The high position is just to store the jumpers, it seems. It looks like 124.3 Ohm to ground with or without the jumper.
So you get 1:121 in low, 4.3:124.3 in mid, and 124.3:243.3 in the high setting. It's nice that the 15 Ohm resistor to ground in parallel to the headphones is already 3 W. I remember I had to beef up the resistors in my 8200R after turning up the volume too much in the headphone position.
So you get 1:121 in low, 4.3:124.3 in mid, and 124.3:243.3 in the high setting. It's nice that the 15 Ohm resistor to ground in parallel to the headphones is already 3 W. I remember I had to beef up the resistors in my 8200R after turning up the volume too much in the headphone position.
The TU-8200R had (has) four 3.3 Ohm 1/2 W resistors to ground, so just 2W total. The jumpers connect the headphones parallel to one, two, or all four of them, with 220 Ohm always in series. So for the TU-8400 the series resistance is lower, and then there is the selectable resistor combination in parallel to the headphones, forming a voltage divider with the 120 Ohm series resistor.
For 600 Ohm headphones the 8200R gives you a choice of 3/4 times 1/4, 1/2 or the full output amplitude, whereas the 8400 offers roughly 1/2, 1/40th or 1/120th of the full output, if I got it right. So the maximum output is similar, but the steps are quite different.
For 30 Ohm headphones the 8200R steps are 1/28, 1/14 or 1/7th, whereas the 8400 has 1/6th, 1/30th or 1/120th; again similar maximum output, but the 8400 allows lower limits.
Due to the 220 Ohm being always in series after the divider, the 8200R has a pretty high output impedance, which can't be good for the damping factor. The 8400 at the low and medium setting has much lower output impedance, due to the 1 Ohm respectively 4.3 Ohm in parallel to the headphones.
The 8400 does not have a selector switch for the speaker output impedance?
For 600 Ohm headphones the 8200R gives you a choice of 3/4 times 1/4, 1/2 or the full output amplitude, whereas the 8400 offers roughly 1/2, 1/40th or 1/120th of the full output, if I got it right. So the maximum output is similar, but the steps are quite different.
For 30 Ohm headphones the 8200R steps are 1/28, 1/14 or 1/7th, whereas the 8400 has 1/6th, 1/30th or 1/120th; again similar maximum output, but the 8400 allows lower limits.
Due to the 220 Ohm being always in series after the divider, the 8200R has a pretty high output impedance, which can't be good for the damping factor. The 8400 at the low and medium setting has much lower output impedance, due to the 1 Ohm respectively 4.3 Ohm in parallel to the headphones.
The 8400 does not have a selector switch for the speaker output impedance?
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Hi Martin, not really, as far as I am concerned. I have used it a few times with my (high impedance) Sennheiser 6xx (at least I think that is what they are called), and it worked fine, but I'm not using it with headphones on a regular basis. I'd think the lower impedance of the TU-8400 is better.
To me these amps seem a bit bulky as headphone amps, although opinions might differ on that, and running them at the very low output power levels needed for headphones, you might get more noise and less of the sonic benefits (2nd harmonic addition) ascribed to this type (single ended) of tube amp.
Here is my old distortion measurement for my TU-8200R with Lundahl output transformers. The lowest data point is 0.1W into 8 Ohm, and as you can see 2nd harmonics are at the permille level (just above -60 dB) in triode and ultra-linear mode:
To me these amps seem a bit bulky as headphone amps, although opinions might differ on that, and running them at the very low output power levels needed for headphones, you might get more noise and less of the sonic benefits (2nd harmonic addition) ascribed to this type (single ended) of tube amp.
Here is my old distortion measurement for my TU-8200R with Lundahl output transformers. The lowest data point is 0.1W into 8 Ohm, and as you can see 2nd harmonics are at the permille level (just above -60 dB) in triode and ultra-linear mode:
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With the 8400, to get 1 mW into a 300 Ohm headphones (which is an ear-splitting 103 dB for my Sennheisers), you need 0.55 V at the headphone output. In the high setting, the headphones are parallel to 124.3 Ohm, so the 0.55 V is over 87 Ohm, and the full voltage over the 120 Ohm plus headphone, and thus also over the 15 Ohm resistor, would be 1.33 V. That would give 0.12 W into the 15 Ohm.
At the medium setting the headphones are parallel to 4.3 Ohm, so 0.55 V over 4.2 Ohm, so you'd need 16 V over the 15 Ohm resistor, which is 18 W, more than the amp can put out and also way too much for that 3 W resistor. The most power you could get into 300 Ohm headphones, while burning 3 W in the 15 Ohm resistor, in the medium setting is 0.16 mW. That might actually be still too loud for listening, which is good, since you would be at the maximum power for those resistors. So, using the medium setting for high impedance headphones might actually work well, and put the amp into a nice power and distortion region. Probably can't hurt to check my math, though.
At the medium setting the headphones are parallel to 4.3 Ohm, so 0.55 V over 4.2 Ohm, so you'd need 16 V over the 15 Ohm resistor, which is 18 W, more than the amp can put out and also way too much for that 3 W resistor. The most power you could get into 300 Ohm headphones, while burning 3 W in the 15 Ohm resistor, in the medium setting is 0.16 mW. That might actually be still too loud for listening, which is good, since you would be at the maximum power for those resistors. So, using the medium setting for high impedance headphones might actually work well, and put the amp into a nice power and distortion region. Probably can't hurt to check my math, though.
Ok, I promise this will be the last set of numbers...
Actually, the low setting might work well for reasonable volume with a high impedance headphone:
If you want say 83 dB instead of 103 dB from 103 db/mW headphones, you'd need 1/100th the power and thus 1/10th the voltage. So say 50 mV over the headphones and the 1 Ohm resistor, that makes 6 V over the 120 Ohm and the 15 Ohm resistors, which corresponds to 2.4 W.
Actually, the low setting might work well for reasonable volume with a high impedance headphone:
If you want say 83 dB instead of 103 dB from 103 db/mW headphones, you'd need 1/100th the power and thus 1/10th the voltage. So say 50 mV over the headphones and the 1 Ohm resistor, that makes 6 V over the 120 Ohm and the 15 Ohm resistors, which corresponds to 2.4 W.