Pretty basic MOSFET power amp.

I'm used to it thumping at switch on. No speaker relay in this basic
design.

But one of them as taken to thumping at switch off. Not always, which
makes fault finding more tricky.

Most likely cause?

--
*Change is inevitable, except from a vending machine*

Dave Plowman London SW
To e-mail, change noise into sound.

Some kind of imbalance in the discharge of something?
I was wondering, if I take out a relay in the speaker feed of a mosfet amp
made by Denon, whether it will just thump or could it cause damage to the
amp or speakers?
Its nice not to have it but the number of times I need to clean the relay
contacts is getting stupid.

Brian

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"Dave Plowman (News)" wrote in message
...
Pretty basic MOSFET power amp.

I'm used to it thumping at switch on. No speaker relay in this basic
design.

But one of them as taken to thumping at switch off. Not always, which
makes fault finding more tricky.

Most likely cause?

--
*Change is inevitable, except from a vending machine*

Dave Plowman London SW
To e-mail, change noise into sound.

On 27/04/2016 11:06 PM, Dave Plowman (News) wrote:
Pretty basic MOSFET power amp.

I'm used to it thumping at switch on. No speaker relay in this basic
design.

But one of them as taken to thumping at switch off. Not always, which
makes fault finding more tricky.

Most likely cause?


**Cosmic rays? Who knows? Without a GREAT DEAL more information, no one
can help you. Brand and model numbers would be helpful at the very
least. A schematic would be better.

Do you REALLY expect useful advice with so little information?

--
Trevor Wilson
www.rageaudio.com.au

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Dave Plowman (Nutcase) wrote:

Pretty basic MOSFET power amp.

I'm used to it thumping at switch on. No speaker relay in this basic
design.


** You posted the same here in December last year.


But one of them as taken to thumping at switch off. Not always, which
makes fault finding more tricky.

Most likely cause?


** Old and bad electros are the cause of almost everything.


BTW; anyone familiar with the famous Crown DC300A ?

Both channels would emit a long squeal about 10 seconds after switch off - it sounded much like a party balloon being let down while squeezing the neck.

There was a note inserted in later service manuals that *no attempt* should be made to fix the problem, as any possible fix would render the amp unstable.




..... Phil

On 28/04/2016 02:44, Phil Allison wrote:

BTW; anyone familiar with the famous Crown DC300A ?

Both channels would emit a long squeal about 10 seconds after switch off - it sounded much like a party balloon being let down while squeezing the neck.

There was a note inserted in later service manuals that *no attempt* should be made to fix the problem, as any possible fix would render the amp unstable.


My preamplifier did that.
http://www.keith-snook.info/wireless...amplifiers.pdf

It wasn't enough to damage the tweeters so I didn't worry about it.

--
Eiron.

On Wed, 27 Apr 2016 18:44:59 -0700 (PDT), Phil Allison
wrote:

Dave Plowman (Nutcase) wrote:

Pretty basic MOSFET power amp.

I'm used to it thumping at switch on. No speaker relay in this basic
design.


** You posted the same here in December last year.


But one of them as taken to thumping at switch off. Not always, which
makes fault finding more tricky.

Most likely cause?


** Old and bad electros are the cause of almost everything.


BTW; anyone familiar with the famous Crown DC300A ?

Both channels would emit a long squeal about 10 seconds after switch off - it sounded much like a party balloon being let down while squeezing the neck.

There was a note inserted in later service manuals that *no attempt* should be made to fix the problem, as any possible fix would render the amp unstable.




.... Phil

So they had an instability they couldn't fix, but sales took it off
the engineers and put it in the market anyway? Why am I not
surprised?

I don't know the design, but assuming it is an op-amp (as most audio
amps are) it shouldn't be beyond the wit of man to get a dominant pole
into the loop.

d

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Don Pearce wrote:


BTW; anyone familiar with the famous Crown DC300A ?

Both channels would emit a long squeal about 10 seconds after switch off
- it sounded much like a party balloon being let down while squeezing
the neck.

There was a note inserted in later service manuals that *no attempt*
should be made to fix the problem, as any possible fix would render the
amp unstable.




So they had an instability they couldn't fix, but sales took it off
the engineers and put it in the market anyway? Why am I not
surprised?


** The DC300A is perfectly stable with any load and all frequencies.

When the AC supply is removed, an internal +10V boost rail above the main positive rail quickly dies removing current from the class A driver stage - which pretty much disables the output stage.

When the DC rails to the input op-amp ( a uA739 ) drop below a working level, the squealing sound arrives. Its not particularly loud and does not depends on load conditions. The frequency is a few kHz, falling in pitch until there is almost no charge left in the main filter caps.

I know about it since I was asked to fix the noise by a customer who owned a DC300A. The solution was to fit a speaker relay with switch on delay and fast off.

I only mentioned this as an example of how trying to fix a small problem can lead to creating a much worse one. I suspect trying to fix inbuilt turn on/off transients in power amps are mostly like this.

IOW - " leave good enough alone ".



.... Phil

On Fri, 29 Apr 2016 19:17:55 -0700 (PDT), Phil Allison
wrote:

Don Pearce wrote:


BTW; anyone familiar with the famous Crown DC300A ?

Both channels would emit a long squeal about 10 seconds after switch off
- it sounded much like a party balloon being let down while squeezing
the neck.

There was a note inserted in later service manuals that *no attempt*
should be made to fix the problem, as any possible fix would render the
amp unstable.




So they had an instability they couldn't fix, but sales took it off
the engineers and put it in the market anyway? Why am I not
surprised?


** The DC300A is perfectly stable with any load and all frequencies.

When the AC supply is removed, an internal +10V boost rail above the main positive rail quickly dies removing current from the class A driver stage - which pretty much disables the output stage.

When the DC rails to the input op-amp ( a uA739 ) drop below a working level, the squealing sound arrives. Its not particularly loud and does not depends on load conditions. The frequency is a few kHz, falling in pitch until there is almost no charge left in the main filter caps.

I know about it since I was asked to fix the noise by a customer who owned a DC300A. The solution was to fit a speaker relay with switch on delay and fast off.

I only mentioned this as an example of how trying to fix a small problem can lead to creating a much worse one. I suspect trying to fix inbuilt turn on/off transients in power amps are mostly like this.

IOW - " leave good enough alone ".



... Phil

I certainly agree that there is very little point for the end user to
try and fix this. But it is a design flaw and should have been fixed
by the original engineers.

Speaker relay? Yes, that will stop you hearing it, but it doesn't fix
the problem.

d

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In article , Don Pearce
wrote:
I certainly agree that there is very little point for the end user to
try and fix this. But it is a design flaw and should have been fixed by
the original engineers.

Speaker relay? Yes, that will stop you hearing it, but it doesn't fix
the problem.

FWIW When designing the 730/732 amps the development *preamp* design at
one point tended to give bursts of about 1MHz about ten secs after being
turned off. It was coming from ICs being used to regulate and stabilise the
rails.

I solved it by not using ICs in the PSU.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Don Pearce wrote:

Phil Allison


So they had an instability they couldn't fix, but sales took it off
the engineers and put it in the market anyway? Why am I not
surprised?


** The DC300A is perfectly stable with any load and all frequencies.

When the AC supply is removed, an internal +10V boost rail above the
main positive rail quickly dies removing current from the class A
driver stage - which pretty much disables the output stage.

When the DC rails to the input op-amp ( a uA739 ) drop below a working
level, the squealing sound arrives. Its not particularly loud and does
not depends on load conditions. The frequency is a few kHz, falling
in pitch until there is almost no charge left in the main filter caps.

I know about it since I was asked to fix the noise by a customer who
owned a DC300A. The solution was to fit a speaker relay with switch on
delay and fast off.

I only mentioned this as an example of how trying to fix a small problem
can lead to creating a much worse one. I suspect trying to fix inbuilt
turn on/off transients in power amps are mostly like this.

IOW - " leave good enough alone ".



I certainly agree that there is very little point for the end user to
try and fix this. But it is a design flaw and should have been fixed
by the original engineers.


** It was not a problem that needed fixing.

The DC300A was marketed as a laboratory & professional use amplifier - not a home audio one.


Speaker relay? Yes, that will stop you hearing it, but it doesn't fix
the problem.


** Speaker muting relays have been standard features in countless home and professional audio amplifiers for decades. Their main purpose is to stop transients at switch on/off from being heard.

The last Onkyo 7.2 channel receiver I serviced had 9 chunky relays, one for each power amp, one in the AC supply and one in the main AC secondary for standby.

They are very much the solution to the problem.



..... Phil