May I have some help regarding..Electrical safety,Star Grounding and Suggested Layout

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A

AndrewT

R.I.P.
Joined 2004
Hi,
that T3.1A fuse will pass >700W for an eternity and ~1400W for tens of seconds, maybe even for a few minutes.
Fit a soft start and fuse at somewhere between T800mA and T1.2A

How much does the output offset vary from a cold start up to fully warm? Try using a hair dryer, set to hot, blowing onto the heatsink.

Experiment the other way. Build a safe to use AC coupled amplifier first. Understand how it works. Understand why all those optional extras are really not optional.
Once you are fully informed then make decisions on which components to delete in a forlorn attempt to blow up your speakers.
 
Tripmaster

Tripmaster

Member
Joined 2008
AndrewT said:
Hi,
that T3.1A fuse will pass >700W for an eternity and ~1400W for tens of seconds, maybe even for a few minutes.
Fit a soft start and fuse at somewhere between T800mA and T1.2A

How much does the output offset vary from a cold start up to fully warm? Try using a hair dryer, set to hot, blowing onto the heatsink.

Experiment the other way. Build a safe to use AC coupled amplifier first. Understand how it works. Understand why all those optional extras are really not optional.
Once you are fully informed then make decisions on which components to delete in a forlorn attempt to blow up your speakers.
Click to expand...

I've just added a Zobel Network (2R8 & 0.1uf) and changed the location of the FB resistor (R3) so as to reduce the offset

FBRes.jpg


When I apply power the light bulb briefly lights...dims and then relights

I cant see any shorts and this only happens when the amp is connected to the PS

Any ideas?
 
A

AndrewT

R.I.P.
Joined 2004
oops.
The two pics show the alternatives.
On the left is cap and resistor
On the right is resistor alone.


Doing the left version with no cap leaves the NFB lower leg open circuit.
The minimum gain for the chipamp to remain stable is ~ 10Times (+20dB).
At a gain of one (NFB open circuit) with or without the output Zobel the chipamp is virtually guaranteed to oscillate.
The chipamp may already be damaged.


Build an AC coupled chipamp with ALL the optional add-ons in place and learn how this works.
 
Tripmaster

Tripmaster

Member
Joined 2008
AndrewT said:
oops.
The two pics show the alternatives.
On the left is cap and resistor
On the right is resistor alone.


Doing the left version with no cap leaves the NFB lower leg open circuit.
The minimum gain for the chipamp to remain stable is ~ 10Times (+20dB).
At a gain of one (NFB open circuit) with or without the output Zobel the chipamp is virtually guaranteed to oscillate.
The chipamp may already be damaged.


Build an AC coupled chipamp with ALL the optional add-ons in place and learn how this works.
Click to expand...

I have added the cap and now get a 25v output on both sides and a 0.7mV DC offset
 
A

AndrewT

R.I.P.
Joined 2004
if you are using the schematic shown then you are still missing the input DC blocking capacitor which also sets the LF roll off frequency and you have no RF attenuation at the input.
The remainder of the output Thiele network is also missing.

Read the National datasheet.
 
Tripmaster

Tripmaster

Member
Joined 2008
AndrewT said:
if you are using the schematic shown then you are still missing the input DC blocking capacitor which also sets the LF roll off frequency and you have no RF attenuation at the input.
The remainder of the output Thiele network is also missing.

Read the National datasheet.
Click to expand...


I have another Chipamp built to the same specs and this sounds and operates OK.

I will Google what you have just mentioned and make the necessary changes.
 
Tripmaster

Tripmaster

Member
Joined 2008
AndrewT said:
Hi,
that T3.1A fuse will pass >700W for an eternity and ~1400W for tens of seconds, maybe even for a few minutes.
Fit a soft start and fuse at somewhere between T800mA and T1.2A

How much does the output offset vary from a cold start up to fully warm? Try using a hair dryer, set to hot, blowing onto the heatsink.

Experiment the other way. Build a safe to use AC coupled amplifier first. Understand how it works. Understand why all those optional extras are really not optional.
Once you are fully informed then make decisions on which components to delete in a forlorn attempt to blow up your speakers.
Click to expand...

Hi Andrew

Can I use a NTC thermistor wired in series for inrush limiting?

If so, what resistance/current rating would be necessary for 2x300va transformers?

Thanks

Richard
 
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