D8444/D8443 Replacement 5" Speakers: 4 Ohm Issues?

[DramaBob]

Hi all,

Newb here, based in Ireland. I just purchased a Philips D8444 and am in the process of restoring/upgrading it. After cracking it open (and evicting two rather large and somewhat disgruntled spiders), I'm getting to grips with speaker replacement. From everything I've read here, the original speakers are weak as a newborn kitten, so upgrading makes a lot of sense. So, here's what I'm going with:

• 2 x Pioneer TS-G520 2-way car speakers (4 ohm) (see https://www.halfords.ie/technology/.../pioneer-ts-g520-coaxial-speakers-689340.html) which should remove the need for the piezo tweeters
• 1 x GRS 8SW-4 Subwoofer (4 ohm) (see https://www.soundimports.eu/en/grs-8sw-4.html)

Now, hopefully, they'll all fit in the case. But that's not my major issue. The issue is that the Pioneers are 4 ohm speakers, and the associated amp is rated at 8 ohms. The logical solution would be to wire them in series to match the amp impedance. BUT, every, and I do mean every, single article I have been able to find on wiring speakers in series describes wiring two speakers in series to a single channel amp.

The amp output in the D8444 has two channels - R+/- and L+/-. And I have no idea how to wire the speakers in series when there are two channels. Do I:

• Wire R+ to the pos terminal of the R speaker, and L- to the neg terminal of the L speaker, and link the two remaining speaker terminals to each other, OR
• Just run both speakers through a single channel, OR
• Just rig 'em up like the original 8 ohm speakers, and take my chances that I won't blow up the amp?

Anyway, I'm a bit lost. All thoughts welcome.

PS: a mate is 3D printing me a copy of the infamous cassette gear! Happy days.

 

[floyd]

If you do end up with taking your chances just using the 4 ohm speakers I would look into using a bigger heat sink to cool the amp ic chip and go easy on it with the volume until you feel it out . I would say pushed hard you might have problems but it's a gamble you take. The circuitry in these things is only designed to handle so much and if you push it too far they go kablooey.

 

[Superduper]

Very few (if any) boombox require 8 ohm speakers. Are the factory speakers 8 ohms?

 

[hopey]

It's stereo so you can't bridge the amp. I have experimented many times on this topic. If you run the 4ohm speakers they run at twice the current so full output will be achieved at half volume (on the Pot). You can use a 5watt resistor in series to get the 8ohms resistance and use the full volume range at the pot.

If your careful and don't run the volume into distortion it will work fine.

 

[DramaBob]

Very few (if any) boombox require 8 ohm speakers. Are the factory speakers 8 ohms?

Hard to tell from looking at the speakers to be honest, but the amp output is rated at 8 ohms for the speakers (and 4 for the subwoofer)...

 

[DramaBob]

It's stereo so you can't bridge the amp. I have experimented many times on this topic. If you run the 4ohm speakers they run at twice the current so full output will be achieved at half volume (on the Pot). You can use a 5watt resistor in series to get the 8ohms resistance and use the full volume range at the pot.

If your careful and don't run the volume into distortion it will work fine.

This sounds like a feasible plan from here. Besides, I'm having the devil's time trying to find 5" 8 ohm speakers, so it might be my only option. But could you explain it to me like I'm 5 how to go about setting up the 5 watt resistor? (thanks in advance)

 

[DramaBob]

If you do end up with taking your chances just using the 4 ohm speakers I would look into using a bigger heat sink to cool the amp ic chip and go easy on it with the volume until you feel it out . I would say pushed hard you might have problems but it's a gamble you take. The circuitry in these things is only designed to handle so much and if you push it too far they go kablooey.

Yep, using the 4 ohm speakers is last resort territory to be fair. But I may yet get stuck doing just that.

 

[Superduper]

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[hopey]

Hard to tell from looking at the speakers to be honest, but the amp output is rated at 8 ohms for the speakers (and 4 for the subwoofer)...

You can measure the impedance of the speaker with a multimeter.

 

[hopey]

This sounds like a feasible plan from here. Besides, I'm having the devil's time trying to find 5" 8 ohm speakers, so it might be my only option. But could you explain it to me like I'm 5 how to go about setting up the 5 watt resistor? (thanks in advance)

You just place it in circuit 4 ohm 5Watt resistor and the 4 ohm speaker that will give you the 8 ohm load.

 

[DramaBob]

Update! Thanks all for your help in resolving my dilemma. I'm going to go the path of least resistance (excuse pun) and line a 5 watt 4.7 ohm resistor into the positive wire to each speaker. I couldn't get a straight up 4 ohm resistor, so it looks like the overall impedance will be 8.7 ohm, which I can live with.

Few pics below - I've had to trim the sides of the frame of the subwoofer to make it fit, and now it's as snug as a bug in there, with about 2-3mm clearance to the rear casing once it goes back on. The gigantic magnet is currently impeding the wiring block from the volume etc pots to the main board, so I'm doing a workaround to fit everything in... More pics to follow as work progresses!

 

[Superduper]

Frankly I think it's horrible advice to suggest or advocate for adding a 4.7 ohm 5W resistor inline. First off, according to the service manual, this product only produces 2wpc. Did you get that? TWO friggen watts per channel. If the original speakers are weak, it's because, get this...... TWO watts per channel.

If that weren't bad enough, in an enclosed electronics box, we are gonna stuff 5 watt resistors in there, so that the amp can waste valuable amplifier (TWO wpc) power heating up those resistors to ensure a toasty cabinet? Where are these resistors going to be mounted? Will it touch anything plastic that it can melt? Because power resistors dissipate energy by converting it to heat. Is the remaining >1wpc going to be able to satisfactorily drive those speakers?

Additionally, Speaker drivers are rated at impedance. NOT resistance, but impedance. Speakers have inductance and reactance, which all varies during the course of audio reproduction. Resistors.... it's straight pure resistance. That's why a 4 ohm speaker will actually measure at probably 3.2 ohms DC. DC isn't the environment that speakers operate in. In other words, not only will it change the resistive load to the amplifier, the original impedance/resistance curve will no longer track like original. Bottom line is, the sound may very well be different.

Here is my position: Get the service manual which is freely available to anyone that wishes to do a search. You'll find that the amp chip is a TDA2004. I couldn't find TDA2004 but a ST does produce a TDA2004R and I didn't do a pinout comparison but a cursory examination does appear that it's pin compatible in terms of VCC/Gnd/IN/Out. So I'm pretty sure this is an modern version of the same amplifier. What does the datasheet say? Yes, it's rated all the way down to 1.6 ohms. I do not get the impression anywhere that this amp is rated at 8 ohms. The only cases I've seen where 8 ohms are required is when an amplifier circuit is run in a BTL configuration. In this service manual, I only see this chip run in a SE configuration. In my opinion, it should be safe to run it with 4 ohms speakers. The only concern is will the power supply be able to supply enough current? I don't know anything about the PS section. But if you are concerned, just don't crank it up so much or for sustained periods of time.

Looks like a lot of work is being put into this project, looks like you care about the sound, otherwise you wouldn't put those upgraded speakers in there. Don't ruin it by following that resistor suggestion. Let the fools do it that way. Download the service manual and see for yourself, then make your own decision. You can download the D8444 service manual by googling it. You can download the ST TDA2004R datasheet here: tda2004r.pdf

Lastly, these units really don't produce a lot of power. Every single milliwatt is essential. A watt can only produce so much sound. Efficiency is key. The more efficient a speaker driver, the more SPL it can output. SPL is the loudness. It's common for a high efficiency speaker to be twice as loud as one that is not as efficient. Your subwoofer looks impressive. Really "pretty" and impressive looking. The question is whether it's all show and no go when it's a 60 ton tank expecting a jet engine but got a corolla engine pushing it instead.

 

[hopey]

Those new speakers look great. There is a real risk of blowing your amp chip as it will distort way before the speakers although distortion can be heard regardless of what causes it. Adding the 5 watt 4.7 ohm resistor will help to protect the amp although losses are inevitable, the trade off may seem worth while when a kid decides to grab the box and start a dance party.

 

[hopey]

Call me crazy, you could run the two way Pioneers in series to get the 8ohm load.

 

[floyd]

Call me crazy, you could run the two way Pioneers in series to get the 8ohm load.

You would have to take the negative from one speaker and the positive from the other then put a wire that Bridges the positive and negative terminals together from both speakers to achieve the eight ohm load. Not sure how all this would work out so I would be careful.

 

[hopey]

You would have to take the negative from one speaker and the positive from the other then put a wire that Bridges the positive and negative terminals together from both speakers to achieve the eight ohm load. Not sure how all this would work out so I would be careful.

I think there is a singe flex through the cone, so just pop off the tweeter and re-jig the connections.

 

[DramaBob]

I think there is a singe flex through the cone, so just pop off the tweeter and re-jig the connections.

I did consider this, but the amp has two output channels (R+/- and L+/-) and apparently can't be bridged. Gah.

 

[DramaBob]

Frankly I think it's horrible advice to suggest or advocate for adding a 4.7 ohm 5W resistor inline. First off, according to the service manual, this product only produces 2wpc. Did you get that? TWO friggen watts per channel. If the original speakers are weak, it's because, get this...... TWO watts per channel.

If that weren't bad enough, in an enclosed electronics box, we are gonna stuff 5 watt resistors in there, so that the amp can waste valuable amplifier (TWO wpc) power heating up those resistors to ensure a toasty cabinet? Where are these resistors going to be mounted? Will it touch anything plastic that it can melt? Because power resistors dissipate energy by converting it to heat. Is the remaining >1wpc going to be able to satisfactorily drive those speakers?

Additionally, Speaker drivers are rated at impedance. NOT resistance, but impedance. Speakers have inductance and reactance, which all varies during the course of audio reproduction. Resistors.... it's straight pure resistance. That's why a 4 ohm speaker will actually measure at probably 3.2 ohms DC. DC isn't the environment that speakers operate in. In other words, not only will it change the resistive load to the amplifier, the original impedance/resistance curve will no longer track like original. Bottom line is, the sound may very well be different.

Here is my position: Get the service manual which is freely available to anyone that wishes to do a search. You'll find that the amp chip is a TDA2004. I couldn't find TDA2004 but a ST does produce a TDA2004R and I didn't do a pinout comparison but a cursory examination does appear that it's pin compatible in terms of VCC/Gnd/IN/Out. So I'm pretty sure this is an modern version of the same amplifier. What does the datasheet say? Yes, it's rated all the way down to 1.6 ohms. I do not get the impression anywhere that this amp is rated at 8 ohms. The only cases I've seen where 8 ohms are required is when an amplifier circuit is run in a BTL configuration. In this service manual, I only see this chip run in a SE configuration. In my opinion, it should be safe to run it with 4 ohms speakers. The only concern is will the power supply be able to supply enough current? I don't know anything about the PS section. But if you are concerned, just don't crank it up so much or for sustained periods of time.

Looks like a lot of work is being put into this project, looks like you care about the sound, otherwise you wouldn't put those upgraded speakers in there. Don't ruin it by following that resistor suggestion. Let the fools do it that way. Download the service manual and see for yourself, then make your own decision. You can download the D8444 service manual by googling it. You can download the ST TDA2004R datasheet here: tda2004r.pdf

Lastly, these units really don't produce a lot of power. Every single milliwatt is essential. A watt can only produce so much sound. Efficiency is key. The more efficient a speaker driver, the more SPL it can output. SPL is the loudness. It's common for a high efficiency speaker to be twice as loud as one that is not as efficient. Your subwoofer looks impressive. Really "pretty" and impressive looking. The question is whether it's all show and no go when it's a 60 ton tank expecting a jet engine but got a corolla engine pushing it instead.

Huge thanks for this really comprehensive assessment! I got the 8ohm figure from the service manual - see screengrab below. So now, I'm in the dilemma of:

1) Line in a resistor, potentially reducing the power to the speakers which is already pretty brutal
2) Say "F*** it" and take my chances doing a straight connection to the 4ohm speakers and hope the amp doesn't blow
3) Learn an awful lot more about electronics and rebuild/replace the amp chip (tbf, this is the option I'm least likely to take...)

Food for thought!

 

[hopey]

I did consider this, but the amp has two output channels (R+/- and L+/-) and apparently can't be bridged. Gah.

I was referring to the two way pioneer speakers you have installed. They have two separate coils one for the woofer and one for the tweeter from factory these are connected in parallel which makes the two 4ohm coils =<4ohms total. If you wire them in series 4ohm + 4ohm = 8ohm.

 

[Superduper]

You are getting overly focused on the service manual specifications. Those specifications are as-built specifications, not what the amp is capable of. For that, just look at the amp chip manufacturers datasheet for that amp. The manufacturer specifications is where you will get the true data on the capabilities a chip is designed for. If you look at the output power data, the final amount is dictated by how the product manufacturer decides to implement that chip into their design. By using higher impedance speakers, it is cutting down on the chips output (throttling it, if you will). It appears that this amp chip is capable of running at 1.6 ohms. See the chart below:



You'll see that at 14.4V and 4 ohms, the chip outputs typically 6wpc. Drop the voltage to 12V and you'll get lower watts, maybe 4. Raise the speaker impedance to 8 ohms and now, you'll probably have 2wpc. This seems consistent with the service manual specs. Of course to get the higher ratings, it is presumed you have proper heatsinking. Whether yours is deficient, I don't know because I don't have one of these. But I do think you are worrying too much. If you blow it, then just change the amp chip and consider adding a more robust heatsink or other means. The chip appears to be readily available still so it's no big deal really.