Greetings friends. I'm putting together a headphone amp based on the 6N1P Headphone Amp which is in turn based on the Morgan Jones 6DJ8 Amp and I had a couple questions about proper usage.
As you can see, resistor R7 runs from output to ground. Does R7 dissipate the load if headphones are not connected? I've read it's best not to have headphones plugged in during power-up and shutdown. I have a switching jack; should additional resistance be added to increase the load when nothing is plugged in?
Any other OTL amp do's and don'ts?
I have HD600 but will do initial testing with AKG k240 (55ohm)
thanks for taking a look!
As you can see, resistor R7 runs from output to ground. Does R7 dissipate the load if headphones are not connected? I've read it's best not to have headphones plugged in during power-up and shutdown. I have a switching jack; should additional resistance be added to increase the load when nothing is plugged in?
Any other OTL amp do's and don'ts?
I have HD600 but will do initial testing with AKG k240 (55ohm)
thanks for taking a look!
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300 Ohm is probably more suitable for this amplifier.
I will however simulate with 55 Ohm output and see what happens.
22 kOhm resistor is probably enough.
I would use this transformer: https://www.toroidal-transformer.co...n242-power-transformer-for-tubeamps-30va.html
This give like 320 VDC as supply.
I simulate with 320V. I changed resistor R1 to 39 kOhm.
As you can see the gain is 21.0 with 300 Ohm load.
This is high. But there is not very much current to feed the load.
So a load of 55 Ohm might be too heavy. But you may experiment to see.
I will however simulate with 55 Ohm output and see what happens.
22 kOhm resistor is probably enough.
I would use this transformer: https://www.toroidal-transformer.co...n242-power-transformer-for-tubeamps-30va.html
This give like 320 VDC as supply.
I simulate with 320V. I changed resistor R1 to 39 kOhm.
As you can see the gain is 21.0 with 300 Ohm load.
This is high. But there is not very much current to feed the load.
So a load of 55 Ohm might be too heavy. But you may experiment to see.
+1 to what lineup said.
I think you should experiment with it as well.
My go headphones are also AKG, the K702 model.
I have built that same circuit, not quite completed yet because I got sidetracked with the circuit mentioned below.
A design by Menno Van Der Veen from 2006 which is a W.C.F. also un-powered for now (trying to get my Regulated P/S to work properly)
Also take a look here.
https://docs.google.com/spreadsheet...G9yZEJ-smI/edit?gid=1006269980#gid=1006269980
Contrary to what is often written and spoken, you really don't need much power with headphones.
The only exception is with the "I dare to be different " designs with crazy low impedances / SPL which thankfully are few and far between.
I think you should experiment with it as well.
My go headphones are also AKG, the K702 model.
I have built that same circuit, not quite completed yet because I got sidetracked with the circuit mentioned below.
A design by Menno Van Der Veen from 2006 which is a W.C.F. also un-powered for now (trying to get my Regulated P/S to work properly)
Also take a look here.
https://docs.google.com/spreadsheet...G9yZEJ-smI/edit?gid=1006269980#gid=1006269980
Contrary to what is often written and spoken, you really don't need much power with headphones.
The only exception is with the "I dare to be different " designs with crazy low impedances / SPL which thankfully are few and far between.
Thank you for taking a look. The HD600 would be the primary headphones for this amp, the AKG k240 would just be for testing purposes, but with the large difference in impedance maybe that's not a good idea.
My main question has to do with connecting the load to the output. Powering up a transformer-coupled amp with no load connected can quickly destroy the OPT - is there such a threat here? I'm worried about the Pop at power-up damaging the headphones, or a tube failure at power-up damaging them, so I'd rather connect the 'phones after a few minutes of warming up.
I already have a PT in hand, the Hammond 269AX. In fact, this thing is 95% built. Here's the power supply:
I'm no good at PSUD2 but I expect 320-350v from this supply.
I'll try elevated AC heaters at first and build a DC supply if hum is an issue, but I'm concerned that the 2A supply is insufficient to provide 1.8A of rectified power.
I've also considered a variant of this amp with 6N6P as outputs, powered by the Antek AS-05T280, 280v@90mA 6.3v@4A, but I don't know what I'd drive it with. Seems this is a nice amp for the 300ohm cans.
w
My main question has to do with connecting the load to the output. Powering up a transformer-coupled amp with no load connected can quickly destroy the OPT - is there such a threat here? I'm worried about the Pop at power-up damaging the headphones, or a tube failure at power-up damaging them, so I'd rather connect the 'phones after a few minutes of warming up.
I already have a PT in hand, the Hammond 269AX. In fact, this thing is 95% built. Here's the power supply:
I'm no good at PSUD2 but I expect 320-350v from this supply.
I'll try elevated AC heaters at first and build a DC supply if hum is an issue, but I'm concerned that the 2A supply is insufficient to provide 1.8A of rectified power.
I've also considered a variant of this amp with 6N6P as outputs, powered by the Antek AS-05T280, 280v@90mA 6.3v@4A, but I don't know what I'd drive it with. Seems this is a nice amp for the 300ohm cans.
w
My simulation with 320V supply tells that 55 Ohm headphones will be a very limited output.
300 Ohms is no problem.
I have simulated with 6H30 tubes at the output stage. = 40 mA in that stage.
And this works with 55 Ohm and of course also 300 Ohm.
Does 6N6P provide more current?
That is why you consider this. I am sure.
300 Ohms is no problem.
I have simulated with 6H30 tubes at the output stage. = 40 mA in that stage.
And this works with 55 Ohm and of course also 300 Ohm.
Does 6N6P provide more current?
That is why you consider this. I am sure.
Lower impedance cans require more current, that makes sense. In the original MJ Headphone Amp, further analysis found more drive power for low-impedance cans by optimizing the plate resistors for both tubes, and adding feedback. 6N6P outputs would provide enough current to drive low-impedance cans without feedback...
Here is my simulation with 6N1P input and 6N6P output.
The current is 52 mA in output. This will drive 55 Ohm easily.
The output level is 1.18 Vpp which correspondent to +10 dB.
Note that this can mean you have to change some resistors in reality.
SPICE models are not always correct.
But this will give a hint how your values should be.
EDIT: Spice simulation removed due to fantasy values.
The current is 52 mA in output. This will drive 55 Ohm easily.
The output level is 1.18 Vpp which correspondent to +10 dB.
Note that this can mean you have to change some resistors in reality.
SPICE models are not always correct.
But this will give a hint how your values should be.
EDIT: Spice simulation removed due to fantasy values.
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@lineup With around 215Vak and 52mA, you're putting about 2x the rated plate dissipation on the 6N6P, no? I wouldn't recommend doing that if you want tubes to last.
This is a White cathode follower stage, which operates push-pull, so you get about 2x the issue current as available output. Throw in a small amount of feedback to clean up the output for lower impedances like the optimized Morgan Jones amp published decades ago on Headwize.
This is a White cathode follower stage, which operates push-pull, so you get about 2x the issue current as available output. Throw in a small amount of feedback to clean up the output for lower impedances like the optimized Morgan Jones amp published decades ago on Headwize.
Check out the datasheet and pick an operating point that respects the dissipation limits. For headphones, running really high voltage probably doesn't offer a lot of advantage since they will only require a few volts before getting to deafening levels (aside from the rare inefficient high impedance models). Just find a nice linear portion of curve with about 2-4V cathode voltage. I'm not on my simulation machine right now, but I think I've played around with ECC99/6N6P WCF in the past with around 120Vak and 20-30mA idle current.
EDIT: Try a search for the optimized Morgan Jones and optimized White cathode follower. There are formulas for determining the optimum plate load resistor as well.
EDIT: Try a search for the optimized Morgan Jones and optimized White cathode follower. There are formulas for determining the optimum plate load resistor as well.
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Well, you have to do the design work somewhere, either with pencil and paper looking at the datasheets or in simulation. SPICE models of triodes can be quite decent, although the variance between individual tubes can make it seem unreliable. Build and see can be risky if making design guesses.
It is a bleeder resistor for the output capacitor, mostly. This will keep the DC voltage at the output limited while the capacitor is charging, but it may be risky to leave headphones plugged in during power up and power down. Some OTL can develop tens of volts at the output during power on/off, so it would always be safer to plug in headphones after pet had stabilized and remove them prior to shutdown. This can be achieved with a relay as well.
The problem with valve powered headphone amp without an output transformer is the lack of galvanic isolation from lethal high-voltages. Some fool proof protection circuitry to prevent lethal voltages appearing on the headphone cable is surely obligatory? Otherwise you could be one shorted capacitor away from a very nasty experience.
Output caps are rated to 450v but ya, fuses inline with the diaphragms do have a certain appeal....
Fuses won't prevent an electric shock - the heart stops well before a fuse blows. Its an unlikely scenario true, but headphone cables can fray and rub etc, you don't want to enter Darwin Award territory... Capacitors can fail, and guess what, 50 or 60Hz is in the audio band so they will let any mains ripple straight through unattenuated. Better to clamp the output voltage to ground +/- some low voltage. Stay safe...