Buzzing Pre-amp

[stevebell]

Hi,
I hope someone here can help with this as its driving me nuts!

About a week ago my system started buzzing like crazy and I had to pull the plug fast. I turned it back on and it worked fine for a few more minutes then started buzzing again 'very loud' even though the volume was turned right down. I did a few basic checks and found that the earth wire had detached itself from the board in the preamp (Heybrook C3) power supply. I re-soldered this but the buzzing is now at a continuous level, whatever the volume setting. Music can still be heard behind this buzzing and the volume controls still regulate the volume of this.

I stripped out the preamp board/power supply (removing the phono stage, volume control board and input selector board) and checked the smoothing cap and bridge rectifier and looked around for dry joints but can't find any obvious fault; although the board under the wire-wound resistors is discoloured.

With no other components attached except for a pair of cheap powered computer speakers, the buzz is loud and continuous, even with the speaker volume turned down. If you pull the plug on the preamp power supply as soon as the leads are plugged into the speakers they buzz. The buzz gets no louder with the power turned on and fitting/removing the earth lead makes no difference. A multimeter on the outputs shows no AV or DC on the output (that I can measure).

The preamp power supply consists of an externally housed toroidal transformer, then in the amp a bridge rectifier, 4700uF cap, wire-wound resistor and a circuit using a CA3140E and TIP31A for regulation of the +38.8V DC supply.

The pre stage is a sprinkling of ZTX214 and ZTX384C transistors, resistors, diodes and several caps.

Any clues as to where to check next as the tracks are very small and I don't want to wade in any further without more direction

Cheers,
Steve

[Jan Dupont]

Does it buzz in area of two times the Main Hz (100-120 Hz) or is the frequency higher ?

[stevebell]

Hi,
Thanks for your reply.

100Hz or thereabouts sounds about right. What's confusing me is that with the pre disconnected from the mains the hum appears immediately you connect the powered speakers (either channel); even with the volume on the speakers turned down. Turning on the pre and cranking the volume up makes no difference to the level of hum/buzz confused:

Cheers,
Steve

[Jan Dupont]

It seems like to either have a ground loop or a missing ground connection/pcb track......

I do though have some problems understanding your gear setup....
Are your speakers active (build in power amps) ???

[stevebell]

Hi,
I guess I'll have to take a another look. The thing is that the main board is only around 5x2" wide and all the tracks and wiring connections on it seem sound.
I've missed something obvious

As for the setup; to avoid damage to my mains system I've connected the uncased pre-amp main board to a set of cheap powered speakers via flying leads soldered to the board.

Cheers,
Steve

[jnb]

Originally posted by stevebell
I turned it back on and it worked fine for a few more minutes then started buzzing again 'very loud' even though the volume was turned right down. I did a few basic checks and found that the earth wire had detached itself from the board in the preamp (Heybrook C3) power supply.

This first sounded like an oscillation. This can start after powering the amp for a short time, perhaps as it is waiting for just the right stimulus for the oscliiation to start.

Then I read about the ground connection. Was it due to the ground connection touching intermittently or is it a coincidence? Have you checked with this ground disconnected again? How does it go when you connect the ground and short the input?

Music can still be heard behind this buzzing and the volume controls still regulate the volume of this.

Sounds like the problem is before the volume control (unless coming back through a feedback loop).

With no other components attached except for a pair of cheap powered computer speakers, the buzz is loud and continuous,

Do you not have the input shorted in this case with the input board detached?

If you pull the plug on the preamp power supply as soon as the leads are plugged into the speakers they buzz.

How are those speakers?

A multimeter on the outputs shows no AV or DC on the output (that I can measure).

How about on the supply rails?

[stevebell]

Originally posted by jnb
Have you checked with this ground disconnected again? How does it go when you connect the ground and short the input?

There is no difference with the ground connected or not, but I will try shorting the inputs later today.

This first sounded like an oscillation. This can start after powering the amp for a short time, perhaps as it is waiting for just the right stimulus for the oscliiation to start.

Which components could cause this on both channels (please don't say any!).

How are those speakers?

Working fine on other equipment, and my main system (power amp/speakers) is fine with another preamp.

How about on the supply rails?

I only have a multimeter but I can't measure any AC on the rails and the DC voltage seems stable. I've tried with the preamp disconnected from the mains and with the external toroidal transformer disconnected and it still buzzes. Tonight I'm going to try and isolate the preamp section completely from the power supply to find out which section is the culprit?

On the issue of grounds, this is what I found. The earth wire from the mains supply passes through the external power supply box (this is where the break was) then into the preamp case. Here it is connected to the casework (currently not in play) and a centre post on the preamp board. The centre post is on the 0V track from the power supply and connects to all of the input and output grounds and the phono stage (currently removed) earthing post. All connections are sound.

Cheers,
Steve

[jnb]

Originally posted by stevebell
Which components could cause this on both channels (please don't say any!).

I find that the input to the active components is the usual place for oscillations to start but it could be other things such as loose or incorrect feedback compensation, or unfortunately routed cables. Since, however, you say it happens to both channels at once, these are unlikely in your case.

I've tried with the preamp disconnected from the mains and with the external toroidal transformer disconnected and it still buzzes.

Do you mean that the speakers buzz when connected to the preamp when the preamp is turned off and out of the wall? And earlier you said they don't buzz when not connected to the pre? That sounds like the preamp is acting like an antenna or magnetic reciever of some sort, am I hearing you right because if not, the ground loop, as ACD said, or a lifted reference is a possibility as you have removed some boards.

The earth wire from the mains supply passes through the external power supply box (this is where the break was) then into the preamp case.

Since the break didn't change anything, it seems lifting the ground on the speakers wouldn't help.

[stevebell]

Originally posted by jnb
Do you mean that the speakers buzz when connected to the preamp when the preamp is turned off and out of the wall? And earlier you said they don't buzz when not connected to the pre?

Exactly, yes. Even with the volume controls on the powered speakers set to zero the buzz is present, and loud, as soon as either phono lead is attached. Increasing the volume makes no difference (negative or positive) to the level of the buzzing.

In fact when the problem first occured in my main system my initial reaction was to turn the volume controls to zero, then to hit the power switch on the pre. It wasn't until I hit the off switch on the power amp that the buzzing stopped. It was after the second time that I found the broken earth in the P/S and when fixing this didn't help I started stripping down the amp, removing anything that isn't necassary to isolate the problem.

I'll see if I can find any more clues tonight

Cheers,
Steve

[jnb]

It doesn't seem likely that it is a signal from the pre and where the volume on the active speaker is dodgy, but shorting the outputs to ground with clip leads just inside the pre case should eliminate the pre as the source of the noise, unless it is a ground issue. Maybe it could be the speaker amp, even if it only has this reaction with this pre.