Head-Fi.org › Forums › Equipment Forums › Headphone Amps (full-size) › AD8620 op-amp OK with 9v for META42
New Posts  All Forums:Forum Nav:

AD8620 op-amp OK with 9v for META42

post #1 of 12
Thread Starter 
I feel sheepish posting this thread since I know nothing about electronics. However, I'm trying to make a decision on a META42 configuration, and I've been wading through threads in an attempt to figure out the different options. (I also looked at tangent's page, but the technical vocabulary mostly went over my head.)

I seem to be getting different opinions on whether the AD8620 is viable with a single 9v battery. Can I get some opinions. If you think not, which amp would you suggest.

Also, if there are any "plain english" threads or web pages on this subject, please let me know.

Thanks.

Edit: To be more accurate, everything I know about electronics I learned during high school and have been busy forgetting for the past 20 years. So I have a vague recollection of terms such as voltage and impedance. Sometimes I even feel like I understand what they mean, but this is probably mostly an illusion on my part.
post #2 of 12
I'm currently driving an 8620-based META42 with two 9v batteries. It has an EL2001 buffer on the input and double-stacked EL2002s on the outputs, which is probably not a good configuration for a portable amp - it draws a constant 28 mA. I intend to hook up a more appropriate DC power supply, I just haven't gottten around to it yet.

I find that when alkaline batteries get down below 6v each (12v total), the sound loses a lot of dynamics. A few hours after that and I'm out of operation. A pair of fresh batteries restores the sound's "color".

Hopefully, someone with a more "portable-friendly" configuration will chime in. Good luck with your project.
post #3 of 12
I use the AD8620 with stacked EL2001 buffers and a cascode current source, all driven by a single 9v battery in my travel rig. It works just great for me.
post #4 of 12
JMT, how long does that amp last with which headphone and at what volume level? Thanks.
post #5 of 12
With most headphones, you can get by with the AD8620 on a single 9V battery, but of course it's a sacrifice. More voltage is certainly better.

As for losing dynamics on your amp with 12V, Pocomo, it's probably not a voltage issue, but instead that you're using alkalines and that the internal resistance of the batteries has risen to the point that you're getting more ripple on the rails than you get with fresh batteries. As the ripple rises, the tautness of the amp suffers, as more and more of that ripple gets back into the op-amp's output in audible amounts.

Quote:
Also, if there are any "plain english" threads or web pages on this subject, please let me know.
My Audiologica pages have some bits that aren't too hairy, technically. More info can be found by searching in the DIY forum, since we've initiated a lot of newbies, and those discussions are archived there for you to learn from.
post #6 of 12
Quote:
Originally posted by tangent
As for losing dynamics on your amp with 12V, Pocomo, it's probably not a voltage issue, but instead that you're using alkalines and that the internal resistance of the batteries has risen to the point that you're getting more ripple on the rails than you get with fresh batteries. As the ripple rises, the tautness of the amp suffers, as more and more of that ripple gets back into the op-amp's output in audible amounts.
Thanks Tangent! I'm learning something new almost every day around here.
post #7 of 12
I agree with Tangent about pocomo's observation: It's not the 12 volts that is the problem, it's the 9-volt batteries with only 6 volts left. I observed the same thing with a single battery.
post #8 of 12
Thread Starter 
I remember reading something on this site about voltage across battery life. If I remember correctly, alkalines were observed to drop off relatively quickly from stated voltage, whereas NiMH's dropped off more slowly. Would type of battery used make a difference?
post #9 of 12
That's an oversimplification, Davie.

Alkalines drop steadily in voltage with time. The service life vs. load curve is linear if the load is about 30mA or less, as with most op-amp based headphone amps. That is, if you halve the load, service life exactly doubles. This is why it makes sense to spec cells in terms of milliamp-hours (mAh): as long as you stay within the cell's load limits, the milliamp value times the hours that it will run at that level always equals the same number. But when you start to exceed the cell's load handling ability, service life goes down faster than the load goes up.

With NiCd and NiMH rechargeables, voltage starts off a bit higher than the rated voltage -- typically 1.4V for a 1.2V cell. Then as you use it, it drops quickly towards its rated voltage, and then stays there for most of its service life. Then as it approaches the end of its service life, it curves rapidly down towards its minimum usable voltage, about 0.9V. The load handling ability of rechargeables is much higher: as long as the load is low enough that they can take at least half an hour to an hour to discharge (e.g. 750 to 1500 mA for a 750 mAh cell) the service life vs. voltage curve stays the same shape, just compressed along the service life axis. When you start exceeding the cell's load handling ability, the curve starts to distort, but it happens at so much higher load levels than with alkalines that it is mostly an academic concern.

Which is better? Well, rechargeables start at a lower voltage than alkalines, but if their service voltage is high enough for your application, they'll stay at a higher voltage for longer than an alkaline. This can be very important. For instance, imagine you have an amp with a single 9V battery, and that the amp starts sounding bad at about 7V with your headphones. An alkaline drops steadily from about 9.5V to about 5V with respect to time, so you end up wasting about 44% of the battery's capacity. A rechargeable battery might start at more like 10.5V, drop quickly to about 9V, and then stay there for virtually all of its service life, so you get virtually all of the power out of the battery. And, since the rechargeable has a lower internal resistance (which is why they can handle higher loads), they will cause less ripple on the power supply rails, so they may help the amp to sound better than an alkaline.

The downside of rechargeables is the initial expense and the fact that they still usually end up having lower lifetimes in practice than alkalines.
post #10 of 12
Thread Starter 
I've already invested in a battery recharger, so I don't think that the extra expense of a 9v NiMH rechargeable would hurt too much.

tangent, I checked out your site. I had no intention of getting into DIY stuff, but you've got me tempted to try building a cmoy.
post #11 of 12
Then I apologize on behalf of the imminent damage to your bank account.
post #12 of 12
Quote:
Originally posted by Davie
tangent, I checked out your site. I had no intention of getting into DIY stuff, but you've got me tempted to try building a cmoy.
JUST SAY NO!!
New Posts  All Forums:Forum Nav:
  Return Home
  Back to Forum: Headphone Amps (full-size)
Head-Fi.org › Forums › Equipment Forums › Headphone Amps (full-size) › AD8620 op-amp OK with 9v for META42