Electronics > Beginners
Building a sound mixer within my guitar amp with line IN & mic input
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Yansi:
You absolutely have to have a C13 in there, if operating from single supply. However, its value shall be increased way above 1uF if you want to use electrolytics, which you want anyway, cause 1uF is just too low to have a decent bass response. Depending on the R14 in there, but I'd guess 22uF may be fine.

Output cap C5 shall be over 20uF also (even 47-100uF - not that uncommon to see).

The higher the value, the lower the AC voltage across it -> less distortion. And also less phase mangling at the lower end of audio spectrum.

And you are still MISSING the input EMI protection! Bypass both ends of C8 to ground using about 470pF caps and stick about 5 to 10 ohm resistor in series with both input pins.

C11 shall be at least 470uF! You need a very low impedance in between the emitters to obtain high gain and low distortion.

Use both C6 and C7 100uF.

Even your frequency plot shows the response on the low end is very inadequate for audio use. You can't have a -6dB roll-off on bass in a mixing desk.

I don't really see the value of Rop (R14, R18), but C9-C12 is likely limiting the frequency response upper end. There is no way this circuit would be that laze to chicken out at -3dB 20kHz.  You can't design audio this way. The frequency response shall be 50kHz+ to get the flattest possible response within 20Hz to those 20kHz.

Ain't nothing special analog mixing desks having bandwidths above 50kHz. It is to help to make transient responses better. Audio signal is not just a pretty sine-wave.  (But understand there is really not much point in pushing the response much further beyond say 70kHz, you will just welcome noise and spurious signal, that may mix on non-linearities in the circuit and cause a perceivable interference).

ALso, note the gain potentiometer shall be the C / Exponential type ("reverse log") for any practical use.
dazz:

--- Quote from: Yansi on June 06, 2019, 08:49:08 am ---You absolutely have to have a C13 in there, if operating from single supply. However, its value shall be increased way above 1uF if you want to use electrolytics, which you want anyway, cause 1uF is just too low to have a decent bass response. Depending on the R14 in there, but I'd guess 22uF may be fine.

Output cap C5 shall be over 20uF also (even 47-100uF - not that uncommon to see).

The higher the value, the lower the AC voltage across it -> less distortion. And also less phase mangling at the lower end of audio spectrum.

And you are still MISSING the input EMI protection! Bypass both ends of C8 to ground using about 470pF caps and stick about 5 to 10 ohm resistor in series with both input pins.

C11 shall be at least 470uF! You need a very low impedance in between the emitters to obtain high gain and low distortion.

Use both C6 and C7 100uF.

Even your frequency plot shows the response on the low end is very inadequate for audio use. You can't have a -6dB roll-off on bass in a mixing desk.

I don't really see the value of Rop (R14, R18), but C9-C12 is likely limiting the frequency response upper end. There is no way this circuit would be that laze to chicken out at -3dB 20kHz.  You can't design audio this way. The frequency response shall be 50kHz+ to get the flattest possible response within 20Hz to those 20kHz.

Ain't nothing special analog mixing desks having bandwidths above 50kHz. It is to help to make transient responses better. Audio signal is not just a pretty sine-wave.  (But understand there is really not much point in pushing the response much further beyond say 70kHz, you will just welcome noise and spurious signal, that may mix on non-linearities in the circuit and cause a perceivable interference).

ALso, note the gain potentiometer shall be the C / Exponential type ("reverse log") for any practical use.

--- End quote ---

Thanks so much Yansi. I'll change all those cap values you mentioned. Some of them I have so low because it didn't make a difference in the simulation (on the top end) and I'm used to guitar designs where that kind of low end response is usually fine. I didn't know a higher capacitance will entail lower distortion (BTW, I just got the book you recommended  :-+)

Great to know that C13 is needed. I'll leave it there.

Rop (R14, R18) is up at 68k (from 22k) to increase the gain a tad, but I'll probably need to tweak that value once it's built to maximize gain without distortion.

The gain pot is indeed an antilog taper in the simulation. It just seemed to increase the gain much more evenly than the other tapers.
Yansi:
Coupling capacitors typically limit only the frequency response on the low-end of the spectrum.

With those 68k,  100pF is definitely the limiting factor (1/2piRC = 23.4kHz). If you want to keep the two feedback resistors 68k, then instead of 100pF I'd lower them to something more reasonable like 33pF. You just want to limit the bandwidth slightly to not allow for high frequency noise and spurious signal, but you do not want (nor need) to low pass filter the audio signal at 20kHz.

Regarding the Rop=68k: It seems your maximum gain of the preamp is over 70dB. That is way to high for this circuit (SNR will become bad with such gain). I  think you should lower the Rop somewhat so the maximum gain will be about 60dB, not more.

The typical input gain preamp in a mixing console may provide a gain of about 10 to 50dB with the 40dB range tunable by the gain pot. If you want more than 50dB gain, you can always crank other pots on the mixing desk, to get additional 10-15dB more ;)

The minimum gain is basically set by the gain of the differential amplifier (opamp) part of the circuit. However, 10dB minimum gain is too much to allow for a line level signal to be injected into the MIC input, hence why mixer desks always allow means for attenuation the input signal by  10 to 20dB. Typicaly, there is a switch "20dB PAD" on the input, or the more common way is to implement two types of connectors: XLR for MIC level and a JACK (called typically also TRS 1/4") that is wired so that it attenuates the signal whenever something is plugged in - typically, just resistors in series with the signal that form a voltage divider together with the input imepdance of the MIC amplifier (10-20kohm LINE input impedance).

Keep in mind that those LINE jacks are TRS (tip-ring-sleev, ie. "stereo" type in laymen terms), but are used to transfer also BALANCED signal. For stereo LINE signal you need two separate TRS jack inputs.
Typically, TIP is the HOT side (positive polarity), RING is COLD (negative) and sleeves the ground as is standard.

This configuration allows for plugging an UNBALANCED signal, using just a TS jack ("mono"), which shorts the COLD (negative) input to ground.

I would recommend you to really look for some mixing desk service manuals/schematics to get a better idea how do these operate internally.

I have never built a full diy mixing desk myself (if I would, I would do it in digital domain now, hihi - I like DSP stuff), but I would encourage to do it. I have only built several specialized blocks, for example a separate quad MIC preamp such as yours, to turn LINE level only inputs on my mixer to another four MIC inputs.
Even if it will not be very useful result for practical use, you will pretty much learn a lot of the stuff during the process, which is why I love electronics.


dazz:
Yeah, it was the 100pF caps that were cutting off the high end. Obviously.
If I lower Rop to 6k7 I get a nice 7 to 50dB range of gain and lots of highs. I'll start there as per your suggestion 🍻
Yansi:
6k7 is a non-standard value, I'd round to 6k8, or even allow for slightly higher gain to fit to the 10dB minimum.

Also a tip - make the gain pot just 5k,  if you will increase the value further, the gain will not drop by much. Instead, it will make most of the turn of the pot do "nothing" and then the gain will go up. A don't forget to use the exponential (reverse log) type, with a linear one the gain can't be controlled nicely, as it will sharply peak just near the end of the turn.

If you will have troubles finding the reverse log ones (seen plenty on fleabay, Aliexpress and Tayda), you can of course use normal log one, however you need to wire it then so the gain is rising with turning counter-clockwise (which is not pleasant, but still better than a linear pot).
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