[dmp]

I am still getting noise when switching.

That doesn't sound right to me - seems you have DC on the output of the last opamp. Maybe try swapping the ICs and see if the problem goes away (U3 & U4). Or you could add a blocking cap but it would be better to just get opamps with the the output Vbias.

About the high frequency - these are pretty noticeable.  You might want to measure the frequency response (I posted some sweeps back on page 9). I'm guessing there is something wrong if you aren't hearing them, especially with the 12k & 16k. The stock 20k is very noticable because the HF boost is full at 20k but has a wide boost all the way into the high mids (~1k ~2k)

[dmp]

What discrete opamp do you have for U3?  If it has a offset voltage that's could be the problem. Maybe stick a blocking cap in to test.

[dmp]

Does my drawing for the HF switching above look OK?

Looks fine

[manulaudic]

I've gone through the thread and there was a mention as though only the mid pot needed to be dual and i traced back the pcb and indeed it seems like the others are working as single pots.

If there anybody that can confirm that it 's be possible to use single pots escept for the mid one?

Manu

[TwentyTrees]

Yep!

If there anybody that can confirm that it 's be possible to use single pots escept for the mid one?

Manu

[manulaudic]

Mid frequency sweep (P4a and P4b) needs to be dual gang. The others can be single gang. If you look at the schematic, 'HI' (Dual 10k Linear) and 'MID' (Dual 10K Linear) are wired so each gang/track is in parallel, so Hi (P2A and B) is working like a 5k single track linear pot, Mid (P3A and B) is also like a 5k single track linear pot. Only one half of P1 is used so it is acting like a single track 22k linear pot. P4 is the only one that really takes advantage of both tracks fully.

Indeed i found this ! i'll go with that
Manu

[Indecline]

Anyone found a way to widen the Q on the midband? I’ve messed with it in spice but no luck.

Anyone interested in some API 500 format projects??

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JLM 40-Segment LED meter in API 500 format

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FS: Studer 169 EQ 500 series -SOLD

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WTB Studer 169 500 Eq

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