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Old 20th June 2004, 08:19 AM   #1
tifojo is offline tifojo  United States
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Default feedforward vbe compensation in class-A

hi,

so, I'm working on a headphone amp, and I'm going to use a class-A emitter follower with no global feedback. philosophical issues aside, there shouldn't be any need for global feedback, since I can just waste a whole bunch of power getting the distortion low in the output stage and I'll have huge supply rails (like, +-32 V) for large emitter degeneration resistors in the input stages.

i'm designing this for the sennheiser 580/600/650 series by the way (300 ohms), thus the large supply rails and the lack of concern about noise from large bias currents.

what I'm concerned about here are nonlinear vbe and non-constant beta in the output transistors, as I see these as the main likely causes of distortion. with modern transistors the beta effect is small, and can be made smaller still by using an output triple or doubling up ouput transistors, but the vbe effect is fundamental.

anyway, what I'm thinking of doing is using a controlled current source in the emitter of the output transistor to make its collector current constant, thus reducing the effects of both vbe vs. Ic and beta vs. Ic. Is there a name for such a system? Has it been tried much? Is there a fatal flaw I'm not seeing? I know it's power-inefficient, but for a headphone amp, who cares...

tim
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Old 20th June 2004, 10:26 AM   #2
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Isn't this just an emitter-follower with a constant current source load?
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Old 20th June 2004, 10:54 AM   #3
sreten is offline sreten  United Kingdom
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Sounds like single ended constant current class A, max efficiency = 25%.

Only problem is without feedback its very difficult to set output voltage.

sreten.
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Old 20th June 2004, 03:19 PM   #4
tifojo is offline tifojo  United States
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in the typical configuration, the current through the output transistor is the sum from the current source and the current driven through the load, so not constant.

for instance, say I'm driving 300 ohm headphones to +-24 V (about one watt, 126 dB spl with senn. 580) with a 100 mA standing current in the output transistor. the current through the load has peak amplitude of 80 mA, so the current through the output transistor varies between 180 mA at the positive extreme and 20 mA at the negative.

what I'm proposing is to actually measure the current through the load with a small series resistor, and subtract it from the current source setting. strictly speaking that introduces a feedback loop around the output transistor via loading effects on the voltage gain stage and the non-constant vbe in the output transistor, but the loop gain is small and the distortion reduction (if there is any) would come from the feed-forward effect instead.

I suppose you could ask why not just use global feedback if I'm going to do all that work...

furthermore it's very wasteful of power...I haven't crunched the numbers, but I really don't care if my headphone amplifier dissipates 20 watts to drive 1 watt into the load! it will just impress my friends more with how warm it gets and what giant heat sinks it has.

might be interesting just to try it and see if it makes any audible difference at all. most of the distortion from vbe and nonlinear loading effects will be second harmonic anyway, and all evidence shows that's the least offensive type.

oh, and of course there will be a global feedback loop to servo out the dc offset, but it will be way sub-audio.
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Old 20th June 2004, 05:51 PM   #5
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The Sennheisers need so little voltage and current that
you probably can do the job with a dumb Mosfet like the
IRF610 operated as a follower with 13.8V (easy power
supply to buy) and biased by a 100 ohm source resistor.

You'll get a little 2nd harmonic, ohhh, too bad
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Old 21st June 2004, 12:05 PM   #6
tifojo is offline tifojo  United States
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P=V^2/R, so the Sennheisers need lots of voltage but not much current.

anyway, I'm dead set on using BJTs, since (a) Doug Self seems to be convinced they're more linear (b) the square root (as opposed to log) voltage drop vs. output current for mosfets makes (a) logical and (c) I've listened to an utterly amazing BJT headphone amp that I'm basically trying to copy.

But yeah, a second harmonic at -60 dB is probably not going to be the end of the world. So perhaps I just won't bother.

Tim
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Old 21st June 2004, 12:12 PM   #7
tifojo is offline tifojo  United States
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oops! didn't see who I was replying to...I'm sure you know that P=V^2/R

oh, boy, and you must have kind of a rivalry with doug self since he's Mr. BJT and you're Mr. Mosfet. anyway, my foot's in my mouth!

by the way, I really dug that DIY op-amp tutorial you used to have on the website...is that still available somewhere? i used to tell everyone who was curious what an op-amp is to go read it...

later,

tim
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Old 22nd June 2004, 03:34 AM   #8
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I'm pretty sure you'll never hear from him again.

However, the answer to your question is that yes, the article is still available. It's on the Pass DIY website under preamps.

And, by the way, while I too know the relationship between power, resistance, and voltage, why do you say that you'll need lots of voltage ? Surely the headphones consume next to no power, don't they? Or am I missing something?
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Old 22nd June 2004, 03:42 AM   #9
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If the headphones are 300 ohm and you use a BJT with
a beta of 100, then the input impedance will look like 30K,
and anything should be able to drive that fine.

p.s. I was Mr BJT until about 13 years ago, and then I
converted for no particular reason.
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Old 22nd June 2004, 04:03 PM   #10
tifojo is offline tifojo  United States
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So, my goal is to drive 1 watt into 300 ohms before clipping. For the Sennheisers, that makes 126 dB SPL, which is the 96 dB dynamic range of a CD plus an optimistic 30 dB of noise from the amplifier and listening room. That's 24.5 V peak, so to be safe I'm going to use +-32 V supply rails. There's also an annoying gap in available power transformers between 24 V and 36 V.

Don't try doing that calculation for your speakers or you'll wind up wanting a 10 kW amplifier!

Anyway, the amp that inspired me uses two cascaded driver stages for the output, presumably to reduce the effects of non-constant beta, and it's that output triple that I'm trying to avoid. From the looks of all the little ferrite beads and funny RC networks he had to throw in, there must have been some irritating stability problems.

All the transistors in that design are obsolete now, though, and now that I do the calculations it shouldn't be a problem to just do a normal two-stage emitter follower with modern transistors, which have more constant beta.

The whole problem with this is that the Sennheisers are just so good, you start to notice very tiny changes in the circuit easily. The guy who built the other amp spent a year and a half tweaking it...
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