Microphony considerations - test

It would be interesting to extend the test to the interconnects. A coax cable is a microphone, especially when connected between a high impedance source and higher impedance sink. Passing a signal may help.

Paul

 

Could do.

The WAD phono stage has 1.2k ohms output impedance and the Radio input on the Quad is 100k ohms.

Is that what you had in mind Paul?

The effect is likely to be small if measurable and separating the pure cable effect from that of the equipment at either end won't be easy.

 

I don't really know. ISTM that considering plausible physical effects in the worst case gives an upper-bound for whatever may be really the case in actual installations. And if you squash a coax its capacitance changes, which will cause a small current flow, which will turn into a voltage. Etc.

Paul

 

Agreed, that was also the thinking behind this test - worst case.
I'll think on it and if anyone has an idea on how we can test some coax let's hear it.

<edit>

How about this.

Long length of coax say 5m, connected to the MM phono input so a nice high 47k ohms.
Short out the far end and place the last 1m of cable directly across the face of a loudspeaker. Then blast it with 100dB+ of music and record the result.
The long cable will allow the SS phono stage to be placed well away from the loudspeaker.

 

Now this I don't need proving - anyone who's ever plugged a guitar or bass into an amp is well aware of cable microphony!

Tony.

 

Does it - in what way exactly? He seems to use it quite happily - nothing appears to be 'broken'.

 

A SS power amplifier making a noise through the speakers when the case is tapped is not something I've come across, ever. Since such things are easily avoidable I'd be interested to see the design. Damaged amplifiers can make odd noises if the internals are loose or connections are bad.

 

I think guitar-amp is a boundary case... Do wireless systems sound better or worse? One would hope worse.

Paul

 

That's why i suggested it, to rule it out as a possible cause, do keep up.

Sorry Basil, I can't remember seeing you round my house, ever. It may not be within your experience that Naim gear pops when walloped, but mine did, i'm pretty sure Ynwans does too. I still use a Naim card based phonostage and that sure as hell knows when I have plopped the clamp from my SME on the shoebox case for that.

It that doesn't tally with your experience, that's fine, no need to be so dismissive. There are no absolutes.

 

It's not damaged - there are no loose bits inside. I'm not saying the manufacturer if it is going to reflect poorly on them - it's not difficult to guess anyway.

 

Not the slightest idea. Back in my day they were only used by poodle-haired twats in spandex clad metal bands. I was in an indie band so stood motionless by my amp.

Tony.

 

No problem, but if you've got a SS power amp and its that sensitive I'd say that is not a good design, given no other power amp to my knowledge suffers this effect.

Even tapping my valve phono stage case only produces the faintest of sound through the speakers. I can't guess the make - PM it if you like, I won't make it public.

 

Warmth?

The carpet was keeping the bottom of the case warmer than other surfaces.

 

Lindsay,

Good guess but I don't see how as many of the boxes on any rack probably see the same temps if not higher after a good thrashing. The difference in sound post-thrashing and on a rack isn't the same as direct to carpet either.

I'm sure the electrical field in most carpet comes into play to some extent; however, it still doesn't explain the consistent differences when the gear is racked.

One thing I regret not trying was isolating how much of the effect was with the various CDPs at the store vs just the amps. I did try this at home later just using my CDS mk1 and found the transport/DAC unit was more sensitive to vibration than other device including any turntable I've ever owned. In other words, the difference in sound with the CDS head unit on a rack vs the carpet was far greater than the preamp or power amp.

regards,

dave

P.S. The same differences with a friend's CDS head unit were experienced with carpet vs rack using his rig and in his home. I had a mid-line passive olive system w/briks at the time and he had a full six pack w/DBLs.

 

Morning Rob,

Well we have been involved with a couple of commissioned two studies, one at Imperial college and the results were most enlightening.

What I can say is that a number of accepted devices used for 'controlling' internal vibration effects of equipment have a 'limited band' of operation, others obtaining measurable effects from 4hz well into the MHz ranges.

We now use this in non audio areas of our business as well as the obvious top great effect.

Why not hire a lab for a couple of days (its not as much as you think) that way you can give concrete credence to your findings, set a precedence lead by example mate.

 

I have seen P.C.Bs. from really big manufacturers that do not have through-plated holes.Some have little eyelets with splayed ends to connect the two sides of the board .These eyelets have been flow soldered on the track side and if your lucky hand soldered on the component side.In other instances I have seen wire links through the holes.Any one of these "joints" can be a source of trouble.

 

Tony, I think a certified lab is required if you want to have results meeting industry standards. However, if you just want to test if something works or not to within quite a large tolerance it's not really necessary. I can't see a way that doing Rob's tests in a lab will cause there to be anything measurable on the audio output when there wasn't before. If he wants to start testing the capability of different isolation devices then maybe something more is required.

What would people like to see as the next step?

I think the ideal test signal would be a square wave with something like a 4Hz frequency. It contains all frequencies and has a repetitive transient pattern to spot easily in noise.

 

Ideal because no amp can hope to replicate it......

 

Nor any speaker. It doesn't matter though, it just needs to be as wide-band and transient as possible.

 

It's not mine; as I said before, a friend owns it. As you stated earlier, one doesn't go round banging on ones amps so it doesn't really mattter. However, the fact that any sound is produced at all is the surprising thing.