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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Fluctuating high-pitch noise in op-amp circuit
Fluctuating high-pitch noise in op-amp circuit
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Old 21st November 2019, 09:05 AM   #31
ubergeeknz is offline ubergeeknz  New Zealand
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Ah. Maybe time to ditch and cut your losses

Last edited by ubergeeknz; 21st November 2019 at 09:06 AM. Reason: Read the first post
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Old 21st November 2019, 09:14 AM   #32
Rolox is offline Rolox
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Quote:
Originally Posted by ubergeeknz View Post
Ah. Maybe time to ditch and cut your losses
This is ******* me off so much. what in hell could cause this?
I've tried every other outlet and disconnect all possible sources of interferences in the house, no results

I don't get it

it won't be a big financial loss but that thing has so much potential to sound good - guess I'll be stuck with my old TDM crossover again. which is dead silent - but doesn't sound half as good.
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Old 21st November 2019, 09:16 AM   #33
Rolox is offline Rolox
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Could it be some failed part somewhere causing this? in the circuit? in the PSU?
I mean come on... it's such a basic device with such basic circuitry... this is nuts.
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Old 21st November 2019, 09:23 AM   #34
ubergeeknz is offline ubergeeknz  New Zealand
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Possibly? Do you know anyone who is geared up to analyze it more closely?
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Old 21st November 2019, 10:20 AM   #35
Rolox is offline Rolox
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OK another update:

I put back the original JRC NE5532 in.

the whistling noise seems to be gone (but I will leave it on all day to check if it comes back...)

...if the whistling noise indeed goes away when I put the slower NE5532 in the circuit, that means either:

-OPA2132 was oscillating and that what we heard (although how can OPA2132 oscillate in all but the most inadequate circuit is beyond me, it's like the easiest chip to use as upgrade) maybe because I put a FET op-amp in a circuit made for bi-polar devices?

-the presence of faster chips makes the circuit prone to pick interferences, but interferences should vary depending on location, etc... here it doesn't seem to.

The reason why I bought this crossover was not to use it with any faster IC op-amp... I wanted to use it with Sparkos discretes, wich are supposedly triple compensated etc to make them almost universal. I wonder how those would behave in this circuit. I know Sparkos Labs would let me re-send them for a refund if they don't work in my device but I need to be sure I don't damage them...
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Old 21st November 2019, 02:49 PM   #36
indianajo is offline indianajo  United States
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Okay, Rf interferance.
Is it in a steel box connected to mains safety ground or water pipe from water table? Is there a choke on each + and - from power supply - I use 11 turns on toroid form. If AC driven, is power supply walled off from circuit by ferrous steel? Toroids move to board side of wall. Do all entry signals to quiet compartment have 33 to 68 pf disk caps to signal ground on them? Does line level exit have a choke in series with it? 11 turns around china marker is usual.
If not you have more work you can do. Plus the feedback resistor bypass caps. What slew rate is OPA amp, 20 v/usec? I'm not looking up, I don't want one. NE5532 is 6.
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Old 21st November 2019, 03:03 PM   #37
Rolox is offline Rolox
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Originally Posted by indianajo View Post
Okay, Rf interferance.
Is it in a steel box connected to mains safety ground or water pipe from water table? Is there a choke on each + and - from power supply - I use 11 turns on toroid form. If AC driven, is power supply walled off from circuit by ferrous steel? Toroids move to board side of wall. Do all entry signals to quiet compartment have 33 to 68 pf disk caps to signal ground on them? Does line level exit have a choke in series with it? 11 turns around china marker is usual.
If not you have more work you can do. Plus the feedback resistor bypass caps. What slew rate is OPA amp, 20 v/usec? I'm not looking up, I don't want one. NE5532 is 6.
Here's a picture; there's no chokes as far as I can tell, and the PSU section isn't separated from the rest. Do you think a simple steel plate around the PSU could help?
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File Type: jpg psu2.jpg (520.7 KB, 86 views)
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Old 21st November 2019, 03:54 PM   #38
indianajo is offline indianajo  United States
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Yes if it is not self-oscillating. Plate has to be bonded so it is connected electrically to the safety grounded case. Transformer can stop RF but the wires to it from line cord have to be short so they don't act as transmitters. There are IEC sockets with RF trap in them if this has unprotected one of those to the line cord.
I tack the 33 pf caps right on top of the feedback resistor with short leads. I don't see them. 22 or 47 could help too.
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Old 21st November 2019, 05:00 PM   #39
Rolox is offline Rolox
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Originally Posted by indianajo View Post
Yes if it is not self-oscillating. Plate has to be bonded so it is connected electrically to the safety grounded case. Transformer can stop RF but the wires to it from line cord have to be short so they don't act as transmitters. There are IEC sockets with RF trap in them if this has unprotected one of those to the line cord.
I tack the 33 pf caps right on top of the feedback resistor with short leads. I don't see them. 22 or 47 could help too.
If you look at the picture there's a thick white line on the board; it looks like it was meant to accept some kind of shield, there are even pads... I should check if those pads are connected to ground.

I tried an Eupen filtered cable with magnetic powder coating, but it's just a add-on, it doesn't come to the IEC socket, it comes between the power strip and the device's mains cable. It didn't help. I put small ceramic caps between hot and ground on the RCA jacks; it didn't help. The RFI (if it is RFI) must enter someplace else. Maybe the whole pCB acts as an antenna...
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Old 21st November 2019, 07:10 PM   #40
indianajo is offline indianajo  United States
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Your line cord acts as an antenna. Inductive trap for RF has to be near the entry of line cord to enclosure.
There are packaged coils you can buy, dead flat screen TV's & switcher power supplies are full of them. About 8 turns standing up. The ones at the exits of PA amps are about 10-14 microH - or 11 to 14 turns 1 cm diameter.
Yes, the line between power supply & quiet zone looks like good place to put a steel bulkhead. Stainless star washers on scratched paint can bond a steel plate to another plate (like the grounded case) if held tight enough. Realize when steel case top is off shielding bulkhead is only effective against hum, not RF.
Whole pcb will act as antenna if RF is not kept out of the case.
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