Cheap replacement output stage for old Chinese CD player? - diyAudio
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Old 14th April 2012, 08:58 PM   #1
jjne is offline jjne  United Kingdom
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Default Cheap replacement output stage for old Chinese CD player?

OK, apologies for the complete newbie question here.

I have an old Chinese CD/VCD player. It works perfectly, and the digital components inside look as if they're fairly good quality (Sony transport and laser mechanism, and what looks like a fairly standard ESS-based DAC/servo chipset which is screwed as a separate PCB to the right of the chassis.

This player is sat unused, and I have a use for it now as it's a specific size (small) and the alternative would be to buy in a bookshelf hifi system for the kitchen (which I'm not prepared to do).

My aim is to use this as the basis for a audio/video system, using a Raspberry Pi to stream audio to an amplifier (both mounted inside the player) and video to a computer monitor sat on top of it.

Now -- the problem. Although the digital stage looks good, this thing is let down very badly by an appallingly bad analogue stage. It has an all-in-one audio/video/power/amplifier board toward the rear. The sound quality from the phono jacks isn't offensive but the noise floor is very high, and the less said about the amplifier stage the better (turn the volume to zero and there is still a loud hiss from the speakers, along with mechanical and signal noise coming through). So all that has to go.

From what I can see, the output from the DAC is jumpered directly from the CD board to this all-in-one monster. I have connected this output directly to the amplifier board I'd like to use in its place, and I get a good, noise-free signal from this, but it isn't line-level so will need a replacement output stage.

Not being too au-fait with the specifications of the raw signal, can anyone confirm what I need to do with this? Would a simple op-amp with a gain of somewhere between 10 and 100 sort this, or is manipulation required?
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Old 15th April 2012, 08:39 AM   #2
gfiandy is offline gfiandy  United Kingdom
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What are you trying to achieve?

DAC output are usually in the 1 to 2 vRMS region whic is line level. They are not alway capable of driving loads but it sounds like you have been able to drive your amplifier from it fine.

Is it too quiet?
If so an op amp buffer with a gain of around 5 will probably increase the volume enough but you will need a volume control, does your system have one?

The standard spec for line level out of a consumer DAC is 2vRMS but lots of equipment doesn't comply with this as there is no legal requirement to meet any specification.

Regards,
Andrew
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Old 15th April 2012, 09:48 AM   #3
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TBPH you would be just as well served to go onto E-Bay and purchase a mini Hi-Fi for 50.00.
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Old 15th April 2012, 10:34 AM   #4
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Technics HD51 Mini Hi-Fi System - VERY GOOD CONDITION | eBay
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Old 15th April 2012, 04:37 PM   #5
jjne is offline jjne  United Kingdom
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Yes, the output is too quiet. Plugging directly into a cheap TA2024 I'm getting a weak, but good quality signal directly from the DAC.

I had intended to mount a volume pot on the side of the unit (and just run that as a passive preamp), with a small rotary switch between the CD player and the output from the Pi. If the output from the DAC had been sufficient (the Pi's almost certainly will be as it's a 3.5mm headphone jack) this would have sufficed; as it is I'm going to have to buffer the output.

The advantage of using this box as a chassis is the existence of a power supply that appears to be oversized, along with the fact that there's plenty of space to mount stuff. There's a 5A/15VAC tap (being used by the existing chipamps), which can be regulated to +12V for the 2024 (or I can run it from a separate PSU), and there's already a 7805 in there which isn't being used to its full capacity so the Pi can be tapped from that.

Yes, I know I could go out and buy a bookshelf unit but it wouldn't be as flexible, or as enjoyable to put together, or as cheap (as I now have the parts) as doing it myself. The player has a decent amount of free space, and a bunch of speaker/phono terminals at the back which can be easily redeployed. I also have a couple of spare pairs of good quality speakers which should go well with the Tripath.

I may end up just buying the mini system anyway but there's no rush for that.

I will put together a buffer with a 5532 I have in the spares box and see how it goes. Thanks for the replies

Last edited by jjne; 15th April 2012 at 05:05 PM.
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Old 15th April 2012, 05:11 PM   #6
gfiandy is offline gfiandy  United Kingdom
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The NE5532 works best as an inverting amplifer as there is no common mode voltage stress on the input stage so I recommend this configuration.

If the signal sounds distorted reduce the gain as it's probably clipping. If it gets very hot it's probably unstable, ensure the decoupling is close and there is no capacitance to ground directly on the output. 100 ohms is enough to isolate the out put, put it in series with the output if you have this problem.

Hope it works out well

Regards,
Andrew
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Old 15th April 2012, 09:37 PM   #7
jjne is offline jjne  United Kingdom
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Thanks for the advice Andrew -- I set up a stripboard with the op-amp (actually a TL072 as it was the only one I could find -- I have a pack of ten 5532s on the way so I'll swap it over later -- shouldn't make too much difference for a simple circuit though right?), set to a gain of around 5 in an inverting configuration.

Seems to be working well, and I will attempt to increase the gain later as it's still a bit quieter than a typical CD player tends to be, but the good news is that the sound seems good to me. The noise floor is pretty much non-existent, and I did not notice any distortion (although given it was on the bench, with a pair of cheap speakers -- didn't want to blow anything expensive lol). It didn't sound particularly any different to the input from my Sansa MP3 player, so I think I have a good basis to move forward.

Thanks again
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Old 15th April 2012, 10:13 PM   #8
jjne is offline jjne  United Kingdom
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BTW -- it might sound like I have a bit of a cavalier attitude to parts selection here -- but the thing is that this is an occasional-use cheap system to sit in a corner of a kitchen and burble away. As such, if I can get something equivalent to the line output of an old portable CD player it'll be sufficient -- there really isn't any point in putting any more effort into it. I wouldn't dream of running such a monster as my main system lol. Equivalent to 100 hifi system = fine in this case
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