere go with light heart because now I had faith in tubes and thought that I could easily find something as good or even better. I went through a series of well-reputed tube amplifiers, each of which was brought to perfectly working condition.
-Fisher 500 mono with Mullard EL-37 output tubes and Telefunken other tubes. I had two of them;
-Scott 299c stereo with all Telefunken tubes including 7189 outputs;
-Scott LK-48 stereo with 7591 tubes;
-Magnavox stereo console amp with 6V6 tubes;
-Bogen DB-10 mono with 6AQ5 tubes;
-Allen 50 W instrument amp with parallel 6L6 tubes (used in church organs and chimes), have a pair of them;
-HK A-300 with 6V6 tubes;
-HK TA-230 with 6V6 tubes (amplifier circuit very similar to A-300).
All were disappointment except HK amplifiers. Fisher and Scott at first impressed me, especially Scotts with their very profound bass, but they were not near like the Belvedere, and in the long run boring. All these amplifiers used global NFB, so I guess what I've heard was a generic sound of an NFB pentode amplifier. The exceptional HK amps also had gNFB, but inside the NFB loop was nestled a feed-forward (or positive feedback) circuit. These amplifiers have wonderfully live sound similar to that of no-NFB triodes.
To conclude on my experience, pentode amplifiers may have no NFB, and the sky doesn't crush on earth. Moreover, gNFB seems to kill the otherwise lively natural sound of pentodes.
-Fisher 500 mono with Mullard EL-37 output tubes and Telefunken other tubes. I had two of them;
-Scott 299c stereo with all Telefunken tubes including 7189 outputs;
-Scott LK-48 stereo with 7591 tubes;
-Magnavox stereo console amp with 6V6 tubes;
-Bogen DB-10 mono with 6AQ5 tubes;
-Allen 50 W instrument amp with parallel 6L6 tubes (used in church organs and chimes), have a pair of them;
-HK A-300 with 6V6 tubes;
-HK TA-230 with 6V6 tubes (amplifier circuit very similar to A-300).
All were disappointment except HK amplifiers. Fisher and Scott at first impressed me, especially Scotts with their very profound bass, but they were not near like the Belvedere, and in the long run boring. All these amplifiers used global NFB, so I guess what I've heard was a generic sound of an NFB pentode amplifier. The exceptional HK amps also had gNFB, but inside the NFB loop was nestled a feed-forward (or positive feedback) circuit. These amplifiers have wonderfully live sound similar to that of no-NFB triodes.
To conclude on my experience, pentode amplifiers may have no NFB, and the sky doesn't crush on earth. Moreover, gNFB seems to kill the otherwise lively natural sound of pentodes.
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Ok, FC = field coil? I suppose this works with permanent magnets, too, e.g. active impededance control (AIC) by 18Sound.
Wound on top of FC - do you have a drawing? I suppose this means the FC gets an additional layer that does not see DC but rather is connected in series with the VC? This would be similar to the AIC winding that sits on the root of the pole piece.
Yes, pole magnetic saturation (actually, near-saturation) can be achieved with a permanent magnet. It is just a matter of using a relatively large magnet with a relatively small pole piece size. Lowther drivers are made this way.
And yes, countermodulation coil in series with VC doesn't have DC current through it. There is no reason why this approach cannot be used in a PM motor.
And yes, countermodulation coil in series with VC doesn't have DC current through it. There is no reason why this approach cannot be used in a PM motor.
That is not true. There are no physical laws preveting a small driver to beat a larger driver on bass distortion if the small driver pumps enough air with high linearity - it is just a harder engineering task. The radiator size relative to the wavelength matters for the directivity but not for the linearity.
we consider harmonic distortion in the bass as less probematic than wide band IMD. Not saying that bass distortion is not important.
It is true that increase of the B field strength in the gap helps to dilute the Force factor Modulation (FFM) but all drivers are running with high B field as limited by the steel and geometry - there is something to be gained but not a huge factor. FC and shorting rings do not work at low frequencies and wrongly design shorting rings can exacerbate FFM. The PTT motor elminates FFM down to DC. That can to some extend be achieved with a counter modulaiton coil if the excess energy loss can be accepted. Moreover, I am not sure how effective counter modulation coils are - there has been some AES papers about the concept but could be interesting to benchmark. Elinination of FFM is equal to having completely position independent inductance and I doubt that a counter modulation coil can achieve this.
Bruno's point is that small amounts of feedback increases distortion and adds higher harmonics. However, after some points of adding more feedback the distortion goes down proportionally ever after. So the point is to have a lot of feedback and that is an engineering challenge. The bad reputation of feedback is simply because not enough feedback has been applied.
Hi
I remember one Pentode mode SE design back in 1992 from Sound Practics
Fortunately someone scanned the issue 🙂
https://www.fidelity-online.de/wp-content/uploads/2016/02/SOUND-Practices-2.pdf
note the partial local feedback only...
.
I remember one Pentode mode SE design back in 1992 from Sound Practics
Fortunately someone scanned the issue 🙂
https://www.fidelity-online.de/wp-content/uploads/2016/02/SOUND-Practices-2.pdf
note the partial local feedback only...
.
No matter how well a compression-style bass driver is designed, it has a non-linearity of suspension: spring force is different at different deflection points of its long stroke. A bass driver with large radiating area (1 m2) will efficiently radiate at almost zero excursion, making suspension non-linearity much less important.
A radiator of a small size comparative to wavelength has one serious disadvantage: inefficiency. Low efficiency must be compensated by increasing driving power, so driver's design is encumbered by the necessity to handle that power, e.g. mechanical strength, heat dissipation, VC modulation of gap' s magnetic field.
I agree that countermodulation coil may be not very effective. Its effectiveness assumes magnetic coupling with VC, which may not be good under different circumstances. Actually, I am not using countermodulation coils in my Magnavox drivers because I run FCs close to magnetic saturation, at which point the coupling between CMC and VC should be almost completely lost. But position-independent (and lowest) Le is achieved because close to magnetic saturation of the pole piece its permeability gets close to unity and VC behaves like an air core inductor.
Mr. Putzeys may be right here, but I believe that if NFB can be avoided, it should be avoided. I believe the effort should be spent on reducing open loop distortion rather than overcoming complications of applying deep global NFB.
Jan, I don't want to repeat the argument that we had a few years ago.
But how about a circuit with low open loop distortion? Does it also need large amount of feedback, or smaller amount would suffice? I remember Motorola application note with a power amp schematic using only 10-12 dB NFB - after they introduced their new low distortion power BJTs.
Well, how should I apply Mr. Putzeys' wisdom in designing my tube amplifiers? Because of output transformer limitations, it is impossible to apply 60 dB feedback, as prescribed.
And not the best application of feedback. Driver's unbypassed cathode resistor increases Rp and thus increases distortion. But definitely this is an example of a no-NFB pentode amplifier.
IIRC, that thing was almost a copy of a Leach amp and had more than 20 dB of feedback. That was considered a LOW feedback amp by SS standards, but for tubes 20 dB gets into high feedback territory. 30 or more with tubes is almost unheard of, but 60 or more is possible from transistors.
From the diagram posted in 270, 20dB of feedback gets useful in terms of keeping the high order harmonics in check, and the more the better. But the single digits can do more harm than good if you’re starting out in the 10% range before feedback.
It’s why SS amps can be stable with 60 dB of feedback - no transformer. The usual topologies for tube amps don’t have 60 dB of gain to throw away.
