Bluetooth amp into vintage radio

Hi guys,

I’ve recently purchased a bluetooth amp (ZK-502C). I’ve installed it into a vintage Falcon with radio-delete using the choke cable location and a modified headlight switch chassis.

It’s completely invisible and it works brilliantly! Use the original choke knob as volume control and it’s great, see attached pics 1-2. Had to drill & tap into the potentiometer shaft and screw the knob shaft into it through the bezel.

I also have 2 other vintage cars, both with original radios (non-working). Have tested the volume potentiometers in both and they seem to work fine, see attached pics 3 & 4. Both old radios are case-ground units and the white wire from potentiometer is volume control.

I want to keep these looking absolutely original, but add the ZK-502C to both in hidden fashion.
Rather than gut the radios to fit the amp, I’d like to just use the existing volume control pot to control the BT amp.
Can you offer any advice on the simplest way to do this?
Think the Blue-Red wire into radio is power, the black is earth and the white is the “wiper” for the volume control. (see pic 5).

I think this gives me the 3 wires for potentiometer??
Can I use these to bypass the potentiometer in the ZK-502C? or is there another equivalent bluetooth amp that will give similar output in similar sized package but allow me to wire in the volume-pot as described?
Does this make sense?

Is it a workable solution?

Do you have a better suggestion?

I would really appreciate your input on this, as I really love the result with the ZK-502c already in my radio-delete car.

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Hi Warren,

Is this the unit you’ve got?

To my knowledge there are two main ways volume control works

  1. A dual-gang well-matched logarithmic pot that actually attenuates the audio signal directly for each channel
  2. A single-gang pot that controls the gain of a preamp or amp in some other way.

You’d need to measure the pot from your car to be sure, but if the wiring you’ve annotated in your image is correct, it seems like your car uses method 2.

Some snooping on a ZK-502C from stock reveals the pot is 10K dual-gang with a switch, and the back of the PCB suggests it going to the IC AC21BP0A932-25A4. The trail ran cold there as an application note schematic for this IC didn’t reveal much, but I think the amp board uses method 1.

This means you might need to make a control circuit to go from that stock pot to the Bluetooth amp, but I think with your skillset it’d be easier to adapt the original knob/shaft to that of the amp, as you did with the Falcon. Perhaps one of our more analog-minded forum members has done this before?

Let me know Warren if you have any questions about what I’ve dug up so far.

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Hi James,
Thanks for the information above.
I was wondering about the MAX9744 BT amp (your SKU ADA1752) as that seems more like method 2 as it has only 3 port holes for pot wires. However it seems to be only 20W rather than the ZK at 50W.
Any thoughts?

Hi Warren
20W or 50W. about 4db difference. Ir would take a discerning ear to hear the difference, or insensitive speakers.
Possibly you like very loud sound or have a noisy car.
Years ago it was considered that the “average” or “normal” listening level for an “average” radio in an “average” house was about 50 to 100 mW (0.05 - 0.1W). Indeed most stipulated alignment levels were 50mW. I mentioned this as it might give you some idea of just how loud 20W can be. Of course as with most things all this is pretty relative and depends greatly on a person’s hearing and the level of background noise.
I do agree that 50W on paper gives you a bit more head room.
Cheers Bob
PS. An example. Years ago I used to service the Police (and others) VHF and HF radio networks in PNG. The 50W base stations were clerverly designed so that if you had a transmitter failure and a spare mobile on hand the 25W transmitter board out of the mobile fitted straight into the base station. This temporarily converted the Base to 25W instead of 50W but sufficed until the 50W TX could be repaired and replaced.
The point I am making is that this arrangement had no user noticeable difference in performance. The difference could be measured of course but as far as the user was concerned all was OK.

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Hi Robert,
Thanks for the information above.
Can I discern from that, that the MAX9744 BT amp (your SKU ADA1752) would be a suitable alternative to the ZK to enable me to connect the existing pot in the vintage radio to the pot ports on the BT amp?

Hi Warren
It is not MY ADA1752. I have no commercial relationship with Core except to make a purchase now and again. Just an interested bystander.
I have no detailed knowledge of the amps under consideration but I am old enough to possibly have come across the car radios in question. But I am afraid “vintage radio” means very little as there were many “vintage” radios about back in the day.
Cheers Bob


Hi James and Warren
Just a couple of observations looking at the pics if that “vintage” radio.
I think the volume pot would hav been single gang. I don’t think car radios were into stereo reception when that was built. Looking at the “tuning box” it could well be inductive tuning which was easier to manage with the station “preselect” systems of the era. It was probably lucky to have some of the circuitry on a PCB.

If the radio was built into the dash I don’t know what you would do cosmetically. You would need to keep the front panel at least even if the tuning knob did nothing. Just there for show. These radios were well before the front access mounting systems of today and I really had the impression sometimes that the radio was mounted and the rest of the car was built around it. A real contortionist effort to gain access sometimes.

Having said all that I think possibly the simplest way to go would be to de-guts the old radio and just use the front panel with tuning knob and volume control. I don’t think you will be able to use the case as because of engine interference etc the case is very well screened and your bluetooth probably would not work. The volume control would probably have the on/off switch mounted on the back. Very hard to find log pots with switch these days so fingers crossed the original is OK.
Cheers Bob

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Hi Bob,
Thanks for the comments.
The radio is from a 1965 Mustang. Yes I want to maintain the original face/dial/etc in the dash and not be able to see any indication of updated audio.
It’s worked brilliantly in my Falcon sedan delivery (the brown dash) , but that was as described using a knob screwed into the shaft of the ZK potentiometer.
Problem for me with the Mustang radio is I’m not sure how to get the knob-shaft of the pot to connect to the BT amp as you can see in the labelled photo, the pot is closed at the rear.
First thought is to cut the rear of the pot off to expose the shaft, but I want to be sure not to destroy it to the point I can’t use the shaft… hence the thought of adapting the wiring from the pot to a BT amp with analog volume.
Just really finding it difficult to find one…

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Hi Bob,

This is off-topic, but we actually carry some of that exact type, though the brand isn’t mentioned, so I’m not sure about how well matched the two gangs would be.


Hi James
It is very likely that radio would be pre stereo broadcast as far as car radios are concerned anyway.
For mono just use one gang and ignore the other
Must be log curve or as commonly called these days “audio log”. Marketing eh ???.

Interesting Core stock a switch pot. Will have to file that info away for future.

Remember the days of reverse log or anti log pots (10kΩ was common). Usually dual gang. I would not even try to find one these days.
Cheers Bob