seeing a small signal loss between preamp and power amp

Thread Starter

Hummusdude

Joined Jun 18, 2020
6
I built an LM3886-based power amp that I found at circuitbasics.com and it works well. But it didn't have any tone controls or attenuation so for fun (and for my portfolio) I designed and built a preamp that uses a Baxandall 2-band active EQ. According to the designer of the EQ (Elliott Sound Products) the tone circuit has a center frequency of about 640 Hz for both treble and bass. I also designed a mixed-signal pcb (my first) that has digital control of the preamp gain and analog pots for the EQ.

The preamp is working pretty much as expected, rather quiet and the output is right where I expected. My music player max output is around 200 mV peak and the input of the power amp is expecting 1V to reach max power. So my preamp puts out 0.97V with 200 mV in and that's close enough for me.

But the output of the preamp drops to about 800mV when I connect it to the power amp. I wasn't expecting this as the LM3886 has very high input impedance. My first thought was that maybe some attenuation is coming from the input filters to the power amp. The designer didn't address using it with a preamp so maybe there's something fundamental to cascading the two that I missed.

Below is the schematic of the LM3886 circuit for reference. It works very well although my scope measurements suggest the gain is closer to 23 than 21 (the designer says R4 and R3 set the gain for the circuit) so perhaps a clue there. The input low pass filter (R1 & C2) cutoff is 234 kHz and the input high pass filter (C1 & R2) cutoff is 1.7 Hz.

For the low pass filter, I calculated the reduction in gain as Log10(234k/640) = 2.56 decades. At 20 dB/dec that gives 51.26 dB. So the reduction in gain would be a factor of 10^(-51.26/20) = 0.00274. That gives about 999 mV. For the high pass filter the math works out to almost exactly the same result. So on the face the input filtering doesn't appear to be the issue.

Another possibility is that I'm not measuring it correctly. I noticed that the preamp output voltage will vary by a few mV depending on where on my board I attach the ground lead of the probe. That makes sense as the the ground plane of the board is going to have areas where the voltage level is not quite earth. But the total output of the amp at the speaker terminals is also showing the drop so I'm inclined to think this isn't the problem either.

I should add that when the preamp is connected to the power amp the drop is clearly happening in the tone circuit. I measure 1.0V at TP3 on my board and 800mV at the output. The designer of the EQ says there should not be any drop at the center frequency regardless of the setting of the pots. I centered the pots anyway just to be sure and set the input frequency to 640 Hz and still the drop.

I know there's a lot going on here and I tried to be concise. Sorry if I missed any important details.
 

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BobTPH

Joined Jun 5, 2013
4,046
Did you use linear pots? If you used audio taper ones, they are not centered at the center.

Also, what signal are you using when you see this drop? Did you try using the 640 Hz center frequency?

Bob
 

Thread Starter

Hummusdude

Joined Jun 18, 2020
6
Adjust the Gain of the Buffer-Amp before the Tone-Circuit, done.
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Yes, getting the game is not a big problem. I was hoping to get some clarity about what was causing the signal loss. I'm pretty new at this stuff so I guess I figured I was missing something crucial.
 

Thread Starter

Hummusdude

Joined Jun 18, 2020
6
I think maybe the problem is that "centering" the tone control doesn't mean centering the pots. It means setting both pots to minimum attenuation.
An excellent point. I used an ohmmeter to check the pots in circuit. They're not 100K in circuit. So I set them to have equal resistance on both sides of the pot.
 

Thread Starter

Hummusdude

Joined Jun 18, 2020
6
Did you use linear pots? If you used audio taper ones, they are not centered at the center.

Also, what signal are you using when you see this drop? Did you try using the 640 Hz center frequency?

Bob
Input signal is 640 Hz. Pots are linear.
 

Thread Starter

Hummusdude

Joined Jun 18, 2020
6
Where does your signal ground connect to your power ground? Did I miss it?
The signal ground attaches to the power ground via the red dotted line. It's hard to see in the pic because of the red edit, but the signal out of the board is right where the red circle is on the left of the pcb. The black and white wires connect the preamp to the power amp boards. So the signal ground from the power amp boards connects to the preamp board and then a single ground wire goes to the power supply. Attached is a diagram of the layout of the power supply for reference.
 

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Thread Starter

Hummusdude

Joined Jun 18, 2020
6
Problem solved.

The issue turned out to be an unreliable connection from one of the pot leads to the board.

Thanks to all for your input!
 
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