How Studer 169 eq mid band works?

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Well-known member
Dec 24, 2008
Could anybody point out what this type bandpass filter is called so I could search more information ?:) ( see Audiox studer 169eq schematics found from here )

All started as I got Tascam m15b console over a year ago. Some of the eq bands tend to oscillate as I turn frequency knob ( scratchy pot, losing contact? ), or sometimes with full boost. It makes really loud squeeck type sound which I would very much like to get rid of.

Now the Studer EQ is only one schematics I found which looks to be similar topology, but it has frequency pot connected different way as Tascam and also has some additional components (such as feedback capacitor :). I'd like to understand why my eq oscillates, and is there some way to eliminate that oscillation for good. I know some may think "Tascam is no good, get rid of it!", but then I (and you!) wouldn't learn anything new ;)

ps. opamps are TLO72


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Simple 1-pole bandpass with top-and-bottom freqs defined by capacitors and dual-pot resistance, and +/- pot feeding the noninverting opamp input through this filter from either the filter's input, or from the (inverted) opamp output.

So there's opposite polarity on either end of the +/-pot, which cancels out to give (ca.) 0 in centre position..

Jakob E.
These are Wien-bridge type filters. They operate by feeding a portion of the opamp output into the positive input. Stability depends very much on the components in the bridge being equal. With age, the pots can develop a high parasitic resistance which may turn the circuit into an oscillator.
The LF & HF control in the Studer 169 operates on a different principle. It is  Baxendall-derived. As such it is a shelving filter. But the MF section is a Wien-bridge EQ; the added caps are DC blockers, to reduce noise of scratchy pots.
Thanks Jakob and abbey!

Well, what do you know, it is a wein bridge oscillator! :D

Looking at the circuit in Figure 1, R1 and C1 produce a positive phase shifted current with respect to the output voltage. When this current meets R2 and C2, these components produce a voltage that is phase shifted in a negative direction. At one frequency the phase shift caused by R1 and C1 will be offset by an equal and opposite phase shift caused by R2 and C2 and the net phase shift will be zero. The circuit is now in danger of oscillating.


Yeah, now that I know what to look for this topology is everywhere.. Seems to be the "british eq" type of thing.
Here is about oscillation

What is the advantage of R13 and R15 on audiox 169 eq schematic?


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tmuikku said:
What is the advantage of R13 and R15 on audiox 169 eq schematic?
They have put DC blocking caps, so they needed to put a resistor to ground on the positive input of the opamp, that is in parallels with the lower section of the dual pot, AC wise. So in order to maintain the balance, they had to put another resistor on the upper section.

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