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Adcom GFA-5002 increase ClassA bias on purpose
Adcom GFA-5002 increase ClassA bias on purpose
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Old 12th September 2019, 02:54 AM   #11
Phloodpants is offline Phloodpants
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Join Date: Feb 2001
Location: Madison, WI
Sorry, I'm wrong about that protection circuit!

The JRC4558 op-amps are definitely for the auto-on circuit. There is no input DC protection, just output protection provided by a UPC1237 chip.

First, the input is unity-gain buffered by one op-amp, then another op-amp boosts that signal by a gain of 122x, which is rectified to charge a capacitor that switches on the 555 timer, which has a time constant around 100 seconds. The remote trigger feeds into this same circuit.
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Old 12th September 2019, 07:37 AM   #12
sckramer is offline sckramer  United States
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Join Date: Nov 2010
Attached the schematic and service manual for reference.
Digging how it sounds, especially for late night listening-- very microdynamic with no hardness, and detail at low volume. Really temped to look around for a 2nd to monoblock them.
Cool, re: the 555 timer... saw that chip and figured it must be the timeout after there's no signal- for standby. (or something like that)

Again, I plan on light use only and keep it turned off, or enable the auto-standby mode. It's very hot while on-- Settled on about 50~55degC (direct on the heatsinks) pulling 80Watt from the wall idle.
Attached Files
File Type: pdf hfe_adcom_gfa-5002_schematic.pdf (155.2 KB, 12 views)
File Type: pdf hfe_adcom_gfa-5002_service.pdf (43.7 KB, 3 views)

Last edited by sckramer; 12th September 2019 at 08:05 AM.
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