[audiogeekery]

Haven't come across it, but generally amp makers give us a high enough input impedance that it shouldn't be a real factor in gear matching. Unless you have a source with an unusually high output impedance? Or is there something else I'm not taking into account.

Output impedance, on the other hand can really impact the freq response for some headphones (looking at you here, Andromeda) and/or speakers. But this is a factor only for the amp that is directly driving the transducer (headphone or speaker). Again, unless I am missing something.

I was also trying to find the input impedance for this thing! I want to hook up my Croft Tube pre amp and phono stage to listen to vinyl but wasn’t sure.
That preamp has an output impedance of < 300 Ω so I’m not sure if it’s okay to try?

Anyone feel confident to chime in? Would be much appreciated!

 

[Hansotek]

I was also trying to find the input impedance for this thing! I want to hook up my Croft Tube pre amp and phono stage to listen to vinyl but wasn’t sure.
That preamp has an output impedance of < 300 Ω so I’m not sure if it’s okay to try?

Anyone feel confident to chime in? Would be much appreciated!

You'll be just fine hooking up to the Croft. You don't really have to worry about impedance matching with the source. Resistance matters a lot more when you're matching an active component to a passive one. Between two active components, it's not much of a concern.

Which Croft do you have, the Micro 25? I want one of those. That phono stage sounds sweet.

 

[audiogeekery]

You'll be just fine hooking up to the Croft. You don't really have to worry about impedance matching with the source. Resistance matters a lot more when you're matching an active component to a passive one. Between two active components, it's not much of a concern.

Which Croft do you have, the Micro 25? I want one of those. That phono stage sounds sweet.

Thank you!! Yes, indeed it is the Croft Micro 25. Love it to bits. I’ll also try the line stage to see if Tubes from there make a difference with the DAC (Mimby) fed to it, but it’s mainly the phono stage and it’s a glorious one.

Hope you manage to snag one, soon!

 

[someyoungguy]

Woohoo, final piece of the tube puzzle arrived today! Tubemonger socket saver, pre-used Ken Rad 6C8G tube and 6922 to 6C8G adaptor off eBay. I’ve also got some ferrite snap ons for the wire on the way from amazon.

First of all I want to say a huge thanks to the people on this forum: I would have never dreamed of doing this or known where to start if it wasn’t for the helpful posts on here. The sound out of this set up is great - fantastic soundstage and sense of space between the instruments, great dynamics.

But secondly, I’ve got the same problem as Zachik - quite a large hum, mainly coming from the right channel, which is interesting in that the right channel is mainly where I have scratch from the volume pot. I noticed after picking up my DAP (which is the source, connected by 3.5mm to RCA cables) that the hum was reduced, so experimented a bit and have figured out the hum is lower pitch and lower volume when I touch the CTH case or connected DAP, and if I do that in combination with holding my other hand over the top of the tube, the hum is reduced even further to nearly gone. Also the hum is the same volume regardless of how far up the volume pot is turned. So it seems to me a combination of grounding and RF interference?

As mentioned I have some ferrite clip ons on the way, so hope that will reduce RF interference. I’ll keep experimenting to see if I can get the noise to reduce as the actual sound quality is awesome. When music is playing the hum isn’t too bad, but immediately emerges from the background when the music switches to a quieter passage.

P.S. I’m using the CTH with B&W P7s. I haven’t got any higher impedance headphones to try it out with.

 

[Zachik]

Woohoo, final piece of the tube puzzle arrived today! Tubemonger socket saver, pre-used Ken Rad 6C8G tube and 6922 to 6C8G adaptor off eBay. I’ve also got some ferrite snap ons for the wire on the way from amazon.

First of all I want to say a huge thanks to the people on this forum: I would have never dreamed of doing this or known where to start if it wasn’t for the helpful posts on here. The sound out of this set up is great - fantastic soundstage and sense of space between the instruments, great dynamics.

But secondly, I’ve got the same problem as Zachik - quite a large hum, mainly coming from the right channel, which is interesting in that the right channel is mainly where I have scratch from the volume pot. I noticed after picking up my DAP (which is the source, connected by 3.5mm to RCA cables) that the hum was reduced, so experimented a bit and have figured out the hum is lower pitch and lower volume when I touch the CTH case or connected DAP, and if I do that in combination with holding my other hand over the top of the tube, the hum is reduced even further to nearly gone. Also the hum is the same volume regardless of how far up the volume pot is turned. So it seems to me a combination of grounding and RF interference?

As mentioned I have some ferrite clip ons on the way, so hope that will reduce RF interference. I’ll keep experimenting to see if I can get the noise to reduce as the actual sound quality is awesome. When music is playing the hum isn’t too bad, but immediately emerges from the background when the music switches to a quieter passage.

P.S. I’m using the CTH with B&W P7s. I haven’t got any higher impedance headphones to try it out with.

Finally! Another person with same issue. Not that I am happy you have an issue, but at least I am not alone... which starting to make me wonder if that is a design issue.
@runeight Alex - I did notice that if I touch certain location with my finger (like the contacts on the back of the XLR4 connector, which I do not use since I use the 1/4"), the hum is greatly reduced! I wonder if there is a grounding issue that causes this hum...
@someyoungguy - for me, the ferrite made NO difference.

Testing with a much harder to drive headphones (HD600) - I can barely hear any hum. I need to pay attention and look hard for it... With AEON Closed - it is definitely there, but not very loud at all. Campfire Cascades - unusable! Super loud hum!!
That makes me wonder whether everyone has the hum, but (by sheer chance) most use harder to drive / less sensitive cans and therefore cannot or do not notice the hum?
Do ANY of the guys that use the 6C8G and experience no hum (or so they think...) got access to Cascade to try?

 

[runeight]

If I may say.........

I know that you guys are having a great time with this particular mod and, apparently, it sounds pretty good too. This is definitely part of the fun of this hobby. I know bc I do this kind of stuff too, but.......

This is not a design issue. You are making physical mods to the amp which are outside the intended use and enclosure. You are sticking big, complex antennae up from the center of the board into the tube socket which is where all the gain of the amp occurs. Any noise introduced into the this part of the amp is likely to be amplified.

Because of this, it could very well be that not every instance of this mod will be as quiet as a mouse.

Remember, the CTH (noted in manual) uses a floating power brick. This means that is does not have an AC wall ground (a situation common to many consumer electronic products). It takes its ground from whatever it plugged into the RCA jacks. Similar to a portable amp. Whether the source has an AC ground or does not, doesn't really make any difference to how the two devices settle their mutual ground. When you touch a ground point on the amp with your finger, particularly with no input connected, you are possibly providing another ground point which may or may not change the noise level.

But, MHO is that the real issue is the major physical mod of the large tube, plate cap and wire, extenders, and adapters. So, just keep this in mind when you're making this mod.

 

[Zachik]

If I may say.........

I know that you guys are having a great time with this particular mod and, apparently, it sounds pretty good too. This is definitely part of the fun of this hobby. I know bc I do this kind of stuff too, but.......

This is not a design issue. You are making physical mods to the amp which are outside the intended use and enclosure. You are sticking big, complex antennae up from the center of the board into the tube socket which is where all the gain of the amp occurs. Any noise introduced into the this part of the amp is likely to be amplified.

Because of this, it could very well be that not every instance of this mod will be as quiet as a mouse.

Remember, the CTH (noted in manual) uses a floating power brick. This means that is does not have an AC wall ground (a situation common to many consumer electronic products). It takes its ground from whatever it plugged into the RCA jacks. Similar to a portable amp. Whether the source has an AC ground or does not, doesn't really make any difference to how the two devices settle their mutual ground. When you touch a ground point on the amp with your finger, particularly with no input connected, you are possibly providing another ground point which may or may not change the noise level.

But, MHO is that the real issue is the major physical mod of the large tube, plate cap and wire, extenders, and adapters. So, just keep this in mind when you're making this mod.

Thanks Alex for the very prompt and informative reply! Much appreciated.
I have a quick follow-up question, if I may:
Granted the design of the ground as described by you above, is there any practical trick (or mod) to add or force a better ground (other than holding my finger to the right spot while listening to music )?
You mention the ground comes from the RCA inputs - any mod to the RCA cables to make them ground better regardless of the DAC they come out of?
Or any mod to the amp itself to add a ground (without doing anything extreme to change the amp itself)?

Alex, I understand we're operating here outside the designed envelope (thank for the explanation), and bad comes to worse - I will only use this mod with HD600 and other less-sensitive cans. But would still be nice to improve / fix the issue if at all possible using "custom" RCA cables or any other trick / mod to improve the grounding.

 

[Marlowe]

Hmm, I was thinking of possibly giving this setup a try. I've gone halfway: I'm using a NOS Tung-Sol 2C51 with an adapter (I already had both pieces in my Valhalla 2, which I've moved to my TV rig with JJ EE88CCs). I think the 2C51 makes small but worthwhile improvements, though not as dramatic as reported with the 6C8G. A good example is the impact of the bass and drums on the remastered Out on the Weekend from Neil Young's Harvest; it sounds great on the stock tube, but just a bit greater with the 2C51. I've been running this for a few weeks without issue. No hum, the amp still runs only slightly warm, the tube remains moderately warm and safe to touch (or even grasp) even after a few hours of play. Given the issues that at least some that have tried the 6C8G tube are having, I think I'll stop with the more modest 2C51 mod.

 

[Phantaminum]

If I may say.........

I know that you guys are having a great time with this particular mod and, apparently, it sounds pretty good too. This is definitely part of the fun of this hobby. I know bc I do this kind of stuff too, but.......

This is not a design issue. You are making physical mods to the amp which are outside the intended use and enclosure. You are sticking big, complex antennae up from the center of the board into the tube socket which is where all the gain of the amp occurs. Any noise introduced into the this part of the amp is likely to be amplified.

Because of this, it could very well be that not every instance of this mod will be as quiet as a mouse.

Remember, the CTH (noted in manual) uses a floating power brick. This means that is does not have an AC wall ground (a situation common to many consumer electronic products). It takes its ground from whatever it plugged into the RCA jacks. Similar to a portable amp. Whether the source has an AC ground or does not, doesn't really make any difference to how the two devices settle their mutual ground. When you touch a ground point on the amp with your finger, particularly with no input connected, you are possibly providing another ground point which may or may not change the noise level.

But, MHO is that the real issue is the major physical mod of the large tube, plate cap and wire, extenders, and adapters. So, just keep this in mind when you're making this mod.

You’re right on this. I think after some experimenting *I* (not MD) can only say this mod is only a good mix with 300 Ohm headphones but planars/sensitive headphones could/will experience issues. As I mentioned before as well this is a try at your own risk mod.

 

[someyoungguy]

Yeah I absolutely understand this is a DIY mod

I’m thinking of connecting a ground wire to the body of the CTH and wiring it to a plug, to plug it into a different socket, effectively adding a ground via a separate plug. I’m only using a portable DAP as a source so maybe using a desktop DAC would provide a better sound...hmm like a Massdrop R-2R

I had been thinking of building a little wooden case/stand for the CTH to provide it a bit of protection, so might try doing that sooner rather than later to see if it reduces the RF interference.

@Zachik That’s a pity about the Cascades, I intend to pick up a pair as an upgrade to the P7s. If worst comes to worst, I’ll look into getting a high impedance headphone to use exclusively with the CTH, maybe some HD6XXs again or preferably high impedance closed backs.

 

[Zachik]

I’m thinking of connecting a ground wire to the body of the CTH and wiring it to a plug, to plug it into a different socket, effectively adding a ground via a separate plug. I’m only using a portable DAP as a source so maybe using a desktop DAC would provide a better sound...hmm like a Massdrop R-2R

Massdrop RDAC (R-2R) is one of 2 sources I have been using to test the CTH... Unfortunately, that is NOT the solution!
@someyoungguy - I will be VERY curious to hear back from you after you experiment with adding ground to your CTH...
Hopefully, @runeight Alex would reply to my previous post asking about ways to mod the RCA input cable or the amp itself to add / improve ground.

I understand everything we're doing here is DIY / at our own risk!

 

[someyoungguy]

Massdrop RDAC (R-2R) is one of 2 sources I have been using to test the CTH... Unfortunately, that is NOT the solution!

Ah right, now that I look at the pics on the Massdrop website I see the plug for that isn't grounded either. Hopefully I'll get some time to trial things this weekend, otherwise it might a few weeks before I get to play around with grounding it to the mains earth.

 

[audiogeekery]

Just an update: Line Out from the Croft Micro 25 preamp worked wonders into the CTH. Still in awe of this smooth volume control!

Now I have more control with premium ECC82 tunes for rolling ahead of the amp. I didn’t anticipate it working as well as it did. Lots of air and wider soundstage, etc. Only tried out the Mimby through the linestage. Tonight I’ll test the phono stage on my HD6XX’s with the CTH.

Thanks all!

 

[Zachik]

Quick update on the 6C8G "mod" and hum:
I am very grateful to @CEE TEE who has sent me a loaner CTH, so I can test to see whether the hum was "by design" (as @runeight suggested it could be) or just my unit...

After testing every permutation between 2 CTH, 2 power adapters, and 2 tube (6C8G -> 6922) adapters - the conclusion is clear (IMHO):
The "6C8G mod" is only meant for high impedance headphones (as noted a few posts ago by @Phantaminum).

So far, I tested with 3 headphones:
1. Campfile Cascade - sensitive, easy to drive and efficient. Un-usable with 6C8G as hum is way too loud.
2. Mr. Speakers AEON Flow Closed (AFC) - less sensitive / less efficient. Hum still there, but substantially lower. Not recommended with 6C8G but could work for some.
3. Sennheiser HD600 - much higher impedance and lower efficiency. This one pairs very nicely with 6C8G.

I have several more cans in my stable - will try some of them with the 6C8G in the next couple days and report back...

 

[FLguy]

FWIW, it seems that there may be other factor(s) at work than just the headphone impedance and efficiency...I tried this mod with a pair of LCD-X (impedance around 20 ohms, 103dB/1mw), and I cannot hear any hum at any volume level. YMMV of course.