Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: NightFlight on January 08, 2014, 06:59:47 AM
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Possibly getting my hands on a Khozmo Acoustics 48 position stepped ladder attenuator as borrow/gift. The pot is worth as much as the retail Crack, so I'm apt to make it work in the circuit if possible. Can I get away with it, or do I have to make a change to get this load to work correctly on the output?
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fimg.canuckaudiomart.com%2Fuploads%2Flarge%2F511711-khozmo_acoustics_48_position_stepped_ladder_attenuator.jpg&hash=91c1b9b33e8aea3be2ca013f155145c437a1f455)
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50k won't be an issue. I use a 50K Goldpoint in mine. The size of the Khozmo may present some challenges with the tight'ish corner location.
Cheers,
Geary
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Thanks for that input. Which headphones are you using?
And... yes indeed it may be snug. Getting the dimensions soon to see if its even possible. However, the fight may be worth it.
I've been working it over in my head and coming out the face of the unit breaks the ease of future mods.
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With the Crack, I am vintage....AKG K240 Sextett's and K140's. Also have AKG K702's, but not a great match with the Crack, so they get a transformer output 396A based amp.
Cheers,
Geary
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Well, I must report this Khozmo 48 step ladder attenuator is a huge success. I've only a couple hours of listening on it, but the difference (for me) was nothing less than jump out at you and smack you in the face type experience. Nothing changed in the sense of overall parameters, but the detail and realism top to bottom is fantastic. Whole new instruments previously buried by the noise floor came up for air! Its so clich-eh, but there you have it. I couldn't be happier with this upgrade. :D
On the other hand, it wouldn't be the type of upgrade that someone who wasn't familiar with my amp would necessarily notice without a prior reference point. Still, I wouldn't hesitate to recommend a good stepped ladder pot for every BH product. I think best appreciated after getting to know the crack for a few months like I have.
It was a bit of a challenge to fit into the existing pot position, but it IS possible. Without the speedball it would have fit without issue, but the left channel A/B board for the speedball is in the way and had to be moved about 5mm left. I'll have to find a way to custom mount it, which shouldn't actually present too much of an issue.
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And I thought squeezing in my Valab was tricky.
Congrats for getting it to fit, I am pleased your efforts have been well rewarded with the improved performance.
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I stuffed a couple Mundorf 250V 100uF MKP in there too. Now I have hold it by the transformer and drop it back in the wood case straight down. It's snug so I have to shove it in the last bit of the way. Ummff.... ummff. :P
That and a Rogers 12BH7A D-getter in the input have really started things cooking.
I'd post a couple pics, but I'm too busy listening. 8)
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Awesome keep us posted on your impressions of the mods.
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I think the Mundorf have provided significant improvement in terms of dynamics. In some cases they're even a bit much. The panasonic electrolytic caps that were stock weren't to bad as the improvement the film caps bring is nice, but not a huge leap. However, that could just be the break-in period since I've only managed to get ~20hrs in on them. The 12BH7A I had in there (I've reverted to my CV4003) is most definitely out of spec in a crack even with the speedball. While nice - the bass is bloomed and distorted - very 'tubey' sound - lush. The difference in overall cohesion once I returned to the CV4003 was very obvious. I wish I knew how to tweak the 12BH7A parameters, its otherwise a very promising tube, it seemed more tactile and musical. The CV4003 has a fantastically 'correct' presentation - but its just not as engaging.
As for the Khozmo... I've adjusted to it and now it just seems normal in terms of the added resolution it brought.
Additionally I've added a 12v cpu fan running at 7v (to slow it and run quiet) under the crack. The internal ambient temp was hitting 80-90C and my film caps are rated up to 85C. The fan has dropped the ambient temp to 40-45C.
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I wonder why your Cracks runs so hot. Mine gets no where near those temperatures. Do you have the Speedball installed which does away with those big hot resistors?
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Do you have the Speedball installed which does away with those big hot resistors?
Those big, hot resistors are replaced by big, hot heatsinks that dissipate the same power.
90C is pretty toasty, this could partially be attributed to the BH7, depending on the voltages present.
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Hi Paul out of interest I just checked with my infrared thermometer. My Cracks been running now for 3hrs 45 mins and the maximum temp on the hottest part of speedball heat sink was 61
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Yeah, if the metal tab on the TIP50 is 60C, that's dandy!
Seeing 90C inside the amp would make me scratch my head about where all that heat was coming from, though perhaps removing the rubber feet and other unusual circumstances could drive the temps up.
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I'm curious - how was the ambient temp measured?
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With a cheap indoor outdoor temperature and humidity gauge which has a wire with a probe you can stick outside the window funny thing when both sensors are inside it always read half a degree difference. For ambient I was using the rooms air temperature.
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90c sounds a bit extreme. I had a probe around my SEX amp and the highest i found was the main transformer ~60c, followed by the chassis plate in the high 40s. Obviously inside the tube reads 100-140c but the elecy bits were easily half of that.
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My 80-90C was measured from non-heat conducting surfaces, with an infrared thermometer. I was seeing this temperature on the surface of the caps, black zip ties holding them, etc. I'll re-measure as I was taking measurements quick. Just to make certain I didn't get measurements from heat producing surfaces such as transistors, sockets or transistors.
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Sounds like you were getting false readings to me, are you sure your meter hadn't switch over to Fahrenheit?
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Yeah, if the metal tab on the TIP50 is 60C, that's dandy!
Seeing 90C inside the amp would make me scratch my head about where all that heat was coming from, though perhaps removing the rubber feet and other unusual circumstances could drive the temps up.
I have the rubber feet on. I understand the need for the crack to breath. The small fan under it certainly can't hurt. With the fan I see 54C on the largest board. Around 51C on heat sinks.
Taking the fan out now and letting it run. I've reverted to the CV4003 running the 5998. Its quite possible the 12BH7A was driving it a bit hard. I'd have check the plate voltages if/when I put it back in.
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Sounds like you were getting false readings to me, are you sure your meter hadn't switch over to Fahrenheit?
LOL yes. The big C or F are obvious.
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nice stepper NightFlight...looks like this will be great build ,any chance for a pic of the Attenuator installed ?
P.S. I think your IR thermometer might be off ,I just measured mine
bottom pic is inside and top of the plate
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Hehe, I see we are all on the same page here.
I stuck my meat thermometer tip in the hottest cooling slot and after 3.5 hours its at 113F
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm8.staticflickr.com%2F7320%2F13044770864_f017f76df9_o.jpg&hash=5f1cb6f0e7794ff87c09601e0a9af476ace38a7c)
Yeah, I need to get another thermometer, I dropped this one.
Shows 111, but temp max was 113. Ambient is 72, so looks like a 41F rise.
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you are only 3C higher then me buddy ..lol ;)
my ambient is 74F
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The transformer is so hot that you will remove your fingers after 5 seconds.
Crack runs hotter than my Stereomour, but after taking it's temperature I'm not worried.
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I believe it was the 12BH7 pushing it hard. The CV4003 keeps the transistor package(s) max temp at 61C. The highest surface temp I measured on the surface of the caps as around 40C. I would call that ambient since they aren't generating heat, so far as I can tell.
However I'm in a small room and the ambient temp can creep up if I'm at it all day. The computer throws heat and the room will creep up to 28C. The house thermostat being downstairs - doesn't realize this and keeps blowing hot air in the room. *sigh*
I have the fan set up now anyhow - it should serve to keep the whole unit cooler and extend the life.
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Pics as requested. Sorry - just really done quickly before I head out.
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Pics as requested. Sorry - just really done quickly before I head out.
Ohh that looks a squeeze, I notice the small speedball board required a little creative offsetting.
Can I ask what sort of range in steps you are normally using out of 48 available?
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It does. Board 'A' is currently floating. I'm looking to find something to cut and bind it down. A custom cut piece of plastic and replacement pair of screws.
As for the stepping, honestly I keep it at around half way around the dial. With 48 positions, I get around 350 degrees of turn out of it. I adjust visually with the screw hole more than the line guide. :P That gets the crack to the sweet spot and then I attenuate with Foobar, which is configured for WSAPI out. The Khozmo is not the best for changing volume level. You get good snaps and clicks changing position. However, it's possibly one of the best steppers out there - truly hardcore IME
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That gets the crack to the sweet spot and then I attenuate with Foobar, which is configured for WSAPI out.
From what i understand you really don't want to do that with a digital source as depending on the DAC it will either re sample and dither the signal, or just drop bits of data altogether. I believe its less of an issue when playing 24bit audio as there is a greater range to work with, but its still preferable not to do it. Also almost all amplifiers are designed around a redbook input level of 2volts, if you start attenuating and feeding a weaker signal you end up either having to increase the gain, or crank the volume up at the amplification stage to reach the same output level, both at the expense of a higher noise floor.
In fact thats a great question to ask John Swenson to explain in the DAC thread...
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The differences I'm adjusting on the source is minimal. It really is imperceptible. This is a totally different thread however.