Crack;Bottlehead OTL

[Todd R]

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Much to my dismay, my crack doesn't have enough juice to suitably push my vintage Fostex T40 (orthos). I wonder if the speedball will fix this? I'm comparing to my G&W T2.6f tube amp which does seem to adequately power them.

The Speedball update isn't going to make the amp any more powerful, it replaces some resistors with a constant current source.
What impedance are those headphones?

 

[Nebby]

If my google-fi is correct, it's got an impedance of 50ohms which is certainly less than the recommended 100ohm min impedance for the Crack amp.
 
 
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Crack is an output transformerless amp designed specifically for running higher impedance (100 ohms or higher) headphones like the many Sennheiser, AKG and Beyer designs.

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The Speedball update isn't going to make the amp any more powerful, it replaces some resistors with a constant current source.
What impedance are those headphones?

 

[Beefy]

Quote:

Much to my dismay, my crack doesn't have enough juice to suitably push my vintage Fostex T40 (orthos). I wonder if the speedball will fix this? I'm comparing to my G&W T2.6f tube amp which does seem to adequately power them.

 
Quote:

The Speedball update isn't going to make the amp any more powerful, it replaces some resistors with a constant current source.
What impedance are those headphones?

The Fostex is just 50 ohms, I think, and quite insensitive thus requiring a lot of current. Not a good match for the Crack at all.
 
The G&W T2.6f looks like it is a hybrid with output MOSFETs supplying a decent current drive and low output impedance, which is why it works a lot better.

 

[Doc B.]

We try to point out everywhere we can that Crack is intended for headphones of 100 ohms or higher impedance. A lower impedance, low sensitivity headphone is going to work a lot better with our S.E.X. kit, which can drive loads down to 4 ohms with 2000 mW of output. A third alternative for extreme cases of low sensitivity is our Paramount amps, which can deliver 8 watts into 16 ohms, and which we use with the notoriously difficult to drive AKG K1000s. Amongst these three choices we should pretty much have every headphone covered, save for electrostats.

 

[Armaegis]

I've read about impedance matching, which is why I was using my low resistance cans with an impedance adapter (I made a couple in 90 and 280 ohm flavours). Overall I found it just gave me better control over the volume and tightened up the bass. The adapters didn't help much with the T40s though. Could it be because orthos are very current hungry cans? (at least from my understanding they are)
 
Impedance matching is generally about optimizing power transfer correct?
 
On the flip side, I liked my Sextetts (600 ohm) better out of the Crack than the G&W.

 

[Beefy]

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Originally Posted by Armaegis /img/forum/go_quote.gif
 
Impedance matching is generally about optimizing power transfer correct?

From my terribly uneducated viewpoint, the benefits of impedance matching for maximal power transfer seem to be grossly over-rated. In the case of your orthos damping factor is FAR more important, so you want as low an output impedance as possible and no series or parallel resistances between the amp and the driver, because they reduce the amp's control over the driver.
 
And yes, your orthos need a lot of current.

 

[Skylab]

One thing I have definitely learned is you cannot just add in-line resistors and think that is "impedance matching".  Impedance matching can only be accomplished with transformers.

 

[Armaegis]

Well what's the point of that Etymotic 4S-4P adapter then?

 

[Beefy]

In that case, it is to deliberately increase the amp's output impedance. The exception that proves the rule.

 

[Doc B.]

Yes, damping factor seems to be an issue as the more common complaint from folks who use Crack with low impedance cans is soft bass, which is typically very well controlled and punchy with high impedance headphones. The low impedance output of amps like the S.E.X. amp means that DF can be lower and they will be able to keep the bass tight on the bottom end with low and high impedance cans, and the S.E.X. can also put out enough power to offer decent voltage swing into higher impedance headphones. So it seems to be a more universal solution. That said, most folks (including me) prefer the Crack with high impedance cans, S.E.X. with lower impedance cans.
 
If one can justify buying several different headphones, it might be most rewarding to optimize the amp setup for each one.

 

[Beefy]

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The low impedance output of amps like the S.E.X. amp means that DF can be lower and they will be able to keep the bass tight on the bottom end with low and high impedance cans, and the S.E.X. can also put out enough power to offer decent voltage swing into higher impedance headphones. So it seems to be a more universal solution. That said, most folks (including me) prefer the Crack with high impedance cans, S.E.X. with lower impedance cans.

I'm curious about this...... I've been looking at your kits, trying to decide on my next project. I'm interested in S.E.X. for my low impedance phones, but am a little confused by the fact that it uses 120ohm resistors in series with the output.
 
Also, I have seen some complaints of the gain of S.E.X. being too high, even with high impedance phones. Can you possibly tell us the overall voltage gain of the Crack and S.E.X. into 32 and 300 ohm loads?

 

[Armaegis]

In my case with the orthos, it wasn't that the bass was soft. The bass actually extended quite well. The issue was that the sound was congested, especially during "busy" or complex passages in the music. At those points, the music became flabby and almost sounded like clipping, especially in the lower end.
 
I'm not looking for a solution per se; more interested in an explanation. I'm assuming it's a power delivery thing, but if there are other factors I'm all ears.

 

[Beefy]

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In my case with the orthos, it wasn't that the bass was soft. The bass actually extended quite well. The issue was that the sound was congested, especially during "busy" or complex passages in the music. At those points, the music became flabby and almost sounded like clipping, especially in the lower end.
 
I'm not looking for a solution per se; more interested in an explanation. I'm assuming it's a power delivery thing, but if there are other factors I'm all ears.

If the bass is soft, it is an effect of the output coupling capacitors being too small. The coupling capacitors form a high pass filter with the 'resistance' of the phones, literally cutting off the bass. You can help improve that by increasing the capacitor size.
 
But in your case, congested flabby bass is almost certainly from poor damping factor. Very little that you can do about that. Apparently, there are alternatives to the 6080 that decrease output impedance from 120 ohms to 70 ohms which may help somewhat, but the Bottlehead forum is a better place to find this info than my memory.

 

[Armaegis]

Hmm... at this point I'm thinking I have a misunderstanding of the term damping factor... what does it mean in technical terms?

 

[Nebby]

Wiki has a good overview of Damping Factor: http://en.wikipedia.org/wiki/Damping_factor