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Old 27th October 2006, 07:10 AM   #1
pgbhat is offline pgbhat  India
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Default First DIY Amp

this is my first DIY amp using LM1875
attached is the schematics and the layout
looking for feedback from the experts here
thanks
pgbhat
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File Type: pdf layout.pdf (24.4 KB, 247 views)
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Old 27th October 2006, 07:12 AM   #2
pgbhat is offline pgbhat  India
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schematics
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File Type: pdf circuit.pdf (18.4 KB, 185 views)
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Old 27th October 2006, 07:23 AM   #3
Wynand is offline Wynand  South Africa
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I'm still a newbie in my own eyes so keep that in mind...

I take it the track making a circle around each amp is ground. I think that'll be a problem as it'll create a ground loop = hum on the speakers.


I'll look some more....
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Old 27th October 2006, 07:29 AM   #4
Wynand is offline Wynand  South Africa
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Try to get the bypass caps as close to the pins as possible (C1, C5 ect).
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Old 27th October 2006, 07:35 AM   #5
Wynand is offline Wynand  South Africa
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I just had a look at the datasheet.

If your input cap and R is 2u2 and 22K, shouldn't you include the 1M resistor aswell. It seems that you used the datasheet schematic. Why not use all of it?

Sorry it's Friday so my head can't seem to remember all the calcs.

Maybe one of the old hands can help out.
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Old 27th October 2006, 07:37 AM   #6
Wynand is offline Wynand  South Africa
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Can you get/solder SMD Resistors? It's easy to solder, you just get a pair of tweezers and wet one of the pads, place the resistor and then solder the other side.

Doing this you can use a smd Res for the feedback resistors (R3 and R5. Reducing a lot of track and freeing more space to get the caps close to their pins.
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Old 27th October 2006, 07:38 AM   #7
pgbhat is offline pgbhat  India
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hello Wynand
thanks for your reply
i will look into all the inputs u have given
is that 1M resistor required as i have put one pot for the input?
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Old 27th October 2006, 08:00 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi Pg,
the Negative Feedback (NFB) RC time constant is 22mS (R9*C7). This will roll-off the bass too early. It should also be lower frequency (minimum half an octave) than the input filter bass roll-off. The input is set to 48mS so the NFB>=68mS. Change the C7 to a non polar 68uF or 100uF.

The schematic makes the classic mistake of showing all the grounds/returns as the same symbol.
They all do a different job and deserve a different symbol. Unfortunately simulators and PCB design software cannot cope with this so we regularly see the same mistake on many schematics.

here goes:-
input ground
amplifier front end ground
power supply 0volt reference
speaker return
decoupling common
Zobel return
transformer centre tap
mains safety earth.

Just giving them names helps to understand the different function.
EACH one should have it's OWN return to a central star ground. But this is uneconomic.
So part two follows:
Combine the RCA input ground + volume control ground + input filter ground + NFB ground. Take this trace off the board as a separate wire to central star ground.
Take the decoupling off the board as a separate wire to central star ground.
Take the speaker return AND the Zobel return direct from speaker terminals to central star ground.
Take the transformer centre tap to the central star ground.
Take the PSU 0v to central star ground.

DO NOT put the central star ground on the smoothing capacitors!!!!!

Take the mains input earth wire direct to a permanent chassis fixing and NEVER remove it. Not even for maintenance. A welded connection (NOT soldered) would be ideal but a permanently bolted connection will do as second choice.
Above the permanent connection add a second nut and secure a wire running to a disconnecting network.
The disconnecting network can consist of any or all of the following all wired in parallel.
Power diode bridge (2 by 25A) wired in inverse parallel,
Power resistor (2r2 to 22r),
High frequency capacitor (22nF to 220nF ceramic 100V),
Power thermistor,
Take a wire from the disconnecting network to either:-
a. the RCA input ground or
b. the input ground connection on the PCB.
Choose which is quieter with your sources.

R11 and R12 will run very hot and increase the ripple on the PSU.
If you decide you really need discharge resistors then change them for a higher value, try 2k2 to 10k.
You could also wire in a series combination of LED and resistor across each pole of the PSU to show LIVE power.

Finally the volume control is going to screw all your input and source impedances and change the input filter turn-over frequencies. It will act like a variable tone control as you adjust the volume. If you set the filters high enough to avoid audible HF effects then one does not effectively filter RF interference. Some amps don't like RF at the input.
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Old 27th October 2006, 08:50 AM   #9
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Hi AndrewT

I have seen on a couple of places that it is said that you do not use the strap between capacitors as your star point. Why is this? I'm just curious!

Gert
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Old 27th October 2006, 09:23 AM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
the 0v common connection between the smoothing caps carries all the high peak pulses that charge the caps. This is a very bad modulation to insert into the power rail of an amplifier.
Add this modulation to the ground reference and you are almost guaranteed to get buzzing at the amplfier output.

Look carefully at how the current gets from the transformer to the rectifier and then to the smoothing caps and how it returns to the transformer.

This route from and all the way back should be SHORT if possible and the LOOP AREA of the flow and return cables should be SMALL (no if possible this time).

When the amplifier is taking no current, the transformer centre tap does not inject any current into the 0v common. For this reason I solder the two centre tap wires together BEFORE I take them to the central star ground. The wire from the centre tap to the central star ground only carries the same current as the amplifier puts into the ground. If the amplifier exactly matches the +ve rail current and the -ve rail current then no current flows in the ground connection to central star ground and no current flows from transformer centre tap to central star ground.

When the amplifier sends current to the load or draws unmatched current from the supply rails then the transformer centre tap injects a matching current into the central star ground.

It follows that the current flowing from the centre tap into the central star ground is exactly matching the load current (smoothly changing audio bandwidth signal) +- a small ClassA bias for the unmatched amplifier currents.

So keeping the charging currents in that SMALL SHORT loop prevents any current and voltage effects on the amplifier ground.

The lack of voltage modulation on the central star ground and the signal grounds running to the amplifier front end is what makes the amplifier completely hum and buzz free. Some don't believe me, Try it.

BTW. this all came from this FORUM.
I was in the wilderness for 30years before I came hear/here to learn.
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