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Old 5th March 2010, 06:36 PM   #101
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Yes, I'm looking for the smallest sized transformers I can get away with while not unduly compromising performance. I'm looking for the lowest VA rating for a 9v transformer powering a Placid single (for the Buffalo II) and the lowest VA rating for a 15v transformer for powering a Placid bipolar (for the IVY III). I'm trying for a fairly compact layout for a transportable setup.

Am I going down the wrong trail by taking the earlier recommendation of setting the placid for the BII at 350-400mA at ~5.5V and for the placid bipolar at 150mA per rail at 15V to calculate minimum VA rating needed?
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Old 6th March 2010, 01:03 PM   #102
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if you are going transportable, why not use battery power? I would think you could survive on slightly less than +/-15vdc, but dont quote me on that. i'm going for fully portable and am using a different output section to achieve this. taking on my first custom PCB to achieve this in a small package, but will be using OPA1632 with some compact discrete diamond buffers.

Brian or Russ, you've probably played with this, how well do you think OPA1632 will cope with a split battery 'bipolar' supply feeding the ref pin of the OPA1632 with the 'center tap' of the battery pack, either passive by just pulling from the middle of the battery, or with a rail splitter.
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Old 6th March 2010, 02:24 PM   #103
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Using a diamond buffer after an OPA1632 is redundant unless you plan on pushing out more than 100ma or so, because the output stage of the OPA1632 is already a diamond buffer capable of about 150ma.

As for the batteries I am sure 12V rails would work fine. The 0V (or GND if you will) point is the mid point when the two 12V batteries are in series.

Also 15VA should be fine for both supplies. I would not go smaller.

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Russ
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Old 6th March 2010, 02:25 PM   #104
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That's an interesting option, qusp. I like it and will definitely look into it, but I'm concerned about current draw and amount of time I can get on a charge.

Thanks for the recommendation on VA rating, Russ!
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Old 6th March 2010, 02:36 PM   #105
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You will, of course, need to use large battery, or recharge very often.
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Old 6th March 2010, 02:46 PM   #106
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Also you will need to watch for uneven discharge.
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Old 8th March 2010, 01:44 PM   #107
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yeah by middle of the battery I meant exactly that, literally a battery as in a collection of cells, not a single cell, strangely we call a single cell a battery. so yeah at the mid point of a series of cells is the passive option, but for the reason Russ mentions I was thinking possibly about a charge monitor as I dont want to pull a ref from the middle and ten have '0' be something else entirely and risk offset, but I did read in the datasheet for the OPA1632 that the internal ref is dominant. only reason i'm thinking of using the buffers is because I will be primarily driving VERY low impedance IEMs at 28R, so I want as much current as I can reasonably muster. I am using 20 x AAA or perhaps 16 x AA depending on whether I use i2s input from a modified iriver or cowon X5L, or the spdif receiver. the other option is using an external battery pack and running some SMD super regulators, this would help with evening out the current draw. still in research phase at the moment. thanks for the tips guys, the OPA1632 really is a great little chip I can see why you chose it for the buffalo, its very adaptable indeed. I did find several circuits using buffers though and they were all driving low impedances. I will of course try without, I have some separate versions of the buffers in question made up and was planning on connecting them via pin headers, the PCB is just for the OPA1632/feedback and decoupling/ termination, charge circuit, possible virtual ground, input and output

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Old 8th March 2010, 01:55 PM   #108
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Unless you are playing at ear bleeding levels 100ma into 28ohm IEMs is far more than enough. In fact 30-40ma would drive you deaf pretty quickly.

The VOCM voltage is arbitrary it can be anything, 0V is also a relative voltage. Especially for portable apps.

Cheers!
Russ

Cheers!
Russ
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Old 8th March 2010, 02:30 PM   #109
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I know the loudness is enough, they go loud enough with straight out of an ipod, but I have found that the rather complex load presented by 6 driver (each ear) IEMs benefits a great deal from more current. I know 0 is relative, but I worry about drift and the '0' point not actually being the middle of (for instance) +12vdc and -12vdc and rather being -1.4vdc when related to these 'end points' especially as being such highly sensitive IEMs, even 20 -30 mvdc offset is a bit of a worry.

with JH13, some of the drivers may be presenting a much lower impedance like 4R and others higher than 28R depending at which frequency etc. they sound much better to these ears when given more current, the amp I use with them at the moment (portable) puts out 1.4W into 8R and it sounds much better than my other portables. I still only use it in the 2x gain setting and rarely get past 9-9.30 oclock, but more power reserve and 18V seems to work wonders with them. I will be trying JUST the buffers too, just unity gain, nothing is set in stone and I also like to use my HD600 @ 300R, though the JH13 are my primary objective, the amp section must do well with them above all others. IME andall my reading confirms this, but lower impedance loads take MORE current, not less. any reading I have done lately of datasheets recommends a buffer after the opamp for use with low impedance loads

I must be communicating badly, because you seem to be throwing things back at me that are expressly covered in my posts?? I would have though the fact I mentioned the ref pin and the '0' point (inverted commas) would say that I understood the concept of ground being relative. not being facetious, just wondering if i'm not making myself understood
I worried about drift, so started getting other ideas on the go and doing some reading on exactly how the ref pin works on the OPA1632 a couple nights ago late, I seem to remamber it always takes the mean between its own internal reference and what is fed to the ref pin (I know its more involved than that), but if it reaches a thresh-hold it will then take total control and only use the internal reference at the penalty of reduced DNR

Last edited by qusp; 8th March 2010 at 02:58 PM.
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Old 8th March 2010, 02:55 PM   #110
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With 15V rails, the OPA1632 can deliver upwards of 2W.
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