Taking the tube plunge!

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FullRangeMan

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Re: Taking the tube plunge!
« Reply #40 on: 23 Oct 2017, 05:22 pm »
If its not impacted by the volume control pot from the amp, I would say its generated by the tweeter or xover parts or even wet inside the enclosure etc imo.
There too are many possibilities to generate a hiss.

Docere

Re: Taking the tube plunge!
« Reply #41 on: 24 Oct 2017, 06:50 am »
Now only question is very very low level hiss out of left channel. Has been there since the beginning I think. Can only tell its there if you put your ear to the speaker; no impact on listening, can't even detectable if your ear is not right up against the speaker (not impacted to volume control).

I've read other places that this is typical of tube amps, in particular with medium to high sensitivity speakers (mine are 94db)....

Thoughts?

Low level hiss typical of valve amps? I run circa 99dB efficient speakers. My first amp build was decent - solid, pretty-well laid out, not fancy - and runs an average power supply and AC filaments/heaters. No hiss, just a clean hum audible foot or less from the main speaker driver (AC filament). No other noise excepting a brief subdued thrum about a second after switch-on. I'm not familiar with your amp; maybe hiss is due to regulator noise, CCS or layout? In any case, it is so low in amplitude that it is hardly worth thinking about.

Still, I'd like to clarify that hiss is not a mandatory cost of admission to tube amps.

Edit: amended post after re-reading the post: hiss is very faint.

Cheers,
Ray
« Last Edit: 24 Oct 2017, 09:19 am by Docere »

FullRangeMan

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Re: Taking the tube plunge!
« Reply #42 on: 24 Oct 2017, 05:11 pm »
Some tube amps even without noise pot have no his or hum, it depends on details of the builder's assembly, such as routing of parts and distance between parts and top table.
So the top table should be aluminum that is not magnetic and still absorbs the noises.

Docere

Re: Taking the tube plunge!
« Reply #43 on: 25 Oct 2017, 03:09 am »
Some tube amps even without noise pot have no his or hum, it depends on details of the builder's assembly, such as routing of parts and distance between parts and top table.
So the top table should be aluminum that is not magnetic and still absorbs the noises.

I don't want to drag this off topic; I'll post once only to make a correction/qualification.

You are correct regarding hiss and hum, but with qualification: hum will be audible to a degree if running directly heated tubes with AC on the filaments and efficient speakers. For example, a quality 2A3 SET build running good tubes and AC filaments will have 2.5 to 3.5mV AC at the binding posts. I've gotten as little as ~1.5mV from a 2A3, but hum was still audible very close to the speaker. PP with AC filaments can get lower, due to cancellation effects. DC filaments, if well-implemented, should result in an essentially silent system.

Aluminium is an inexpensive material that is light, is easily worked and has okay-ish shielding properties for audio; exposed aluminium oxidises, making electrical bonds (earthing, 0v signal grounding) problematic. Steel is a better shield of audio-frequency EM radiation, for example from audio transformers. Steel does not shield from high frequency RFI (in the GHz range) like aluminium, but that is not an issue in domestic tube audio circuits. The Austenitic Stainless Steels are nearly ideal: common 302 grade is essentially non-magnetic, has good shielding properties for audio, is strong, does not oxidise under domestic conditions, can finish nicely, is not crazy expensive... but can require specialised skill/tools to work.

Cheers,
Ray

FullRangeMan

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Re: Taking the tube plunge!
« Reply #44 on: 25 Oct 2017, 03:20 am »
Interesting, I had read good reports on 10mm acrylic and Corian as top plate, both looks good and are resonance sink similar to Teflon.