(Solved) Changing the gain of the MC33078 OP-AMP?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 10, 2021, 09:36:16 pm
content: Hey all, I just spent 4 hours reverse-engineering the circuit of the Sound Blaster SB0220 microphone amplifier circuit. Attached is a schematic image that I made; I'm fairly confident there are no errors, and I have 0 idea how the whole thing works (I know practically nothing about OP-AMPS). I've drawn the schematic in such a way that it nearly mimics the actual PCB layout of components. The second image attached shows what the PCB looks like, with important landmarks drawn onto the image.The datasheet of the op-AMP ST MC33078 can be found (https://www.st.com/en/amplifiers-and-comparators/mc33078.html)How I currently use the sound card is shown in image 3; by applying 12v to the LM7805, attaching my microphone to the in-built 3.5mm jack, and then using aligator clips to connect to the wires which have the amplified signal.I want to increase the gain of the OP-AMP, because currently it's not very amplified. Based on my schematic, which component(s) should I change? I'd preferably *not* want to change the LM7805, as that supplies power to the STAC9708T and other components, and I don't know the values of the ceramic capacitors.Thanks.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 10, 2021, 10:20:05 pm
content: Attached is a *simplified* circuit diagram, drawn in a more traditional OP-AMP style. I still have no idea how it works lol. Help?

author: magic
author_rank: Super Contributor
timestamp: April 10, 2021, 10:44:21 pm
content: That's just two inverting stages with gains of 2.4x and ~9x, respectively. Gain is the ratio of the two resistors on IN- as usual, and IN+ is just some virtual ground. The first stage automatically biases the DC level of the signal to the virtual ground.The red capacitors impose some bandwidth limit and bandwidth will be reduced if you increase the feedback resistor.Reducing 10kΩ OTOH will increase loading on the mic and reduce low frequencies.It's probably easier to build a replacement from ground up on a perfboard and leave the poor card alone.

author: SuzyC
author_rank: Frequent Contributor
timestamp: April 10, 2021, 11:03:48 pm
content: In the PC control panel, under sound, in most MSoft OS's, there is a check-box option to increase mike gain by x10.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 10, 2021, 11:14:10 pm
content: Quote from: magic on April 10, 2021, 10:44:21 pmSo I assume output stage that attaches to the STAC9708 is the higher-gain stage, correct? Also, I don't really understand what you mean by "The first stage automatically biases the DC level of the signal to the virtual ground.".Quote from: magic on April 10, 2021, 10:44:21 pmWhat about changing either the 24k or 18k resistors on the OP-AMP? That way there will be no change to the mic load?Quote from: magic on April 10, 2021, 10:44:21 pmWhile I agree, I'm doing this to sort of challenge myself to repurpose old hardware (this is an old PCI card which I likely won't ever plug back into a PC, but if I can still use it and reduce ewaste, fantastic) and learn a bit at the same time.Drawing both of the schematics was a fantastic learning experience for me, and being able to tinker with (the correct) resistors and understand how they affect the op-amp will also be a further good experience.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 10, 2021, 11:16:34 pm
content: Quote from: SuzyC on April 10, 2021, 11:03:48 pmCurrently have my microphone at 100, and mic boost at 0db.Adding any microphone boost SIGNIFICANTLY increases noise, and even having my microphone at 100 also has introduced a lot of noise. Since the MC30778 appears to be a fairly good OP-AMP with very low THD+N, I want to increase the gain of the OP-AMP, so I can decrease my PC's gain.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 10, 2021, 11:32:31 pm
content: So I'm looking at my schematic again, and why is it that the output of one OP-AMP feeds into the input of the other? By having two stages instead of one, is less noise introduced for the same amount of gain? Also, what would this type of OP-AMP circuit design be called?

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 11, 2021, 12:32:37 am
content: I now have another question; I'm watching Dave's video on OP-amps, and I saw him mention the virtual ground at approximately 30min in, and how it is produced.Since my OP-Amps input is 5v (from the LM7805), does that mean the max theoretical output voltage can be is 2.5V?

author: mikerj
author_rank: Super Contributor
timestamp: April 11, 2021, 02:26:52 pm
content: Quote from: LooseJunkHater on April 10, 2021, 11:32:31 pmThere are two gain stages in this case because the microphone signal is routed to two different devices that have different signal level requirements. The second gain stage goes to the "TAD" connector, which stands for "Telephone Answering Device". Iit was possible to connect a soundcard to a modem to create an answering service or for hands free telephone operation back when land lines still ruled.The first stage connects to the STAC9708T codec which is the one of interest to you. The gain of this stage is determined by the 10k and 18k resistors connected to pin 2 of the op-amp. Doubling the value of the 18k resistor will double the gain, but the capacitor you see connected between pins 1 and 2 of the op-amp rolls off the frequency response at a frequency determined by both the capacitor value and the 18k resistor. Just doubling the resistor will halve the cut-off frequency i.e. greatly reducing the top end of your microphone signal, to restore this you would need to halve the capacitor value.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 15, 2021, 11:58:40 pm
content: Alright one last question; since my OP-Amps input is 5v (from the LM7805), does that mean the max theoretical output voltage can be is 2.5V, due to the use of the virtual ground?

author: ledtester
author_rank: Super Contributor
timestamp: April 16, 2021, 12:31:14 am
content: Quote from: LooseJunkHater on April 15, 2021, 11:58:40 pmWith a 5V supply difference the MC33078 is only able to get within a volt or so of the supply rails. Have a look at Figure 3 "Output voltage vs. Supply Voltage" on page 6 of the datasheet. On the x-axis look at supply voltage = 2.5V since that corresponds to a supply difference of 5V.So if your supply rails are +5V and 0V, the output can range between about 1V and 4V.The output voltage swing will also depend on the output impedance. Table 3 on page 4 gives you an idea of what that is like for voltage rails of +/- 15V and various output loads.More on output voltage swing:https://microchipdeveloper.com/asp0107:output-voltage-swingIf your virtual ground is 2.5V, the maximum amplitude of the op-amp output will be 4-2.5 = 1.5V.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: April 16, 2021, 12:39:56 am
content: That was a fantastic explanation that I understood!!! Thanks!