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JLH 10 Watt class A amplifier
JLH 10 Watt class A amplifier
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Old 22nd November 2020, 12:19 PM   #7471
kronzilla is offline kronzilla  Belgium
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Quote:
Originally Posted by OldDIY View Post
Bare copper wire on insulated racks with petals (like 1950)
But it's not good for the capacitpr's they will dry out very fast because they have the same temperature as the heatsink...
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Old 30th November 2020, 01:07 PM   #7472
Navitatis is offline Navitatis  Greece
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There is a pair of JLH 1969M amplifiers (MOSFET variation) and a 36V/5A SMPS to power them. During power on there is a very ugly loud noise on the output, as well as a current spike for a few seconds and then everything settles.

If the voltage is increased slowly using a bench power supply, the problem is mitigated and seems safe for the loudspeakers.

The SMPS cannot be soft started slowly enough to solve the issue, so I'm looking for a solution from the DC side, between the SMPS and the amplifier.

I've read that 36V DC is too high for conventional relays to operate reliably so I fiddle with a possible circuit using a mosfet to ramp up the voltage to the amplifier, as shown on the image.

My knowledge in electronics is limited while I'm climbing the learning curve, so I need some experienced opinions about this idea.

Regards
Attached Images
File Type: png Screenshot_20201130_142556.png (54.6 KB, 267 views)
Attached Files
File Type: asc test ps with mosfet n-channel.asc (1.3 KB, 3 views)
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Old 30th November 2020, 01:28 PM   #7473
kozard is offline kozard
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Perhaps try a slow turn-on LDO? Perhaps try the 5A LT1084 in a slow turn on configuration set for minimum dropout? Maybe try 1.5V dropout?

I wonder if a simple capacitance multiplier/ripple eater would have a sufficiently slow turn-on?

LT1083 kits are inexpensive and should be a fairly easy modification to add slow turn-on.


You should also strongly consider a speaker protection board with something like 240VAC/30A relays and a boot delay. I assume you are not using a speaker protection board?
Attached Images
File Type: png LT1083 LDO.png (360.1 KB, 260 views)
File Type: png Slow Turn On Modification.png (66.3 KB, 259 views)
File Type: png 30A 240VAC Speaker Protection Board.png (343.1 KB, 249 views)

Last edited by kozard; 30th November 2020 at 01:36 PM.
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Old 30th November 2020, 03:09 PM   #7474
Navitatis is offline Navitatis  Greece
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Hey Kozard, thanks for replying!

I have a few delay+speaker protection boards available but I don't want to use them in this amplifier because 1)already has a coupling capacitor at the output, 2)sounds clean (to me) and I'd prefer to avoid additional artifacts in the signal path.

The SMPS used is already regulated and smoothed enough with extra CLC filter, so additional LDOs or capacitance multipliers will add unnecessary complexity without obvious benefit, as I understand.

My point is only the slow turn-on, that as you mention, should be a fairly easy modification. Is there any suggested schematic of such modification?

Is the one I posted above valid for the purpose?

Or, alternatively, has anyone successfully solved the issue with the turn-on thump on mosfet variations of JLH 1969? There was some discussion about it in this thread long ago (i.e. page 67) but I couldn't spot any permanent solution.

Regards
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Old 30th November 2020, 05:13 PM   #7475
OldDIY is offline OldDIY
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Click the image to open in full size.There are various schemes for 1969M. The correct one has 0.5 ohm at the source and 220 ohm at the gate.
Capacitor C1 reduced to 4.7 microfarads.

Last edited by OldDIY; 30th November 2020 at 05:35 PM.
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Old 30th November 2020, 05:16 PM   #7476
Rallyfinnen is offline Rallyfinnen  Sweden
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*Maybe* increasing the value of the bootstrap cap could calm thing down a bit. This way the current in the output stage goes up slower. However, the input and output caps always needs to be charged when powering up the amp, so I think it will be hard to make it totally quiet.

An ugly way to do it could be to short the output during startup. Nothing in the signal path when the relay is opened, and the inrush current to the output cap should be limited to the Iq setting.
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Old 30th November 2020, 05:44 PM   #7477
kozard is offline kozard
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Quote:
Originally Posted by Navitatis View Post
Hey Kozard, thanks for replying!

I have a few delay+speaker protection boards available but I don't want to use them in this amplifier because 1)already has a coupling capacitor at the output, 2)sounds clean (to me) and I'd prefer to avoid additional artifacts in the signal path.
I believe that the relay will add totally inaudible and undetectable artifacts. At least several orders of magnitude below the output capacitor.

The purpose of adding the board is to have the boot delay. The amplifier stabilizes before the relays turn on (and thus eliminates the pops and bangs).



A simple solution (a bit like the standby switch on my tube amplifiers) would be two mechanical switches. The first turns on the power supply the second connects the speakers to the output capacitors. You just need to remember to turn them on with the right sequence and a little delay. I am naturally familiar with doing something like this with my tube amplifiers. That is a very simple and low cost solution.



Quote:
Originally Posted by Navitatis View Post
The SMPS used is already regulated and smoothed enough with extra CLC filter, so additional LDOs or capacitance multipliers will add unnecessary complexity without obvious benefit, as I understand.

My point is only the slow turn-on, that as you mention, should be a fairly easy modification. Is there any suggested schematic of such modification?

Is the one I posted above valid for the purpose?
I have several LT1083 regulators and many LM317/LM337. However I have not modified any to slow turn-on since I have the boot delay on the protection boards. However I understand that the slow turn on schematic (that I posted from Ti LM317) works with the LT1083/4/5 also.


I realize that the SMPS might be sufficiently regulated already. That is why I suggested just a minimal drop-out LDO or ripple multiplier setup just for the slow turn-on.


I have experimented with SMPS for my mini1969 and several class AB amplifiers also. So far I have not need additional filters or regulators. I have added additional LC filter sections and I have not been able to detect any difference even when I put my ear close to the tweeter. I have done experiments with the left channel running SMPS and the right running a regular transformer/rectifier/filter caps. I can move back and forth between the two speakers and I have not been able to detect an audible difference.




Quote:
Originally Posted by Navitatis View Post
Or, alternatively, has anyone successfully solved the issue with the turn-on thump on mosfet variations of JLH 1969? There was some discussion about it in this thread long ago (i.e. page 67) but I couldn't spot any permanent solution.

Regards

I don't know of such a solution, but there are many knowledgeable and helpful individuals on this forum and I am sure they will reply if they know of a circuit modification solution.

Last edited by kozard; 30th November 2020 at 05:48 PM.
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Old 30th November 2020, 05:45 PM   #7478
OldDIY is offline OldDIY
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Click the image to open in full size.
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Old 30th November 2020, 07:06 PM   #7479
john_ellis is offline john_ellis  United Kingdom
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I would suggest a capacitance multiplier would work. A complementary Darlington pair with MJ2955 and BD139 with the MJ2955 emitter to positive, and a 1mF capacitor from the base of the BD139 to ground, and 1k resistor to positive should work.
Simulated turn-on thump was 6V peak with the circuit "as is" (at least, my simulated version) and only 0.25V with the slowed risetime.

You will end up with something like a 2V drop across the series pass transistor, perhaps your PSU could be tweaked a little?
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Old 30th November 2020, 07:24 PM   #7480
kozard is offline kozard
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Quote:
Originally Posted by kozard View Post
I realize that the SMPS might be sufficiently regulated already. That is why I suggested just a minimal drop-out LDO or ripple multiplier setup just for the slow turn-on.

I meant "Capacitance Multiplier" or ripple eater. Too late to edit. Sorry about that.
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