Help with LM338k slow start circuit

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
Hi there,
I posted here before and everyone was very helpful, I'm hoping I can get some assistance again!
I have a voltage regulator circuit with an LM338k it is powering a D-Class amp:
lm338k.JPG
It works great except when I turn it on a get a bit of annoying popping in the speakers. I was thinking converting the circuit to a slow start could help smooth this out a little and put less strain on the amp. (I have also added a mute switch to the amp which works but I'd rather not have to use it every time).
The data sheet for the LM338 has a brief example of a circuit with slow start using a PNP transistor.
LM338 slow turn ON power supply circuit.png
I am just not too sure about correct component values for my intended output (the example is for 15v regulation, and I can't see anything else in the datasheet explaining the circuit). I can see where most of my components would fit in. Obviously R1 and R2 would remain the same - setting the output voltage, and I imagine the protection diodes could remain in place.. but how do i determined the value for R3? and is the input cap separate to the ripple smoothing cap in my original circuit? (0.1uf is drastically different to 4700uf...) Would my C3 cap translate to the C1 cap in this schem?

Any help would be very much appreciated!
Thank you!
 

dannyf

Joined Sep 13, 2015
2,197
I was thinking converting the circuit to a slow start could help smooth this out a little and put less strain on the amp.
you probably want to experiment it a little before you invest it in the beast. your mute idea is much better and more effective.

btw, I would not put any regulated power supply (unless you are talking about some really expensive ones) on a dynamic power amp: they aren't meant to drive a dynamic load.
 

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
you probably want to experiment it a little before you invest it in the beast. your mute idea is much better and more effective.

btw, I would not put any regulated power supply (unless you are talking about some really expensive ones) on a dynamic power amp: they aren't meant to drive a dynamic load.
Thanks for the reply dannyf!
Sorry, I am still fairly new to circuits and electronics.. Why would a regulated power supply be no good for my amp?

This is the amp that it is running:
http://sun-pec.com/Pics/SunPec/PCB/SUNPEC-EMO-PCB07-Mould-Spec.pdf

it all seems to work really well (except for those speaker pops at turn on I am trying to iron out), what would be a more suitable power supply?
 

ronv

Joined Nov 12, 2008
3,770
Hi there,
I posted here before and everyone was very helpful, I'm hoping I can get some assistance again!
I have a voltage regulator circuit with an LM338k it is powering a D-Class amp:
View attachment 117777
It works great except when I turn it on a get a bit of annoying popping in the speakers. I was thinking converting the circuit to a slow start could help smooth this out a little and put less strain on the amp. (I have also added a mute switch to the amp which works but I'd rather not have to use it every time).
The data sheet for the LM338 has a brief example of a circuit with slow start using a PNP transistor.
View attachment 117776
I am just not too sure about correct component values for my intended output (the example is for 15v regulation, and I can't see anything else in the datasheet explaining the circuit). I can see where most of my components would fit in. Obviously R1 and R2 would remain the same - setting the output voltage, and I imagine the protection diodes could remain in place.. but how do i determined the value for R3? and is the input cap separate to the ripple smoothing cap in my original circuit? (0.1uf is drastically different to 4700uf...) Would my C3 cap translate to the C1 cap in this schem?

Any help would be very much appreciated!
Thank you!
I think you can build it as shown. You need to keep your big smoothing cap on the input.
C1 sets how fast the voltage rises. I would keep C3.
 

Thread Starter

PegasusIII

Joined Nov 12, 2016
18
I think you can build it as shown. You need to keep your big smoothing cap on the input.
C1 sets how fast the voltage rises. I would keep C3.
Ok thank you!
So if I kept the values for R3, C1 and C2 from the 15v circuit it would still work for my 22v configuration? What is the role of R3 in the slow start circuit?

Does this make sense or are there superfluous components in there?:
lm338.JPG
 

Alec_t

Joined Sep 17, 2013
10,606
At switch-on, C9 will charge up via R3. While it is charging Q1 conducts and pulls down the voltage on the ADJ pin of the IC, thus keeping the regulator output low.
I would query whether C2 is necessary. It slows the transient response of the regulator.
 
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