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Old 26th January 2005, 02:33 PM   #21
Joules is offline Joules  United States
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dhenryp - How do you cut the aluminum and how do you keep the ribbon straight wile rolling the waves in to it? How much tension do you put on the ribbon when you install it between the magnets.
keep up the great work - we'er all behind you 200%
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Old 26th January 2005, 05:39 PM   #22
dhenryp is offline dhenryp  United States
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Default Transformer calculations

CV - GREAT post! That was exactly the kind of information I needed to get me started.

Based on plugging in parameters from my torroids I can see that they have the (probably fatal) flaw of too small a cross section. A very rough calculation says that my F-150-W core has a cross section of ~ .257 CM^2 as compared to the 2 CM^2 in your assumption. Working this through seems to indicate that it is too small to handle the kind of input voltages I need to reasonably power the ribbon.

As luck would have it, I ordered two additional cores yesterday AM when I ordered the new magnet wire and they look like they could work very well.

Here are the calculations for these new toroids and where I use other formulas from the net, I'll post a reference link.
------------------------------------------------------------------------------
New torroids:

Model FT240-77 (2.4" OD, 1.4" ID, .5" thick - Type 77 ferrite)

http://www.oselectronics.com/ose_p88.htm

u = 1800
mh/1000 turns = 2740
Max flux density of type 77 = .46 tesla
------------------------------------------------------------------------------
Here we go:

----------------------------------------------------------------------
CALCULATE DESIRED PRIMARY INDUCTANCE
-----------------------------------------------------------------------
Primary Reactance = "CV" Rule of thumb 5-10 times desired impedance

Xl = 2Pi * f* L
http://www.allaboutcircuits.com/vol_2/chpt_3/2.html

L = XL / (2Pi * f)

L = 8 * 10 /(6.28 * 1000)
L = .0127 ~ 13 mh

Xl = desired impedance in ohms
2Pi = 6.28......
f = crossover frequency = 1000hz
L = inductance in Henry

__________________________________________
CALCULATE PRIMARY AND SECONDARY TURNS
------------------------------------------------------------------
Primary Turns = 1000 SQRT (L / toroid mh per 1000 turn)
http://www.oselectronics.com/ose_p88.htm

L = Primary Inductance in mh

Primary Turns = 1000 SQRT (13 / 2740)
Primary Turns = 68 Turns

Primary impedance = Turn Ratio ^2 * Ribbon resistance
Turns Ratio = SQRT (Primary impedance / Ribbon resistance)
Turns Ratio = SQRT (8/.028) = ~16
------------------------------------------------
Secondary Turns = Primary Turns / Turns Ratio
Secondary Turns = 68 / 16 ~ 5 Turns (rounded up)
-------------------------------------------------------------------------
CALCULATE MAXIMUM INPUT VOLTAGE AND POWER
------------------------------------------------------------------------
A = Torroid Cros Section = 1.25cm *1.25 cm = 1.56 cm ^ 2
A = .000156m^2
-----------------------------------
N = V / (4 * F * A * B)

(Formula different than CVs but mostly units and maybe CV forgot Frequency term?)

http://www.magmet.com/lamination/eur...ansformer.html

N = Primary Turns
V = Max RMS Primary Voltage
F = Xover Frequency
A = Torroid cross section in M^2
B = Max flux denisty in torroid in Teslas
4 = constant (something to do with converting peak V to rms?)

V = N * 4* F * A * B
V = 68 * 4 * 1000 * .000156 * .46 = 19.5 V rms
-------------------------
P = V^2 / R
P = 19.5 ^2 / 8 = 47 watts

P = Max power applied to primary without saturating core
R = Reflected impedance at primary

----------------------------------------------------------------------------
SUMMARY FOR FT240-77 BASED RIBBON TRANSFORMER
----------------------------------------------------------------------------
Primary = 68 turns (I'll probably use 16AWG wire)
Seconday = 5 turns (I'll use 14 AWG wire)
Impedance @ Primary = 8 ohms
Max power at Primary = 47 watt (more than enough)
--------------------------------------------------------------------------
I hope I didn't botch the math too badly. Let me know if I did. I got a feeling I'm going to be using the edit button on this one.

Denis
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Old 26th January 2005, 05:44 PM   #23
dhenryp is offline dhenryp  United States
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Quote:
Originally posted by Joules
dhenryp - How do you cut the aluminum and how do you keep the ribbon straight wile rolling the waves in to it? How much tension do you put on the ribbon when you install it between the magnets.
keep up the great work - we'er all behind you 200%
I just put the foil on a flat surface then put a straight edge on top of the foil. Use a sharp/new razor blade and hold the blade at a low angle to the surface. Press firmly and cut slowly. It's not too hard to get clean 8-9 inch strips, at least with the thickness of foil I use.

I haven't got the "straight waves" part down yet. Mine tend to drift in one direction or another. Maybe practice will make perfect. I think if you get it right to the first order, everything else is cosmetic.
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Old 26th January 2005, 05:59 PM   #24
gl is offline gl  United States
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Hi Joules,

To add to Denis's input, I use a piece of flat glass and wet the foil lightly with water so it holds down. I use a straight edge made of hard plastic or well finished metal. A metal ruler is generally too rough. Use the single edged razor blades that they sell at paint stores and like Denis says hold them at a low angle. Go slowly. Change blades often.

Graeme
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Old 26th January 2005, 08:09 PM   #25
dhenryp is offline dhenryp  United States
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Default Other thing that affect sound quality?

Assume I can eventually get the right size and type core and calculate the windings so that the impedances, inductances are OK and the thing can transfer power to the ribbon (big assumption).

What other properties of the transformer core and/or winding technique that will influence distortion, phase issues or other aspects of sound quality?

E.G. Is it best to put primary on one part of the core and secondary on the opposite side (assuming there is room for both with one layer) or is it best to lay the Secondary on top of the Primary?
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Old 26th January 2005, 08:54 PM   #26
Joules is offline Joules  United States
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Electra-print audio ( www.electra-print.com ) seem to specialize in audio transformers with "wa-zoo" core material for line level xformers and out put xformers. Maybe they might be of some help.

Maybe replacement ribbons for the Raven's or others ribbons tweeters are available, I understand they are relitivly inexpensive and might be easer to setup and use in your Device. ... maybe ...I think ...
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Old 27th January 2005, 01:24 AM   #27
dhenryp is offline dhenryp  United States
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Quote:
Originally posted by tiroth
Repost of Talema data from my samples. I can send you a 35VA unit if you want to experiment. Maybe the leakage inductance is too high though. The smaller cores are way better on this measurement.

I looked at the Digikey site and could not find the Talema transformer. Can you give me a pointer. When I find the data sheet and understand how I could use it I will take you up on yourkind offer of a sample.

Thanks!

Denis
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Old 27th January 2005, 01:27 AM   #28
dhenryp is offline dhenryp  United States
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Quote:
Originally posted by LineSource
I have built amps that can direct drive 0.1 ohm Apogee ribbons and was not happy with the sound. I then converted to a 5.8 micron ribbon that increase the restance to 0.35 ohms, tripled efficiency, and it sounded wonderful with the direct drive amp. Much better than with a transformer.

LineSource - Can you point me to a source for 5.8 micron aluminium? The 12 micron stuff I'm using is the thinest I've been able to find.

Thanks,

Denis
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Old 27th January 2005, 01:37 AM   #29
dhenryp is offline dhenryp  United States
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Quote:
Originally posted by Joules
Electra-print audio ( www.electra-print.com ) seem to specialize in audio transformers with "wa-zoo" core material for line level xformers and out put xformers. Maybe they might be of some help.

Maybe replacement ribbons for the Raven's or others ribbons tweeters are available, I understand they are relitivly inexpensive and might be easer to setup and use in your Device. ... maybe ...I think ...

I looked at the Electra-print page and it looks like these are for vacuum tubes. I think these are quite different from the point of view of the impedances they match, the current they drive to the output and the voltages involved. I could be wrong, but they don't look like they transfer to ribbons (plus they look expensive).

I have seen a guy inthe UK that sells matching Xformers for an old ribbon (Decca?) that is quite reasonable ($60-70 ?). I'm inclined to move forward with a DIY design because:

1. It's interesting (at least to me)
2. buying a pre-built might work, but it might not and I might never understand what the problem was.
3. There is at least the potential to come up with a design that matches my situation better.
4. There is also the potential to come up with a cheaper Xformer, although at the rate I'm going I will end up spending MORE money. Maybe it will help the next person making a DIY ribbon.

Thanks for the suggestions and the encouragement!

Denis
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Old 27th January 2005, 01:41 AM   #30
tiroth is offline tiroth  United States
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The Talema/Amveco potted toroids are items TE70000 and up.

The TE70000 - TE70005 are dual 7V up to dual 22V, 1.6VA. There are six part numbers per decade, so this same naming convention continues with TE70085 being the largest (50VA) potted toroid.

The "mini" transformers use the same convention and the part numbers are TE62000 and up.

It helps to have a catalog, Digikey sells all sorts of stuff you'd never find on their site.
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