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Old 16th October 2010, 06:21 PM   #1
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Default BJT Bias?

I'm looking for a tube bias mechanism that has a higher voltage drop and lower impedance than an LED string. Here's what I came up with. Initial testing shows lower overall distortion than LED bias, particularly for 3rd and higher harmonics, and way lower distortion than resistor + cap bias, but a bit of a rising frequency response. There is only a ~.5db rise from 40Hz to 15K, but it is audible as being a little bright. Any thoughts on viability, or issues that might crop up? Or what BJT to use? I might try a mosfet, too, to see if that's any better.
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Old 16th October 2010, 07:15 PM   #2
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Sziklai pair could hold your PNP emitter at a more constant current,
and let either an NPN or N-MOSFET dump the big current swings...
(dump currents could optionally be returned directly to the plate)

I don't actually know if that would solve your rising response issue?
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Old 16th October 2010, 08:14 PM   #3
DF96 is offline DF96  England
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Zeners can be noisy, but this might not matter too much in an output stage. I'm not sure about the rising response - could it be the base-collector capacitance of the BJT shunting the zener at high frequencies? If so, a capacitor across the zener would solve both problems.
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Old 16th October 2010, 11:20 PM   #4
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No clue if this could even work?

If U1 was a MOSFET instead of an 811:
Would need resistor from Q1 collector to
ground. To bleed constant current across
the threshold voltage of the MOSFET.
This would hold emitter of Q1 and 6SN7
loadline at the same constant current.

Probably nothing constant when driving
811 in A2. Pay $$$, take you chances.

6SN7 plate feedback enforces Mu.
Or so Triodlington theory would have it...

R1 R2 might could be pre-amp filaments?
Assuming no requirement to elevate...
Then again, might be better a pot.
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Last edited by kenpeter; 16th October 2010 at 11:49 PM.
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Old 17th October 2010, 12:22 AM   #5
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I get lower THD with a resistor.
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Old 17th October 2010, 01:35 AM   #6
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If loudspeaker was a simple resistor, I could see the logic in that.
Depends how, where, and why you measure THD.

Triode makes low THD with real load voltage feedback to plate.
Current feedback from cathode is a stab in the dark guestimate
whenever load impedance is electromechanical moving target.

Current mode damps no energy stored in mechanical resonance,
so am not fallin' for oversimplified Ampere turns reasoning either...

6SN7 plate makes for Mu in example above, R4 is mostly hidden
by the collector of Q1. Before anyone points out the conflicting
fact that I too have drawn R4 an unbypassed cathode resistor.

Last edited by kenpeter; 17th October 2010 at 01:51 AM.
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Old 17th October 2010, 05:43 AM   #7
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Ahh, the "ignorance" defense...

"We don't know what the load is, so we can't tell if one thing or another is better!"

So wait, why was the question asked in the first place?!

But there is something useful that we CAN use. It's called logic. You don't have to take it off with your shoes when you step inside the forum, you know.

It's reasonable to assume a loudspeaker is a linear element. A very complex one, but still linear. You increase the voltage (or current) to the speaker, it gets louder. Frequency has absolutely nothing to do with it. If it's linear, and if it works at one frequency, then it will work at all frequencies, simultaneously. That's all that's needed.

Since a loudspeaker is a linear element, it is true, in all cases, that if you reduce the distortion of the signal source, you WILL reduce the distortion at the output.

This isn't rocket science.

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Old 17th October 2010, 06:51 AM   #8
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Quote:
Originally Posted by Sch3mat1c View Post
I get lower THD with a resistor.
I don't.
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Old 17th October 2010, 08:45 AM   #9
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What's the distortion with a zener string I wonder? I have 3 amplifiers with LED cathode loads and want to try zeners on one of them.

Sometimes I wonder if we aren't just creating another regulated power supply underneath the cathode, but if it's good and simple, there are benefits. I am very interested in this direction.

I found, with LED's as cathode loads, an extended freq response both up high and down low, maybe that's why you think your cct may be a bit "bright", because you are hearing high freq detail you didn't hear before? Just a thought...

Sala's shunt section of his DC shunt supply might be a good candidate for this task, but still has the difficulty of fine-tuning the required voltage. Maybe you could get some ideas from that schem.

Last edited by Ian444; 17th October 2010 at 09:05 AM.
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Old 17th October 2010, 03:19 PM   #10
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Yes creating regulated virtual ground under the cathode,
thereby real ground becomes our fixed bias. Same exact
things, only completely different...

Zenier string under the cathode? I'd say stick with 5~6V
types, as those have the most stable tempco... You need
bleed some extra milliamps through them to assure they do
not operate on the knee of the curve.

Also nothing wrong wit Crusher's emitter follwer, cept HF
bypass might be a good upgrade as previously noted...
Zeniers supposed to be white noise generators, probably
true. Yet strange I have never encountered a situation
where I actually been able to hear that effect...
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