Thank you very much for your order. I boxed up your board earlier today so you should have the design documentation by now.
You'll need to change D4 and D8 to 12-15 V types. That'll lower the turn-off point to about 15-18 V.
Tom
You'll need to change D4 and D8 to 12-15 V types. That'll lower the turn-off point to about 15-18 V.
Tom
I've started to order the parts on the BOM but, not surprisingly, plenty of items are on backorder! I can substitute passive parts but most of the semiconductors are unavailable for several months at the usual suppliers.
BC546B, BC547B are interchangeable. uA78L05 is a substitute for the LM78L05. Any MOSFET in D2PAK/TO-263 will work. Lower on-resistance is better.
Tom
Tom
Thanks for the help Tom.
These two MosFets are either in in stock or due to be available within days - both sets of specs look OK to me - any thoughts?
AUIRFS3306TRL Infineon / IR | Mouser United Kingdom
IRFS4310ZTRLPBF Infineon / IR | Mouser United Kingdom
Cheers
These two MosFets are either in in stock or due to be available within days - both sets of specs look OK to me - any thoughts?
AUIRFS3306TRL Infineon / IR | Mouser United Kingdom
IRFS4310ZTRLPBF Infineon / IR | Mouser United Kingdom
Cheers
IRFS4310ZTRLPBF is rated for 100 V (same as the MOSFET I specify). The AUIRFS3306TRL is "only" rated for 60 V. If you're using the Guardian-86/686 with an LM3886-based amp, either will be fine.
Tom
Tom
Thanks again Tom. I'll be using the Module in a ZM SissySIT project, +/-24V rails, so I ordered the 60V mosfets because they were actually in stock rather than on a short backorder lead.
Taken from Neurochrome's website:
"The Guardian-86 will also disconnect the speaker if the amplifier outputs more than ±700 mV of DC."
I'm no expert but from what I've read 700mV of DC are dangerous for loudspeakers. Therefore, I would appreciate any clarification for the following questions:
Is 700mV a safe value to disconnect the speakers?
Wouldn't it be advisable to use a lower value?
I don't think it's possible, but I would like to be sure if one can't change the value to disconnect the speakers?
Thanks.
"The Guardian-86 will also disconnect the speaker if the amplifier outputs more than ±700 mV of DC."
I'm no expert but from what I've read 700mV of DC are dangerous for loudspeakers. Therefore, I would appreciate any clarification for the following questions:
Is 700mV a safe value to disconnect the speakers?
Wouldn't it be advisable to use a lower value?
I don't think it's possible, but I would like to be sure if one can't change the value to disconnect the speakers?
Thanks.
The Guardian-86 and Guardian-686 are designed to disconnect your speakers in the event of a catastrophic amplifier failure. They are not designed to replace DC servos or AC coupling capacitors.
Any well-designed audio amplifier should have low DC offset. It seems many of the Class D module makers design for less than 100 mV. I generally expect less than 10 mV in a speaker amp and less than 1 mV in a headphone amp.
When an amplifier fails, it tends to output the full rail voltage on the speaker terminals. In such cases the Guardian-86/686 will disconnect the speaker, thereby preventing further damage.
I could potentially/maybe see a scenario where the DC servo in a power amp failed, which resulted in high DC offset on the amp. That's unlikely as it requires the DC servo to fail with its output stuck at 0 V, which seems like a pretty far-fetched scenario. In such scenario you'd get a pretty loud pop on startup, which I would hope that you'd go investigate. I doubt there would be any damage though. Even 700 mV is far below the signal swing commonly applied to speakers. 700 mV would result in 61 mW dissipated in an 8 Ω speaker. As mentioned, this scenario is fairly unlikely to happen in reality. I would expect an amp with a broken DC servo to present a significant DC voltage at its output. Certainly more than the 700 mV needed for the Guardian to trip.
It's interesting how some have now locked into the trigger point of the protection circuits. "Why 700 mV? Why not 0 V?" First off, a 0.00 V trigger point would be impossible to implement. There will always be a DC offset, both in the amp and in the detection circuit. The closest I've ever come to 0.00 V DC offset was in the design of the LMP2021 where the offset ended up being 400 nV (typical), 5 µV (worst case). So there is that.
Secondly, the sensitivity of the protection circuit affects its reaction time. DC protection circuits work by lowpass filtering the voltage across the speaker terminals. If you want high sensitivity (low trigger voltage) you need to push the cutoff frequency of that lowpass filter way down. This, in turn, means that the protection circuit becomes much slower and it may not disconnect your speakers in time to save them when something goes wrong. If you choose to not lower the cutoff frequency of the lowpass filter you end up with a protection circuit that disconnects the speaker on every loud bass thump. That's not my idea of good circuit design.
Another factor to consider is reliability. I chose a 700 mV trip point because that allows me to use a pair of NPNs for the detector. Those devices provide reliable protection. If you want a more sensitive circuit, you'll likely need an opamp in there. This adds complexity, which lowers reliability.
I did consider fancier lowpass filtering. It would be kinda neat to use a micro controller to sample the voltage across the speaker terminals and then filter it digitally. That could create a lowpass filter with a very steep cutoff that could allow for a low trigger voltage while also having a reasonable reaction time. But many would frown at the idea of doing anything digital at the output of an amp and at some point the idiom "stuffing 10 lbs of sh.... uh ... "dung" ... in a 5 lb bag" also came to mind.
I hope that sheds some light on the design tradeoffs.
Tom
Any well-designed audio amplifier should have low DC offset. It seems many of the Class D module makers design for less than 100 mV. I generally expect less than 10 mV in a speaker amp and less than 1 mV in a headphone amp.
When an amplifier fails, it tends to output the full rail voltage on the speaker terminals. In such cases the Guardian-86/686 will disconnect the speaker, thereby preventing further damage.
I could potentially/maybe see a scenario where the DC servo in a power amp failed, which resulted in high DC offset on the amp. That's unlikely as it requires the DC servo to fail with its output stuck at 0 V, which seems like a pretty far-fetched scenario. In such scenario you'd get a pretty loud pop on startup, which I would hope that you'd go investigate. I doubt there would be any damage though. Even 700 mV is far below the signal swing commonly applied to speakers. 700 mV would result in 61 mW dissipated in an 8 Ω speaker. As mentioned, this scenario is fairly unlikely to happen in reality. I would expect an amp with a broken DC servo to present a significant DC voltage at its output. Certainly more than the 700 mV needed for the Guardian to trip.
It's interesting how some have now locked into the trigger point of the protection circuits. "Why 700 mV? Why not 0 V?" First off, a 0.00 V trigger point would be impossible to implement. There will always be a DC offset, both in the amp and in the detection circuit. The closest I've ever come to 0.00 V DC offset was in the design of the LMP2021 where the offset ended up being 400 nV (typical), 5 µV (worst case). So there is that.
Secondly, the sensitivity of the protection circuit affects its reaction time. DC protection circuits work by lowpass filtering the voltage across the speaker terminals. If you want high sensitivity (low trigger voltage) you need to push the cutoff frequency of that lowpass filter way down. This, in turn, means that the protection circuit becomes much slower and it may not disconnect your speakers in time to save them when something goes wrong. If you choose to not lower the cutoff frequency of the lowpass filter you end up with a protection circuit that disconnects the speaker on every loud bass thump. That's not my idea of good circuit design.
Another factor to consider is reliability. I chose a 700 mV trip point because that allows me to use a pair of NPNs for the detector. Those devices provide reliable protection. If you want a more sensitive circuit, you'll likely need an opamp in there. This adds complexity, which lowers reliability.
I did consider fancier lowpass filtering. It would be kinda neat to use a micro controller to sample the voltage across the speaker terminals and then filter it digitally. That could create a lowpass filter with a very steep cutoff that could allow for a low trigger voltage while also having a reasonable reaction time. But many would frown at the idea of doing anything digital at the output of an amp and at some point the idiom "stuffing 10 lbs of sh.... uh ... "dung" ... in a 5 lb bag" also came to mind.
I hope that sheds some light on the design tradeoffs.
Tom
Tom,
Thank you for your thorough and enlightening answer. You certainly shed a whole lot of light on your degin's tradeoffs, so much so that I plan to place an order on your webstore soon.
On Neurochrome's website the option to select Guardian-86 unpopulated PCB boards is not available, does that mean there is no stock? And, if so, do you have any idea when will you have them back in stock?
Thanks.
Thank you for your thorough and enlightening answer. You certainly shed a whole lot of light on your degin's tradeoffs, so much so that I plan to place an order on your webstore soon.
On Neurochrome's website the option to select Guardian-86 unpopulated PCB boards is not available, does that mean there is no stock? And, if so, do you have any idea when will you have them back in stock?
Thanks.
Yep. I'm out of the unpopulated Guardian-86 boards. Going forward I will only offer the fully assembled version.
If you'd rather solder it yourself, I suggest the Guardian-686.
Tom
If you'd rather solder it yourself, I suggest the Guardian-686.
Tom
Hi Tom,
Few months ago I built Modulus-286 integrated with your preamp in one box and I'm happy with it as a pig in mud .
It has been fitted with 2x Guardian-86 But unfortunately switching it of still comes with pops in the speakers.
Any advice ?
Thanks,
Stef
Few months ago I built Modulus-286 integrated with your preamp in one box and I'm happy with it as a pig in mud .
It has been fitted with 2x Guardian-86 But unfortunately switching it of still comes with pops in the speakers.
Any advice ?
Thanks,
Stef
Sorry. I missed your question. Apparently one can now watch a thread and not receive email notifications. I've changed that so that I will now be notified of new posts here.
Would you shed some light on how you solved the problem?
Tom
Would you shed some light on how you solved the problem?
Tom
No problem Tom.
It was simple. I just changed my toggle switch for a one with more pins to cut of supply to the guardians at the same time when I power off the whole thing.
It takes a minute or two before the supply voltage falls below 22 V so I imagine the guardian wasn't disconnecting the speaker when power was down.
It was simple. I just changed my toggle switch for a one with more pins to cut of supply to the guardians at the same time when I power off the whole thing.
It takes a minute or two before the supply voltage falls below 22 V so I imagine the guardian wasn't disconnecting the speaker when power was down.
Thank you for your reply.
Mains is ±235V with 2× 25 VAC 150 VA power transformers with ±27.5 of secondary output what gives me ±36.5V out of V Power-686.
I had 2×24 VAC Hammond before, which I had on a shelf from other project, but it was so bloody noisy and I replaced it with Toroidy ones.
Stef
Mains is ±235V with 2× 25 VAC 150 VA power transformers with ±27.5 of secondary output what gives me ±36.5V out of V Power-686.
I had 2×24 VAC Hammond before, which I had on a shelf from other project, but it was so bloody noisy and I replaced it with Toroidy ones.
Stef
tom,
for your specified mosfet IRFS4310ZTRLPBF, i cannot see any details regarding the current carrying limits when used under normal operating conditions, i.e. as link from amplifier output to the speakers.
for your specified mosfet IRFS4310ZTRLPBF, i cannot see any details regarding the current carrying limits when used under normal operating conditions, i.e. as link from amplifier output to the speakers.