Puzzle of the Day - Aleph 5 Amp Quandry

karlsushi

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Dear knowledgeable and experienced DIYers,

See if you can work this one out (no spoiler here, I don't know the answer!).

Bear with me as there's a bit to get your head around.

I am having an issue with my Joe Henry built Pass Labs Aleph 5 power amp clone. The other day it decided to pop the left channel. Turns out, it was just a fuse.

After a fair amount of to-ing and fro-ing, it appears that the amp seems unhappy with my Chord DAVE connected to it. It works absolutely fine with my other DAC and preamp in the chain, but every time I connect the DAVE, it blows the same fuse on the same left channel. Never the right.

The DAVE has a variable output mode (using an internal digital volume control) or a fixed line level mode so it can be used as a straight DAC with another preamp. One bit to note (perhaps) is that it has an unusually high line level output at 6V (it's a Chord thing!).

When using DAVE in preamp mode, it goes pop. When used as a DAC with my other pre, no fuse blows.

Other really odd thing is that the DAVE was used fine with the Aleph 5 for many hours previously, but they have just decided recently to stop liking each other.

Finally, the DAVE works absolutely fine with my Aleph 3 amp (virtually the same design as the Aleph 5, but at 30W as oppose to the 5's 60W output). In theory, this rules out an issue with the DAVE.

I am in contact with Joe Henry and needless to say he is scratching his head with this one. The amp is most likely going to have to go back to him for some tests, but thought I'd post here to see if anyone had any ideas at all.

Thoughts appreciated.

Thanks in advance

Karl

Edit: Seems the amp has now decided to stop working with anything connected. Now get a loud buzzing from both channels with my preamp connected, DAVE or no DAVE, so it's going to be off to Joe for repair ☹️
 
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toprepairman

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The answer is a doddle.
The amplifier is only conditionally stable, (poor design or build or both) so under certain circumstances it is oscillating at a frequency above audio, it could be as low as 25 kHz but could be right up to radio frequencies 100's of kHz or even MHz.
When it oscillates it is going full blast and loading the power suppy hence the severe hum/buzzing.
Joe H should have recognised this immediately.
 
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karlsushi

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Well, it is on its way back to Joe so hopefully he can work out what's going on.

In terms of the design, you can have a chat with Mr Nelson Pass about his poor design!

Which presumably leaves the build being an issue!

But how does this explain that the amp worked perfectly fine for around 10 months using those same two preamps!?
 

toprepairman

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Yes that's odd, has anything else changed, interconnects or speaker cables?
It'll be interesting to hear what he finds.
 

karlsushi

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Amp is now fixed.

I was reluctant to leave my Chord DAVE with Joe for testing and after a number of tests, he never found exactly what the issue was because he couldn't get it to fault like it did for me.

But he decided in the end just to replace the whole left channel board given that it was only the left channel that was blowing a fuse.

He built the amp for me last March and so he carried out the works completely free of charge under 'warranty'.

If nothing else, this demonstrates to me what a great service he is providing that he was prepared to undertake such extensive work to get it resolved, albeit some might say that is what would be expected in the circumstances.

Fingers crossed it stays fixed, but its playing away beautifully on the end of the DAVE currently, so here's hoping.

Sorry I don't have an answer for the puzzle!
 
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bencat

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In my view most of what Robert recorded is brilliant and any King Crimson is fine .
 

karlsushi

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A quick update for anyone interested.

Amp was working fine for a couple of weeks after the left channel was replaced, but it blew again last week. Fuse didn't go this time, but smoke billowing out the case :eek:

Lovely as it was while it was working, the A5 has now gone back to JH for a refund :(

To be fair, he didn't quibble the return and has refunded me in full. Reckons its the first amp he's ever had returned for a refund.

Now that it seems an actual component has faulted, he may be able to get to the bottom of what was causing it, so I'll update this thread if he comes back and confirms.

On a positive note though, I now have some cash in the bank to look for a new amp :)
 

karlsushi

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Finally it seems the puzzle has been solved. Message from Joe Henry as follows:

"it was a small curly piece of metal swarf under the left hand power supply board and the grill bottom shorting the negative 34 volt rail and blowing the fuse intermittently and nothing to do with the amplifier boards at all".

He sent his sincere apologies.
 

hifinutt

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Wow thanks for update .been using my f5 monos now for a fair while . Have pondered those neurochrome amps he has but henry ( top repairman) has not been too positive about switch mode psu, s which JH neurochrome have i think
 

Idlewithnodrive

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Finally it seems the puzzle has been solved. Message from Joe Henry as follows:

"it was a small curly piece of metal swarf under the left hand power supply board and the grill bottom shorting the negative 34 volt rail and blowing the fuse intermittently and nothing to do with the amplifier boards at all".

He sent his sincere apologies.
Major kudos for the man, working it out, informing you and, unusually for this country at the moment, treating you courteously as a paying customer 👍
 
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