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Old 21st December 2013, 11:07 PM   #1
rjm is offline rjm  Japan
Richard Murdey
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Default Push pull class A bias current vs. noise, is there an optimal compromise?

Simple question with most likely a complex answer. I'm posting it here rather than in the headphone forum as the guys over there rarely get further than worrying over which op amp to use...

On one hand, it is easy to determine class A bias current: decide on the maximum power output, calculate the maximum current for the minimum load, and set the bias current to be slightly in excess of the peak value (1.4x rms).

For a headphone amp, (arbitrarily) specifying 20 mW, 16 ohms, rms is 35 mA, peak 50 mA.

Thing is, for a headphone amp its fairly easy to built it out for higher bias. The more bias current, the more linear the circuit gets for the same output current. At the same time, the noise from the output devices also increases with current, forcing the designer to choose: low noise or low distortion?

Is the device noise issue real or not significant in typical applications (e.i. does the input noise upstream dominate)?
Is there an ideal compromise point or is it better to just up the bias to the practical maximum and forget about it?
Once you've decided on the bias current, how to you go about choosing the best transistor to achieve lowest noise?

LTSpice sim attached, though its a crutch. It would be better to calculate the device noise rather than rely in model simulation.
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File Type: png push-pull test.png (29.4 KB, 253 views)
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Last edited by rjm; 21st December 2013 at 11:12 PM.
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Old 21st December 2013, 11:22 PM   #2
jcx is offline jcx  United States
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for sensitive iem with 130+ dB SPL re 1 Vrms the only low noise choice is a ouput stepdown transformer
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Old 22nd December 2013, 04:17 AM   #3
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Ok, I can accept that. For regular headphones 100-120 dB and a direct pp output stage though .... is transistor noise something to worry about, and if so, how to pick a set of transistors to match the bias current for lowest noise?
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Old 22nd December 2013, 05:55 AM   #4
wayne is offline wayne  United States
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The noise will be dominated by the input stage. Simulations show noise down in the dirt for both just bias for low harmonics or best sound.
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Old 22nd December 2013, 07:16 AM   #5
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That would have been my intuitive conclusion. Good. Thanks.

My listening evaluation suggested a slightly higher noise floor, and I was wondering if the increased bias current was the cause. Your answer says no, and actually the measurements I've just made show its not a noise floor increase per se. but interference of some kind (hash rather than white noise). I'll have to look more carefully at the grounding and bypassing layout on the new boards.
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Old 22nd December 2013, 12:41 PM   #6
AndrewT is offline AndrewT  Scotland
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try adjusting the Re value to suit the peak output current value that your load may demand.

A loudspeaker demanding typically 100mApk to 1Apk tends to have Re ~ 0r1 to 0r47

A headphone amp demanding 10mA to 100mA would tend to use Re ~1r0 to 4r7

A line driving amp might have to deliver 1mA to 10mA and would suit Re 10r to 47r.

Adjust the Vre to optimally bias the output stage and you end up with bias currents that roughly scale with the load current demand.
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Old 24th December 2013, 12:14 PM   #7
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Andrew, is Re still set for the output current in Class A, when the bias current is larger than the peak output current?
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Old 24th December 2013, 05:29 PM   #8
AndrewT is offline AndrewT  Scotland
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does the load need 100mA pk or 100mAac?
Let's assume 100mAac.
the peak value will be ~140mApk.
A push pull amplifier biased to 70mA can output a theoretical 140mA, without half switching off.
But that last bit gets half the transistor into the region where they are starting to turn off.
So set the bias to 80mA.

Assuming a 1pr output stage optimally biased into ClassAB.
We know that Vre should be around 26mV.

Re = 0.026 / 0.08 = 0r325 use 0r33

Set the bias to 26mVre and the amplifier operates in optimal ClassAB.
The Maximum ClassA current is ~140mA.
The amplifier stays in ClassA when driving a 100ohm headphone to 10vac (14Vpk)
That is far too loud when sensitivity is usually between 90 and 100dB/mW
A 32ohm headphone could be driven to ~3.2Vac and stay in ClassA. Still very loud.
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Old 26th December 2013, 10:20 AM   #9
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Indeed loud. The idea, somehow, is that larger bias currents are desirable if the thermal budget will allow, even if the maximum class A power delivery far exceeds the real world demand. LTSpice insists that it leads to lower distortion. I'm not entirely convinced.

Up to 200 mA is practical for low voltage headphone amplifiers. Now ... does that really sound better? At the moment my thinking is that 20-30 mA should work just as well. I plan to try out different settings and see if I can hear any difference.

The optimal Vre is 26 mV? That seems entirely reasonable and all, but I don't follow the reasoning exactly. Since it allows the derivation of Re for the selected bias current, though, I'm all good.
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Old 26th December 2013, 10:29 AM   #10
AndrewT is offline AndrewT  Scotland
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There is much discussion on Vre for setting optimal ClassAB bias.

If that is used for a push pull ClassA amplifier and for some reason the load demands extra current that exceeds the ClassA value then the amplifier transitions into ClassAB with the least crossover distortion while that extra current is being sourced.

If the load never demands that extra current then the amplifier remains in ClassA and there is no crossover distortion.

For the demands of headphones this seems an ideal way to drive them.

The only thing that needs some extra thought is choosing a topology and devices that that suit the low noise required.
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