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the CTH (Compact Tube Hybrid) Rev A thread... - Page 36

post #526 of 577
Quote:
Originally Posted by cfcubed View Post

I'd expect that noise to go away w/proper casing, grounding incl grounding the pot.
So I got some spare time to put the amp into case. The buzz is a bit more silent but still there whenever I touch the volume knob.
When you said ground the pot, does that mean I should connect it to the case too? It's isolated now.
Edited by Lil' Knight - 1/27/13 at 3:36am
post #527 of 577

If the case is metal I'd connect it to SG (which = IG input ground).  And if the volume pot is not mounted to grounded metal then you should run a small ground wire to it.

On the RK27s I loosen one of the 2 little screws on back, slip a small/thin wire under it, screw it back in & run that to IG/SG/case ground.  One way or another grounding that knob.

post #528 of 577
I'm using the Hammond case with plastic panel so the pot is totally isolated. Also, the pot used is TKD 601 which does not have the screws like those of the RK27. Looks like I should just connect the ground pin to SG then.
post #529 of 577
Thread Starter 

I had this issue exclusively with CTHs. Earlier on in this thread cfcubed suggested the same thing about tying the pot to input ground or SG -- either one works. After loosening the screw on the RK27 and making a small lead going to IG the buzz went away completely. With my "Wire" amp I don't have the same issue. No need to tie the pot to GND.
 

post #530 of 577

The need to ground the pot when using a metal knob is not uncommon and more a standard practice.  Don't want it being an antenna for noise or a path for static shocks to go directly to your input stage.

It's no big deal to do it either, if there's no easy way just solder (file, flux, solder) a ground wire to a thin big washer or sheetmetal cutout & thread it onto inner side of pot shaft.


Edited by cfcubed - 1/28/13 at 4:39am
post #531 of 577

I picked up a set of new headphones recently and it turns out that I have a 60Hz or 120Hz hum (don't really have a way of figuring out what it is, exactly - can't find any good comparison sounds on youtube) in my CTH that I could not hear with the K501 (due to the headphones' low sensitivity). I've ensured that it is not my source introducing this hum. The case of my volume pot isn't grounded to anything and nothing changes when I touch the volume pot so I don't think that's anything. I'm using the Rev. A version of the BoM so C3H is an updated 330uF. I have noticed that the buzz gets a little more noticeable at 6.3V heater supply (6CG7). My other tubes are a 12AU7 and a 12BH7. I have tried using a surge protector and running it straight out of the wall. Disconnecting inputs doesn't change anything. I'm pretty clueless when it comes to troubleshooting so that's about as far as I got. Any help?


Edited by Ikarios - 1/30/13 at 8:02pm
post #532 of 577

>  a set of new headphones recently and it turns out that I have a 60Hz or 120Hz hum

 

It's been a long time since we've had a report of this with CTH...  Hum using newer BoM parts and possibly not related to grounding.

 

Exactly what/which are your new headphones?

You say touching the vol knob has no impact, does touching its (metal?) case have any?

About where is the vol knob in its rotation when you notice the hum & do you only notice the hum between music passages/during the quietest parts (or is it more noticeable than that)?

 

If this is not a ground issue then you seem to be on the right track WRT C3H, which was raised to suppress such noise.  

R18 values also have some impact here - I wonder what value yours are & if they are std BoM/100R.

post #533 of 577
Quote:
Originally Posted by cfcubed View Post

>  a set of new headphones recently and it turns out that I have a 60Hz or 120Hz hum

 

It's been a long time since we've had a report of this with CTH...  Hum using newer BoM parts and possibly not related to grounding.

 

Exactly what/which are your new headphones?

You say touching the vol knob has no impact, does touching its (metal?) case have any?

About where is the vol knob in its rotation when you notice the hum & do you only notice the hum between music passages/during the quietest parts (or is it more noticeable than that)?

 

If this is not a ground issue then you seem to be on the right track WRT C3H, which was raised to suppress such noise.  

R18 values also have some impact here - I wonder what value yours are & if they are std BoM/100R.

 

I believe I adhered to the CTH Rev. A BoM for all resistor/cap values, apart from the LED. The headphones I hear the hum on are HD700s, which are, from the specs, much more sensitive than my K501. I hear the hum at all rotations of the volume knob, even at the lowest, although it's pretty faint, and difficult to pick out if music is playing. It's similar to the faint drone of an air conditioner in another room, or a buzzing fluorescent light. If I get to about 12 o'clock in the rotation the amp starts to get noisy, with some DAC noise, but that's at almost unlistenably loud levels. I listen at around 9/10 o'clock at most. I've tried touching pretty much every exposed piece of metal on the outside of the case - RCA grounds, headphone jack, volume knob, exposed area of metal case, all to no avail. The only thing that changes the hum a bit is fiddling with the AC jack's nut (loosening and tightening) but my back panel is pretty tight back there and I don't want anything accidentally bridging. I suppose I'll have to take it apart and take pictures anyway, though.


Edited by Ikarios - 2/1/13 at 10:43am
post #534 of 577

How do I wire up an LED under the Tube?  What pads do I use for it?

 

Thanks

post #535 of 577

You can use PCB's embedded e12 green/on/latch LED pads.  If you're already using it think the circuit will still behave if you parallel in another LED (test this).

Think I had to enlarge the PCB hole under the tube socket (e.g. w/a reamer), shrink-wrap LED leads & glued LED in socket's hole.

post #536 of 577

Thanks.  Still waiting on a part from ups, and then I can finish it up

post #537 of 577

I search and searched and maybe I just missed it, but is there any information on how much power the CTH can put out?  I'm looking into using it for some HE-500s and I've read that people are doing this successfully.  I'm just curious if we have ballpark numbers.  The HE-500's supposedly need 2 watts at 32 ohms.

post #538 of 577

IIRC runeight made a post about that within the last few years, e.g. how many mW CTH's buffer should put out before leavhing class A.  Guess you can use google search against this site or consider pinging him about it.

 

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).


Edited by cfcubed - 6/23/13 at 4:23am
post #539 of 577

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.


Edited by holland - 6/23/13 at 9:42am
post #540 of 577

I was looking for a substitute for 220uH inductor (L2H) which is available at mouser since I won't want to place an order from farnell just for a single item.

Would this one work :

http://in.mouser.com/ProductDetail/ABRACON/AIUR-02H-221K/?qs=C7smiywbPwWHRe3jq6sAEA==

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