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Old 4th May 2009, 08:53 AM   #1
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Default Bias Current Mythbuster?

Hi,
I read from Pg 91 Audio Reality by Bruce Rozenblit

"Here goes another myth down the drain, There is no precise bias current setting for optimum operation. There are only range of operation. Generally speaking, the heavier the bias current, the warmer the tube will sound. Some people think warmer is better. I definitely do not. My design are based on maximizing accuracy. What is critical here is not the absolute level of current , but the balance of current. Output transformer cannot tolerate much of DC current imbalance. Too much imbalance will cause premature saturation & severe distortion."
....

He then went on describing how he adjust the bias voltage of the tube to obtain current balance in both sections of the pp output stage.

As an engineer (not EE but ME) I can understand the science behind the PP transformer but really don't know what to believe any more regarding the part in the bias point now!

I raise this subject here to hear what the great minds here has to say and hopefully, will help clear my thoughts.

Cheers

Ken
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Old 4th May 2009, 10:46 AM   #2
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Certain transformers require perfect current balance or they saturate as described. These are ones without airgaps such as toroidals. These tend to have low hysteresis which is good when the current direction changes because micro detail is preserved at these very low signal levels.
In the case of airgapped transformers hysteresis will be much higher and will tend to produce loss of micro detail at low signal levels. This is why SE transformers can be better at micro detail. However some people prefer to run their output transformers with a bit of DC imbalance as this minimises hysteresis effects at low signal level.

Personally I opt for the none airgapped transformers run at zero offset. My prefered transformer is the toroid. The advantage are
1-wider bandwidth
2-less hysteresis induced blurring at crossover - better micro detail.
3-almost total cancellation of residual power supply hum from the finals. In theory if the current balance were perfect you could run with no power supply capacitance and still have zero hum !!

Shoog
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Old 4th May 2009, 11:52 AM   #3
monyse is offline monyse  Canada
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In push-pull operation is hard to avoid imbalance especially as tubes are aging and this may require often rebiasing to get back to the sound you were used to; sometimes you may even have to change one or both tubes though they are still good but impossible to balance. I prefer to use balanced pp schematics where non or very little DC is flowing through the primary winding like the circlotron or bridge arangements ; these arengements require more components ( tubes or power supplys etc. ) but allow for not expensive single end transformers without a gap, often power toroidal transformers without compromising to much on the sound.
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Old 4th May 2009, 03:33 PM   #4
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Quote:
Originally posted by Shoog

3-almost total cancellation of residual power supply hum from the finals. In theory if the current balance were perfect you could run with no power supply capacitance and still have zero hum !!

Shoog
That will be cool! Has anyone attempted?

What about the part on range of operating bias?


Cheers

Ken
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Old 4th May 2009, 07:50 PM   #5
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Quote:
Originally posted by Shoog

In the case of airgapped transformers hysteresis will be much higher and will tend to produce loss of micro detail at low signal levels. This is why SE transformers can be better at micro detail.

Shoog

huh? aren't SE transformers air-gapped? Now I'm confused....
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Old 4th May 2009, 08:04 PM   #6
Jeb-D. is offline Jeb-D.  United States
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Quote:
huh? aren't SE transformers air-gapped? Now I'm confused....
They are, and have rather large hysteresis. However, a conventional SE amplifier operates on a linear part of the BH curve. Current never changes direction, there is only varying amount of current flowing in one direction. So hysteresis doesn't really matter for convention SE transformer.

For a push pull transformer, current does change directions so hysteresis becomes more important. This makes air gap undesirable in push pull transformers. But this also means that it is important to keep the current matched fairly close between each half of the winding. An equal current flowing through each half magnetizes the core in opposite directions which cancels each other out. So the transformer sees no DC when each side is matched.

Parallel feed (parafeed) single ended transformers are like push pull transformers, but require a coupling cap to keep DC out. Since there is no opposing current source in SE to cancel DC effects on the transformer.
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Old 4th May 2009, 08:13 PM   #7
rknize is offline rknize  United States
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They are air-gapped but they are inherently unbalanced because there is only one tube drawing current through the primary. So you don't have to overcome hysteresis in the core.
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Old 4th May 2009, 08:29 PM   #8
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Quote:
Originally posted by Shoog

3-almost total cancellation of residual power supply hum from the finals. In theory if the current balance were perfect you could run with no power supply capacitance and still have zero hum !!

...in pauses, because your music will be inter-modulated with 120Hz and it's harmonics.
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Old 4th May 2009, 11:35 PM   #9
Sheldon is offline Sheldon  United States
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Like Wavebourn said. As soon as you have signal the transformer is unbalanced, hence the intermodulation. It's easy to see here (compare post 92 and 91) http://www.diyaudio.com/forums/showt...247#post801247

Sheldon
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Old 4th May 2009, 11:39 PM   #10
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So, as soon as it will have no hum in pauses, you may run it even without a power supply at all!
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