Prosonic PQR-9850 Amp / speaker mod

Chuckmeter

Member (SA)
Dec 11, 2017
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I've read all sorts of opinions about boombox acoustics and class D amps. Not that many actual implementations. So I decided I would find out for myself...

The "iPod Viking" is a perfect candidate for a power vs. size ratio update. Definately, there's room for enhancement.

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I gathered a few parts...

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This is work in progess but pretty much done as this point. Here's the back cover...

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I've replaced the transformer with a more capable SMPS

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I've used the transformer posts to mount a step-up converter that bumps up the voltage going the amplifier. The amp can use from 9 to 24 volts. Theoritically it should work at full power (24 volts) using D batteries. For how long remains to be seen. Not a big concern...

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The relay in the bottom is the amp power switch

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And the amp itself. It is a T1-M from HiFimeDIY. There's no precise spec for its output power but if set to the highest voltage it should likely push around 25 WRMS into each channel.

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At this point I still need to work on the front part. I'll be back with more pictues.. Stay tuned !
 

Chuckmeter

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Dec 11, 2017
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Here's where I get the signal to the new amplifier...

The built-in power amp is a pair of Sanyo LA4440 ICs. Looking up the datasheet for this part I can see they are set up in bridged configuration, signal input (therefore preamp output) happens at pin 2. Indeed, following the trace to this pin on the pcb brings us to the volume pot.

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Now for the front cabinet...

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New speakers mounted with 3-way xovers... These are much heavier than the original drivers and fill much more space.

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At the bottom is the headphone / ext. speakers jack PCB. The headphone jack has a pair of switches that will direct the audio to either the speakers or - if hps are connected - to limiting resistors, then back into the jack.

I've disconnected the audio from one of these switches and attached it to the headphone jack permanently. I use the newly freed switch to cut the power to the new amp's power relay. Therefore, connecting headphones will turn off the new amp.

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Now going to reassemble everything back together and testing it out... I'll be back !
 

Reli

Member (SA)
Dec 24, 2010
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USA
Nice, be sure to add a few strips of Dynamat or Rockmat in strategic places (especially on the back panel) to reduce resonance.
 

Superduper

Member (SA)
Reli said:
Nice, be sure to add a few strips of Dynamat or Rockmat in strategic places (especially on the back panel) to reduce resonance.
And maybe a ton of bracing and perhaps never ship this thing. Seriously, that is a lot of speakers and added weight to that thin shell, you don't want this thing to Humpty Dumpty itself when you crank it, or impact it.

BTW, the existing amps in bridge configuration is plenty stout, 19wpc is possible with 18VDC supply (don't go over). That is no slouch and those amps are frequently used in car applications. The weak point is the existing power supply which wouldn't be able to supply enough current for that kind of output, and the speaker system which are probably a joke compared to what you stuffed in there. BUT the Sanyo LA4440 amps are old school AB class amps which you may (depending on how sensitive your ears are) prefer to D class amps. If you find that you have unacceptable hum you can't get rid of with that new amp, or sound quality issues, you may want to see how the existing amp sounds with those new speakers and a more beefy PS. Anyhow, just a thought.

BTW, my disco robo uses those exact same Sanyo amps and it is loud enough to fill my entire 2800 sq ft house with sound from those little (but beefy) speakers. I imagine if the disco robot had 6.5's instead of the 4 inches, how much stronger it would sound.
 

goodman

Member (SA)
Jun 14, 2011
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Bulgaria
Wow, this must heavy...
So many speakers, power supply and circuit boards...
New electronics into old boombox.
Waiting to see the final result and demo video of course... :yes:
 

Chuckmeter

Member (SA)
Dec 11, 2017
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Montréal
It's quite an improvement as far as the speakers are concerned. Lower, stronger bass and the mids are so much better than before.

But the box itself proves to be a major limitation as the volume is increased. Quite a few random resonances

As it is now, it's an obvious SQ improvement at low to moderate volume, but it cannot play very much louder than before. Still working on it.
 

Chuckmeter

Member (SA)
Dec 11, 2017
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Montréal
The "magic cassette" is the Delitape app running on my old 2nd gen iPod touch.

Although it's soft touch, the transport buttons are mechanical, there are no switches there. I mounted a small board with push buttons underneath the cassette mechanism. They connect to the iPod through an inline remote control that I have gutted for that purpose. They also connnect to a BT board that was added later on.

It can't record anymore, otherwise it's got all its original functions working. Tape, radio, aux jacks. When on the aux input, the tape selector switch is used to select between the iPod, BT or actual aux jacks.
 

Brutus442

Member (SA)
Jan 7, 2012
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Toronto, Canada
Amazing project.

I also have the 5890 and love this mod!
I'm assuming you made a new bezel for the ipod rather than using the existing cassette door? Either way it's extrordinary and the sound is 100x better, well done!
 

Fatdog

Well-Known Member
Staff member
May 3, 2009
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Chuckmeter said:
The "magic cassette" is the Delitape app running on my old 2nd gen iPod touch.

Although it's soft touch, the transport buttons are mechanical, there are no switches there. I mounted a small board with push buttons underneath the cassette mechanism. They connect to the iPod through an inline remote control that I have gutted for that purpose. They also connnect to a BT board that was added later on.

It can't record anymore, otherwise it's got all its original functions working. Tape, radio, aux jacks. When on the aux input, the tape selector switch is used to select between the iPod, BT or actual aux jacks.
Simply ingenious!
 

Chuckmeter

Member (SA)
Dec 11, 2017
23
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Montréal
I have tried a few ideas to mitigate flaws with the enclosure. Here's what gives the best results...

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I made speaker gaskets out of Blue tack. This stuff has excellent damping properties, and combined with the speaker frame it adds stiffness at the weakest points of the front panel, resulting in greatly reduced vibration and a more "neutral" sound. Drum kicks, for example, suddendly sound more natural and less "plasticky". Also used it to fill the seam between the scale window and the case and around the cover of the SMPS to eliminate rattle at these spots. Lastly, I used it to attach the crossover boards to the cabinet and the cassette door to the iPod.

I "sealed" the back openings of the tweeter and mid drivers with Gorilla tape. I should have done this right from the start, but since the back of the box is opened I thought it wouldn't build enough internal pressure to become a problem I didn't give it much thought. Couldn't have been more wrong. It can now crank the volume way louder before it starts "rattling", if it ever does.

Among other things I have tried with little to no effect at all :

- I covered the inside of the cabinet with Plasti dip paint, which is a paint with a rubber finish. It is supposed to "deaden sound" but doesn't make any difference. Still I like the idea of having a thin layer of rubber between mating plastic parts.

- Filled in the voids around the drivers with glass wool. No effect at all.

- Sealed the drivers with silicone caulk. That is what I used before trying blue tack. I guess it was correct (but messy) as a gasket replacement but doesn't provide any structural advantage.