Review and Measurements of DarkVoice 336SE Headphone Amp

[solderdude]

Can't wait to see the teardown of how they put this thing together

a preview of what to expect.

The schematic will be similar to this one but with other output cap values and R7 being 1k:

 

[Schackmannen]

I got this amp a year ago to use with my HD6XX and it worked well for a while but then i started hearing an audible hum from it even while nothing was playing. Tapping the tubes could sometimes make it go away for a while but it wasn't a permanent solution. Bought new tubes for it since people said the stock ones were crap. One of the new tubes (i think the big one) did help remove the hum a bit but the new smaller tube just made it a lot worse with the amp making loud pops and stuff like that. Another thing i had a problem with was that turning the volume knob past 12 o clock did not increase output power at all.

I think i read somewhere that someone did a teardown of theirs and that a lot of things inside were squished or otherwise looked faulty so I've been wanting to do the same to mine since i suspect that mine is the same, but im not sure how safe it is. @amirm or anyone else that knows, if the unit has been on the shelf for a long period of time without having any power connected, is it safe to open up? Or could there be anything in there that might still hold a charge for a long time and possibly give me a shock?

 

[solderdude]

Another thing i had a problem with was that turning the volume knob past 12 o clock did not increase output power at all.

I have seen a lot of the cheapy ebay hybrid amps use a linear potmeter as a volume control.
This gives the exact similar effect of having very little changes between 12 o'clock and max setting.
One would expect a log type pot for these amps though.

if the unit has been on the shelf for a long period of time without having any power connected, is it safe to open up? Or could there be anything in there that might still hold a charge for a long time and possibly give me a shock?

When you leave it off for a extended period (days) there MAY still be some charge on the big capacitors.
Difficult to spot any so called 'bleeder resistors' that some designs have to safely self discharge the circuit slowly after it has been switched off.
This would cost a few cents tops yet I rarely see it applied.
You can measure the remaining voltage with a cheapy ($ 10) multimeter across the terminals in 200VDC setting on the big capacitors in the rear (on the small PCB).
Safe to open up and have a look though, this means not touching anything...

When one has to work on it one must discharge the big capacitors by shorting the big caps with a 8 Ohm resistor or so.
Don't short them with a wire, certainly not when it was just switched off.

 

[SIY]

So the "designer" used a 6080 as a straight cathode follower, then compounded the issue with a 1k load resistor. The gain of the first stage is not used to reduce the distortion, either. Capacitive coupling guarantees high output impedance at low frequencies. There's also some questionable omissions of safety parts, and both the measurements and the user reports suggest that grounding was not done properly.

A lot of small changes could have improved this unit greatly without a huge cost impact. But that's engineering, not "design."

 

[Jkdjedi]

Shame you didn't test this with a burned in tube, new tubes need to be "baked" in to get rid of the hum, I pull out the power tube and leave the preamp tube connected, leave it on for 48 hours, after that... no hum. And the RAytheon is a very good tube for the monies, ($7) This Tube amplifier was designed for high impedance headphones, that is it. However, using a 5998 power tube is been said to lower the impedance output to play nice with lower impedance cans. You should take a couple of weeks with this the Darkvoice and give it a chance, I think you'll be blow away.

 

[Dogen]

Shame you didn't test this with a burned in tube, new tubes need to be "baked" in to get rid of the hum, I pull out the power tube and leave the preamp tube connected, leave it on for 48 hours, after that... no hum. And the RAytheon is a very good tube for the monies, ($7) This Tube amplifier was designed for high impedance headphones, that is it. However, using a 5998 power tube is been said to lower the impedance output to play nice with lower impedance cans. You should take a couple of weeks with this the Darkvoice and give it a chance, I think you'll be blow away.

I haven’t tried that method yet; what’s happening physically to 6SN7 the tube while it’s “burning in”? I’m reluctant to try this on one of my more expensive tubes. Is it really just prematurely aging the tube?
And how is it that hum is coming through with the volume at zero? That I can’t understand.

 

[Dogen]

So the "designer" used a 6080 as a straight cathode follower, then compounded the issue with a 1k load resistor. The gain of the first stage is not used to reduce the distortion, either. Capacitive coupling guarantees high output impedance at low frequencies. There's also some questionable omissions of safety parts, and both the measurements and the user reports suggest that grounding was not done properly.

A lot of small changes could have improved this unit greatly without a huge cost impact. But that's engineering, not "design."

I’d be interested in exactly what small changes a good engineer might make to the schematic. I’d be willing to try some things out.

 

[SIY]

I’d be interested in exactly what small changes a good engineer might make to the schematic. I’d be willing to try some things out.

On the road today and tomorrow, I’ll try to fill in some details later this week. But certainly a servo for the output, feedback to the first stage, and a safety resistor from the volume control wiper to ground will be included.

 

[SIY]

I haven’t tried that method yet; what’s happening physically to 6SN7 the tube while it’s “burning in”? I’m reluctant to try this on one of my more expensive tubes. Is it really just prematurely aging the tube?
And how is it that hum is coming through with the volume at zero? That I can’t understand.

The first few hours a tube is run, the getter heats and absorbs some of the residual gas. This can be more significant in tubes that have been stored for a while. Morgan Jones described an oven method to accomplish the same thing in his excellent book, Valve Amplifiers.

 

[SIY]

Oh, and speaking of that book, there’s a superb if somewhat complex tube headphone amp design in it. That’s the way it should be done, ideally.

 

[westyjeff]

@amirm Let's set up a date to get my Cayin HA-300 to you. You have tested some lower end tube headphone amplifiers, lets see what a top tier product will do.

 

[Dogen]

The first few hours a tube is run, the getter heats and absorbs some of the residual gas. This can be more significant in tubes that have been stored for a while. Morgan Jones described an oven method to accomplish the same thing in his excellent book, Valve Amplifiers.

Thanks to your earlier mention, I picked up that book. Just getting started with it - exactly what I was looking for.

 

[RayDunzl]

And how is it that hum is coming through with the volume at zero? That I can’t understand.

Generally speaking, the volume knob on an analog amplifier attenuates the voltage of the input signal. It doesn't modify the output.

The rest of the amplifier circuitry hums along unmolested by the volume control, ready to amplify whatever it is fed (or otherwise picks up).

 

[gvl]

One of the new tubes (i think the big one) did help remove the hum a bit but the new smaller tube just made it a lot worse with the amp making loud pops and stuff like that.

Clicks and pops are most likely because of the crappy tube sockets loosening with time, it's worse on some tubes vs others. You need to bend the springs in each hole toward the center a bit with a small screwdriver or some other pointy tool. Make sure the caps are discharged before attempting this. Deoxit on tube pins helps too temporarily. Speaking from experience.

 

[304290]

I

Sounds like it is about 10 years old... while things have changed in the last 10 years I'm not entirely sure that the market was "dominated" by 300 ohm headphones 10 years ago...

I didn’t specifically say 10 years ago. This amp is actually much older than that and yes when it first came on the scene, mainstream headphones were senn hd580, 600, 650 and beyer dt770, 880, 990 and the likes. This was before the days of planers. And yes there were plenty of low impedance cans. But the higher impedance ones were more popular.

 

[gvl]

What can be better than a Darkvoice headphone amp? 2 Darkvoice amps!

 

[solderdude]

needs moar toobzzz

 

[amirm]

Or even more:

 

[solderdude]

I have this crackle ... lemme check which tube it is ...

 

[RayDunzl]