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Old 1st Dec 2020, 3:25 pm   #1
vampyretim
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Default Alchemist Kraken amp

Hi all,

I've been trying to repair an Alchemist Kraken amp which has gone down on one side. It has a BD250DC and another transistor that has no identifying numbers, I assumed that they were BD249's. Anyway the BD250DC was a short and it also took out a TIP32C and three MPSA92's. So I've replaced all of the above and left the unidentified output transistor in place as it was thankfully fine.

I cannot find a service manual anywhere and have no idea how to set the base bias. The amp sort of works albeit it has nowhere near the gain of the working channel but I'm getting no distortion on my scope however as soon as I install a speaker I'm getting a lot of noise and my lamp limiter is showing as a short. All dry checks across the output stage check out okay. I'm seriously hampered without a schematic it will take me hours to draw the circuit out. Does anyone have a schematic and has anyone ever worked on one of these before? There's obviously another transistor down somewhere but I would really expect to see one side of the sine wave distorting if there was. I will pull every component off the board and test it if I have to.

Any help greatly appreciated.
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Old 1st Dec 2020, 5:26 pm   #2
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Default Re: Alchemist Kraken amp

I think you're on the right path.

Manufacturers of boutique amplifiers never give out circuit diagrams. If they did, everyone else could steal all their secrets! Or customers/reviewers could see how mundane the internals are.

It sounded like you'd taken all the semiconductors out and tested them by the way you phrased things. You might have something wounded but not entirely dead that breaks down at working voltage but looks OK on transistor tester voltages? Otherwise testing all the diodes and resistors might make sense if silly currents have been on the go.

Tracing out a diagram might be worth it if the thing is likely to come your way in the future.

David
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Old 1st Dec 2020, 7:24 pm   #3
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Default Re: Alchemist Kraken amp

Hi David,

I haven't checked all the transistors I just went backwards from the output stage until I no longer had shorts. The amp is DC coupled.

I guess I'm just going to have to draw the thing out which is not something that I will relish doing.

I really think that these boutique amps are more hassle then they're worth, my ears don't really hear an awful lot of difference between Hi Fi amps and I think that amps like this are massively overpriced.

Tim.
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Old 1st Dec 2020, 10:16 pm   #4
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Default Re: Alchemist Kraken amp

Hi Tim,

I consider boutique amps risky to bother with. You're likely to get an owner who afterwards complains that it sounds 'granular' or 'has less air'. As these effects are essentially psychosomatic there isn't anything you can do about it. Installing some very expensive components quite unnecessarily might do it, otherwise a witch-doctor needs to be called in.

I don't have the task ahead of me, so it's all too easy to say that it's worth tracking out just to see how plain it is inside. If you're really lucky, you might spot a design whoopsie.

David
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Old 28th May 2021, 2:47 pm   #5
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Default Re: Alchemist Kraken amp

Hi all,

I've traced out the circuit (a while ago now) but thought that I would upload. What are your thoughts on the circuit?

There are a few unmarked components. I assume that the second output transistor is the complmentary equivalent of the BD250C, what are your thoughts?
Attached Files
File Type: pdf IMG_20210528_0002_NEW.pdf (113.0 KB, 149 views)
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Old 28th May 2021, 2:59 pm   #6
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Default Re: Alchemist Kraken amp

I was Chief Engineer with Alchemist Products for a while. I am the designer of the Axiom/Maxim integrated but not the Kraken and all its offshoots. The complementary part is indeed BD249. More commonly though we fitted TIP35/TIP36 output devices and these were the now "unobtainium" "F" type with the highest voltage rating. Higher power amps in the range needed this higher voltage capability but you should be OK with "C" types for the Kraken.
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Old 28th May 2021, 9:21 pm   #7
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Default Re: Alchemist Kraken amp

There seem to be too many "blobs" around the TIP31- Both the .47R and .33R resistors appear to be shorted.
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Old 28th May 2021, 10:45 pm   #8
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Default Re: Alchemist Kraken amp

Quote:
Originally Posted by vampyretim View Post
Hi all,

I've traced out the circuit (a while ago now) but thought that I would upload. What are your thoughts on the circuit?

There are a few unmarked components. I assume that the second output transistor is the complmentary equivalent of the BD250C, what are your thoughts?
No output inductor?
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Old 28th May 2021, 11:59 pm   #9
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Default Re: Alchemist Kraken amp

It's got the shunt RC of half of the full Zobel, but the RL to the speaker is missing. Sometimes this gets done for religious/fashion reasons but then the section doesn't really provide the intended isolation from destabilising effects (boutique cables).

There seems to be several small errors floating around, like the quiescent current setting emitter resistors for the compound output transistors. For some reason the designer preferred a string of diodes to the more common Vbe multiplier. The drawing needs a few re-draws to get things into a more conventional format.

The input transistors are really a long-tailed differential pair, and once drawn that way, the jig-saw will assemble itself more neatly and make functions more obvious.

At least it has transient catching diodes on the output.

I haven't sorted out the VAS yet.

No secret sauce detected. The great secrecy can only be done to generate pseudo-mystique.

David
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Old 29th May 2021, 12:33 am   #10
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Default Re: Alchemist Kraken amp

I noticed the errors but it's nearly there

No "Thiele network" was originally fitted but later models from well after I left had one retro fitted... well just the inductor anyway! It was just a ferrite cored Toko type thing and probably saturated! I fit a proper one with hand wound air "cored" inductor and damping resistor when I get one in for repair.

It sounded great but measured.... well lets say if you wanted a triangle wave generator @ 50W then feed it with 25Khz! It slewed like a sloth!
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Old 29th May 2021, 2:05 am   #11
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Default Re: Alchemist Kraken amp

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well lets say if you wanted a triangle wave generator @ 50W then feed it with 25Khz! It slewed like a sloth!
I'm not surprised. Still sounding OK fits in with the idea that you can't hear a component on 75kHz, whether or not a 25kHz fundamental is present. If you want an amplifier that will produce 25kHz signals with reasonable fidelity to the original waveform, then substantially more elaborate hardware is needed. If done, you can then stick a passive filter in front of it and have confidence that it can track anything which can get in.

David
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Old 29th May 2021, 4:55 pm   #12
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Default Re: Alchemist Kraken amp

Hi guys,

I noticed the faults last ninght in particular the shorted components in the output stage.

Please check out my revised version.
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File Type: pdf IMG_20210529_0005_NEW_0001.pdf (288.2 KB, 111 views)

Last edited by vampyretim; 29th May 2021 at 5:18 pm.
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Old 29th May 2021, 9:24 pm   #13
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Default Re: Alchemist Kraken amp

330R is used on both outputs from base to rail. The emitters of the drivers go directly to the collectors of the outputs and then each goes via the respective 0r33 and 0r47 to form the output midpoint.
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Old 29th May 2021, 9:57 pm   #14
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Default Re: Alchemist Kraken amp

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Originally Posted by Jez1234 View Post
330R is used on both outputs from base to rail. The emitters of the drivers go directly to the collectors of the outputs and then each goes via the respective 0r33 and 0r47 to form the output midpoint.
Thanks Jez, I will redraw it.
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