You mean going with a lower value? Have not tried that yet, but will do so. This prototype is so open air that it is bound to be picking up hum from all over the place.
For a speaker amp with a 25:1 OPT, not a big problem. For a headphone amp with a 5:1 or 4:1 OPT and a lightly driven tube, it's an issue. Gain structure again.
This about the tribute 1:2. From Pieter Treurniet. Translated by me.
As you can read from Rob's story. 1:2 Interstages are critical. The source impedance (driver tube) and primary DC current determine the important part of the result.
Therefore it is important to know the application. If things don't go well you'll have drop of the highs below 20kHz and/or dropof of the lows as a result of the combination of Rp and induction of the primary of the IT.
With an 1:2 you have voltage gain; is this really necessary? And with a 1:2 IT you'll halve the current( do you then have enough drive for the final tube?)
With a 1:2 you'll run the risk of lesser results in a particular application.
1:1 IT's are easier in all aspects, especially wrt HF bandwith.
50mA IT's will cost € 240,- per pair ex VAT. They will be poured in an aluminium enclosure (powdercoat black or lightgray) Core material will be 0.05mm silicium iron c-core.
If this concerns a groupbuy some discount may be given. I'm swamped in work until the beginning of april. Somewhere in the middle of april I'll be able to plan in this work.
As you can read from Rob's story. 1:2 Interstages are critical. The source impedance (driver tube) and primary DC current determine the important part of the result.
Therefore it is important to know the application. If things don't go well you'll have drop of the highs below 20kHz and/or dropof of the lows as a result of the combination of Rp and induction of the primary of the IT.
With an 1:2 you have voltage gain; is this really necessary? And with a 1:2 IT you'll halve the current( do you then have enough drive for the final tube?)
With a 1:2 you'll run the risk of lesser results in a particular application.
1:1 IT's are easier in all aspects, especially wrt HF bandwith.
50mA IT's will cost € 240,- per pair ex VAT. They will be poured in an aluminium enclosure (powdercoat black or lightgray) Core material will be 0.05mm silicium iron c-core.
If this concerns a groupbuy some discount may be given. I'm swamped in work until the beginning of april. Somewhere in the middle of april I'll be able to plan in this work.
One thing I would ask is the 4P1l isn't going to drive the 307a any where near the grid, so why use the mu output of the driver?
Is the intent that one would precede this headamp with a preamp? In that case we would really have a nice design that will cover all.
What is the Rp of the 307a in triode mode? Anyone know where we can find a spice model of a triode 307a ? The curves look like a mu of 6 which is a nice advantage over the 2a3 and 300b.
Pieter Treurniet has seen this thread. As you may or may not know. Pieter is the owner and winder of Tribute Transformers. He is willing to contribute with his ideas re an output transformer (for instance wrt multiple secundaries to make it more universal as a headphone amp)
But he does need a concrete schematic to be able to do the above.
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This is what I use for rough work.
* 307a LTSpice model triode strapped
.subckt 307a P G K
CPG P G 0.55p
CGK G K 15p
CPK P K 12p
*koren 8
Bp P K I=(0.01369103481m)*uramp(V(P,K)*ln(1.0+(-0.591620367)+exp((5.574041
845)+(5.574041845)*((6.396026735)+(-4.173415004m)*V(G,K))*V(G,K)/sqrt((-1.840130
29e-005)**2+(V(P,K)-(7.11503529))**2)))/(5.574041845))**(1.704405957)
*koren 5
*Bp P K I=(0.0130446811m)*uramp(V(P,K)*ln(1.0+exp((11.13979944)+(11.13979944)*
(6.559627252)*V(G,K)/sqrt((-0.6688132954k)+(V(P,K))**2)))/(11.13979944))**(1.713
912611)
*koren 4
*Bp P K I=(0.01184706113m)*uramp(V(P,K)*ln(1.0+exp((11.13293476)+(11.13293476)
*(6.572692852)*V(G,K)/V(P,K)))/(11.13293476))**(1.73295172)
.ends 307a
* 307a LTSpice model triode strapped
.subckt 307a P G K
CPG P G 0.55p
CGK G K 15p
CPK P K 12p
*koren 8
Bp P K I=(0.01369103481m)*uramp(V(P,K)*ln(1.0+(-0.591620367)+exp((5.574041
845)+(5.574041845)*((6.396026735)+(-4.173415004m)*V(G,K))*V(G,K)/sqrt((-1.840130
29e-005)**2+(V(P,K)-(7.11503529))**2)))/(5.574041845))**(1.704405957)
*koren 5
*Bp P K I=(0.0130446811m)*uramp(V(P,K)*ln(1.0+exp((11.13979944)+(11.13979944)*
(6.559627252)*V(G,K)/sqrt((-0.6688132954k)+(V(P,K))**2)))/(11.13979944))**(1.713
912611)
*koren 4
*Bp P K I=(0.01184706113m)*uramp(V(P,K)*ln(1.0+exp((11.13293476)+(11.13293476)
*(6.572692852)*V(G,K)/V(P,K)))/(11.13293476))**(1.73295172)
.ends 307a
Pieter was an active member here at diyaudio and I learned a lot about transformer theory from his posts. Seeking for one of his posts these says I saw the 'account disabled by request of member', which I found quite sad. So Pieter, if you are reading this, please come back
Hallo Erik. I can't speak for Pieter. But I will act as a proxy poster for a dht headphone amp. So I'll start a new thread soon. Mainly because I'm not sure this thread is going forward in a particular direction. And also dht tubes deserve nay demand the very best output transformers possible or else what is the point.
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Did you know dark ale came along because someone forgot the malt in the dryer? Spreading threads can take leaps. I also think the transformer is the main key, tho.
Staffan
Not for orthos or old AKG's, the gain is only 8, so the best you' do is drive the 307A all the way with an input sensitivity of 2 Vrms, generally you want an amp to have a sensitivity of under 0.5V to account for low level recordings. Recent measurement I find showed that Hifi man inflates their headphone sensitivity ratngs by 5 to 6 db/mW, this is common in the indusry. I measured my old AKG sextettes to be 7db lower than their sales brochure. This is why we can't design a head phone amp around marketing specs.
Dark ale is disgusting.
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I agree the output transformer is the key.
This thread we have shown that the 4P1L is not enough power for all the top50 dynamic headphones. The 2A3 is good for all but 2, and the 307A/PX4 good for all dynamic headphones. Also the 307A/PX4 offers the lowest low level distortion as an output vs anything short of a gm70.
We can can continue arguing about the gain/driver stage but I think it comes down to a personal choice, ie its not possible to drive a 307A to its fullest via a DHT, some will resort to an extra gain stage, some to IDHT's, some will drive an input transformer with their DAC, and some don't need the extra gain.
So I think we are done with our meandering for the output tube and hence are ready for a transformer spec for a 307A/PX4 (for those of us who can't find a PX4 or 307A the 2A3 can generally use the same OPT.)
Gaining a transformer designer's interest was a hidden agenda with this thread, so I think it good for a spin-off thread to focus on a world class headphone output transformer.
I vote for permalloy (nickel) parafeed. The loss in power and low level detail from secondary DCR/inductance with headphone impedances is something I haven't found "speaker" transformer designers to overcome with singlefeed iron cores. Just an example the last 5k/32 ohm transformer I had wound had a secondary DCR of 12 ohms, so over 33% loss in power from the resistive divider formed between the secondary and the voicecoil. This results in more distortion since the tubes work hard for the extra 33% output to hold the same volume and the distortion kills detail.
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I certainly would not want parafeed, but don't let that stop you.
I don't want parafeed either but I don't see a means to avoid it, I see it as a necessary evil with headphones.
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?