[money4me247]

   
The issue with the Bravo amps and high sensitivity cans is that it has a ton of gain (+30dB), where the gain on the LD 1+ is both lower and adjustable.  With high sensitivity and a fixed line level input, you'll barely have any adjustment in the volume pot on the Bravo.
 
  -Mike

thanks for your quick response. besides what you've said about the volume, are there any other potential issues or inadequacies based on the specs? I am just primarily worried about sound quality.
 
I've also read somewhere that you should have the output impedance of the amp 1/8 of the headphones, but from my experience a 4ohm impedance headphone amp must be truly rare as I haven't found anything like that.

 

[HOWIE13]

Isn't 92.5dB relatively low sensitivity for these types of headphones?

 

[mhamel]

  @ UmustBKidn: 
Fascinating and very helpful reply. The Audiotubes website has lots of information. It's all very bewildering though, especially when there are different ways of assessing tubes, different machines measuring different things, and on top of all that there is the question of whether the machine is up to date with its calibration.
I guess in the days when tubes were ubiquitous as an electronic component people were used to all these measurements. Today, it seems to me that the machines may be useful if well calibrated and interpreted correctly, but could also be grossly misleading. I think I will go for what looks like 'reasonable value' tubes and keep my fingers crossed that they will sound OK.

 
I buy and sell some tubes, and before I really got into it, I invested in a good tester.   After looking at the options for vintage testers, the costs/challenges to get them rebuilt and keep them properly calibrated, I went with a modern digital tester.  http://www.maximatcher.com/maxipreamp.html   There are others out there at lower and higher price points as well.   There are some eBay sellers that are very good about testing/matching, but there are others that I also think are full of crap.   I've seen where someone posts photos of the test results of the tubes he/she sells, and they use the same photo in every ad.   Every tube shows exactly the same result?  Not a chance.    
 
That's not to say that vintage testers aren't good.  For someone with the knowledge, time and experience to keep them in top shape, they can be very accurate.  In addition, sellers like Brent Jesse have curve tracers that let them plot the response curves of the tube and compare it to the manufacturer's data sheet, giving an even more accurate result on how the tube performs compared to spec.   There's a lot of time involved, that's a big part of why he's expensive.
 
When I sell a tube, I test it after a brief warm up, then again after at least an hour or two to see how it has stabilized.  I record the results, then listen to it through both my tester, which is ultra-sensitive to any sort of tube noise, and within an amp.  Finally, it gets tested again after being fully warmed up to make sure the results are consistent.  Every tube gets a label with the exact results that come out of the tester, and I'm happy to explain them, how those results compare to the published data sheets, etc.
 
With the Bravo amps, since there's only a single tube involved, it's easier than having to find matched pairs or quads, but you do want to make sure the tube's triode sections are as well balanced as you can find.  It's also helpful to look for a tube that's been tested for gain between the sections as well.
 
   -Mike

 

[HOWIE13]

@Mike
Thanks for the advice about the Maximatcher. I'll look into that. It would be nice to think that all sellers are as thorough as you are!

 

[MaN227]

its interesting how long these have held interest

 

[mhamel]

  @Mike
Thanks for the advice about the Maximatcher. I'll look into that. It would be nice to think that all sellers are as thorough as you are!

 
Howie,
 
If you'd like a decent modern tester without completely breaking the bank, check out the Tube Imp.
 
    -Mike

 

[HOWIE13]

@Mike.
Brilliant-it's ordered!
Thanks again.

 

[Evshrug]

Never heard of them. I tend to avoid anything with "cryo" in it - that's more snake oil IMO.

Not personally tried, but here's another vendor: tubedepot.com. They are insanely expensive, $299 for a Mullard 12AU7. You're better off taking your chances on ebay lol.

Another vendor, not personally tried, but cheaper than the above: www.vacuumtubes.com. His prices seem a bit more in line with reality.

More tube sources is always a boon.

I'm not 100% sure cryo treating provides a benefit (although superconductors usually need to be kept extremely cold before they start to super-conduct, so it seems it MAY have some legitimacy though I suspect the molecular "alignment" gets scattered a bit more every time the tube heats up, that's basic physics), but I figure they're at least decent samples of tubes.

The issue with the Bravo amps and high sensitivity cans is that it has a ton of gain (+30dB), where the gain on the LD 1+ is both lower and adjustable.  With high sensitivity and a fixed line level input, you'll barely have any adjustment in the volume pot on the Bravo.

  -Mike

2x What Mike said.

To elaborate:
The bravo amps are just too loud for many sensitive headphones, you have to keep the volume turned down really really low to keep from damaging your hearing. When potentiometers (volume knobs) are turned down that much, they create a channel imbalance which will either sound like one earcup is louder OR just off and unnatural (good manufacturers go to great lengths to match the drivers in their top headphones). The other problem, LOUD enough doesn't always equal GOOD. Headphones need a good supply of current to have the dynamic range to supply high-energy notes like low bass and complex music passages with lots going on... It tightens up music and allows them to play smoothly. I hate the song, but Kanye West's "Gold Digger" percussion is a great example of (poor mastering and) a sound that will easily distort if the headphone drivers can't handle the excursion or if they don't get enough current to prevent clipping.

thanks for your quick response. besides what you've said about the volume, are there any other potential issues or inadequacies based on the specs? I am just primarily worried about sound quality.

I've also read somewhere that you should have the output impedance of the amp 1/8 of the headphones, but from my experience a 4ohm impedance headphone amp must be truly rare as I haven't found anything like that.

There are many factors influencing that, but to look at it that simply is a myth. What do you think even happens when you don't have a lot of headphone impedance to dampen the amp output impedance?

Exacerbated by that designer of the O2, the real situation where having good headphone:amp dampening is important is when the headphone doesn't have a flat impedance curve... Check Inner Fidelity's impedance curve measurement for your headphone before you start obsessing about amp output impedance. For example, Sennheiser's HD headphones often have spikes in their imp curves (so bass would be boosted and hazed over), while the AKG Q701 and Beyerdynamic DT880 (and pretty much my entire headphone collection, even the V-MODA M-100) all have relatively flat imp curves and barely change at all. Many home theater receivers have headphone output impedances over 100 or 120 ohms, but plug your headphones into them... do they sound like crap?

 

[money4me247]

Response is in BOLD.
 
Quote:

  To elaborate:
The bravo amps are just too loud for many sensitive headphones, you have to keep the volume turned down really really low to keep from damaging your hearing. When potentiometers (volume knobs) are turned down that much, they create a channel imbalance which will either sound like one earcup is louder OR just off and unnatural (good manufacturers go to great lengths to match the drivers in their top headphones). The other problem, LOUD enough doesn't always equal GOOD. Headphones need a good supply of current to have the dynamic range to supply high-energy notes like low bass and complex music passages with lots going on... It tightens up music and allows them to play smoothly. I hate the song, but Kanye West's "Gold Digger" percussion is a great example of (poor mastering and) a sound that will easily distort if the headphone drivers can't handle the excursion or if they don't get enough current to prevent clipping. Thank you for that helpful information. I have noticed that I am keeping the headphone volume knob at pretty low dial, but I haven’t noticed any clipping or other audio problems with my pairing even with “Gold Digger.” Any specific audio problems I should be looking out for?
 
There are many factors influencing that, but to look at it that simply is a myth. What do you think even happens when you don't have a lot of headphone impedance to dampen the amp output impedance? mmm… I honestly have no idea. Sorry if the answer is supposed to be obvious 

 
Exacerbated by that designer of the O2, the real situation where having good headphone:amp dampening is important is when the headphone doesn't have a flat impedance curve... Check Inner Fidelity's impedance curve measurement for your headphone before you start obsessing about amp output impedance. For example, Sennheiser's HD headphones often have spikes in their imp curves (so bass would be boosted and hazed over), while the AKG Q701 and Beyerdynamic DT880 (and pretty much my entire headphone collection, even the V-MODA M-100) all have relatively flat imp curves and barely change at all. Many home theater receivers have headphone output impedances over 100 or 120 ohms, but plug your headphones into them... do they sound like crap? Headphone graphs from Inner Fidelity for the HE400: http://www.innerfidelity.com/images/HiFiMANHE400.pdf. The impedance graph looks pretty flat to me. Does that mean that it is okay that I am driving the low impedance, high sensitivity  HE400 with the Bravo tube amp?

Spoiler: Bravo V3 specs refresher

 

In Put Power	DC24V
Input Sensitivity	100mV
Input Impedance	100KOhm
Out-Put Impedance	20~600 Ohm
Gain	30dB
Frequency response	10Hz-60KHz +/- 0.25dB
Signal/Noise Ratio	>90dB
Dynamic range	84.6dBA(300 ohm) 89.8dBA(33 ohm)
THD	0.016%(300 ohm) 0.45%(33 ohm)I
MD + Noise:	0.045(300 ohm) 0.42(33ohm)
Dimension	79mm (D) X 130mm (W) X 44mm (H)
Input	Stereo RCA x1, Stereo 3.5mm x1
Output	Stereo 6.35mm x1

 

 

[HOWIE13]

I would agree in that I have never been persuaded by the eight times rule. The bass is meant to become somewhat uncontrolled, but I have never noticed that.
For Classical Music and Jazz the V3 volume knob is no problem for me driving even sensitive headphones. Maybe it's because Classical music is less compressed, with wider dynamics and generally lower output. I find the useful knob range is from about '20 to' to just over the '12 o'clock' position. 

 

[UmustBKidn]

  lol... well, when you die in a few decades & i get unfrozen by future technology, we can discuss snake oil vs real science 

 
...oh wait, we won't b/c you were not cryogenically preserved 

 
So cryo-preservation of human beings is real science?
 

 

[DefQon]

Cryo tubes is bigger marketing bs scam than cryo cables.
 
Anyway that's not the point.
 
High gain on the Bravo amps with variable sensitivity headphones? Easily fixable. Of course given the craptastic and improper components used for this particular basic class A circuit. Swap the 10K pot for a 50 or 100K (if your source can accept it). The stock pot has to go, volume doesn't go up step by step in terms of db but one little turn after 11'O clock on the dial and you go from small volume to loud.

 

[HOWIE13]

@DefQon
I've never noticed a particularly sensitive point whilst turning the V3 volume knob. Have just tried again now and volume increase is smooth and I can't detect that particular problem.
I wonder if the source input voltage might make a difference? Maybe a faulty unit? 

 

[DefQon]

  @DefQon
I've never noticed a particularly sensitive point whilst turning the V3 volume knob. Have just tried again now and volume increase is smooth and I can't detect that particular problem.
I wonder if the source input voltage might make a difference? Maybe a faulty unit? 

 
I'm referring to the v1. Haven't pulled a v3 apart before.

 

[money4me247]

interesting stuff. i've ever noticed an issue finding the appropriate volume as I can adjust the volume on my laptop & my music player after turning the knob.