[Zulkr9]

I ordered a used acopia A28MT300 from ebay. I heard these work well, can anyone chime in ?

 

[trollchu]

Checking the Drop page for this, they don't actually list an input impedance, does anyone know how to test for this? The LCX lists at 10 kohms, could this be identical? I'm considering the possibility of testing a preamp before the CTH in my system, has anyone tried theirs with a pre? Did the CTH take to it well?

 

[roderickvd]

The inputs are connected directly to a 20 kohm pot. Should not require a preamp unless you want to switch sources or color the sound more.

 

[HTSkywalker]

Hybrid amps are normally good all rounders with both planars and classic cans with a touch of tubey sound

 

[roderickvd]

I replaced the stock WIMA interstage capacitors with Miflex KPCU-02's. I chose the Miflexes because I like them in my Eufonika tube amps, they are attractively priced, and last but not least: they fit.

The KPCU's have twisted leads that are too thick to go through the holes on the PCB, even when you untwist them and take a single strand. So I sacrificed two resistors from the parts bin and clipped off their leads:

Fastened with self-adhesive cable tie mounts, it fits to a tee:

To keep cross-posting to a minimum, subjective impressions here: https://www.superbestaudiofriends.o...alli-tube-hybrid-amp.4866/page-54#post-416142

 

[trollchu]

The inputs are connected directly to a 20 kohm pot. Should not require a preamp unless you want to switch sources or color the sound more.

Yes, my intention was to color the sound, preferably with an octal and to add an analogue EQ. I don't usually use EQ, but I've been playing around adding a bass boost from 55 to 220 hertz. It's been pretty satisfying alongside the strong mids of a Raytheon I've rolled in, I got the idea that an octal based sourced might play very well before my CTH then.

 

[AudioPileUp]

Just looking to see if anyone has any suggestions for me in terms of what to try/troubleshoot for my CTH amp which would not turn on yesterday morning.

Swapped out the tube, no dice.

Checked the voltage on the AC adapter, it's fine, 27.9V measured with my meter vs. 28V, figure that's close enough.

But when I plug in the adapter, see a spark at the back of the unit. I opened it up to see if the power is making it to the board, and I can't seem to find 28V across any of the pins on the power inlet. (checked all combos of the 3 pins on the bottom of the board, unless there's some sort of coating on the solder that would stop this approach?)

Checked for continuity between the two metal pieces inside that same component (where power cord plugs in) and got 27 ohms. That was with no tube in it, and power switch off. No clue what's normal there.

Also no sign of any failures on the board. Nothing looks burned. No bulging caps etc.
When I try to power on, power light does not come on, tube does not glow.

(Posted on /r/headphones as well but not much there in terms of suggestions)

Cheers!


Update: confirmed that the pins outs have continuity to the inputs on the power jack.

And tried another power adapter. It was 24V with a display on it. It sparked also when connecting, and the display dropped to 2.5V when connected, meaning some sort of short in the amp somewhere?

 

[HTSkywalker]

Just looking to see if anyone has any suggestions for me in terms of what to try/troubleshoot for my CTH amp which would not turn on yesterday morning.

Swapped out the tube, no dice.

Checked the voltage on the AC adapter, it's fine, 27.9V measured with my meter vs. 28V, figure that's close enough.

But when I plug in the adapter, see a spark at the back of the unit. I opened it up to see if the power is making it to the board, and I can't seem to find 28V across any of the pins on the power inlet. (checked all combos of the 3 pins on the bottom of the board, unless there's some sort of coating on the solder that would stop this approach?)

Checked for continuity between the two metal pieces inside that same component (where power cord plugs in) and got 27 ohms. That was with no tube in it, and power switch off. No clue what's normal there.

Also no sign of any failures on the board. Nothing looks burned. No bulging caps etc.
When I try to power on, power light does not come on, tube does not glow.

(Posted on /r/headphones as well but not much there in terms of suggestions)

Cheers!


Update: confirmed that the pins outs have continuity to the inputs on the power jack.

And tried another power adapter. It was 24V with a display on it. It sparked also when connecting, and the display dropped to 2.5V when connected, meaning some sort of short in the amp somewhere?

Definitely a burned component or a loose solder, it may be visible.

 

[AudioPileUp]

Definitely a burned component or a loose solder, it may be visible.

Figured as much at this point. Thanks for the input HTSkywalker...

I do hate letting things break and not being able to fix them. Also hate the current throw away culture that's prevalent with electronics.

On the other hand, I did try pretty hard to diagnose, and can be less guilty upgrading now. Cheers!

 

[HTSkywalker]

Figured as much at this point. Thanks for the input HTSkywalker...

I do hate letting things break and not being able to fix them. Also hate the current throw away culture that's prevalent with electronics.

On the other hand, I did try pretty hard to diagnose, and can be less guilty upgrading now. Cheers!

Best check with the tech support docb@bottlehead.com or create a thread on their technical forum.
Beware If you need it repaired by them there is a minimum charge of $125
Good luck and cheers

 

[Aerocraft67]

Early runs of CTH have a known issue with volume noise. Here's a story of one with a happy ending.

My CTH started with a trace of volume pot scratchy noise when I bought it used four years ago. Made noise only when I turned the knob. It got worse over time, to where it was making nose in the right channel when not moving (although only audible in silent passages).

I ran down old threads about addressing this. Decided to try a heavy dousing of the volume pot with contact cleaner.

Couldn't find DetoxIT with a spray stick on Amazon so I got BW-100. Then I discovered I needed a Torx driver for the amp housing screws. Unsure of the size, Got a set of them. Size T10 worked best.

Removed the bottom plate and backplate screws and pulled the circuit board out. I worked the glued-on volume knob off without too much trouble. Just kind of pulled alternative edges of it up from the front plate, rocking back and forth. Popped off reasonably easily, well before I worried about breaking it.

That revealed a nut and washer around the knob stem. Must be metric because I couldn't get an imperial socket to fit it. No problem, it twisted off easily with a Vice Grip plier. Removed that and the washer. That was all I cared to disassemble.

I sprayed the bejezus out of the volume pot, from the front and back, trying to penetrate in the stem socket in front and the cubicle pot housing in back. Twisted the stem after each application. Gave the other components a little toot as well.

Let it dry a couple minutes and reassembled. Two of the three screws on the bottom just turned infinitely during reassembly—not sure if they were already stripped, but I didn't crank on it all that hard. That was the only casualty.

Procedure successfully removed the noise. Knob also rotates easier now. Must have really been gummed up.

Saved me $600 on a Lyr+. Much as I'd like to match the rest of my Schiit stack, that's a big price for a small upgrade.

 

[roderickvd]

That's great to hear! I have had mostly the same experienced. Mostly, because yes the contact cleaner did wonders. But what I never managed is to get the volume knob off. Believe me, I applied so much force and still it would not come off. I'd prefer to change the volume pot to reduce the slight channel imbalance at low levels, but that's not in the stars for me.

 

[HTSkywalker]

Again the same old question now
Any audible difference between the supplied wall wart and an LPS ?

 

[Aerocraft67]

That's great to hear! I have had mostly the same experienced. Mostly, because yes the contact cleaner did wonders. But what I never managed is to get the volume knob off. Believe me, I applied so much force and still it would not come off. I'd prefer to change the volume pot to reduce the slight channel imbalance at low levels, but that's not in the stars for me.

I had your posts in mind when I noted the volume knob removal.

The scratch started to come back, so I applied another round of contact cleaner. This time I didn't dis-assemble the housing. I just pulled off the knob, removed the nut and washer, and sprayed downward into the stem housing best I could, twisting the stem each time. Seems to have resolved the re-appearance of the scratch.

The knob is a bit loose at this point. I could probably put a little shim in there or something to firm it up. Don't want to glue if I have to keep applying cleaner.

 

[Aerocraft67]

I briefly compared CTH to my new Pietus Maximus. CTH held its own, and provided a desirable dose of tube sweetness. PM was otherwise the superior amp.

Fetching less than $100 on eBay with shipping, CTH remains a compelling value. For that price, I’m keeping it for a bedside secondary rig. Mainly feeding HD 6XX.

I presume the latest Vali is the better recommendation for entry-level, hybrid tube today. But CTH may well provide a superior synergy match for someone dialing that in.