Greetings Friends. I've been thinking about OTL amps for headphones a lot lately, inspired by the MJ/6N1P post a few weeks back. Most of the posts and articles I've found seem to focus on optimizing the amp to drive low impedance headphones, but I have a pair of HD 600 so I want to build an OTL amp to drive Hi-Z cans.
The 3x-dual-triode topology seems an ideal place to start, with each channel using half the first tube as driver and then both halves of a full tube wired as a White cathode follower, or SRPP, then capacitor-coupled to the output. Tube choices include 6DJ8 and the Soviet 6N1P, 6N6P, and 6N23P. Initial schematic:
One thing I've already run across, the large output coupling cap is required to deliver flat bass response to Lo-Z cans, but with a higher Z, I can get away with a much lower value, perhaps 60uF. That allows the use of film caps, such as this 60uF 400v from WIMA, $15, so 4x the price of a comparable EL cap. Is a DC-LINK cap suitable as an output cap? Is it worth the $?
How do I reckon the output impedance of such an amp? Depends on the tube, I should think. Cans are 300ohm, 1/8 of that is 37.5, so output Z should be >40ohm(?)
Does the output Z affect any other parts of the circuit? Bypass caps, for instance?
Related, there was a headphone amp put out by Schiit called the Volkvangr, it had no output transformers or caps, just one 6N1P and 4x 6N6P per channel. How would such a thing output power?
Thanks for taking a look!
will
The 3x-dual-triode topology seems an ideal place to start, with each channel using half the first tube as driver and then both halves of a full tube wired as a White cathode follower, or SRPP, then capacitor-coupled to the output. Tube choices include 6DJ8 and the Soviet 6N1P, 6N6P, and 6N23P. Initial schematic:
One thing I've already run across, the large output coupling cap is required to deliver flat bass response to Lo-Z cans, but with a higher Z, I can get away with a much lower value, perhaps 60uF. That allows the use of film caps, such as this 60uF 400v from WIMA, $15, so 4x the price of a comparable EL cap. Is a DC-LINK cap suitable as an output cap? Is it worth the $?
How do I reckon the output impedance of such an amp? Depends on the tube, I should think. Cans are 300ohm, 1/8 of that is 37.5, so output Z should be >40ohm(?)
Does the output Z affect any other parts of the circuit? Bypass caps, for instance?
Related, there was a headphone amp put out by Schiit called the Volkvangr, it had no output transformers or caps, just one 6N1P and 4x 6N6P per channel. How would such a thing output power?
Thanks for taking a look!
will
O hey that's another thing! I thought servos were little motors that make the model airplane flaps go up n down. What kinda servos are you talking about?
Servo -> Serve.
There are lots of little things in engineering that do some given function in order to serve your benefit. Motor servos, hydraulic servos, and electrical servos.
In this case the servo in question is a circuit that measures the output impedance of the circuit and makes some adjustment somewhere else. For the schiit amp it would measure the output voltage and compare it to ground. If the output voltage of the amp is 0 DC voltage, then everything is fine. You can plug your headphones directly into that circuit without any issue.
If the circuit starts to go above or below ground, the servo (opamp in this case) will tweak it's output voltage in order to adjust the output back down to 0 volts.
Here is a website that explains what a circuit servo can do and how you can design one yourself. https://www.tubecad.com/2017/09/blog0397.htm
1. An electronic servo, or an electronic servo plus a mechanical device, almost always use some sort of negative feedback to create a servo loop.
Sometimes a two Selsin [Selsyn] motor loop is called a servo loop.
There are many characteristics of most of those servo loops:
Closed loop
Overshoot
Undershoot
Hysteresis
Lag time
[Sometimes] Oscillation
Anything that makes it go Open Loop
If things go wrong, there can be all kinds of results.
2. Many people never thought about it this way . . .
Some of the earliest servo loops are Biological:
Insulin loop
Breathing rate loop
Heart rate loop
Compound loops, such as driving around the corner without crashing.
When these biological loops go bad (open loop, overshoot, undershoot, or oscillation) it often is a reason to call 911.
$0.03
Sometimes a two Selsin [Selsyn] motor loop is called a servo loop.
There are many characteristics of most of those servo loops:
Closed loop
Overshoot
Undershoot
Hysteresis
Lag time
[Sometimes] Oscillation
Anything that makes it go Open Loop
If things go wrong, there can be all kinds of results.
2. Many people never thought about it this way . . .
Some of the earliest servo loops are Biological:
Insulin loop
Breathing rate loop
Heart rate loop
Compound loops, such as driving around the corner without crashing.
When these biological loops go bad (open loop, overshoot, undershoot, or oscillation) it often is a reason to call 911.
$0.03
Selection of the output cap is critical as a LP filter is created between C4/C5 and R9(?) but too low a value can also increase LF distortion? oy vey.
If both triodes are identical and the lower cathode is bypassed, output Z can be estimated as:
ra2/([mu+2]*ra + mu2R4)
Where ra is the internal anode resistance of the tube. However, it is rather difficult to get an accurate figure for the WCF on paper. In reality, with a 6DJ8 it's likely to be somewhere around 40 ohms, ish
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