[DjBobby]

  When I first got the Little Dot MkII, I measured the DC offset on startup and noted that it was pretty high, hence the warning not to use it as a preamp for a DC coupled power amp. The DC current will be amplified by the power amp and blow out speakers/headphones. The preamp output comes off the headphone out directly.  I finally read the manual carefully and it said to plug in the headphone before turning the amp on.  Not sure if that's a good idea.  I made this little video to show why.
 

 
Without a load, it takes even longer to stabilize.  I would wait for at least 5 minutes after turning on the amp before plugging in headphones.

Thanks for the measurements. Still not understanding why would David ZheZhe insist on plugging the headphones before turning on, to protect the amp or the tubes?
Btw. did you have a chance to measure the output impedance? 

 

[drteming]

The DC voltage without a load is bonkers crazy. It went over 100V! Even when it settles down, it fluctuates quite a bit going from a couple of mV's up to around 50mV so I can't really get a measurement to calculate the output impedance.

I think having a load turning on and off the amp protects the amp, but I don't feel like sacrificing expensive headphones. Hmm, that gave me an idea. Will pop down to Radio Shack later today.

 

[drteming]

The DC voltage without a load is bonkers crazy. It went over 100V! Even when it settles down, it fluctuates quite a bit going from a couple of mV's up to around 50mV so I can't really get a measurement to calculate the output impedance.

Duh! That was stupid. Will measure the AC voltage with a 60Hz test tone.

 

[drteming]

Well, I actually measured the AC voltages with a 120 Hz test tone with the attenuator set at half-way.  The left channel measured 0.187 V under load and 0.574 V open.  The right channel measured 0.207 V under load and 0.605 V open.  So Z for the left was 68.3 ohm and the right was 63.4 ohm.  The load was a pair 33.2 ohm Vishay-Dale RN60 resistors, with measured resistance of 33.1 ohm.
 
With the 33.1 ohm load, the startup voltage spike went to about 120 mV and then settled down, so it's safer if you have a low impedance headphone to leave it plugged in, though not ideal.
 
For fun, I measured the voltage with a 2.2 ohm load, and it never went above 5 mV.

 

[DjBobby]

  Well, I actually measured the AC voltages with a 120 Hz test tone with the attenuator set at half-way.  The left channel measured 0.187 V under load and 0.574 V open.  The right channel measured 0.207 V under load and 0.605 V open.  So Z for the left was 68.3 ohm and the right was 63.4 ohm.  The load was a pair 33.2 ohm Vishay-Dale RN60 resistors, with measured resistance of 33.1 ohm.
 
With the 33.1 ohm load, the startup voltage spike went to about 120 mV and then settled down, so it's safer if you have a low impedance headphone to leave it plugged in, though not ideal.
 
For fun, I measured the voltage with a 2.2 ohm load, and it never went above 5 mV.

Thanks a lot for this. So it seems it has quite highish Z and secondly it's not so nice to have HD650 all the time plugged in....

 

[drteming]

A switched 1/4" stereo socket and 2 33.2 ohm resistors, and we got a Little Dot MkII/headphone protector.
 

 
 
Turn on the amp with this plugged in and wait for a minute or so before plug in the headphone. Protects the amplifier by giving it a load at startup, and protects the headphone by keeping it away from the voltage spike.

 

[TrollDragon]

  Without a load, it takes even longer to stabilize.  I would wait for at least 5 minutes after turning on the amp before plugging in headphones.

So you have the resistor bridging the two channels???
Didn't see the YT description in the inline video...

With a proper headphone load I get none of those results at all... This is on the MK IV, which has very similar circuitry to the MK II.

Possibly an issue with your amplifier, is it from MassDrop as well?

 

[drteming]

[quote name="TrollDragon" url="/t/284835/review-of-the-little-dot-mkii-tube-headphone-amp-pre-amp/495#post_13080859"[quote="trolldragon, post: 13080859, member: 175867"]
With a proper headphone load I get none of those results at all... This is on the MK IV, which has very similar circuitry to the MK II.


Possibly an issue with your amplifier, is it from MassDrop as well?
[/quote]


It's a MassDrop amp. I've replaced all the resistors and film caps though. I don't think it's the amp because it did the same thing before I replaced the guts. Also, the warning about against using it as a preamp for a DC coupled amp makes sense for this behavior.

 

[TrollDragon]

It's a MassDrop amp. I've replaced all the resistors and film caps though. I don't think it's the amp because it did the same thing before I replaced the guts. Also, the warning about against using it as a preamp for a DC coupled amp makes sense for this behavior.

With the LD being OTL, there are no transformers to stop B+ from heading out the LO jack with a tube short failure, that is why they don't recommend connecting to DC amps.
It would destroy the amp speakers and all.

The B+ would could do a really fine job on your headphones, since the Headphone Out and Line Out are in parallel.

 

[drteming]

With the LD being OTL, there are no transformers to stop B+ from heading out the LO jack with a tube short failure, that is why they don't recommend connecting to DC amps.

It would destroy the amp speakers and all.


The B+ would could do a really fine job on your headphones, since the Headphone Out and Line Out are in parallel.

I wonder if it's the output caps. The MkIV has a pair of 330 uF caps compared to the 220 uF in the MkII. I had ordered a pair of 330 uF and a pair of 470 uF caps late last week so I should be getting them in a few days. We'll see what happens after I switch them out.

 

[drteming]

The 470 uF caps were too tall to fit in the case, but the 330's fit just fine.  I got the same series as the original 220 uF caps: Nichicon CS.  With the 33.1 ohm load, there was still the same DC voltage spike on startup, so my little box will stay attached to the Little Dot.  The upside is that with my Fostex TH-500RP (48 ohm), there is noticeably better bass extension than before.  With the DT990's (600 ohm), there's no difference.
 
My understanding is that when tubes short out, it is usually on startup, so it may not be a bad idea for folks with OTL amps to build a dummy load like my little box to protect their headphones.

 

[serman005]

I am ramping back and briefly hijacking the conversation several levels--with apologies. I am wondering if I can use an ipod Classic with the Little Dot? Not sure about the connections there. I'll be using mostly HD600, HE400S, and HD598. Thank you very much!

 

[DjBobby]

  I am ramping back and briefly hijacking the conversation several levels--with apologies. I am wondering if I can use an ipod Classic with the Little Dot? Not sure about the connections there. I'll be using mostly HD600, HE400S, and HD598. Thank you very much!

No problem, you just need one 3,5mm to stereo RCA cable. For the better quality you could use FiiO L9, bypassing iPod's internal amp, and on the other side female 3,5mm to stereo RCA.

 

[OctavianH]

Which DACs are you using with the Littledot? I paired at the moment with iDAC2 and the sound is decent, but I wait for some new tubes to improve the sound.
 

 

[YungFrieza]

How do these pair with stock HD-800's? I just bought them and don't really have an amp for them. Thanks!