Looking at a schematic from a C Audio RA series amps. I noticed there is capacitors placed between the pots. Is this to block DC that the pots could create? Schematic attached:
The pots do not create DC but the opamp will have a small DC offset which should be blocked. Also the inputs are DC coupled and so any DC present there would be amplified and passed along.
Pots can be noisy when turned if DC is present. The caps at the output are good practice to also block DC.
Pots can be noisy when turned if DC is present. The caps at the output are good practice to also block DC.
Thanks Mooly, yes, it makes sense now you have explained it.
It seems that the opamp is being run on 18V supply rails dropped via 18v Zeners from HT rails. The Opamp is TL074 which the max rails seem to be 15V. It is common to run opamps at the specified absolute voltage max limit set out in data sheets? I'm guessing they are pushing the opamp hard to get a larger output voltage swing?
It seems that the opamp is being run on 18V supply rails dropped via 18v Zeners from HT rails. The Opamp is TL074 which the max rails seem to be 15V. It is common to run opamps at the specified absolute voltage max limit set out in data sheets? I'm guessing they are pushing the opamp hard to get a larger output voltage swing?
The maximum supply for the TL0 series of opamps is 36 volts and it is fine to run them at that 24/7. Its not an issue at all.
The TL074 is FET input opamp and that actually means that the DC offset from the opamp will be very close to zero at all times. The one on the diagram is shown as an MC33079 which is not FET input and so would have much more significant offsets. Another difference is that the TL0 series (which tbh I am a huge fan of for audio) have quite a limited output current drive ability compared to some other opamps and that is something that can be important in true professional audio applications. In other words a TL0 will not fully drive a 600 ohm load. For most uses that is not important but that is one of the known downsides to the TL0 series. Around 2k is the minimum loading to still achieve full voltage swing at the output.
Could someone have swapped the opamp in the past?
The TL074 is FET input opamp and that actually means that the DC offset from the opamp will be very close to zero at all times. The one on the diagram is shown as an MC33079 which is not FET input and so would have much more significant offsets. Another difference is that the TL0 series (which tbh I am a huge fan of for audio) have quite a limited output current drive ability compared to some other opamps and that is something that can be important in true professional audio applications. In other words a TL0 will not fully drive a 600 ohm load. For most uses that is not important but that is one of the known downsides to the TL0 series. Around 2k is the minimum loading to still achieve full voltage swing at the output.
Could someone have swapped the opamp in the past?
I'm going to disagree with Mooly, something I rarely do. TI says here the TL074 is a 30V part which I've always seen by every manufacturer as "Absolute Max Voltage" of 36V, which means if the regulators/zeners are a little bit overvoltage and you have 36.1V, the manufacturer no longer guarantees it will work. I recall a few years back going around with someone intentionally operating an opamp at the absolute max.
https://www.ti.com/product/TL074
But now TI has muddied the waters. Not only does that page suggest a higher VCC rated TL074H (40v operating voltage), the TL074 data sheet lists both of these parts, and under "Features" it says 40V for all parts. Yet under Absolute Maximum Ratings p. 12 it says:
https://www.ti.com/lit/ds/symlink/tl074.pdf
Supply voltage, VS = (V+) – (V–) All NS and PS packages; All TL07xM devices 36
All other devices 42
Also, read note (1) on that page.
I didn't find the thread, but I found where I mentioned it earlier. My question at the end remains.
https://www.diyaudio.com/community/threads/lm3886-schematics-pcb.162099/page-34#post-6637181
https://www.ti.com/product/TL074
But now TI has muddied the waters. Not only does that page suggest a higher VCC rated TL074H (40v operating voltage), the TL074 data sheet lists both of these parts, and under "Features" it says 40V for all parts. Yet under Absolute Maximum Ratings p. 12 it says:
https://www.ti.com/lit/ds/symlink/tl074.pdf
Supply voltage, VS = (V+) – (V–) All NS and PS packages; All TL07xM devices 36
All other devices 42
Also, read note (1) on that page.
I didn't find the thread, but I found where I mentioned it earlier. My question at the end remains.
https://www.diyaudio.com/community/threads/lm3886-schematics-pcb.162099/page-34#post-6637181
🙂 It is allowed.
That is a new one on me... interesting. TI did their own thing with the LM833 offering two versions, one the original and one with a quasi output stage. It needs great care in ordering to make sure you get exactly the version you want.
Does this work, post #79684:
https://www.diyaudio.com/community/...orch-preamplifier-part-ii.146693/post-4675868
That's quite "interesting" that you could do that and not damage the chips, and maybe within the timeframe but that's not the thread I'm thinking of. It was maybe 40-80 posts, the poster was obstinate that he could run an opamp at Absolute Maximum Ratings and not expect any problem, wanting to get the largest peak-to-peak signal output as possible. I and several others were trying to talk him out of it.
I can't disagree with what you say on maximum ratings, they are just that, an absolute that should never be exceeded. Coincidentally another thread also has TL082's running on a Zener stabilised 18v supply.
I suspect these things become more significant once you start running the chip at multiple 'maximums' such as supply, temperature of the package and perhaps with it also running into a short circuit (which should be survivable indefinitely).
Its an interesting one 🙂
I suspect these things become more significant once you start running the chip at multiple 'maximums' such as supply, temperature of the package and perhaps with it also running into a short circuit (which should be survivable indefinitely).
Its an interesting one 🙂
The classic problem is if the bias current for some subsequent stage flows through the pot wiper terminal - if the pot is even a bit scratchy this causes the subsequent stage to saturate on any intermittency (slamming its output to one of the rails which is usually very very loud). However if the bias current is down in the picoamp range this effect is strongly reduced as stray capacitance can fill in the gaps. This is why you usually see FET opamps in volume and tone circuitry, as this reduces the number of caps needed, or at least allows the wiper cap to be much smaller in value.
It seems later in production life these amplifiers had 'flat pack' output MOSFETs fitted and change in opamp to MC33079 as stated in the schematic.
I think there is plenty of amps out there that run the opamps on 18v rails.
I think there is plenty of amps out there that run the opamps on 18v rails.
I think C4 and C14 are related to the opamp for inverting for bridge mode in case there is DC offset in that opamp?
I was wondering why the designer didn't use 1% resistors around input opamp and a pot in the feedback loop around the opamp for better CMR precision. I'm sure tight production costs were considered here. The use of BF422/23s too plenty of them in whole circuit. I believe they have low COB
I was wondering why the designer didn't use 1% resistors around input opamp and a pot in the feedback loop around the opamp for better CMR precision. I'm sure tight production costs were considered here. The use of BF422/23s too plenty of them in whole circuit. I believe they have low COB
These were very commonly used in CRT drive circuitry such as here. Also used in push pull CRT drive stages.