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BobEllis 6th November 2006 07:57 PM

DC Protection and Clipping Indicator
I've come up with this for a combination DC protection/Speaker delay/clipping indicator. I'd originally hoped to this on a single board with a power controller/soft start circuit, but I couldn't do within the free EAGLE board size limit without going SMD. I really don't want to go SMD, so it is two boards. (The controller should be published soon)

This board consists of two of Rod Elliot's project 23 clipping indicators and a copy of the Hafler DC protection/speaker delay circuit. All are powered by the amp's rails, although the relay power is separate and could easily be powered by an auxiliary power source. Power for the clipping indicator opamps can come from a +12V supply or the amp rails.

Each clipping indicator has two inputs and is completely independent. The power rails can be tied together if the amp has a common supply. One input from the first clipping indicator can be routed to the other so that a single connector can be used to bring power and signal to the board.

Options include:

Single clipping LED for

1. up to four channels with a common PSU

2. two channels with independent supplies. (dual mono)

3. 4 channels with two power supplies (dual stereo)

2 clipping indicators for

1. 2-4 channels with a common power supply.

2. 2-4 channels sharing two power supplies.

The protection circuit is pretty much a straight copy of the hafler circuit. Keeping the input resistors different means that the circuit will open the speaker relays if the two channels fail to opposite rails.

Since the circuit is rail powered, when the rails come up it delays closing the speaker relays for a few seconds.

I made two changes to the circuit:

I doubled up the relay control transistors so I can be comfortable using two relays to control a four channel amp. High hfe BC546C should ensure that the LM339 has no trouble driving a pair of relays. I plan to use this with a 4 channel amp that shares a common supply.

My mains power is subject to blinking in the summer - it cuts out just long enough to cause thumps in my system, but not long enough to open the speaker relay on my DH500. Taking a cue from Rod Elliot, I added a spot for zeners in series with the protection diodes across the relays. This should make the relays open a little more quickly.

As I write this up, I realize that I don't have DC protection on two of the inputs. I'll add that but in the meantime comments, constructive criticism and suggestions welcome.

Schematic -

Board layout minus the ground plane -

Nordic 6th November 2006 07:59 PM

Nice !

Care to draw up a BOM?

BobEllis 6th November 2006 09:31 PM


1n4007 10
1n4148 - 4
1n5242 4
1N5240 2
BC546C 2
MPSA42 4
MPSA92 4
TL072 2
LM339 - 1

1 f 16V 2(shown as a film cap - may jsut keep it that way for .07 difference)
.39 f 3
10 f 16V NP 1 C104 shown as a film cap - needs to change.
22 f 100V - 1 - C105 shown as a film cap - needs to change.

1K 10
100K 9
27K 1W 2 (value depends on rail voltage)
2K7 2W 2 (value depends on rail voltage)
12K 4 (value depends on rail voltage)
150K 8
27K W 2
22K 3
3K9 2
1M 1
10M 2
4K7 1
1k5 2W 2 (value depends on rail voltage)

2 Pin Headers or Molex 2 pin KK headers 5
4 Pin headers or Molex 4 pin KK headers 2

I figure under $10 for parts. Add your relays to this. I plan to use flange mount relays so no board required.

lineup 6th November 2006 09:59 PM


Great work
and a very useful circuit for all us amplifier builders.

It has those basic functions for protection we can benefit from,
if we care for our projects and speakers to be a little more safe.
And do not want those bad troubles.
At least those which can be avoided.

Thanks for sharing BobEllis.

Can't be too difficult to build, for those who can make or some way get PCB for it.
Of course there has to be some instructions added
so we know how to setup and adjust parameters.

But I guess we will get this knowledge by and by
if we keep on asking questions in this dedicated topic.

lineup :cool:

BobEllis 6th November 2006 10:28 PM

Thanks for the encouragement, Lineup. I plan to have our friends at Advanced Circuits make me some boards. If there is enough interest I could do a group buy of boards and parts.

I've had an offer to integrate the soft start/power controller part of my idea into a single board by someone with a full copy. I'm working on that now. The features I plan to incorporate there are:

Soft start - including thermal shutdown if soft start relay fails.

Power on with either
1. Momentary pushbutton
2. 12VDC trigger
3. audio input

Second delay to drive muting relays - initiates when soft start trips.

Momentary pushbutton mute relay control.

Connections for thermal fuse protection - kills main power when overtemp exists. You'll be able to use several sensors or jumper them out if not needed.

Standby/ power on LED

On-board transformer for aux power.

I'm thinking that I will score the boards between the soft start section and the clipping/DC protect circuit so they can be mounted separately if desired. (or eliminate the clipping indicator for preamp use) I'm wondering if it might make sense to make one large panel with the muting relay circuits too. Have all sections scored and ready to break apart - it might make production more cost effective.

Thoughts? Missing features?

lineup 6th November 2006 11:16 PM


Originally posted by BobEllis
Thanks for the encouragement, Lineup.
I plan to have our friends at Advanced Circuits make me some boards.

If there is enough interest I could do a group buy of boards and parts.


Wait for some time with those good plans of Group Buy and making some boards.

Let this topic run for some weeks.
Like with software, there is always good with some debugging period.
Before an official public STABLE release.
1. Alpha versions
2. Beta versions
3. Release Candidate versions, RC1, RC2 ..
4. Release
5. Release fixes and patches

My guess is that this Protection and Clipping Indicator Circuit
is in stage 2. Beta or possibly in stage 3. RC1 = Release candidate 1

As I say, if people will have a good look at your circuit
and suggest modifications for improvements
and so avoiding problems in some unusual situations,
you might have a very good release version in some weeks or a couple of months.
Of course, the most of debug and testing you will do yourself.
Using other amplifiers with different power supply and various levels of everything.

Just because everything works with one amplifier, doesn't mean it will work like this in another.
It may work, if it is a very good and flexible circuit.
But until some further testing and other people have had a look at it,
we cant know.

I know you wont take this as critics, from me.
I only wish the best for any good project.
It is just so much better, to make sure circuits are 95% reliable
and works in >=90% of all cases
before start make and offer PCB to other people.


BobEllis 7th November 2006 12:18 AM

Hi Lineup -

I appreciate your suggestions. I am nowhere near ready to order anything like a production board. I was just trying to publicly outline my intentions for this project.

Since the circuits presented here already are basically copied from others, I think they are fairly close to being releaseable. I will breadboard them over the next few weeks.

The rest of it will be lifted from application notes. I have no formal electronics education, so I will be looking for all the guidance I can get. I will breadboard the circuits before ordering prototype boards and test those before going to production. If I can get all the sections into a single panel that fits the size limits, I can take advantage of Advanced Circuits 3 for $33 each special - and have a prototype board set complete with soldermask and silkscreen.

I have found circuits that I plan to copy for using a 4013 flip flop for the pushbutton operation. Are the switches properly debounced? Have I connected it properly for startup in the powered down state? Hopefully someone in the know will check once I post a circuit.

The audio detect circuit will also be borrowed from Rod Elliot with the addition of buffers on each of the inputs. When I breadboarded the circuit I found that with my sound card the crosstalk was unacceptable as the circuit is published. I will add a bit of gain in the buffers since I found sensitivity too low.

The 12V trigger will use a TIL-194 AC transistor output optocoupler and a resistor to limit the current. This will help prevent ground loops. Pretty simple.

The flip flop, audio detect and 12V trigger will supply current (through diodes to isolate each) to the base of a BC546C to activate the mains relay. This will trigger the delay setup using a BC556C to drive the soft start relay. this will trigger the next delay to activate the muting relays. (I have thousands of both of these, so why not use them. ;) )

I'd better get back to working on the schematic and post it for review. :smash:

BobEllis 7th November 2006 01:09 AM

I'll post the schematic of the soft start portion in pieces to make it easier to read.

Here is the basic flip flop circuit that I plan to use for the power on/off and mute functions. Not shown is the implicit power connection. Does this seem reasonable and debounced? Will the output be low on initial power up?

The momentary switch connects to the header on the left. Teh port labeled power connectst to the relay driver.

Samuel Jayaraj 7th November 2006 01:53 AM

What is the threshold/trigger voltage of the DC sense circuit?

Just an idea - what if a Leach style Clip indicator were to be used, it may be possible to sense overcurrent as well in most output configurations? If the sense circuit is made adaptable, this would be a very comprehensive protection board.

Another thought - let the relay drive be take off points on this board so that the relays can be located on the amp boards itself making for short signal paths and less heavy guage wiring.

BobEllis 7th November 2006 02:08 AM

The clipping indicator threshold can be set by adjusting component values. Rod uses 3V below the rails, I plan to use 5V for a little extra margin. See Project 23

I tried to wrap my mind around the Leach circuit since it has so few parts, but couldn't figure it out well enough to scale it for various rail voltages.

The DC protect threshold is 1.8V below 10 Hz, adjustable with resistor values should that prove too high.

I plan for all relays to be off board for exactly the reasons you describe. Pin Headers or Molex .100" KK series connectors can be used.

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