Amplifier Distortion, DC-Offset, and You!
- Thread starter EchoWars
- Start date
Bill Ferris
Lunatic Member
BTW. Fuzzbuster, most service manuals indicate a period of warm up time(usually for SS, a 1/2 hour) before checking and adjusting circuits.
And it`s best to let a long unused, or unknown past electronic gear be powered up, after initial power up and check out for serious issues, to let it run for a minimum of several hours with the cover/s off and monitored for stability, output transistors over heating, & original caps becoming hot, or very warm, also, be on the lookout for unusual cooking electronic smells.
Probably best connected to a low watt light bulb configured DBT during this check out.
I you are aware of this electronic equipment checkout safe procedure, please feel free to ignore my suggestions !!
Kind regards, OKB
tubino
A guy with too many tubes
I just checked a BGW 750B I’ve had for a week. I checked with different DMMs and kept coming up with under 2mv per channel. Great!
But I have a BGW 750C that was working fine and then... stopped a week ago. It has a relay that is designed to protect the speakers, and it shuts the outputs when there is DC offset. So I suspect that is what happened, and I can try to measure what is at the relay (if I can figure out which spots), but what can I do if there is a lot of DC offset? This is a seriously built pro audio amp, yet all the looking I’ve done at the manual and schematic do not indicate any way to adjust it.
But I have a BGW 750C that was working fine and then... stopped a week ago. It has a relay that is designed to protect the speakers, and it shuts the outputs when there is DC offset. So I suspect that is what happened, and I can try to measure what is at the relay (if I can figure out which spots), but what can I do if there is a lot of DC offset? This is a seriously built pro audio amp, yet all the looking I’ve done at the manual and schematic do not indicate any way to adjust it.
iaRIVR
Active Member
I will buy that solution RJolly87 and leave well enough alone!
If I can get my amp to stabilize there or better, I'll be satisfied. Nature of the beast, apparently.
watson-greg
New Member
View attachment 1266513 
Hi,Im into big power (several KW) crowns and dyna416s bridged and have a couple dual trace scopes on line at all times to see the offset,waveshape,clipping etc 100% of the time as well as a Storage scope so I dont miss the real time sig with alarm limits set and do a THD test and RTA running fullyiome to watch for peaks from absorbant people hanging with me . yea the friends do load down some music ranges LOL. i come from the high power tube era so way too critical of Bias-yes imBIased LOL again.
and yea with all the DC coupled amps nowadays its got to be watched with caps drifting ESR and op amps going into Hf osc. caps on the series ckt input to driver transistors is not as bad as it seems just be large enouth UF to let the 15Hz through.
Oh wait I know someone who argues that some DC is really just good Bass signal at its lowest frequency of 0hz when hes drinking. clean bass is like a Heavenly thing. says me 5string Fender jazz bass. throuhg a Benheiger subharmonic synthesizor X2.
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watson-greg
New Member
View attachment 1266658 
.since there is no indoor building DB compression.I do real inductive load but on new arrivals after a few hrs on warmup and connected to medical grade line Isolation x former i put dummy load 1kw wirewound resistor and want 0-10mv maxx. alsn check for any ac or dc from each output to ground. some amps have to lift the earthed ground at input to clear hum at full tilt.
and use balanced inputs instead, most pro stuff has this option.
many Crown pro amps run matched pnp/npn outputs and self measure this and use a diff opamp config to offset any dc on the collector line. I made and designed semiconductors and the Vbe and bandgap of .7 v is way generalized for bias on volts. only A good curvetracer will do for real clean Hi Fi. like my old buddy 575.
also I run some Techron Gradient amps (10ks & M-600s)that run from DC to 100k so I have to be sure the inputs to them are 0,0000 Vdc. they were made to run/drive MRI coil magnets so theres no room for error there. will and have smoke a voice coil in 3 seconds and set the driver on fire. speakers dont like Any DC. doesent hurt to put a 10000uf cap in series for startups.
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watson-greg
New Member
pppurfect mv. less than 50mv in typ .
any one ever experment with magnets around their el34 output tubes to creat a field to offset elecrron flow in the new field- like a magnetron cavity cept at audio freq ?
An external field bias.
you Rf engineers know where im going.
this can be a new area of sales for you eseroteric guys
we used high power mags in particle accelerators and implnters to focus electrons and permantly implant electrons into semiconductors to change the crystel lattics and increase intrinsic doping. so it can effect solid state units tooooo.
im going to dig out some 6v6 units,scope,tube tester etc. stand by. who can hear the electron beam change???
any one ever experment with magnets around their el34 output tubes to creat a field to offset elecrron flow in the new field- like a magnetron cavity cept at audio freq ?
An external field bias.
you Rf engineers know where im going.
this can be a new area of sales for you eseroteric guys
we used high power mags in particle accelerators and implnters to focus electrons and permantly implant electrons into semiconductors to change the crystel lattics and increase intrinsic doping. so it can effect solid state units tooooo.
im going to dig out some 6v6 units,scope,tube tester etc. stand by. who can hear the electron beam change???
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Deserves its own physics thread, start one in the tube forum.
RJolly87
Well-Known Member
To add, I had a Sony STR-1800 that I checked after acquisition. At start, it showed some bias. At idle after running a little while, it showed 0 bias.
Yes, nothing, no millivolts both channels.
I couldn't believe it myself.
That probably explained why it sounded do awesome!
Yes, nothing, no millivolts both channels.
I couldn't believe it myself.
That probably explained why it sounded do awesome!
watson-greg
New Member
yes, bias is current to the output base drive to overcome .7 vbe turn on of the device/
not the emitters. they are the emitter resistors, transistor Beta, etc.
you got the general idea though-us engineers are just picky. LoL
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