Help with boom box modification

[Califauna]

I have a W-King D8 Bluetooth boom box which I'd like to modify with some external speaker output jacks and some buttons to switch the internal speakers on and off.

From factory the specs of the boom box are:

-Power Input: DC 5V
-Speaker Output Power: 2 x 25W ( confirmed by Youtube channel Hartdegen review as 12.5W RMS per channel)
-Drivers:
-2 x Woofer, 70mm, 10Ω, 70mm,15W
-2 X Tweeter, 8Ω, 30mm,10W x 2
-Frequency Response: 60Hz - 20kHz
-Battery Type: Li-ion ( 2 x 4000mAh)
-Battery Voltage: 7.4V ( I measured 8v)
-True Wireless Stereo (TWS): Support
-Technology: BT4.2, A2DP, AVRCP, HSP, HFP.

-The measured resistance at each amplifier output (amp side, speakers disconnected) is 1.2 million ohms.

The tweeters have a resistor in parallel creating a passive high-pass filter.

Here are the circuit diagrams from factory and for the modification I'd like to make.

Factory:
Diagram 1.
Factory.jpg

And here with my modification:
Diagram 2.
Mod1.jpg


As you can see, I have added:

1. Jacks to connect to external speakers.
2. A higher level high pass filter function on each side (HPF #3 and #4), within which you can see that the factory high-pass HPF #1 and #2 are also nested.
3. Switches 1 and 4, which (I hope) allow the built in speakers to be switched into the passive high pass filter configuration which I have added , using capacitors 2 and 3. This could be handy when connecting to external speakers which can better handle the bass part of the audio. For calculating the value of capacitors 2 and 3 in these added HPFs, I have considered the added external speakers (connected through the jacks) to be the 'resistor', and that this external speaker nominally has an impedance of 4 ohms. Regarding this I have some questions below.
4. Switches 3 and 4, which allow the built-in speakers to be turned off entirely (eg. when I want only external speakers to be used).

Regarding the HP filter, typical diagrams and explanations are that a resistor is always necessary in parallel with the load (in this case a speaker through which passes filter audio), sometimes with a connection to ground. Eg:

Diagram 3.
Highpass withgound.jpg

Am I right in thinking that in fact an extra resistor is not always necessary for a filter like this to work, as the driver itself acts as an inductor and resistance in high pass filters with drivers in circuits like, for example, in this book, which shows the simplest type of HPF?:
simplestHPF.jpg

Among other explanations of why a resistor is included in such filter circuits, I have read that 'it is to avoid 'thump' when connecting a load, and to allow a steady state at all times' (+ capacitor capacitor always able to discharge) . However, I am not concerned about an initial 'thump' when connecting a speaker. Also I don't see why this 'steady state', or continually being able to discharge/charge is necessary or advantageous. Actually I see that as a disadvantage potentially, as the amplifier/battery power is being left to drain away through such a resistor completing the circuit (?), even when the external speaker is not connected.
FFI also see explanations indicating that a resistor helps prevent current leaks to ground, as in the picture above with ground connection. However, as it is a portable speaker in this case, there is no ground connection, so why would a resistor be necessary there?

Another explanation I've read is that "if you want to predict the filter's performance you need to know how much resistance is in series with the signal. A theoretical source with zero impedance will not be affected by the capacitor. If the signal is coming from something which has significant output impedance, then an additional resistor may not be needed."

Regarding that explanation, I DO know the resistance which is in series with the signal, (4ohms - not a theoretical 0 ohms), so why the need for an additional resistor in parallel with it?

I also have the following questions:

I figure that the addition of capacitors 3 and 4 will continuously affect the cutoff frequency of HPF#1 and #2 (factory installed HPFs), which filter the signal to speakers 1 and 4, because the resistance of the parallel 'resistor' each sees will vary. I figure this because, taking HPF#3 for example, the 'resistor' it sees when speaker 5 is connected and switch 1 is open will no longer be just speaker 2, but the combined resistance of speaker 2, capacitor 3, and speaker 5 all in parallel. Thus the 'resistor' of HPF#1 will vary continuously as the frequency of the signal into capacitor 3 changes (the changing audio signal). Perhaps it will also be affected slightly by varying amounts of back EMF produced by speakers 2 and 5? How much is all this likely to affect the filtered output of speaker 1 in this scenario?

Will typical 3.5 mm audio jacks be able to handle the current flowing through speakers 5 and 6? I also have phono plugs, but there isn't much space to work with inside the enclosure part where the electronics are, so I'd like to go with smallest components if possible. I'm also not sure what rating the switches would have to meet. When S1 and S2 are closed and SP5 is connected, I have calculated that theoretically the peak current through switch 1, switch 2, and jack 1 (when all the signal is above the cut-off frequencies), would be I=V/R = 7.4v / (1/((1/8Ω)+(1/10Ω)+(1/4Ω)) = 3.515a. This is ignoring any back EMF induced in the speakers (if indeed this is a real issue).

I notice the rating for switches is often higher when used in 110 Volts circuits than when the same switch is used in a 240v circuit. Is this something to do with the rating being partly determined by the power (P=IV), not just the current flow through the component?

Am I right in thinking capacitor 3 will do nothing (remain infinite resistance) when switch 1 is closed?

Thanks for your explanations, corrections, suggestions, and possible additional ideas/functionalities that might be useful and could be added while the box is open.

[RJSV]

From my limited experience:
   I see 3 uF cap. can be switched into the circuit, that does 'nothing' as that's large value, so just blocks (any) d.c. .
   Often, like if you have a 10 inch 'woofer' to connect,  the big speaker just is incapable of much audio output, at high enough frequency, like 6 khz, so no filter needed, unless your actual music is way way too much treble, anyway.
   For high end, having inductor to ground is, actually more like a 'low end reduction', as the inductor will prevent higher freq from being shuttled to 'ground', that being a virtual ground for portable device. That's your cable shields etc. maybe more literally separate from earth ground. (Confusing I know, but portable devices, and airplanes, they just, almost, 'pretend', but electromagnetic fields DO respond to local 'commons and metal cases.)
  Consider a circuit, completely encased in metal box: That is going to have electric field behavior not much different, if you go use an added ground rod, literally.
Some of that is a whole subject (voodoo to some).

   I see a cap. 122 uF: that looks more like something a modelling software made, for you, a bizarre value. Why not 126. 76 uF ? See my point, above a couple uF that has no discernable change, it's just a bigger huge value, except when that's a dc supply cap or some electric car motor power circuit.
   Your mini jacks should be fine, at moderate power.
But I don't see the value of switching the internal speaker, optionally, thru a dc blocking cap.
   I think if you want, you could put a small value cap in SERIES with, say, 32 ohms, with a switch that shorts out the cap. That will drop the tweeter volume, as a 'high f cut', that can be more of a 'reduce everything', but that will give effectively what you wanted (I think).

   The resistor 'bleeding dc' continuous, not sure where you read that, but not good of course. Was that added, or from original circuit
   Not sure about that 120 megohm output reading, or if that was in any context a useful reading. ()

   Nice.  What do you prefer to listen to ?
    Do you seek to listen with high f end reduced ?
    I always like to dial most high f out of audio...Too scratchy sounding...
  -- Rick

[RJSV]

Oops, sorry, I meant, for a little HF cut you could put 32 ohms in series with a small cap (250 pf ?) AND connect one end to ground, other end to speaker positive: that will either reduce 'everything', or, when open switch, will reduce 'more' highs. Similar relative to each other, but then you turn actual volume up very slightly to compensate, so effect overall is same as emphasis on low end...

[RJSV]

...hey, Wait a second:
   That picture looks to me like TWO little full range speakers, plus a PIEZO TWEETER. ?
Are you sure you looked at that disk, all smothered in glue goop?  That sure looks like a piezo disc, in which case it does not need any filter, it simply won't produce much low end, at all.
   You could try, suggested a simple switch, just as a 'high cut', just takes the piezo disk out of circuit.
Just sayin.

   As to 'back EMF' from speakers, I haven't heard, but not an exclusive audio pro. For the switch rating, no worries, unless you've got a guitar amp or something w real power, 50 watts or such.
  Oh, and for calculating power output, why the 7.4 volts, is that measured output ? I suspect, maybe you have applied a formula I can't follow logic, without seeing details of calculation, not just a result.
I just don't believe that's 7.4 volts while exceeding rated power by several factors...looks like paper fantasy.
Measured values, formulas help folks focus best.

[RJSV]

Wait...yet again I see something interesting,
   Looking at speaker jacks added, only 1 Jack is the norm, bringing up a factor I've never thought about before:. Most stereo jacks bring out the two speakers as ,1 WIRE, each channel, right and left, plus 1 ground or common.
Your schematic, starting with having TWO jacks, actually brings out what is sometimes called 'H' outputs, that is neither wire is ground, they just both are relative only to each other, voltage-wise.
   I would expect to see only ONE jack, as same as would be for using headphones; just a left and right signal, plus a ground.
 
   INTERESTING I never thought of that...(shows how much I know, lol).
   I think maybe one of those outputs, is a ground, on the circuit board if you trace it, one ground and one '+', for each side, (right and left side).

[Califauna]

Hi RJHoward,

The idea behind adding HPFs for SP2 and SP3 is to be able to block all low frequencies to the internal speakers so those frequencies can be handled when an external speaker is connected. Not really about getting a particular EQ or anything like that.

I'm not sure what you mean by 'ground' in this context, unless by ground you mean V-. A circuit like this would be 'V-', or just '-' right? (as it's a portable speaker there isn't a ground).

The tweeters are dome tweeters.  https://cdn.shopify.com/s/files/1/0225/4317/1659/products/71-bC8YiK1L_1024x1024@2x.jpg?v=1557078219

With back EMF I refer to the motional EMF of the driver generated by the drivers themselves as they act as an inductor. https://www.edn.com/loudspeaker-operation-the-superiority-of-current-drive-over-voltage-drive/
https://mynewmicrophone.com/the-complete-guide-to-speaker-impedance-2%CF%89-4%CF%89-8%CF%89-more/

In my modification diagram (factory.jpg), C3 and C4 are 132uF because I'm trying to create a HPF and that's the value I get for calculating a cut-off value of 300 Hz, according to the formula fc= 1/(2πRC).  I use R=4 because I am working on the basis of an ohms ratings for the connected external speaker of 4 ohms. I believe this external speaker becomes a load in parallel with the filtered drivers for the drivers (eg. SP1 and SP), acting like the resistor in the typical HPF filter circuits, like the one I have included.

The 3.3 uF caps shouldn't be doing nothing. Not sure what you mean there. They came installed with the speaker from factory, and are filtering out the high low frequencies from the tweeter. As far as I can tell, they are in there because they create a HPF with a cut-off of 4.8 KHz using fc=1/(2πRC), where r=10 and C=3.3uF.

[RJSV]

   I think I follow, mostly:
   When you plug in ext speaker you want only 300 Hz and above to be from internal speakers. So I see how you get the cap value. (Right ?)
    I think, after you have confidence not going to damage anything, re: power ratings of switches and Jack's, then the rest of it can be anticipated and confirmed by listen testing and observing. Although, some direct result info might be elusive.
   So, I think you are at that point, already.

   One place for caution, is that too many parallel speaker loads are there, 10 ohms, 8 ohms, and 4 ohmsis going to be, what;. some 2.1 ohms load on the device outputs.  I'm not expert, there, but super cautious approach I might put a series resistor, simply to not load the amp so much. Perhaps 4 ohms in series, on each channel.
   Plus I see, now, how that 3.3 uF cap will cause tweeters to handle frequencies above 4.8 khz cutoff.

   Some of the rest, is just language I'm not following, like you state:  "filtering out the hf from the tweeter".
I think that meant: 'highpass to drive the tweeter's.
Now, still don't see any resistor, across the tweeter, but my take on that is:
   The dome tweeter is a piezo device(?), and so by putting a resistor in parallel, you are 'conducting all frequencies equally, thru resistor, but conducting hf more preferentially, thru the driver itself (capacitive piezo element). Right ?
And so the driver gets, preferentially more current as frequency goes up, thus hf sound is relatively enhanced, compared to all frequencies.
   Again, though, that's more to 'argue', rather than a flaw that will damage components. (So go ahead, won't hurt to try, I agree with circuits proposed, somewhat).

   Now, to the ground issue: it's just a common, in portable device. But still, are those speaker outputs 'ground referenced' ?
Or, OK, 'referenced to common'?  Like, is V- supply also the device circuit 'ground', or is it bipolar ? Sounds like 5 volts single supply, in which case the minus 5V is your common.
   If you can, look up 'H configuration output driver'. That means both output wires oscillate plus and minus. Otherwise, it can be 'Plus' output oscillates, while negative output (V-) just sits at common, 0 volts.
Benefit of 'H' driver is you can get more AC voltage swing, with audio.  For motor driving, you can get both directions of rotation.
   I still don't completely follow, as literature saying the two woofers only on ONE driver, but schematic seems to show full stereo. ?? But, again, that's just arguing.

   Oh, and you probably see higher current ratings on comparable looking items, but at 120 volts you will have twice the current, for exactly comparable product. That's just double the current, if you want same power, at 120 VAC, that the other device has, at 240 VAC.

[Califauna]

Some of the rest, is just language I'm not following, like you state:  "filtering out the hf from the tweeter".

Thanks yes that was a mistake. I meant to say 'filtering out the low frequencies from the tweeter' I have edited the post.