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A Very Compact Hybrid Amp - Page 76

post #1126 of 2213
Possible indeed. At least we have some idea that some tubes are culprit to some of the noises we hear
post #1127 of 2213
I too find the 6n1p to be noisier than 12au7 varieties, although within this group the individual tubes vary. I have four 6n1ps and each has a different amount of noise and more or less noise on one side or the other.
post #1128 of 2213
I have also found when running 12AU7's that the background hum is minimal. The 6n1p's bring it out more. I haven't decided if it's simply due to gain (as adamus suggested) or if it's something particular to the 6 volt heater mode in general.

I haven't spent much time worrying about it mainly because I don't notice it at normal listening volumes, and because I have settled on using one tube for the last several weeks. I'm really loving the sound of the CBS-Hytron 7318 the best of all the tubes I've tried so far. It really makes my DT880's shine...
post #1129 of 2213
Odd, the one sachu made for me is totally silent but I don't use IEMs so can't test with really sensitive drivers but I did wazz it up full and touched various parts... nothing!

I really like this amp.
post #1130 of 2213
This "tube noise" that, I think, we've reached a consensus on, at least that 6n1ps are noisier, is not volume dependent. In fact, I hear it with the pot all the way down. It also does not involve touching anything.

The noise that gets louder when the pot is touched is, at least for me, volume dependent and not present when there is a source on the input.
post #1131 of 2213
Thread Starter 
If the tube noise increases with increasing mu then the problem is most likely noise at the input grids or ground noise from the line or other devices.
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 #1132 of 2213
I'm not so sure it is mu related. For instance, my 6211 is pretty quiet and is 27 amplication factor to 6n1p's 33, not so much difference, and no louder than 12bh7 at only 15 (actually my 12bh7 is noisier than it.)

This actually makes me think it is about higher current draw.
post #1133 of 2213
Thread Starter 
you mean higher draw from the heater supply? That certainly fits your pattern.

This is possible since higher draw means larger current spikes in the heater rectifiers and filter caps. More radiation and less good filtering.

cfcubed, do your heater circuit calculations permit someone to put a larger cap in for C3H? This would be a very interesting test.
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 #1134 of 2213
Quote:
Originally Posted by tacitapproval View Post
This actually makes me think it is about higher current draw.
My hotel room is a bit loud, but I can hear the very faintest buzz with volume all the way down -

Very low with 12AU7s
Slightly louder with 8416
Same with 6922
Slightly louder still with 6H30

Which would lead credence to the heater current theory..

BUT I don't recall hearing this except on a couple of tubes in the past...
post #1135 of 2213
Quote:
Originally Posted by wiatrob View Post
My hotel room is a bit loud, but I can hear the very faintest buzz with volume all the way down -

Very low with 12AU7s
Slightly louder with 8416
Same with 6922
Slightly louder still with 6H30

Which would lead credence to the heater current theory..

BUT I don't recall hearing this except on a couple of tubes in the past...
Maybe the hotel room would be quieter if you send the hookers away?
post #1136 of 2213
runeight, did my scope pics give any clue to the hum?

also, whilst it was being scoped, i did notice the offset wanders between 0 and 3mv quite quickly, would this indicate anything unusual or is it just the behavious of the buffers and servos
post #1137 of 2213
Alex, is it possible to wire two of these boards in bridge mode with dual supplies? OR does it suffer the same problems as a balanced config would..the dual splitters having an imbalance between them?
post #1138 of 2213
Thread Starter 
Quote:
Originally Posted by adamus View Post
runeight, did my scope pics give any clue to the hum?

also, whilst it was being scoped, i did notice the offset wanders between 0 and 3mv quite quickly, would this indicate anything unusual or is it just the behavious of the buffers and servos
adamus, what were you measuring in that picture?

offset wander 10mV or less is ok and normal. even more than this would be ok.
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 #1139 of 2213
output signal with inputs shorted.
post #1140 of 2213
Thread Starter 
what's the scale on the y axis? and can you do some measurments on different tubes with different heater currents?
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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