Noobman Cutting Head

[EpicenterBryan]

Also welding or replacing with a one-piece V. Epoxy is not working.

Looks like you are using feeler gauges, probably stainless. If you can make an extra set and mail them to me, I'm happy to spot weld them for you at 90 degrees.

Torque tube needs a larger 'footprint' at the leaf spring.

Is that an issue with the diameter needed for your screw to mate?

Nylon nut that fastens the torque tube to the leaf springs should be metal for a solid connection.

You can always add super glue at that point. I assume you did the same where the guitar string is connected at both the the nylon screws. Nice idea on the bracket for the end of the piano wire. It should be stout enough. You could always beef it up. Try pulling forward on the torque tube with an magnifier glass at the body end connection. If you see the tube move forward, you will need to beef up the bracket.

Bryan

[CHAPPERS]

Looks like you are using feeler gauges, probably stainless. If you can make an extra set and mail them to me, I'm happy to spot weld them for you at 90 degrees.

You truly are a legend Bryan! Thank you. I may well take you up on that.

I have this idea I'm thinking about right now which is to use a ball joint instead
of the leaf spring/wire arrangement.

Found some lovely little graphite/phosphor bronze helicopter control arm ball joints at the model shop. It seems to me they would perform the same function and minimize any unwanted frequencies or weird harmonics that the leaf/wire could possibly induce.
Do you think in theory that might work?

Watched a 6N on Ebay go to $1400+ today. So I went to the store and bought a few parts to start making my lathe, buying one is a pipe dream at that kind of money.

Snagged 2 nice bearing mounts with bearings that perfectly fit a lovely old lead screw.
Goodness only knows if I can build a successful lathe, but loving the process.

Cheers for the input Bryan, much appreciated!

[EpicenterBryan]

I have this idea I'm thinking about right now which is to use a ball joint instead
of the leaf spring/wire arrangement

I know exactly where you are coming from on this. But I think you either mis-stated the idea, or are missing something. The point of the V-spring (along with the torque tube) is to allow the full range of movement, but prevent the stylus from going out of perpendicular to the surface of the disc. A ball joint would provide the same range of motion, but would not prevent rotation of the torque tube. The ball in the socket is allowed to rotate which is exactly what you are trying to avoid. That is exactly what the V-Spring prevents.

What I think you intend, is something like a U-joint. That would allow the movement similar to the spring, but if one end is fixed, it would not allow the torque tube to rotate (which would be connected to the other end). Capiche?

B

[CHAPPERS]

Rotation! Darn, I see. Hmmm, now I'm thinking again...

Time to start on the lathe I think.

I really need to start making some cuts, otherwise it's
all just guess work from this point really.

Thanks again!

Cheers Bryan

[oldskoolhead]

nice work, im just a noob as well lol so i just have 1 question about those drivers you are using, i can see the attraction of them as how they work looks ideal for this application but is the fact they only produce lows of 300hz not a problem? is this a compromise for ease of use or is there another reason you didnt choose a driver that would cover the lower ranges?

[CHAPPERS]

nice work, im just a noob as well lol so i just have 1 question about those drivers you are using, i can see the attraction of them as how they work looks ideal for this application but is the fact they only produce lows of 300hz not a problem? is this a compromise for ease of use or is there another reason you didnt choose a driver that would cover the lower ranges?

Cheers! Great question.

Honestly I used them because a few people in these forums have had good success with them.
And I'm a total noob, so it seemed a good place to start!

I haven't actually made a cut yet, or played with set-up and EQ, but my guess would
be that once the RIAA curve is applied, the drivers are working above 300hz.

But that's a total guess, I have way more questions than answers at the moment,
but slowly getting my head around it.

Hopefully Bryan will weigh in with an expert answer.

[opcode66]

If you look at the frequency response that Bryan is measuring here http://lathetrolls.com/viewtopic.php?f=15&t=5371&start=100 you'll see he has a distinct drop off starting at 300 hz. What you are referring to not only explains what we are seeing, but Bryan is essentially demonstrating the drop off in response from 300 hz and lower.

He is trying to work out feedback for his cutterhead to flatten this response. Though, I think, there will be areas that will never be flattened. Even with feedback or a well constructed EQ curve. The drivers themselves have physical constraints due to their construction. This is why I started by building transducers first. Then I moved on to building a head.

Below is a curve Bryan plotted.

file.jpg

[CHAPPERS]

Interesting stuff, I figured off-the-shelf drivers were more a starting point than the final design.

Thanks for the input, driver research is next on my list!

Cheers.

[opcode66]

Food for thought. Making your own transducer is not that hard. You then have ultimate control to tweak your design. This is important once you get to the feedback stage. Changes to the transducer affect resonance and how much power you need to drive at certain frequencies. And, along the way, you end up learning a lot about magnets, electromagnetic fields, weight vs resonance, designing to avoid rattling parts, and springs.

[CHAPPERS]

Thanks Opcode, looking forward to digging into this further.
I can see how the balance of all these things are critical to
cutting something you'd actually want to listen to.

And that's the goal!

Cheers.

[opcode66]

A cutterhead is the culmination of so many principles in perfect harmony it baffles the mind.

[Podorvanov]

Think this is about as far as I can get without a lathe to test on.

The 'moving mass' - pushrods, torque tube, head and brad (as a placeholder stylus) weigh in at just under 5 grams total. An aluminum head would be a little lighter.

Pushrods are nylon with carbon fibre tips.

Currently, what look like the leaf spring are just a stiff bracket (no need for piano wire). The flexibility is allowed by the rubber bush. The idea is to be able to experiment with both arrangements.
I have a design for a bracket for the piano wire option that simply bolts on.

I've seen various designs, so I wanted this to be as flexible as possible for fine tuning and modifying.

I suspect that will burn way more hours than building this did!

I ran some audio through it with standard eq and there's a good strong vibe in the stylus.

What are your thoughts guys? Think it'll work?

Also, are the Russian cutting stylus on EBay usable? Bearing in mind I'm bound to trash a few before I get this all dialed in.

Cheers guys.

Chappers

image.jpg

Hello. He wanted to know what kind of driver you are using to build its cutting head. Please tell us their name, model, and if you can link to where you bought them. Thank you.

[CHAPPERS]

The drivers are Dayton Audio BCT-2 Transducers available here:
http://www.parts-express.com

I haven't tried them yet, but as Oldskoolhead and Opcode 66 pointed out, the low frequency response taps out at 300hz, so they will almost certainly need modification, if not replacing altogether.

Something to think about if you're planning on getting some!

[oldskoolhead]

[/quote]I haven't actually made a cut yet, or played with set-up and EQ, but my guess would
be that once the RIAA curve is applied, the drivers are working above 300hz.

But that's a total guess, I have way more questions than answers at the moment,
but slowly getting my head around it.

Hopefully Bryan will weigh in with an expert answer.[/quote]



i thought applying the RIAA curve wouldnt eliminate the frequency altogether but just lower the gain on it or the amplitude of the lower frequency so it fits a smaller space on the vinyl, like turning the volume of it down, so a 20hz signal will still be there but with less gain, so played at -20db.
maybe lower frequencies could be achieved with the driver by recording it at a different pitch?

[CHAPPERS]

i thought applying the RIAA curve wouldnt eliminate the frequency altogether but just lower the gain on it or the amplitude of the lower frequency so it fits a smaller space on the vinyl, like turning the volume of it down, so a 20hz signal will still be there but with less gain, so played at -20db.
maybe lower frequencies could be achieved with the driver by recording it at a different pitch?[/quote]


You're totally right, not sure what I was thinking.

Now I'm really interested in how to modify the drivers to achieve a better low frequency response. I have no idea what component affects this, so need more research.

Any thoughts?

[markrob]

Hi,

Its nonsense that the drivers tap out at 300 hz. You are looking at the published acoustical response of the driver and assuming that has any bearing to its performance as a cutter. In fact, the drivers have response down to DC when operating as a mechanical coupled system. You can see this for yourself. If you apply a DC voltage to the head, it will deflect in proportion to the current flowing in the coil. The low frequency limit will be based on your audio chain including the power amplifier. You can easily cut down to very low sub sonic frequencies with the right signal chain. High frequency performance will be your main issue and this also has nothing to do with the acoustical response of the drivers. The important parameters are:

1. Moving mass of the driver (and all of the other moving elements in your design).
2. Power handling limits and thermal time constants (how fast the coil heats when power is applied).
3. Motor efficiency (how much force is generated per amp of current).
4. Electrical time constant (due to the inductance and coil resistance).

All of these factors interact and have a large effect on the performance of the head. For example. You might find a driver that can handle 60 watts RMS and assume its a better candidate than another rated at 10 watts RMS. This might be true, but in general, large power handling comes with increased moving mass. So more force will be required to accelerate the voice coil. In the end, you might find that you need more power to move the bulkier high powered driver than the lighter low powered unit. It gets tricky to optimize for this application.

Mark

[opcode66]

I guess the response curve that Bryan plotted that shows a distinct drop starting at 300 is nonsense too then?

[oldskoolhead]

just a thought at recording at a different pitch, this may have an effect on the RIAA curve and require some adjustment to the curve on recording so if you adjusted the pitch so 20hz played at 300hz you would need a -20db slope at 300hz so when it is played back in real time the 20hz is at -20db unless my logic is illogical? lol

[opcode66]

If you do half speed cutting you have to drop the riaa by 1 octave, correct.

[EpicenterBryan]

Hopefully Bryan will weigh in with an expert answer.

Expert? HaHa!

Two things:
Those drivers have a stated resonance of 530hz +/- 30%.
http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/GD-003.pdf
Since these are surface transducers and have an aluminum part that threads into the driver, I believe that spec is with that part attached. When these are mounted in my head it's around 600hz.
https://lathetrolls.com/viewtopic.php?f=15&t=5371

The plot Todd re-posted below is of the driver on my test bed with no load, and without that big alunimim part. In that case, the resonant frequency is lower - around 300 hz. Yes, the frequency drops off but it doesn't mean the driver can't produce low frequencies. It just means it will take more power to get there or a modified EQ curve (or feedback).

Bryan