That is right, but what I meant was not gain in power, but 3 dB gain in efficiency due to acoustic coupling of drivers.
In parallel, the 6 dB gain in sensitivity is offset by a 6 dB reduction in amplifier gain, and the 3dB increase in efficiency is offset by 3 dB reduction in power when the current clips - so youre right back where you started. And the amp runs hotter.
The gain, gm times RL, and therefore sensitivity increases when you put them in series. Same reason it gets louder at fundamental bass resonance. Still stuck with the power reduction, which puts the max SPL output back where you started. But does draw less power to make the same SPL so there is a realizable gain in that case.
The gain, gm times RL, and therefore sensitivity increases when you put them in series. Same reason it gets louder at fundamental bass resonance. Still stuck with the power reduction, which puts the max SPL output back where you started. But does draw less power to make the same SPL so there is a realizable gain in that case.
Your "air motion index" has no sense at all - voice coil in the magnet field, plus spider compliance do not know how many grams is the membrane weight and how many grams is the mass of the air, they"see" only the sum of those two - i.e. Mms!
Also, your "best driver" - plasma tweeters have extremely high measured distortion:
http://www.roger-russell.com/ionovac/ionovac.htm
Measured IM distortion is bigger than 2% !!!
Test of plasma tweeter Acapela Ion TW1:
https://mapoulin.wixsite.com/audiobymartin/plasma-tweeter-part-2
Spoiler from the test: "Distortion rises to totally unacceptable levels ..they literally torture you. I mean they sucks big time, sorry, its really what I think.
Bottom line is that for me, they're just toys to play with.
Bottom line:
A quality Bullet type super tweeter, lighter, more efficient and much cheaper will outperform the plasma tweeter any day."
Besides, Mms mass does not cause nonlinearity, as @lrisbo explained before:
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the advantage of low Mms is mainly higher sensitivity. The following reduction in needed motor force for a given SPL can of course potentially reduce the distortion of the motor. On the flip side, lower Mms means less bass extension in a passive system for a given box volume. I am personally also fascinated by these old drivers. Just 11g for a 12” is quite something. We must just not forget the laws of physics.
So, you are promoting "Unobtainium" driver (Magnavox FC) to DIY folks as the ultimate midrange driver!? How is this relevant to anything?
About the Magnavox FC measured parameters - you are contradicting yourself!
Please explain how Magnavox FC parameters are better than this modern, of-the-shelf driver:
Eighteen Sound 6ND410:
Sensitivity = 102 dB 1W/1m
Mms = 8.2 g
Qms = 2.2
All the parameters which you are promoting as crucial to the good sound, are better than Magnavox FC!
Also, lightweight and thin membrane of the 12" Magnavox FC is prone to unacceptable resonances - just compare it to the smooth frequency response of Eighteen Sound 6ND410.
Nonsense!
Bonus - measurements of plasma tweeter, by Paul W. Klipsch:
http://www.readresearch.co.uk/loudspeaker_papers/klipsch_modulation_distortion_article_1.pdf
(Spoiler) Measured distortion of plasma tweeter: 30% !!!
6ND410 is indeed outstanding driver. One thing good about it is that it was designed with midrange performance as a goal. Not like Scan Speak or Purifi where design goal was bass performance, and, as a result, midrange performance was poor.
Unfortunately, 6ND410 cannot be used as a substitute for the Magnavox in this project . It's useful frequency range is 500-4K, so its low end is way higher than what is needed to match the Neo-8. In the proposed system, the mid should have 120 and 800 crossover points. The 6ND410 will require something in addition to cover the 120-500 gap.
6ND410 cannot be compared to the Magnavox. Yes, it has lower Mms (8 g vs. 11.4 g), but it's radiating surface is only 1/4. It has Qms of 2.2, but this is at the resonance frequency of 2x (120 Hz). The Magnavox has infinitessimally small Qm at 120 Hz. 6ND410 has relatively high Le of 0.4 mH, and resonance peak extending into lower midrange, making it not very suitable as a load for a no-NFB pentode amplifier.
6ND410 is a pro driver designed to handle large amounts of power. High power requirement demanded sturdy construction, resulting in relatively high moving mass. In the project of this thread, high power handling is not required, so a better speaker with a lighter cone can be used.
Magnavox drivers are not something that a DIYer cannot easily procure. Very large numbers of them have been made. Any time there are at least 4-5 listings on eBay, and they are quite affordable. Of course, it would not be a solution for a manufacturer, but this particular DIY forum.would hardly exist if we reiied solely on currently manufactured merchandise.
Unfortunately, 6ND410 cannot be used as a substitute for the Magnavox in this project . It's useful frequency range is 500-4K, so its low end is way higher than what is needed to match the Neo-8. In the proposed system, the mid should have 120 and 800 crossover points. The 6ND410 will require something in addition to cover the 120-500 gap.
6ND410 cannot be compared to the Magnavox. Yes, it has lower Mms (8 g vs. 11.4 g), but it's radiating surface is only 1/4. It has Qms of 2.2, but this is at the resonance frequency of 2x (120 Hz). The Magnavox has infinitessimally small Qm at 120 Hz. 6ND410 has relatively high Le of 0.4 mH, and resonance peak extending into lower midrange, making it not very suitable as a load for a no-NFB pentode amplifier.
6ND410 is a pro driver designed to handle large amounts of power. High power requirement demanded sturdy construction, resulting in relatively high moving mass. In the project of this thread, high power handling is not required, so a better speaker with a lighter cone can be used.
Magnavox drivers are not something that a DIYer cannot easily procure. Very large numbers of them have been made. Any time there are at least 4-5 listings on eBay, and they are quite affordable. Of course, it would not be a solution for a manufacturer, but this particular DIY forum.would hardly exist if we reiied solely on currently manufactured merchandise.
Assuming that Mms is unimportant for speaker sound quality will lead one to the ridiculous implication that a good-sounding speaker may have Mms of 100 g or even 1 kg. This is not worth any serious debate. Any speaker designer worth his/her salary does everything possible (within design constraints) to reduce Mms. In the example of the 6ND410, why would they use aluminum-wound VC?
Plasma speaker was a dream that did not survive implementation realities. It only performed well at highest audio frequencies and very low levels. But there it was indeed unsurpassed.
Ionovac etc. tried to make a practical plasma tweeter - and failed. The problem was not the one of the principle, but of how to extend the modulation depths of the plasma cloud while keeping it linear. Then horn-loading to increase very, very low sensitivity at tremendous power input.
It was a mistake trying to make it commercial.
My purpose of invoking plasma tweeter was to illustrate the benefit of low Mms, not to promote the commercial implementations of this design.
Ionovac etc. tried to make a practical plasma tweeter - and failed. The problem was not the one of the principle, but of how to extend the modulation depths of the plasma cloud while keeping it linear. Then horn-loading to increase very, very low sensitivity at tremendous power input.
It was a mistake trying to make it commercial.
My purpose of invoking plasma tweeter was to illustrate the benefit of low Mms, not to promote the commercial implementations of this design.
Why do you say the midrange of Purifi drivers is compromised? what is you reasoning? In terms of distortion? So far no valid arguments have been made. It might of course not be the best match for a low power tube amp. Is this why it is 'poor'?
Poor midrange?! Nonsense! ScanSpeak (and Purifi) drivers are used in the world's best high-end loudspeakers.
You are wrong, again. It's useful frequency range is 120 Hz - 4 kHz.
"It's radiating surface is only 1/4". So what?!
Also, 6ND410 has double amount of Xmax.
Bottom line: 6ND410 has lower distortion than Magnavox at the same SPL - and that is what is the most important.
Did I mention 6ND410 has lower Mms than Magnavox? Oh, you noticed it, also... So, because Mms is the most important parameter (according to you), why are you choosing driver with higher Mms?
Nonsense!
Maybe you don't know, but Qms is a measure of the mechanical damping of the driver. Mechanical damping is a result of the mechanical properties of the spider and the surround. As you know (or don't know) spider and surround are attached to the cone and chassis all the time, even at frequencies away from the resonance frequency. 😉
Qms and Qes are measured at resonance frequency Fs because it is convenient and accurate, not because they are "infinitely" small at 2 x Fs.
On the contrary - resonance peak is very useful at lower midrange, especially for a no-feedback pentode amplifier. It will equalize the frequency respnse all way down to 120 Hz.
???
You are contradicting yourself:
6ND410 has lower mass than Magnavox 12" driver!!! 8.2 grams compared to 11.4 grams!
At all DIY forums the most numerous projects are for new, off-the-shelf drivers. Try searching this forum, it will open your eyes.
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It is not worth any serious debate if you isolate a single parameter (Mms) away from other important parameters. Drivers with Mms of 100 g and higher are wonderful sounding (especially at low frequencies), because they have much stronger motor. On the other hand, if the Mms=zero is the panacea, why all plasma tweeters have unbearably high distortion?
In 6ND410 they used aluminium VC because the reduced mass (over copper VC) was gone to the heavier, thicker and stronger cone, to eliminate cone break-up resonances. Magnavox FC has thin cone full of nasty resonances, in spite of heavier cone assembly.
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All plasma tweeters, including commercial and DIY, with or without horn, exhibit extremely high distortion - in spite of zero Mms!
In your one-minded reasoning, that is the final proof that low Mms is the worst possible design for the loudspeaker driver.
Cone breakup cannot be prevented by making the cone stiffer. Stiffness only shifts breakup to higher frequencies,and actually makes breakup nastier, with prominent dips and peaks at breakup frequency. Light paper cones have good self-damping. In the Magnavox driver, I was hoping to pinpoint cone breakup by identifying the first peak and/or dip between 500-2,000 Hz, but didn't see anything prominent.
6" Scan Speak sounds mediocre in midrange - in my experience. There is objective reason for that: bad waterfall, primarily caused by relatively high moving mass. All mid-bass drivers are same in this regard.
Just a quick glance at frequency response of 6DN410 is enough to see that it rises steeply from 100 Hz and only levels at 1,000 Hz. It is similar to frequency response of the Neo-8. From the point of working range, 6DN410 is like Neo-8 sans high frequency extension. It makes no sense to use it in the project.
The speaker debate here is becoming circular. I believe that I have stated my position well enough, and will no longer participate in pointless bickering.
6" Scan Speak sounds mediocre in midrange - in my experience. There is objective reason for that: bad waterfall, primarily caused by relatively high moving mass. All mid-bass drivers are same in this regard.
Just a quick glance at frequency response of 6DN410 is enough to see that it rises steeply from 100 Hz and only levels at 1,000 Hz. It is similar to frequency response of the Neo-8. From the point of working range, 6DN410 is like Neo-8 sans high frequency extension. It makes no sense to use it in the project.
The speaker debate here is becoming circular. I believe that I have stated my position well enough, and will no longer participate in pointless bickering.
I didn't say "stiffer" anywhere. I said: "thicker and stronger". Good damping is characteristic of "thicker" cones and is the opposite of "stiffer".
Nonsense! Too light and thin paper cones have nasty resonances.
May we see those measurements?
Your experience is totally opposite of the all audiophiles in the world.
May we see the waterfall diagram of the Magnavox FC driver? Compared to ScanSpeak?
This statement is enough to conclude that you don't know the loudspeaker basics. All midbass and woofer drivers are measured in large enclosure or in large flat IEC baffle, so drivers with low Qes (like 6DN410) exhibit high-pass filter characteristic in the high midrange band, but if you mount them in a appropriate enclosure volume, than the low frequency range will go up in SPL level. Also, current drive will equalize (boost) low frequency band of the 6DN410.
Yes, because you are avoiding to answer all questions (mine and from @lrisbo) which shatter in pieces your wrong believing.
I believe all forum members here have seen how illogical and contradicting is your reasoning (and position).
Pointless bickering?! You are wrong on all fundamental issues about loudspeakers!
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Examples of no-NFB pentode amplifiers
I have close experience with 3 different no-NFB pentode amplifiers.
1. 1934 kit amplifier
1934 was exciting year for tube afficionafos. The first power pentode, Type 47, and improved indirectly-heated low mu triode Type 56 became available. I don't know who offered a DIY amplifier kit with these tubes, but what I have is a kit amplifier: point-to-point wiring job is undoubtedly amateur.
I got the amp with what I believe is original matched quad of Sylvania 47s. The original Thordarson interstage transformer was shot, so I replaced it with historically correct Ferranti AF-3c. Here is the schematic:
The circuit is classic transformer- coupled 2-stage amplifier. Input transformer has primary taps for low and high level inputs. Volume control pot (wirewound!) Is connected across the secondary. Original interstage transformer was 1:2 step-up, but Ferranti is 1:3.5. The amplifier is on separate chassis from power supply. The power supply was missing, so I had to improvise it. One strange thing is low value of 56 cathode resistor bypass capacitor, which should emphasize high frequencies. But then there are 500 pF capacitors from 47 grids to ground that should cause opposite effect. I don't understand the rationale of this, but the amp has good balance of lows and highs.
Frequency response is 60-8,000 +/- 3 dB. Both lows and highs are largely limited by the interstage transformer.
The amplifier has nice warm mellow vintage sound. With a Magnavox speaker, the bass is tight and not boomy. I really enjoy the sound of this amplifier. It perfectly matches 78 rpm playback. I am a fan of 1920s - 30s popular music, have a big collection of of shellacs, and listen to them often.
I have close experience with 3 different no-NFB pentode amplifiers.
1. 1934 kit amplifier
1934 was exciting year for tube afficionafos. The first power pentode, Type 47, and improved indirectly-heated low mu triode Type 56 became available. I don't know who offered a DIY amplifier kit with these tubes, but what I have is a kit amplifier: point-to-point wiring job is undoubtedly amateur.
I got the amp with what I believe is original matched quad of Sylvania 47s. The original Thordarson interstage transformer was shot, so I replaced it with historically correct Ferranti AF-3c. Here is the schematic:
The circuit is classic transformer- coupled 2-stage amplifier. Input transformer has primary taps for low and high level inputs. Volume control pot (wirewound!) Is connected across the secondary. Original interstage transformer was 1:2 step-up, but Ferranti is 1:3.5. The amplifier is on separate chassis from power supply. The power supply was missing, so I had to improvise it. One strange thing is low value of 56 cathode resistor bypass capacitor, which should emphasize high frequencies. But then there are 500 pF capacitors from 47 grids to ground that should cause opposite effect. I don't understand the rationale of this, but the amp has good balance of lows and highs.
Frequency response is 60-8,000 +/- 3 dB. Both lows and highs are largely limited by the interstage transformer.
The amplifier has nice warm mellow vintage sound. With a Magnavox speaker, the bass is tight and not boomy. I really enjoy the sound of this amplifier. It perfectly matches 78 rpm playback. I am a fan of 1920s - 30s popular music, have a big collection of of shellacs, and listen to them often.
When I first got this PPP 47 amplifier, I thought of upgrading it to modern standards. The Ferranti AF-3 interstage (introduced in 1924) has unique and revolutionary design. The windings are sectioned, 6 primary sections and 4 secondary. The secondary is placed between the two layers of primary. The three layers are wound on cages that fit into each other, thus the dielectric between sections and layers is air. With 1:3.5 step-up ratio (very unfavorable for transformer's high frequency response), the high frequency roll-off begins at 8 K. Because of modular design, the transformer can be easily taken apart. Just changing the ratio to 1:1 would extend frequency response well beyond 20 K. In 20s - 30s, step-up transformers were needed to supplement the low voltage gain of tubes like 01A. Now we don't need such "amplifying transformers".
However, on hearing how good this amplifier sounds, I changed my mind and left the amp as it was.
However, on hearing how good this amplifier sounds, I changed my mind and left the amp as it was.
Gary Pimm's 47 amplifier
It may be relevant to recall Gary Pimm' s 47 amplifier here. Mr. Pimm's amp is a two-stage DC-coupled circuit using Schade feedback from 47 plates to grids. This is what he wrote about sound quality of his amplifier:
"Sound quality: very clean and dynamic. A more romantic sound than my 300B amp... The 47s really sound a whole lot like 45s".
Mr. Pimm tried substituting 47s with 6BQ5s in his circuit, and commented that 6BQ5s sounded good, but didn't have the refinement of directly-heated tubes.
What Mr. Pimm said fully applies to the no-NFB 47 amplifier. Thus, the nice sound can be attributed to output tubes rather than to Schade feedback. Mr. Pimm described a number of modifications, each of which furter improved the sound. But the foundation for the improvements was directly heated pentodes as output tubes.
With this in mind, the output tubes for the no-NFB pentode amplifier in this project will have to be directly heated.
It may be relevant to recall Gary Pimm' s 47 amplifier here. Mr. Pimm's amp is a two-stage DC-coupled circuit using Schade feedback from 47 plates to grids. This is what he wrote about sound quality of his amplifier:
"Sound quality: very clean and dynamic. A more romantic sound than my 300B amp... The 47s really sound a whole lot like 45s".
Mr. Pimm tried substituting 47s with 6BQ5s in his circuit, and commented that 6BQ5s sounded good, but didn't have the refinement of directly-heated tubes.
What Mr. Pimm said fully applies to the no-NFB 47 amplifier. Thus, the nice sound can be attributed to output tubes rather than to Schade feedback. Mr. Pimm described a number of modifications, each of which furter improved the sound. But the foundation for the improvements was directly heated pentodes as output tubes.
With this in mind, the output tubes for the no-NFB pentode amplifier in this project will have to be directly heated.
2. 1937 car radio
Philco C-1550 radio was an option in Chrysler cars beginning from 1937. The radio would add $100 to the $670 price of 1937 Plymouth. I restored and tested this radio as part of my car project.
The set, including radio, vibrator HV power supply, and speaker, is packed in 10.5"×8"×7.5" box.
Things are packed pretty tight inside.
The speaker is 7" FC:
Field coil is running on 6 VDC from car onboard power, dissipating about 10 W. The driver has lightweight paper cone similar to that of Magnavox. I wish I could have more drivers like this, but it means buying whole set and paying for 25 lb shipping. These radios are not rare and prices are reasonable.
The audio part is no-NFB SE amplifier using type 42 output pentode.
Philco C-1550 radio was an option in Chrysler cars beginning from 1937. The radio would add $100 to the $670 price of 1937 Plymouth. I restored and tested this radio as part of my car project.
The set, including radio, vibrator HV power supply, and speaker, is packed in 10.5"×8"×7.5" box.
Things are packed pretty tight inside.
The speaker is 7" FC:
Field coil is running on 6 VDC from car onboard power, dissipating about 10 W. The driver has lightweight paper cone similar to that of Magnavox. I wish I could have more drivers like this, but it means buying whole set and paying for 25 lb shipping. These radios are not rare and prices are reasonable.
The audio part is no-NFB SE amplifier using type 42 output pentode.
Here is the schematic.
The audio amplifier is 2-stage. Voltage amplifier is the high mu triode Type 75. This tube has amplification factor of 100 and is more linear than 12AX7. 75 cathode resistor is unbypassed. 75 is RC-coupled to the Type 42 output stage. 42 is 6-pin equivalent of the octal 6F6. Electrically, it is also equivalent to the Type 47, with exceptions that the latter has filament cathode and somewhat lower voltage and power ratings. 42 is fixed-biased. Capacitors 44, 45, and 49 roll-off high frequencies, which were considered detrimental for limited-bandwidth AM broadcast and were anyway beyond speaker's HF response.
According to 42 data sheet, power output at 250 V is 3.7 W at 7% THD (for comparison, 2A3 is 3.5 W at 5% THD). 0.1 Vrms across volume pot drives the amplifier to full power.
Evaluating the amplifier, I used external 250 V power supply with 5x current capability. Speaker's FC was run at 18 W. The sound was fantastic, similar to 2A3 SET driving a high-efficiency speaker. Although I didn't have a 2A3 SET at the time for side-by-side comparison, I know how 2A3 sounds. Even if 42 sounded not as good as 2A3, the difference won't be night-and-day. I believe that Philco speaker contributed greatly to overall sound quality. The sound was loud at a fraction of a watt, louder than my comfortable listening level. The low frequencies weren't something special, but quite adequate for vocals, piano, and chamber music such as jazz, string quartet, or small chamber orchestra. The highs were missing, as one would expect. This can be easily corrected by removing response-shaping capacitors and adding a super-tweeter.
The audio amplifier is 2-stage. Voltage amplifier is the high mu triode Type 75. This tube has amplification factor of 100 and is more linear than 12AX7. 75 cathode resistor is unbypassed. 75 is RC-coupled to the Type 42 output stage. 42 is 6-pin equivalent of the octal 6F6. Electrically, it is also equivalent to the Type 47, with exceptions that the latter has filament cathode and somewhat lower voltage and power ratings. 42 is fixed-biased. Capacitors 44, 45, and 49 roll-off high frequencies, which were considered detrimental for limited-bandwidth AM broadcast and were anyway beyond speaker's HF response.
According to 42 data sheet, power output at 250 V is 3.7 W at 7% THD (for comparison, 2A3 is 3.5 W at 5% THD). 0.1 Vrms across volume pot drives the amplifier to full power.
Evaluating the amplifier, I used external 250 V power supply with 5x current capability. Speaker's FC was run at 18 W. The sound was fantastic, similar to 2A3 SET driving a high-efficiency speaker. Although I didn't have a 2A3 SET at the time for side-by-side comparison, I know how 2A3 sounds. Even if 42 sounded not as good as 2A3, the difference won't be night-and-day. I believe that Philco speaker contributed greatly to overall sound quality. The sound was loud at a fraction of a watt, louder than my comfortable listening level. The low frequencies weren't something special, but quite adequate for vocals, piano, and chamber music such as jazz, string quartet, or small chamber orchestra. The highs were missing, as one would expect. This can be easily corrected by removing response-shaping capacitors and adding a super-tweeter.