| Riku-78 |
I am positive that there are around 1 billion threads about the subject, but didn't find what I was looking for. I did try to search, I swear.
So here is a quick question. We all know that National chip amps run optimally with +/-40v, which means a +/-28 (40 / sqrt(2) = ~28.28) volt tranny is needed to get the maximum output. 28 volt transformers just aren't available anywhere. Using a +/-30v transformer doesn't ever appear to be recommended, because they produce +/-42.43 volts after full rectification.
Now, LM3886 and LM4780 datasheets state that maximum supply voltage with no input signal is 84 volts, i.e. +/- 42. Isn't it fair to assume that as soon as the amp starts working and draws some current, i.e. when there IS an input signal, the voltage in the supply rails drops below the 84 volts (+/- 42 volts) maximum? Just a 0.86 volt drop would be enough.
However, if I use a +/-30 volt transformer in my active speaker project, I'll face another problem. The active crossover is going to power its opamps using the same toroid as the amplifiers and a 7815/7915 or a 7812/7912 pair. These regulators don't accept over 40 volt input voltages. Can I just put a voltage divider before the regulators? The voltages on the inputs of the regulators will drift with the load, but I guess it would be rather easy to keep them in the acceptable range. |
|
|
| Nuuk |
Why don't you just use a transformer with 18 volt secondaries?
If you want to use a higher voltage, use a transformer with 25 volt secondaries.
If you must use a transformer with 28 volt secondaries, get one custom wound.
IMHO - it is far better to power pre amps/buffers/crossovers from their own transformer. Remember, if you don't, you will need some way of delaying the power amp or you will get a very nasty noise when you power up and may even damage your speakers (particularly as there will be no crossover components between amp and drivers). ;) |
|
|
| Riku-78 |
True, true. There are 2 reasons to use the same power supply for both the power amps and the opamps:
1) Power suplies are expensive.
2) I'm going to build the amp in the speaker cabinet, so I don't have much room to waste.
I think I need to protect the drivers somehow. There is a circuit on the ESP site to protect the drivers from both DC and the power-on. What do you think, does that suffice to protect my drivers?
However, running the amps with +/- 35 volts is probably ok. Guess I'll be using two bridged LM3886s for the tweeter, and a parallel LM4780 for the woofer, so I should have enough power to not to fry my drivers. Even with the lower voltage. |
|
|
| tlmadsen |
Ther is no problem in using a 30 V AC transformer, but you need good cooling of the LMxxx.
Use the 317/337 regulator to get the +/-42 V down til something like +/- 15 V. They only have a input/output dif of 37 V, but no low max. input.
(the 7815/7915 will work with a higer than 37 V input most of th e time)
Regards
Thomas |
|
|
| Riku-78 |
Thanks for the info! Just what I wanted to hear. ;)
It appears that 317 regulators aren't available at the local electronic components store. Any idea where to get them? Or any good alternatives? Or can I couple a 337, which is available, with a 217? |
|
|
| Nuuk |
STOP
Don't do anything until you have read through the posts on this forum and through all this . :att'n: :att'n: :att'n: |
|
|
| Riku-78 |
I've read your (?) site and a lot of stuff on both this forum and the ESP site.
I'm trying hard to stick with readily available designs (and even better, PCBs) as much as possible. |
|
|
| Nuuk |
| quote: | | I've read your (?) site and a lot of stuff on both this forum and the ESP site. |
But why not stick with an 18 volt transformer then?
And you will have seen the stuff about driver protection/switch on delay.
If you want a ready-made active design that goes down to 40 Hz (easily OK for a single sub), there is a design on the site including details of the active crossover. You only have to substitute the amps for some chip amps and you are there. ;)
I appreciate that you are trying to save money although that doesn't seem to be an issue with your search for drivers, but I would still recommend a separate transformer for the crossovers (you only need a small one).
Keep your amps close to the speakers, but not in the box. Put your crossovers in a case with a selector switch and volume control and have that close to your CDP. |
|
|
| pinkmouse |
Why do you need to bridge two 3866s for the tweeter? A single chip should be more than enough unless you have a very low crossover.
I would recommend the use of a 25V or lower traffo, as Nuuk does. Yes, going for a 30-0-30 theoretically gives you more power, but in practise you won't even notice the difference in perceived loudness. It also gives you more margin for error, rather than pushing every component to the extreme, and the whole thing blowing up if your mains voltage fluctuates. |
|
|
| Riku-78 |
| A sufficient transformer for the opamps doesn't cost that much. But they're all relatively big physically. I guess I just have to try to find a very small transformer. |
|
|
| Riku-78 |
| And about the power... It's not like I'm willing to break all the safety glass windows in the neighbourhood with my monitor loudspeakers. Just want the amplifiers to be powerful enough to not to fry the tweeters. Heard too many stories about tweeters that were destroyed by underpowered amps. Besides, I plan to use the ESP PCB to build the LM3886 amp for the tweeter, and the board is designed for two amps. Just have to have both inverting and non-inverting tweeter outputs on the filter board. |
|
|
| woody |
First do you already have the transformer? If so you might
want to measure the output voltage after your bridge rectifier
and power supply caps with a 100ma load.
There are a couple of reasons for this. First a low power
transformer raited at say 1 amp 30 v will probably be a bit
higher if you are only loading it for .1 a or so.
Then your real line voltage at your wall outlet will probably
not be 115v. It will probably be a bit higher or lower. In Atlanta
Ga. I have seen it as high as 127V ! either or both of these
could things could give you an output voltage a bit higher
than expected then we forgot to put in that you will need to
figure on ~1.5v lost in the bridge rectifier. |
|
|
| Riku-78 |
Haven't bought the transformers yet, still trying to find affordable ones... The wall outlet in this part of the World is nominally 230 volts / 50Hz, and I've heard it can really be pretty much anything from 225 to 235 volts.
I will probably use an oversized, say 500VA, torus. +/-25VAC.
That still leaves the question about powering the XO. |
|
|
| joensd |
Talking about rail voltage and no mention of speaker impedance?
Mmmh, yes the chip would have big "idle power" at +-40V but shouldn´t be a big concern unless you connect your 4Ohm speakers and suqeeze out 50W which is almost uncoolable for one chip. I´ve tried it and burnt me fingers.
For a 4Ohm speaker I would vote for 18V secondaries as well but with a 8Ohm you might go up to 30V. That said I would only use 30VAC if I had it lying around. As it seems you´d still have to buy it for for 25VAC or so. |
|
|
| Nuuk |
The transformer for your crossovers can literally fit in the palm of your hand and won't take up much space.
Also, if you want to include all this in the speaker cabinet as you originally stated, aren't you going to need separate transformers for each channel? Or were you intending running an umbilical from one speaker to the other? :confused: |
|
|
| Riku-78 |
I just realized that the tweeter I'm going to use is 4 ohm... Probably better not to use bridged LM3886s to drive it. Hangover and designing stuff don't mix, really. A while ago I tried to convert 3 inches to human-understandable units and got 15 cm.
:headbash:
Indeed, I will need two power supplies. I'm building two identical active speakers that don't share any components (the way active studio monitors are usually made). That way, even if I fail to achieve every other goal I have set for my project, I will at least have [insert a big number here]dB channel separation... I assume you're saying that I don't quite need a 500VA transformer to drive a single channel?
:D
Would it be a working solution to just order 6 of those parallel LM4780 boards and bridge two of them to drive each woofer (8 ohm) and one to drive each tweeter (4 ohm)? |
|
|
| Nuuk |
| quote: | | I assume you're saying that I don't quite need a 500VA transformer to drive a single channel? |
My set up had 300 VA per channel to drive both the woofer and tweeter.
You really seem to be a bit hung up on the power needed to drive these things. Without a passive crossover to soak up power, you will honestly find that a single LM3886 per driver will be more than sufficient.
But please don't put all the electronics in the speaker 'boxes'. That will adversely affect the sound for sure! :att'n: |
|
|
| Riku-78 |
| In the sketches I have drawn the amp is isolated from the acoustic cabinet using 6 mm vener. The relatively thin material may cause some nasty resonances, and I may end up using 19 mm MDF, but as the cabinet will not be very wide (~200 mm total) the 19 mm MDF side panels (and internal corner baffles) may be sufficient to support the structure. However, the point is that the amp / crossover / power supply thingy will not be inside the 'box.' Just built as a part of it, in a subenclosure in the bottom of the cabinet. |
|
|
| Riku-78 |
| Or do you mean that all electronics stuffed in a small subenclosure cause some electric interference that affects the sound? |
|
|
| Riku-78 |
Thanks for the link. :)
I guess I'll live with the 25 volt secondaries. |
|
|
| imix500 |
| Meyer Sound uses medium to large amps in almost every cabinet they make- they have been primarily self-powered for years and they are some of the best sounding and performing boxes out there. As long as you account for the volume of the amp module and seal the module or put it in it's own chamber in the cabinet, it works really well. You can also, with some careful design, use the ports of the box to pull air through the amp for cooling. With forced air cooling across the heatsinks through the box, it also works really well when the amp is being heavily loaded- in Meyers case when the amp switches from AB to H. |
|
|
| jarkaa |
Hi, I am driving my tweeters with opa548 and trafo is 60VA +-15V.
for both channels
Tweeters efficiency is about 92dB.
I can tell you that they can get as loud as my ears can take, so I think that for nearfield monitors you can cope with 100-200VA tranny for the whole system. |
|
|
| daArry |
I had a beefy 200VA 2x30VAC tx I used in my LM3886 amp, which I regulated down to +/-24V using LM338T's, and then from that I made +/-15 (using reg 78/915's) for the pre section...
Anyway, all's still workin (and soundin great) and i've still got 10 fingers! |
|
|
| Nuuk |
| quote: | | Anyway, all's still workin (and soundin great) and i've still got 10 fingers! |
That's a big advantage over the rest of us who have only eight! :devilr: |
|
|
| daArry |
very good, very good.... ;)
would be handy tho thinking about it! mebbe some of us will do what some of those funky lemurs done and evolve sets of customised 'finger tools'.... a pokey and teaspoon would be good for starters! |
|
|
| Nuuk |
| quote: | | would be handy tho thinking about it! mebbe some of us will do what some of those funky lemurs done and evolve sets of customised 'finger tools'.... a pokey and teaspoon would be good for starters! |
Well, at last a use for genetic engineering! ;) Want to do hi-fi in your next life? Request an extra hand or two. :D |
|
|
| Riku-78 |
| So... What exactly are the benefits of using a pair of LM3886s instead of LM4780s? Does LM3886 sound significantly better? |
|
|
| Nuuk |
| quote: | | So... What exactly are the benefits of using a pair of LM3886s instead of LM4780s? |
Not necessarily. I was just suggesting a 'simple' plan using one amp for each driver. But there is no reason not to use the LM4870's if you prefer. ;) |
|
|
| Riku-78 |
One reason why I would like to use an LM4780 for each driver instead of LM3886 is that being able to either wire the channels in parallel or bridge them gives me more room to fool with the impedances.
Another reason is the power... I've already said it and I say it again: I don't want to blow up the neighbourhood. I just want to have enough power to always stay on the safe side. Ever heard of a 50 y/o audio enthusiast who has never destroyed a driver by using an undersized amp? I'm trying hard to be one a quarter century from now.
:cool:
And the power is particularly important with the low crossover point I'm going to use.
;)
The only drawback I can think of in using LM4780s is the increased complicity of the design.
:xeye:
About the heat dissipation... I knew many commercial designs use the vent airflow to cool the amps. This is a great idea, but I think I will be using an simpler approach. The whole backpanel (or perhaps just a part of it) of the cabinet will be covered with an aluminium plate to which the amps are connected. This is just easier to design, and the looks don't really mattter, because with my woodwork skills the cabinets will look horrible, anyway...
:D |
|
|
| Nuuk |
| quote: | | Ever heard of a 50 y/o audio enthusiast who has never destroyed a driver by using an undersized amp? |
Well, I'm 56 and I have never blown up a speaker with an under-powered amp. I don't quite know where you have got this paranoia from.
Neither have I yet heard of anybody building a Gainclone who has done so - with any chip including the LM1875, and with any power supply!
On the contrary, listen to the evidence. Jarkaa told you
| quote: | | Hi, I am driving my tweeters with opa548 and trafo is 60VA +-15V. |
|
|
|
| Riku-78 |
Wow... Everyone I know have blown some driver... Of course, that must have been caused by sound levels that cannot have sounded good any more.
Driving the tweeter with, say, 40 W gives around 99dB sound pressure, which is quite a bit more than enough. No need to push to the limits. But how about the woofer? It's 8 ohm and has somewhat lower sensiitivity. |
|
|
| Nuuk |
| quote: | | Wow... Everyone I know have blown some driver... Of course, that must have been caused by sound levels that cannot have sounded good any more. |
May be you and your friends now need hearing aids, not active speakers with mega PSU's! :D
For the record, my active speakers used an 8 ohm woofer with 90 db sensitivity and were powered by 38 watt amps. I never came close to destroying the drivers or being able to listen at full volume!
OK, here is an open question to anybody on the forum. Have you ever blown a driver by using an under-powered amp? |
|
|
| jarkaa |
Hehe, never blown any elements.
As I described my tweeter setup before, I can tell you that it can be very LOUD.
At normal listening level tweeter drains about 0.1-0.15w (measured about 0.9V-1.1V at tweeter terminal). It means that even with a few watts you will more likely damage your membrana tympanica (sorry about the latin name but I do not know the english word for the membrane inside your ear) before your tweeter blows. |
|
|
| Riku-78 |
| quote: | | sorry about the latin name but I do not know the english word for the membrane inside your ear |
The English word for "tärykalvo" is "tympanic membrane" :D
Or "ear-drum", which is quite a bit more common. :)
But to get back on topic, I think you guys have been convincing enough. The question in my mind is no longer "what are the benefits of using LM3886", but rather "what are the benefits of using LM4870." As they both seem just as good (for my purpose, anyway), and there are tons of readily available designs with LM3886 on the web, I guess I just use those.
So I just added myself on the LM3886 grouporder page... |
|
|
| planet10 |
Can you tell us about your speakers... drivers used etc. One has to watch the siren-song of big numbers... those numbers are into a resistor, and speakers -- even directly driven ones do not resemble resistors. making sure the amp can deliver into a real load is n=more imporatant than big numbers -- one of the reasons i like to use 18-20V secondaries on my clones -- that is the sweet spot for power delivery into a realistic load (i do plan on trying some higher voltage stuff, but will use them with 16 ohm speakers).
dave |
|
|
| planet10 |
| quote: | Originally posted by Riku-78
"what are the benefits of using LM4870." |
A 4870 is essentially 2 3886s in a single package (ie it is more compact, but probably a bit harder to get the heat out)
dave |
|
|
| carlosfm |
| quote: | Originally posted by Nuuk
Neither have I yet heard of anybody building a Gainclone who has done so - with any chip including the LM1875, and with any power supply! |
I tried bi-amping my Epos ES11 speakers with LM3886s (regulated +/- 30V PSU) for bass and LM1875 for treble with regulated +/- 25V or unregulated +/- 21V (with 15V trafos).
Either way, the results were very good. |
|
|
| imix500 |
| Nuuk, in response to your question to all, I have never blown a driver using an under-powered amp. It's really quite hard to do if you know what you're doing and your gain structure throughout the system is correct. Meaning everything clips together, which is how I run a system- the way it should be run. I have however replaced HUNDREDS of elements in speaker clusters from stadiums to concert halls to arenas due to under powering. I've re-drivered one particular cluster 4 times with 20 drivers in it. Heh, I've installed 40 motor start caps in that cluster too help block dc to the horns from the amps to lessen my service calls. It happens just as often in huge complex systems as it does with a stereo in someones living room. Often in the big systems it's because of budget, and the right size amps were "too expensive", and 90% of the time that decision comes back with avengence. The same can be said of home audio on occasion. Although it's been my experience that the components are better matched power-wise in that world. |
|
|
| Riku-78 |
| quote: | | Can you tell us about your speakers... drivers used etc. |
I haven't bought any parts yet, but my plan is to use Peerless CSX 176 H woofers, Vifa DX25TG-05-04 tweeters and a very low crossover point. The cabinet will be a PMC style compact transmission line. The whole project is really a cabinet design experiment--the speakers may end up sounding great, and just as well like a pair of plastic multimedia speakers. In the latter case, I can always tear them down and build bass reflex cabinets. |
|
|
| planet10 |
| quote: | Originally posted by Riku-78
cabinet will be a PMC style compact transmission line. |
Do model it in MJKs software before you build it...
dave |
|
|
| joensd |
| quote: | | I can always tear them down and build bass reflex cabinets. |
Take Dave´s advice, don´t believe you´ll fail with the software.
And before building a BR you can always use the Peerless Pipes cabinet:http://geocities.com/rbrines1/ |
|
|
| peranders |
| quote: | Originally posted by Riku-78
I am positive that there are around 1 billion threads about the subject, but didn't find what I was looking for. I did try to search, I swear.
So here is a quick question. We all know that National chip amps run optimally with +/-40v, which means a +/-28 (40 / sqrt(2) = ~28.28) volt tranny is needed to get the maximum output. 28 volt transformers just aren't available anywhere. Using a +/-30v transformer doesn't ever appear to be recommended, because they produce +/-42.43 volts after full rectification.
Now, LM3886 and LM4780 datasheets state that maximum supply voltage with no input signal is 84 volts, i.e. +/- 42. Isn't it fair to assume that as soon as the amp starts working and draws some current, i.e. when there IS an input signal, the voltage in the supply rails drops below the 84 volts (+/- 42 volts) maximum? Just a 0.86 volt drop would be enough.
However, if I use a +/-30 volt transformer in my active speaker project, I'll face another problem. The active crossover is going to power its opamps using the same toroid as the amplifiers and a 7815/7915 or a 7812/7912 pair. These regulators don't accept over 40 volt input voltages. Can I just put a voltage divider before the regulators? The voltages on the inputs of the regulators will drift with the load, but I guess it would be rather easy to keep them in the acceptable range. |
If you are prepared to blow the LMxxxx you can test a 2 x 30 VAC transformer. If the amp has worked one week or so it will probably withstand the voltage especially if you have a stable mains voltage (not too much voltage peaks).
If I wanted to give a good engineering choice 2 x 28 V would have been correct. www.elfa.se has 2 x 28 V AC :nod:
Using LM317/337 is a good idea since they will take 12+35 volts = at least 50 volts. |
|
|
| sek |
Hi,
| quote: | | Have you ever blown a driver by using an under-powered amp? |
This is a never ending story, mostly among pro musicians. The story goes like this:
When a solid state amplifier is overdriven and goes into clipping, it approaches a square wave signal at it's output, because as the input signal waveform is clipped, it becomes flat on it's top. :rolleyes:
If you use an amp with too low power (whatever that means), overdriving it will apply this sqare wave to the speakers. Woofers are usually filtered with a low pass filter. But as tweeters have no low pass filter (on top of the audible frequency range), they are exposed to the high power at high frequencies. I'm talking of tens to hundreds of kHz and up.
It now depends on the tweeter what it makes of it. Can it handle power at higher frequencies? Probably not, because it isn't designed for this abuse.
Another story tells that an overloaded amp applies DC at it's output, because of the flatness of the distorted waveform. Well, this can only sound sensible if asymmetries in the distortion lead to an offset of the 'sqarish' wave. But this affects woofers only, as tweeters (usually) employ a high pass filter.
To contribute to the question: I have never managed to destroy a tweeter with an 'underpowered' amplifier. But I have heard stories about it. Like abusing the amplifiers way above their specifications. Usually, the tweeters fail short before the amps would have. Or connecting the tweeter wrong. Or setting the crossover frequency too low, so that the actual reason of the failure was simply too much power.
To sum this up, tweeter failures due to underpowered amps is almost always due to wrong usage of the system. This almost exclusively involves people who can't care for the power levels a 'user' applies, e.g. rented equipment, musician on stage (can't mix while playing...) and such.
The reason why 50+ people report of such problems at home is due to the way power amplifiers used to fail 50 years ago. ;) The high frequency power required to fry a tweeter would only come into play at distortion levels one couldn't stand at home any more. Not to mention the terrible loudness. With a modern solid state amplifier (e.g. chip amps and their protection circuits), this particular problem will probably never occur, while overpowering it with two LM3886 is very likely at too high listening levels...
Selecting an overpowered amp for a tweeter is no cure to the actual problem: wrong system use. But it poses a completely different risk: frying the tweeter due to too much power, which sounds, looks, feels and costs exactly the same! The only solution: use an appropriate amp, and use it right. ;)
Sebastian. |
|
|
| agrobler |
Have fried three drivers.... but only because I got LM3886's to oscillate... Tweeter - adjusted Active X-O while powered... :bawling: Two mids... openloop on active X-O... all six channels, luckily only two mids connected... searching for groundloops and not thinking... and 100 000 uF PSU caps meant switching off poweramp did not help... at first. :headbash:
Powerful amp means that drivers get fried in shorter space of time when clipping |
|
|
| planet10 |
The more you guys talk about blowing tweeters, the more i think a tube amp that compresses instead of clips is a good choice for tweeter amp -- hmmm i have some nice "small" EL95/EL84 OPTs that would be nice for a tweeter amp.
In my hifi days i saw a lot of tweeters smoked by high feedback Japanese amps pushed beyond their limit -- these amps did not clip gracefully... a large part of the problem could be how the amp recovers from clipping as much as it is the clipping itself.
dave |
|
|
| carlosfm |
| quote: | Originally posted by sek
The reason why 50+ people report of such problems at home is due to the way power amplifiers used to fail 50 years ago. |
No need to go back 50 years.
30~35 years ago transistor amps with no protection were quite common.
People made parties with low power transistor amps.
Output transistors fried and automatically the speakers too.:cool:
Alas, on these amps without protection if you powered them on without speakers they would burn the output stage.
Loose contacts with speaker cables were a real problem.
Some people didn't know or sometimes forgot this and had problems.
That's how my father repaired a Sinclair transistor amp (anyone remember these?) for some five times.
The guy was always fiddling with his system and sometimes forgot to connect the speakers to the amp.
Bang!:cool:
These chips we are using a really a peace of mind. |
|
|
| FBsHT |
Has anyone mentioned the regulation problem with using a 30X2 transformer?
Sure 30V * sqrt(2) = 42.4V which might be OK, but this is the output voltage at load. If the transformer has 10% line regulation the low/no load voltage will be closer to 33V.
33V * sqrt(2) = 46.7V! This has a much lower probability of not smoking. This isn't accounting for variation in you mains either. If your tranny is designed for 30X2 with 115V input, then 120V mains would up your output to 34.4V (no load).
34.4V * sqrt(2) = 48.7V!
It is probably best to keep your tranny to 25X2 or lower. The output power difference will be small and your amp will be happier.
That being said, I have a four channel LM3886 based amp that I built many years ago (1997?). I am running it with a salvaged pioneer transformer that supplies +/- 43V rails with low load.
The amps are still running with no problems. The scary part is that these are driving 4 ohm speakers!:bigeyes: |
|
|
|
