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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 18th March 2012, 08:06 PM   #21481
jan.didden is offline jan.didden  Europe
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John, I think you should say what to select for, you're the one setting the requirements.
Is opamp Iq really a priority? A few mA or even 10mA on a 100mA class-A amp?

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Old 18th March 2012, 08:17 PM   #21482
bcarso is offline bcarso  United States
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I'd probably try a 5534 first, partially because of the flexibility of the external compensation. Iq is 4mA typ., 8mA max., and that 8mA is probably a sort-of CYA n-sigma outlier number.

From the schematic I saw it looks like there will be at least ~300uA flowing all the time, so your output devices will stay on all the time to some extent.

Of course, insist on genuine parts! I can only imagine what sort of counterfeits there may be out and about.
 
Old 18th March 2012, 08:32 PM   #21483
mlloyd1 is offline mlloyd1  United States
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John Curl's Blowtorch preamplifier part II
for an op amp with a "hot" output stage, why not the old AD811?
or the less old AD815?

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Last edited by mlloyd1; 18th March 2012 at 08:34 PM.
 
Old 18th March 2012, 08:42 PM   #21484
bcarso is offline bcarso  United States
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Quote:
Originally Posted by mlloyd1 View Post
for an op amp with a "hot" output stage, why not the old AD811?
or the less old AD815?

mlloyd1
Blindingly-fast current-feedback amps tend to oscillate without much provocation, and I suspect the AD811 is no exception. If the output devices (as yet unspecified) contribute both significant gain and phase shift, I think things are likely to scream. And the quiescent current is 16.5mA, which with beta-of-even-50 output devices is a whole bunch of collector current. But why not try it?
 
Old 18th March 2012, 09:00 PM   #21485
john curl is offline john curl  United States
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I have built a version of this design in my lab. However, it is somewhat different than what I am putting out here. The principles are the same.
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Old 18th March 2012, 10:03 PM   #21486
scott wurcer is online now scott wurcer  United States
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Quote:
Originally Posted by mlloyd1 View Post
for an op amp with a "hot" output stage, why not the old AD811?
or the less old AD815?

mlloyd1
The AD815 does well without external help on headphones. I documented well the thermal response at low frequencies (DC) just for you guys since the xDSL folks don't care.
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Old 18th March 2012, 10:17 PM   #21487
steveh49 is offline steveh49  Australia
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Quote:
Originally Posted by ThorstenL View Post
My preferred way of making a "low impedance load driver for Op-Amp's" has a similarly low parts cound and is also targeted for 100...200mA Iq...

A total of 3 parts (2SC5171 or 2SA1930 depending if we want to pull the Op-Amp output up or down to put it into SECA, LM317 and one resistor) and we are SE Class A, including the Op-Amp output.
That's about 1 ma (order of magnitude) base current in the output transistor. Is that enough to turn off one of the opamps transistors reliably?

Now a question of definition - how would this sort of output stage be classed (SE-A, PP-A, AB etc) if one transistor were on for the entire cycle, while the other was off at idle (due to DC current drawn from the stage) but turned on for part of the cycle due to the AC current draw?
 
Old 18th March 2012, 10:58 PM   #21488
john curl is offline john curl  United States
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Bcarso, you are gaining insight!
However, remember 100ma Iq, everyone, that is a primary design choice. That leaves behind virtually any normal op amps, without an added output stage.
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Old 18th March 2012, 11:24 PM   #21489
john curl is offline john curl  United States
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Let's look at another 'trick' with this design. What happens if we accidently short out the headphone amp? Even without a 'buildout' resistor?
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Old 18th March 2012, 11:24 PM   #21490
john curl is offline john curl  United States
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What about thermal runaway?
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