Although not my first 3886 build but this time i'm doing it for my own use 
I sourced couple of PCBs which has a really nice & usable layout. Very straight forward build, nothing fancy & i am not following pcb schema. I set the input impedance to 22k & gain around 27dB. Skipped feedback compensation network & didn't use any capacitor between the Inputs but added RF blocker(from signal In to ground, 235khz low pass). As input cap i used 1uf mkt from Vishay & for Ci i'm going to use 100uf bipolar. I planned 100uf Nichicon muse BP for that place but their footprint is little bigger so placed a new order for 100uf 16v Elna RBD bipolar & will settle this later. Anyway as on board supply decoupling cap i used 1uf film with 470uf 63v electrolytic per rail. I don't have solid polymer or X7R ceramic at the moment so used general purpose elco with 1uf film instead.
On the other hand for PSU i made my own board, 3x4700uf per rail without bypass capacitors. 220VA transformer & 35amp bridge.
I sourced couple of PCBs which has a really nice & usable layout. Very straight forward build, nothing fancy & i am not following pcb schema. I set the input impedance to 22k & gain around 27dB. Skipped feedback compensation network & didn't use any capacitor between the Inputs but added RF blocker(from signal In to ground, 235khz low pass). As input cap i used 1uf mkt from Vishay & for Ci i'm going to use 100uf bipolar. I planned 100uf Nichicon muse BP for that place but their footprint is little bigger so placed a new order for 100uf 16v Elna RBD bipolar & will settle this later. Anyway as on board supply decoupling cap i used 1uf film with 470uf 63v electrolytic per rail. I don't have solid polymer or X7R ceramic at the moment so used general purpose elco with 1uf film instead.On the other hand for PSU i made my own board, 3x4700uf per rail without bypass capacitors. 220VA transformer & 35amp bridge.
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The amp is now singing & i must say it has excellent sonic qualities. No hum or noise & very warm but natural sounding! For sparkling highs i intentionally kept the input cap as low as possible, 1uf 
(..of course my spkrs are not good below 50hz). Anyway as preamp i temporarily added a gain stage using TL072A but a weird thing is happening 
When i turn off the amp(..& preamp at the same time) after 1 or 2 second a loud 'Quachhh'/whistle like sound occurring. Initially i thought it is happening because of small preamp PSU caps(1000uf/rail before the 7xxx regulators), i mean discharging faster than amplifier PSU caps. So i changed those to 2200uf but the problem is still there. I also used anti-parallel diodes to safely discharge regulator output caps but no help. I'm sure the problem is from the preamp side. Don't know what's the solution :-( Otherwise everything is fine.
(..of course my spkrs are not good below 50hz). Anyway as preamp i temporarily added a gain stage using TL072A but a weird thing is happening When i turn off the amp(..& preamp at the same time) after 1 or 2 second a loud 'Quachhh'/whistle like sound occurring. Initially i thought it is happening because of small preamp PSU caps(1000uf/rail before the 7xxx regulators), i mean discharging faster than amplifier PSU caps. So i changed those to 2200uf but the problem is still there. I also used anti-parallel diodes to safely discharge regulator output caps but no help. I'm sure the problem is from the preamp side. Don't know what's the solution :-( Otherwise everything is fine.
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I think the preamp is oscillating or probably there is a ground loop problem; because the amplifier section is very quiet. When i connect the preamp a consistent low pitch hum & white noise appears. Also that weird turn off thump(preamp). Let's see what i can do
will share everything about this.....
will share everything about this.....
If your preamp produces a thump when it turns on/off you'll get the same thump amplified if you leave the power amp on while toggling the power on the preamp. This is why you see recommendations of powering up from the source towards the load (speaker usually) and powering off from the load to the source.
Preamps produce noise. This noise will be amplified by the power amp because that's what power amps do. They amplify.
A quiet amp is a good thing. I'm not sure how you could conclude that your amp is oscillating from that. Do you have an oscilloscope so you can check for oscillation?
Tom
Preamps produce noise. This noise will be amplified by the power amp because that's what power amps do. They amplify.
A quiet amp is a good thing. I'm not sure how you could conclude that your amp is oscillating from that. Do you have an oscilloscope so you can check for oscillation?
Tom
I said probably the 'preamp' is oscillating, amplifier section is fine. Also i think there is a ground loop problem. I'll report back soon.
Hum is Hum usually power supply or assumed ground loop.
TL072 considered low noise, but if the gain is set high or high value resistors can be noisy.
thump is usually DC offset on power up/down hasn't stabilized yet.
so even with coupling caps it can still thump.
TL072 is compensated so DC offset usually around 3mV can be 1mV on some packages.
once voltage drops below 2.5 volts or comes up from 2.5 volts. can be anything.
Unity gain stable but again below 2.5 volts, on shut down it probably lets off a little squeal.
would have to see the design, usually resistor on the input helps and small 75 ohm on output helps
then small value capacitor across feedback resistor will keep it from squealing.
or too much gain or technically unity gain stable but again once voltage rails fall below 2.5 volts
that goes out the door. so gain of 1 to 3 still good rule of thumb.
Home amps around 30 to 60 watts no big deal usually for power up order.
but still helps a lot.
Being a old school PA guy around 1000 to 5000 watt amps.
On power up the amp is turned on last. power down turned off first.
Noboby wants to hear or pay for a 5000 watt THUUUUUUMP...Lol
No matter how many fancy soft starts, relays, mutes or limiters etc etc
guarantee the bigger the amp and system , more likely its gonna thump
TL072 considered low noise, but if the gain is set high or high value resistors can be noisy.
thump is usually DC offset on power up/down hasn't stabilized yet.
so even with coupling caps it can still thump.
TL072 is compensated so DC offset usually around 3mV can be 1mV on some packages.
once voltage drops below 2.5 volts or comes up from 2.5 volts. can be anything.
Unity gain stable but again below 2.5 volts, on shut down it probably lets off a little squeal.
would have to see the design, usually resistor on the input helps and small 75 ohm on output helps
then small value capacitor across feedback resistor will keep it from squealing.
or too much gain or technically unity gain stable but again once voltage rails fall below 2.5 volts
that goes out the door. so gain of 1 to 3 still good rule of thumb.
Home amps around 30 to 60 watts no big deal usually for power up order.
but still helps a lot.
Being a old school PA guy around 1000 to 5000 watt amps.
On power up the amp is turned on last. power down turned off first.
Noboby wants to hear or pay for a 5000 watt THUUUUUUMP...Lol
No matter how many fancy soft starts, relays, mutes or limiters etc etc
guarantee the bigger the amp and system , more likely its gonna thump
@tomchr Guessing, because while prototyping that preamp i completely forgot about layout rules. Opamp input & output+supply traces shouldn't be close to each other. On the other hand i'm using separate transformer for amplifier & preamplifier, so both wire from the Center Tap should be soldered on the same point in the PSU board. Btw spkr returns(-) are directly from amplifier boards.
@WhiteDragon I've used 1Kohm in series with non-inv input & 100ohm as output resistors; input impedance is 100k. I didn't use any capacitor in parallel with feedback resistor because i only have 47pf NPO ceramic & that's too big for that place i think. Gain is around 6dB.
Edit- Amplifier output DC offset is quite minimal, 1-3mV max. I followed Toms advice & used 33k + 100uf to eliminate turn off thump. Power supply +/-30vDC.
@WhiteDragon I've used 1Kohm in series with non-inv input & 100ohm as output resistors; input impedance is 100k. I didn't use any capacitor in parallel with feedback resistor because i only have 47pf NPO ceramic & that's too big for that place i think. Gain is around 6dB.
Edit- Amplifier output DC offset is quite minimal, 1-3mV max. I followed Toms advice & used 33k + 100uf to eliminate turn off thump. Power supply +/-30vDC.
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Hum problem solved, it was my biggest mistake to connect both CT(trnsfrmr) at the same point. It was polluting preamp power supply. But how to stop the op-amp from squealing (weird turn off thump)??
Here is the preamp schema.... for supply decoupling i used 0.1uf ceramic+22uf elco. btw my amplifier is ac coupled.
Here is the preamp schema.... for supply decoupling i used 0.1uf ceramic+22uf elco. btw my amplifier is ac coupled.
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Tried different op-amps but problem remains the same, scary loud squealing after 2 or 3sec of power off 
Finally the problem is solved! The source of that squealing was from a mp3 player. The amplifier is dead silent 
& no turn on/off pop when not connected to that mp3 player. I'll change the p.supply location of that player 
& no turn on/off pop when not connected to that mp3 player. I'll change the p.supply location of that player 