Hello,
i'm new to solid state... I was reading a shematic (phono preamp) with a TL072 op amp, and if i calculate the gain, i find 1260...
Is it really possible to amplify that much? Is a phono input so low?
I made 492200 / 390 ...
Thank you for the help...
i'm new to solid state... I was reading a shematic (phono preamp) with a TL072 op amp, and if i calculate the gain, i find 1260...
Is it really possible to amplify that much? Is a phono input so low?
I made 492200 / 390 ...
Thank you for the help...
That's also true, but at low frequency (~20 Hz) the stylus amplitude in the vinyl is very small. The voltage induced in the phono cartridge is a few tens of uV.
It is not just resistance. What is the Reactance of C4 10nFd?
160k ohms at 100Hz, 16k at 1kHz, 1,600 ohms at 10kHz. So just C4 and R2 define a gain of like 16,000/390 or 41 at 1kHz. I have grossly over-simplified but the trend is like EdGr says: sloping over the whole audio band with a wiggle in the middle. The so-called "RIAA curve" (RCA's New Orthophonic).
Time to go back to basics. In the 70's op-amps were all the rage. +/- 15V supply (so that is max output swing - minus overhead)
But let's take a 1K resistor to ground on the inverting leg of the op-amp input (the - side). And a 10K resistor as the feedback.
Then we get 10X amplification. https://www.arrow.com/en/research-and-events/articles/fundamentals-of-op-amp-circuits
Now, when you insert a RIAA feedback circuit, you get a RIAA amp.
https://sites.google.com/site/solidstatephonopreamp/the-build/riaa-equalization
There is PLENTY of information on the internet on this. And NEVER +/- 120V 😎
Good luck.
But let's take a 1K resistor to ground on the inverting leg of the op-amp input (the - side). And a 10K resistor as the feedback.
Then we get 10X amplification. https://www.arrow.com/en/research-and-events/articles/fundamentals-of-op-amp-circuits
Now, when you insert a RIAA feedback circuit, you get a RIAA amp.
https://sites.google.com/site/solidstatephonopreamp/the-build/riaa-equalization
There is PLENTY of information on the internet on this. And NEVER +/- 120V 😎
Good luck.
Calculating the network with reactances of the caps at 100Hz, 1kHz & 10kHz:
A (100) = 360x (51dB)
A (1k) = 78x (37dB)
A (10k) = 14x (23dB)
Given the MM specification of approx 2.0mV @ 1kHz (thus 0.2mV @ 100Hz & 20mV @ 10kHz):
100Hz: 0.2mV * 360 = 72mV
1kHz: 2.0mV * 78x = 156mV
10kHz: 20mV * 14x = 280mV
This may look strange, but the actual time constants in the RIAA network are 3180μsec, 318μsec and 75μsec.
Hence the assumptions of 0.2 @100 and 20 @ 10k are a bit off, a factor 2 approximately (rule of my thumb).
Then:
100Hz: 0.2mV * 360x * 2= 144mV
1kHz: 2.0mV * 78x = 156mV
10kHz: 20mV * 14x : 2= 140mV
The TL072 should have an open loop gain of well over 360x at 100Hz, it is 95dB => 56000x, so the gain margin is 95 - 51 = 44dB (160x)
Better do a simulation for more accurate results, I leave that to the simu-enthusiasts.
Attached the datasheet of the TL072, open loop gain and phase at page 20 upper right corner.
A (100) = 360x (51dB)
A (1k) = 78x (37dB)
A (10k) = 14x (23dB)
Given the MM specification of approx 2.0mV @ 1kHz (thus 0.2mV @ 100Hz & 20mV @ 10kHz):
100Hz: 0.2mV * 360 = 72mV
1kHz: 2.0mV * 78x = 156mV
10kHz: 20mV * 14x = 280mV
This may look strange, but the actual time constants in the RIAA network are 3180μsec, 318μsec and 75μsec.
Hence the assumptions of 0.2 @100 and 20 @ 10k are a bit off, a factor 2 approximately (rule of my thumb).
Then:
100Hz: 0.2mV * 360x * 2= 144mV
1kHz: 2.0mV * 78x = 156mV
10kHz: 20mV * 14x : 2= 140mV
The TL072 should have an open loop gain of well over 360x at 100Hz, it is 95dB => 56000x, so the gain margin is 95 - 51 = 44dB (160x)
Better do a simulation for more accurate results, I leave that to the simu-enthusiasts.
Attached the datasheet of the TL072, open loop gain and phase at page 20 upper right corner.
Attachments
Great, thank you for the replies and information... So when a designer claims 30 dB for a power amp, it's supposed with 1 KHz frequency?
A power amplifier has no frequency correction like a RIAA, so that should would apply from (near) dc to up maximum power bandwith frequency. 30dB at 1kHz should also be the same at 20Hz and 20kHz.
Save power amplifiers with a bad frequency response, and save power amplifiers with tone control and/or loudness filtering inside the closed loop gain. Those can do real strange things.
Save power amplifiers with a bad frequency response, and save power amplifiers with tone control and/or loudness filtering inside the closed loop gain. Those can do real strange things.