Need help with DC biasing an audio input - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: May 15, 2021, 03:12:47 am
content: I'm trying to integrate an example circuit from Maxim into my existing preamp schematic but I'm not sure how to deal with the low-pass filter. Maxim's circuit is from their app noteQuoteabout how to bias an audio input signal so that it will work with their digital pot. The dpot can't deal with negative voltages so the go to solution is to positive bias the input and then decouple it afterwards. FWIW, the app note is very interesting reading about why it's not a good idea to use a dpot in the same way you'd use an analog pot.It seems like the filter cap should go after the HO node and before the wiper node to create a low pass filter. But if I did that it seems like the changing value of the wiper would impact the corner frequency of the filter.In my schematic I haven't yet added a decoupling cap. I'm thinking I will put it between the pre-amp and tone stage. The op amps are supplied with +- 15VDC and I would be biasing the input to +2.5 VDC, so plenty of head room in the op amp.

author: moffy
author_rank: Super Contributor
timestamp: May 16, 2021, 04:11:50 am
content: Do like the Maxim diagram shows and add the unity gain buffers shown, then place your 4.7 gain amplifiers after the buffers. You might want to AC couple from the output of the unity gain buffer to the input of your 4.7 gain amplifier.

author: hummusdude
author_rank: Regular Contributor
timestamp: May 16, 2021, 04:04:01 pm
content: That seems prudent. I wondered about the 33 uF coupling caps on the output. In their app note they reference the large capacitance on the input having something to do with the overall input impedance. It also says the output coupling cap is optional if their is coupling further on down the line, which I have in my power amp input. It's 4.7 uF...plenty big enough for low end. I wasn't planning to remove the DC bias before the power amp input since my signal op amps are at +- 15 VDC supply and the preamp output swing is 1Vppk max. Even with the DC offset that still leaves plenty of headroom, at least to my inexperienced point of view.

author: moffy
author_rank: Super Contributor
timestamp: May 17, 2021, 09:43:13 am
content: Always remove any DC offset from audio. A 1uF capacitor in series with the 10k resistor you have as the input to you 4.7 amplifier gives lower than 20Hz low frequency roll off. The cap can then be MKT(plastic) and non polar, for lower distortion and easier implementation.

author: Zero999
author_rank: Super Contributor
timestamp: May 17, 2021, 11:33:20 am
content: Quote from: hummusdude on May 15, 2021, 03:12:47 amYou need to bias the digital potentiometer at 2.5V and make sure the voltages on all terminals are withing 0V and +5V.In your circuit it's biased at 0V, so would be subject to negative voltages, which is probably fine, if it's <100mV or so, but will cause dignificant distortion beyond that.Just bias the points connected to 0V, to 2.5V, on your schematic and AC couple to the next stage. Use the TL431, or similar to generate 2.5V.

author: hummusdude
author_rank: Regular Contributor
timestamp: May 17, 2021, 03:03:42 pm
content: I breadboarded Maxim's bias circuit and it's working perfectly so I'll probably go with that.

author: moffy
author_rank: Super Contributor
timestamp: May 18, 2021, 07:25:34 am
content: Good choice.