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Old 13th February 2013, 09:44 AM   #441
Stixx is offline Stixx  Germany
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Quote:
I see it as a necessary evil with headphones.
Not necessarily all headphones, since there are high impedance headphones as well. The [theoretical] loss is much lower when you are driving HD800's with an impedance of about 350 Ohm via Lundahl LL1689 OPT with a secondary DCR of about 15 Ohm (depending how they are connected...)

But I fully agree that choosing really good output transformers is the way to go for either route...
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Old 13th February 2013, 10:17 AM   #442
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Originally Posted by Radium View Post
If 2A3 class tube is considered, then consider transformers like LL1623: 4x13.4 : 8x1 turns ratio, 0.4 Ohm per each secondary, so if all in series that is 3.2 Ohm. LL1620 and LL1627 provide higher and lower turns ratio respectively, with the same secondaries. Is neither of these really an option here, with so many ways to connect primaries and commutate secondaries for different headphones?
Yes you are right. The only problem is they are relatively heavy and bulky because of the power handling at low frequency.

I have already played with the LL1627 and LL1623. For example, if you use the LL1627 as 1.2K/8R transformer with 32R headphones you will get about 4.2K primary impedance. Having 18H low end cut-off (-3 db) will be around 6-8 Hz with valves like the 2A3 and the 4P1L. Insertion loss will be very low around 0.2 db or even less with 4P1L.
The same LL1627 can be used as 0.65K/8R for 64R headphones with similar results.
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Old 13th February 2013, 11:39 AM   #443
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Originally Posted by 45 View Post
For example, if you use the LL1627 as 1.2K/8R transformer with 32R headphones you will get about 4.2K primary impedance.
I am thinking about commutation of secondaries (quality paid with complexity) for headphones with an order of magnitude difference in impendance (32 and 300Ω, plus 8Ω for small speakers) vs. a resistive ballast (simplicity paid with distortion). So just for a test, I put together an xls for LL1627, LL1623, LL1620, LL9202 that computes impendances of all possible combinations of their coils and the headphones of interest (32, 38, 52, 64 and 300Ω). For the tube I want I need 2500~3000Ω.

LL1620 (without 52Ω), LL1623 (without 300Ω) and LL9202 (without 300Ω) can have secondaries easily commutated to accept all interesting headphones and 8Ω speakers. And 300Ω can be commutated with a shunt resistor to match it to one of the other values (i.e. 64Ω).

So I do not see a desparate situation here, if small extra complexity (and a microcontroller to run all those relays and R-sensors) is not a taboo.
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Old 13th February 2013, 12:25 PM   #444
regal is offline regal  United States
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Quote:
Originally Posted by Radium View Post
I am thinking about commutation of secondaries (quality paid with complexity) for headphones with an order of magnitude difference in impendance (32 and 300Ω, plus 8Ω for small speakers) vs. a resistive ballast (simplicity paid with distortion). So just for a test, I put together an xls for LL1627, LL1623, LL1620, LL9202 that computes impendances of all possible combinations of their coils and the headphones of interest (32, 38, 52, 64 and 300Ω). For the tube I want I need 2500~3000Ω.

LL1620 (without 52Ω), LL1623 (without 300Ω) and LL9202 (without 300Ω) can have secondaries easily commutated to accept all interesting headphones and 8Ω speakers. And 300Ω can be commutated with a shunt resistor to match it to one of the other values (i.e. 64Ω).

So I do not see a desparate situation here, if small extra complexity (and a microcontroller to run all those relays and R-sensors) is not a taboo.
Can you post the spreadsheet?

One thing I am not sure of is if we think about a 3K:38 transformer with a single secondary, think about how much lower a winding ratio this is than a 3k:8 transformer. As we all know the lower the winding ration the better th quality. With the LL's they use multiple secondaries so I think of that benefit or potential opportunity is lost?
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Old 13th February 2013, 12:42 PM   #445
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Originally Posted by Radium View Post
a resistive ballast (simplicity paid with distortion).
In principle is not a good deal because it is already difficult to keep distortion low even with a proper OPT. We are dealing with a situation where you need very little power for high SPL (i.e. headphones) in comparison to loudspeakers.
Anyway there is nothing wrong if you try. At least you can be sure!

Quote:
Originally Posted by regal View Post
Can you post the spreadsheet?

One thing I am not sure of is if we think about a 3K:38 transformer with a single secondary, think about how much lower a winding ratio this is than a 3k:8 transformer. As we all know the lower the winding ration the better th quality. With the LL's they use multiple secondaries so I think of that benefit or potential opportunity is lost?
With Lundahls you can have several optimal combinations because the single section terminations are brought out. You just need to do this for the secondary. The primary sections are meant to be all in series for minimum capacitive load.
The lower turn ratio is not better a priori. It depends on the application but generally speaking is typically worse already when dealing with impedances in the Kohm range. If you compare a 5K IT with 1:1 ratio and a 5K/8R OPT with identical geometry except the serial-parallel connections of the secondary the latter is a lot better because the leakage inductance and the stray capacitances of the secondary are neglectible.
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Old 13th February 2013, 12:56 PM   #446
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Actually a turn ratio of 4:1 (which is approx. what you need for 5K/300R) is already a lot better than 5K:5K.
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Old 13th February 2013, 01:46 PM   #447
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Originally Posted by regal View Post
Can you post the spreadsheet?
Attached. But there may be silly mistakes in there, so verify before making any decisions.

Quote:
Originally Posted by regal View Post
With the LL's they use multiple secondaries so I think of that benefit or potential opportunity is lost?
I am looking at a rather narrowly specified design: single-tube EML AD1 (grid driven directly by high-voltage I/V DAC, at -45V) at 250V on anode and 60mA, as it is meant to be, which gives about 4W at 8Ω load. Having read your initial requirements (in general a good match with what I want) and the following discussion, the transformer for a widely varying Z loads is the main complication, and it looks like multiple secondaries with some combination of secondaries' commutation and gentle shunting (for odd loads) may accomodate all potentially interesting headphones plus 8Ω speakers at a cost of relatively small increase in overall complexity (added Reed relays and a microcontroller).
Attached Files
File Type: zip Lundhal-Z.zip (12.8 KB, 22 views)
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Old 13th February 2013, 06:59 PM   #448
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Hello

This may be something to add to the conversation.

A couple or 3 years ago I had Edcor design a transformer for 300 ohm HD600 headphones. The requested transformers were specified as 3 watt, 50 ma idle current, and 5K input : 300 ohm output with a 40% tap (to experiment with ultra-linear connection). I ordered 10 to avoid the design fee. The Edcor model number is GXSE3-300-5K. If you want to try a pair for your prototype; they cost me about $20 each back then.

DT
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Old 13th February 2013, 07:01 PM   #449
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I want two opts, YHPM

Last edited by merlin el mago; 13th February 2013 at 07:11 PM.
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Old 13th February 2013, 07:03 PM   #450
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Pieter's take on a DHT headphone amplifier.
Pieter's take on an all DHT headphone amplifier.
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