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Old 25th February 2013, 04:39 AM   #1
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Default LM3875 gainclone too much treble

Hi

Just getting my LM3875 to work properly ... sound is good ... but compared to my Gainclone LM1875 ... the LM3875 has more treble and less bass based on my listening ... isnt it normal ...?

How to get the Treble tamed down to balanced out the frequency response? ... Suggestions?

TQ
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Old 25th February 2013, 05:11 AM   #2
Jay is offline Jay  Indonesia
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Schematic?

Bigger supply caps, bigger feedback caps, input pF decoupling, feedback compensation, output coil, etc.

But better amps normally have "more" treble (actually poorer amps lack the treble). When this treble can be reproduced without distortion, then you have the high-end sound. Distortion may come from cheap tweeter or inferior crossover. You may want to fix those, or kill the disturbing treble and be happy with med-fi.
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Old 25th February 2013, 05:19 AM   #3
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schematic used is the directly one from NS
optional components ... (capacitor) not used ...
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Old 25th February 2013, 02:21 PM   #4
JMFahey is offline JMFahey  Argentina
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Well, that amp is absolutely flat, so , as suggested above, "the other amp" must be lacking treble.
And, personally, I *would* use the "optional" feedback cap.
Not much sense having an amp with DC response, until you have a speaker with DC response
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Old 25th February 2013, 10:33 PM   #5
djoffe is offline djoffe  United States
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Your design is missing many "optional" components that aren't optional. National/TI shows them later in the data sheet. Mostly things like the output inductor, damping resistor, and zobel network. Without these, you may get transient signal dependent oscillations that probably would brighten up the sound.

Another poster suggested of these kinds of amps, that if your heatsinks are too small, the chip temperature rises to the point where the Spike protection kicks in, which makes a sound rich in high frequencies.

You can get an idea of what kind of heatsinking is appropriate from the data sheet, or taking a look at an example amp build with LM3886s, which is a very similar chip.

Akitika GT-101

Note that this version shown is single supply, so no thermal washer is needed under the chip...this decreases thermal resistance, and keeps the chip cooler.
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Old 26th February 2013, 03:47 AM   #6
gootee is offline gootee  United States
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What rail voltages?

What transformer VA rating (per rail)?

What values of reservoir capacitance per rail?

What cap values are used for Cs?


If everything else is OK, you would also want Cs (local decoupling and bypass caps) to include 0.1 to 1 uF high-frequency bypass cap directly at each power pin, and also another 470 uF to 2200 uF (or more) for local decoupling, to be able to supply transient current demands with low-inductance (i.e. short length!) connections (preferably using three or more smaller paralleled electrolytic caps with values that sum to the desired total capacitance per rail, mounted as closely as possible the chip's power pins).

For your power supply, you should have a minimum of roughly 14,000 uF of reservoir capacitance per rail. That is based on calculating the capacitance required to be able to supply the current to produce a 5-Amp-peak sine wave at 20 Hz without causing the rail to dip by more than 1 Volt, assuming that sufficient-current recharging pulses are being provided every 8.3 ms (60 Hz mains; add 15% or so for 50 Hz mains). Using 3X 4700 uF would work nicely, for that.

But I would probably want roughly at least 3X 14000 uF per rail, which should be able to support 5 Amps DC current into 8 Ohms, continuously, for rails rated at 100 Watts RMS (40 Volts peak), without letting the rails dip by more than 1 Volt, assuming 120 Hz recharging pulses with sufficient current are available. That should cover almost any "worst-case" signal, at max output power.

For the equations, see the post at Finalizing TDA2050, LM3886 , which were explained and derived at Power Supply Resevoir Size .

Your transformer VA (Volt-Amps) rating should be AT LEAST 1.5X to 2X the rated maximum RMS output power, just to account for heatsink losses (i.e. the amp is not 100% efficient). But I would use at least 3X the max rated RMS Watts for the VA rating, unless I had significant headroom in the rail voltage.

Cheers,

Tom

Last edited by gootee; 26th February 2013 at 04:03 AM.
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Old 26th February 2013, 04:21 AM   #7
gootee is offline gootee  United States
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I meant to say, first, that you probably do not have added treble, but, rather, a lack of bass.
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Old 26th February 2013, 05:02 AM   #8
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Power supply i am just using 4700uf for the positive and negative rail ... for the transformer i used a large toroid but i didnt know the VA rating because it didnt state on the transformer (it was actually salvaged from a broken amplifier i have)

BTW here have some photos of the completed and working LM3875 amplifier i built
Attached Images
File Type: jpg IMG_4392.jpg (982.2 KB, 633 views)
File Type: jpg IMG_4395.jpg (787.7 KB, 579 views)
File Type: jpg IMG_4396.jpg (941.3 KB, 536 views)
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Old 26th February 2013, 08:12 AM   #9
JMFahey is offline JMFahey  Argentina
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Quote:
But I would probably want roughly at least 3X 14000 uF per rail,

Well, if cost is not an objection ..... be my guest.

But what danny is using now is fine.
And the transformer looks big enough.
Pity out of the 80000 Chinese characters I still need to learn, oh, 79999?
So for now I can't read the transformer label .
Just give me some time.
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Old 26th February 2013, 11:16 AM   #10
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ok ... there written was 22V 0V 22V ... another set of wire 22V 0V 6V but the latter set were not used since it was used by the previous broken amplifier for their unknown purpose ...
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