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Old 19th October 2013, 11:32 PM   #351
mm7 is offline mm7  Canada
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Quote:
Originally Posted by ChrisA View Post
It would be educational to try elevated AC heaters and measure the buzz with AC.
I tried. It is amazing! Buzz is less audible. But it is more hum.
May be due to sloppy wiring because DC was a target.
The heater wires are twisted, but there are inch or two untwisted wire between the sockets. Heaters between sockets are connected sequentially, with parallel connection inside a socket. So total Vh is 12.6V with 6.3 in each envelope.

Here is AC buzz/hum spectrum.
60Hz is louder, but higher harmonics are quieter, which sounds better for ears.
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Old 20th October 2013, 12:06 AM   #352
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R8 and R9 in the PSU schenatic are probably too large and this may be contributing to the buzz. Most tubes like to have a heater cathode resistance of about 20K which implies reducing your values to about one fifth of their prenet values and increasing their rating appropiately.

In practice, with large amounts of smoothing capacitance,you do not get current spikes 100 times the average current. They tend to be limited by the dc resistance of the transformer secondary and the ESR of the capacitors. I have never measured spikes greater than 10 times the average current. Buzz caused by diode current spikes can be cured by bypassing them with 100nF.

Cheers

Ian
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Old 20th October 2013, 04:35 AM   #353
ChrisA is offline ChrisA  United States
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Originally Posted by mm7 View Post
..
Here is AC buzz/hum spectrum.
60Hz is louder, but higher harmonics are quieter, which sounds better for ears.
I think now it is absolutely certain what the cause of this buzz is.

That is what I was getting at. Except in extreme cases AC can work fine. You could install a hum balance pot and better layout can maybe reduce the 60Hz by 3 to 9db. For most applications that is good enough. But for recording you need even better. That is when people resort to DC heaters. It is not needed in most normal HiFi amplifiers.
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Old 20th October 2013, 04:39 PM   #354
mm7 is offline mm7  Canada
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I targeted this preamp as a recording, not Hi-Fi. I know it is very naive for a first project. And it is ambitious and challenging, that is what I like for this hobby. So terms and costs are second priorities for this one.

What I've got from the AC experiment that even a large "ripple", which the 12V AC is, does not cause very loud hum when applied to upper tube's heaters that are 40V negatively "elevated". So DC with some mV ripple should be fine.

What creates buzz is some internal inductive coupling in the transformer. Or EMI from (over)loaded transformer? Not too much sure. I know if it is unloaded it does not buzz.

I am going to feed heaters from a separate power transformer. like one of these ones: Hammond Mfg. - Class 2 - Energy Limiting Transformers - ("BA" - "BE" Series)
though I do not quite understand what "energy limiting" means.
I like that they are boxed in shielding and are "Rugged, low noise, cool running design".
I hope this will not produce more buzz (EMI) than the current one.

What do you think?

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Old 20th October 2013, 05:40 PM   #355
ChrisA is offline ChrisA  United States
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I think(?) power limiting means they are safe to short. Their internal resistance is so high that they can't burn up. I've not used this exact model but I've seen transformers like this used for door bells and lawn sprinkler timers. They are designed to mount to a standard steel electrical junction box. The AC main side mounts using standard conduit threads.

The version I've seen sell at the local hardware store and are rated very low, like 50 ma. These Hammond parts have bigger spec'd current. I've never seen a "bell transformer" as they are called used for audio

I have used "wall wort" plug in AC power supplies and then led the power to my project using a coaxial plug. The plug-in boxes are kind of the same thing. In fact my Art "Tube MP" preamp uses a 12VAC wall wort to power a 12AX7 tube.
ART Tube MP Studio Mic Preamp | Musician's Friend

Yes, who would have guessed that a 12 volt "ripple" would not sound so bad? I think your logic is right, cutting 12V to 12mV by going with DC should reduce the 60Hz hum by 60dB. The only thing working aginst that logic isthat rectified AC has a 120Hz, not a 60Hz fundamental. The 120Hz is easier to hear.
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Old 20th October 2013, 08:27 PM   #356
mm7 is offline mm7  Canada
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Originally Posted by ChrisA View Post
I have used "wall wort" plug in AC power supplies and then led the power to my project using a coaxial plug. The plug-in boxes are kind of the same thing. In fact my Art "Tube MP" preamp uses a 12VAC wall wort to power a 12AX7 tube.
ART Tube MP Studio Mic Preamp | Musician's Friend
Tried with wall wart 12V. Buzz/hum is not audible.
On spectrogram it does not go higher then hiss.

I think it is the way to go.
The only concern, for safety, is there a way to stop +90V elevating voltage to go to wall wart cable?
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Old 22nd October 2013, 06:51 PM   #357
mm7 is offline mm7  Canada
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OK. Buzz is fought.
Is hiss at -90..-100 dB OK?
Hiss is quite audible when the attenuator is cranked up.
Can it be reduced?
Is input transformer the only solution?
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Old 24th October 2013, 10:46 AM   #358
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Originally Posted by mm7 View Post
OK. Buzz is fought.
Is hiss at -90..-100 dB OK?
Hiss is quite audible when the attenuator is cranked up.
Can it be reduced?
Is input transformer the only solution?
If your output hiss is better than -90dBu I would be amazed. How did you measure it?

Cheers

Ian
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Old 24th October 2013, 01:40 PM   #359
mm7 is offline mm7  Canada
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I plugged output into sound card E-MU 0404. And analyzed captured signal with Jack Audio Analyzer.
If you take a look at above post #351 you will see screenshot of the tool.
Currently I do not have buzz spikes. Hiss is on same place.
If I speak loudly or yell signal goes up to -20dB.
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Old 24th October 2013, 02:09 PM   #360
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Quote:
Originally Posted by mm7 View Post
I plugged output into sound card E-MU 0404. And analyzed captured signal with Jack Audio Analyzer.
If you take a look at above post #351 you will see screenshot of the tool.
Currently I do not have buzz spikes. Hiss is on same place.
If I speak loudly or yell signal goes up to -20dB.
OK, that signal is the noise spectrum. It is not easy to work out the noise over the entire bandwidth from that spectrum. Just about all it is safe to say is that the noise level is not -90 to -100dB. If and its a big if, that spectrum show the noise per root Hertz, then to find the noise level over the entire bandwidth (such as you might read on a meter connected to the output) you need to multiply by the square root of the bandwidth. The square root of 20,000 is 141 which, converted to dB is 43dB. So, given the above assumption about the spectrum, then if the noise bumps along at -100dBu then the actaul noise over the whole 20KHz bandwidth is 43dB higher than this i.e. -57dBu

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Ian
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