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the CTH (Compact Tube Hybrid) Rev A thread... - Page 38

post #556 of 596
Originally Posted by gurubhai View Post

Followup : Connected a 24V/ 2A transformer and the TB+ is now 95.9V. Currently have a BEL ECC82 plugged in listening to a T50RP, no clipping and its whisper quiet. Sounds pretty darned good too.smile_phones.gif
Thanks for the help guys.
That's good news. So holland was on right track WRT low A/C - TB+. What was the quoted voltage and amperage of the first adapter that didn't work?
Of course these A/C adapters are just transformers in a box, no active accomodation of load and direct relation to line voltage.
BTW I've had good results, perhaps w/easier to drive cans, with 0.8+A adapters and we spec 1+A IIRC.
Edited by cfcubed - 7/10/13 at 7:10am
post #557 of 596

The other adapter was 24V, 1A ac so it should have worked but it didn't. Unloaded, it was outputting about 23.5VAC unlike holland's adapter which is closer to 27VAC. My current trafo outputs around 25VAC unloaded and seems to be working fine unless the mains dip too low.

post #558 of 596
Originally Posted by cfcubed View Post

I've HE-500s myself & prefer my 2-channel b22 to CTH for them, O2 if something smaller is required (e.g. @ work).  Same for when I'd owned LCD-2s.  IMO & to my ears these orthos do not need warmness that tubes can impart, they benefit more from the drive of something like any b22.  When I do sometimes miss the option to color/contour the sound w/tube swapping I swap in the CTH & use a tube w/sparkle like chrome plate Siemens (and it performs well & sounds quite nice).


Originally Posted by holland View Post

Wow, CTH.  This brings back memories, what a great little amp with some neat features like the self zeroing output diamond buffer.


Anyhow, I'm not Alex, but the CTH uses a class AB diamond buffer with approximately 20mA idle.  It should peak out around 40mA before exiting class A.  I am not sure what the voltage swing is, but for the HE-500 it shouldn't matter, it will be current limited.


I would guess the voltage swing should be about 13Vp-p, roughly 5Vrms.  I'm guessing roughly 185mW rms.  Heat in the small enclosure is a constant battle, I'm not sure it can sustain that.  Note, no calculations done, and no measurements taken.  I'm not really sure if it can hit that before it clips.  Anyhow, for 38ohm you're likely looking at around 60mW of class A.  That should land you around 105dB-ish before coming out of class A.


Depending on how much SPL headroom you actually need or want to target, should dictate your headphone amp requirements.  I guess if you need 120dB peak out of your HE-500, then the popular DIY amps out there are EHHA and Beta22.  The SOHA2 should land you around 112dB, in stock form.  You can get more with a bigger heatsink.


Edit: keep in mind, it's all theoretical.  One would need to bench the amp to see where it lands in terms of THD to determine what's acceptable or not.


To close this loop, I got my HE-500's yesterday.  I'm quite happy with the HE-500's on the CTH but think it is current starved.  The bass thumps and extends, don't get me wrong, and details in the mids and highs are wonderful, but bass can loose some slight details.  IMO we are talking minor, but still.  At reasonable to moderate loud volume levels I find it quite nice (volume control at 12pm).  Saying that, I also ran the HE-500's from an old 20 watt speaker amp headphone out (Sanyo JA-220), and the speaker amp's power is able to drive the HE-500's to incredible detail from lows to highs.  So, I also prefer the HE-500 with something with more power, but the CTH and HE-500  sounds great enough that if it was my only option, I don't think I'd worry too much about upgrading.  It would be the last 10-20% type upgrade only.

Edited by civilmonkey - 7/11/13 at 11:19am
post #559 of 596

Just finishing off my CTH build, had the board for awhile but a few other projects got in the way. I have a question regarding the transformer hookup, my plug is off board, mounted to the back panel. I've noticed in a few pictures of others builds, they've wired their off board AC-in jacks to S1 and S2 instead of the sockets provided for the BOM listed PCB mount jack. 


My question is: Am I ok to wire the center pin of the jack to the rear socket, and outer rim to the front socket, so I can keep the switch?

post #560 of 596
Yes, A/C has no polarity.
post #561 of 596

Well, fired up my CTH last night without any sparks, but that was the only good news.

Did all the startup measurements and everything seemed fine, so hooked it up to a source and some old 600 ohm cans, Nothing!

No output at all! Spent hours verifying all the components, everything looks good, no solder bridges either, checked with a loupe.

Retook the measurements:

OG -> SG = 11.98v

OL -> SG = 11.97v

OR -> SG = 12.00v

TB+ -> SG = 98.8v

P1 -> SG = 77.9v

P6 -> SG = 78.v

P4 -> P5 = 6v


On turn on, Relay snatches at about 20 seconds.

I'm using a new 6DJ8 Amperex.


Not really sure where to look next, all my measurements thus far look pretty good?

It's gotta be a circuit unrelated to the ones I've already checked.


Any suggestions??

post #562 of 596

Ooops! Wired up the output jack to the pins that disconnect when the plug is in! DOH!

Man does this thing sound sweet!

post #563 of 596

NIce! Glad to see this wonderful amp is still being built. Congrats!

post #564 of 596

Hey gang, does anyone know the 'ma' load on ICP the 24V reg? I want to try out one of Bellesons new mini regs, but it only supports 225ma load. TIA!

post #565 of 596

Off the cuff I'd say a big no to an ICP w/225ma load rating.  It's powering output buffer, rail splitter + e12/relay in CTH and gets (very?) warm at its 1A rating.

The best way to know its draw would be to lift its output pin & bridge back using an ammeter (> 1A capable).

post #566 of 596

Hey Guys - need a little help. I'm trying to troubleshoot a CTH; here's what I have.


OL, OR, OG   - ~12V


I latch fine.


Right channel has proper output, no output on the left channel. Tube pin 1 is ~95V. TB is ~95V check component values, and the left channel op amp. Pot is soldered correctly. Thoughts?

post #567 of 596

What is tube pin 6? Is this a new build ?

post #568 of 596
Originally Posted by Misterrogers View Post

Right channel has proper output, no output on the left channel. Tube pin 1 is ~95V. TB is ~95V check component values, and the left channel op amp. Pot is soldered correctly. Thoughts?

Well you know where to look (pin 1) & the bad symptom (TB+ should be 95V - 105V and plate pins 1 & 6 about 80V.)   So we know you are up to here in CTH's setup instructions:

If TB+ is good but pins 1 and 6 are far from 80V the tube opamps may be bad or a component value may be wrong.
If everything in the tube circuit tests good but the plate voltages are still far off, the tube is bad.


I find the problem curious because its a "no sound" problem, IIRC more often you get some sort of sound even if these measurements are off...  But it's been a long while since I've debugged/helped debug something like this.

I assume your right-channel/pin 6 measurements are proper (e.g. plate about 80V). 

If so you've probably done this but swapping ICR & ICL to see if problem follows & of course trying different tubes.  Then crawling though the left vs right input stage components & being extremely careful making/comparing live board measurements.  Maybe 1st comparing resistances at places between them on non-live board would show something.  And of course searching CTH threads for the plate pins measuring same as TB+ problem.

Edited by cfcubed - 3/6/14 at 4:18am
post #569 of 596
I am beginning to gather parts for the CTH build. My main 'phones would be the Koss KSC35 (60ohms @ 101db) and the only tube I am going to use is the 6f8g.
Any suggestions regarding parts choice for this build? smily_headphones1.gif

Also, which one of these would be the correct adapter? I think the 6f8g->6ca7 was mentioned before, just want to make sure I got the correct one.


post #570 of 596
Thread Starter 

I believe those adapters should work.


I know it's up to you but why not try any of the 6DJ8 or 12AU7 variants that the amp was designed around?


Also, curious why you need an amp for the KSC35s? Aren't they fairly efficient phones to begin with?


Parts wise... I'd suggest using the longer Hammond case... you can fit in the Alps RK27 pot and 1/4" jack with the longer case. Also go for the upgraded the 732-1429-ND 1A L2H. This will all up to 800mA filaments e.g. 6N6P tube. Slap on at least a partial heatsink on the LM2595 if you're going to run the 6N6P. Also, get a multi-rotary switch like this 633-MRK206A-RO. This will allow you to use more than 2 heater voltages. Both of my CTHs have 4,6,8, and12v heater voltages. I think most people just do two - 6 and 12v so it's up to you.

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