KiCAD: Velleman K4700 DC Offset Detection PCB - updated

[graham-r]

OK, so I wanted to use a Velleman K4700 DC Offset Detection PCB to protect speakers when using my aging Crimson Elektric CS1500 Power amp......

But there was no way the Velleman PCB would fit in the remaing space in the CS1500 as it was too long (approx 125 x 55mm). So, once again having waaaaaay too much time on my hands I made my own version that would fit in the approx 80 x 80mm space available - as well as adding some useful features to make it more flexible.

The schematic used is identical to the one published by Velleman with the addition of four decoupling caps. The transformer and relay 'footprints' and connections are identical to those supplied by Velleman although I used different components (RadioSpares and Omron respectively).

The PCB's above don't show the transformer, instead they show a connector (J3) so an off-board AC supply can be used as an alternative. The entire transformer section of the PCB (with rectifier diodes) can be cut off and placed remotely and connected using a cable between J4 and J5.

External DC can also be connected via J5 (approx +/- 8v).

This PCB above is a slightly revised version to those I had made, electrically they are identical.

I will publish the gerbers on Github fairly soon if anyone else wants to get some made.

 

[Arkless Electronics]

I built up a Velleman one a few days back and it didn't work correctly! Thick end of 2 hours work later and I found the problem.. a hairline crack in a PCB track which was not visible even under a hand magnifying glass.... prob some contamination of PCB at etching stage... £70 labour I can't really charge for

 

[graham-r]

I built up a Velleman one a few days back and it didn't work correctly! Thick end of 2 hours work later and I found the problem.. a hairline crack in a PCB track which was not visible even under a hand magnifying glass.... prob some contamination of PCB at etching stage... £70 labour I can't really charge for

That is not good, I have had a couple of these fail for no obvious reason, perhaps they also had cracks in the tracks ?

Mind you for about £15 you can't expect too much !

 

[graham-r]

Here a couple of pics of the finished modules - they work as expected....... but, one of the LED's doesn't seem to be doing what it should, some investigation needed.

The above are how it is intended to work in 'normal' use - with the transformer in place (this does not need J4/J5 as they are connected by tracks). In fact to save cost all the connectors could be left off and hard wiring used - the unused two-way is used to connect a manual mute facility by connecting then pins together.

 

[Arkless Electronics]

That is not good, I have had a couple of these fail for no obvious reason, perhaps they also had cracks in the tracks ?

Mind you for about £15 you can't expect too much !

It wouldn't surprise me! Everything about it is pretty poor quality... including Chinese electrolytics of a brand I've never heard of, "Hitano".

 

[Arkless Electronics]

Here a couple of pics of the finished modules - they work as expected....... but, one of the LED's doesn't seem to be doing what it should, some investigation needed.

The above are how it is intended to work in 'normal' use - with the transformer in place (this does not need J4/J5 as they are connected by tracks). In fact to save cost all the connectors could be left off and hard wiring used - the unused two-way is used to connect a manual mute facility by connecting then pins together.

Nice job there! If the LED issue is that yellow LED does not fully extinguish then it seems they all do that...

I was going to get another one for my own use but I'll design and build one from scratch on veroboard. I could copy their design but I suffer from "not invented here syndrome" It will cost me more for the parts alone than the Velleman kit of course.

 

[martin clark]

@graham-r - very neat!

 

[chrisn]

I knocked out a Naim tweeter last week when I turned the active crossover off before the power amps both of which had apparently operational Velleman units.
I love active Naim speakers, but can I really be bothered anymore?

 

[mega lord]

Ouch, sorry to hear that. It could become very expensive all this driver destruction.

 

[graham-r]

The circuit diagram - it seems unnecessarily complicated.....

 

[graham-r]

It wouldn't surprise me! Everything about it is pretty poor quality... including Chinese electrolytics of a brand I've never heard of, "Hitano".

The (failed) one I looked at had Jamicon caps (at least that is what is printed on the outside !), so perhaps they have cheapened the quality of components ?

Another possibility is that the kit itself was a fake ?

 

[graham-r]

I knocked out a Naim tweeter last week when I turned the active crossover off before the power amps both of which had apparently operational Velleman units.
I love active Naim speakers, but can I really be bothered anymore?

I have had a similar experience when the 0v wire fell off of a JLH class A amp I was building/testing - the K4700 didn't catch the resulting oscillation and took out the (thankfully cheap) drive units of the grot speakers I was using.

But at least it made an amusing noise
SWMBO: "Did you do that"
Me: "It wasn't me it was the amp blowing up"

 

[Vinny]

Loads of boards on EPay from China, most of them small. I have not checked but in all probability you can see the component details.

Unfortunately I would assume that it would be entirely possible for them to fail unsafe??????

 

[graham-r]

Nice job there! If the LED issue is that yellow LED does not fully extinguish then it seems they all do that...

I was going to get another one for my own use but I'll design and build one from scratch on veroboard. I could copy their design but I suffer from "not invented here syndrome" It will cost me more for the parts alone than the Velleman kit of course.

It isn't the yellow (wait) LED as that works as it should - I always thought dimming after the mute period was intentional.

The other LED (error) isn't lighting at all - there only appears to be about 2 volts across it when it should be lit, that should be enough to light it. The LED is very old (1970's probably) so likely not that efficient so may not be glowing at all.

The cost of the parts for this version turned out not to be anywhere as much as you may think - the resistors/caps were the most costly as I had to buy the ones I did not already have in 25's or 50's. The transformer and relays were £3.52 and £1.56 respectively (+VAT). The transformer is over-specified for this so a smaller one may be cheaper.

The total cost of the PCB's in this instance was approx £20 for five, this will vary as JLCPCB had a £3 offer on.

 

[graham-r]

Loads of boards on EPay from China, most of them small. I have not checked but in all probability you can see the component details.

Unfortunately I would assume that it would be entirely possible for them to fail unsafe??????

Quite possibly, if the detection circuit fails to detect DC then you are in trouble, the Velleman design will cause the relays to drop out if the power goes.

 

[Vinny]

the Velleman design will cause the relays to drop out if the power goes.

Any board that didn't, would be worthless, I would suggest. The Chinese ones here certainly satisfy that requirement as two power supply transformers for them failed.

 

[Arkless Electronics]

It isn't the yellow (wait) LED as that works as it should - I always thought dimming after the mute period was intentional.

The other LED (error) isn't lighting at all - there only appears to be about 2 volts across it when it should be lit, that should be enough to light it. The LED is very old (1970's probably) so likely not that efficient so may not be glowing at all.

The cost of the parts for this version turned out not to be anywhere as much as you may think - the resistors/caps were the most costly as I had to buy the ones I did not already have in 25's or 50's. The transformer and relays were £3.52 and £1.56 respectively (+VAT). The transformer is over-specified for this so a smaller one may be cheaper.

The total cost of the PCB's in this instance was approx £20 for five, this will vary as JLCPCB had a £3 offer on.

With the parts I will be using it will cost a lot more than the kit or the figures you give above. Chassis mounting 12VA transformer alone about £10 plus VAT and I'll probably use heavier duty relays. I may well add protection against oscillation.

I have designed the trickiest parts of an "ultimate" protection unit which uses phased mosfet and relay switching so as to provide the lowest possible resistance and distortion when connected and be able to safely break even the output from 300W++ amplifiers without relay contacts welding together etc. Other possible features include over driving protection for all drive units by averaging the power within the bandwidth handled by each driver. It was mainly a design exercise and "to prove a point" and I'm not likely to do anything with it...

 

[S-Man]

I have had a similar experience when the 0v wire fell off of a JLH class A amp I was building/testing - the K4700 didn't catch the resulting oscillation and took out the (thankfully cheap) drive units of the grot speakers I was using.

The K4700 only protects against dc. It is not designed to protect against high levels of ac "signal".

 

[Arkless Electronics]

Protection against a huge pulse of power into a tweeter in an active system is a tricky one. An in line cap between amp and tweeter to prevent most of the low frequency part of the pulse seems about the best choice...

 

[davidsrsb]

Active tweeter amplifiers need some sort of power limiting. Tweeters have very limited excursion limits and you should respect that