EMRFD Message Archive 6119
Message Date From Subject 6119 2011-04-13 22:00:34 David low power audio amp 6120 2011-04-13 22:11:36 n7ve Re: low power audio amp
8 ohm ear buds? I normally see these in the 16 to 32 ohm range and they seem to work fine on the output of op-amps. When you select ear buds, be sure to look for ones with at least 104 dB spl/mW. The best I have seen are up to 112 dB spl/mW. The only ones that I have seen higher than this were actually specified xxx dB spl/***1 V*** which is not at all the same thing and seems to be designed to hide the lack of sensitivity.
106 to 112 dB spl (sound pressure level) per mW is a ton of volume for a tiny 0.5v pk-pk signal. I normally find 50 mV pk-pk a comfortable listening level6122 2011-04-14 03:43:00 Tim Re: low power audio amp
Most often a 600 ohm:8 ohm unit.
This has the additional advantage in a direct conversion receiver, of making direct conversi6124 2011-04-14 11:51:38 KK7B Re: low power audio amp
If the headphone volume is too low with a series resistor, I'm more likely increase the op-amp output stage gain by raising the feedback resistor value than adding a transformer--but I've done both.
Looking back through a dozen or so of my designs, I tend to use a series resistor with op-amps, and a transformer when designing discrete transistor headphone amplifiers.
Re-read Dan's post and note those numbers--they are useful to have on hand when sketching receiver block diagrams intended to drive headphones.
Best Regards,
Rick
6126 2011-04-14 13:02:45 Glen Re: low power audio amp
Some earbuds are mighty efficient, and with no AGC,
a direct-conversion receiver can deliver some
literally ear-popping transients. A series resistor
is a good, simple way of limiting the damage. Dan
suggests diode limiters on top of that.
Another way of increasing the impedance load of
low-Z earbuds is to wire the left & right elements
in series. You can do that at the phono-plug jack.
Most often, a "mono" source will wire left & right
in parallel, to deliver the same signal to each
element. This reduces 32-ohm elements to a 16-ohm
load. If you float the connection that is common
to both elements (this point is usually grounded),
then the right "hot" lead goes to the op-amp output
and the left "hot" lead goes to op-amp ground. You
now have a 64-ohm load seen by the op-amp. Be careful
that you properly "un-ground" the comm6127 2011-04-14 13:15:26 Gary, WB9JPS Re: low power audio amp 6129 2011-04-14 19:39:10 kb1gmx Re: low power audio amp 6130 2011-04-14 20:17:39 David Re: low power audio amp
Some time ago after reading one of your technical presentations I measured all my earphones/earbuds for sensitivity and volume levels. My earbuds do fit in the category of .5Vpp for a comfortable listening level and these were spec'd at 106 dB/mW. For this design I want a higher level to just over 1.5 Vpp capability for possible different headphones or speaker. The impedance of my earbuds are rated at 16 ohms but in parallel they become closer to 8 ohms. This was confirmed by varing the output impedance using decade boxes.
I have experimented with either dual diodes in the feedback loop or limiting diodes on the output and prefer the former as a more graceful approach.
I did not get to using a series resistor in the output but now think it necesary thanks to everyone's comments. Several circuits I reviewed do indeed use series diodes such as the Tuthill 80 which I built (2 47 ohm in parallel) while other circuits vary from 150 to 470 ohms in series. They also combine the series resistor with a low pass filter/RFI choke.
Since I'm designing for CW I use a 700Hz tone through an attenuator as my source. Monitoring the output I did notice a significant output degradation going from 16 to 8 ohms. But certainly the various circuits all behave better closer to 100 ohms and above. It just never occured to me to use a make up resistor to keep the op amp happy. But going back to my original question - don't they make op amps with higher output capability or are they called audio amps?
Oh and my R2Pro/modified A/B amp now blasts my ears off as I found the output electrolytic was leaky. In fact, I discovered a surprising number of caps that were bad in my parts bin. Note to self to test before using in the future. Be suspicious about electrolytics. The Art of Electronics has a wonderful discussion on A/B amps and it was worth spending several evenings going through various biasing schemes.
I would like to ask the question what your optimal CW design would look like in terms of poles and gain per stage? I have read your filter presentation where you hint of more elaborate designs you've made. I'm going to work on the filter section next and any comments would be appreciated to keep my from reiventing the wheel.
6132 2011-04-15 02:38:39 Johan H. Bodin Re: low power audio amp
I get a similar uncomfortable feeling from out-of-phase mono in headphones. It
sounds like having the sound source both inside and outside the head at the same
time. Really brain-twisting...
73
Johan SM6LKM
-6133 2011-04-15 03:09:20 Tamás Fábián Re: low power audio amp
follower can be used with an emitter resistor of a few hundred ohms,
possibly a couple of KOhms, if the drive level is big enaugh. If the signal
from the previous stage is filtered, and the output is correctly biased,
this can be done with three elements: transistor, emitter resistor, output
capacitor. Five elements, if one feels more comfortable with limiting
diodes. Would be a very low cost solution.
If gain is needed, same could (?) be done with a common emitter circuit too.
Hm. seems interesting. I always have more spare transistors than op-amps, or
straight LM386-s. ;)
Any thoughts?
On 15 April 2011 05:17, David <davidschuh@yahoo.com> wrote:
>
>
> Dan, et al.
>
> Some time ago after reading one of your technical presentations I measured
> all my earphones/earbuds for sensitivity and volume levels. My earbuds do
> fit in the category of .5Vpp for a comfortable listening level and these
> were spec'd at 106 dB/mW. For this design I want a higher level to just over
> 1.5 Vpp capability for possible different headphones or speaker. The
> impedance of my earbuds are rated at 16 ohms but in parallel they become
> closer to 8 ohms. This was confirmed by varing the output impedance using
> decade boxes.
>
> I have experimented with either dual diodes in the feedback loop or
> limiting diodes on the output and prefer the former as a more graceful
> approach.
>
> I did not get to using a series resistor in the output but now think it
> necesary thanks to everyone's comments. Several circuits I reviewed do
> indeed use series diodes such as the Tuthill 80 which I built (2 47 ohm in
> parallel) while other circuits vary from 150 to 470 ohms in series. They
> also combine the series resistor with a low pass filter/RFI choke.
>
> Since I'm designing for CW I use a 700Hz tone through an attenuator as my
> source. Monitoring the output I did notice a significant output degradation
> going from 16 to 8 ohms. But certainly the various circuits all behave
> better closer to 100 ohms and above. It just never occured to me to use a
> make up resistor to keep the op amp happy. But going back to my original
> question - don't they make op amps with higher output capability or are they
> called audio amps?
>
> Oh and my R2Pro/modified A/B amp now blasts my ears off as I found the
> output electrolytic was leaky. In fact, I discovered a surprising number of
> caps that were bad in my parts bin. Note to self to test before using in the
> future. Be suspicious about electrolytics. The Art of Electronics has a
> wonderful discussion on A/B amps and it was worth spending several evenings
> going through various biasing schemes.
>
> I would like to ask the question what your optimal CW design would look
> like in terms of poles and gain per stage? I have read your filter
> presentation where you hint of more elaborate designs you've made. I'm going
> to work on the filter section next and any comments would be appreciated to
> keep my from reiventing the wheel.
>
>
>6135 2011-04-15 09:31:30 popmedkoh Re: low power audio amp
> sounds like having the sound source both inside and outside the head at the same
> time. Really brain-twisting...
>
> 73
> Johan SM6LKM
>
> -6136 2011-04-15 13:06:34 Andy Re: low power audio amp
all hear it, but some people might not tell which one is right,
especially with earphones which are somewhat "unnatural" to begin
with.
I frequently used to flip the phase back and forth when I was
listening on the job at a broadcast radio station. I started doing it
to test whether the audio feed was right or not. After a while I got
pretty good at just telling, without doing an A/B test. So I guess I
trained my ears+brain to just know which was which.
There was at least one LP that was released with an entire side cut
out-of-phase. It's interesting that the record company ... I think it
was Columbia ... didn't notice it in time to stop the release. (It
must have been difficult to cut, since the wiggles would have been
mostly up-and-down rather than side-to-side, and some disk cutting
equipment was said tn not handle the up-and-down undulations that
well.)
Many other LPs from a certain age had portions that were out of phase,
either because it was the rear channel in "quad" recordings, or
perhaps for effect (voice on "Dark Side of the Moon" comes to mind).
These days I am easily fooled by earbud earphones, which sometimes
SEEM to have the same effect as out-of-phase channels ... perhaps
depending on exactly how the buds are sitting.
Or maybe I am just getting old.
Andy6137 2011-04-15 13:36:19 davidpnewkirk Re: low power audio amp 6139 2011-04-15 19:56:34 Harold Smith Re: low power audio amp
JVC and others) to directly drive headphones in their consumer products,
through a 22 ohm series resistor. It would easily drive a 16 ohm phone to
painful volume. That application was stereo, which points out the
possibility of using the same or similar dual opamp and driving each side of
a stereo headphone. That would give the option of mono (drive both with the
same, in phase) or any other two sources you'd like.
de KE6TI, Harold
6141 2011-04-16 11:11:57 Steve Re: low power audio amp 6142 2011-04-16 11:25:15 David Re: low power audio amp breadcrumbs
Earlier comments on the using the series resistor still bother me from an efficency point of view. Why introduce an additional load equivalent to the earbuds just to limit o/p current? You basically cut the power efficiency in half to the realm of a class A amp. Or am I missing something?
This brings up the question of using low impedance earbuds in the first place. I use them a lot while listening to podcasts so I'm comfortable using them. They are also light weight, small and convenient to take on QRP trips. A quick search resulted in a few earbuds with impedances in the several hundred ohm range but they were expensive.
The academic excerise of building several class A/B amps was enjoyable. Its somewhat rewarding to tweak the lowest quiescent current from these designs while maintaining minimal cross over distortion. Rick's R2Pro design and Todd's (QRP Homebuilder)experiences with A/B designs were great references. Output efficiencies in my mind do not seem to warrant all the fuss though at lower end audio speaker requirments in the range of several hundred ma. The trade off of packing extra battery power makes this marginally important.
Dan Tayloe's designs are intriguing and for this rx iteration I will follow his recomendations.
I've laid down so many breadcrumbs in the design forest I'm in danger of getting lost.
6143 2011-04-16 13:06:28 Andy Re: low power audio amp breadcrumbs
> from an efficency point of view. Why introduce an additional load
> equivalent to the earbuds just to limit o/p current?
headphones are plugged into a stereo jack, the mono plug shorts out
the right channel.
Without the resistor, the amp needs to be happy driving into a dead
short indefinitely.
That might also be why the radio previously mentioned, intentionally
drove the headphones in series, even if it made them out of phase.
That type of connection accommodates either mono or stereo headphones,
without needing separate drive and current limiting.
Andy6144 2011-04-16 14:07:28 kb1gmx Re: low power audio amp breadcrumbs 6145 2011-04-16 19:59:16 David Re: low power audio amp breadcrumbs
6146 2011-04-16 20:28:36 kb1gmx Re: low power audio amp breadcrumbs 6147 2011-04-16 21:40:01 n7ve Re: low power audio amp breadcrumbs