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PSU: 30vdc, 400mah, 12 watts .. what to use it for

post #1 of 19
Thread Starter 
I was at Goodwill today and found a 30 vdc, 400 mah, 12 watt PSU brick. Is it too much for a CMoy, PIMETA or whatever amp?
post #2 of 19
400 mAh, you mean it's a battery?

How big is this thing?
post #3 of 19
Thread Starter 
I think it's about 3"x5"x3" brick PSU. I beleive it was some old HP device AC/DC brick PSU. If it'll work for anything I'll pick it up tomorrow, it was like $1.99
post #4 of 19
I have two of these HP bricks. I haven't used them for anything "yet" because at any given moment I had something more suitable, but yes, it is usable as a basic front-end to a supply for a headphone amp.

It's an unregulated, typical AC-DC wart with (I forget specifics but roughly...) typical discrete rectifier and 4700uF capacitor after it, so as far as ~400mA warts go, it has a higher uF capacitance/current ratio than most which could translate into moderately lower ripple given any particular following output regulation.

IN other words, the current is too low for a speaker amp, but (most, at least featured on headfi) if you build a typical spec sheet linear regulation circuit after it (LM317 type, LM7824, or the reference here, a "Tread" from Tangent), it should do fine for a headamp as a well-endowned unregulated wall wart part of the equation.

If you were really ambitious, you would even find that there is enough room in the brick cavity that if you took the dimensions of free space remaining between the transformer and the bottom of the brick, you could spec that size circuit board and build a complete linear regulated supply inside it's case. As always, you'd be trading time vs money, it would be a hobby-tweak thing to do since you can buy finished products with ~ 24V regulated output. The benefit in developing only one design / finished PSU is low, but if you developed the accompanying ciruit board and applied it to multiple warts/projects then it would save some money if you are adept at etching your own circuit boards.

So in summary, if you were to use the typical Tread board for a 24V output to a moderate or lower current headamp (400mA or lower, but really it depends on the size of heatsink you find acceptible beyond about 200mA amp avg. demand), it is a good deal on a wart at $2.

Personally, I'd prefer a "24v, 500mA-1A" wart for most of the headamp projects here, as the most commonly spec'd running voltage for a headamp seems to be 24V rail-to-rail, but in some cases, certain cans, even a 12V/200mA (or higher, depends on amp & class A situation) would suffice.

Insufficient evidence to conclude whether you need 30V before regulation or not. All I can say for certain is that it's a good wart for it's specs at $2, but your purpose has to need a ~ 30V wart or close enough to that .
post #5 of 19
Quote:
Originally Posted by mono View Post
All I can say for certain is that it's a good wart for it's specs at $2, but your purpose has to need a ~ 30V wart or close enough to that .
Yah, like a PPAv2 with OPA627/OPA637 that LOVE those volts
post #6 of 19
Thread Starter 
Thanks mono, I'll go tomorrow to get it, and scrounge for more stuff
post #7 of 19
While we had the active topic I decided to pull out my variant of this wart and take a couple measurements. Unloaded voltage is 37.6. At 140mA current the (DC output) voltage is 33.4. At 260mA, 31.5V.
post #8 of 19
Thread Starter 
Hewlett Packard
Model C2175A
Input: 120VAC, 60Hz, 180mA, 23VA
Output: 30VDC, 400mA, 12W

Using my cheep Craftsman DMM, I got 40.50~41.0VDC no load. I would test amperage, but I don't think my DMM can test that high. It's a Craftsman Pocket Meter 82351.

I do not get any Ohm reading from DC+ or DC- to either AC prong.

In order to test the mA, I need to hook it up to a device & test it with the leads in parallel right?
post #9 of 19
Quote:
Originally Posted by Joshatdot View Post
Hewlett Packard
Model C2175A
Input: 120VAC, 60Hz, 180mA, 23VA
Output: 30VDC, 400mA, 12W

Using my cheep Craftsman DMM, I got 40.50~41.0VDC no load. I would test amperage, but I don't think my DMM can test that high. It's a Craftsman Pocket Meter 82351.

I do not get any Ohm reading from DC+ or DC- to either AC prong.

In order to test the mA, I need to hook it up to a device & test it with the leads in parallel right?
Not in parallel - in series with either the positive lead or the negative lead, but not both. You actually wire the DMM into the circuit as if it's a single series resistor with the probes as the "resistor leads" on either end. That way, current from one of the power leads flows right through the DMM on its way to the load.

P.S. If mono's measurements are representative, you may only get 27-28V under load after passing through the LM317 on a TREAD.
post #10 of 19
Yes at anything over roughly 200mA, the margin is too small to get 30VDC regulated output with the one I have (at expected temperature range) but it seems to have a slightly lower voltage than Joshatdot's.

National LM317 Datasheet (Jun. '05, pg. 6)
post #11 of 19
Thread Starter 
I have not tested mA yet, but will this be a good cheep PSU for whatever low mA amp?

I found a nice 24VDC 500mA regulated ac/dc convter/psu on digi-key, I think I can even get 1000mA of the same psu.
post #12 of 19
A low current amp, say a CMOY, Pimeta, other class A/B designs often seen around here, will not need much of a wall wart for AC/DC conversion stage (or only isolation and voltage drop from AC line, wall outlet) of the PSU.

First you'll have to decide what voltage you want your amp to use. 24V is popular, and more versatile than 30V since a few opamps may not like higher voltage and seldom do you "need" voltage as high as 30V (as +-15V rails I mean) except with particularly high impedance, low efficiency headphones.

You can either pick PSU parts according to some ideal for the amp(s) to be powered, according to price or convenience of parts you find that are close enough to the ideal, or just use what you have at any moment and adapt it as necessary to suit the job.

For smaller amps I like the little wall-warts, ideally anything between 14-24V and at least 200mA current. I like to have (very roughly, +-30% or so) double the current rating expected from the amp when there isn't undue cost or difficulty in finding warts or transformers to suit that job. I don't double in other cases like a power amp (for speakers). By doubling you may find the transformer runs a bit cooler as well, it it a PITA to have to crack open a transformer that's sealed just to replace a thermal fuse buried in the transformer windings (most people would just throw it away).

The regulated PSU from Digikey, assuming it is linear not switching type, appears fine for many amps based on what info we have about it. You could also or instead build an acceptible supply from the HP brick you have.

On a very low current headamp, a self-built or self-added linear regulation stage will have less heat to deal with from any chosen voltage drop. For example you could use the HP 30V brick with a Tread set to 24V output for powering a CMOY without much concern about heat, only a very small 'sink might be prudent, or may not really be necessary at all.

Suppose instead you had a highly class-A biased PPA2 and were also looking to take the HP 30V brick and get 24V out of a Tread. Since it uses multiple times the current, the linear regulator produces that multiple times the heat. Actually a little less than that multiple since the voltage from a wart or brick that's unregulated will drop some with increasing current demand from the load. Previously I posted some numbers that are a good example of that with the HP brick.

Choosing a transformer/brick with a voltage just above that of the desired output from the Tread et al. linear regulator stage (plus 3V or see the chart I linked above to account for the dropout at any particular current) instead of a lot higher, will make heat produced by the regulator, a lower level that is easier to manage.

Not sure if this addresses your question or not Joshatdot, there are multiple ways to arrive at an acceptible result. Since you already have that HP brick, yes it will tend to be among the cheaper options to finish into a (linear) supply. Many people choose an LM317 based linear regulator, one example of which is the Tread from Tangent.

You could design your own on protoboard or buy the board or whole kit from Tangent, or if the application didn't require noise *as* low you could even put an LM7824 linear regulator after the HP brick for a simplier, cheaper design that might even be air-wired instead of needing a PCB. Going with LM317 isn't necessarily much more expensive, it could cost as little as $1 or so, more depending on your inventiveness or parts you already had on hand.

It's up to you, how you want to tackle it. Buying the ready-made wart from Digikey is certainly quicker and easier but if you need multiple warts for several amps it can start getting expensive. There are also some finished linear warts from various surplus eletronics sites for about $4-5 but tend to be only 15V or lower output, often 12V.
Example 1 | 2
They may not have the type or size barrel plug you want though, and shipping raises prices a lot. I tend to just add some LM7824/7815/etc and LM317 plus suportive parts onto other orders when I run low so I always have a way to build the *next* linear supply I may need without having the shipping costs for a separate order of a couple dollars worth of parts.
post #13 of 19
Joshatdot, I just came across one of your other posts where you mentioned building a Pimeta. As seen from my prior measurements of my HP brick at 140mA (roughly an upper boundary for what a Pimeta might consume depending on how many buffers you stacked and other configuration choices), you could use the HP brick and a following linear regulation stage to get 30V output, or 24V output (or around this range).

Based on my wart if you had 140mA current demand from the load, 33.4V just before the linear regulator stage, and 24V regulated for the Pimeta then you'd have;

( [Brick voltage @ desired load] - [regulated voltage to amp] ) * current (in amps) = heat.

(33.4 - 24) * 0.140 = 1.3W

1.3W is enough that you'd use a small heatsink on the regulator, but it wouldn't need be anything fancy, even a ~ 1/8" thick scrap of 1 x 1" aluminum sheet metal would suffice in most cases, though if it gets much larger you then need a place to mount it so the weight/strain on the linear regulator's leads isn't excessive. At that 1.3W you could even 'sink the regulator to a larger, electrically isolated, area of copper on a PCB if you were designing or making your own regulator boards, or the whole regulator plus Al sheeting sink could be bolted lying parallel on the protoboard if you chose that route.
post #14 of 19
Thread Starter 
I don't have anything to plug in this HP Brick PSU. Can I ghetto rig some light bulbs or something to draw the PSU's max mA output?

Also I still am not 100% sure how to measure mA with my DMM.
post #15 of 19
in a PPA you will use rail spliters anyway, so why limit the voltage to 24 or 30? anything in between will work just fine. using 28V will give enough voltage headroom for the LM317 to work, even when it is loaded.



If size is not big deal, you can get Two 12V switching PSU's to make +/- power supply, this way you don't have to use a spliter to generate a virtual ground. 12v PSU's are common in thrift stores like the Goodwill. Add some extra large caps as super ripple fileters and some fast (Low ESR electrolytic, Tantalum, large ceramic) caps to improve transient response. Then you get an ideal PSU for any headamp project.
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