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Old 4th February 2007, 04:52 AM   #11
Pano is offline Pano  United States
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Yeah, I know, I know. You aren't alone. But I do not automatically get my hackles up at descriptions like that. Other technical stuff bugs me far more. But maybe I have not read enough issues of Stereophile.


Quote:
Originally posted by AndrewT
I have never yet managed to make a cased stereo amp as quiet as an uncased mono floating ground amplifier.
That's funny Andrew, because for me -- it's the opposite!! Maybe it has to do with what's in the air where we are? I keep meaning to haul an amp up the side of the mountain far from electric power lines to have a listen. No 60Hz power for miles. Wonder what that will be like?

In a sailboat far at sea, with only battery power, how much shielding would you need? Of course that does not negate the questions of case connections, vibrations, etc.
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Old 4th February 2007, 06:28 AM   #12
BWRX is offline BWRX  United States
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Quote:
Originally posted by panomaniac
I have yet to hear proof that vibration makes a difference in solid state amps, but there are reasonable arguments as to why it "might."*
A couple years ago I brought in a battery powered SI T-amp to demo on some multi-thousand dollar NHT speakers that belonged to a university AES sound demo room. During my presentation of class d amps (during which I demoed the SI T-amp which actually wowed just about everyone in the room when they were told the amp powering the speakers was a 10W/ch 8AA battery powered class d amp with an iPod as a source), the professor asked everyone to close their eyes and raise their hand if they heard any difference in sound quality over the next 30 seconds. Everyone closed their eyes and he then proceeded to pick up the SI T-amp and shake the living daylights out of it. No one raised their hand. While continuing to shake the amp up and down he asked everyone to open their eyes. Everyone saw what he was doing and he commented that vibration has no audible effect on the sonic quality of an amp. Perhaps his explanation was flawed because he should have stated that vibration has no audible effect on the sonic quality of a SI T-amp. Other amps have yet to be tested

For the record, the SI was in its standard injection molded plastic case.
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Old 4th February 2007, 07:31 AM   #13
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Quote:
Originally posted by BWRX
While continuing to shake the amp up and down he asked everyone to open their eyes. Everyone saw what he was doing and he commented that vibration has no audible effect on the sonic quality of an amp.
That's not a particularly good test of whether an amp is microphonic. How fast can you shake you arm? Maybe 5 Hz or so? 5 Hz isn't audible and isn't the sort of thing that one is concerned about with regard to amplifier vibration sensitivity. If he had pounded on it it would have been a better test.

The real concern is whether the sound from the speakers can induce vibrations in the amp chassis and whether those vibrations can cause audible effects. ALL amps are microphonic if the speakers are playing loud enough, close enough to the amplifier. This question, like so many others in audio, boils down to degrees.

My amp is built solidly enough and I don't listen to music so loudly that any audio induced vibration will ever be audible. A lot of consumer amps are pretty flimsy and people do strange things with them like set them on top of speakers. A lot of subwoofers come with amps built in. In such cases it makes sense to question the sensitivity of the amp to vibration.

Here's an experiment to try. Open your amp and tap on the input coupling cap (if it has one) with a pencil eraser while the amp is powered up. You'll clearly hear thumping from the speaker.

OK, so you have a microphonic amp, now what? Do magic spikes, sorbothane balls, or magnetic levitaton do any good? I doubt it. Best bet- move the amp away from vibration sources- either mechanical or acoustical.

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Old 4th February 2007, 08:29 AM   #14
Nordic is offline Nordic  South Africa
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My latest preamp is a wood construction... perfectly quiet unless I position it crappy in respect to the other hifi components....

Its not going to vibrate either... must weigh like 4kgs.

I suppose everything is down to the scale of the observer to the observed item... but if you move a wire in a magentic field you induce a voltage over it... now there is lots of dc carrying wires in an amp... all you need is to move them around in the field...might be wrong here though...just theory
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Old 4th February 2007, 09:14 AM   #15
Pano is offline Pano  United States
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Quote:
Originally posted by I_Forgot
Here's an experiment to try. Open your amp and tap on the input coupling cap (if it has one) with a pencil eraser while the amp is powered up. You'll clearly hear thumping from the speaker.
I'll have to try that one. It's similar to the pot test I did. Ran 9 VDC thru the pot. Took the wiper output to my soundcard mic input via a DC blocking cap. Got no signal when thumping it.

This sort of test ought to be easy to do on an amp. Set up something to vibrate it, then record the output.

Apart from vibration, perhaps there are other issues, as has been mentioned higher in the thread. Such as eddy currents induced into the case and wiring. If the case is part of the circuit, those currents might affect the musical signals.
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Old 4th February 2007, 10:44 AM   #16
AndrewT is online now AndrewT  Scotland
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Quote:
Originally posted by panomaniac
..........That's funny Andrew, because for me -- it's the opposite!! Maybe it has to do with what's in the air where we are? I keep meaning to haul an amp up the side of the mountain far from electric power lines to have a listen. No 60Hz power for miles..........
are you saying that for you, electrical interference through the air (as against coming in through the mains) is your over-riding concern above all else?

I am surprised.
I have lived in towns before my present rural location, but even here I have 6kV overhead cables just 50m away, four different Cell-phone transmitters within 2km, all my normal domestic appliances and lots of commercial road and site vehicles moving around and I am not aware of ANY airborne interference.

But, I very definately contaminate my sound by connecting the mains borne interference that comes in via the safety ground.
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Old 5th February 2007, 01:44 PM   #17
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TNX to all! I'm a bit surprised because til now i've supposed that sound of enclosures, box and similar was a quite academic rather than real question and I've thought what happen to me was dued mainly to my clumsies. But is not so. Especially GRollins and AndrewT gave in thread some considerations which are excepionally in touch with many thoughts I've worked out in order to catch the essence of this curious and stimulating problem. Which appear to be just neglected but absolutely NOT unknown.

Til now I've conducted some experiments only on some old and cheap CD players (2 Philips, 1 Sony and 1 Pioneer). With the partial exception about a Philips CD600 (which come in the session yet modified), experiments a modifies left untouched the internal circuitry and concentrate mainly on rearranging the enclosures from an electrical viewpoint.

Philips enclosures are all plastic with metal reinforcement (especially top covers) which don't play any explicit electrical task (there are unconnected from all, even from the main ground); screening function reely on the ground plane existing on the upper side of PCB

Sony and Pioneer instead are mounted in a metal enclosures which act as a general shield.

Although the final result from a sonic viewpoint is extremely similar for all four CDP, the strategy of modification was quite different in the two kind of CDP.

Philips required the *adjoint* of a second ground plane, connected to the ground output, which cover the downside of the PCB. In the case of CD600 this auxiliary ground plane was obtained connection the bottom metallic cover to a ground; in the case of CD471 (the other Philips I own) the second ground plane was obtained glueing on the bottom of plastic case an aluminium foil (cut from a kitchen reel) and connecting it to the output signal grounds with a clip.

Sony, Pioneer and, I suppose, any other equipment with the same kind of enclosures (jap consumer style), required instead a systematic breaking of electrical continuity of the metal components of enclosures (top and bottom covers, rear panel and, if metallic, front panel).

Rear panel was connected on the output signal ground via a dedicated piece of wire (*not* via the outer ground ring of RCA pinout!). Bottom cover was left connected to a single ground point (on PSU) insulating or lefting they open any other connection to circuit ground.

The edges where top, bottom and rear covers meet and joint were insulated with insulating tape. Hence electrical continuity between panels was assured by a *single* screw lefting the others insulated or unmounted.

The mains trasformer was remounted electrically insulated from the bottom cover of the enclosures to decouple magnetically the former from the latter.

CONCLUSIONS - The modified equipments were of cheap type i get around; the same kind of tweaking may be done (and was done by a friend) on a low priced dvd-all standards players which can be found on big stores without any adverse advice.

In all the cases the sound has *inequivocally* (and somewhat unexpectedly!) changed. Here if the sound changed for better or for worse is not a question; the question is the changing *itself* due only to a rearrangement of ground *external* to the ground really pertaining the active circuits (from schematic and topological viewpoint, nothing is changed from before the tweaking).

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