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Old 29th November 2009, 01:28 PM   #1
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Default I need more bass!

I have been upgrading an old technics se-m100 (130wpc, dual mono, c 1980's) to learn some theory and skills before trying to build an amplifier myself.
I replaced all the electrolytic capacitors, and put silmics in the signal path. I have changed the internal wiring for signal stages. It has an inbuilt dac with 4 pcm 63's, I have replaced the capacitors and op-amps with two LM4562NA. I set the bias and DC offset carefully- it's very stable.
Despite all this the bass is still a bit thin and clinical, though vocals are now much better.
My reference is a Sugden a21a, and I would like to get close to that sound balance!
The PSU has 4 10,000uf capacitors on the board (71v), which the schematics say can't be changed to a different spec. I have this need to put some 22000 100v capacitors in there instead, but this will be a bit of money! Would I be better cutting my losses? Will it make a difference? The amplifier boards sadly do have an ic on each (something labelled 5218p) - can I change these? Any thoughts very gratefully recieved! Best wishes, Jo
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Old 29th November 2009, 02:28 PM   #2
AndrewT is offline AndrewT  Scotland
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130W into what load impedance?
71V for what?

130W into 4r0 can be got from +-38Vdc to +-41Vdc
130Winto 8r0 can be got from +-51Vdc to +-55Vdc.
You should not need smoothing caps any higher than 63V

What high pass filtering is built into the amplifier? This may be setting your bass limit.
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Old 29th November 2009, 03:10 PM   #3
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Quote:
Originally Posted by AndrewT View Post
130W into what load impedance?
71V for what?

130W into 4r0 can be got from +-38Vdc to +-41Vdc
130Winto 8r0 can be got from +-51Vdc to +-55Vdc.
You should not need smoothing caps any higher than 63V

What high pass filtering is built into the amplifier? This may be setting your bass limit.
You could teak they feedback splitter capacitor a bit bigger to increase low end.
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Old 29th November 2009, 04:25 PM   #4
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Hi thanks for the replies. Sorry I wasn't clear- it's 130w into 8ohms (in theory). I can't tell if it has a filter from the schematics, but am just learning electronics again, so that doesn't mean much! Here is a dissassembly pic in it's native state, where you can see the PSU capacitors which I was thinking of replacing. I can't upload the schematics pdf as it is too big, and I am new to the forum. As you can see they are an odd shape and size, but I could probably squeeze bigger ones in (the DAC sits in the middle, between the two). Although this is all practice, I am getting to rather like the amp, so making it sound right would be great! Apologies again for my ignorance, I am still reading the books! Thanks again, best wishes,
Jo
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Old 29th November 2009, 05:03 PM   #5
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Quote:
Originally Posted by Jo90 View Post
My reference is a Sugden a21a, and I would like to get close to that sound balance!

Not going to be easy. The Sugden being class A simply has a different tonal balance. The original Technics caps may be drying out or simply have a leaner tonality. I am not sure i remember Technics amps having earth-shaking bass. It may also be the topology. Who knows? Plus there is the preamp section too unless you are only comparing the power amps.

The amp appears to be dual mono with only 10mF per rail which seems marginal. Upping the capacitance can only help although it may be necessary to replace the rectifiers as well.

Last edited by analog_sa; 29th November 2009 at 05:09 PM.
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Old 29th November 2009, 07:26 PM   #6
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Great thanks for that. On balance, I am going to have a go at the PSU capacitors. I'll track down the specification of the regulators from the schematics, to see if they need changing also. If it all goes pop, I will hopefully have learnt something about PSU's! I will let you know if I set the house on fire! Thanks again,
best wishes,
Jo
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