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Apt Amplifier Model 1 amplifier question - buzzing

ViperZ

ViperZ

Well-Known Member
Hi all,

I rebuilt Apt Power Amplifier Model 1 amplifier, but have this weird condition that I don't understand.

When no input is connected, there is loud buzzing (transformer-like) through only the right channel (it's much quieter in the left channel). As soon as I connect RCA input from my preamp to that channel (even with no signal), the buzz is gone completely.

Service manual has this note "The input jack chassis ground is the ONLY chassis ground in the amplifier". Well, OK, but still doesn't explain why only the right channel buzzes.
DC offset (not adjustable, measured at the output terminals) is 10mV per channel. Each channel has bias adjustment trimpots, but these have no effect on buzzing.

So is it the ground missing from the right channel?

Thank you.
 
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Definitely not RCA cables - connecting preamp actually 'fixes' the issue.

I traced ground connections from the power supply to both amp boards - on signal side, and on power side - both are well connected, I even resoldered them. If I lift the power board out of the amp (hence disconnect it from chassis), both channels buzz identically. But if I put it back (hence connect the chassis to the ground traces), only right channel buzzes. Not sure if it's the ground loop, or the way ground wires are ran through the amp to the board... Asking for wisdom...
 
So this problem didn't exist before you did the work on the unit?

What about screw connections - ie: those to the chassis thru the PCBs? If the RCA is the only ground connection to the chassis, then could there be another ground connection you have inadvertently made to the chassis?
 
It didn't work at all when I started working on it. Leaked cap in left channel, broken protection relay, etc

When I disconnect that screw connection to chassis, both channels buzz identically. So wouldn't that mean that there is no other connection to ground from that channel?
 
Hey Pete..
Well is deep. You know I try to help out with USA or AU Uk mates. etc..

Okay.. No sm..
ViperZ said:
When no input is connected, there is loud buzzing (transformer-like) through only the right channel (it's much quieter in the left channel). As soon as I connect RCA input from my preamp to that channel (even with no signal), the buzz is gone completely.
Click to expand...

To me? the interpretation means the power amp and pre must be connected... thus via the ground chassis. Same could be (could be) if connecting another pre' thus a ground wire between chassis needed. Perhaps matched set for reference in some user manual?
-----------
Just to note.. do not unscrew a pcb grounded screw to test!! I wonder if something else blew.. and not mention of fuses okay etc Might check those for over rated.

I appreciate OP might be beginner, too. :D
bink
(perhaps)
 
does the unit have its own ground connection to mains supply ? like 3 wires ? if not try reversing the power plug .
 
temporary connection between input rca.s ground shell might tell a story . or connecting one or the other input rca to each channel in turn . or just try shorting rca inputs to avoid other things .
 
Ok, let me turn on my apt 1, which is is great condition, without the cables and see what it does.....

(Buzzes like a busted 1940's tube radio)

You're fine. Its normal.
 
Gazdatronik said:
Ok, let me turn on my apt 1, which is is great condition, without the cables and see what it does.....

(Buzzes like a busted 1940's tube radio)

You're fine. Its normal.
Click to expand...

Very interesting. Thank you. Does one channel buzz louder than the other by any chance? It really bothers me that I have two supposedly channels, connected to the same ground reference point. Checked and resoldered all ground connections, but one channel buzzes way louder than the other.

petehall347 said:
does the unit have its own ground connection to mains supply ? like 3 wires ? if not try reversing the power plug .
Click to expand...

The unit has a polarized 2-prong power cord, which I did not reverse - looks like factory connections there.

Binkman said:
Hey Pete..
Well is deep. You know I try to help out with USA or AU Uk mates. etc..

Okay.. No sm..


To me? the interpretation means the power amp and pre must be connected... thus via the ground chassis. Same could be (could be) if connecting another pre' thus a ground wire between chassis needed. Perhaps matched set for reference in some user manual?
-----------
Just to note.. do not unscrew a pcb grounded screw to test!! I wonder if something else blew.. and not mention of fuses okay etc Might check those for over rated.

I appreciate OP might be beginner, too. :D
bink
(perhaps)
Click to expand...

Holman wrote a book on this amp instead of a service manual. I read the whole thing (honestly), and nowhere does it say that the preamp must be connected at all times. I have worked and owned many amps, and never have I owned an amp that requires INPUT to be connected to it.

And no, nothing blew. If all my amps would blow up upon disconnecting the input or even upon disconnecting the chassis from the amps, I would probably have no amps by now.
 
petehall347 said:
temporary connection between input rca.s ground shell might tell a story . or connecting one or the other input rca to each channel in turn . or just try shorting rca inputs to avoid other things .
Click to expand...

Where would you suggest connecting the RCA ground shell to? It's soldered onto the PCB. I am reading close to zero on pretty much every ground path that I can reach without disassembling each channel.

This amp has a weird ground connection scheme. There are at least two ground paths from power/protection PCB to each amp board. One path is from RCA connector shell. The other path is through large filter caps to each board. One other is through shielded signal cables (shields - I don't see if the shield is connected on each board, it's at the bottom of the boards).

But like I said, my main concern is that channel buzzes louder than the other, which doesn't seem uncommon for this particular amp model.
 
ViperZ said:
Where would you suggest connecting the RCA ground shell to? It's soldered onto the PCB. I am reading close to zero on pretty much every ground path that I can reach without disassembling each channel.

This amp has a weird ground connection scheme. There are at least two ground paths from power/protection PCB to each amp board. One path is from RCA connector shell. The other path is through large filter caps to each board. One other is through shielded signal cables (shields - I don't see if the shield is connected on each board, it's at the bottom of the boards).

But like I said, my main concern is that channel buzzes louder than the other, which doesn't seem uncommon for this particular amp model.
Click to expand...
The amp sensitivity is fairly high, it picks up a lot of background hash if the power isnt clean and the cables are junk, this I know firsthand. Other than that, your dc offset looks good.

As for the one channel louder buzzing, this amp I have read has one peculiarity(and i will paraphrase and maybe get some things wrong) in that the position of certain wires that run alongside the amp board need to be deliberatley placed. The techs that built them originally used bic pens with notches cut in the end, to use as tools to place the wires to minimise noise.

Perhaps one channel has one of these wires closer to where they should not be?

That is a probable explanation but no reason to get bent out of shape about it. If the amp gets quiet when connected and one channel is not obviously louder, Id say let it alone. Or sell it to me. I could use another.
 
The internal wiring is definitely something to note. A lot of wires come out of the left side of the power board, so the wires that go to the left channel are much shorter, and the wires that go to the right channel run right along the power transformer. When everything is buttoned down, it's very difficult to even move those wires. I actually did try to rearrange them with long wooden q-tips. Worth trying again I guess, as I had the whole amp taken apart at one point.
 
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