Operating the regulators in the thermal shut-down zone is certainly not a good idea anyway, and since this is class A, it will not be an isolated event, but a permanent situation caused by insufficient heatsinking.
That would be the thing to worry about, as a first priority.
If it does happen, the 20mA short-circuit current of the opamps will go into various parasitic junctions of the regulators, not the healthiest of situations, but not damaging either, and no worst than 180°C Tj undergone by the regulators in the first place.
A resistor in series with the outputs of the opamps would mitigate somewhat the effect, but 100ohm will make little difference on this point, and could already degrade the performances.
So I think it is better to live with it....
I guess I'll start at 30mA and work my way up, the problem is not so much the actual heatsinks but the sealed enclosure I plan on using. A pretty tea tin is really the equivalent of a tiny oven, but I suppose mounting the regulators on the bottom would help to a certain degree.
I can actually live with the low Iq since this particular amp will be driving some tiny output transformers. Really more of a bomb-proofed project than hifi.
I can actually live with the low Iq since this particular amp will be driving some tiny output transformers. Really more of a bomb-proofed project than hifi.
Hi guys!
Unfortunately I ran into some troubles with my build of Elvee's amplifier... I made a set-up a few days ago in order to ***** the behavior at different frequencies.
I made some ~8ohm rezistive loads, connected them to the board and powered it up... Previous tests with this amp, where I used a pair of 4 ohm speakers went well, although at some point I had the feeling that the sound is a little bit distorted. I blamed it, at that time, on the low impedance speakers...
Now, with the set-up I made, I saw that the output was oscillating
. I could measure a 1.24MHz distorted sine wave with a pk2pk amplitude of 0.8V. This amplitude was higher before the output cap and the sine less distorted. I also noticed that the opamp was getting a little hot (rough measurement with my finger).
This happens no matter if the input shorted to ground or connected to the signal generator.
The amplitude of the osculation is reduced when I lower the supply voltage to around 7V.
Any hints what may cause this?
The oscilation remains and modulates any signal I feed from the signal generator.
Maybe i'll manage to post some screenshots in a few days...
Unfortunately I ran into some troubles with my build of Elvee's amplifier... I made a set-up a few days ago in order to ***** the behavior at different frequencies.
I made some ~8ohm rezistive loads, connected them to the board and powered it up... Previous tests with this amp, where I used a pair of 4 ohm speakers went well, although at some point I had the feeling that the sound is a little bit distorted. I blamed it, at that time, on the low impedance speakers...
Now, with the set-up I made, I saw that the output was oscillating
. I could measure a 1.24MHz distorted sine wave with a pk2pk amplitude of 0.8V. This amplitude was higher before the output cap and the sine less distorted. I also noticed that the opamp was getting a little hot (rough measurement with my finger).This happens no matter if the input shorted to ground or connected to the signal generator.
The amplitude of the osculation is reduced when I lower the supply voltage to around 7V.
Any hints what may cause this?
The oscilation remains and modulates any signal I feed from the signal generator.
Maybe i'll manage to post some screenshots in a few days...
Well, I made some changes in the input stage. I've attached a pdf of the schematic.
Could this be the cause?
The Zobel network is in place, although I remember not finding 4.7ohms and placing two resistors in parallel to get a value as close as possible. I think it's somewhere around 4.9ohms...
The bias current seems stable at ~850mA. I did not trace it on the oscilloscope, only looked at the meters on the bench power supply. Also, the voltage on the rails is stable. What could cause oscillations on the power rails?
I used the LME49722 for this build and maybe this could also be the cause.
I hope I'll catch one or two days with more spare time to solve this problem... But right now I'm pretty tied up...
It would be very nice if someone else could build the amplifier with my layout to check if it's a layout fault or not. I've attached the full Eagle project.
Could this be the cause?
The Zobel network is in place, although I remember not finding 4.7ohms and placing two resistors in parallel to get a value as close as possible. I think it's somewhere around 4.9ohms...
The bias current seems stable at ~850mA. I did not trace it on the oscilloscope, only looked at the meters on the bench power supply. Also, the voltage on the rails is stable. What could cause oscillations on the power rails?
I used the LME49722 for this build and maybe this could also be the cause.
I hope I'll catch one or two days with more spare time to solve this problem... But right now I'm pretty tied up...
It would be very nice if someone else could build the amplifier with my layout to check if it's a layout fault or not. I've attached the full Eagle project.
Look no further: you have replaced a 4MHz part (LF353) by 55MHz one, more than 12x faster. Did you really expect it would go unnoticed?
First thing to do, replace your LME by a LF353, TLO82 or similar, to make sure there are no other problems, and then if it works properly you can begin tackle the grafting of the LME into the circuit.
I guess you will need to slow it down to the level of the LF353: place 4.7pF to 47pF capacitors directly between the output and the (-) input of each opamp.
I cannot guarantee it will do the trick, you may need to revert permanently to a more reasonable amplifier, or adopt other compensation strategies.
That's an unmapped territory.
Yeah... I was afraid it was from the LME amplifier.
I also wondered what would happen when using this fast amplifier in your schematic, without actually thinking about proper compensation. Indeed, the feedback loop is purely rezistive (at least until a certain frequency), so the addition of a negative pole to limit the frequency response of the amplifier would be the minimum necessary compensation.
So yeah, I just did what I read on the nwavguy's blog not to do!
Now... I must get my hands on lower speed amplifiers...
I also wondered what would happen when using this fast amplifier in your schematic, without actually thinking about proper compensation. Indeed, the feedback loop is purely rezistive (at least until a certain frequency), so the addition of a negative pole to limit the frequency response of the amplifier would be the minimum necessary compensation.
So yeah, I just did what I read on the nwavguy's blog not to do!
Now... I must get my hands on lower speed amplifiers...
It would make things one thousand times worst: the opamp would see the non-scaled version of the phase-shifted feedback signal in its full horror...
Did it do the trick for you?
When the bandwidth of the main gain element is grossly excessive, there are few options left.
The capacitor can be viewed as a feedforward or TMC compensation bypassing the output stage at higher frequencies
apologies for resurrecting an ancient thread, how large do the output caps need to be ? Can I get away with something like 1000uF/16V . My main concern is size. I designed a compact board to fit on top of a heatsink. I see in this prototype build the output caps are 1000uF, will that limit low frequency ?
Attaching a board layout file.