Atma-Sphere Cutting Lathe

Gregadd

WBF Founding Member
Apr 20, 2010
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#21
Are these the subjects of the thread?
No The subject of the thread is that an audio manufacturer has decided to make vinyl with his own cutting lathe. All the rest has been discussed ad nauseum in other threads. I cautioned Tom about the same thing.
 

Gregadd

WBF Founding Member
Apr 20, 2010
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#22
I dont think i ever heard an atmashphere unit.
Cheers to Ralph to pursue his idea of fidelity it just aint mine in the area of source fidelity thats all I
My point then is not to gain some kind of gotcha argument points. It is however intended to suggest that perhaps you attributed some classic tube shortcomings that are solvable and indeed have been significantly addressed in Ralphs designs.
 
Last edited:

Gregadd

WBF Founding Member
Apr 20, 2010
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Metro DC
#23
500dm_ph.jpg Hafler DH 500
 

Gregadd

WBF Founding Member
Apr 20, 2010
7,156
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Metro DC
#24
It seesm that criticism iof Mosfets was not in iine with your own choice of amplifiers and a statement that it is as good as it needs to be.
 

Atmasphere

[Industry Expert]
May 4, 2010
949
157
455
St. Paul, MN
www.atma-sphere.com
#25
Are you about to say that a tube amp, an OTL tube amp has better fidelity and grip on a mechanical device than the best solid state gear? Thats all I am saying. Cheers to Ralph to pursue his idea of fidelity it just aint mine in the area of source fidelity thats all I meant.

In regards to propaganda, surely mine is not on par with the audio hi ends? If it is then I should be quite proud, aahem, I guess....
OK, I can easily say that an OTL has a 'better grip' on the cutterhead than any transistor amplifier.

There is a reason we chose the Westerex system- its cutterhead was designed for tube electronics and has an easy impedance that is hard for transistors. That is to say, its nominally about 10 ohms. The lowest impedance is about 8, and shows an impedance curve similar to a loudspeaker, but with the impedance peak moved up slightly in frequency. IOW it occurs at the resonance of the device, which is in the midrange rather than the bass. Above the peak the impedance slowly rises.

This works out ideal for the M-60, which has wider full power bandwidth than the stock transistor amps (yes, Westerex made a solid state system called the 1700 series electronics). Due to the RIAA curve, its really the highs where you need to have power for proper bandwidth extension. The impedance curve of the cutterhead is an ideal match for an OTL, even one of moderate power.

Embarking on this project has been a learning experience, one that we have been working on for close to 20 years. At this point I can safely say that an OTL is the ideal amp for a Westerex cutterhead. In most cutter systems, the power amp uses loop negative feedback, and then there is a 30 db feedback amplifier that is looped around the cutterhead and the amplifier. This is a recipe for instability. If you model this using Chaos Theory, this is powerfully evident. Transistor amps by their nature, when used in this application need to use feedback. Unfortunately such use increases odd ordered harmonic distortion, something already a problem for transistors since the human ear uses odd-ordered harmonics to determine how loud a sound is (this is also the reason why transistors sound bright even though they measure flat on the bench).

So we made some lathe cuts that employed 4 different amps on one LP side- all using the same musical selection. First the stock (rebuilt) cutteramps, 2nd a stock (rebuilt) Dynaco ST-70, third, a 10-watt SET, and finally the modified M-60 (the mod was to eliminate the slight amount of feedback and add a fuse in series with the output, something any cutteramp should have). So what we created was a lathe cut that allowed us to audition the differences on any system, and in so doing we found those differences to be audible on any system too.

The transistor amp emphasized the highs. The ST-70 played with the same speed but place more emphasis on space. The SET placed emphasis on dynamics (no doubt the result of its distortion characteristics), the M-60 placed emphasis on full bandwidth, space and detail.

Now I should also point out that one thing about a cutterhead is how unlimited it is as an audio operation. They are easy to drive, so the 10-watt SET actually had power to spare. The stock cutter amps are rated at 125 watts, which means that they make about 1/2 of that at full power into the cutterhead at 20KHz (something that would instantly fry the cutterhead if that were to actually happen). By comparison the M-60 can make about 80 watts into the same load at the same frequency. That is why we used the M-60 rather than our larger amp, the MA-1. But unlimited? A few watts is all that is needed to make the cutter sing. It can cut grooves without distortion that no arm or cartridge combo would ever have a hope of tracing- it would literally throw them right out of the groove. Bandwidth is crazy- easily from 5Hz to +50KHz with no worries. Really the limit is not in the cutterhead but the playback, and its the mark of a good engineer that knows what the cartridge will be reliably able to handle.

So its really about the low power distortion of the amplifier. Most transistor designs fail in this area. What is needed is an amp that has distortion linearly decreasing to zero as power falls to zero- they will be inherently smoother and more transparent in this application. Its simply not a job for most transistors.
 

Atmasphere

[Industry Expert]
May 4, 2010
949
157
455
St. Paul, MN
www.atma-sphere.com
#31
global
hvbias said:
How do you get the output impedance to such a low 2.3 ohm without feedback?

I am coming up with 280 (6as7 plate impedance), divided by 14x2 = 10 ohms.
You did not include the mu of the tube, which is part of the equation. Rp/(mu+2) source: Radiotron Desigher's Handbook.
 

MylesBAstor

Well-Known Member
Apr 20, 2010
11,223
8
585
New York City
#33
OK, I can easily say that an OTL has a 'better grip' on the cutterhead than any transistor amplifier.

There is a reason we chose the Westerex system- its cutterhead was designed for tube electronics and has an easy impedance that is hard for transistors. That is to say, its nominally about 10 ohms. The lowest impedance is about 8, and shows an impedance curve similar to a loudspeaker, but with the impedance peak moved up slightly in frequency. IOW it occurs at the resonance of the device, which is in the midrange rather than the bass. Above the peak the impedance slowly rises.

This works out ideal for the M-60, which has wider full power bandwidth than the stock transistor amps (yes, Westerex made a solid state system called the 1700 series electronics). Due to the RIAA curve, its really the highs where you need to have power for proper bandwidth extension. The impedance curve of the cutterhead is an ideal match for an OTL, even one of moderate power.

Embarking on this project has been a learning experience, one that we have been working on for close to 20 years. At this point I can safely say that an OTL is the ideal amp for a Westerex cutterhead. In most cutter systems, the power amp uses loop negative feedback, and then there is a 30 db feedback amplifier that is looped around the cutterhead and the amplifier. This is a recipe for instability. If you model this using Chaos Theory, this is powerfully evident. Transistor amps by their nature, when used in this application need to use feedback. Unfortunately such use increases odd ordered harmonic distortion, something already a problem for transistors since the human ear uses odd-ordered harmonics to determine how loud a sound is (this is also the reason why transistors sound bright even though they measure flat on the bench).

So we made some lathe cuts that employed 4 different amps on one LP side- all using the same musical selection. First the stock (rebuilt) cutteramps, 2nd a stock (rebuilt) Dynaco ST-70, third, a 10-watt SET, and finally the modified M-60 (the mod was to eliminate the slight amount of feedback and add a fuse in series with the output, something any cutteramp should have). So what we created was a lathe cut that allowed us to audition the differences on any system, and in so doing we found those differences to be audible on any system too.

The transistor amp emphasized the highs. The ST-70 played with the same speed but place more emphasis on space. The SET placed emphasis on dynamics (no doubt the result of its distortion characteristics), the M-60 placed emphasis on full bandwidth, space and detail.

Now I should also point out that one thing about a cutterhead is how unlimited it is as an audio operation. They are easy to drive, so the 10-watt SET actually had power to spare. The stock cutter amps are rated at 125 watts, which means that they make about 1/2 of that at full power into the cutterhead at 20KHz (something that would instantly fry the cutterhead if that were to actually happen). By comparison the M-60 can make about 80 watts into the same load at the same frequency. That is why we used the M-60 rather than our larger amp, the MA-1. But unlimited? A few watts is all that is needed to make the cutter sing. It can cut grooves without distortion that no arm or cartridge combo would ever have a hope of tracing- it would literally throw them right out of the groove. Bandwidth is crazy- easily from 5Hz to +50KHz with no worries. Really the limit is not in the cutterhead but the playback, and its the mark of a good engineer that knows what the cartridge will be reliably able to handle.

So its really about the low power distortion of the amplifier. Most transistor designs fail in this area. What is needed is an amp that has distortion linearly decreasing to zero as power falls to zero- they will be inherently smoother and more transparent in this application. Its simply not a job for most transistors.
Thanks Ralph for a really informative description of the technical aspects of your project and cutting lathes in general. I know I learned a lot!
 

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