I was quite suprised at how good the live sound was on a Massive Attack concert I went to recently. Hadn't seen an outdoor one before and wondered if the bass would come across well. It was in fact the most powerful bass i've ever heard, literally "trouser flapping" and the vibrations in your chest from it actually made it difficult to breathe . The overall quality of sound was good too, a bit too bass heavy, but at least it wasn't bright and nasty, my ears were fine after.

Generally, in shops the sound is poor. I find the best is when they use Hi-Fi speakers and multichannel amps, though this only works well in smaller shops. Virgin use compact tannoy speakers and they appear to use a lot of amplifiers for them all rather than 100v distribution, they sound respectable. In our local HMV they have a good system on the top floor. It is a pair of big club speakers, they look cheap and are made of plastic but used to sound very good. Now though someone has altered the settings and it has a horrible boomy bass boost sound to it. The speakers look far to big for the room and are mounted on the walls up high but it did manage to sound pretty good.

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Angry!? I'm absolutely electrolytic!

Will have to make do with myspace now I guess...

 

Know what you mean about feeling the bass they always run too loud at Donut gigs (at least for me!).

You have to bear in mind the massive power they run at these gigs!, I remember reading an article years back about the PA for an ABBA tour of Australia. The first thing they had to do was order (well in advance) an independent 3 phase mains supply to be provided, in order to cope with the demand - they also always carried sacks of salt and large metal spikes, so they could make their own earth system if they didn't like that provided. All the amps (banks and banks of them) were top end, Crown and such. Lighting and sound were on different phases, to help prevent any noise from the lighting getting to the amps.

Powers are even higher (and cheaper) these days, if I remember correctly the first band to run 1,000,000 watts was the Rolling Stones - late 60's early 70's?.

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[quote=Nigel Goodwin]
< And I repeat again as well, what has that got to do with the matching INTO the preamp - quoting
< formulas from a book doesn't help anything, do you have actual examples of microphone
< transformers being impedance matched to the preamp input impedance?.
[end QUOTE =Nigel Goodwin
.................................................. ...............................
In my final attempt to show that impedance matching is
required for best noise performance the ultimate in low level
amplifiers, I include the following from respected companies
on the Internet
..................................................
1. http://www.jensen-transformers.com/an/an002.pdf
see right side of p2
..............................................
2. http://www.cinemag.biz/mic_input/mic_input.html
3. http://www.jensen-transformers.com/mic_in.html

.............................................
4. http://www.shure.com/ProAudio/Produc...diotransformer
quote
Do's and Don'ts of Audio Transformers

Do use a transformer to match impedances.
Do use a transformer to increase or decrease signal level by up to 25 dB.
Do use a 1:1 transformer to isolate problem components in an audio chain.
Do not use a transformer to increase signal level by more than 25 dB.
...............................................
5. http://www.kandkaudio.com/mccartsetup.html
6. http://www.midi-classics.com/c/c4283.htm
++++++++++++++++++++++++++++++++++++++++++++++++++
It is important to keep an open mind.
Especially when there is so much evidence
that apparently negates what we are sure we know.

hawk2eye

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hawk2eye

 

Hawk2eye,
Your 1st example has an 80 ohm source connected to the 42k impedance of an input transformer. That is far from being matched.

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Uncle $crooge

 

1000W surely? a million watts is madness! I did wonder how much power they used at that concert, and then it's not just the sound but lights, equitment etc. I didn't see any generators so presumably they had a special supply, or they were round the back somewhere out of sight.

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Angry!? I'm absolutely electrolytic!

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1kW of audio power sounds twice as loud as 100W.
In TOA's demo room I had a subwoofer with two very efficient 15" drivers connected to a bridged amp producing 1800W cleanly at max and got clipping sometimes. The sound was so clear that it didn't seem like so much power.

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Uncle $crooge

 

1000W is common place, my small Behringer PA is 800W - I can't remember the year the Stones first used 1,000,000 watts, but it was a LONG time ago, certainly 70's and may have been 60's.

I would expect many modern club disco systems to output in the 10's of kilowatts?.

Here's a quick 'modern' quote I found:

"The Rolling Stones are also one of the most agile organizations in the world. They move 250 employees with every concern, have the capacity to generate four million watts of electricity, lay eight miles of sound and electric cable at every venue, assemble the world’s largest television screen, position 1,500 spotlights, and build a stage so tall that it actually requires aircraft warning lights under U.S. Federal Aviation Administration rules, all in record time."

What do you think they do with 4 megawatts of electricity

As I mentioned earlier, the ABBA concert arranged for their own supplies to be laid on - but any regular venue would probably have it permanently there.

As you say, a generator is a possibility, a truck sized one can generate an enormous amount of power, and they can be whisper quiet.

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All those are literature from transformer manufacturers, and none seemed to suggest feeding into a matching impedance preamp?. What they did suggest, is a loading resistor on the output of the transformer (to reduce ringing in the transformer), this then feeds in to a higher impedance input preamp - as all valve microphone preamps did.

I'm quite prepared to keep an open mind, but you've yet to give a single example of matching source impedance to preamp input impedance in ANY microphone preamp?.

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I tried out building a new circuit around the LM386, which I found at the RadioShack, since everyone pretty much agreed an op-amp would work out better. I checked out it's data sheet as well as this page here, to come up with my circuit. The 20x gain the LM386 defaults to is reasonable, but I think I'll modify it with a rheostat to tweak it at will. I have two questions that strike me immediately. First, the input 10uF cap is electrolytic, and the second page referenced shows it with the positive towards the input. Looking at the internal schematics LM386, this seems acceptable since that side goes to ground. Is that acceptable? Secondly, what's the function of the 0.1uF cap and 100 ohm resistor to ground on the output? The original spec was for a 0.05uF cap, but I substituted it for 0.1uF which I had on hand. I'm guessing this circuit acts as some sort of filter to lessen noise. Is this the case, and is my substitution acceptable?


In response to the discussion of impedance coupling, for microphones, at least, wouldn't having matched impedance create a load on the mic itself, thus causing distorted input?

TJ

Attached Images

amp2.GIF (3.5 KB, 11 views)

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KF4GAU

I haven't burned myself with the soldering iron in at least.. Ow that's hot!

 

It isn't an opamp. It is a power amp. It is pretty noisy if it has a gain more than 20.

The input DC voltage of the LM386 is 0V so if the capacitor is fed from a grounded volume control then it doesn't matter which way the capacitor's polarity faces.
At first I thought 10uF is way too high a value. Then I realized that it would help reduce noise if the volume control is 1k to 10k ohms.

It is 0.05uF and 10 ohms. 0.1uF might work but I would use 0.047uF instead. It stops the LM386 from oscillating at a high frequency when the inductance of a speaker causes a high load resistance, like your application without a speaker.

A matched impedance doesn't cause distortion, it causes the output to be half.

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Uncle $crooge

 

Well, the packaging used an op amp symbol within the DIP, but I understand the difference. I also snagged a 741 op amp while I was there too. If I'm going for more than 20x gain, would building a circuit around the 741 be any less noisy than the LM386? How much clean gain should I expect from the 741? Perhaps a greater amount of gain could be achieved by running the signal through both the LM386 and 741 in series, or perhaps two of the same?

For something completely different, does anyone know of a cheap local source of small tin boxes to house little projects like this? RadioShack has plastic boxes, but I'm thinking this really should be shielded by using a metal container, right?

TJ

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KF4GAU

I haven't burned myself with the soldering iron in at least.. Ow that's hot!

 

An old, old, old 741 opamp is one of the 1st and one of the noisiest ever made. Its max frequency is only 9kHz at full output.

A preamp needs a low noise wide bandwidth audio opamp like a TL071 or one much better.

To amplify a 1mV mic to 1V, you need a voltage gain of 1000. Even a wide bandwidth TL071 has its response drop above only 3kHz with such a high gain, so a TL072 dual opamp could be used with each opamp in series with a gain of 32 for a response that is flat to 100kHz.

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Uncle $crooge

 

Are you sure? Wouldn't that oscillate?

Using two op-amps in series would double the phase shift as well as the gain, so you're sure to get oscillation.

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They will be completely separate and won't oscillate. Each opamp will have a gain of 32.

If the negative feedback was around both of them in series then they will oscillate.

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Uncle $crooge

 

Ahhhh, after taking a look at the spec sheet for the 741 and the TL071, I see what you mean! The bandwidth is much wider, the THD is two orders of magnitude lower and the slew rate is incredibly better for the TL071. I'm learning a lot on this board. I'm pretty sure I can snag a TL082, which looks as if it would work the same as a TL072, so I can try building both in series.


I'm still pretty unclear on how the 0.5uF cap and 10ohm resistor on the output prevent an LM386 from oscillating. Using google, I found a lot of info on how oscillation can start and how to prevent it, but not with this particular method. I'm guessing the point is to provide a guaranteed minimum amount of AC current leaks off in the presence of a high load resistance?

TJ

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KF4GAU

I haven't burned myself with the soldering iron in at least.. Ow that's hot!