Realistic STA 2080 receiver question
Mother Mooch
Posts: 129
Ok, guys, here i am again 
Thanks To Cnh (a very good member here) has set me up with a nice Yamaha AVR to listen to my Monitor 12's. Thanks Cnh
A buddy of mine brought over and old Realistic STA 2080 High current receiver to test out the Polks, But it's kinds strange though, When hooked up even at low volume, the speaker excursion is outrageous, They sound ok, and crank pretty good, but the speakers bounce the whole way in and out with the music? Now i hooked, the Yammy back up and no problem, Hook the Realistic back up and it's still pumping those babies out. Now at higher volume, it will make the cones distort, but even at low volume, these things are jumping!
Any ideas at all? Bad amp? I checked polarity and it's fine, and have the play station (which i use for a cd player hooked into the aux. It's almost like what they would do with turntable rumble but with the beat?
Steve
Thanks To Cnh (a very good member here) has set me up with a nice Yamaha AVR to listen to my Monitor 12's. Thanks Cnh A buddy of mine brought over and old Realistic STA 2080 High current receiver to test out the Polks, But it's kinds strange though, When hooked up even at low volume, the speaker excursion is outrageous, They sound ok, and crank pretty good, but the speakers bounce the whole way in and out with the music? Now i hooked, the Yammy back up and no problem, Hook the Realistic back up and it's still pumping those babies out. Now at higher volume, it will make the cones distort, but even at low volume, these things are jumping!
Any ideas at all? Bad amp? I checked polarity and it's fine, and have the play station (which i use for a cd player hooked into the aux. It's almost like what they would do with turntable rumble but with the beat?
Steve
Post edited by Mother Mooch on
Comments
-
How would that Rat Shack receiver sound at the bottom of a dumpster?The world is full of answers, some are right and some are wrong. - Neil Young
-
Check for DC "offset" at the speaker terminals with a DMM of the offending amplifier/receiver. If you see any significant DC voltage there (steady or fluctuating)... you've got trouble (likely the output devices or the drivers). DC output from a "direct coupled" solid state amplifier can easily destroy a loudspeaker driver (i.e., burn the woofer's voice coil open).
on the other hand...
What source are you using? If, perchance, it's a turntable, the LF excursion you see could be rumble or acoustic feedback, and may indicate very flat and extended LF response of the Rat Shack (and, perhaps, the presence of a fixed LF rolloff on the other
component). EDIT -- oops. Playstation/AUX. Nevermind.
EDIT^2: Info on the STA-2080 on pg. 12 of http://www.radioshackcatalogs.com/catalogs/1981/
(in case you're interested). Was this one of the R/S receivers with MOSFET outputs...? -
Voltage offset? I was an electrician at one time in my life, but it was all big stuff. this little stuff trows me off. What would i check and with what device? I have an omh meter and that's about it! and yes i think it's mosfet
Thanksmhardy6647 wrote: »Check for DC "offset" at the speaker terminals with a DMM of the offending amplifier/receiver. If you see any significant DC voltage there (steady or fluctuating)... you've got trouble (likely the output devices or the drivers). DC output from a "direct coupled" solid state amplifier can easily destroy a loudspeaker driver (i.e., burn the woofer's voice coil open).
on the other hand...
What source are you using? If, perchance, it's a turntable, the LF excursion you see could be rumble or acoustic feedback, and may indicate very flat and extended LF response of the Rat Shack (and, perhaps, the presence of a fixed LF rolloff on the other
component). EDIT -- oops. Playstation/AUX. Nevermind.
EDIT^2: Info on the STA-2080 on pg. 12 of http://www.radioshackcatalogs.com/catalogs/1981/
(in case you're interested). Was this one of the R/S receivers with MOSFET outputs...? -
Here's a roughly correct explanation :-) Most relatively modern solid state amplifiers have a circuit topology that is symmetrical, fully-complementary and direct coupled, often all the way from input to output. Ina power amplifier, this is done with two (relatively) high-voltage DC power supply "rails" (one positive and one negative) and pairs of complementary (calassically, NPN and PNP) output devices. If the amplifier is properly balanced, there will be zero volts DC at the output and the AC (music) signal will be passed with a low frequency limit that is at or very close to DC (zero Hz). Earlier solid state amplifiers used a single power supply and a capacitor to remove the DC from the outputs, which also rolls off the low frequency response of the amplifer.
If, for example, an output transistor in the push-pull pair shorts, it is possible for that DC "rail" to be present at the output... in a high power amplifier, this could be 50, 60, 70 V DC or even more. This would energize the voice coil of the woofer as an electromagnet. The voicecoil isn't made to handle continuous high current DC and it can burn open. This can happen at the "driver" stage before the outputs, too. If there are no capacitors in the signal path, the results can be similar.
I've toasted a couple of drivers thus, and therefore learned to always check for DC at the outputs before I ever risk a speaker on an unknown direct-coupled SS amplifier :-)
I had an Onkyo TX-2500 pass through the "shop" that had flutter similar to your description. Sho'nuff, a DMM showed DC "spikes" of several volts occuring randomly. I didn't bother to troubleshoot further (Solid state stuff is too hard for my little brain to figure out; tube circuits are much more straightforward).
Hope this helps.
EDIT: In practice, many amps have an "offset" adjustment to balance to zero DC voltage at their outputs. Here's a primer on that topic: http://www.audiokarma.org/forums/showthread.php?t=5634 -
Come to think of it, in fairness, cone flutter such as you describe can also be symptomatic of a form of amplifier instability (oscillation) known as "motorboating", which could be caused by bad 'passive' components (resistor, capacitor, inductor), a loose connection in the feedback loop, poor "lead dress" allowing wires in the circuit to act as an antenna; stuff like that.
EDIT: DMM is a digital multimeter. If you're old like me, you might have a VTVM (vacuum tube voltmeter) and/or a VOM (volt-ohm meter). Any of these is adequate for the job :-) A VTVM or DMM is a high input impedance device and won't load the circuit under test. A $10 "pocket tester" VOM has a 'sensitivity' of probably 1000 ohms per volt and will load the circuit somewhat (Ohm's law and all that), so the result won't be as accurate -- but that's not really an issue in a case like this.
Realistically (no pun intended) DC offset of plus or minus 100 mV or so won't be a hazard. If you see DC on the order of volts, though, something ain't right. -
Too me Sir, sounds like you know what you are talking about! do you do repairs?mhardy6647 wrote: »Come to think of it, in fairness, cone flutter such as you describe can also be symptomatic of a form of amplifier instability (oscillation) known as "motorboating", which could be caused by bad 'passive' components (resistor, capacitor, inductor), a loose connection in the feedback loop, poor "lead dress" allowing wires in the circuit to act as an antenna; stuff like that.
EDIT: DMM is a digital multimeter. If you're old like me, you might have a VTVM (vacuum tube voltmeter) and/or a VOM (volt-ohm meter). Any of these is adequate for the job :-) A VTVM or DMM is a high input impedance device and won't load the circuit under test. A $10 "pocket tester" VOM has a 'sensitivity' of probably 1000 ohms per volt and will load the circuit somewhat (Ohm's law and all that), so the result won't be as accurate -- but that's not really an issue in a case like this.
Realistically (no pun intended) DC offset of plus or minus 100 mV or so won't be a hazard. If you see DC on the order of volts, though, something ain't right. -
Not as such; just for fun.
I am no expert on anything solid state, either. -
How would that Rat Shack receiver sound at the bottom of a dumpster?
Hey, come on! Some of that 70's vintage Realistic gear was all right."Don't forget to change your politician. They are like diapers they need to be changed regularly, and for the same reason."
