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Old 18th August 2012, 04:17 AM   #7791
system7 is offline system7  United Kingdom
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I'm very interested in your notion that the Ariel is, er, rubbish with a transistor amp, Lynn. Especially as I've decided that the Vifa P13WH is about the best polycone in town and want to buy some! I'm also planning on adapting your crossover for a small "BBC LS3/5A" type 2-way.

Surely that is just tonal differences with a valve (and significantly high output impedance) optimised speaker? But we are currently interested in the very symmetrical Susan Zeus power Mosfet amplifier, so I'll go with that for now...

Click the image to open in full size.

Now never mind discussion of harmonic distortion artefacts and Class A or A/B, what leaps out at me is the evidently superb common-mode rejection from those expensive centre-tapped transformers. That along with whatever output impedance it has must define it. For those who are unfamiliar, Common-mode is noise picked up from the earth rail. Earth is in fact a vague concept, it is better to think of it as a reference voltage. You must then ask yourself what reference the output signal is relative to. Sometimes it is the power rail, sometimes it is the conventional earth.

Common-mode rejection is a huge topic, made worse by the need for volume controls and circuit input/output protection. For interest I include a Balanced Differential amplifier circuit which is happy as long as both arms are symmetrical reactively and driven by zero source impedance.

You also ask yourself whether an op-amp circuit driven by a long tailed pair is going to be better or worse than one of John Linsley-Hood's simple little one transistor Class A line amps. I know which I'd soonest have. JLH.
Attached Images
File Type: jpg Susan_Zeus_Power_Amplifier_Schematic.JPG (32.8 KB, 792 views)
File Type: jpg balancedopamp.JPG (29.3 KB, 782 views)
File Type: jpg Long_Tailed_Pair_Amplifier.JPG (21.9 KB, 785 views)
File Type: jpg J. L. Linsley Hood Amplifier.JPG (24.3 KB, 782 views)
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Old 18th August 2012, 10:10 AM   #7792
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Quote:
Originally Posted by system7 View Post
I'm very interested in your notion that the Ariel is, er, rubbish with a transistor amp, Lynn. Especially as I've decided that the Vifa P13WH is about the best polycone in town and want to buy some! I'm also planning on adapting your crossover for a small "BBC LS3/5A" type 2-way.

Surely that is just tonal differences with a valve (and significantly high output impedance) optimised speaker? But we are currently interested in the very symmetrical Susan Zeus power Mosfet amplifier, so I'll go with that for now...
Not really a matter of output Z; it's easy enough to add a 1~2 ohm series resistor to a conventional transistor amp - and all it sounds like is an even worse transistor amp, with degraded dynamics and more edge and grit than usual.

Change to nearly any valve amplifier, and the feeling is immediate relief, along with far more vivid tone colors and more interesting dynamics. Not a small difference at all.

The Ariels, for better or worse, reveal substantial differences - sometimes amazingly large - between amplifiers. I certainly didn't plan on this, but that's how it turned out. I've heard transistor amplifiers sound everything from murky and dull, with unintelligible vocals (Krell), to unbearably bright and zippy (some prosound amp with internal fans). Most just sound somewhat bright, with noticeably flattened tone colors, like MP3 at 128kbps.

Valve amplifiers restore musical tone colors to real-life levels, but many have lots of obvious added-on colorations of their own. SET amplifiers can sound tubby, lush, or edgy and hyper-detailed, depending on how the driver stage and power supply were done. PP amplifiers are typically more neutral, although higher-watt (6550, KT88) PP pentode amps can have a fatiguing, grainy quality, particularly if 6DJ8's are used as drivers.

Some of the SET amplifiers combine neutrality with vivid tone and dynamics, but they tend towards the exotics - 845's and larger tubes, which require heroic drivers and power supplies. It's easier to get there with moderate-power pentodes and direct-heated triodes. Small details of phase-splitters have a large effect on PP sonics. Not a fan of the "concertina" or split-load phase splitter at all.

One thing I can tell you; despite moderate efficiency, if you use the Vifa 5.5" driver in MTM pairs with a good crossover, you will hear very subtle differences between amplifiers that are simply not audible with most commercial speakers. What would be a slightly noticeable difference on say, a B&W high-end speaker, will be glaringly evident on the paired Vifa's. I surmise the very good objective performance of the Vifa - which has superb, near-electrostat time response - allows the listener to hear subtle differences more easily, while the time-decay crud (resulting from out-of-band breakup) of most midrange drivers masks amplifier differences.

Strictly a guess, but drivers with very good time-decay performance let your hear "deeper" into the amplifier, revealing subtle problems with balance under dynamic conditions, or nonlinear capacitances that affect forward-path time delays (in other words, stability margins that are program-modulated).

Although it's kind of a mixed blessing, the Ariels do make good analytic tools to evaluate small changes to amplifier topology, or audition different flavors of DACs. You will hear the difference between delta-sigma and ladder DACs, for example.

As for the Susan amplifier, I have no idea at all what it would sound like. I've never heard one. It is radically different than a complementary-symmetry Class AB feedback amplifier, so it's a safe guess it sounds different as well. In what way, though, I have no idea. When I first auditioned the Amity amplifier, I had no idea what it would sound like - I just knew it would sound different than a Williamson, Dynaco, Fisher, or Marantz, and it did.

Last edited by Lynn Olson; 18th August 2012 at 10:35 AM.
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Old 18th August 2012, 01:34 PM   #7793
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Quote:
Originally Posted by system7 View Post
I'm very interested in your notion that the Ariel is, er, rubbish with a transistor amp, Lynn. Especially as I've decided that the Vifa P13WH is about the best polycone in town and want to buy some! I'm also planning on adapting your crossover for a small "BBC LS3/5A" type 2-way.
The "susan" is going to be brutally honest about the qualities of the output devices.

As with anything driving a 90db plus efficient speaker the first watt is going to be the most important - where any heavily "A" oriented amplifier is going to "win". Beyond that, with out feedback you're in a trade off between microdynamics (my words for Lynn;s "can you hear the room") vs macro dynamic - where the chorus and the orchestra and the organ get serious. And I/M and its friends enter the room. Think about 6 minute into the adagio for spartacus or any massed chorus (eg Carl Orff's Carmina Burana )
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Old 19th August 2012, 01:44 AM   #7794
system7 is offline system7  United Kingdom
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Thanks for the clarification, Lynn. I think you're just saying the Ariel WORKS BETTER with valve amplification.

On a rare foray into the Solid State section here, we had a look at Mark Houston's DIY Class-A 2SK1058 MOSFET Amplifier.

Click the image to open in full size.

Quite a valve-like 8 ohm output impedance and Class A current drive via Mosfet with a mere 5W available. Nelson Pass would look for something more special in output devices, but it's the efficient loudspeaker and tonal response that needs some thought with this sort of amplifier. As thoglette remarks, some speakers like the low inductance Fostex full range can lose it a bit with complex music.

Enter the WLM La Scala again. I find this speaker fascinating. 2.2mH inductance Visaton W200S 8" bass and of all things, a Visaton TW-70 CONE tweeter. Almost a relic of the valve driven sixties.

Click the image to open in full size.

The only filter I could get working properly was a simple one, quite Robin Marshall in fact. I used the old trick of adding a 6 ohm resistor to estimate the difference when driven by a valve amp. The bass loading that was best and least affected with either type of amp (Solid State or Valve respectively in the diagrams) was stuffed port aka aperiodic, here with 30 litres box. I suppose it is possible to use Zobels and small bass coils to get the impedance flatter so it is tonally similar with all amps, but I hope you can see how it works anyway.
Attached Images
File Type: jpg WLM_LaScala_Clone_Circuit.JPG (22.1 KB, 98 views)
File Type: jpg WLM_LaScala_Clone_Transistor_Drive.JPG (60.0 KB, 95 views)
File Type: jpg WLM_LaScala_Clone_Valve_Drive.JPG (61.0 KB, 77 views)
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Old 19th August 2012, 10:34 PM   #7795
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For anyone interested in a PP source follower power stage, including Lynn's suggested hybrid, here are some interesting graphs.

First the Current through the Mosfets in my hot-rodded version of Susan's amp. Shown are the push and pull Mosfet currents with the output transformer configured 4:1 delivering 5.5v peak into 4 ohms. This is close to 3.8 Watts which is much louder than needed for the Beyond Ariel speaker, I think.
The currents swing from around 1.1 to 1.75 amps.

Next is the capacitance curve for the Fairchild FQA34N20 Mosfet. To address the non-linearity of MOSFET capacitance, it is desireable to pick a part that has nearly non-changing capacitance in the operating region. Note the bottom right of the graph where I have annotated the 40v bias point for the amplifier. The controlling capacitance for this follower topology is CRSS (reverse transfer capacitance). CRSS is found in the lowest line in the graph and is specified as 55pf typical, which is where I operate it. Notice how this capacitance is effectively flat over the entire range, which range is +/- 11volts about the 40v operating point. The +/- 11 volts are need to provide the 5.5 volts at the output side of the OPT in this example, the condition for the current waveforms of the first graph.

This Fairchild part if effectively flat for a range that is double that, +/- 22v (11v peaks at output). Even +/- 30volts isn't bad (15v peaks output).

How loud are a pair of Beyond Ariel speakers at two meters with +/- 15volt peaks at the terminals?

The area to avoid on the second graph is below 10v. Note the Y-axis is linear.
Attached Images
File Type: jpg PP Mosfet currents OPT4to1 delivering 5.5v peak into 4R.jpg (63.9 KB, 83 views)
File Type: jpg FQA34N20 capacitance.jpg (146.6 KB, 58 views)

Last edited by charliemb; 19th August 2012 at 10:36 PM.
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Old 20th August 2012, 02:08 AM   #7796
soongsc is offline soongsc  Taiwan
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Interesting how this thread is turning in an amp thread.
Lot's of problems with amplifiers is that the amplifier and power supply are treated as separate components. Secondly, whenever we talk about the source end of either amp output or power supply output, we ignore the return aspects of the current flow. These ignorance lead to a trend where devices that have more separation between the high current loops and low current loops tend to sound smoother. Simple device have simple current loops, which make optimization much simpler.
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Old 20th August 2012, 02:59 AM   #7797
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The new loudspeaker is around 99~100 dB/metre/2.83Vrms, with a load between 12~20 ohms over the working range.

The bass driver is an Altec/GPA 416B (Alnico magnet, 16 ohm version), the HF horn is an Azurahorn AH425, the large-format compression driver is either a Radian 745Neo (Neodymium magnet, 16 ohms) or Altec/GPA 288 (Alnico magnet, 16 ohms), and the passive crossovers are 3rd-order lowpass at 700 Hz (transitional between Butterworth and Bessel) for the 15-bass driver, and a 4th-order highpass at 700 Hz (transitional between Bessel and LR4) for the HF driver.

If the Radian 745Neo is used, it needs to be attenuated by 14 dB, which is accomplished with a custom autoformer with a 16-ohm shunt resistor between the autoformer and the lowpass filter. The highpass filter is designed for a 16-ohm resistive load.

The woofer uses a standard Zobel inductance compensator between the lowpass filter and the woofer, and the lowpass network is designed for 12-ohm resistive load.

No inband equalization is required for either the woofer or HF horn system. The system is time-aligned with all drivers in-phase; the lip (outer edge) of the AH425, by lucky coincidence, is about 1/2" forward of the front face of the bass cabinet, with about 2" of vertical spacing between the top of the cabinet and lower edge of the AH425. The bass cabinet is about 4.5 cubic feet and uses a resistive vent.

Last edited by Lynn Olson; 20th August 2012 at 03:06 AM.
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Old 20th August 2012, 10:55 AM   #7798
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Sounds like a sexy design and perhaps a worthy alternative for those who have aspirations to collect the bits and pieces to build a M19.. GPA has now just released their own crossover for the M19, making it a one stop shop for all the bits except the horn etc..

I read somewhere that Bill at GPA says there is really no audible difference between the Alnico 416 and the Ferrite version, so maybe that could help shave the cost down a bit for some?

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Old 20th August 2012, 11:42 AM   #7799
DaveCan is offline DaveCan  Canada
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Here's the store page, wanted to add it to my last post but editing time expired..

http://www.ebay.com/sch/$billatgpa/m.html?_nkw=&_armrs=1&_from=&_ipg=&_trksid=p3686
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Old 20th August 2012, 01:29 PM   #7800
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Quote:
Originally Posted by Lynn Olson View Post
The new loudspeaker is around 99~100 dB/metre/2.83Vrms, with a load between 12~20 ohms over the working range.
Regarding the new speaker being nominal 16 ohms... If that's the case then 4:1 output mode on any Susan Zeus-type power output stage may not be optimal. Instead, 2:1 might be optimal.

Either way, 16 ohms reflected back on the primary side of the OPT will look like either 256 ohms or 64 ohms. Either are a piece of cake to drive and will be very low in distortion.
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