stereo preamp advice

[PRR]

I never understood how this overvoltage protection approach works. I mean, it will limit input voltages by V± ± 0.7V but the absolute maximum limits for many opamps dictate input voltages no higher than the supply.

"No higher" can't mean 0.000,000V- that would imply some kind of zero offset or a perfect rectifier. Also any series resistance prevents infinite/very-high current, the real culprit.

Yes, we can blow-up gear with the 'protection' shown. But it is not so easy.

[ElectricDruid]

Yes, we can blow-up gear with the 'protection' shown. But it is not so easy.

"Pretty difficult", even!! And usually that's enough. We're mostly trying to avoid being heavily penalised for our inevitable moments of stupidity, not protect the thing against deliberate attack.

[snk]

I can also post my layout, but I don't know if it would be useful?

it is always useful. but with a circuit diagram. and photos of your build. easy peasy. please.

Hello
This is already a first post with the layout I designed : it is the dual version of the verified layout by TagboardFX (which is obviously mono/single channel, like the original).
I have included the suggested mods (capacitors and resistors values, etc).
I made it dual channel.
I used a fixed resistor value (5.1k) instead of the antilog 500k pot.
I used a 100k dual gang output volume pot.
I also added some components at the input for polarity protection (a 1n5817 diode) and decoupling (100R resistor and capacitor).





Later today I will be able to post images of the build, and run some further testings on my (not working) build.

[antonis]

Dunno if I missed something but what's the reason of implementing 2 single op-amps instead of a dual one..??

Can't see either the need nor the implementation of single op-amps extra pins for compensation/off-set trimming..

[snk]

Dunno if I missed something but what's the reason of implementing 2 single op-amps instead of a dual one..??

Can't see either the need nor the implementation of single op-amps extra pins for compensation/off-set trimming..

It's just that I don't know how to do it 
I thought about it, it occured to me, but I don't have the knowledge to design such a circuit. And as the Mxr Microamp circuit was very simple, I thought it would be ok and easy enough to gather 2 of them into a single enclosure.

[antonis]

It's just that I don't know how to do it 

Fair enough..

If we do it for you in the schematic form, are you willing/able to "transfer " it on layout board..??

[snk]

If we do it for you in the schematic form, are you willing/able to "transfer " it on layout board..??

At least, I'd be willing to try 
Thank you a lot for the kind proposition
I can't guarantee that it would be fine at the firt attempt, but I can try, it doesn't seem that much complicated.

[fryingpan]

If we do it for you in the schematic form, are you willing/able to "transfer " it on layout board..??

At least, I'd be willing to try 
Thank you a lot for the kind proposition
I can't guarantee that it would be fine at the firt attempt, but I can try, it doesn't seem that much complicated.

It's actually easier.

[duck_arse]

your bias setup doesn't work. on your layout I see a 100k from each oppie pin 7 to a 47k to ground each. that's not what the circuit shows. then I stopped trying to follow.

[snk]

your bias setup doesn't work. on your layout I see a 100k from each oppie pin 7 to a 47k to ground each. that's not what the circuit shows. then I stopped trying to follow.

Yes, you're right : I obviously messed up the values (it should be the same values : 100k on the original schematic).

[duck_arse]

also, while we are discussing backwards electros - that 3u3 on the input - you have 1M [or something - 22M?] pulling one end to ground and the other end connected to about half supply. check which end connects where ....

[snk]

also, while we are discussing backwards electros - that 3u3 on the input - you have 1M [or something - 22M?] pulling one end to ground and the other end connected to about half supply. check which end connects where ....

Ok, I think I get it. I had the + towards the audio input, but it should "look" towards the biased current. So a 180° rotation is required. Right?

[antonis]

At least, I'd be willing to try 
Thank you a lot for the kind proposition
I can't guarantee that it would be fine at the firt attempt, but I can try, it doesn't seem that much complicated.

Here you are..
(there are some items values mods and electros correct polarity..)

[snk]

Hello
Thank you all for the help! Life has gotten a bit too busy these last weeks, so I could not test any further this project. But I am not giving up, I just to find some free time to mess around with it
I will let you know how things work !