I've searched the site for something similar to my problem, but there isn't anything that fully complies to my situation. So here it goes:
Looking for ideas on where to start. The amp was refurbished by me. Original schematic, no mods. Changed all electrolytics and transistors on the input board. Replaced feedback wiring (original wiring was weak/corroded at the input board connection). As is known and common knowledge, this amp model has a bit of very low level buzz (I wouldn't call it hum, it is more a kind of 60hz plus lots of harmonics), and this buzz can be heard with your ear pegged to the mid woofer and/or tweeter. But it is inaudible a few inches away. Quite normal.
Next step in refurbishing this amp, was to add speaker protection. I installed a Chinese completely assembled rig, which includes a 120V to 12V mini toroidal transformer. So it was very easy to get this thing going inside the amp's chassis. Got the 120V from the power distribution strip inside the amp, which feeds the amp's transformer after the power switch. So the speaker protection board is powered up/down by the main power switch as well.
The rest of the Speaker Protection board is wired this way: Input ground, directly to star ground point. Input positives, wired straight to each channel's output stage positive.
Speaker posts are wired this way: grounds go directly from the speaker posts to the star ground point. I saw no point in wiring the speaker grounds through the speaker protection board, as this device is common ground and does not switch the ground, only the positives. speaker post positives (+) go to the speaker protection device's positive outputs.
Input board is wired this way: main ground wired directly to star ground point. feedback grounds, directly to star ground point. Feedback positives, directly to each channel's output stage positive.
The amp works OK, but now the "buzz" is appreciably higher than before. It can now be heard (albeit faintly) a few feet (4-5 ft away) on a quiet morning through 90db sensitivity speakers.
The star ground is bolted down this way, from top to bottom:
Input board ground
Feedback ground
Speaker protection board input ground
Speaker negative posts
Chassis Ground
Before doing anything, I'd like to read your ideas on how to reduce or, possibly, eliminate this buzz.
Cheers!
Luis
Looking for ideas on where to start. The amp was refurbished by me. Original schematic, no mods. Changed all electrolytics and transistors on the input board. Replaced feedback wiring (original wiring was weak/corroded at the input board connection). As is known and common knowledge, this amp model has a bit of very low level buzz (I wouldn't call it hum, it is more a kind of 60hz plus lots of harmonics), and this buzz can be heard with your ear pegged to the mid woofer and/or tweeter. But it is inaudible a few inches away. Quite normal.
Next step in refurbishing this amp, was to add speaker protection. I installed a Chinese completely assembled rig, which includes a 120V to 12V mini toroidal transformer. So it was very easy to get this thing going inside the amp's chassis. Got the 120V from the power distribution strip inside the amp, which feeds the amp's transformer after the power switch. So the speaker protection board is powered up/down by the main power switch as well.
The rest of the Speaker Protection board is wired this way: Input ground, directly to star ground point. Input positives, wired straight to each channel's output stage positive.
Speaker posts are wired this way: grounds go directly from the speaker posts to the star ground point. I saw no point in wiring the speaker grounds through the speaker protection board, as this device is common ground and does not switch the ground, only the positives. speaker post positives (+) go to the speaker protection device's positive outputs.
Input board is wired this way: main ground wired directly to star ground point. feedback grounds, directly to star ground point. Feedback positives, directly to each channel's output stage positive.
The amp works OK, but now the "buzz" is appreciably higher than before. It can now be heard (albeit faintly) a few feet (4-5 ft away) on a quiet morning through 90db sensitivity speakers.
The star ground is bolted down this way, from top to bottom:
Input board ground
Feedback ground
Speaker protection board input ground
Speaker negative posts
Chassis Ground
Before doing anything, I'd like to read your ideas on how to reduce or, possibly, eliminate this buzz.
Cheers!
Luis
Can you post a picture - if the speaker protection includes a transformer onboard, where is it located (Idoubt it is a very well designed power supply on a protection board)? Close to the input and you might be picking up noise. You might find that grounding the protection directly to the speaker terminals does lessen the hum and possibly add a 1nF cap from negative post to chassis as another layer of RF rejection. Good Luck.
Good idea on moving the protection board ground from the star ground to the speaker terminals… or even better, further away, to the chassis ground… that ground is only used to measure DC offset, by the IC on the board. It should be totally inconsequential to use chassis ground.
the protection board does include a small toroidal transformer. And a couple of caps. the board is fixed directly to the amp’s chassis, but it is not grounding itself to the chassis (Already tested this).
will post some photos as soon as I can during the weekend.
cheers,
Luis
the protection board does include a small toroidal transformer. And a couple of caps. the board is fixed directly to the amp’s chassis, but it is not grounding itself to the chassis (Already tested this).
will post some photos as soon as I can during the weekend.
cheers,
Luis
I removed the DC protection board, rewired everything back to normal. Added an earth loop breaker using a bridge rectifier, a 33 ohm resistor and a 0.47uf poly cap. And all is fine now.
I think that's the way. :^) These are highly reliable and difficult to blow amplifiers if they are maintained properly, and as long as you change those pesky driver transistors to something with more Vce like the MJE15032/33... Re-cap it, and you don't even need to do the big filters... You're good to go.
That protection board would fail many years before the amp will.
That protection board would fail many years before the amp will.
My guess is the buzz is EMI from that switching power supply on the protection board. The 555 has its circuitry spread out over some pretty big loop areas, fields spreading out, so it prone to pick up noise.
Thanks Chris. Your blog has been a great inspiration for my hobby. Yes, I’ve changed all small signal transistors and the driver transistors, using modern onsemi parts, per your BOM/parts list. also changed the few electrolytics on the input board. heatsinks added to the VAS transistors. Also, new and slightly higher specc’ed onsemi bridge rectifiers.
I’m getting a nice <2mV DC offset at the speaker terminals with inputs shorted. Stable bias and a nice warm (guessing 40-45C) rear heatsink after hours of use. I think this 555 is ready for many years to come.
Next projects, a couple of GTP’s with DC leaking issues, and recapping a working room heater, I mean, GFA-5500…
Cheers,
Luis
