Do I need coupling caps for my audio pre-amp? - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: April 01, 2021, 08:10:00 pm
content: I'm sure this sounds crazy but I'm wondering because I built a pre-amp circuit that found online that claimed to be very quiet and it actually is. I get why you'd want a coupling cap with a BJT or if you're powering a logic chip with an unregulated supply and/or at high frequencies.With 1Vppk sinewave in I get a very clean signal out. The screen show shows both channels output. My music player maxes out at 660mV ppk out so no problem there. I built this to drive an LM 3886 power amp and at max gain on the pre-amp I get 18.6V pk out into 8 ohms resistive and still very little distortion. My circuit uses 7805 and 7815 linear voltage regulators to power the op amps and logic chips so perhaps that has something to do with why the signal is so clean? I do see about 200 mVDC offset but the op amp has tons of head room so not sure if that is an issue.Am I missing something?

author: ve7xen
author_rank: Super Contributor
timestamp: April 01, 2021, 09:03:07 pm
content: Do you mean coupling caps (in the signal path) or decoupling caps (on the power supply rails)? Your narrative mostly suggests the latter, but you also mention DC offset and the title suggest the former.Do your opamps need dedicated decoupling caps in this case? Probably not, audio is relatively slow, and I'm going to assume that with such a simple circuit, your layout puts the regulator output cap near to the opamp supply pins anyway. For testing this you'll want to run at the maximum frequency and maximum amplitude your design is intended to handle.Do you need AC coupling caps in this design? Well, if you're getting 200mV DC on the output, that is way too much, and if you change nothing else then yes you would need to add AC coupling at the output (or the input to the next stage). That does seem like an abnormally high amount of DC for this design though, I don't see any obvious-to-me place it would be coming from. Are you sure your TL072s are genuine? By eye, this offset also only appears on CH1.

author: hummusdude
author_rank: Regular Contributor
timestamp: April 01, 2021, 10:21:17 pm
content: Yes, I meant coupling. thanks for clarifying. I sometimes mix the two up.I tried a few values of coupling caps at the output of the pre-amp and 2.2nF seems to work well. There's no more offset ( on either channel) and frequency response is still good at the low end. Curious that the 2.2uF caps at the output of the pre-amp stage didn't block the dc. As for genuine I sourced them at Mouser so I'm assuming they're fine.QuoteMy thought as well. I haven't yet started on the pcb layout but currently the longest distance is about 6 inches from the 7805 to the Arduino Nano and it doesn't seem to be having any problems. The only thing it does is control the various triggers to drive the dpot that is part of the feedback resistance of the opamp. I imagine that all of the logic will be much closer to the 7805 when I do the layout.

author: ve7xen
author_rank: Super Contributor
timestamp: April 01, 2021, 11:03:09 pm
content: Quote from: hummusdude on April 01, 2021, 10:21:17 pmI assume that they are blocking any DC in the first stage, and that the DC is coming from the final stage. Does twiddling the pots affect the DC bias at all? I'm not sure where it could be coming from in this circuit. To my understanding, there isn't enough gain available here to push the input offset voltage that high, and the TL072 is JFET input and has tiny bias currents.The capacitance you need for the coupling caps will be defined by the corner you're looking for, but also the load impedance. If that's known because this is part of an integrated system, then you can calculate the capacitor you need based on the load impedance. Otherwise you'll probably want to assume at most maybe 10k. You'll need about 1uF to put the corner below 20Hz with a 10k load.

author: hummusdude
author_rank: Regular Contributor
timestamp: April 01, 2021, 11:44:27 pm
content: QuoteThat was my guess as well but I forgot to mention that I had disconnected the pre-amp from the tone circuit for testing and that is when I noticed the offset on the pre-amp output. The gain is quite low...about 1.8 or so. The reason for the pre-amp is that my power amp doesn't give full gain with the 660 mV ppk that my phone/laptop (even less output) can provide.QuoteI understand 1/(2*pi*f*c) but I confess calculating the input impedance of the tone circuit is a bit above my pay grade. The 2.2uF on the output of the pre-amp was the designer's choice. As I said before, I'm surprised that that didn't filter out the offset. But I'm a new tech and quite a bit about the design of these different sections is beyond my training. I'm attaching the schematic for the power amp section for reference in case that helps.

author: hummusdude
author_rank: Regular Contributor
timestamp: April 03, 2021, 02:05:32 am
content: I rebuilt the breadboard with custom cut #22 guage wire instead of those ubiquitous but rickety jumpers and not only is it working great but the dc offset disappeared as well. So it seems the offset was probably a result of poor grounding or perhaps a ground loop. I literally went from 6 breadboards to 3. And the extra 2.2 nF cap that I used to remove the offset had the effect of decreasing the output voltage of the pre-amp stage by a factor of 4. I guess it created a mis-matched impedance.Next step is to do the layout of the PCB and get some review here before I send it off to manufacture. Thanks for your help!