SSL 9k5 - Alternative Input Gain Selector - Thoughts/Ideas?

[GrannyGremlin]

I think (yeasr ago) I tried using a tapering resistor with a linear pot in this sort of configuration & it didn't work too well. I think this is because it works when you feed the signal into the top of the pot & take the signal off the wiper, but the 9k doesn't use the rev log pot like this it only uses 2 of the connections & therefore you don't get the log law happening. You could just work out the resistors values you need to make a log switched attenuator & then hook them to the switch in reverse.

Wrong. Slugging resistors are good only for potentiometers connected as voltage dividers.
In the 9k, the pot is connected as a rheostat (aka variable resistor).

Oh sorry, I just assumed it would have been a voltage divider. With the things I've been working on lately I haven't seen a variable resistor gain control in a while.

.... [looking up the schem] Yep; says 2.5k though.

 

[abbey road d enfer]

Commenting so i remember to come back later- but I am sure I had an online calculator that would tell you the resistor values you needed to calculate a log or rev log taper switch. You enter the number of steps, and the total resistance and it spits out the values.

If I’m remembering it correctly it should work for this configuration. I will do a look today.

This is not enough, because the gain of the preamp is not a direct function of the pot's resistance.
One must use the global gain equation to calculate the steps.

 

[pvision]

How many steps do you need? use a 6 x 2 switch and have six steps. Should be fine if you have a pad as well

Maybe have line level on the last stop, then then three 3dB steps followed by two 6 dB steps. With a 20 dB pad that gives you attenuation steps of

0
3
6
9
15
20
(21)
23
26
29
35
41
47

 

[abbey road d enfer]

How many steps do you need? use a 6 x 2 switch and have six steps. Should be fine if you have a pad as well

There's no "pad" as such in the 9k.
It is constituted of two cascaded variable-gain stages, each sharing half the gain.
It would be quite possible to use two separate pots, but it would require a dose of attention from the operator, because there could be situtions where the first stage producs too much gain, resulting in early clipping, or too little gain, resulting in extra noise.
SSL'schoice of a dual pot takes care of this issue, and offers enough gain control to make a pad unnecessary in most practical cases (minimum gain is about 11dB).

 

[frejo89]

Just to make sure we're talking about the same thing, the unobtainium part is a DUAL 2.2k RevLog, isn't it?
Well, I think AML's offer is hard to beat, unless you're prepared to place a MOQ order with Ohmeg.
You may want to try setting up a group buy...
Now you can use on of these. You would have to recalculate the resistors, but it seems like the cheapest source for a 23 position rotary switch;
You may even find a source for the switch alone.
Or your original idea of a 12-position.
https://www.ebay.com/itm/284528695825?itmmeta=01J2NM5EAFGNYRQFRZB662EJ1E&hash=item423f3b5a11:g:f2wAAOSwOQlhkQ3u&itmprp=enc:AQAJAAAA4BCIRKiIPUgRZw9QzJOHERE1BOlgoPEDkxPUSU9KJV9nuIIXCEx8VBheQm1wWprSq8FDvYospwgxX6YWL82Hb303wJgXve4/9oXI39yMxa7sOHPGn/MwVWPPYtAdi898ctQp6KJD9J13rOyXTUfcTpF6bsm6dElW1++ChNHI4Co/vgw5k8e1TwojIi3Z10UJYG0laANRvi0ZpBuK8I8vy4RAAHDGP9iPiPtrOkSSFtifxMv7G/ruOc2K5hM0zTKfcY4lSAwCcRx7crrcR444p3HYMUaVpPxcpTLoPuoi0hd7|tkp:Bk9SR6zllbSVZA

yes, you are basically right - the AML offer is principally a good one. But due to the minimum order value, you have to choose 7 of them.
This will be the price incuding shipping costs:




Because its UK -> EU you have to add another 20€

in total you will reach round about 80€ for the use of 1 (or two, in case you need the 47k too) and I disagree to do that

 

[frejo89]

You could get the same potentiometer sold by AML directly from Omeg UK.

Yes you could do that, too. I already asked them to make me an offer.

There is also a minimum order value there (around 4 or 5 pieces) and I think it came out to nearly the same invest, because one piece was around 3 pounds more expensive than the AML ones.

 

[abbey road d enfer]

in total you will reach round about 80€ for the use of 1 (or two, in case you need the 47k too) and I disagree to do that

Unfortunately, it's the sad situation with potentiometers.
I'm afraid any alternative will be equally costly.

 

[Harpo]

from #391 of the SSL 9K / 4K thread, just run the numbers with your wanted steps ...

 

[amplexus]

This is not enough, because the gain of the preamp is not a direct function of the pot's resistance.
One must use the global gain equation to calculate the steps.

Right, I was simply thinking about creating a dual gang rev log pot with a dual deck switch- not about calculating the db/step of the switch

 

[amplexus]

Unfortunately, it's the sad situation with potentiometers.
I'm afraid any alternative will be equally costly.

This is the real answer. A decent quality dual deck rotary isn’t exactly a $2 part, plus resistors plus your time to build the switch (unless one considers one’s time to be free)

 

[frejo89]

Thank you all for your time and your thoughts !

just to expand this a little further for the moment:

What will happen if you would use maybe 2k rev log or 2,5k rev log - as replacement?
Could you really do that ?

 

[Rob Flinn]

Thank you all for your time and your thoughts !

just to expand this a little further for the moment:

What will happen if you would use maybe 2k rev log or 2,5k rev log - as replacement?
Could you really do that ?

You will just have a slightly different gain range, but it won't really affect anything much because the 9k has more gain than you would ever need unless you are recording a nose flute with a ribbon mic at 100m. Also the tolerance on many pots would mean you could end up with quite close to these values anyway.

 

[frejo89]

hi guys =)

After much deliberation, I ordered the omeg pots from AML - also simply because it is the usual way

After installing them, I noticed something that I had completely forgotten about the omegs. They are scratchy as hell....
not the feeling when turning, but the audio signal on the gain pot

I disassembled them and tried everything possible. Cleaning the conductors, setting the wipers pressure..
contacting spray (although I cant stand it at all)
nothing of that helped. Not even with other pots (I had ordered several specifically).

Does anyone have any idea how to get rid of the scratching?

Relating to the SSL 9k it comes in a short position at the half of turning cw and after round about 75%

 

[abbey road d enfer]

hi guys =)

After much deliberation, I ordered the omeg pots from AML - also simply because it is the usual way

After installing them, I noticed something that I had completely forgotten about the omegs. They are scratchy as hell....
not the feeling when turning, but the audio signal on the gain pot

I disassembled them and tried everything possible. Cleaning the conductors, setting the wipers pressure..
contacting spray (although I cant stand it at all)
nothing of that helped. Not even with other pots (I had ordered several specifically).

Does anyone have any idea how to get rid of the scratching?

Have you checked there is no DC across the pots?

Relating to the SSL 9k it comes in a short position at the half of turning cw and after round about 75%

What do you mean "comes in a short position"? Loses signal?

 

[frejo89]

thanks for your answer.

Have you checked there is no DC across the pots?

yes, not really detected DC there.
regarding the schematics, maximum of 29mV between 1E and 1S and 2E and 2S.



At the end of turning CW there is a massive oscilated hiss - but I think thats due to the massive gain, it provides.

Have you checked there is no DC across the pots?

What do you mean "comes in a short position"? Loses signal?

sorry for the bad wording here.
It should be "it happened in at a short range"

 

[abbey road d enfer]

thanks for your answer.


yes, not really detected DC there.
regarding the schematics, maximum of 29mV between 1E and 1S and 2E and 2S.

Seems to be too much to me.

 

[frejo89]

Ok, then. Any advices on this?

I did everything as described. I got the current PCBs by PCBgrinder (Bruno2000, REV 4.2)

I already did the well-known phase fix and calibrated the HPF - but the Problem is on the gain pot.

I do not know other fixes on this.
So if you have an advice for me, I appreciate

 

[abbey road d enfer]

Ok, then. Any advices on this?

I did everything as described. I got the current PCBs by PCBgrinder (Bruno2000, REV 4.2)

I already did the well-known phase fix and calibrated the HPF - but the Problem is on the gain pot.

I do not know other fixes on this.
So if you have an advice for me, I appreciate

Assuming you use the recommended MAT02 or MAT12 or a similar monolithic dual transistor, the offset voltage at the emitters should be less than 50uV, so I would look for something wrong in the bias.
For example what are the voltages at the outputs of the two input 5534's. Do these voltages vary with the position of the gain pot?

 

[frejo89]

Assuming you use the recommended MAT02 or MAT12 or a similar monolithic dual transistor, the offset voltage at the emitters should be less than 50uV, so I would look for something wrong in the bias.
For example what are the voltages at the outputs of the two input 5534's. Do these voltages vary with the position of the gain pot?

Firstofall thanks for your time!

I got a MAT02 EH - the offset is the already at 20mV there. thats to much... Could it be a fake one? (Photo attached)

When I got the correct 5534s, than yes, they do vary.

IC22 (to R73) goes from -2,07V to -2,79V
IC29 (to R109) goes from -2,02V to -1,2V

I recognized that already my (filtered) 3-Rail PSU by DIYRE had a little offset:
+15,76V against -15,84V

 

[abbey road d enfer]

Firstofall thanks for your time!

I got a MAT02 EH - the offset is the already at 20mV there. thats to much... Could it be a fake one? (Photo attached)

Even if it was a fake, but functional, the offset would not be so huge.

When I got the correct 5534s, than yes, they do vary.

IC22 (to R73) goes from -2,07V to -2,79V
IC29 (to R109) goes from -2,02V to -1,2V

That seems wrong to me.
Since I have never built a 9k nor have one available to measure, I can't be of more help.
I would suspect some resistor value; can you certify the values of resistor pairs R106/R70, R105/R69, R109/R73 ?
Also check C82/C58, one may be leaky. It happens with styroflex.

I recognized that already my (filtered) 3-Rail PSU by DIYRE had a little offset:
+15,76V against -15,84V

It shouldn't matter.