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Old 16th November 2013, 12:52 AM   #61
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Quote:
Originally Posted by Art M View Post
If that is a protection relay, it looks totally inadequate with a contact
rating of 7A/ 30VDC for such a powerful amplifier.
Yes, and I put 2 in parallel for good measure.
I looked at many models and I didn't want one too big in there.
If you have a better one to suggest, I will take a look at it. Not too big though.
I will still use 2 in parallel in any case. And there is one other for the mains power detection, I kept the same model to make things easier. I am considering adding one more for muting the input, but that takes so much room, I'm more interested in using a solid state switch like the J111/112. But this may be for a future version, maybe when I make use of the flat packs, which will take up less pcb real estate and that will allow more stuff on that precious space.
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Old 16th November 2013, 08:22 AM   #62
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Originally Posted by spookydd View Post
I am wary of the flat packs when it comes to power dissipation. You need a larger number of them compared to the metal cases.

Eventually I will make a version of my design with flat packs, as they are more convenient and easier to mount on heatsinks. Plus we can design for a forced cooled tunnel heatsink and that is one of my plans for the bigger amps for my low frequency speakers.
The Lo Tim uses 2pair of 250W 175C To3 devices.
I used 3pair of 230W 150C To264 plastic packages to ensure I got Tj about 25C lower than for an equivalent 175C To3 device.
You have to run the plastic package cooler, as well as allowing for less heat flow through the Thermal interface.
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Old 16th November 2013, 08:29 AM   #63
AndrewT is offline AndrewT  Scotland
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Two relays in parallel do not help with longevity of the contacts, nor does it help with preventing burning/welding of the contacts on opening.
Paralleling helps with reducing the impedance across the contacts and the connecting wiring.

The problem, even with a dual contact relay, is that one pair of contacts opens before the other pair. The time difference only needs to be in the microsecond range and the last opening contact then suffers the burning as the arc establishes across the tiny gap that is just starting to open. It is getting that arc extinguished, that becomes the priority.
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Old 16th November 2013, 09:40 AM   #64
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Quote:
Originally Posted by AndrewT View Post
Two relays in parallel do not help with longevity of the contacts, nor does it help with preventing burning/welding of the contacts on opening.
Paralleling helps with reducing the impedance across the contacts and the connecting wiring.
I was thinking about the impact on the damping factor, which is reduced somewhat by having less added resistance. Plus since the relay's contacts have insufficient current handling, it doesn't hurt to parallel them. But If there are better relays that can be used, of course I'll take a look at them, but I want to avoid the huge ones though.

They need to be fast to release, if possible, so they are better protection.

Quote:
Originally Posted by AndrewT View Post
The problem, even with a dual contact relay, is that one pair of contacts opens before the other pair. The time difference only needs to be in the microsecond range and the last opening contact then suffers the burning as the arc establishes across the tiny gap that is just starting to open. It is getting that arc extinguished, that becomes the priority.
I think an input muting relay would be nice, if it mutes first before the output relays open. And the use of the output relays for emergency protection is rare, so if we hardly make use of them to switch off while loaded, then it should be too much of a problem. The key is to design so they are not routinely stressed.

Those relays are rather cheap too and easy to get.

Any good relay suggestion is welcome.
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Old 16th November 2013, 12:23 PM   #65
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I recalculated the protection resistors values for higher value to reduce dissipation, mostly in R26/34 and D3/4. The current in D3/4 was about at maximum when acting for protection or near clipping. The needed power handling for R26/34 was high, requiring bigger resistors. The 1W resistors should now suffice.

I am considering the option to use a solid state muting switch on the input. I've seen the J111/112 types used a lot in such situations, it's simple and should work. I wouldn't place it in series with the signal path, but it can short a resistor when muting.
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Old 16th November 2013, 05:20 PM   #66
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Here is one of the relays I considered before going with the songchuan:

Invalid Request

It does have more current capacity, but it's also bigger.
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Old 17th November 2013, 08:26 AM   #67
AndrewT is offline AndrewT  Scotland
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Re-wire your XLR sockets.
PIN1 goes to Chassis right next to it's respective socket. NOT to Signal Ground.
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Old 27th November 2013, 05:57 PM   #68
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Re-wire your XLR sockets.
PIN1 goes to Chassis right next to it's respective socket. NOT to Signal Ground.
You're quite right! (as always )

And now that I looked at it, for an unknown reason, I had also swapped pins 1 with 3. Duh!

The thing is, now I'm not sure what I'll do those pin1, because I can't connect that to the pcb ground plane, neither to the signal ground, and there is no easy physical way to connect it to the chassis. And there is the matter of the external ground connection, not an actual pin in the plug, but the plug casing pin, which I had hooked via jumpers to the choice of signal ground or main ground plane. This isn't good now. I will need to change all this.
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Old 27th November 2013, 06:08 PM   #69
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More changes will be coming real soon on this ongoing design:

- I will do away with the output relay and replace that by SSRs. I am still pondering on that change, as I really think cutting the ground side speaker return is a good idea, but at the same time, the amp output side should really be also cut, so perhaps doing both sides might be nice.

- I will also do away with the fuses on the rails and use SSRs instead, probably triggered by the same signal as the output switches, not certain yet, but seems like a possibility.

- I may remove the zobel from the main pcb and move it to the speaker plug itself, although this is an uggly wiring to do... I may end up with 2 zobels, one on the amp's output before the output R//L and the other farther out on the plugs. I may even think about tapping the feedback on the outer side of the R//L, so that would be included inside the loop.

- I am also looking at an input muting, which I would want triggered in the proper order when muting the output. I definitely don't want to use the VI limiter's trigger to trip the amp's protection on the SSRs, as I don't want a simple soa trespass to stop the show.

This amp is kind of like my test bed for tweaking design choices. Then subsequent versions will probably go further, and eventually those will be using the flat packs.
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Old 27th November 2013, 06:49 PM   #70
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Hi spookydd
Looking at all these massive preparations and renderings you've made for this classic LTP/VAS topology, my suggestion would be to do some more research among DIY-ers who already have experiences with this amp and ask them would they still choose or recommend this sch or direct you elsewhere. I think such a great project deserves to have deep second thought. All the best
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