Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a conventional
power supply?

Comments welcome. ;-)

--
*A computer once beat me at chess, but it was no match for me at kickboxing.

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

In article , Dave Plowman (News)
wrote:
Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Some amp designs might 'thump' anyway as the bias points passed magic
values that caused various parts of the circuit to 'wake up'.

Comments welcome. ;-)

Personally, such thumps have never bothered me. The only change I made to
my Armstrong 700s was to eventually bypass their output relays. After a
decade or more they did as I'd predicted originally and started to give
unreliable connections. So I shorted across them.

Provided the thump doesn't bother the speakers or blow you across the room
I simply regard it as an audible 'switched on' indicator.

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

"Jim Lesurf" wrote in message
...
In article , Dave Plowman (News)
wrote:
Common way to stop the thump you can tend to get with a MOSFET amp
at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it
might
be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Some amp designs might 'thump' anyway as the bias points passed
magic
values that caused various parts of the circuit to 'wake up'.

Comments welcome. ;-)

Personally, such thumps have never bothered me. The only change I
made to
my Armstrong 700s was to eventually bypass their output relays.
After a
decade or more they did as I'd predicted originally and started to
give
unreliable connections. So I shorted across them.

Provided the thump doesn't bother the speakers or blow you across
the room
I simply regard it as an audible 'switched on' indicator.


Surely the risk is d.c. offset as almost all modern amps and certainly
a MOSFET amp would have split supplies.

I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!


--
Woody

harrogate3 at ntlworld dot com

On 22/12/2015 6:06 AM, Woody wrote:
"Jim Lesurf" wrote in message
...
In article , Dave Plowman (News)
wrote:
Common way to stop the thump you can tend to get with a MOSFET amp
at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it
might
be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Some amp designs might 'thump' anyway as the bias points passed
magic
values that caused various parts of the circuit to 'wake up'.

Comments welcome. ;-)

Personally, such thumps have never bothered me. The only change I
made to
my Armstrong 700s was to eventually bypass their output relays.
After a
decade or more they did as I'd predicted originally and started to
give
unreliable connections. So I shorted across them.

Provided the thump doesn't bother the speakers or blow you across
the room
I simply regard it as an audible 'switched on' indicator.


Surely the risk is d.c. offset as almost all modern amps and certainly
a MOSFET amp would have split supplies.

I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.

--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?
Or any amp with a regulated PSU.

--
Eiron.

On Tue, 22 Dec 2015 08:25:21 +0000, Eiron
wrote:

On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?
Or any amp with a regulated PSU.

Finite output impedance sees to that, strictly. But on a practical
level, if your amp will produce double the power into 4 ohms, you've
over-designed it for 8 ohms.

d

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

On 22/12/2015 7:25 PM, Eiron wrote:
On 22/12/2015 02:24, Trevor Wilson wrote:
On 22/12/2015 6:06 AM, Woody wrote:


I built one based on the Ambit PCB but then built the JLH regulated
supply which has relay protection for the L/S. One channel on that
decided to sit at about 120mV and there was no way I could get it
down. The supply rails were almost identical so I can only assume that
something has slipped in the amp design.

Without a doubt though the best power amp I have ever heard. Does 112W
into 8R and 224W into 4R - and that takes some doing!



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


Not even an ME1500?

**Not even the mighty ME1500, with it's 5kVA power transformer and
310,000uF of filter capacitance.

Or any amp with a regulated PSU.


**Nope. Output device saturation is the problem.


--
Trevor Wilson
www.rageaudio.com.au

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

Trevor Wilson wrote:



**It's also impossible. No amplifier can exactly double it's power into
lower impedance loads. Your measurements were wrong.


** High power, class D types with a PFC switching PSU get very close.

Regulated DC rails and super low internal impedance is the key.


..... Phil

Dave Plowman (News) wrote:

Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it might
be better to ramp up the AC into the amp - even if it uses a conventional
power supply?

Comments welcome. ;-)



** Ramping up the AC with a Variac will show you if the idea works or not - usually its no help or even makes the thump worse.

What you normally see is the output swings DC from the get go and then at some point it suddenly jumps back to zero.

The problem is not restricted to MOSFET amps, most amps thump at switch on unless there is a speaker muting relay.

My own BJT amp has no thump if switched on after a long break, but a rather loud one if cycled back on after 10 or 20 seconds - cos one rail drops faster.



.... Phil

In article ,
Phil Allison wrote:
Dave Plowman (News) wrote:

Common way to stop the thump you can tend to get with a MOSFET amp at
switch on was a relay on the speaker output.

It occurred to me that in these days of cheap high power triacs it
might be better to ramp up the AC into the amp - even if it uses a
conventional power supply?

Comments welcome. ;-)



** Ramping up the AC with a Variac will show you if the idea works or
not - usually its no help or even makes the thump worse.

Sadly, not something I possess.

What you normally see is the output swings DC from the get go and then
at some point it suddenly jumps back to zero.

If I feed it from the bench power supply and start that at zero volts no
thump at any point as you turn it up to the working voltage.

The problem is not restricted to MOSFET amps, most amps thump at switch
on unless there is a speaker muting relay.

This one is pretty loud. ;-)

My own BJT amp has no thump if switched on after a long break, but a
rather loud one if cycled back on after 10 or 20 seconds - cos one rail
drops faster.



... Phil


--
*It's o.k. to laugh during sexŒ.Œ.just don't point!

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