Hello there...
At the moment I am trying to build my own preamp for a STK463-Amplifier. Because I have no expirience with building any kind of audio-devices, I am hoping to get some tips from here 😀
Here´s my schematic:
And here´s the PCB-Layout:
Finally the routing on the PCB:
Now my questions:
I think there´s no problem with routing under the input-caps. Or am I wrong?
Are there any problems with placing the powersupply between the input-caps?
Thanksalot 😉
At the moment I am trying to build my own preamp for a STK463-Amplifier. Because I have no expirience with building any kind of audio-devices, I am hoping to get some tips from here 😀
Here´s my schematic:
An externally hosted image should be here but it was not working when we last tested it.
And here´s the PCB-Layout:
An externally hosted image should be here but it was not working when we last tested it.
Finally the routing on the PCB:
An externally hosted image should be here but it was not working when we last tested it.
Now my questions:
I think there´s no problem with routing under the input-caps. Or am I wrong?
Are there any problems with placing the powersupply between the input-caps?
Thanksalot 😉
You have the ground under the input caps, that's ok.
I don't get why the ground traces are separate from the massive area of copper. It doesn't make sense, I would leave just one huge piece.
And personally I don't like too much 5532. Not bad, but it can be better (LM4562 is on eexemple, there are others too).
And... a gain of only 2X (6dB)? Why?
I don't get why the ground traces are separate from the massive area of copper. It doesn't make sense, I would leave just one huge piece.
And personally I don't like too much 5532. Not bad, but it can be better (LM4562 is on eexemple, there are others too).
And... a gain of only 2X (6dB)? Why?
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The purpose of R1 is to correct for the offset caused by the input bias current of the op-amp. Hence, its resistance should be R3||R7 or 5 kOhm (use 4.99 kOhm, 1 %).
I strongly suggest using a star ground (google it!) topology and avoiding the copper pours. Using a star ground will give you better performance with regard to hum and noise. I've been able to lower the induced hum of a few amps by as much as 30~40 dB by implementing star ground -- and using the proper node in the circuit as the star ground.
I suggest that you move C3, C4 closer to the IC to lower the supply impedance at HF. That's the idea behind those caps...
~Tom
I strongly suggest using a star ground (google it!) topology and avoiding the copper pours. Using a star ground will give you better performance with regard to hum and noise. I've been able to lower the induced hum of a few amps by as much as 30~40 dB by implementing star ground -- and using the proper node in the circuit as the star ground.
I suggest that you move C3, C4 closer to the IC to lower the supply impedance at HF. That's the idea behind those caps...
~Tom
The reason for using the NE5532 is that i got 50 of them for free, so why not use them? 😀
The groundplane has been set-up. Makes the wiring more easy (forgot how to use it in Eagle 😱) Also C3+4 moved closer to the chip. The "powerbuffer" has been doubled up to 2x100µF per rail (looks nicer without the hole between the input-caps^^).
What are "normal" gains for an preamp and how can i set the gain to the desired value?
The groundplane has been set-up. Makes the wiring more easy (forgot how to use it in Eagle 😱) Also C3+4 moved closer to the chip. The "powerbuffer" has been doubled up to 2x100µF per rail (looks nicer without the hole between the input-caps^^).
What are "normal" gains for an preamp and how can i set the gain to the desired value?
That depends on what you are trying to do. A CD player with a high output won't need any amplification at all if you are feeding a power amp with a 100mv sensitivity.
On the other hand an old tape deck with DIN outputs feeding a power amp with a 1 volt sensitivity will.
You set the gain by varying the feedback factor.
In your circuit the gain is R3+R7 divided by R7. So that'S 10e3 +10e3/10e3 which is 2. Increasing R3 will therefore increase the gain, as will reducing R7.
If you are using 5532 (which are great for straight gain block use) then I would consider AC coupling R7 (the feedback return) to minimise offset problems and keep R3 the same as R5 and remove (short out) R1
The most obvious oversight is the lack of a build-out resistor on the op-amp outputs. The resistor decouples from the capacitance of the cable and also limits current in the event of a short. Adding capacitance on the opamp can cause ringing and instability. The build out is important even if the next stage is on the same PCB. Low value 27-100 ohms is common.
G²
Which bit 🙂
The STK power amp will need a certain voltage at the input to deliver full output to the speaker. CD players put out the most voltage, and can often be used with no preamp at all. You can just feed the output straight into the power amp via a volume control.
If you use older stuff, then that usually has a much lower output and if connected as just described wouldn't be loud enough... so you need a preamp... just as you have drawn. How much gain you need depends on the equipment concerned but it's nothing to get worry over.
Make the preamp... and if the output (volume) isn't enough it's just a case of altering one resistor (R7) per channel... the feedback "return" resistor. Making the resistor lower increase the gain.
The 5532 isn't the best for absolute DC precision... there will be a small DC offset at the output. I'm assuming the input to the power amp is AC coupled (with a cap) so this problem doesn't really matter anyway.
Volume controls should not have any DC applied to them, even a few millivolts as it makes them crackly.
Where is your volume control going ?
Attachments
huge sized parts can be easylly become microphonic ...Due to mechanical size it is easy to "pick up things on the way "
so here and hear what i read on a greek forum and please advice
MK series capacitors designwise have foil like the elctroyltics so if you connect the outerfoil side and related connector of the side of the circuit with the lower impentance then the "microphonic" behavior of the capcitor is a lot better
there is also advice on how to locate which conector is which ...
does it make any sense to your opinion ????
so here and hear what i read on a greek forum and please advice
MK series capacitors designwise have foil like the elctroyltics so if you connect the outerfoil side and related connector of the side of the circuit with the lower impentance then the "microphonic" behavior of the capcitor is a lot better
there is also advice on how to locate which conector is which ...
does it make any sense to your opinion ????
The schematic is o.k. I would't use the additional 100uF cap. In addition I'd lower R5 to 47k or 22k if the offset voltage is your concern.
Unfortunately the grounding isn't well done as the return path for your blocking caps is quite long. You should strictly route the power supply traces as shown in your schematic. From the right side of your 100n caps to pin 4/8 of the op-amp. Use a solid signal ground plane after the power supply decoupling caps. Think from input (power supply) to output (signal / amp). Ground plane shound be as large as possible without any galvanic coupling for eddy power supply currents.
Unfortunately the grounding isn't well done as the return path for your blocking caps is quite long. You should strictly route the power supply traces as shown in your schematic. From the right side of your 100n caps to pin 4/8 of the op-amp. Use a solid signal ground plane after the power supply decoupling caps. Think from input (power supply) to output (signal / amp). Ground plane shound be as large as possible without any galvanic coupling for eddy power supply currents.
It's a high brow name for "Direct Current Coupling".
Or it's not transformer or capacitor isolated.
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