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Old 15th December 2011, 08:09 PM   #21
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^ Yes that will work, and the higher voltage will allow higher wattage. I'd still regulate the opamp power rails.

Last edited by !; 15th December 2011 at 08:12 PM.
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Old 15th December 2011, 08:35 PM   #22
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Excellent! thank you, now after the advice, I definitely plan on regulating the power supply to the filter amps, they can accept up to ±17V is there an optimal power?
Would there be a large difference between that and say ±12V?

The smaller transformer would be easier to find, more affordable, and on top of all of that the largest signal the board would be intended for wouldn't exceed 6V p-p.

On the actual board just unwanted signals will be attenuated and such so no boosting would be done.

Oh and also, say the power supply is ±30 or maybe even ±35V for the amplifier chip, is it possible to regulate such? I have a feeling the current draw will be nearing 4 amps if not more. Most regulators (that I have seen) max out at about 1.5 amps..
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Old 15th December 2011, 10:35 PM   #23
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^ I mentioned 17V only because it was the least voltage drop tolerable using a regulator to keep heat down, based on the amount of current to also power the tweeter amp. +/-12V is fine, generally speaking I'd look at transformer options, picking one easily sourced, cost effective, where you're dropping the least voltage necessary. For example if you used a 2 x 12VAC transformer and LM7812/LM7912 regulators (or LM317/337) for +-12V you wouldn't even need a heatsink on them when only supplying a few dozen mA for the opamps.

There are various ways you can regulate your power amp rails. Some feel it is worthwhile and some don't (most people here do not do it for chipamps). There are higher current regulators or you can add a higher power transistor to handle most of the current. Either way this can be a significant amount of additional heat, probably not worthwhile if your power supply is being placed in the speaker cabinet.
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Old 15th December 2011, 10:37 PM   #24
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For a common signal voltage, regulating the op amp supply voltage at a common ±12-15 volts should be fine. I wouldn't think the power amps require a regulated voltage. It is possible to regulate it if need be.
The popular LM317 in a TO220 package is guaranteed for only 1.5 amps. Under the right conditions it can supply much more. By itself, though, still not enough for your power amp.
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Old 15th December 2011, 11:22 PM   #25
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Somehow another option escaped me in my previous post. If you are using power amp chips with the same voltage, so you can use the same transformer for both HF and FL for the two drivers, there is no particular need for a second transformer to only power the opamps.

You have not mentioned the # of opamps you will use and I am too lazy to look up the current draw on the previously mentioned opamps so I'll randomly pick a count of 3 opamps and 10mA per. With 2 x 24VAC transformer you can drop the DC supply voltage down ~20V to get +-12V regulated output for the opamps and still only have about 0.6W heat dissipation at the regulator which is reasonable enough for something that isn't battery powered.

Last edited by !; 15th December 2011 at 11:25 PM.
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Old 16th December 2011, 12:32 AM   #26
ZLyzen is offline ZLyzen  United States
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With 2 x 24VAC transformer you can drop the DC supply voltage down ~20V to get +-12V regulated output for the opamps and still only have about 0.6W heat dissipation
What you're saying to do is drop the voltage with a regulator to about 12V for each rail?

However if I bump up the power amp supply to somewhere around ±30-35V than this might be problematic right?

and yeah, I'm already worried about the head from the LM4780 chip (which will be sinked (sunk?)) and it will be in the cabinet (which is ported) I'm hoping the transformer won't give off too much heat but that will be flat-mounted to the back of the cabinet (toroidal), which will be a rigid metal panel for all the connections and plugs.

Last edited by ZLyzen; 16th December 2011 at 12:39 AM.
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Old 16th December 2011, 01:10 AM   #27
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^ Q1: Yes Q2: It doesn't have to be a problem. If you use a regulator with a floating ground like LM317/337, you will be ok with this large a voltage difference. It can handle up to 40V difference between input and output which would be barely crossing the threshold where I'd put a heatsink on it at ~ 1.2W heat... if it were per my prior example, 3 x 10mA opamps.

Mount your amp circuit board next to the cabinet rear. Notch out the circuit board or the cabinet rear if necessary so the amp chip's tab/heatsink interface can be flush with the outside wall of the cabinet, then you can screw a heatsink to the back of the cabinet and the amp chip to that -OR- you could avoid notching it out if you sandwiched a piece of plate aluminum between the chip and 'sink as large as reasonably possible for best results (and of course heatsink grease between them). External mounting will significantly improve the effectiveness of the heatsink so a smaller size can be used as well as keeping other parts inside cooler, though generally it looks better if you go for a short fin height, larger length and width shaped 'sink.

If the amp chip you use has a bare metal heatsink tab, be sure to electrically insulate it from the heatsink if that tab is internally connected to the negative power rail (see the spec sheet for any amp chip you consider using) and the heatsink is external.

The transformer will survive the heat a lot better than the capacitors will, but unless it were potted with a very flat surface to interface the back panel, you can't expect to transfer much of the heat out of the amp case by only mounting it next to a metal panel, nor would you necessarily want to as the insulators in some jacks you're mounting there might become fatigued from thermal cycling... unless you were sure they were made with something very heat resistant like teflon. Not that this is certain to be a problem, it all depends on how hot it gets.

Last edited by !; 16th December 2011 at 01:20 AM.
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Old 16th December 2011, 01:53 AM   #28
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Okay, well I guess a final question would be is it more effective (I know more practical that's for sure) to only use one transformer?

I think that's what I'll end up going with: regulating the unregulated amp chip's supply with a couple LM317/337's for the filter.

I've seen inside one particular active monitor which was roughly the same cabinet/speaker size that I'm planning for, but a much higher wattage and they used a heatsink internally: DIY

This is preferred for me not only for looks but also because such edges on a cabinet can cause distortion or unwanted coloration of the sound?

I am familiar with the heatsink compound and insulation especially if I make that back panel the earth ground.

I'll have to see what ends up being better, the transformer at the bottom of the cabinet or mounted on the back panel.

The spec sheet for the filter amps states that-
output current under ±17V and 600Ω load is typically 26mA
The load would be the input resistance of the power amp right? (if this was the filter that directly fed to one half of the power chip)
and Yeah I plan to use

1 chip initially
half for unbalancing a balanced input (if needed)
half for input sensitivity/volume, whatever you'd like to call it

then the signal would be spilt
each side would get 2 entire chips
so 4 op amps per channel

3 each for the crossover and the final for LF or HF adjust, however there might not be a LF adjust as the HF can just be adjusted, thus the final amp for the LF would probably be a low cut for frequencies below my sub/mid's rating

so 5 chips, 10 op amps

still a small current that they would need, but not negligible, I would probably mount the regulators to the heatsink as well just incase.
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Old 16th December 2011, 04:54 AM   #29
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It should work fine with either 1 or 2 transformers, but I suppose we could say using only one is more effective if/when it's going to have regulated power to the opamps anyway.

That someone built the linked active monitor the way it was, is not a proof that it won't die prematurely from heat. It's sort of a corner-cutting budget design that I wouldn't spend time trying to duplicate. Here's one example of capacitor failure due to either heat or cap defect, KVR: Krk RP8 hum and although someone mentioned capacitor plague in that topic, that was typically failures seen when caps are filtering switching supply circuits, not 50/60Hz linear PSU circuits.

You don't mention whether it's an E-core or toroidal transformer but either should be mounted so their magnetic field is away from (particularly low level) audio signal leads and the speaker magnets.

The regulators have their metal mounting tabs at output voltage (LM317) and input voltage (LM337) level, they should be electrically isolated from the heatsink.
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Old 16th December 2011, 08:12 AM   #30
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Quote:
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I just noticed a small error there.

I'll rectify tomorrow, the principle are not in cause anyway.


Simple curiosity: has someone noticed what this error is?
I will not post a corrected version after all, it would be a waste of time.
Matthew 7:6, etc
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