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Old 1st April 2005, 07:53 AM   #1
sq_guy is offline sq_guy  Australia
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Lightbulb Help with car amp tweaking

Hi all,

I am using cheap car amp and i really want to learn what things can be done to improve its sound.

I know it might be a wise decision since the amp itself is not really a genesis, audison or other expensive brand. The keywords here are "learning" and "hobby"

So here's the guts:
- Electrolite cap before toroid: 2 * 1000 mF
- Electrolite cap after toroid: 2 * 4700 mF
- Rectifier bridge: 2 * YG 225D2
- Amp transistor D718 & B688 (made by KIA?)
- Coupling cap: couldn't find any ???

Now... I need a guidance on what can be replaced with higher quality without too much modification to the circuits.

I am thinking to replace the electrolites with higher value and higher quality such as panasonics or elna to improve bass.

I don't have a deep knowldege about electronics but I think the transistor is not mosfet type. Is it possible to upgrade it to better type?

Anymore suggestions?
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Old 1st April 2005, 08:16 AM   #2
Did it Himself
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I'd be wary of mucking about the changing the caps in an SMPS. It can cause instability and worse performance if you don't really know what you are doing nor have access to an oscilloscope.
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Old 6th April 2005, 11:46 AM   #3
Pulse-R is offline Pulse-R  Australia
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I'm about to replace the input op-amps and decoupling caps in my car amp
it's noise is around -105 dB or so - far too noisy for SQ
I'm hoping to get it below 120dB - to at least come close to the 24/96 feed to it
I'm pretty sure it's the input chips because I can 'turn down' the noise by reducing the input sensitivity.
The caps are to spice up the sound a bit - the depth is a little shallow, and lack well-defined bass
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Old 13th April 2005, 06:18 AM   #4
Clipped is offline Clipped  Thailand
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you can change the caps to low esr types with the same values if you want to play it safe.
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Old 13th April 2005, 11:59 AM   #5
sq_guy is offline sq_guy  Australia
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Thanks Clipped. If it seems like the only way I might do that. Any comments on the transistor? Is it replaceable with different model?
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Old 13th April 2005, 12:26 PM   #6
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Changing to low ESR types may not be beneficial as the inherent resistance in the 'normal' caps may prevent ringing. For playing with stuff like this you really need to know what you are doing and have access to measurement kit.
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Old 22nd April 2005, 12:47 AM   #7
robjak is offline robjak  United States
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Default Simple amp mods?

Hi

You should first get or trace a schematic of the amp before modding so you really know what each components function is.

If you haven't done so already, you should read up on the "objective" causes of NFB amp distortion first to see the complexities involved in what you are trying to do. Simple parts swapping of output devices is usually not possible without redesigning the amp, increasing bias, affecting thermal stability, etc.

Here's a DIY source for the power transistors. Sanken LAPT's are considered the best but you will need to redesign to use them.

http://www.ampslab.com/components_power.htm

The same applies to the SMPS. You can easily cause things to "blow up" in a hurry there.

I am not saying mods can't be done but you really need to know what you are doing, almost to the point of becoming an amp designer yourself! So if you want a challenge here you go:

http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm#2

I don't agree with everything he says but he provides a good foundation to build on.

A simple mod that D.Self suggests on that page is to increase the size and voltage of the "feedback network blocking cap". He also uses a diode across it but I haven't experimented with that yet.

If your amp uses a cap in the feedback network (most do) it may be too small and will cause significant distortion in the bass. This will be clearly audible "subjectively" Don't skimp here go BG NP or OS-CON since it's only one cap/channel.

I cant recommend messing around with the power supply output caps becuase they are spec'ed along with the toroid to provide the best transient response. If you want to improve the supply of "instantaneous" current to the output stage you could add low esr caps close to the output transistors so as to keep them from greatly affecting the power supply caps. But to do this as others have mentioned may require a 'scope to make sure the power supply or amp doesn't do "funny" things when heavily loaded.

Another thing you may want to try is to make a "composite" cap bank that will provide better line regulation for your amp. This cap must be wired in between the fusing and the power supply.

By "composite" cap I mean a bunch of small identical caps wired in parallel to provide much better discharge current and speed than a larger stiffening cap. To be effective many caps 10 - 40 need to be used depending on the needs of the amp. Because of space constraints amp manufacturers don't leave enough room in the amp for you to do this mod "inside" the amp so you will be forced to make a box that sits under your amp that contains the caps.

If you are still interested I can elaborate further...
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Old 29th April 2005, 06:53 AM   #8
sq_guy is offline sq_guy  Australia
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Hi Robjak,

Thank you for the detailed explaination. After thinking about it I decide to do more resarch and learn more before attempting any mods.

Thanks!
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Old 2nd May 2005, 10:53 PM   #9
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Default bridging amps

ive got two amps. ones a jensen 600watt and the other is a sony(watts unknown.) is ti possible to bridge them if so how would i do it? any help appreciated.
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Old 3rd May 2005, 09:45 AM   #10
Pulse-R is offline Pulse-R  Australia
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It can be very difficult to bridge two dis-similar amplifiers, there are many factors including the impedances of the output stages, etc.

Generally, bridging two amps is only done if they are identical types.
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