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AC detected in what is supposed to be DC

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texaslonghorn

New Member
Hi. I am sure this has been covered, but I couldn't find a thread that 'splained it to me in terms I can understand (IQ=shoe size). I have a solid state audio amp that has a noticeable buzz/hum in the output. After several tests I have eliminated any ground loop. Sounds like 60hz but I put it on the scope and it appears to be a very dirty signal a little closer to 70hz (???!!!). Today, I placed a DVM across the DC output legs directly after the filter caps (pretty much standard set up: AC in to transformers, then to bridge rectifiers, then to filter caps out to the signal boards). The DC readings are fine, but I am still getting about 0.1 VAC. Does that sound acceptable? I thought that there would be far less - even nondetectable - AC at that point. The bridges are brand new as are the caps. This is my first time trying this and am in learning mode. Any and all thoughts are appreciated. Thanks.
 

Papabravo

Well-Known Member
I understand that there is 100 mV RMS ripple on the DC output. If the supply is a 12V supply then that is quite ordinary. Was your undertanding that the filter capacitors would remove the ripple down to undetectable levels? Sorry, but it just doesn't work that way.

Check out the section on "Smoothing"
https://www.kpsec.freeuk.com/powersup.htm#smoothing
 
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MikeMl

Well-Known Member
Most Helpful Member
If you are in a 60Hz country, and are using a full-wave bridge rectifier, the ripple will be at 120Hz.
 

mbarazeen

Member
i dont beleive that your problem is from ripple... do you get the same when no input? since at no input it will take only less current no chance for you to have ripple.

check the biasing of your amp circuit. you can check in several steps to ensure it has not go off.

if you have doubt on supply check it using a battery.
 

texaslonghorn

New Member
Thanks. A few clarifications: Yes, I am in the US, so we are talking about 60hz. Second, the DC power output is supposed to be 55V each leg. I took it to a tech to have the bias set, but I haven't check that myself yet. I kinda think mbarazeen is on the right track here for my ultimate problem. I did a bit more checking last night and found a couple bad diodes I replaced, so that may help.

Please keep the ideas coming. Like I said, I am still learning an appreciate any comments. Thanks
 

Papabravo

Well-Known Member
I don't understand why you are talking about "each leg". A schematic sure would be helpful. I don't understand why we have to keep asking for one when people come to the forum with questions.
 
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mbarazeen

Member
may be you are meaning as each leg a dual supply, do you get +55 & -55 when measured respect to ground? or you are getting the voltage between a transistor instead??

also another way to proach is to divide your board into two, disconnect propper coupling capacitor from preamp stage to power stage, check the audio at that point that is from preamp stage, if the noice is there or not, also repeat it with final out put too.

make sure you dont disturb the biasing when you remove any component for checking
 
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Speakerguy

Active Member
Hey TexasLonghorn,

The AC ripple on your power supply lines sounds fine. It is to be expected and the PSRR of the amplifier should kill it out - hardly any high power amp runs off a regulated linear supply (most all are just rectified & filtered). Under load the ripple will be in the several volts range fyi. I suspect the 70Hz noise peak on the scope to be some sort of artifact. If you're feeding 60Hz in with no input, then you should only see 60Hz or its harmonics (120Hz specifically) unless something very weird is going on.

It would be very helpful if you could post a schematic.
 
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texaslonghorn

New Member
Okay - here are the relevant schematics. Thanks.
 

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  • main board schem.pdf
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Speakerguy

Active Member
Oh ****, you're working on a Bryston. Nice amp! I'll keep looking at the schematic, just suprised.
 
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texaslonghorn

New Member
Once it gets up and running I hope it will be nice. This one is beyond warranty (ie. >20 years old) and needed work. I have replaced all the electrolytics with Blackgates (except the filter caps, which are UCC supplied direclty by Bryston), updated all the power transistors to the type Bryston now uses, had several parts silver plated, checked all solder points and reset where needed, replaced most of hte signal diodes and low power transistors, replaced a few ceramic caps in the signal path with high end films and silver micas, replaced the signal wiring with pure silver solid core wire/teflon insulation, added Dynamat for chassis vibration damping, and cleaned all PCB contact points. Now if it will simply work.....
 

audioguru

Well-Known Member
Most Helpful Member
It was a nice amplifier 20 years ago.
But it is old. Bury it and replace it with a new amplifier.
 

texaslonghorn

New Member
I am going to get it working, sell it and get an Aleph. Besides, when I am done with the Bryston, not much other than the transformers will be original.
 

Papabravo

Well-Known Member
Okay - here are the relevant schematics. Thanks.
So how is the AC line cord connected to the transformers? Is there a hidden connection between the narrow little boxes at the bottom of the backboard?
 

Speakerguy

Active Member
It was a nice amplifier 20 years ago.
But it is old. Bury it and replace it with a new amplifier.
Has class A/B amplifier technology changed in 20 years? :)

I have an Adcom GFA-555 mkII. It too was a nice amplifier 15 years ago. Still is :)
 
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audioguru

Well-Known Member
Most Helpful Member
You have fixed many problems with the old Bryston amplifier but there are probably many more that you didn't think about.
Newer designs might be better.
 

texaslonghorn

New Member
Problem found - my tech had reversed the hot and neutral on the incoming AC line connections to the backboard. Now no noise, no clipping indication, no blown fuses. After a burn in on my bench for the day, I will hook it up to my system and listen.
 
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