I have a question about this preamplifer i like to build.
Here is the schematic:
I've already built an amp, now I want to make the preamp above.
Thing is I like to put the pot (VR1a) before the preamp but don't know what cap should I leave. The 22uF or the 1uF? see the mid-section of the schematic.
Also if someone could explain why the pot is there and not at the beginning of the circuit I would greatly appreciate it 🙂
Here is the schematic:
I've already built an amp, now I want to make the preamp above.
Thing is I like to put the pot (VR1a) before the preamp but don't know what cap should I leave. The 22uF or the 1uF? see the mid-section of the schematic.
Also if someone could explain why the pot is there and not at the beginning of the circuit I would greatly appreciate it 🙂
The pot is where it is because it gives 100% defined conditions for its operation. Its fed from a low impedance point (1st opamp). Placing the pot at the front means that you get the full noise contribution of the first stage all the time. With the pot where it is now the noise is reduced as the volume is reduced.
If you want to move the pot then you can couple pin 1 to pin 5 directly and omit all the other parts.
You could also omit the second opamp altogether and just copy the parts at the output to pin 1 of the first opamp.
It really is better as it is though.
If you want to move the pot then you can couple pin 1 to pin 5 directly and omit all the other parts.
You could also omit the second opamp altogether and just copy the parts at the output to pin 1 of the first opamp.
It really is better as it is though.
Thanks for the answer Mooly!
One more question about the circuit, what will be the difference if I put normal electrolytic caps instead of those non-polarized(NP) ones?
One more question about the circuit, what will be the difference if I put normal electrolytic caps instead of those non-polarized(NP) ones?
If your music source or power amp has DC on the pin, and you point polarized electrolytic caps the wrong way, they wlll blow up. That is if the DC is more than a volt or two off - polarized will usually take a volt backwards. The usual practice is to use polarized caps, and point the minus out both ways, in and out. This assumes your driver and load will be nearly zero volts compared to the analog ground of this circuit. This could be true if your power transformer has a center tap that is connected through a high value resistor to the safety ground of the wall plug. If the plus or minus of the power transformer is referred to safety ground with a high value resistor, the inherent DC bias must be accounted for with your input and output caps. I usually put 470 k or a megohm resistor to connect the op amp analog ground to something - The ring of the power amp RCA cable usually.
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Difference. None whatsoever imo. Just be sure to fit the caps according to the DC conditions present in your circuit. That simply means measuring the voltage across the cap and making sure its fitted to match.
Also do as Indiana suggests and add a high value resistor to ground on the input side of the input cap.
The 4.7uf input cap is larger than needed, and here you could use a small 0.47uf film cap. If you do then increase the 22k resistor to around 470k. There is no noise penalty doing that when the input is in use and connected to something.
post1 sch
I would move the final 100r from the output to right next to pin7 of the dual opamp.
This resistor isolates the opamp output from parasitic capacitances and locating it far away from pin7 introduces the risk of unknown parasitics affecting performance.
And I agree with Mooly.
Add a grounding resistor to the input side of the input DC blocking capacitor and to the output side of the output DC blocking capacitor. These can be >>100k, I use 2M2 for this grounding duty.
The resistor takes any leakage current through the coupling capacitor/s to audio ground and prevents a voltage building up on the in/out if the interconnect is disconnected.
Hot plugging with a charge on the coupling capacitor can damage equipment.
For a power amplifier I use input filters set to 0.68us to filter RF and other interference and 80ms to filter/block DC and other VLF non audio signals.
I increase these by one octave for earlier stages like pre-amplifiers, or Buffer, etc.
i.e. use ~0.3us and ~160ms as the filters before one reaches the power amplifier.
Adopting this philosophy for your preamp would result in 1.5uF feeing into 100k for 150ms.
This will not remove any low bass audio signal and not add significantly to the power amplifier roll -off.
To match this, the input filter shown as 4u7F & 22k can be:
4u7F & 33k
3u3F & 47k
2u2F & 68k
1u5F & 100k
All of these capacitor values can be obtained at reasonable cost in polypropylene but can become a bit big.
You will get virtually identical performance using MKT or MKS in much smaller sizes and much reduced cost.
Because these capacitors do not act as filters for the wanted audio signal they contribute no audible distortion.
As an experiment you could temporarily replace the input capacitor 1u5F (& 100k) with a pair of 4u7F (16V or 25V) polar electrolytic placed back to back. And compare the sound quality.
I would move the final 100r from the output to right next to pin7 of the dual opamp.
This resistor isolates the opamp output from parasitic capacitances and locating it far away from pin7 introduces the risk of unknown parasitics affecting performance.
And I agree with Mooly.
Add a grounding resistor to the input side of the input DC blocking capacitor and to the output side of the output DC blocking capacitor. These can be >>100k, I use 2M2 for this grounding duty.
The resistor takes any leakage current through the coupling capacitor/s to audio ground and prevents a voltage building up on the in/out if the interconnect is disconnected.
Hot plugging with a charge on the coupling capacitor can damage equipment.
For a power amplifier I use input filters set to 0.68us to filter RF and other interference and 80ms to filter/block DC and other VLF non audio signals.
I increase these by one octave for earlier stages like pre-amplifiers, or Buffer, etc.
i.e. use ~0.3us and ~160ms as the filters before one reaches the power amplifier.
Adopting this philosophy for your preamp would result in 1.5uF feeing into 100k for 150ms.
This will not remove any low bass audio signal and not add significantly to the power amplifier roll -off.
To match this, the input filter shown as 4u7F & 22k can be:
4u7F & 33k
3u3F & 47k
2u2F & 68k
1u5F & 100k
All of these capacitor values can be obtained at reasonable cost in polypropylene but can become a bit big.
You will get virtually identical performance using MKT or MKS in much smaller sizes and much reduced cost.
Because these capacitors do not act as filters for the wanted audio signal they contribute no audible distortion.
As an experiment you could temporarily replace the input capacitor 1u5F (& 100k) with a pair of 4u7F (16V or 25V) polar electrolytic placed back to back. And compare the sound quality.
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OK I've redrawn the schematic with the suggested changes. The resistor at the input is changed from 22K to 33K, added those ground resistors and shifted the 100ohm resistor closer to the pin7 of the op amp.
Please check if the schematic below is correct:
What is off please tell me to correct it 🙂
Please check if the schematic below is correct:
What is off please tell me to correct it 🙂
As drawn...
Be sure to fit C1 and C2 according to DC potential in your amp. R11 is not required. R8 is not required.
What would be better imo would be to replace R2 with a 390 or 470k and use a 0.47uf film cap for C1
Be sure to fit C1 and C2 according to DC potential in your amp. R11 is not required. R8 is not required.
What would be better imo would be to replace R2 with a 390 or 470k and use a 0.47uf film cap for C1
I would remove R10 rather than R11 and change C6 to 1u5F
Now that you have removed the ferrite bead, I would increase R3, somewhere between 510r and 2k
C1 should be plastic film, or >>4u7F electrolytic
C2 & C7 should be bi-polar since there should be AC signal only at these locations.
Follow datasheet for supply rail decoupling.
Now that you have removed the ferrite bead, I would increase R3, somewhere between 510r and 2k
C1 should be plastic film, or >>4u7F electrolytic
C2 & C7 should be bi-polar since there should be AC signal only at these locations.
Follow datasheet for supply rail decoupling.
I built the preamp but I'm having problems with the volume control. Thing is, the volume won't go down to zero-mute even if the pot is set to the lowest position.
There is still some signal/music going through him.
Without the preamp, the volume control worked fine.
Tried other pots, the one now is 50K stereo log. The others were 10k,470R and 250k none of them solved the problem...
Tried rewiring it to the utmost front, still the same issue.
Even tried to lower the gain on the LM, still the issue remains. Currently running out of options. Any suggestions on what to do are welcomed.
There is still some signal/music going through him.
Without the preamp, the volume control worked fine.
Tried other pots, the one now is 50K stereo log. The others were 10k,470R and 250k none of them solved the problem...
Tried rewiring it to the utmost front, still the same issue.
Even tried to lower the gain on the LM, still the issue remains. Currently running out of options. Any suggestions on what to do are welcomed.
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if you're SURE the pots are good, other possibilities are a wiring error or some parasitic signal leakage. can you show pictures so we can see the build construction?
mlloyd1
mlloyd1
Check the wiring of the vol pot.
Getting the wires wrong can lead to an odd control law.
One of our renowned Power Amplifier assemblers posted an incorrect vol pot wiring diagram many times on this Forum !
If that proves OK, then check the minimum resistance of the wiper to audio Ground end of the track. If this is not 0r0 then some audio will leak through.
If you measure 1r0 for a 50k vol pot, then the maximum attenuation is -93.98dB, not "mute".
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