6AL5 limiter sidechain hpf?

Does anyone know how to add a high pass filter to the sidechain of a tube limiter like a Gates Sta Level or UA 175 using a 6AL5 tube?

-Paul

Hi,
For the Sta level you could try to change the values of C8 and C9. The lower the value the less the compressor will react on low frequencies.
Best
Bernd

Thanks for the suggestion Bernd.
Yes, I was thinking about putting a different selection of cap values on a switch. But I'd really love to know how Retro implements the variable pot on their version of the 176. Any ideas?

Bump, Curious about adding sidechain hpf to the 176 detector circuit.

Hmmm interesting old thread! Maybe a kind of blend pot? It should be a double concentric pot with 2 pairs of caps on each side or something like that.... or 2 caps in serial and a pot that bypass one of the 2 caps.... i ve made a 4 pos selector in my rs124 builds with 4 pairs of apparaid caps.... a pot instead of this would be much better!

From the Retro 176 Manual:

4.1.5 6AL5 Detector and Ratio Switch
The output transformer has four sets of input taps to accommodate the four ratio settings.
Each set of taps is brought out to the ratio switch. The ratio switch then feeds the 6AL5
full wave rectifier through the Sidechain Highpass filter network and Asymmetry mode
switching. Bias for setting the compression threshold and ratio is derived from a resistive
ladder also on the ratio switch. The 6AL5 detector output feeds the attack/release
controls and timing capacitors before driving the gain reduction stage.

4.1.7 Sidechain Highpass Filter
The Sidechain HPF applies additional resistance across the detector input coupling
capacitors effectively creating a first order highpass filter. The filter is disabled through a
switch when the filter control is set at minimum.

Looks like switching in resistance to R36 & R37 after the coupling caps C6 & C7.

So the .5u and 220k = 1.44Hz DC filter.
Looks like adding a dual 1k DPDT Switch pot in parallel with R36 & R37 will bring the cutoff up to 318Hz before the detector.

Perhaps the variable resistors should be in parallel with the coupling caps ?

Here's a pic of the Retro HPF pot. Still trying to figure out were to wire it in reference to the Coupling Capacitors.

I think this post helps solving the mystery of why strapping a variable resistor across C6/C7 on the 176 creates a sidechain HPF https://electronics.stackexchange.com/.../cut-off... the example looks at a differential low pass filter but swapping capacitors and resistors gets us to the right place. A 10k rev log pot wired as variable resistor followed by a 1k resistors should be very close to what Retro does.

Great thread!

This makes more sense now as I was not familiar with differential filters. So in this example R1 would be the variable resistor for the adjustable high pass?
Series capacitors C1=C2=C*2
Then parallel variable resistor cutoff is ƒc = 1/(2πRC)

That’s my current understanding after thinking about the issue for a few years… on the 175/176, the side chain capacitors are the big 0.5uf Gudeman oil can/paper in oil. I have all the parts to build a pair and hope to test this soon!

Here we go.

https://www.ti.com/lit/an/slwa053b/slwa053b.pdf?ts=1711498840637&ref_url=https%3A%2F%2Fwww.google.com%2F