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A Very Compact Hybrid Amp - Page 75

post #1111 of 2203
5/4/09 at 6:50pm
Quote:
Originally Posted by sachu View Post
Can't wait for it..expecting it any day now..I have a bunch of other tubes to tide me over for the moment though.
I cant wait to hear what you think. It should livin things up a bit, most tubes are too warm and rolled off or bright like the RCA cleartop but the Siemens is is nice and clear and smooth and has a tiny kick in the bass department. If you try it and hate it, send me a PM ok.
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post #1112 of 2203
5/4/09 at 7:01pm
Anyone else had any luck with iem's and the CTH? Or, is this unreasonable?

Also, what would be the negative result of overspecing R18 to 200ohms? 150 is definitely quieter than 56 was. Tell me why this is a bad idea.
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post #1113 of 2203
5/4/09 at 8:36pm
Quote:
Originally Posted by tacitapproval View Post
Anyone else had any luck with iem's and the CTH? Or, is this unreasonable?

Also, what would be the negative result of overspecing R18 to 200ohms? 150 is definitely quieter than 56 was. Tell me why this is a bad idea.
I believe I posted that the CTH worked with my Sennheiser IE8 without noise, but with the Mullard tube I was using there was more bass than I cared for (an IE8 problem I've been complaining about).
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post #1114 of 2203
5/4/09 at 8:42pm
The ie8 is even more sensitive than the sa6, so hmmm... I wonder what accounts for my problem.
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post #1115 of 2203
5/4/09 at 10:02pm
tacitapproval - runeight's suggestions aside, I do get the faintest hum, noticeable between songs w/my D2000s, on both my CTH builds under certain conditions. Don't recall it being noticeable w/my less efficient, hi-Z DT-880s.

I do recall some others mentioning they detected this too, but I think what cans you use, what you place the CTH near (electrically speaking), what tube type used, and perhaps other things affect this. E.g. I found a couple of my 12AU7 variants the most quiet in this regard.

So don't think its unique to your build. Would like to track it down in my build(s) someday, but as the amp sound so good & the hum level is not detectable when musics playing, its low priority.
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post #1116 of 2203
5/4/09 at 10:18pm
My experience concurs with yours cf. Tube definitely has the greatest impact on the hum. It is more a niggling issue only because I often want to use this bedside at very low volume with iems, otherwise any noise from it is pretty negligible. To that end, I went ahead and ramped up r18 to 200ohms (actually 221, it's what I had)--noise is somewhat lower (not gone) and I don't notice any obvious degradation of signal quality.
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post #1117 of 2203
5/4/09 at 10:20pm
I had some hum when using it with my Gamma1 DAC running on DC power. Using USB power eliminates it.
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post #1118 of 2203
5/4/09 at 10:30pm
The noise I am describing is not source dependent.
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post #1119 of 2203
5/4/09 at 10:55pm
Are you using shielded cables from the input to the pot and then to the board? I had some hum while I was using unshielded wire. Once I replaced the input and output cabling with shielded, it resolved the hum problem..
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post #1120 of 2203
5/4/09 at 11:37pm
I did have shielded cable on it. I ended up removing them because of space and flexibility concerns, this in turn lead to problems with some stray shielding wires causing havoc on the board. It also did not alleviate the noise, although I can't now say whether or not it improved things versus the unshielded wires, as I don't remember well enough.
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post #1121 of 2203
5/5/09 at 1:03am
Thread Starter 
If the noise is dependent on the tube there are really only five possibilities:

1. Coming from the input. Possible if the input wires are exposed.
2. Coming from the B+. Possible, but the B+ is very well filtered and the CCS really rejects ripple.
3. Radiation from the PS section directly into the tube. This is also possible given the close spacing and the vertical diodes. But if it doesn't happen on all of the amps then it's hard to see why this would happen in some cases and not in others.
4. General EM from the surroudings. I guess your noise is not location dependent?
5. Grounding. This is always the toughest nut to crack. But the star ground scheme should solve this provided that the pot is wired correctly.

You might check out Q3P and Q4P, but this is a long shot. Definitely not a heater problem because the heater is running at 150kHz behind the chokes.
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post #1122 of 2203
5/5/09 at 1:11am
Question, would increasing the value of C4 improve the bass response? Or is the input impedence into Q4/Q5 sufficiently high that it would not make a noticeable difference increasing C4 to 470nF/1uF?
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post #1123 of 2203
5/5/09 at 1:55am
Quote:
Originally Posted by adamus View Post
tomorrow i will try grounding the pot, it gets much worse if i touch the pot...

i think the 8416 will be extremely hard to beat in this amp. even with the very slight hum, the 8416 is sounding superb.
All my amps have isolated pots including the CTH (still uncased). They all had that "hum" noise you describe, that same irritating hum that gets worse when you touch the pot. The wire trick from pot shaft to SG have eliminated all my hum/noise issues (aside from a recent noise I discovered with my SOHA II where it needed a ground loop breaker but that's another story).

My CTH is dead silent regardless of source or volume level, HOWEVER, this is only true if I'm using my 12AU7 tube. I can always hear a different kind of hum simply by swapping the tube with either one of my 6N1Ps (with heater set to 6.3v). The hum is very faint, and does appear to be slightly stronger in the left channel.
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post #1124 of 2203
5/5/09 at 2:14am
mines also close to zkool's 12au7 its quiet. With 6n1ps it has an irritable hum and they are interesting tubes, im beginning to really like.
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post #1125 of 2203
5/5/09 at 2:45am
Quote:
Originally Posted by zkool448 View Post
All my amps have isolated pots including the CTH (still uncased). They all had that "hum" noise you describe, that same irritating hum that gets worse when you touch the pot. The wire trick from pot shaft to SG have eliminated all my hum/noise issues (aside from a recent noise I discovered with my SOHA II where it needed a ground loop breaker but that's another story).

My CTH is dead silent regardless of source or volume level, HOWEVER, this is only true if I'm using my 12AU7 tube. I can always hear a different kind of hum simply by swapping the tube with either one of my 6N1Ps (with heater set to 6.3v). The hum is very faint, and does appear to be slightly stronger in the left channel.
Cheers Zkool448, I'll ground the pot and see what happens, I will also switch to shielded wire.

I switched in a 6n1p and as you suggest, the hum did get stronger but if i recall correctly, the 6n1p is a higher gain tube, so it could just be magnifying residual hum.
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