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Old 17th February 2013, 07:57 PM   #531
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Thanks Bas! I had not looked for Pieter's name, spelled correctly, before and was exhibiting my typical murican blindness to other territory's. I hope Pieter will forgive me.
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I'm sure he will. I spelled his name in the german version years ago. And he still replies to my mails.
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Old 17th February 2013, 08:48 PM   #532
45 is offline 45  Italy
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Originally Posted by regal View Post
It depends on who makes your transfo, my builder just quadrupled the secondary left the primary/core the same as for a 45 and I got a "nice" 10 ohm DCR secondary, the tota efficiency of the transfo gave about 50% less power than an "ideal" model, this is what I am "warning" folks about. Most transfo winders have a formula that is based on a 25:1 primary/core size and a headphone secondary is just considered a low quality "add-on." Rather than an opportunity to take advantage of the 11:1 ratio.

So hence my interest in the dual 8 ohm secondary "balanced out headphone drive" idea/rumor. Trafo builders have years of experience with 5k:8 transformers.
It depends on how one makes it. If one has just a formula for a specific combination then he is not very much in control.... Anyway you really don't need a University degree to optimize the windings to get a balanced and efficient transformer! The maths is simple. It is more about practical experience.
If you had considerably less efficiency then it had not been properly "re-calculated" but this has nothing to do with the fact that one needs a lower turn ratio for higher load. In principle a turn ratio of 11:1 has no advantage on 25:1 if the primary impedance has to be the same. Actually can be worse! The very good choice of wire gauge is such that you can practically get identical results without changing the primary in most cases.

Last edited by 45; 17th February 2013 at 09:01 PM.
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Old 18th February 2013, 06:19 AM   #533
regal is offline regal  United States
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Regal,


As for your concerns with your current transformer. Loses are not defined by the secondary alone. To get within a few % of the total power loss you must take the square root of the turns ratio and multiply the primary DCR by that, and, add it to the secondary DCR to approximate the infinitely reflected losses between the two. There is a core loss but it is insignificant enough to be ignored in a properly designed transformer.

In a general sense any SE OPT with more than 100 ohms primary DCR is going to have a high insertion loss. A 50% loss is pretty ridiculous, but 45 tubes require a fairly high inductance and with their desired current flow and the resultant gaps, you are stuck with either high DCR, a large (14 pounds 3.75" X 4.55" core perimeter) M core transformer, an amorphous core or a nickle C Core transformer, to generate the required inductance. Much better to accept a bit of flavoring and use a lower mu tube, like the 300B or 2A3.
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If you had considerably less efficiency then it had not been properly "re-calculated" but this has nothing to do with the fact that one needs a lower turn ratio for higher load. In principle a turn ratio of 11:1 has no advantage on 25:1 if the primary impedance has to be the same. Actually can be worse! The very good choice of wire gauge is such that you can practically get identical results without changing the primary in most cases.


I think we finally have gotten to the heart of the matter and why I have held off a design. With a 4P1L single-feed output odds are an 11:1 (5k:32) transfo is going to give the same power delivery as a 25:1, because unless we find a transformer winder willing to design from scratch the primaries on both will be the same, and hence the 50% loss of power I experienced.

So how do to proceed? I think we have two general choices at this point that make since:

#1 3A5 (Mu-follower Gyrator DC coupled)-4P1l Parafeed Nickle or Amorphous Core OPT

#2 ECC40 - 307A (or 2A3) Single Feed Mcore OPT (Balanced output 8+8 secondaries)

I would chose a 841/VT-51 with a gyrator mu follower for #2 but I am finding this tube very difficult to source. Of course there are variations on the two ideas, but basically we have #1 a smart 1W amplifer and #2 a fun 2W amplifier.
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Old 18th February 2013, 11:58 AM   #534
45 is offline 45  Italy
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Regal, the transformer you need already exists. It is the Lundahl LL1627.
Primaries will always be all in series regardless of the load. Secondaries will be in configuration C for a turn ratio of 11.3 (i.e. 4.1-4.6K for 32-36 ohm), configuration D for a turn ratio of 8.5 (i.e. 4.6K for 64 ohm) and configuration F for a turn ratio of 4.25 (i.e. 5.4K for 300 ohm). You always use all secondaries and power loss is very low in all cases around 0.1 dB! The only thing you need to specify is the gap for a lower operative DC current. If you ask for 40-50 mA rather than the standard 90 mA, you will get the right inductance for 4-5K (i.e. about 25-30 H).
You could think about some way to swap terminations around (for example using jumpers or similar stuff) until you don;t find the right set-up and headphone so that you don't have to solder every time you change headphone. You could also ask for the best core they have (thin lamination) or the amorphous.

Last edited by 45; 18th February 2013 at 12:00 PM.
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Old 18th February 2013, 01:26 PM   #535
regal is offline regal  United States
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Originally Posted by 45 View Post
Regal, the transformer you need already exists. It is the Lundahl LL1627.
Primaries will always be all in series regardless of the load. Secondaries will be in configuration C for a turn ratio of 11.3 (i.e. 4.1-4.6K for 32-36 ohm), configuration D for a turn ratio of 8.5 (i.e. 4.6K for 64 ohm) and configuration F for a turn ratio of 4.25 (i.e. 5.4K for 300 ohm). You always use all secondaries and power loss is very low in all cases around 0.1 dB! The only thing you need to specify is the gap for a lower operative DC current. If you ask for 40-50 mA rather than the standard 90 mA, you will get the right inductance for 4-5K (i.e. about 25-30 H).
You could think about some way to swap terminations around (for example using jumpers or similar stuff) until you don;t find the right set-up and headphone so that you don't have to solder every time you change headphone. You could also ask for the best core they have (thin lamination) or the amorphous.
I agree this would be a good off the shelf choice. And I think the dual 8 ohm secondaries opens things up for Electra-Print and Ecore transformers which are a good value for custom stuff.

But I am with Pano we have options know and have gotten to the root-cause of the transfo issues so I am very enthused that there is some resolution to the transormer issue.

Time to start buildig stuff.

We basically have narrowed it down to two types of amplifiers: 1. low output all DHT 4p1l output 2 stge amp, and 2. high power 2A3/307A output ( will need to be three stage amp fo all DHT.)


We really have to watch sensitivity. the 3A5-4P1L is a good setup for an all DHT SET headamp. The plates would be paralell, gyrator loaded DC coupled to the 4P1l so as to allow positive grid drive. Its probably the only true 2 stage all DHT design we have with more power than a 6c45pi or 6EM7 spud SET.

Does anyone have experience with the 841 DHT, gain of 30 ?
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Old 21st February 2013, 12:16 PM   #536
regal is offline regal  United States
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I found a nice set of NOS 307A's for cheap so they are still available from the usual places The 4P1L & even 2A3 don't give a much advantage in power over a 6E5P or similiar spud amp, it doesn't make a lot of since to drive them with a super tube/pentode that would pump out 2 watts on its own either.

But the 307A gives more power and it is a fairly sensitive tube.

To keep it all DHT the only way would be a three stage amp.

An input tube like a 30, 01A, #26. Then an "output" stage consisting of a gyrator mu out 4p1l dc coupled to the 307A. I am having a mental block on what "stacked" supplied SSHV's would look like. This would make a nice all DHT headphone amp/5W speaker amp. Does anyone have a schematic of what a "stacked" pair of SSHV's looks like included the rectifiers/prefiltering?
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Old 21st February 2013, 12:20 PM   #537
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Congratulations for the 307A regal. I'm also interested in a 3 stage to make a little amp due to the fact that I have on hand 26, 4P1L & 307A, any suggested schematic?
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Old 21st February 2013, 12:49 PM   #538
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Yes I hope Ale doesn't mind me posting this. I can work it up for a 26-4P1l-307A, but I will need help with the stacked power supplies.

I personally would lean to a 30 input simply because it is easier to deal and less microphonic from what I understand, But I have #26 and 4P1l's on hand and no 30's so I guess it would be worth it to figure out the microphonics.

I see the amp being built with the #26 and 307A on top and the ugly 4p1l hidden underneath 4 Clean watts class A should be very easy for this amp, probably 5-6w with A2.

Also as I lost Iko with my spice THD #'s and I wanted to mention that Spice is predicting much higher THD that Ale has been measuring even as OTL preamp. I look at LT spice as a great tool for setting operating points determining zero grid thresholds, power levels, etc but think the THD isn't accurate because its based on an infinite low noise floor, the THD predicted from spice is good for relative comparison and little else.
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Old 21st February 2013, 07:34 PM   #539
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The spice model is not accurate in A2, so THD is not that representative.
You probably want to consider carefully to go down the DC coupled route with stacked supplies. I'm currently building a 4-65a SE in A2 which is proving to be very demanding due to the supplies. Not sure whether is worth the effort (and size and cost) for a headphone amp.

BTW: 30 is more microphonic than 26 :-)
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Old 21st February 2013, 08:20 PM   #540
iko is offline iko  Canada
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Originally Posted by regal View Post
Yes I hope Ale doesn't mind me posting this. I can work it up for a 26-4P1l-307A, but I will need help with the stacked power supplies.

I personally would lean to a 30 input simply because it is easier to deal and less microphonic from what I understand, But I have #26 and 4P1l's on hand and no 30's so I guess it would be worth it to figure out the microphonics.

I see the amp being built with the #26 and 307A on top and the ugly 4p1l hidden underneath 4 Clean watts class A should be very easy for this amp, probably 5-6w with A2.

Also as I lost Iko with my spice THD #'s and I wanted to mention that Spice is predicting much higher THD that Ale has been measuring even as OTL preamp. I look at LT spice as a great tool for setting operating points determining zero grid thresholds, power levels, etc but think the THD isn't accurate because its based on an infinite low noise floor, the THD predicted from spice is good for relative comparison and little else.
That should be interesting. In my opinion the different thd results we get are due to the poor transformer models we have. I don't read too much into that.

I put together a prototype 4P1L-307A headphone amp but didn't have proper OPTs so I gave up for now. One thing led to another and I ended up with a D3A-4E27 SE prototype... off topic.
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