amek 9098 stepped to pot gain control conversion

[angelothewolf]

Hi there, Im having trouble to choose the right pot to replace the gainswitch in this preamp. what you think? reverse log? what value?

[angelothewolf]

I try diferent pot values, but all of them seems to have the same effect, a lot of gain comes up just in the last millimiters of the pot travel...

[Leo_Gnardo]

reverse log? what value?

Reverse log or reverse audio looks like the solution all right. The value ideally would be the same as the string of resistors, about 155K. It's easy enough to find reverse pots intended for repairing Fender amps, 50K and 3M. If you could find a 250K reverse, you could parallel a resistor to scale its max value to 155K, that might be an acceptable solution. But - where to find a 250K RA ? ? ?

[angelothewolf]

well I try a 10k reverse log and it worked the same way. in the last millimiter of pot travel came a lot of gain sudently. Im taking the signal right after the 4700uf caps and ferrite "F9"

[Leo_Gnardo]

well I try a 10k reverse log and it worked the same way. in the last millimiter of pot travel came a lot of gain sudently. Im taking the signal right after the 4700uf caps and ferrite "F9"

Then it's no wonder Amek used a switch. Look at the way the resistor values change as the switch is rotated from zero gain to max. First step is 150K, then 2K7, then hundreds of ohms, tens, down to 2 ohms at the end. Unless you had a very specially made pot, you would wind up with the effect you are having. I don't see a good pot solution, you'll have to seek out a switch. Yes, what a pain in the neck.

[angelothewolf]

well, I got the swith. the thing is that i am doing this for a client who want a pot instead. If it were mine I would never do that actually. I wonder if there is a way to mod the circuit to work with a pot in there...

[olddawg]

Can you simply just set it up for maximum gain and pad the input? I doubt he would hear the difference in the different setup.

[km6xz]

That would lower signal to noise ratio to worst than Behringer level. The key for pro console signal chain is each stage operated at its optimum DR and gain in series.
The front end is using the common mode gain reduction as it is, which is needed for optimum balanced line operation. That common mode shunt is why the low values are used in the gain switch.

[olddawg]

That would lower signal to noise ratio to worst than Behringer level. The key for pro console signal chain is each stage operated at its optimum DR and gain in series.
The front end is using the common mode gain reduction as it is, which is needed for optimum balanced line operation. That common mode shunt is why the low values are used in the gain switch.

I guess so... And I'm sure you are correct. But I'm an old hack and a good noise floor was always my best friend . It hides a multitude of evils in rock and roll, lol!

[angelothewolf]

This equipment is from a dubbing studio. The owner claims that he needs a smooth gain control to adjust level by hand, while he's recording voice, due to instantaneus level increasing...

[angelothewolf]

.The front end is using the common mode gain reduction as it is, which is needed for optimum balanced line operation. That common mode shunt is why the low values are used in the gain switch.

Can explain this a little more in detail?

[dmeek]

I wonder if the "150K" is a misprint and is actually 15.0K. That would make the resistor string a total of about 20K.
A reverse audio pot should work. Soundcraft uses a similar preamp in some of their mixers.

[angelothewolf]

I wonder if the "150K" is a misprint and is actually 15.0K. That would make the resistor string a total of about 20K.
A reverse audio pot should work. Soundcraft uses a similar preamp in some of their mixers.

It is 150k...

[olddawg]

This equipment is from a dubbing studio. The owner claims that he needs a smooth gain control to adjust level by hand, while he's recording voice, due to instantaneus level increasing...

Odd... Seems like he would do that at the console channel level. Varying mic sensitivity can cause a multitude of evils. Golden Ears I'm sure.... Wonder if he could tell the difference if you put a $30 Art mic preamp in a different box?

[angelothewolf]

Odd... Seems like he would do that at the console channel level. Varying mic sensitivity can cause a multitude of evils. Golden Ears I'm sure.... Wonder if he could tell the difference if you put a $30 Art mic preamp in a different box?

Well, it is not a console. it's a rackmount unit. Preamp with EQ. I dont really know if the guy has golden ears or whatever... Im just thinking in how can I replace that switch with a pot.