Good afternoon all, my 70s integrated amplifier suffers from emi / rfi type interference particularly in the phono section. The phono section features op amps which I believe can be culprits for that kind of interference. My question is... would it be possible to use the newer type of op amp with emi filtering as a possible cure. The current op amps are LM1458 and have VCC + and - of 15.5 v applied to them. Any suggestions for a suitable replacement would be very welcome.
Also I would like a little more gain from the phono stage as it can be fairly quiet on some records. would lowering the resistor value a little at the beginning of the phono input stage do this? It is currently a 1k resistor so would say an 800 ohm resistor give it that bit extra.
Many thanks.
Also I would like a little more gain from the phono stage as it can be fairly quiet on some records. would lowering the resistor value a little at the beginning of the phono input stage do this? It is currently a 1k resistor so would say an 800 ohm resistor give it that bit extra.
Many thanks.
Those audio chips have a GBW of 1Mhz considering that modern audio chips have many times that --IE- LM833=16Mhz I don't think its your chips that are causing the problem.
Many electronic devices in your home can cause this even a DECT home phone base or a neighbour .
I have spent years with the UK public on precisely this subject relating to various household electronic devices and it requires a process of elimination .
The phono section of an old amplifier is usually the most sensitive part of it so first step disconnect any input leads from it ---do you still hear any interference when the volume control is advanced ?
Are you sure its radiated interference and not a faulty resistor/capacitor in the amp ?
Move the amp to a different room --- still hear the noise ?
Rotate the amp to present a different angle to any RF interference.
There is a LOT more to this ,the above is only preliminary tests.
Many electronic devices in your home can cause this even a DECT home phone base or a neighbour .
I have spent years with the UK public on precisely this subject relating to various household electronic devices and it requires a process of elimination .
The phono section of an old amplifier is usually the most sensitive part of it so first step disconnect any input leads from it ---do you still hear any interference when the volume control is advanced ?
Are you sure its radiated interference and not a faulty resistor/capacitor in the amp ?
Move the amp to a different room --- still hear the noise ?
Rotate the amp to present a different angle to any RF interference.
There is a LOT more to this ,the above is only preliminary tests.
Low pass filter close to input connectors would probably help.
Otherwise OPA1678 is relatively new part with really good performance and low price. There is paragraph describing "EMI Rejection Ratio" in datasheet so I guess TI took this matter in considerations in design of this chip.
Otherwise OPA1678 is relatively new part with really good performance and low price. There is paragraph describing "EMI Rejection Ratio" in datasheet so I guess TI took this matter in considerations in design of this chip.
Try first moving the turntable very close to the amplifier, and fold up the turntable leads into
a small bundle. This can help reduce the problem if it is mainly pickup from the leads.
If still problems, try ferrite beads on the wire from each phono input hot lead RCA socket to the board.
While you're in there, add a 0.01uF ceramic capacitor from each RCA ground lug directly
to the chassis, with very short leads. Add a ground lug if necessary.
a small bundle. This can help reduce the problem if it is mainly pickup from the leads.
If still problems, try ferrite beads on the wire from each phono input hot lead RCA socket to the board.
While you're in there, add a 0.01uF ceramic capacitor from each RCA ground lug directly
to the chassis, with very short leads. Add a ground lug if necessary.
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Thank you for your reply. I dont think the chips are causing the problem but was hoping something newer/ better may filter out the problem. The amp already has some tiny rated disc capacitors from the rca leads to chassis ground. This has had me baffled for years so just hoping a swop of the op amps may improve things.
Thanks for all the comments so far, great stuff.
Thanks for all the comments so far, great stuff.
As I said new audio chips have a higher GBW factor which will INCREASE the RF pickup ,fix the noise problem first before you change the audio chips .
Here is the definitive RF help in audio applications in easy to understand information in separate boxes with detailed colour photos, its an easy reference for those wanting a simple help and advice PDF.
Microsoft PowerPoint - AES-RFI-SF08.ppt - AES-RFI-SF08.pdf
Here is the definitive RF help in audio applications in easy to understand information in separate boxes with detailed colour photos, its an easy reference for those wanting a simple help and advice PDF.
Microsoft PowerPoint - AES-RFI-SF08.ppt - AES-RFI-SF08.pdf
OPA1678 has built-in RF filtering and input noise voltage and current densities that are suitable for MM cartridges, but it makes sense to first try split-core ferrites at the end of the cable that goes into the amplifier input, or some of the other suggestions made in this thread.
Really old designs (and this one has to be from the late '70s or so) often have inadequate input filtering, not the least in order to keep input capacitance low (in those days, AT70/AT71E would have been popular OEM cartridges, and those work best with 47 kOhm || 100-200 pF, cable included, leaving ~70 pF max for the phono input). Their grounding setup may leave them generally more susceptible to RF ingress as well (the aforementioned tiny disc caps being an indication of such a layout - you usually have the least trouble when input ground is bonded to the back panel, but don't even think about a retrofit).
Is a schematic of the amplifier in question available?
Technics integrated amps of the '80s and '90s generally had rather comprehensive input filtering, with series inductors and all. A lot of manufacturers were too cheap for that.
Is a schematic of the amplifier in question available?
Technics integrated amps of the '80s and '90s generally had rather comprehensive input filtering, with series inductors and all. A lot of manufacturers were too cheap for that.
Thanks for your reply. It's probably from 1975 and I have never managed to find a scematic for it. It is similar to some of the old sherwood designs but different especially in the phono stage.
I am thinking about putting small ferrite beads arond the axial legs of the input resistor. I have seen a scematic for another phono pre amp that shows this. Presumably it would help in emi filtering but I guess could cause other problems.
Thank you everybody for your suggestions.
I am thinking about putting small ferrite beads arond the axial legs of the input resistor. I have seen a scematic for another phono pre amp that shows this. Presumably it would help in emi filtering but I guess could cause other problems.
Thank you everybody for your suggestions.
Those ferrite beads certainly won't hurt but I doubt they'd be very effective below, say, 10 MHz. You may be better served by replacing it with an R-L combo. Board layouts from those days can certainly be .... less than ideal ... from an RF POV.
MC1458 are dual µA741s so fairly crummy by modern standards. I'd try some 4558s, which should still be slow enough for the sloppy layouts of the day (though faster already) and lower noise to boot.
I rather doubt 1975, I don't think I've ever come across any consumer audio device with "modern" DIP package dual opamps pre-~1978 (even though MC1458 would technically have been available since 1971 according to the datasheet). The Japanese used the TA7136P single SIP-7 affair (an oddball by today's standards) as early as 1974, but generally speaking your average audio device in 1975 was all-discrete save for the tuner section.
BTW, the assertion that "higher slew rate = more EMI-prone" seems dubious to me. For all I know, the lower a current the (bipolar) input transistors are running at, the more prone they are to RF rectification. Basically it comes down to the ratio of device current and rectified current.
MC1458 are dual µA741s so fairly crummy by modern standards. I'd try some 4558s, which should still be slow enough for the sloppy layouts of the day (though faster already) and lower noise to boot.
I rather doubt 1975, I don't think I've ever come across any consumer audio device with "modern" DIP package dual opamps pre-~1978 (even though MC1458 would technically have been available since 1971 according to the datasheet). The Japanese used the TA7136P single SIP-7 affair (an oddball by today's standards) as early as 1974, but generally speaking your average audio device in 1975 was all-discrete save for the tuner section.
BTW, the assertion that "higher slew rate = more EMI-prone" seems dubious to me. For all I know, the lower a current the (bipolar) input transistors are running at, the more prone they are to RF rectification. Basically it comes down to the ratio of device current and rectified current.
RF pickup is usually caused by unwanted rectification in the input transistors, I don't think the opamp gain bandwidth product is much to do with this, whereas the input transistors' efficiency as detector diodes is, and also the ease an RF signal can get from the input jack to the chip.
JFET input opamps are typically a lot less sensitive to this interference as the input devices don't have a forward biased junction.
I suppose high bandwidth devices may pass on the issue to the next stage in the chain so its not clear whether that's an advantage or not.
You definitely want to try to stop RF getting to the chip in the first place, once rectification has happened its too late. 100pF ceramic across the input jack right on the jack is probably a good start and is usually not going to exceed the load capacitance budget unless the leads are long.