[Misterrogers]

I drive my HE-5's every day with a slightly modded CTH, and it has the chops (P->P) to wake it up quite nicely, so I think you'll be fine with the HE-4. SOHA II is a very nice amp too - seems it's primarily a cost choice.

 

[kchapdaily]

thanks for the advice. im leaning toward the cth i think. do i need to change any components to match the impedance of the amp to my
he-4's? and if i do go with the soha II, would i need to change anything on its bom either? im not quite clear on impedance matching, but my understanding is that if my amps impedance is greater than my headphones, thats bad. he-4 is a low impedance can, and i havent found anything on the output impedance of either the cth or soha II

 

[Mullet]

The CTH has an output impedance of 104ohms with R18. R18 = 100ohms. The natural output impedance with a "jumpered" R18 is 4ohms. Thus 104 ohms with R18 in place. If you're after the lowest output impedance possible then jumper R18L and R18R with two leads. R18 was designed to counter the super high gain of the amp. A 6DJ8 has 33 mu and a 12AU7 has a mu of 18. If you used high sensitivity headphones with a CTH with no R18 then you wouldn't get past 8 or 9 o'clock on your pot without the volume being too high.
 
Furthermore, my current understanding of planar magnetic headphones is that their damping factor isn't really effected by an amps output impedance because their impedance and phase doesn't change over the frequency spectrum. Thus no bass loss or weird frequency response. I can say that I'm fine with the defaults of the CTH R18. I guess you could always jumper them first and if it works for you then stick with it. Or conversely throw in the default value of 100ohms for R18 and if you don't like it take 'em out.

 

[kchapdaily]

ok perfect. if i go with the cth, i will set it up in the standard configuration, and if it doesnt sound right, i will jumper R18. i appreciate the response, very helpful!

 

[BK_856er]

Goes without saying, but socketing those positions makes changing the R18 values quick & easy and low risk.
 
The little CTH drives my LCD-2 very nicely.  Certainly not lacking in the bass department.  Pretty sure I'm using the default R18 value or something close to it.  Sounds so good I haven't been motivated to go back in!
 
BK

 

[kchapdaily]

thats definitely good to hear. the he-4's wont sound quite as nice as the lcd-2's though 

. does anyone have a basis for comparison between the cth and the soha II? also, is this the best place to get the board for the cth? http://joeaudiophile.com/?product=compact-tube-hybrid-amplifier-printed-circuit-board

 

[cfcubed]

You could search here (or use google to) and look for SOHA II vs CTH impressions, there may have been some.  That link you posted is the only place to get the board

 

[Misterrogers]

And rumor has it, that that place (Joe Audio) will soon have full kits up, with some awesome custom, predrilled aluminum cases with hand rubbed cherry wood end caps

 

[mmayer167]

^ this. Very cool

 

[kchapdaily]

wow a kit would make this a whole lot easier, but maybe not as fun 

. i think i was going to go with a metal body and wooden endcaps for this case anyway, so that would certainly be cool. any idea what kind of timetable they have for release?
 
also cfcubed: i thought i searched a while ago, but turns out i forgot what i found. searched again, and answered my question. soha II wins, but marginally, and its more expensive to build. its also got a larger footprint. going to build the cth first, but will eventually be building the soha II anyway.

 

[Misterrogers]

heh - to each their own; I never found the sourcing of parts that fun I'll let you know when I have some photos of the cases. Cases should be available in a week or so, kits about the same timeframe.

 

[BK_856er]

I've never heard the SOHAII, but the CTH has tremendous grins/$ and grins/cm2, and it holds its own against many larger and high-dollar setups.  The component density can be challenging, but it's a very satisfying project that you can literally "hold in your hand" and admire.  In a lot of ways it's my favorite amp.  Go for it!
 
BK

 

[mrhk13]

Quick question for Mullet on gain of the CTH. I'm using Sony MDR-SA3000 headphones, 70 Ohm impedance, sensitivity of 100 dB/mW. I find that with the 12AU7 tubes I barely make it to 9 o'clock on the volume; with the 6-series I can just about do 8 o'clock. Based on your last post, should I increase the value of R18 to lower the CTH gain and so get more fine control over the volume? If so, any suggestions on the value for R18? Thanks!
 

 

[kchapdaily]

@misterrogers: beautiful. depending on price, i will certainly be ordering one of those kits. if i can avoid sourcing parts right now, i would like to. im a busy college student, and i dont have as much time as id like for projects unfortunately.
 
@BK: glad to hear you enjoy the cth so much! im looking forward to putting one together 

 

[Mullet]

Quote:

Quick question for Mullet on gain of the CTH. I'm using Sony MDR-SA3000 headphones, 70 Ohm impedance, sensitivity of 100 dB/mW. I find that with the 12AU7 tubes I barely make it to 9 o'clock on the volume; with the 6-series I can just about do 8 o'clock. Based on your last post, should I increase the value of R18 to lower the CTH gain and so get more fine control over the volume? If so, any suggestions on the value for R18? Thanks!
 

 

High sensitivity headphones would benefit from higher R18 values. I'd say try 150-200 ohms to get more out of your pot. Keep in mind with 1/8 damping factor you'd want way less than even 104ohm output impedance. 70/8=8.75ohm output impedance is what you'd want if you were going by the book. I guess at that point you'd have to decide if the damping factor really means anything to you and your ears.
 
Otherwise, you could up your pot to 100k and see if that helps. I'm not sure what the input impedance of the CTH is though. I'd assume 50k was picked for a reason. From what I know with other amps like "The Wire" I had to up the input impedance to 500k ohms to match a 50k pot. It was advised to not use the pot to correct gain problems. Hopefully, someone else will chime in with more info on if changing the pot to a higher value will help or is correct in this situation.
 
Most of my headphones (orthos) are at the 12 o'clock range with a 50k pot and 12AU7 tube. My 32ohm Magnums are maybe at 11 o'clock or a tiny bit less.