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Old 30th March 2015, 06:34 PM   #1
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Default Led biasing or fixed biasing?

Can anybody in the know, clear this up for me.
- What are advantages and disadvantages of both implementations in power amplifier output stages?
- Why would one choose one over the other?
I am only asking, since both are as complex and costly as each other.
Also "fixed bias" means a negative supply with trimmers to adjust bias.

Last edited by KenTajalli; 30th March 2015 at 08:41 PM. Reason: confusing question
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Old 30th March 2015, 06:35 PM   #2
M Gregg is offline M Gregg  United Kingdom
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The two are completely different..

And dependant on power..

The LED idea does not use electrolytic caps so there is no aging and cap failure..

Fixed biasing requires a DC bias supply..how are they as expensive as each other?

A DC supply requires supply caps and trimmers and fail safe design. (Fixed bias)

LED in low power can cause problems not enough current to stabilise or light the LED..


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Last edited by M Gregg; 30th March 2015 at 06:40 PM.
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Old 30th March 2015, 06:37 PM   #3
DF96 is offline DF96  England
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LED biasing is almost the same as fixed biasing, at least for small signal Class A stages. LED bias is cheaper and simpler.
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Old 30th March 2015, 06:42 PM   #4
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Quote:
Originally Posted by DF96 View Post
LED biasing is almost the same as fixed biasing, at least for small signal Class A stages. LED bias is cheaper and simpler.
I take "small signal class A" as a valid answer for cheaper and simpler.
I suppose I should have added:
" In Power amp final stage" as it is mostly there that one may consider either.
Say 10W to 40W amps.
Also as "fixed Bias" I meant negative supply with trimmers to set the bias.
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Old 30th March 2015, 08:30 PM   #5
Magz is offline Magz  United States
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For small signal stages fixed bias can be as simple as a battery. I use a lithium 9V in my preamp and it should last up to 10 years. It's been 4 years so far and it still measures as new.
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Old 30th March 2015, 08:43 PM   #6
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by KenTajalli View Post
I am only asking, since both are as complex and costly as each other.
With LED bias, some possible voltage is 'wasted', so output swing (i.e. output power) is reduced compared with fixed bias.
You need a lot of LEDs to handle the current and acheive a useful bias voltage with most power valves. Mounting all those LEDs can be inconvenient. I'd say this qualifies as 'more costly' than simple fixed bias.
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Old 30th March 2015, 08:53 PM   #7
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Quote:
Originally Posted by Merlinb View Post
With LED bias, some possible voltage is 'wasted', so output swing (i.e. output power) is reduced compared with fixed bias.
You need a lot of LEDs to handle the current and acheive a useful bias voltage with most power valves. Mounting all those LEDs can be inconvenient. I'd say this qualifies as 'more costly' than simple fixed bias.
good points, but we are counting pennies.
What I am after is , are there real benefits in LED biasing that I can not see?
Compared to overall cost of an amplifier, and the effort to put one together, the cost/effort difference between two implementations is real small.
Why did anyone think of LED biasing?
What issue(s) were they trying to fix?
What new issues did they create?
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Old 30th March 2015, 09:04 PM   #8
TheGimp is offline TheGimp  United States
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LED biasing provides very low dynamic resistance in the cathode circuit of a tube output stage.

It is considered an alternative to cap bypassed resistor in fixed bias.

They were addressing the capacitor and it's deficiencies.

for a power output stage, it takes an array of LEDs. Several strings of LEDs in series have one small resistor for balance when the strings are tied in parallel. The LEDs need to be select3ed for (1) low dynamic impedance and (2) matching voltage drop.
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Old 30th March 2015, 09:09 PM   #9
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Originally Posted by TheGimp View Post
It is considered an alternative to cap bypassed resistor in fixed bias.

They were addressing the capacitor and it's deficiencies.
I am confused, aren't you describing cathode bias?
Fixed-bias as in no cathode resistor, and a separate supply and pots to adjust bias (in power-amp output stages).
Call it manual bias if you like.
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Old 30th March 2015, 09:13 PM   #10
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Quote:
Originally Posted by KenTajalli View Post
I take "small signal class A" as a valid answer for cheaper and simpler.
I suppose I should have added:
" In Power amp final stage" as it is mostly there that one may consider either.
Say 10W to 40W amps.
Also as "fixed Bias" I meant negative supply with trimmers to set the bias.
What you are actually referring to is "Grid bias". Grid bias is when you adjust a negative supply with a trimmer to reference the signal input. "Fixed bias" requires the cathode to be elevated with a (hefty) resistor, while the grid is referenced to ground.

Grid bias is a nice solution when you don't have enough B+ to do "Fixed bias". It has the disadvantage that you need an additional negative supply secondary from your mains transformer. It also has the disadvantage that you immediately introduce new AC noise into your audio signal.

Fixed bias : Has the advantage that you won't be introducing new AC noise into your signal, such as with "Grid bias". You also don't need an additional negative supply winding off your transformer. It has the disadvantage that it will need higher B+ than "Grid bias". So perhaps not something for the novice builder... Also has the disadvantage that designs often need to bypass the cathode resistor with an electrolytic capacitor. I won't go into all the things you might not want this electrolytic capacitor though. You also need a substantially rated cathode resistior to do Fixed bias.

In any case, I would chose Fixed Bias over Grid Bias any day for anything you aspire to become Hi-Fi. Just my opinion though...

Ian

Last edited by soulmerchant; 30th March 2015 at 09:23 PM.
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