Sidechain/HPF in a Compression Pedal - When, and Why?

[superheavyfunk]

I'm a long time compression lover/user. In fact, it's the only effect I'd keep if I had to pare down to just one. I've gone thru a few of them (not as many as some of you but a fair amount, to be sure), but I've never used one that had a sidechain or HPF built in.

The only two that I'm aware of are the Empress and the Apex. The Empress comp has an input for sidechain and, as I've read, one can wire up a cap into a 1/4 slug and use the circuit as an HPF. The Apex only has the HPF option, and it's built-in on a variable pot.

My question is: When and why would one want an HPF in a compressor? I understand that having the HPF causes the compressor to ignore the lowest frequencies and allows the comp to primarily react to the frequencies above whatever point you've set it to. But *why* someone would want that is what I'm wondering about. In my experience, it's the higher strings (and their corresponding frequencies) that I'm trying to get to pop out more... I'm never short on bass. So I compress my signal in order to bring the bassier notes down to a level that's more even with the higher ones. It seems to me that having an HPF in a comp would achieve the opposite effect - which is cool if that's what you're going for, but I've never played a bass that lacked in low end and needed to have only the db reduction in the highs. *shrug*

I haven't been able to find a video that does a really good job of showing the difference between a traditional compressor and one that uses an HPF... plus, I suspect it's probably more of a "feel" thing than an audible thing, but as I said, I've never tried one. Can anybody provide some insight?

 

[BurningSkies]

Cali 76CB has a great sidechain HPF. When? All the time. Let your lows stand, and they don't trigger compression so you don't end up pumping your signal all the time if you like lows.

 

[mmbongo]

I just got an Apex a few days ago, and i have it side by side with my Keeley Bassist right now.

Now I'm no compressor expert, and I don't know all the technicalities. But kinda like you, I was curious about the side chain thing, and here's what I feel about it...it's kinda neat because once you get the compression set for your higher strings the way you want, it could tend to make your lower strings squishy sounding because they are louder. Dialing up the HPF allows more of the low strings to come through so they stay bigger sounding if you want. That's about the best way I can put it

 

[superheavyfunk]

once you get the compression set for your higher strings the way you want, it could tend to make your lower strings squishy sounding because they are louder.

Squishy sounding, huh? Do you mean that, unlike a traditional comp, it doesn't "tighten up" the sound? Let's say that I wanted a Marcus Miller, or a Tool sound (they're more similar than many ppl think, imo)... Does having the HPF on make that more difficult?

Im not really looking for that sound... Just trying to get a handle on how the side chain thing comes across. What kind of music do you play? In the end, does the apex get you where you want to be for that genre? Do you play in a band?

 

[keatz]

I was totally in your corner of thinking when trying to work it all out in my head, but once i got a cali76 and turned up the hpf the proof was in the pudding. Sounded and felt alot more open and natural while still bringing up the highs and articulates. Traded it for an FEA dual band compressor and as lovely and versatile as it is, would take the cali76 back in a heartbeat.

 

[Wasnex]

I'm a long time compression lover/user. In fact, it's the only effect I'd keep if I had to pare down to just one. I've gone thru a few of them (not as many as some of you but a fair amount, to be sure), but I've never used one that had a sidechain or HPF built in.

The only two that I'm aware of are the Empress and the Apex. The Empress comp has an input for sidechain and, as I've read, one can wire up a cap into a 1/4 slug and use the circuit as an HPF. The Apex only has the HPF option, and it's built-in on a variable pot.

My question is: When and why would one want an HPF in a compressor? I understand that having the HPF causes the compressor to ignore the lowest frequencies and allows the comp to primarily react to the frequencies above whatever point you've set it to. But *why* someone would want that is what I'm wondering about. In my experience, it's the higher strings (and their corresponding frequencies) that I'm trying to get to pop out more... I'm never short on bass. So I compress my signal in order to bring the bassier notes down to a level that's more even with the higher ones. It seems to me that having an HPF in a comp would achieve the opposite effect - which is cool if that's what you're going for, but I've never played a bass that lacked in low end and needed to have only the db reduction in the highs. *shrug*

I haven't been able to find a video that does a really good job of showing the difference between a traditional compressor and one that uses an HPF... plus, I suspect it's probably more of a "feel" thing than an audible thing, but as I said, I've never tried one. Can anybody provide some insight?

The only reason I can think using an HPF in the sidechain would be beneficial is if you use a lot of bass boost on the instrument. IMHO, that tends to cause compressors to key in a rather unmusical way. By filtering the low boost out, the keying should be more natural. I personally prefer to EQ after compression.

I believe you could also run a parametric EQ in the side chain, so you could shape the signal however you want.

 

[J Posega]

IME, using a sidechain HPF while compressing is useful when you have a very bassy signal but you need to control the full-range more evenly. The HPF filters out the lows, and personally, I like to use one set anywhere from 40-80hz on bass. This means these frequencies pass through the compressor untouched, and allows you to compress the frequencies above that without the lows and sub-lows triggering the compressor.

Since compression can make loud things softer and soft things louder, you could use a higher-set HPF to really focus the compressor on just the mids/highs, which could potentially make them louder/ more even than just a regular compressor working across the full spectrum of frequencies.

It's a great feature if you don't run an HPF in the front of your board because it gives you more control over the frequencies you really hear, rather than the ones you mostly feel. That being said, I prefer a separate HPF at the front end of my signal cutting the lowest lows so that all my pedals react to the stuff I'm hearing, not the stuff I'm feeling.

 

[keatz]

That being said, I prefer a separate HPF at the front end of my signal cutting the lowest lows so that all my pedals react to the stuff I'm hearing, not the stuff I'm feeling.

Ive been meaning to try taking this approach with my fea dual band comp. Send the lowest lows straight to the end of the chain, and run the rest through all my effects then blend together before hitting the preamp.

 

[Wasnex]

IME, using a sidechain HPF while compressing is useful when you have a very bassy signal but you need to control the full-range more evenly. The HPF filters out the lows, and personally, I like to use one set anywhere from 40-80hz on bass. This means these frequencies pass through the compressor untouched, and allows you to compress the frequencies above that without the lows and sub-lows triggering the compressor.

Since compression can make loud things softer and soft things louder, you could use a higher-set HPF to really focus the compressor on just the mids/highs, which could potentially make them louder/ more even than just a regular compressor working across the full spectrum of frequencies.

It's a great feature if you don't run an HPF in the front of your board because it gives you more control over the frequencies you really hear, rather than the ones you mostly feel. That being said, I prefer a separate HPF at the front end of my signal cutting the lowest lows so that all my pedals react to the stuff I'm hearing, not the stuff I'm feeling.

It may not mean the frequencies are totally untouched, rather their impact on compressor action is normalized a bit. I drew a crude graphic. Black is the original signal from the bass with a boost in the low end. Yellow is the HPF applied to the side chain. Red is the resultant modification to the signal used to develop the compressor key.

 

[boomertech]

The only two that I'm aware of are the Empress and the Apex. The Empress comp has an input for sidechain and, as I've read, one can wire up a cap into a 1/4 slug and use the circuit as an HPF. The Apex only has the HPF option, and it's built-in on a variable pot.

Our Opti-FET has a 3-band EQ in the side-chain along with send/return jacks so that you can put any EQ or HPF/LPF that you desire before the detector circuitry.

-Frank

 

[boomertech]

It may not mean the frequencies are totally untouched, rather their impact on compressor action is normalized a bit. I drew a crude graphic. Black is the original signal from the bass with a boost in the low end. Yellow is the HPF applied to the side chain. Red is the resultant modification to the signal used to develop the compressor key.

View attachment 3070066

Good graphic. The way I try to explain it is that, essentially what filtering/EQ'ing in the side-chain doing is modifying threshold sensitivity by frequency.

-Frank

 

[scubaduba]

Seymour Duncan Studio Bass and Doc Lloud Photo Death Ray have a sidechain of sorts too.

 

[boomertech]

My question is: When and why would one want an HPF in a compressor? I understand that having the HPF causes the compressor to ignore the lowest frequencies and allows the comp to primarily react to the frequencies above whatever point you've set it to. But *why* someone would want that is what I'm wondering about. In my experience, it's the higher strings (and their corresponding frequencies) that I'm trying to get to pop out more... I'm never short on bass. So I compress my signal in order to bring the bassier notes down to a level that's more even with the higher ones. It seems to me that having an HPF in a comp would achieve the opposite effect - which is cool if that's what you're going for, but I've never played a bass that lacked in low end and needed to have only the db reduction in the highs. *shrug*

First I need to give a bit of compressor theory...
There are different types of amplitude detection that are used in the compressor control circuitry (this control circuitry is called a side-chain and every compressor has side-chain circuitry). The two types of amplitude detection are RMS and peak. RMS is an averaging of amplitude levels over a period of time, whereas peak is instantaneous. RMS detection works great for applying compression more in line with the response with human hearing; where very quick transients are not causing the compressor to overreact when fast attack times and high ratios are used.

Peak detection is great for protection of speaker systems and distortion prevention within the systems audio electronics. So very fast signal transients are captured by peak detection when attack times are very short. A lot of times, these fast transients along with longer release times, can cause the higher frequencies to sound swallowed-up or over-compressed, because the whole audio range usually gets the same gain reduction with a typical full-range compressor.

With that explained, I can now get to your question.
Since the largest peak transients from a bass guitar are the lowest fundamental and its second harmonic frequencies of the note played, a HPF will reduce the sensitivity (or modify) the threshold for the range of those peaky frequency ranges. This has the affect of reducing the effect of the higher frequencies being swallowed-up by the lowest, peaky frequencies. It kinda does the same function of RMS detection so that your comp is reacting in a more natural (human hearing response) way.

I hope this one way of describing the function of HPF in the side-chain helps.

-Frank

 

[mmbongo]

Squishy sounding, huh? Do you mean that, unlike a traditional comp, it doesn't "tighten up" the sound? Let's say that I wanted a Marcus Miller, or a Tool sound (they're more similar than many ppl think, imo)... Does having the HPF on make that more difficult?

Im not really looking for that sound... Just trying to get a handle on how the side chain thing comes across. What kind of music do you play? In the end, does the apex get you where you want to be for that genre? Do you play in a band?

I've only had my Apex for a few days so I can't really answer those questions yet. And I'm not sure it's going to dethrone my Keeley Bassist, the Keeley just seems to have something magical going on that makes it know exactly how a bass should sound. But for your other questions, yes I play in two bands. Both loud rock & roll. I have the Keeley dialed to even things out and make things fatter. Not sure if the Apex does this as well, I haven't tried it with either band yet.

 

[superheavyfunk]

Our Opti-FET has a 3-band EQ in the side-chain along with send/return jacks so that you can put any EQ or HPF/LPF that you desire before the detector circuitry.

Woah, that's cool! So, how does the EQ work? One would turn say, the mid knob clockwise and it'd react less to the corresponding frequencies as you turn it further? What are the main frequencies that are affected?

I hope this one way of describing the function of HPF in the side-chain helps.

That's super helpful! Thank you.

 

[boomertech]

Woah, that's cool! So, how does the EQ work? One would turn say, the mid knob clockwise and it'd react less to the corresponding frequencies as you turn it further? What are the main frequencies that are affected?


That's super helpful! Thank you.

Here are the plots of the three EQ controls:

The Mids control has a +/-8dB boost/cut range centered at approx 250Hz. So, if you turned the mids control fully clockwise your threshold sensitivity will become 8dB MORE sensitive at 250Hz... the compressor will react stronger for those frequencies.

-Frank

 

[keatz]

So @boomertech would i be correct in saying that a semi equivalent way of a hpf sidechain on a db-cl would be to run the threshold control a fair amount higher on the low side, letting more lows through before compressing?

 

[boomertech]

So @boomertech would i be correct in saying that a semi equivalent way of a hpf sidechain on a db-cl would be to run the threshold control a fair amount higher on the low side, letting more lows through before compressing?

That would be similar to what the HPF would be doing to the threshold. I would also drop the x-over freq a tiny bit, because the threshold level adjustments affects all of the low-band evenly, whereas a HPF slope will affect the lowest frequencies stronger, which will cause them to be less sensitive at the threshold detector.

A great way to set up a DB-CL is to plug an unused plug into the High output. This will allow you to just hear the Low-band side. I've found it very helpful.

-Frank