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Old 2nd February 2013, 12:27 PM   #161
45 is offline 45  Italy
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Originally Posted by Rod Coleman View Post
OK, what peak voltage do we need to aim for?

And does the impedance vary over the frequency range? Is 38 ohm only a nominal?

We should be clear about the goals. If the Orthos are so different, it may not be optimum to have a single design here.
It depends very much on the actual headphone as there can be huge differences in terms of impedance vs frequency curves. Usually the nominal impedance given in specifications is true in the midrange. Then, depending on the design and technology, you can have quite some variations both in module and phase at low or high frequencies. Typical Sennheiser, for example, look very much like a tiny woofer with a peak in the module around 90-100 Hz where phase rotation in about +/- 20-25 degrees. At high frequencies there is a modest slope where impedance is higher (typical inductive behaviour). Other types instead can have a really flat (resistive) impedance from mid to low frequencies and pretty huge variation at high frequency where phase rotation can be as big as +/-45 degrees.
As a rule of thumb the Pout can range from 10 mW to 1W. Having already the phones is a clear advantage as you can taylor the amp to that specific case.


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P.S. Not necessarily the amp has to be very low Zout. Maybe this is true in most cases. There are cases where high Zout (i.e. a constant current amp) is the winner.
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Old 2nd February 2013, 12:43 PM   #162
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Originally Posted by merlin el mago View Post
Will be a good idea OPT with multi tap secondaries 4-8-32-300 ohms this way we have headphone & power amp? or if can't give very good perfomance will use two OPTs: one for headphones & other for loudspeakers?
we need to decide on a tube first. The more I look at it the 4P1L just doesn't have what we need as an output tube, the curves barely allow a 100vp-p swing. Would be better off just building a spud D3A or 6C45pi SET.

I think at least a 2A3 is required to make this project worthwhile, and at that size yes it makes sense to ask for an 8 ohm tap to serve horn/small fr speakers as well.
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Old 2nd February 2013, 01:03 PM   #163
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Here's my serving suggestion for the 4П1Л (4P1L) Triode, to meet Dave's 200V+ swing.

Please see the load line on the triode curves. Best load is 5300 ohm and an operating point of 180V 35mA, if the curves are reliable. I think Ale has some actual measurement, for double-checking.

With ordinary high impedance driver we should be able to swing 90V to 270V [180V] at the anode, for symmetrical grid swing.

Drive the 1st grid with a cathode follower and we will get more like 40V to 320V symmetrical and very linear - 280V swing!

for 38 ohm reflected load, the turns ratio is about 12, so you get ~23V peak to peak on the headphones with cathode follower drive or 2.5W rms, which is the expected value for the 4П1Л.
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Old 2nd February 2013, 02:45 PM   #164
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Thanks Rod, do you think a two stage 4p1L with the driver gyrator mu follower loded/cap cpoupled would have enough to drive the grid poitive for close to the 280V swing?
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Old 2nd February 2013, 03:06 PM   #165
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Originally Posted by regal View Post
we need to decide on a tube first. The more I look at it the 4P1L just doesn't have what we need as an output tube, the curves barely allow a 100vp-p swing.
Hi Regal,
What is your basis for this conclusion? 4P1L can easily provide 100vpp swing and more, you can swing the anode up to 100vrms in class A1. Here is a real example for a 5K3 load as suggested by Rod. The operating point is really good anywhere between 230-250V:

Click the image to open in full size.


Also, 4P1L is extremely linear valve. Here is the valve's intrinsic distortion at 30Vrms:

Click the image to open in full size.

Very interesting thread indeed. I like Rod's approach. I started experimenting with the 6e5p in cathode follower with a autobias circuit to avoid caps in the signal path. I never progressed with the idea. Here is the old thread: 6e5p WCF headphone amp with no cap!

Cheers,
Ale
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Last edited by mogliaa; 2nd February 2013 at 03:12 PM.
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Old 2nd February 2013, 03:47 PM   #166
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Thanks for the input Ale...
I posted a link to your site ealier in the thread...
I will read your other thread...
encourage you to stick around this one..
dennis h
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Old 2nd February 2013, 04:15 PM   #167
regal is offline regal  United States
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Originally Posted by mogliaa View Post
Hi Regal,
What is your basis for this conclusion? 4P1L can easily provide 100vpp swing and more, you can swing the anode up to 100vrms in class A1. Here is a real example for a 5K3 load as suggested by Rod. The operating point is really good anywhere between 230-250V:
Ale
Just being devils advocate before I invest in the iron. The load lines are obvious to me that the 2A3 is more linear swinging 200Vpp than the 4P1l.
But it could very well be acadimic. Just want to make sure we get a solid upgrade over a 6c45p1/C3g/D3A type spud SET (they are tough to beat with practically no caps in the signal path, very pure.)

Have you tried driving a 4P1L with another 4P1l into class A1 with a gyrator mu-follower/cap load? Honestly would prefer keeping this 2 tubes max (i.e. no cathode follower.)
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Old 2nd February 2013, 04:16 PM   #168
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Originally Posted by regal View Post
Thanks Rod, do you think a two stage 4p1L with the driver gyrator mu follower loded/cap cpoupled would have enough to drive the grid poitive for close to the 280V swing?
Driving the grid positive to +8 or 12V is needed for full power, and we should allow for 10-15mA of 1st-grid current to do it properly. This means dc-coupling, in effect, since this high quality design will not suffer to have a large-value cap.

I think that some kind of auto-biassing can be realised to make this simple - I will draw it up when I get a minute.
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Old 2nd February 2013, 06:22 PM   #169
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According to my experiments, 4П1Л in A1 triode mode with 250V B+ and 5K load is capable to produce 1.8 W of pristine clean output power. When driven in A2 up to +12V on G1 it is capable of 2.5 W of pristine clean power. I described the result of experiments in this topic: One more 4P1L SE
The final schematic can be used for headphone amps as well, with appropriate output transformer.
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Old 2nd February 2013, 06:27 PM   #170
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Here is schematic: if to use the transformer with 50 Ohm secondary you can drive 5W Grados. For 2W 32 Ohm you don't need to parallel output tubes, one is enough.

Click the image to open in full size.

6J5P and 6J9P are extremely linear and musical tubes. DHT in this positions give zero advantage.
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