BrianGTs LM3886 input resistor R1

[Russ White]

If you are using a cap on input and Ci can you simply omit and jumper the pads for R1 as there should be no DC offset?

Thanks.

[Peter Daniel]

This resistor is there to provide certain isolation between amp input and outside world. In most cases you don't really need it, and presently, I'm not using such. Even replacing Caddock or Vishay with a short, brings improvement in low level detail and neutrality.

Sometimes using that resistor may equalize some brightness that other GC components may introduce, and in such cases Vishay S102 works quite well as it adds certain smoothness and more body. Caddock, OTOH, seems to be more neutral, preserving same signature of trebles, but it may be somewhat bright.

The value of that resistor is not critical; National suggests 1k, I've been using 220R ( I was probably influenced by Alephs schematics ). But anything between 220R and 1K should be fine. Higher resistor value brings slightly stronger signature, but this is very slight effect.

So, you can remove that resistor, and the circuit should be working fine without it (in most cases at least). You may even notice an improvement.

[Russ White]

OK, I got impatient and just tried it and hoped for no smoke.

It worked fine, and DC offset is still 0. Sounds great.

[Russ White]

Thanks for the kind reply Peter. That was exactly the kind of info I was looking for!

[Russ White]

would bypassing signal ground in a similar manner also be helpful in some cases?

[Peter Daniel]

What do you mean exactly?

[Russ White]

I mean putting a similar value resistor as R1(if you use one) in series with SG prior to attaching SG to the PCB. I think I used the term "bypass" incorrectly. Maybe I should say isolate.

[Peter Daniel]

SG has to be connected directly, but you can try separating SG from OG by 3 ohm, or similar, resistor. This would require cutting one trace though.

[Russ White]

The reason I asked is because on a preamp I am working on right now I have a small problem with humm and hiss when connected to my GC. Nearly none however when connected to headphones.

The preamp is a opa2228 with 2 buf634s one for each channel.

Well, I put 1K resistors on the outputs of the BUF635s and a 1.75K resistor in sereies with the power ground running from the power supply to my star ground. The helped a lot for the humm, but I am not sure if I am harming the circuit.

The power supply is fed by a 30V wall wort and vortual ground is created with a simple volateg divider with 4.7K resistors.

Could I use even higher values for my isolation resistors, as I still have some humm?

[jleaman]

Originally posted by Russ White
The reason I asked is because on a preamp I am working on right now I have a small problem with humm and hiss when connected to my GC. Nearly none however when connected to headphones.

The preamp is a opa2228 with 2 buf634s one for each channel.

Well, I put 1K resistors on the outputs of the BUF635s and a 1.75K resistor in sereies with the power ground running from the power supply to my star ground. The helped a lot for the humm, but I am not sure if I am harming the circuit.

The power supply is fed by a 30V wall wort and vortual ground is created with a simple volateg divider with 4.7K resistors.

Could I use even higher values for my isolation resistors, as I still have some humm?

I'm Also starting to have Problems with my gain clone's and passive pre-amp When connecting another amp ithe other amps hum useing a active xover it hums too. Disconnecting the gain clone amp and using any other type of amp it works just fine and no hums. What could it be ?