"Motorboating" Pre Amp

Hello,

Let me first introduce myself. Im Ryan, I have a very basic understanding of electronics but im looking at putting a little project together. The project is a small system to power 4 speakers which are around the house.

The idea is to have 2 small amplifier board powering two speakers each, with a Bluetooth audio receiver linked by a pre-amp.

Now the problem, I purchased a £6 pre amp kit for maplain's (to bot get some practice and as a kind of "proof of concept") all built and working fine apart from a low 1 second "thud" coming from the bass. The thud is considerably louder then the volume of the speakers.

The pre amp is being taken from the line out of a PC to a standard Hi-FI amplifiter and a pair of bookshelf speakers (Again this is just to test and not the final design)

The amp is powered by a 12v 750mA AC-DC converter (Just a spare)

Any ideas why this is occurring?

Apologies if this is in the wrong place.

 

Can't say much without a schematic...normally this kind of problem happens when their is problem in the feedback circuit of the pre amp or the power amp

So you can't control it with volume control ,so it may be the feedback circuit of the power amp.try disconnecting the pre amp section and check if still its motorboating...if yes the problem is in power amp

Their may be other problems...

 

Thanks for the advise,

The powe ramp works fine without the pre-amp. The thud does change volume with the volume adjustment on the pre-amp however is louder then the base noise

 

Schem: http://www.maplin.co.uk/media/pdfs/N47FL.pdf

 

Variations of current in the power amp causes the supply voltage to jump up and down. The very simple biasing of the preamp amplifies the jumping voltage causing motorboating PUT, PUT, PUT sounds.

Filter the biasing of the preamp better, filter the supply voltage better or regulate the supply voltage to the preamp.

 

Thud only at turn on or turn off?

Or all the time noise?

 

How would the "jumping voltage" from the power amp be getting to the preamp? He said he is powering the preamp with a separate 12v 750mA AC-DC converter. The power amp is a "standard Hi-Fi" unit, presumably with its own connection to the AC Mains.

RyanW, are you certain that you haven't accidentally swapped one or more of the capacitors, installing them in the wrong positions?

If not, could you try connecting something on the order of 1000 uF in parallel with (or in place of) the 100 uF C7?

Why is C4 100 nF polyester while C5 is 100 nF ceramic? Seems very odd. You could try paralleling (or replacing) those with larger capacitances, especially if making C7 larger doesn't eliminate the problem.

 

The title of this thread says MOTORBOATING but Ryan says only one singleTHUD for a duration of only one second, not continuous motorboating sounds.
Ryan does not say when the THUD sound occurs.

Ryan says an AMP is powered from a 12V adapter but did not say if the AMP is this preamp, his power amplifier or both.

The preamp has a very high fixed gain of 100 and the volume control is at its output, not at its input. The input to the preamp is probably overloaded by line level from a PC but the preamp is probably designed to use a dynamic microphone for its input.

 

thanks for all the help so far

 

This formatting might make it more clear:

Continusly at 1 second intervals, the volume of the "thud" is relational to the volume set on the preamp"

The pre-amp is powered by the 12v adapter, the amp is a standard cambridge audio A1 amp.

I have also tried using audio output (headphone jack) from a mobile phone and issue persists.

i will try what was suggested above and see if the issue is resolved.

Im also trying to get a battery pack to power it to rule out the 12v adapter

 

The stereo preamp is extremely simple. It uses an old stereo opamp that I have never seen during my entire audio career. The opamp has diodes between its inputs to prevent the input transistors from a differential voltage that is too high LIKE A LINE LEVEL input.
The preamp probably works fine if its input is a low level dynamic microphone.
EDIT: It was designed to be the preamp for a dynamic record player cartridge (remember them??).
The "thuds" might occur each time the input is overloaded.


If the repeating "thuds" are in both channels then the noise might coming from the power supply.

 

Motor boating can also be caused by ground loops.

 

ESPECIALLY when the gain of the preamp is way too high at 100.

 

I might try with a much lower input source like a cd / tape player with volume set to low.

 

Hi Again,

Let me clarify my set up an issue to avoid any further confusion.

Set up.
Source (PC, phone, MP3) - Pre Amp (Where issue lies) - HiFi Amplifier - Speakers

The issue:

The pre-amp causes the speakers to make a repetitive and constant "thud" coming from the sub on the speaker. This issue also exists (I found out) when no source is selected. So I assume the "thud" is coming exclusivly from the pre amp.

So, what in the pre amp could cause this thud?

 

By "no source is selected" i mean, the end of the 3.5mm is not plugged into anything.

 

The preamp has way too much voltage gain that is not reduced with its volume control at its output. Also it uses an old unusual IC that has diodes between its inputs.

Why are you using a preamp anyway when your signal sources can drive the power amp without a preamp?
Do you use shielded audio cables?

1) Reduce the voltage gain a lot.
2) Replace the preamp IC with a more modern one.

 

I plan on using this, along with a Bluetooth audio receiver and two small power amps to power some speakers placed around the house. Speaking to a few people i know who have the bluetooth - amp set up, they say the volume isnt fantastic, the idea is to use the pre amp to both improve quality and make it a bit more audible.

 

Most audio power amps have plenty of gain so the volume control can be set to as loud as the amp can go or less.

The bluetooth audio receiver should have a gain of 1 like a piece of wire.
Then the preamp is not needed.

Oh. Are you trying to get 10W from a 1W amplifier? A preamp won't do it, instead it will cause the power amplifier to produce more clipping distortion.