Hacking up $60 MP3 player

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
Hi

I just got gifted an MP3 player. It's actually one of those cheap combination MP3/Video CD/Audio CD players with an inbuilt low-power amp.

Sound is OK apart from the fact that the high frequencies have been severly rolled off, I suspect poor filtering implementation has caused this, also there is a bit of hum.

I'm planning to disconnect the amplifier first, to rid the system of the hum. I'm a little lost as to the filtering bit, can someone point me to some Cd player analogue stage schematics?

Thanks

Sangram
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
Since no one bothers to reply, here's a little work-in-progress.

I hit the hum problem first, but disconnecting the amplifier has done no good. The hum remains. However, the amp has not been reconnected. The transformer is running a little cooler. The hum originates in the digital stage. The analogue stage has noise below my hearing level, so I'll assume the hum is not from there...

The really bad news is the power supply regulation. There is a single 7805 powering the display and the main decoder board. This regulator is mounted on a fairly large (by 7805 standards) heatsink, which is still getting very hot, I estimate around 80c. I believe the IC is near its limit, if not beyond it a little bit. The output is steady at about 4.98 volts, but the heat! OMG! It's dropping 7 volts, the input at 12 volts is way too high for this sort of thing.

I plan to separate the two supplies and add a second regulator for the display. Alternatively, use a 7809 before the 7805 to drop the voltage down to acceptable limits for the 7805.

I concentrated on the analogue stage last night.

There is an LM324 powering the output. Eeech!! I need to get rid of that analogue part immediately. The sad part (or, if you like it, the good part) is that the power supply for everything except the power amp is mounted on the board. Ridding myself of the board will help me get separated power supplies for the three stages: Display, analogue and digital.

Anyway, I changed the feedback loop of the LM324 (Implements filtering through its feedback loop, so reducing the capacitance and lowering the overall gain did tame the distortion a bit. Needs some more but I'd rather build fresh than spend time on a loser like this) and initial results sound promising, The highs are back, but with some associated jangly midrange. I spent about three hours looking for antialiasing filters for CD player outputs, but with no success.

MP3 sound OK, Audio CDs sound pretty bogus to me. All dynamic range is crushed, and there is a lot of distortion during dynamic passages. As in if I'm playing 'Coming back to life', specially the intro, I notice major distortion when a note is played on the guitar. This extends to all audio CDs, with varying degrees of irritability depending on the CD. Though the primary use of the player will be to play MP3s in a standalone configuration (I plan on reconnecting the power amp later) I would like it to have acceptable audio CD performance too. Help, anyone??

The digital section is made of something called 'Sunplus' Never heard of the company before. I can't do much about it, except throw the whole thing in the bin. Then I might as well buy a new player... I looked around but there's no information on the chipset(Sunplus 711/702, the 702 is the decoder for the video, dunno the 711) so if anyone's heard of it I'd be grateful for any info...

After work today I'm going to pick up some more stuff for the player, maybe a new PSU section...
 
LCD display generating noise?

sangram said:
The hum originates in the digital stage. <snip> I plan to separate the two supplies and add a second regulator for the display

A separate reg for the display is probably a good idea. Some part of the lcd display, the backplane I think, is driven with a low frequency square wave from somewhere, perhaps internally generated. Maybe this is what you can hear?

GP.
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
Thanks for the help guys. Where do you get your info from? I'm desparate to learn more, so please post any good links on this sort of stuff. I follow HPotter's post with devotion, and have learned quite a bit from it. Alas I have no place to get those exotics from so I'm content to sit and wait for my NAD player.

Well I'll check with my guitar today what note the sound is. Offhand I remember being about 500 or so but I'll need to check what note it really is. AFAIK it's not 800, but I'll need to check..

It's actually one of those light-on-dark displays. Not LED, but a vacuum Flourescent type. Like you get on most micro-compo systems, but so bright you can read by it. I hate it and dimming it may be a good thing, once I figure out how, any help there would also be great, but I'll tackle that inanother thread...

I may actually be a little gypped on that square wave thing, because again if I remember correctly the display driver is located on the digital board. Which means that it actually may not help to separate the supplies, so we'll see. And seeing as we don't get any good electronic stuff in India, I might run out of options for any drastic changes.

I will try separate regulators and some better filtering, maybe some passive filtering and a regulator using a TIP 3055 or similar to bring the voltage down, it might help more than that poor little 7805.

On another note, I found SunPlus. It's a Chinese company making those chips :bawling: so I'm a goner if I need to brickwall it, because I'm pretty sure there's a horrible high frequency distortion coming off the digital stage that needs to be filtered. And I have no clue how to it. I need help on that really bad.

I've spent about seven hours on Google with 'CD player audio output schematics circuit diagrams' in every possible combination and have only seen ads and reviews of CD players. I'm at a loss for information sources...
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
Maczrool: Great idea. I'll start looking there, so long as I can find it on the web.

Another update.

The entire digital board is pretty lo-quality stuff. Fortunately the retailer from who the MP3 player was picked up is also an enthusiast and is planning on introducing his own brand of player, so is following the poject keenly. As a part of this he has agreed to swap the board around with a better one based on Samsung chips. Far from the best but more acceptable.

I switched all the resistors to metal film (analogue stage only) and the caps to nice polyester caps from the ceramics. I also added some power supply filtering caps and small 0.1 ceramic bypass caps for the regulators and the analogue supplies. Small improvements, but now the analogue stage is so transparent the digital stage sounds bad, even worse than before...

I reconnected the amp as well. It was earlier taking input from the digital board, so it was completely unfiltered... I rerouted the wiring to feed it from the final analogue out. The problem is the wiring for the tone controls are made with flat ribbon wire and the length of travel is extensive, so it's picking up a huge amount of RFI. I need to convert it to 3-pin shielded cable. That's tonight and tomorrow, and I'm making up a pair of small bookshelves for it from junk I already own.

I am in the process of building a completely new analogue stage using TL071s, metal film resistors, tantalums and polyfilm audio grade caps. That's this and next week.

The supplies are another pressing question. The supplies for all stages are completely separate. No question. The transormer has 5 secondaries. One for the power amp. One for the 5V. One for the 8V. One for the 3.5 volts for display. One for a square wave. Yes, I found it, it's the funniest thing I've seen. There's a pair of 100 ohm resistors connected across the secondary. The midpoint of those resistors is grounded via a 5.1 V Zener diode. I presume this generates a crude square wave of an amplitude PP of around 20 volts, when the zener conducts on every half-cycle. No rectifiers, this is a funny one.

That's the good news. The problem is, the hum will not go away.

I suspect it is the shortcoming of the digital board itself, not the suppply which is now squeaky clean according to me. Once this is changed over (the single most important change) I will first work on the transport and servo, and then move over to the analogue stage.

Expect more updates soon!

Meantime, does anyone know of a nice drop-in replacement for the LM324 that will be good for audio?
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
Well, can I drop a TL074 into the place of the LM 324, or not??

Over the last week I changed the filter some, but there's still some way to go. The sound has cleaned up, specially after moving to shielded wire for the long 20" runs.

The bookshelves are now complete and the output is good enough for casual listening to MP3 and audio CDs, but the bookshelves have decent resolution so audio CDs are still a problem.

I have managed to source out most of the components for my filter, and got a good filter design program off the TI site, and all the values are quite close to commercially available stuff, and I start building it tonight, based on a pair of TL071s or a single TL074, depending on the filter I'm finally choosing.

Should I use a Butterworth or a Bessel? What kind of phase linearity should I be looking for at 20 KHz? I've plotted a sample graph that gives me about 75 deg phase shift at 20 KHz or so, about half that at 15 KHz, using a two-pole Bessel filter which I think is better than what I've got now (I think it's a single-pole filter). I'd like some help on this.

I also need some help on grounding.

The source of the hum may be the fact that grounding is a little suspect. The chassis ground is given through the heatsink tab of the regulators (7805/7808) who are connected to a grounded heatsink. The Power amp has its own PCB which is powered by a separate 12V winding. The only other grounds are the ones coming in through the signal wire (from dac to filtering/output, and from output to amp). Maybe the player calls for slightly better grounding?? What are the rules for connecting digital/analog grounding together??
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
maczrool said:


For your grounding, you should use ferrite beads and bypass caps on all the digital components.

Stu

Umm caps I understand, .1 ceramics, but ferrite beads? Are these used to decouple the supply to ground or go in series to supply? And how many turns of what wire or is the value unimportant and I can raid a pats bin or and old radio for the beads?
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
maczrool said:
The ferrites (they don't have to be beads) should go between the (in series) power supply pins on your components, especially the digital ones.

Stu

OK, so in parallel to supply, right? One to +5V and one to ground?Won't they short out the battery? Or should it be series - one to power supply in, and one to the + supply for the IC? :confused:

A ferrite is a small coil of copper wire, a few turns, on a ferrite core, right? Or have I got the device itself wrong?? :xeye:
 
Sorry for the confusion. Use ferrites in series from your power source, hopefully some kind of regulator, to the supply inputs on the ICs. Please don't connect the ferrites across ground and your power supply.

Ferrites are generally EMI filters made of a ferrous material. If you use beads, then yes, you wrap turns of wire around them and place them in the circuit. I am using surface mount ferrites rated at 600 ohms and 500ma in the manner described above.

Stu
 
Your friend is correct. For some reason, I thought you wanted to replace a TL074 with an LM324, which should work. For the other way around, you could implement a virtual ground to get negative and positive supplies from a single supply for the TL074. If it's only 5 volts, then it's also not enough. You would need to increase it. I think it wants at least +/-7.

Stu
 

Sangram

Moderator
Paid Member
2002-09-25 11:01 am
India
After a long holiday from a soldering iron - I have actually been playing and performing music - than hacking up my music systems - I finally opened up the beast again.

I have now changed the internal wiring completely. Circlotron (post #3) was absolutely correct, the display is generating noise. When the display square wave drive is disconnected, behold - the HF noise disappears.

Unfortunately I cannot run it for long periods (some of my CDs have 100+ songs) like this so just some careful decoupling has managed to alleviate the problem somewhat. I've also implemented a star ground and rewired it so that the signal has the shortest possible path around the chassis.

Unfortunately the amp and the control circuitry are located on opposite ends of the chassis, with the display and transport in the middle, so I am still picking up motor hum, pickup movement and switching (sounds much like if you try to draw in air between the gap in your teeth), and the display noise in a steady tone. Much reduced, But audible still through my 100 dB+ efficient boombox speakers (they sound lousy but will go ridiculousy loud with their 3-watt amplifiers).

I am trying out some chokes for the power amps too, as they are susceptible to HF noise. In car radio implementations a choke does help improve the poor (~30dB) PSRR of these chipamps - don't cringe - LA4440 (16 w/ch usable)... I did not unfortunately get beads, so these large EI-laminated core chokes will have to do.

With all these mods the player is sounding usable without having to switch off in the first 2 minutes. I have discarded the idea of using a TL074. Too much trouble, not enough space for all the transformers required. I will then have to remove the amp circuitry, which I do not want to do for it defeats the basic purpose - this was supposed to be a standalone.