[Strogian]

I have this.. I think.. CMOY amp that JMT built for me a while back, and it takes 2 9V batteries right now. But, as I have learned a wee bit about electronics, I feel like modifying it to also let me plug it in to the wall. So I went over to Radio Shack and got a 15V (up to 1000mA) adapter -- that was the biggest they had. The guy there said that it shouldn't be a problem, but I could take it back if it didn't work.

Now, I want to be confident about this before I actually do anything, so I used my oscilloscope (I still have no multimeter that I'd trust

) to test voltages. Turns out, the two 9V batteries put out around 18V with the amp off, but only slightly less than 15V when it is turned on. (and it goes down a bit further when I turn the volume up) This makes me think that the 15V power supply will be perfect for this amp -- maybe even a bit better than the batteries. What do you guys think?

 

[tangent]

If the battery voltage sags 3V when you start to pull power from them, they're probably about to die, or you're pulling way too much current from them.

Batteries drop in voltage over time, so it's perfectly reasonable that your batteries happen to be at 15V at the moment, not that they jump back to 18V when the power is removed. Alkaline batteries are useful down to about 0.9V per cell; there are 6 cells in a 9V battery, so around 5V.

Regarding your Radio Shack power supply, it will either be an unregulated supply or a switching-regulated supply. Neither is ideal for audio; you may get some additional noise with the supply relative to batteries.

Make sure you didn't get an AC-AC supply! I seem to recall that there is a 15VAC output unit at Radio Shack.

 

[Strogian]

Quote:

Originally posted by tangent If the battery voltage sags 3V when you start to pull power from them, they're probably about to die, or you're pulling way too much current from them. Batteries drop in voltage over time, so it's perfectly reasonable that your batteries happen to be at 15V at the moment, not that they jump back to 18V when the power is removed. Alkaline batteries are useful down to about 0.9V per cell; there are 6 cells in a 9V battery, so around 5V.

All I know is where the trace is on my oscilloscope, which was given to me a couple weeks ago.

I might have screwed up something before, so I'll do it again right now.

First I measure it up there at 18V (actually, closer to 19V now as I check it again). When I put them into the amp, it's down to... okay, now 16.5V. I switched scales to get the 16.5 reading though, so maybe that affected the accuracy.

If I check both on the bigger scale, it looks like it's going from 18V down to 17V. OK, so maybe switching scales wasn't a good idea.

So it looks like they drop by about 1V when being powered by the amp. And then if I rotate the volume control up, there is another dip in voltage when I pass a certain point. That make sense?

Quote:

Regarding your Radio Shack power supply, it will either be an unregulated supply or a switching-regulated supply. Neither is ideal for audio; you may get some additional noise with the supply relative to batteries. Make sure you didn't get an AC-AC supply! I seem to recall that there is a 15VAC output unit at Radio Shack.

Definitely not an AC-AC supply.

There was actually an 18V of that kind there. Here's what is on the box is "Output: Regulated 15VDC, 1000mA" So what sort of supply would you suggest for me? (yeah I could probably find that in a quick search.. I'll do that now.

)

EDIT: more information on the power supply -- if I hook it up to the o-scope, I have to pretty much go to the lowest scale to see any variation in voltage. (this is with no load ... or max load ... what is it called if the only circuit is through a huge resistance?) It looks like about a 300Hz (1cycle/0.003ms) fluctuation, with an amplitude of 1-2 mV. DC voltage was right on at 15V, though. I don't know if this matters to anyone, but I gathered the data, and I want to put it down somewhere.

 

[tangent]

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All I know is where the trace is on my oscilloscope, which was given to me a couple weeks ago.

A free oscilloscope, being used as a voltmeter...gee, I can't say I'm confident in your measurements. I'm not saying they're wrong, but you're using a tool meant for qualitative measurements to make quantitative measurements, and it's probably really old and probably no longer calibrated.

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there is another dip in voltage when I pass a certain point. That make sense?

Battery voltages drop quickly only when the load increases. I don't see why turning the volume up would cause a rapid increase in the load, unless the amp began oscillating.

Incidentally, checking for oscillation is a fine use of an oscilloscope...

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Regulated 15VDC

It's a switcher, then.

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what sort of supply would you suggest for me?

The switcher may be okay, just not ideal. The worry is how much noise it puts out under load. Attach it to your amp (make sure you get the polarity correct!), plug in the headphones and start playing music or a test tone through the system at normal listening volumes. Then look at the power rails with the scope set to AC coupling mode.

A good linear-regulated supply will have less than 1mV of ripple in this situation. That's what I'd recommend.

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what is it called if the only circuit is through a huge resistance?

You'd say the amp has a no load, and the power supply has a minimal load. ("No load" for the power supply would be when the amp is turned off.)

 

[Strogian]

Aah! I've tried everything I can to find a linear-regulated supply (online), and I still can't find one!

Where do you usually get yours?

 

[tangent]

The ever-popular Elpac WM080 is a linear regulated power supply.

It isn't always obvious whether a supply is a linear or switching type from their listings on the distributor sites. You have to look at the specs. Do a search here on the forum, where I've posted several times before about specs you can look for that will indicate a switcher instead of a linear.

Alternately, you can build your own, either out of raw parts, or out of the various PCBs available.

 

[Strogian]

tangent! THANK YOU for answering all my questions!!

found the post you're talking about:
http://www4.head-fi.org/forums/showt...threadid=20878

That WM080 looks good (I notice it says "0.5%" ripple as opposed to the "1%" that was listed for everything I had seen). But, it is 24 volts, and I only need 18. (for some reason, it's not a very common number to find...?) All I know right now about electronics tells me to put a couple resistors in there and hope the amp draws the same amount of current all the time. Do you have a better solution? I SUPPOSE I could just use 2 9V supplies, but that would be cheating.

Or build my own.. hmmm... I've gotta be able to find this thing somewhere!

 

[tangent]

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I only need 18.

Chances are excellent that all of the power supply caps in that amp have a voltage tolerance of at least 25V. The next common step down from that is 16V, which is of course too low if you're using an 18V supply. You might ask JMT, or open the amp and look for yourself.

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not a very common number to find.

That's right. The most common values are multiples of 12 and 15V.

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I SUPPOSE I could just use 2 9V supplies, but that would be cheating.

If you were that set on the 18V value, you'd be better off to build a custom linear with exactly the value you want.

 

[Strogian]

Quote:

Originally posted by tangent Chances are excellent that all of the power supply caps in that amp have a voltage tolerance of at least 25V. The next common step down from that is 16V, which is of course too low if you're using an 18V supply. You might ask JMT, or open the amp and look for yourself.

Oh, they are rated for 25V! WM080 it is then.