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Switch on thump

C

Chris

Jan 1, 1970
0
I have a guitar pre-amp and separate power amplifier .
When the power amp is already on for a while , should I then power up the
pre-amp , there is a loud ' thump ' .
Can I modify the switch on the pre-amp to get round this problem .
I'm in 230VAC UK .

Thanks
Chris
 
K

Kitchen Man

Jan 1, 1970
0
I have a guitar pre-amp and separate power amplifier .
When the power amp is already on for a while , should I then power up the
pre-amp , there is a loud ' thump ' .
Can I modify the switch on the pre-amp to get round this problem .
I'm in 230VAC UK .

Not stated but inferred is that when switching the two on together, or
after the pre-amp has been on only a short time, there is no audible
thump. This suggests to me that there is an abundance of energy built
up on the pre-amp output during use that is discharging through the
power-amp when you turn on the latter. It would appear that the
solution is to power down the pre-amp prior to using the power-amp,
and to power them up together when it is desired to use the power-amp.
I don't know how modifying the switch on either the pre- or power-amp
would alleviate the problem, unless it is paralleled with the
power-amp switch to ensure they always power-up together. In that
case, plugging them both into a switched power strip should do the
trick.
 
B

Bob Masta

Jan 1, 1970
0
I have a guitar pre-amp and separate power amplifier .
When the power amp is already on for a while , should I then power up the
pre-amp , there is a loud ' thump ' .
Can I modify the switch on the pre-amp to get round this problem .
I'm in 230VAC UK .

Thanks
Chris
The thump probably comes because the preamp outputs are
AC-coupled through capacitors to the power amp. (There
are probably also caps in the power amp input.) Thumps
are especially likely if the preamp runs off a single supply
so its internal output is at half the supply, and relies on
the cap to block this. When you first turn it on, that internal
output point rises quickly to half of the supply, and that
fast rise is essentially AC that passes right through the cap
to appear as a thump. To prevent the thump, you'd need
a special slow-turn-on circuit design.

If the preamp has balanced + and - supplies, this is
still a possible problem if the circuit doesn't come on
in a controlled fashion.

So no, there is no simple switching arrangement to
cure this. A complicated arrangement would be to
have a time-delay such that the preamp is allowed to
power up and charge its output cap into a resistor
to ground simulating the power amp, and then after
the voltage across the cap is reduced to a very low
value the switch connects the power amp.

Best regards,





Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator
 
B

Bob Eldred

Jan 1, 1970
0
Chris said:
I have a guitar pre-amp and separate power amplifier .
When the power amp is already on for a while , should I then power up the
pre-amp , there is a loud ' thump ' .
Can I modify the switch on the pre-amp to get round this problem .
I'm in 230VAC UK .

Thanks
Chris

Turn on the pre-amp first and let it settle for 10 seconds or so before
applying power to the power-amp. Do the reverse when turning off, turn the
power-amp off first then the pre-amp.

The problem is caused by large swings in voltage in the circuitry as power
comes up and amplification is established. Many amplifiers and
pre-amplifiers suffer from this defect. Circuit designers usually go to
great pains to insure their products don't do this by incorporating some
form of muting circuitry in the design. Apparently your pre-amp does not
have this.

One way to solve it is to use a time delay relay to connect the speakers to
the power-amp only after the power has been applied for some number of
seconds. Then any power up thumps will simply not get to the speakers no
matter where they originate. Some amplifiers may require a dummy load when
not connected to the speakers which can be accomplished with a double throw
relay.
Bob
 
J

John Fields

Jan 1, 1970
0
I have a guitar pre-amp and separate power amplifier .
When the power amp is already on for a while , should I then power up the
pre-amp , there is a loud ' thump ' .
Can I modify the switch on the pre-amp to get round this problem .
I'm in 230VAC UK .
 
D

DBLEXPOSURE

Jan 1, 1970
0
Bob Masta said:
The thump probably comes because the preamp outputs are
AC-coupled through capacitors to the power amp. (There
are probably also caps in the power amp input.) Thumps
are especially likely if the preamp runs off a single supply
so its internal output is at half the supply, and relies on
the cap to block this. When you first turn it on, that internal
output point rises quickly to half of the supply, and that
fast rise is essentially AC that passes right through the cap
to appear as a thump. To prevent the thump, you'd need
a special slow-turn-on circuit design.

If the preamp has balanced + and - supplies, this is
still a possible problem if the circuit doesn't come on
in a controlled fashion.

So no, there is no simple switching arrangement to
cure this. A complicated arrangement would be to
have a time-delay such that the preamp is allowed to
power up and charge its output cap into a resistor
to ground simulating the power amp, and then after
the voltage across the cap is reduced to a very low
value the switch connects the power amp.

Best regards,





Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

Bob might have it but, if the preamp is of any quality, it really
shouldn't do this. If you have a Digital volt meter you might want to check
and see if the is any DC at the output of your preamp, there should be none.
When you power up the preamp the output might be jumping from 0VDC to 1 or
2VDC, this would cause a pop. If this is the case, you might want to take
it to you local tech and have it fixed, or at least compare it to one of the
same model to see if it is a poor design.
 
J

Jasen Betts

Jan 1, 1970
0
I have a guitar pre-amp and separate power amplifier .
When the power amp is already on for a while , should I then power up the
pre-amp , there is a loud ' thump ' .
Can I modify the switch on the pre-amp to get round this problem .

possibly. more likely you'll need to add a de-thump unit inside the preamp.

something that will disconnect the output until the pre-amp has stabilised
(probably about 1/4 second after turn-on)...
 
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