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A Very Compact Hybrid Amp - Page 88

post #1306 of 2213
tada

boxed up and the hum is gone. tiniest bit of white noise but no annoying distant wasp. I believe we have cracked it

Thanks guys!
post #1307 of 2213
Quote:
Originally Posted by adamus View Post
tada

boxed up and the hum is gone. tiniest bit of white noise but no annoying distant wasp. I believe we have cracked it

Thanks guys!
Congrats adamus. Step-by-step details/instructions please!
post #1308 of 2213

If experiencing a bit of hum w/CTH PCB rev 1.1 & prior, raising C3H seems the answer.

BoM case:
  1. remove existing C3H (47uf). Replace D5H (1N5820) with 1N5818 to gain room for larger C3H if necessary. Watch polarity.
  2. replace w/330uf* 50v cap that fits. Low impedance preferred. Use 35v rated cap if space demands it. Watch polarity.


Non-BoM case:
  • if space below PCB insufficient for cap, follow "BoM case" procedure above.
  • if space below PCB sufficient for cap, supplement existing C3H with 270uf -> 330uf* 50v cap that fits. Low impedance preferred. Use 35v rated cap if space demands it. Watch polarity.

* Edit/update: My personal listening tests indicate 247uf of low impedance capacitance is sufficient to eliminate any audible hum/noise contributed by CTH heater supply (in my setup). Scope showed minute improvement over this for 330uf AFAIR. Scope showed no improvement going to 470uf & above AFAIR.
As my posts note, comparisons were done against battery heater supply w/onboard CTH SM heater supply disabled... That's part of the process that lead to this suggestion.

Above revisions & findings support standard CTH tube heater target loads (up to 4W (tube series heater voltage * series amperage draw)).
Anyone experimenting w/heater draws > 4W must do just that - experiment
post #1309 of 2213
Nice work guys..though I have never had a problem with any of mine the guy I sold my prototype CTh to has a slight hiss/hum with IEMs..wonder if these mods would make that go away too.
post #1310 of 2213
awesome - I'll hold off on the fix until I get the case done. cheers
post #1311 of 2213
Regarding the 35V, watch the unloaded voltage (no tube). The reason why 50V was spec'd on the stock configuration is because unloaded 24V rectified is about 31V accounting for the diode drops. Unloaded transformers also run higher (though there will be the buffer load), and some may use different transformers. That's getting uncomfortably close to the 35V rating. For those that don't care (like me for my own personal build), I'll use 35V, but if building for others or you do care, use 50V.

In all likelihood, once Alex blesses it he'll move for a 50V replacement, seeing as the current value is 50V and going lower may have longevity issues.
post #1312 of 2213
Quote:
Originally Posted by holland View Post
Regarding the 35V, watch the unloaded voltage (no tube). The reason why 50V was spec'd on the stock configuration is because unloaded 24V rectified is about 31V accounting for the diode drops. Unloaded transformers also run higher (though there will be the buffer load), and some may use different transformers. That's getting uncomfortably close to the 35V rating. For those that don't care (like me for my own personal build), I'll use 35V, but if building for others or you do care, use 50V.

In all likelihood, once Alex blesses it he'll move for a 50V replacement, seeing as the current value is 50V and going lower may have longevity issues.
its a very good point you make.

unloaded rectified on my builds have been about 34 volts..so a 50 Volt cap is a must in my case.
post #1313 of 2213
Thread Starter 
Hey guys, nice work. I hope this solves the problem for everyone and we can get back to business.

Holland is right about the cap voltage. It may be hard to fit a 50V cap into the space available, even with a smaller diode so if you can't then use 35V. As long as the voltage stays below 35V you're probably ok. Temperature has a much larger effect on the life of electrolytics than voltage. But, running a 35V cap at close 35V will shorten its life.

When wiatrob is ready to make new boards we can address this problem. Hopefully there is enough room to move things around for this cap.
Dr. Cavalli gained notoriety with his first DIY amplifier projects. His success has blossomed into Cavalli Audio, a world leader in amplifier design.
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post #1314 of 2213
Alex + CFcubed, thanks for your help on this one.

I now have the feintest of noise, and i can live with it whereas before it was intrusive between tracks and silent sections.

Giving it a good listen now, its good and a very different sound to the bijou...... but the bijou is still your best imho. Its so rich and spacious.
post #1315 of 2213
Heh. If only my invisibility cloak worked when the wife comes up w/the honey do list

Only kidding of course. Nice when empirical proves theoretical tho.
post #1316 of 2213
my humble apologies! (check the edit)

any reason why this little beasty wouldnt work with a 6n6p? too little gain.

The heater current is big (700ma +) but looking at the circuit it should be able to handle it.... maybe with a heatsink on the switcher.
post #1317 of 2213
Quote:
Originally Posted by adamus View Post
any reason why this little beasty wouldnt work with a 6n6p? too little gain.

The heater current is big (700ma +) but looking at the circuit it should be able to handle it.... maybe with a heatsink on the switcher.

It does - I just called it the 6H6P (my Cyrillic is bad) in my six volt listening report from last night...
post #1318 of 2213
Quote:
Originally Posted by adamus View Post
my humble apologies! (check the edit)

any reason why this little beasty wouldnt work with a 6n6p? too little gain.

The heater current is big (700ma +) but looking at the circuit it should be able to handle it.... maybe with a heatsink on the switcher.
As wiatrob stated, it does. I've used it in mine for a while, till I checked the temps. Since then, I've added some venting to my prototype so I should be able to use it again.

I know wiatrob is running a 1A version, and I'll be joining that later on. Woo, woo, 900mA heater tubes...because it's there.
post #1319 of 2213
Quote:
Originally Posted by wiatrob View Post
Hmmm, what if you do? I keep thinking about a special HeadphoneAddict Edition.
The CTH HA SE - Hmmm... Is that one with an on off switch and an orange LED to light up the tube? (I bought an APS "1/8" male to RCA female adapter", so I don't need RCA jacks anymore).

I am trading off my EF1 and will likely start to miss having a little compact tube hybrid around, now that Blutarsky has my TTVJ Millett Portable Hybrid and my son has the Starving Student. My Qinpu A-3 tube hybrid is much bigger and more of a speaker amp, that just happens to drive many headphones via pre-amp out.
post #1320 of 2213
Thanks to everyone who made time to solve this hum problem.

My search at Digikey shows that a 50V 330uF Nichicon PW is the best option for the BOM case as far as what they have in stock. The dimensions are 10x20mm which should fit by my measurements, and the top of the cap will be level with ICH (allowing for a heater switch above).
...of course the 5mm leads will have to be bent slightly to fit the 3.5mm holes.
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