Cool find! Very similar noise specs to ad743/745, all the other parameters are better.
Absolutely not, that will directly raise the noise floor. Feedback resistors should be as low as the opamp can drive, 1k or so. The switch in the feedback network is a likely a noise problem, its contact noise will be amplified enormously as its in the sensitive part of the circuit, and should be removed and permanently soldered. Try tapping the switch with a pen or similar to see how microphonic it is...
https://eu.mouser.com/Semiconductors/Amplifier-ICs/Operational-Amplifiers-Op-Amps/_/N-4h00g?Keyword=ada4625-1&FS=True
what do you mean?
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Hi Dan,
Given that the company that manufactured this preamp is now out of business, could you at least give the make and model of the unit? You wouldn't be divulging any sensitive information.
The application you have there is pretty common, and you might even find the circuit was lifted from the app notes for the TL0 series op amps. Also, given the cost of this unit (you hinted it was not expensive), the shocking find that they used the TL0 series chips isn't a surprise. In fact, it could have used the 4558 originally (the single op amp version) and it wouldn't be any worse than many commercial products. I think we're both very used to seeing what we would consider complete rip-offs.
-Chris
Given that the company that manufactured this preamp is now out of business, could you at least give the make and model of the unit? You wouldn't be divulging any sensitive information.
The application you have there is pretty common, and you might even find the circuit was lifted from the app notes for the TL0 series op amps. Also, given the cost of this unit (you hinted it was not expensive), the shocking find that they used the TL0 series chips isn't a surprise. In fact, it could have used the 4558 originally (the single op amp version) and it wouldn't be any worse than many commercial products. I think we're both very used to seeing what we would consider complete rip-offs.
-Chris
You should be aware that the LT1028 has bias-current cancellation that injects correlated current noise into both inputs. This badly degrades the noise performance when the impedances at the two inputs are very different.
I can see no use for the LT1028 in audio. In a one-stage MM RIAA amplifier, my calculations show that it is considerably noisier than a 741. Nuff said...
I can see no use for the LT1028 in audio. In a one-stage MM RIAA amplifier, my calculations show that it is considerably noisier than a 741. Nuff said...
Thanks for responding. It was never my intention to use the LT1028 (far too expensive for this). I was surprised at how much quieter it was than the original TL071. However in double checking it is only appreciably quieter on MC where the two impedances are closer. In MM, not much difference.
Chris, I never said the company was out of business. This is an obsolete product from a very much alive company. Since I have a signed NDA, I can't say much more.
Chris, I never said the company was out of business. This is an obsolete product from a very much alive company. Since I have a signed NDA, I can't say much more.
Just out of curiocity, what source impedance did you use and did you measure with A-weighting or ITU-R 468 weighting?
TL071: 18 nV/sqrt(Hz) voltage noise and almost no current noise
LT1028: 3.4 pA/sqrt(Hz) current noise with unbalanced impedances and almost no voltage noise
Break-even source impedance: 18 nV/3.4 pA ~= 5.3 kohm
With RIAA- and A-weighting, that's roughly the effective impedance of a cartridge with an inductance of 0.25 H. It's around 0.16 H with RIAA- and ITU-R 468-weighting. For higher inductance MM cartridges, the TL071 should outperform the LT1028.
TL071: 18 nV/sqrt(Hz) voltage noise and almost no current noise
LT1028: 3.4 pA/sqrt(Hz) current noise with unbalanced impedances and almost no voltage noise
Break-even source impedance: 18 nV/3.4 pA ~= 5.3 kohm
With RIAA- and A-weighting, that's roughly the effective impedance of a cartridge with an inductance of 0.25 H. It's around 0.16 H with RIAA- and ITU-R 468-weighting. For higher inductance MM cartridges, the TL071 should outperform the LT1028.
I'm not using any kind of measurement here as of yet. Strictly audible noise. My next step is to put them on the test rig, and measure noise and distortion. Then I'll find the best compromise IC for MM and for MC. It will probably be neither of these opamps.
I'm also using just what I have on hand, so as to not spend any money on this.
You are right. The TL071 does outperform the LT1028 on MM. I hadn't checked that initially.
I'm also using just what I have on hand, so as to not spend any money on this.
You are right. The TL071 does outperform the LT1028 on MM. I hadn't checked that initially.
That will more or less boil down to choosing the op-amp with the lowest overall noise for a source impedance of about 1.2 kohm.
For a 500 mH moving-magnet cartridge and using RIAA- and A- weighting, the effective cartridge impedance is about 12 kohm.
The sensitivity of a moving-coil cartridge is typically about one tenth of the sensitivity of a moving-magnet cartridge.
When you noise-optimize for about a tenth of 12 kohm, noise current will affect the SNR with a moving-magnet cartridge about as much as the noise voltage will affect the SNR with moving coil.
For a 500 mH moving-magnet cartridge and using RIAA- and A- weighting, the effective cartridge impedance is about 12 kohm.
The sensitivity of a moving-coil cartridge is typically about one tenth of the sensitivity of a moving-magnet cartridge.
When you noise-optimize for about a tenth of 12 kohm, noise current will affect the SNR with a moving-magnet cartridge about as much as the noise voltage will affect the SNR with moving coil.
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