Wattage Meters

oldrocker

Playing around and noticed this on my old SX780.

Left meter appears less responsive in it's movement.

Seems to be consistent on various music sources.

Anyone else have this situation?
I wonder if a cleaning of some sort would help or perhaps that channel is just weaker?

mhardy6647

So, here're my thoughts -- "as is" and FWIW

If you feed the identical, mono channel to both signals, do you still see an offset? If you alter balance (i.e., adjust the receiver's balance control) to make the meter levels identical, does it sound balanced or out of balance?

Looks like a small difference (maybe ca. 3 dB?) -- could it be real?

Did both meters used to track each other better?

It's fairly likely (tho' not a slam dunk) that one meter is slightly offset relative to the other one.

There is probably a zero adjustment on the meter (I don' t know this for a fact -- but there usually are).

Do the "ballistics" of the lagging meter match the other meter's (in other words, to the seem to move with the same velocity if the signal's identical for both channels)? It is possible that the "suspension" (for lack of a better word) of one meter is damaged or out of "tune" relative to the other one. The meter movements are sort of like an analog (think "Swiss jeweled") watch movement. They can get borked (remember this thing is like four decades old!).

A mono signal will be helpful to troubleshoot, methinks. A low level, constant amplitude tone, or something like FM radio static (muting off), in mono, should be a helpful tool to troubleshoot.

hifiengine has the service manual (and owners manual, too) for the SX-780, if you don't have it/them: https://www.hifiengine.com/manual_library/pioneer/sx-780.shtml

If you don't have an account, or if you have trouble setting one up (it's free and well worth the effort!) and want the files -- send me a PM if you like.

F1nut

Those things aren't exactly accurate to begin with.

tonyb

F1nut wrote: »

Those things aren't exactly accurate to begin with.

Yep....eye candy really, like those lights on the old equalizers.

mhardy6647

F1nut wrote: »

Those things aren't exactly accurate to begin with.

There is that.


Plus, in components like that Pioneer, they're not "really" power meters at all -- they're voltmeters, calibrated to read the output voltage of the power amp as Watts into a nominal (theoretical) 8 ohm load. They're pretty much inaccurate by definition.

That said, one would think that the same output (voltage) from both channels (e.g., at steady state, such as a single frequency sine wave) should give pretty much the same reading if the zero and full-scale points of both meters are calibrated.

Come to think of it, if the zero's off on one meter, it should be apparent with both meters at rest.

heiney9

Wattage measuring is not linear, it's algorithmic so those meters are averaging. That said stereo signals don't have the exact same content so it's possible that one side "reads" differently than the other. But, and here's the big but......those meters probably aren't accurate enough to pick up the differences in signal between the l/r channels.

They are eye candy at best and probably are functioning as well as they were intended to. Those meters are not a measurement of true wattage output. The numbers on the label of the meter are an approximate average.

H9

mhardy6647

Well, there are two things going on here (at least).

1. The range is indeed logarithmic in terms of power (log linear, i.e., 0.1, 1, 10, 100 watts, etc.) -- the decibel (dB) scale is linear, though (-20, -10, 0, 10...). The establishment of the "Bel" (we use deciBels, i.e., decibels, because the Bel unit is inconveniently large for AC and SPL measurements), named for Alexander Graham "Telephone Boy" Bell, was a deliberate linearization, 'cause back in the old, pre-calculator days, it was kind of hard to work with nonlinear quantities. Heck, that's what slide rules were for!

2. The meter "averaging" actually has nothing to do with the logarithmic nature of audio reproduction per se; it is a (deliberate) function of a meter's "ballistics". The inertia of the meter itself tends, of course, to damp its response, but there are very deliberate choices made (in "pro" level meters) about the averaging characteristics. The "VU" meters of old-time radio stations (and better "prosumer" and pro tape decks) have a very specific peak averaging "algorithm" in their ballistics.

It is "possible" to make peak reading analog meters, but of course there's still some damping and/or hysteresis due to the moving mass and the mechanics of the meter movement.

VU meters are also calibrated (!) to a specific reference point for signal reproduction -- the VU characteristic is irrelevant, though, to the current case. A peak reading meter, by definition, is not a "VU" meter... nor is a wattmeter.


The meters in an SX-780 don't likely meet any particular standard, but, yeah... they should be able to discriminate some difference in signal level (e.g., 3 dB, which corresponds to about a two-fold difference in power).

Yeah, basically eye candy.

mhardy6647

These meters use a little coil of wire within a fixed magnetic field, sort of like a moving coil phono cartridge, so it's easy to imagine that issues of "compliance" and moving mass impinge upon the movement of the meter when it 'detects' a voltage.



source: http://www.tpub.com/neets/book3/7b.htm


Here's a nice write-up on meters from the excellent NEETS resource:

http://www.rfcafe.com/references/electrical/NEETS-Modules/NEETS-Module-03-1-11-1-20.htm

oldrocker

As always, THANKS everyone!!!!!

FWIW, sounds ok to these old ears, just mostly a visual thing.

oldrocker

Thanks Mr. Hardy for the hifi link, got that one myself..

decal

K_M

We have that Exact same old receiver in our basement I think. Not working anymore, but years ago had a friend interested before it blew, he checked the meters and some other stuff due to the age of it, and commented that the meters, were not exactly accurate as to their readings, but they were accurate as far as the relative changes they showed on their markings or something like that.

Never sold it, someone shorted it out first and it smelled really bad for a while..