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Old 27th January 2013, 07:34 PM   #31
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I have a pair of Lundahl LL1689. Any use? My AKGs are 62 ohm

http://www.lundahl.se/pdf/1689.pdf
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Old 27th January 2013, 08:12 PM   #32
jpak is offline jpak  United States
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Originally Posted by regal View Post
I know enough about output transformers to know that their is potential to do better than an 8 ohm secondary, hopefully if we can all settle on at least the output tube we could catch a winder's interest to really design a nice OPT for headphones.
Is there any benefit to having multiple secondaries off the transformer based on the headphone's impedance? I was looking around and it looks like some high end transformer coupled amps designed by Pete Millet like TTVJ 307a and Apex Pinnacle have multiple secondaries selectable depending on the headphone's impedance.

Frank Cooter has used Electra-Print OPTs for some of his dynamic headphone amps. For this DHT design were you aiming for something higher/lower priced? I agree with Frank that DC coupled is the way to go. And CCS load both stages.

Last edited by jpak; 27th January 2013 at 08:25 PM.
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Old 27th January 2013, 08:42 PM   #33
kevinkr is offline kevinkr  United States
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Changed title to Headphone amp as head amp is frequently used to signify a pre-amplifier used to boost MC cartridge signal levels to MM cartridge signal levels, hence confusing.
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Old 27th January 2013, 08:49 PM   #34
45 is offline 45  Italy
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Originally Posted by andyjevans View Post
I have a pair of Lundahl LL1689. Any use? My AKGs are 62 ohm

http://www.lundahl.se/pdf/1689.pdf
With or without gap?
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Old 28th January 2013, 07:42 AM   #35
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Ah yes - forgot about that. They are PP so for SE use looks like parafeed. Unless, that is, the PP transformers are routinely gapped for 5mA. If that's the case I could use a 26 at 4 or 5 mA.

Kevin - do you know the gap on PP Lundahl transformers like the LL1689, 1635, 1660 etc?
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Old 28th January 2013, 08:34 AM   #36
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Originally Posted by regal View Post
I received a couple quotes for single-feed 5k:40 output transformers.

For a pair made by the brits its a bit over $400, with guarantee to design for lowest secondary dcr/inductance M6.

For pair made in sin city its about half that price but not guarantee on high dcr, probably M19.

Then there are china elcheapos called BEZ's that are 4k: 32-64-100-200 for only $100 shipped.

No response from the parafeed experts Magnequest, so rule that one out. Cinemag has some interesting mic ouput transformers that could be good for parafeed, but probably too small for orthos's power.

So its either britain, LV-USA, or China. Have no idea which direction to take.
I'm not surprised that Sowter are the only company that will actually do exactly what we want - even if we only order one pair.

It's my personal experience that Sowter will deliver what they promise, and with quality that will last for anyone's lifetime. With that in mind, the cost is not high.

My opinion is that it would be better to use a bipolar transistor to make the impedance transformation, that to use a cheap, or compromised transformer.
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Old 28th January 2013, 09:17 AM   #37
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My opinion is that it would be better to use a bipolar transistor to make the impedance transformation, that to use a cheap, or compromised transformer.
Are you thinking about an electrolytic cap on the output then?
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Old 28th January 2013, 09:25 AM   #38
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Are you thinking about an electrolytic cap on the output then?
Never!

A compound triode - bipolar stage, with a 2nd-order dc bias controller, to give zero dc offset, no output capacitor at all.

I'll finish drawing it, if there's interest.
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Old 28th January 2013, 09:27 AM   #39
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Yes please!
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Old 28th January 2013, 09:27 AM   #40
45 is offline 45  Italy
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Originally Posted by andyjevans View Post
Ah yes - forgot about that. They are PP so for SE use looks like parafeed. Unless, that is, the PP transformers are routinely gapped for 5mA. If that's the case I could use a 26 at 4 or 5 mA.
I was hoping for the PP version because I have a proposal you might like....

You could use the 9+9:1 connection so that for 62 ohms load you get 20K plate-to-plate impedance and go for an ECC40 Class A push-pull!
At 250V/6mA (bias about -5.5V) you would get about 0.7-0.8W at 1% THD.
You need no more than 4V RMS drive which you can get from another ECC40 in concertina configuration. This is able to give 30V RMS at 1% THD! At 4V out THD should be a lot less.....It uses the same 250V supply voltage.

The original Class A PP amp in the datasheet uses 30K plate-to-plate and about 5 mA for 0.5W Pout while the single ended version uses 15K at 250V/6mAfor .28W Pout.

Last edited by 45; 28th January 2013 at 09:29 AM.
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