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Description:
In this application, we are building a gaincard like amplifier. This
application type is named gainclone in audio world. To take a satisfactory
audio response, we are adding a Linkwitz equaliser to the feedback line
and adding bass compensation also.
We are using LM3886 which is the revised version of its brother LM3875.Briefly features of LM3886 are;
Maximum Output Power: 68W RMS - 108W Peak
THD: %0.03 at 60W
SNR: 110dB at 60 W - 92.5dB at 1W
PSRR: 120dB
Protection Circuitries: DC/AC Short circuit protection and thermal protection
Output Class: Conjugate AB-A
These are the parameters that we are interested in. Honestly this
values are much more better than many of HI-FI amplifiers sold in the
market. Especially it is hard to find 110dB signal to noise ratio. And
another property is, when there is no input, this amplifier is quite
like a dead. It is nearly impossible to hear any noise when you stick
your ear to the speakers.
The circuit below is the recommended design by National Semiconductors
engineers. As you can see it is a plain and applicable circuit.
Essentially we did any change to the input line hence we didn't add
extra noise. Rin is input level potentiometer, Rb is current limiter
potentiometer and Rf1-Ri are gain resistors. In this application, gain
is 20k/1k=20 (26.4dB). 22uF capacitor provides the gain curve start
from about 7 Hz so it filters unwanted low frequencies.
This is our design. As you can see, negative feedback line is revised. In this
way, about 7dB is gained at 30Hz-70Hz band, and reached about 34db at
peak point at 30Hz.
When 26v DC supply is applied to the given design, with 4 ohm
speaker load and 2Vp-p input signal it gives about 65W RMS power. For
users who want to use 8 ohm, we recommend +/- 38V voltage supply. Of
course less than this value may be applied too.
Supply circuit is shown below, It is simple too. There are two KBU
series (6A) bridge diodes and 2x10000 uF capacitors. As a general
rule, 2200uF stabilization capacitor is used per ampere but we are used
to use 4700uF per ampere. In our circuit, 2 Amperes will be drawn per
line and we use 10000uF capacitors. We recommend you to use 2x10000uF
capacitors per each line if you will use the amplifier in party mode.
Generally in HI-FI applications, toroid transformers are used. But this
kind of transformers are expensive and large in volume. Advantage of
this transformers is they have very low leakage magnetic flux so
they can be placed in the same box with amplifier. But we choose to
place transformer and amplifier in separate boxes so we are using standard
transformers. Transformer values should be;
For 8 Ohm - standard mode : 220 / 2 x 24v (with middle out) at least 250W
For 8 Ohm - part mode: 220 / 2 x 24V (with middle out) at least 350W
For 4 Ohm - standard mode: 220 / 2 x 18v (with middle out) at least 250W
For 4 Ohm - party mode: 220 / 2 x 18v (with middle out) at least 350W
Application:
As you can see PCB components are tight but as a result it is
compact ,small and noise free. Especially short negative feedback line
supports this function.
Assembly is not very clear but it gives perfect results. You should use %1 tolerance - metal strip resistors.
This is the PCB side of the board. You can see some components are
connected to this side to keep the power line very short.Line under the
ICs is fed by solder. This process takes time but not so difficult. You
should not omit this step.
Click here to download PCB, layout, connection diagram files.
Notice that there are two types of LM3886 IC. One of them is unisolated LM3886TA
and the other one is isolated LM3886TF.
Not to mess with isolation, we used TA model and connected it to the
heatsink by using silicon gress. For 8 Ohm and TA users, we recommend
2.8°C/W heatsink. This means about 5cm x 10cm, 8 or more layered simple
aluminum heatsink. For TF users this value is 1.8°C/W and this means
double size of the heatsink that we said just before.
Placement of circuits in box is given in the downloadable .rar file. You should open aeration holes on your
boxes. You can use standart speaker terminals for speaker outputs and
RCA terminal for signal inputs.
Notes:
1 - RCA connector's ground terminal must be connected to the middle
of the AF coded terminal block directly and using the shortest way by using a
thin wire.
2 - RCA connector's right and left signal input terminals must be
connected to the rightmost pins ( from PCB view) of the sound level
potentiometer by using LIYCY cable.
3 - Take two output cables (again use LIYCY cable) from the middle pin of the
potentiometer and connect them AF coded terminal blocks' right and left
terminals.
4 - Don't use the amplifier without the potentiometer.
Author: Dr.Hardware
Circuit-Projects & Diyaudiotr
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-Hazar