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Old 9th June 2010, 02:58 PM   #1
Galgo is offline Galgo  Israel
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Question How to amplify an Amplifier !?

Hello there,
I hope this Q is in the right place - it's related to amplifiers in general I guess.
I also hope it's not too stupid/insulting...

I recently experimented with few DIY Class D amplifiers and got very good results - mainly drived from my HTPC and SANSA FUZE.

Now I want to help a friend improve the sound reproduction of an entry level All-In-One Panasonic system he bought - it's one of these complete 5.1 speakers/DVD/Tuner/Amplifier systems with almost no I/O ports for external equipment.

The speakers are really tiny satellites which might be ok for TV/movies but defiantly not for music.

I was able to source a nice pair of KEF CRESTA 2 bookshelf speakers for him and connected them to the Front L/R channels instead of the original noise boxes...

It's already a significant improvement but I feel the integrated "30W/CH MAX RMS" don't have enough power to seriously drive them (sound quality is another subject of course...)

The unit obviously doesn't have any line level output... (maybe only headphone out - not really sure...) and we'd like to avoid avoiding the warranty by open the chassis nor adding separated components or replacing this system all together...

So - is there any way to DIY and amplify the already amplified signal coming from the front speaker connectors (while the other speakers still function and controlled through the unit of course...)?

There are many compact Class-D modules I can use and hide - but how can I use them on an amplified signal instead on line level?

Would something simple as just using a BIG resistor reduce the signal to line levels or is it more complicated than this?

thank you,
Gal
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Old 9th June 2010, 03:10 PM   #2
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Quote:
Originally Posted by Galgo View Post
Hello there,
[snip]So - is there any way to DIY and amplify the already amplified signal coming from the front speaker connectors (while the other speakers still function and controlled through the unit of course...)?

There are many compact Class-D modules I can use and hide - but how can I use them on an amplified signal instead on line level?

Would something simple as just using a BIG resistor reduce the signal to line levels or is it more complicated than this?

thank you,
Gal
Hallo Gal,

It's easy to reduce the speaker output signal in level with two (smallish) resistors or a potmeter and feed that to another amp to drive the speakers. But be aware that the sound will be louder only, NOT of better quality!

If there is a headphone output that will be far better quality-wise. You will need to experiment to see how loud that will be compared to the other speakers; you could use a potmeter between the headphone output and the input of the extra amp of course.

jd
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Old 9th June 2010, 03:19 PM   #3
Galgo is offline Galgo  Israel
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O.K.,
I guess I can try- any insights regarding resistors/Pot values? why do you say "smallish" resistors - wouldn't they have to dissipate significant power/heat?

The problem with the headphone out is that it automatically mute the other speakers and I couldn't find a way to change that... other than that it's obviously a better solution...

BTW - If I'll indeed "implant" an additional amp fed from the speakers out connectors - which wire should I leave longer - the one before the secondary amp and POT (which is now similar to an "interconnect") or the one after the secondary amp? total length before the speakers is around 2.5m - and I currently use Biwire to help .
I know it won't "improve" quality too much - but at least I'll prefer to reduce additional distortions to a minimum...
Gal
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Old 9th June 2010, 03:56 PM   #4
Galgo is offline Galgo  Israel
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Well - I checked the unit again - it's a PANASONIC SC-PT150 manual here:
Panasonic - ideas for life - Page not found

It has no Headphone out - just Mic in
At least It has a function to set volume level separately for each channel - so perhaps only a minimal potmeter or resistor is indeed required...

Amplifier spec. are :
DIN Output Power: Dolby Digital Mode
Front Ch 25 W per channel (5 ), 1 kHz, 1 % THD
Surround Ch 25 W per channel (5 ), 1 kHz, 1 % THD
Center Ch 25 W per channel (5 ), 1 kHz, 1 % THD
Subwoofer Ch 25 W per channel (5 ), 100 Hz, 1 % THD

Total DIN Dolby Digital mode power 150 W
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Old 9th June 2010, 04:12 PM   #5
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Originally Posted by Galgo View Post
wouldn't they have to dissipate significant power/heat?


Nope. Your new power amp will have a sufficiently high input impedance to allow for a pot in the kohm range - per example 2k. The sound quality won't be improved at all though, you will just be able to avoid driving the original amp into clipping. A much better plan will be to tap line level signal somewhere.
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Old 9th June 2010, 04:16 PM   #6
Galgo is offline Galgo  Israel
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Thank you...
I wonder how difficult it'll be getting the line level signal...
I won't be able to do much experimenting (it'll have to be a quick and elegant mod) - but maybe someone has any idea how this unit looks from the inside and were should I tap it?
Gal
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Old 9th June 2010, 04:20 PM   #7
Mooly is online now Mooly  United Kingdom
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Use a 10k preset across the speaker outputs, once you are happy with the level substitute with fixed resistors.

I take it the outputs are "ground referenced" and not from a bridge output... check the speaker negatives go to signal ground.
Worth checking to avoid problems.
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Old 9th June 2010, 04:24 PM   #8
Mooly is online now Mooly  United Kingdom
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Also you will have audio on the 21 pin scart connector... can't remember the pins off hand. You could perhaps make a line feed from those, trace them back and see where they come from (a big IC probably ) and either add a feed in parallel or build and add a small unity gain buffer.
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Old 9th June 2010, 04:28 PM   #9
Galgo is offline Galgo  Israel
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I'm not sure it outputs audio through the SCART - not by the manual at least...
It seems that unit can get many audio INPUTS but nothing goes out apart from the amplified signal for speakers...
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Old 9th June 2010, 04:30 PM   #10
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Originally Posted by Mooly View Post
Also you will have audio on the 21 pin scart connector... can't remember the pins off hand. You could perhaps make a line feed from those, trace them back and see where they come from (a big IC probably ) and either add a feed in parallel or build and add a small unity gain buffer.
Great idea Mooly! The scart is standardized, no? Should have the info somewhere on the 'net.

Edit: bingo: http://pinouts.ru/Home/Scart_pinout.shtml

jd
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