Panasonic cassette Walkmans run out of bass for a reason — two coupling caps, confirmed on the SX73

Panasonic tape Walkmans get thin in the low end once you turn them up. I compared the RQ-SX53/SX73 service manual (PCB + schematic) with Sony WM-EX610 / EX2000, then did the same mod on an RQ-SX97F. It is not a “Panasonic sounds warm” thing. It is two small capacitors.

**Stock SX53/SX73**

- Amp → headphone jack: **C63 / C66 = 68μF / 2.5V** (1206)

- Volume pot → amp input: **C43 / C44 = 1μF** (0603), then 1.5k

68μF into 16Ω is a high-pass around 150Hz (about 73Hz into 32Ω). The 1μF after the pot sits in series with the pot’s Thevenin resistance, so the cutoff moves with the volume knob. Past about 6, bass gets even thinner. There is also a loudness network on the board. Stack those and the bottom end goes slack.

**What Sony actually does**

WM-EX610 (TA2123AF):

- Volume pot **RV301 goes straight into** the power-amp input. No extra coupling cap after the wiper.

- Amp outputs L / R / virtual-ground COM are all biased at 0.6V. **No series output coupling cap.** Headphones sit between L/R and COM, so DC cancels.

Sony tape decks in this generation are DC-coupled at the jack via a third virtual-ground amp. Sony CD / MD players that still use a grounded sleeve usually put **220μF** on the output. Neither approach is “68μF plus another 1μF after the pot.”

**The mod (SX53/SX73 reference designators; same idea already done on an SX97F)**

1. C63 / C66: 68μF → **220μF**, 6.3V is enough. Ceramics sound more open than tantalum here. Watch X5R voltage coefficient — at ~0.8V bias you will not get the full marked value.
   
   
2. C43 / C44: **replace with 0Ω** (or a solder blob). That is the Sony tape-deck input hookup.
   

AN7501 has no third virtual-ground output. The sleeve is real ground, so you cannot delete the output caps. You can only make them bigger. Short the input couple and jump the output to 220μF, and you are copying the two Sony habits that this Panasonic IC can actually accept.

On the 97F the change is obvious: extension comes back, and bass no longer collapses when you turn it up.

Panasonic (松下) literally reads “loose underneath.” The schematic agrees.