30 odd years after their production years, what are easy stereo upgrades on your old C4 ?
In this post, I’d like to share some of my findings to what makes for basic upgrades to what you’re sitting with, without stepping into the car audio customization routes. Or the ways of BOSE (OEM).

Systemic power

Obviously, more power to drive whatever speakers in use, makes sense. With 1DIN options, I see no point in touching any explanations, perhaps only about the period-correct Blaupunkt DAB radios.

Front speakers in doors (limo & avant)

Photo courtesy of Filamade.

Integrated in the door sides are from Audi factory a set of Nokia 4,5″ three-screw points speaker elements. As one can imagine, they are less than ideal for finding a direct bolt-in replacement.

While some opt for a slight modification to fit 5,25″ component speaker kits, the simplest solution is mounting a 4″ component speaker kit, using 3-to-4 screws adapters.

Rear speakers on shelf (limo)

Photo courtesy of Filamade.

This is where I’m slightly insecure one what goes as standard on the C4s, but to my understanding all sedans (limo) had a component set integrated into the shelf in the rear. These are then put in a bayonet fixing with a set-screw.

They are best suited to be replace with 5,25″ speakers, also utilizing a third party 3D printed adapter solution. and should work being either component speaker kit or even a full-tone to provide decent mid bass.

Audi C4 speaker adapters (front door & shelf speakers)

The maker of the above mentioned is Filamade, as found on Etsy here.
Both adapters are made of sturdy ABS plastic, and are a direct fit to OEM fixations on the Audi C4s.

Rear speakers in doors (avant, or retrofit)

Seeing how the avants does not have the rear shelf, they opt to have their OEM rear speaker kits integrated in the rear door sides. But there could also be the option of a retrofit.

Mainly targeting Audi 80 (B4, 1991 onwards), Votex offered 8A0 051 606 – a retrofit kit for mounting a set of tweeters and speakers in the rear doors of the Audi 80 and the Audi 100. Noticeable is how it was advertised as for ‘Audi 80, Audi 80 Avant, Audi 100‘, which I read as intended for the Audi 100 limo only. That makes sense with the avants featuring speakers in the rear doors from factory.

Votex 8A0 051 606 – speaker retrofit kit for Audi 80 and Audi 100.

Replacing the speakers in the rear doors should be a rather straight forward operation, considering either a 5,25″ or even 6,5″ sizing for mid bass that is challenging to achieve with 4″ in the front doors.

Sub-woofer supplement

In general terms, I found that the OEM speakers from the early 1990s were a bit lack-luster especially on the lower-end bass. Without a need for a lot of bass, I went the route of installing an active sub-woofer (8″) for easy placement. And hopefully as a sufficient supplement.

Our Audi 100 is running a Blaupunkt retro-DAB head unit, meaning it is providing RCA output. Had I not had a modern interface head unit, I would strongly consider installing a Focal Impulse D-class 4 channel amplifier which connects directly into the factory ISO-plug. Not only would it benefit more power to any aftermarket speakers, but also plug’n’play with Focal’s ISUB active sub-woofers.

Placing this in our Audi 100 limo, my initial idea was to fit it beneath the passenger seat. There is not enough room under the seat’s sub-frame, but beeing a front-wheel drive car in my case, there was space beneath the rear seat on the right hand side (where quattro cars have their battery).

With a mere 24 x 18 x 7 cm sizing, the Phoenix Gold ZB8P was chosen. That turned out a wise move.
RCA cable went through to the head unit on along the gear shifter, whilst power came via the right hand side threshold and a further routing up into the water tray.