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-   -   Class A, DHT driven, headphone amp (http://www.diyaudio.com/forums/headphone-systems/146788-class-dht-driven-headphone-amp.html)

iko 1st July 2009 04:49 AM

Class A, DHT driven, headphone amp
 
I heard so much about directly heated triodes that I finally decided to build a DHT based headphone amp. There are not that many schematics for such a thing out there, especially for my requirements:
* low cost; most output transformers are quite expensive, so I'd like to avoid them if possible
* relatively simple to build
* not use exotic tubes
* not use dual rail psu for mosfet (or maybe, just to get rid of the output cap)
* direct couple between DHT driver and output stage

I've been thinking that something like a 3a5, or some other DHT as the driver and a mosfet output stage could work, but have no idea if it's a disaster or not, I have no experience with such things. Should I just save for some output transformer and have a 71a output stage?

For now, here's a rough sketch. The topology is simple enough, tube driver with ccs load, directly coupled to a mosfet output stage with a current sink below it.

In any case, what do you guys think of this circuit, anything that outright stinks?

(I loath that 100uF C2. How can I get rid of it?)

Code:

Harmonic        Frequency        Fourier        Normalized        Phase          Normalized
 Number          [Hz]          Component        Component        [degree]        Phase [deg]
    1          1.000e+03        2.416e+00        1.000e+00            0.41°            0.00°
    2          2.000e+03        1.758e-03        7.276e-04          -45.57°          -45.98°
    3          3.000e+03        3.010e-04        1.246e-04          57.20°          56.80°
    4          4.000e+03        4.995e-05        2.068e-05          159.87°          159.46°
    5          5.000e+03        9.582e-06        3.966e-06        -113.65°        -114.06°
    6          6.000e+03        3.279e-06        1.357e-06          -62.77°          -63.18°
    7          7.000e+03        2.551e-06        1.056e-06          126.37°          125.96°
    8          8.000e+03        2.099e-06        8.687e-07          -20.93°          -21.33°
    9          9.000e+03        3.580e-06        1.482e-06          -91.21°          -91.62°
Total Harmonic Distortion: 0.073845%



http://www.cs.toronto.edu/~relu/audi...p-3a5mos-6.png

http://www.cs.toronto.edu/~relu/audi...mos-6-freq.png

rman 1st July 2009 05:05 AM

Hello.

Quote:

(I loath that 100uF C2. How can I get rid of it?)
No big expert here but I believe that to drive a low impedance load like a pair of headphones, you either need an output transformer, or a big output capacitor. If I am wrong someone please correct me.

Cheers.

arnoldc 1st July 2009 06:26 AM

Sorry, but by looking at it you could find more answers at the solid state section?

I would personally would want that 71a OPT output option ;) but with an amplification factor of only 3, you will need a driver for it...

iko 1st July 2009 06:37 AM

Hm, is there a way to move it to the solid state section?

Gordy 1st July 2009 10:56 AM

Some quick observations:

A tube amplifier stage with a solid state follower output stage is a fine idea.

The so-called ‘magic’ of Directly Heated Triodes (in my opinion) is simply down to their linearity, so make sure you operate it in a linear section of it’s characteristics.

I am no expert in solid state ‘discrete’ circuits so will leave the analysis to others, however if I were on your path I would do it like this:

DHTriode voltage amp with CCS load
DC cathode (despite the theoretical limitation of voltage gradient across the cathode)
Capacitor coupled to next stage (yup, it’s simple and it works, and gets rid of your M4 FET)
Follower based on something simple such as Project 83 at ESP …http://sound.westhost.com/project83.htm (but with suitably adjusted power).

Don't worry too much about the output cap for now.

The gain requirement of the triode will be determined by the requirements of your ‘phones (impedance and sensitivity). In general head for quality not quantity. For most ‘phones a continuous 60-ish mW is more than enough, so if you can peak at 1/4 Watt then all is well.

: )

Miniwatt 1st July 2009 12:26 PM

Quote:

Originally posted by rman
a big output capacitor.

See C2: 100 uF ;)

iko 1st July 2009 04:12 PM

Gordy, thanks! In fact I came across this ESP bit some time ago and totally forgot about it in the meanwhile :blush:

You know, I had tried the schematic above with RC coupling but got poor frequency response unless I used a large cap, hence the direct coupling. I might actually try both versions, with the mosfet, and with the cap (I just want to refrain from having to buy fancy caps).

As for headphones, I have the run of the mill Sony MDR-V6, claiming 106 dB/mW, 63 ohm at 1kHz. Lack of lots of power (read loud) is not something I worry too much about.

You say I shouldn't worry too much about the output cap, but considering its size, it would have to be an electrolytic. A couple of "better" output caps will probably run at prices similar to some output transformers. The all of a sudden a power triode + transformer output doesn't look so unappealing, considering that it does away with the extra psu needed for the mosfet output (just thinking aloud here).

rman, I was thinking perhaps a DC servo might work, as wavebourne has done (somewhere else on this forum).

bigwill 1st July 2009 04:40 PM

IMO the coupling capacitor isn't a problem

dsavitsk 1st July 2009 05:33 PM

Quote:

Originally posted by Gordy
Don't worry too much about the output cap for now.

Quote:

Originally posted by bigwill
IMO the coupling capacitor isn't a problem
The coupling cap is going to undo any goodness done by the DHT. It is the bottleneck of the amp.

Quote:

Originally posted by Gordy
The gain requirement of the triode will be determined by the requirements of your ‘phones (impedance and sensitivity). In general head for quality not quantity. For most ‘phones a continuous 60-ish mW is more than enough, so if you can peak at 1/4 Watt then all is well.
1/4 watt is way more than you'll likely need.

For using a DHT for low impedance phones, have a look here: http://www.ecp.cc/less-pressivo-plus-plus.html

astouffer 1st July 2009 06:46 PM

You could always use a DHT for gain and a buffer chip like a BUF634 or HA-5002.


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