Where did you get the OPAs? Most online are fakes. If you didn't use a reputable company, they might be cheap fakes.
Not sure how experienced as a phono designer you are, but there are a few very good reasons behind having a pre-pre stage with 10x gain for mc carts to get the best possible SNR with sufficient headroom and +-5V supply is absolutely ok for that. A lot less nightmares can happen with a +-5v supply .
Why you opted for a 40db stage for mc carts ...I have no ideea.
Then what makes you think it's not an isolated case?
DC blocking was used by famous manufacturers in very high valued models with no bad consequence.Static buildup can destroy any capacitor, especially electrolitic ones that you absolutely need for mc inputs.You're not allowed to have it by turntable's own design.
Did you have the cartridge shield, tonearm, turntable chassis connected to ground? If you didn't use some corck on the platter there shouldn't be any static buildup in a normal commercial turntable.Have you used a modified cartridge that has no ground pin connected to shield?What made you put an emphasis esd protection in the first place? Only the fact that this can happen?
Last edited:
I refrained from mentioning internal ESD protection diodes as I couldn't see how conditions to burn them would be met in your case. They are rated for 10 mA continuous and several hundred mA pulse. I know first hand that they sustain repeated 50 mA pulses with 50 ms duration.
Well, I have several burned TI opamps with low resistance between supply pins. 🙁
They were all burned accidentally on the breadboard with applying at inputs voltage above supply rails at the wrong end of input current limiting resistor.
Having resistor at MC input (<1K) would protect opamp or save expensive cartridge if failure conditions were not coming from the MC input.
I would suggest that too. However, due to the high capacity, which is definitively necessary for a sufficiently low f3, an electrolytic capacitor (polarized cap) must be used.
This means, that instead of a symmetrical supply voltage, an asymmetrical voltage is used (e.g. +30V instead of +/-15V, GND is in this case the neg. pole at the same time).
Furthermore, it should be noted that although there is now DC protection, there is no protection against switching on and off noises (switch on pops).
Basically the same issue with amplifiers and loudspeakers is well known - however, most people do not think that these voltage peaks, which may always arise during the switching on and off process, can also destroy the coils on MC pickups.
Using the MC input as an output and connecting it to another source input is a good way to examine it.
In a similar case I started this thread:
https://www.diyaudio.com/community/...-le-pre-pre-from-magazine-laudiophile.344343/
I have not yet been able to find a satisfactory solution to this in a commercial available MC prepre resp in a RIAA preamp with MC input.
Actually one need a speaker protect unit e. g. with TA7317P - go to
https://pdf1.alldatasheet.com/datasheet-pdf/view/86921/TOSHIBA/TA7317P.html
but with relay contact appropriate for very small signal levels - maybe one of this versions:
https://www.teledynedefenseelectronics.com/relays/Pages/home.aspx
found under
https://www.diyaudio.com/community/threads/best-audio-relays.153935/page-6
P.S.: this thread is filed wrong - better it would be filed under "Analog Sources" (Cartridges, Phono Stages)
Last edited:
You might be interested in this thread, especially the post here:
https://www.diyaudio.com/community/threads/coupling-caps-for-phono-prepre.360126/#post-6340351
https://www.diyaudio.com/community/threads/coupling-caps-for-phono-prepre.360126/#post-6340351
I would definitely look at the power supply angle. Did the OP verify that in operation? What sort of power supply is used? Schematic?
BTW That soldering makes me feel very underperforming ...
Jan
Agree. Split-supplies should always have diodes to prevent reverse voltage especially during power-on and off. Reverse voltage would cause the OP's problem.
Ed
No oscillation, all that things are in initial post.
So once again, circuit is stable as rock, no oscillations, power supplies are simmetrical in power up period with soft start behaviour, no voltage drift or spike on iputs during power on or off, voltage on input is 0V, 0,00mV if you like. Voltage is stable, ultra low noise, etc.
No begginers mistakes here.
You can see in my initial post that this happened during playback, not during power on or off, no nothing.
I will put beads or small ferrite rings on input traces or wires. They will absorb good portion of static spikes , as their inductive character represent high impedance for short spikes of energy, plus some RF attenuation.
You don´t want to add anything in that place, it is a ultra low level signal input section, you try to avoid every component that can add distortion or noise.
Even parallel load resistors are super low noise type, resistors add current noise, in dynamic signal condition resistor can add another type of of noise as signal changes.
I am aware of those problems, I also design ad repair digital, and mixed signal stuff for long time now.
What does the MC IN ESD protection look like? Schottky's to the positive and negative supplies, which would probably also prevent latch-up triggering from pulses at the input, or some transzorb-like thing?
I'm sure a big semiconductor company like TI has a failure analysis department that can track down the failure and make beautiful electron microscope pictures of the failed components on the chip. The trick is to get them to believe that it is in their interest to do so. That will be difficult when you don't buy millions of these chips.
Fair enough 🙂 it was just the most likely scenario I thought. If there were an issue with some weird chip behaviour I would have thought there would be other reports (globally, not so much on here).
You say in post #1
Which suggests you have either experienced or are aware of similar failures with this chip. Do you have any references?
I had on repair couple (maybe 4) of commercial phono preamps which used OPA1641 or 1611 with same issue, one of brand/model was named in initial post.
So no, this is not isolated problem just on mine design, or my ignorance, as some members wish for .
I'm not panicking, I'm just warning and looking for a solution, OK?
There were reports also that some respected fet/tube phono preamps alto burned couple of cartridges, so it´s not just my fabrication of problem.
So no, this is not isolated problem just on mine design, or my ignorance, as some members wish for .
I'm not panicking, I'm just warning and looking for a solution, OK?
There were reports also that some respected fet/tube phono preamps alto burned couple of cartridges, so it´s not just my fabrication of problem.
Last edited:
That is interesting then and does swing the balance toward a chip issue or a very very specific issue with the way it is being used that hasn't been documented or encountered before.
No easy answer then 🙁
No easy answer then 🙁
Connect two anti parallel diodes between the two inputs and you will never have the problem again.
That's my experience.
Hans
That's my experience.
Hans
Assuming a electrostatic discharge triggers a catastrophic latchup - you have two options
1) Keep the trigger impuls off your pcb. I have some experience on this topic and can say this may be very difficult due to the nature of such pulses. Typical ESD pulses have amplitudes of several kV at rise times of 1nsec giving current rates in the ballpark of 10amp/ns. These incredible current rates generate a 10V voltage spike along 1nH - something like 1mm of wire. It is evident that there is no practical way to shunt these even with big SMD MLCC cap or with any clamping diode. All you can do is insert some series impedance that reduces current amplitude.
2) The electrostatic impulse has little energy content and is not destructive per se. It becomes destructive with a powered chip when the pulse triggers internal parasisitc thyristors. If you limit the supply current below the latching current, current flow will stop immediately after triggering and nothing is burned. I propose 47Ohm SMD resistor in each supply line of each IC with local blocking caps of 10nF.
Just my 2c
1) Keep the trigger impuls off your pcb. I have some experience on this topic and can say this may be very difficult due to the nature of such pulses. Typical ESD pulses have amplitudes of several kV at rise times of 1nsec giving current rates in the ballpark of 10amp/ns. These incredible current rates generate a 10V voltage spike along 1nH - something like 1mm of wire. It is evident that there is no practical way to shunt these even with big SMD MLCC cap or with any clamping diode. All you can do is insert some series impedance that reduces current amplitude.
2) The electrostatic impulse has little energy content and is not destructive per se. It becomes destructive with a powered chip when the pulse triggers internal parasisitc thyristors. If you limit the supply current below the latching current, current flow will stop immediately after triggering and nothing is burned. I propose 47Ohm SMD resistor in each supply line of each IC with local blocking caps of 10nF.
Just my 2c
