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EmAtChapterV
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Amplifier stability with low-R, high-L loads?

Post: # 26990Unread post EmAtChapterV
Thu Sep 05, 2013 4:19 pm

As I slowly teach myself electrical theory as it pertains to record cutting, a question has come up that I'm wondering if someone can answer before I start buying magnet wire and winding coils.

Any given amplifier has a maximum current it can deliver across a load, from a maximum voltage rail, but not both at the same time. This is why the rated power for the minimum allowable impedance is less (often much less) than twice that of a load of double the resistance. (The old faithful workhorse I've been using since the mid-nineties is specced for 18V/2.25A/40W at 8 ohms, 14V/3.5A/50W at 4 ohms minimum.)

But this assumes a normal distribution of power throughout the frequency spectrum and normal, minimal inductance in a moving-coil system. (My calculations put the drive coils of a Neumann head at somewhere around 45 μH: 4.7 ohms DC, 7.5 ohms at 10 kHz.) But what happens with a moving-iron system with a massive, multi-layered coil? My Fairchild was rewound by a previous owner to 4 ohms DC, but was around 1.64 mH: 5 ohms at 100 hz, 14 ohms at 1 kHz and 107 ohms at 10 kHz!

In the interests of increased efficiency, I'd like to rewind it with heavier gauge wire, increasing the current capacity and bringing the inductance down to 0.83 mH. It'd be a nominal 7.1 ohms and rising at 1 kHz, but this would leave the amplifier trying to deal with 2.4 ohms at 100 hz - not an ideal situation. The question is, with the RIAA recording curve engaged, the signal at 100 Hz is down 13.1 dB, or at 22% of its original power, so given that the limitation of the amplifier at low impedence is the current it can provide, and only a quarter of the ordinary current is needed at that frequency, how safe is it? Is it still at risk of oscillation?

I know I should just throw a 2 or 3 ohm 10 or 20 watt resistor in series with the coil and call it a day, but I'm curious about how best to get the maximum efficiency out of the entire system without burning up power as unnecessary heat. I tried sourcing 300μF film capacitors and discovered they cost $50 apiece. And at some point in the near future I realize I'm going to have to admit that 18V RMS isn't going to cut the mustard anymore - time for a bridged mono amplifier.

----

Incidentally, I was trying to calculate the maximum current capacity for drive coils in the various gauges, and came up with this. Can I get a sanity check on these numbers?

AWG27 - 985 mA continuous, 2.35A in short bursts
AWG28 - 775 mA continuous, 1.86A in short bursts
AWG29 - 615 mA continuous, 1.50A in short bursts
AWG37 - 95 mA continuous, 225 mA in short bursts - this seems to be the go-to gauge for 500 ohm moving-iron heads.

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markrob
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27006Unread post markrob
Fri Sep 06, 2013 10:48 am

Hi,

This is a big set of questions. Not sure I know the correct answer here, but I'll throw some stuff out there for your consideration.

1. Is your amp solid state or tube based?
2. How did you come up with your inductance values?
3. Your impedance calculations seem wrong. Remember that the real value is the square root of the sum of the squares of the dc resistance and inductive reactance at the frequency of interest (z = 2 * PI * F * L).
4. Don't forget that there is some back EMF that you also have to fight against (this tends to reduce the actual current flow for a given applied voltage).
5. The head is really a current mode device since force developed is proportional to current (f = B*L*I where B is the magnetic field strength I is current and L is the length of the conductor in the field).
6. At high frequencies, you really need a large voltage rail swing to get the required current to flow in the coil due to the inductance (not really more power at this point). RIAA makes this even tougher (then current might matter).
7. When is comes to max current in a conductor, it gets complicated because that depends of how much temperature rise you can live with. You can look up the fusing current for a given gauge in a table

http://en.wikipedia.org/wiki/American_wire_gauge

and use this as an absolute maximum value (e.g. 29 ga = 12A). But even this is reduced once you pack it into a coil and place it into an enclosure at a given ambient). The values you provided may be a good ballpark, but you'll probably want to run some tests when installed to see how you are doing. You can use the fact that copper has a tempco of about 4000 ppm/c and measure actual coil temperature under operating conditions. If you PM me with your mail address, I have a nice pdf from Ferrotec that shows a method for doing this for speakers (applies to us a well).

I'm not sure there is a optimum here. In fact, its not clear what you want to optimize. The force equation says we should maximize the length of wire in the field so that a minimum of current is required, but as you cram more wire into the bobbin, you end up with increased inductance and DC resistance as the wire size is reduced. Given that the head will never represent a fixed load to the amp, I'm not sure what is required to make the best match.

Hope this is some food for thought.

Mark

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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27007Unread post EmAtChapterV
Fri Sep 06, 2013 2:19 pm

This is very helpful, thank you! Especially the correction about the square-sum.

I came up with the inductance values through this calculator: http://www.66pacific.com/calculators/coil_calc.aspx . The bobbin in a 541A is 19 mm in diameter and 6 mm tall, with a 3 x 6.5 mm rectangular inner core. My main reason for wanting to rewind is maximizing efficiency - it had been wound to be as close to 4 ohms DC as possible, not to be full. There was quite a bit of extra space left.

I'm also comparing it to the research the BBC did on the same model - http://downloads.bbc.co.uk/rd/pubs/reports/1948-32.pdf . The resonant frequency and sensitivity in mine are far, far off from what they report as factory specs. I suppose perhaps I need some stronger damping material to accomplish this.

The amplifier I'm using is solid-state, an old Yamaha. It also has a weird rise above 16 kHz I've never been able to explain, and despite serving me reliably and well, it was probably never designed for the wringer I've put it through over the years.

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markrob
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27009Unread post markrob
Fri Sep 06, 2013 4:14 pm

Hi,

Given the amp is solid state, you don't have to worry so much about gross impedance mismatch like you would for a transformer output tube based amp. The SS amp looks like a very low output impedance as compared the load you are driving. So as long as you don't reach its current limit, it will try to drive the load. Stability is usually a problem driving an capacitive loads due to phase lag. I don't think an inductive load (phase lead) is an big issue or there would be trouble driving speakers. One of the reasons you see 400 watt amplifiers called out on pro cutting setups is that as you get to the higher frequencies, you need drive voltage available to beat out the head inductance and force current into the head. If you check out the Cook patent on his feedback head, he adds a extra feedback loop around the power amp to turn the amp into a current source rather than a voltage source. He does this by using a small value resistor in series with the head and feeds this voltage (which is the head current) back to an input summer. With this type of setup, you would see the head drive voltage increase at 6 db oct for a constant input voltage once you get above the L/R time constant of the head.

Does this 16Khz rise you see happen into a resistive load? That sounds worrisome to me as it indicates a possible stability problem. I'd look into that further.

Your inductance calculator is nice, but it looks like its setup for air core windings. Once you stick the hunk of iron from the armature in the gap, the inductance will rise quite a bit. This also brings up the issue of saturation (not an issue with an air core inductor). For example, I've found that the Presto 1C and D heads hit saturation well before overheating is an issue. One thing to be careful of is that once you reach saturation, the inductance falls way off quickly and you can have large current demands.

Mark

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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27021Unread post EmAtChapterV
Sat Sep 07, 2013 2:25 pm

"Wind my own coils", I thought. "How hard could it be?" :roll:

Turns out the bobbin on a Fairchild pops apart into its three constituent pieces at any sort of sideways pressure. Pressure like a heavier gauge wire being wound next to the flanges. I got 23 turns into the first attempt, 59 turns into the second. Both of them looked like a dog's breakfast after the fourth or fifth layer - heavy wire doesn't like to turn 90-degree corners. Ugh. Some glue setting overnight and another try tomorrow, and if that doesn't work, a switch to using lighter gauge wire and an impedence matching transformer.

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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27022Unread post EmAtChapterV
Sat Sep 07, 2013 3:31 pm

Well, so much for that - the bobbin disintegrated in my hands when I attempted to glue it. :evil: Guess what I get to try to replicate from scratch tomorrow?

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Re: Amplifier stability with low-R, high-L loads?

Post: # 27061Unread post ejemmons
Fri Sep 13, 2013 1:23 pm

I am curious - what glue did you use that killed your bobbin? I rewound one in 1968 with zero hassle, but I used whatever gauge wire it had been, it drove quite well with a pair of 807s from a 16 ohm tap, amp I built myself, about 50 watts or so. Anyway I am watching with great interest! Is the bobbin phenolic? Been a LONG time....
Scully "500" with Westrex 3DIIa,
RA-1574E amps.

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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27063Unread post EmAtChapterV
Fri Sep 13, 2013 2:39 pm

It wasn't the glue that was the issue, so much as the bobbin being 65 years old and brittle. The flanges were supposed to fit over the edges of the core, but when I tried to press them back together, the core snapped and collapsed. I think it's probably phenolic, it looks like the same material as the drum that used to be over the motor shaft in my Rek-O-Kut, but almost paper thin.

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Re: Amplifier stability with low-R, high-L loads?

Post: # 27090Unread post EmAtChapterV
Mon Sep 16, 2013 5:18 am

Attempting to glue back together the broken pieces of the original bobbin, as well as hedging my bets by making a new one from two guitar picks and a soft-drink straw:
Dscg0513x.jpg
The original bobbin in three pieces before the center section shattered, along with the first two failed attempts at coil winding:
Dscg0505x.jpg
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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27142Unread post EmAtChapterV
Wed Sep 18, 2013 12:03 am

And here's what I wound up with (no pun intended) this evening:
Dscg0515x.jpg
The original (repaired) bobbin on the left now has 7m/ 23 ft of 26 AWG wire, for a DC resistance of a smidgen under 1 ohm, in 156 turns for a (calculated) inductance of 0.436 mH. I'm going to run it in series with a 2.2 ohm 10W resistor and see where that gets me. Tomorrow it gets installed.

The homemade bobbin, because of the larger core, only has 140 winds. I haven't checked the resistance or calculated any of the other variables for it yet. It will be a backup, or maybe I'll rewind it with a lighter gauge, I haven't decided yet.
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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27250Unread post EmAtChapterV
Tue Sep 24, 2013 4:49 pm

So I tried bench testing it, before discovering I need to wrap everything in permalloy or supermalloy (neodymium magnets are attracted to stainless-steel cutter overheads and leadscrews!), and while I got a good strong signal, there was what sounded like intermodulation distortion. Back to studying and trying to figure out exactly what the current saturation point of the armature is and how close it's getting with a low-impedance, high-current coil, calculating the inductance curve...

And this occurred to me: I don't have any AWG31, but I do have a bunch of AWG34 - half the surface area of a cross-section. Winding one long coil of #34 would get me an estimated 33.7Ω DC, 82.2Ω at 1 kHz and 751Ω at 10 kHz, far too high to be useful unless I put an impedance-matching transformer in there. But... what if I cut it in half, and ran two strands of #34, half the length, interleaved in parallel from the same terminals on the same bobbin? Then I'd have 8.4Ω DC, and relatively the same magnetomotive force in ampere-turns acting on the armature while it only "saw" half the saturating current... but what would happen with inductive coupling? What would the inductance curve look like - #34 or #31? How badly would the amplifier pitch a fit?

Also, I thought I found a local place to remagnetize the (very weak) stock magnet... until I phoned them up and found out their remagnetizer had KILLED two of their employees, and had since been decommissioned. Yikes!

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Re: Amplifier stability with low-R, high-L loads?

Post: # 27252Unread post markrob
Tue Sep 24, 2013 5:05 pm

EmAtChapterV wrote:So I tried bench testing it, before discovering I need to wrap everything in permalloy or supermalloy (neodymium magnets are attracted to stainless-steel cutter overheads and leadscrews!), and while I got a good strong signal, there was what sounded like intermodulation distortion. Back to studying and trying to figure out exactly what the current saturation point of the armature is and how close it's getting with a low-impedance, high-current coil, calculating the inductance curve...

And this occurred to me: I don't have any AWG31, but I do have a bunch of AWG34 - half the surface area of a cross-section. Winding one long coil of #34 would get me an estimated 33.7Ω DC, 82.2Ω at 1 kHz and 751Ω at 10 kHz, far too high to be useful unless I put an impedance-matching transformer in there. But... what if I cut it in half, and ran two strands of #34, half the length, interleaved in parallel from the same terminals on the same bobbin? Then I'd have 8.4Ω DC, and relatively the same magnetomotive force in ampere-turns acting on the armature while it only "saw" half the saturating current... but what would happen with inductive coupling? What would the inductance curve look like - #34 or #31? How badly would the amplifier pitch a fit?

Also, I thought I found a local place to remagnetize the (very weak) stock magnet... until I phoned them up and found out their remagnetizer had KILLED two of their employees, and had since been decommissioned. Yikes!

Hi,

That sounds like a clever way to get you in the ballpark of using #31 without having to order some up. I'd give it a try and see where you stand.

Mark

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Fela Borbone
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27260Unread post Fela Borbone
Wed Sep 25, 2013 10:27 am

EmAtChapterV wrote: Also, I thought I found a local place to remagnetize the (very weak) stock magnet... until I phoned them up and found out their remagnetizer had KILLED two of their employees, and had since been decommissioned. Yikes!
Wow! terrible story!
I usually overheat the speakers motors to dismantle them,they loose their magnetism, but I manage to remagnetize ferrite myself, rubbing a strong neodymium(be carefull with your fingers if try),more or less like this site proposes:
http://www.forcemagneticsolution.com/showlink.asp?id=263
I test their magnetic power with a nut and spring scale, after and before(close magnet circuit in both cases)this measure is not very accurate,but I think the magnet is back to life.
What are the advantages of profesional remagnetizers? more magnetic force?home method only works for ferrite?

Hope my post is not too innocent....

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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27264Unread post EmAtChapterV
Wed Sep 25, 2013 1:21 pm

What are the advantages of profesional remagnetizers? more magnetic force?home method only works for ferrite?
Yes and yes. The "rub it with a neo" method only works for ferrite, and will only bring it up to maybe 35 to 40 percent of what it could be at full saturation. The B-H hysteresis curve of Alnico magnets (which I think is what was on mine? I'm not sure) means they're easy to demagnetize around stronger magnets (like Neos, as happened with mine) but difficult to remagnetize. It takes around 3 or 4 Teslas if my research is correct - almost MRI-machine magnetic levels. It appears to involve peanut-butter-jar-sized electrolytic capacitors and an LP-sized ring of a good half-mile (at least) of several hundred turns of #18 wire - a good recipe for a cardiac-arrest-inducing jolt of electricity and/or a faceful of flying high-velocity improperly-secured magnet.

And neodymium kind of scares me now, even without electricity involved. The main component in my "improved" system is this beast: http://www.kjmagnetics.com/proddetail.asp?prod=BX0X0X0-N52 . It can and has magnetized the entire Rek-O-Kut overhead, and attracted screwdrivers from over a foot away. Securing the head to the lathe, approach it very very slowly, and suddenly BANG, it's pulled out of your hand.

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Re: Amplifier stability with low-R, high-L loads?

Post: # 27274Unread post EmAtChapterV
Thu Sep 26, 2013 7:05 pm

Okay, with the lack of usable magnets, I'm back into "angels dancing on the head of a pin" mode now. Specifically, I'm trying to define how voltage versus current (ie. impedance) affects the magnetomotive force, efficiency, distortion and harmonics versus the magnetic saturation point in a moving-magnet system. It seems like I was going the wrong way using heavier wire.

One Tesla is defined as 1V x 1 second / 1 meter^2, and 1N x 1A / 1 meter. But if the saturation point of a hypothetical armature is also 1T, how does the volume of the armature factor into it? And if you double the impedance of the circuit and apply the same amount of power, you'd have SQRT(2) x 1 second / 1 meter^2, and 1N x SQRT(0.5) / 1 meter. And isn't saturation more about current than it is about voltage...? And the BHmax of any given magnet (the magnet around the armature, not the armature itself) is in MgOe, and magnetomotive force acting on the armature is measured in Ampere-Turns... argh. Bleh. I can't science properly.

I realize there's no free lunch here, so to speak. But wouldn't efficiency improve by using the finest wire you could get (thus highest impedence and lowest current), and either have many, many parallel windings converging to one at terminals outside of the magnetic field, or having one long winding and a gigantic impedance-matching transformer well away from the head itself? I now have pages and pages full of numbers and formulae, and a shiny, polished numeric keypad on my computer keyboard, and I'm still not quite there yet. Brain hurty.


Edit, for the record: the Fairchild 541A bobbin fit 157 turns of AWG26 totalling 6.5 meters (0.87 ohms) which would have handled 1240 mA continuous, for 195 Ampere-Turns. Rewinding it with AWG34 resulted in 954 turns totalling 42.1 meters (3 parallel lengths of 14.02 meters at exactly 12 ohms each, total 4 ohms), which will handle 194 mA continous, for 185 Ampere-Turns. So 95% of the efficiency and 15.6% of the current.

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EmAtChapterV
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Re: Amplifier stability with low-R, high-L loads?

Post: # 27296Unread post EmAtChapterV
Sat Sep 28, 2013 12:02 pm

And it looks like someone got here ten years before I did: http://www.google.com/patents/US7024014 . This bears further examination and experimentation.

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