Subsonic Filters & Film Caps

[AmCan]

I want to have a discussion about subsonic filters often seen on vintage preamps.

I have spent an extensive amount of time upgrading a HK Citation 17 preamp and up until this point I have paid very little attention to the Subsonic Filter Board but 2 things have come to light recently that has me thinking differently about this board.

#1 I purchased a Citation 11 that was extensively upgraded by a highly regarded technician mostly because I wanted to take the wood sleeve from it for my 17 and then resell the 11. However, it gave me a chance to see how an IEC Socket was installed and what caps were put in it. I was surprised to see that the ONLY place the film caps were upgraded were on the “Input Board” (as it was called on the invoice). Well there isn’t an “Input Board” in the schematic. The tech was refereeing to the Subsonic Filter and Hi/Low Cut Board. So based on this then one should always have the Subsonic Filter engaged. I kept asking myself, “Why was this done?”

#2 The answer became more evident to me as I was reading the Great Capacitor Shoot Out (GCSO). Engaging the Subsonic Filter preempts the circuit and then becomes the first board in the chain on the schematic. The authors of the GCSO clearly indicated that the sooner you get the best caps possible into the chain the better the realization is of their sonic benefits.

Further to this discussion is that having restored most of my preamp or what I assumed were the most essential boards I am about to turn my attention to my Citation 19 amp. In doing so I was going to order V-Caps for the main output caps incorrectly thinking that this was the most critical place. These caps happen to be the same value as the output caps on my pre. I put Mundorf Supremes on the output board of the pre with the Vishay 1837 by-pass caps. Based on what I have learned I should probably switch the Supremes to the power amp and put the V-Caps in the pre, right? AND/OR, I really should take a look at putting V-caps on the Subsonic Board and have it engaged all of the time as it will also feed the phono stage. So getting back to the point of this discussion, what is the value of the subsonic filter and how will it affect the integrity of the audio signal? Will the subsonic filter truncate the sound considering the advantage of vinyl is that there is no truncation?

[picowallspeaker]

Great question , thou !
I also changed some caps in my preamplifier , leaving only the 1 µF electrolytic ,which is...in the subsonic filter !! Then lately I also re-discovered the beauty and perfection of vynil and turntable . And with some records at medium volume it really becomes unbearable to see those cones do the HOP ;
but putting the Subsonic filter it really does nasty things , no good ,you know ,it puts a veil...
So I think it's a real problem ,when listening at medium/ high volume ,but it has to be related with the whole : mechanical damping /isolation of the turntable =>electronic chain =>speakers and feedback make the circle again.
So the first things to do would be to eliminate or correct the possible causes ,in other words to put every piece of the chain outside audio band resonance.

[DF96]

Originally Posted by AmCan

So based on this then one should always have the Subsonic Filter engaged.

Why? Only engage it when you need it.

The authors of the GCSO clearly indicated that the sooner you get the best caps possible into the chain the better the realization is of their sonic benefits.

There is some truth in this, in the following sense: capacitor distortion only matters when there is some signal voltage across the cap. Most caps in most circuits do not have signal voltage across them. The exceptions are those caps which define the frequency response. These could be shunt caps to set the HF end, or series (e.g. coupling) caps to set the LF end, or RIAA network caps.

Other things being equal, you get less distortion at lower signal voltages and with more linear dielectrics. So the caps which define the frequency response (such as a subsonic filter) should be near the start of the circuit where signal voltages are small. Later, where voltages are bigger, the caps can be bigger too so they end up with less voltage across them. Note, of course, that the voltage across a coupling cap may be much smaller than the signal voltage at that point in the circuit.

A perfect cap will modify the frequency response. An imperfect cap will also add some distortion. No cap can add quality. Therefore there is no reason to have the subsonic filter engaged unless your source contains unwanted subsonic frequencies. The only exception to this is if you happen to prefer the sound of the distortion added by a particular capacitor.

[bbggg]

Originally Posted by AmCan

the advantage of vinyl is that there is no truncation?

Excuse me??

[picowallspeaker]

Bbggg , he only states that the media is limited only physically .
Except Riaa equalization , which doesn't incorporate any HP or LP filter deliberately. Digital media must take in consideration Nyquist etc etc .
Modern HD files and also SACD just shift the LP pole much higher .

[AmCan]

Thank you all for the detailed replies. This all makes sense to me. A capacitor is a filter that alters the frequency response and adds distortion (or flavor if you will). Now I understand why some people like preamps that consist of only a transformer; nothing there to adulterate the signal.

I do have a well damped system as I work with silicone for a living (prosthetics) so I make my own isolation footers. I don't NEED the subsonic filter but was inquiring to see if the circuit board could be taken advantage of to improve the quality of the signal but I got my answer to that. The last thing I want is an attenuated signal.

In response to vinyl not being truncated that is exactly what I meant. Unlike CDs that are cut off from 20-20, Vinyl is not systematically limited. The school of thought is that the extended and smooth frequency wave playback of vinyl contributes to the harmonic build up in the room and adds to the acoustic perception of space in the music. Even though most of us only hear between 20hz to 20khz the recorded playback above and below this frequency range contributes to the build up of standing waves in the room.

[SY]

Originally Posted by AmCan

Thank you all for the detailed replies. This all makes sense to me. A capacitor is a filter that alters the frequency response and adds distortion (or flavor if you will). Now I understand why some people like preamps that consist of only a transformer; nothing there to adulterate the signal.

...except the transformer, which is likely to be far more imperfect than a capacitor.

[bbggg]

Originally Posted by picowallspeaker

Bbggg , he only states that the media is limited only physically

He must have been joking then. There are a gazillion filters inherent in analogue signal processing, including mechanical ones (eg the cartridge/tonearm resonance). Fretting about a capacitor is rather rich in this context.

[AmCan]

Originally Posted by SY

...except the transformer, which is likely to be far more imperfect than a capacitor.

LOL

But of course .... so is that why capacitors are added to the preamp stage then roughly speaking? I realize that perhaps this is a basic theory question but is your response a matter of active vs passive preamp, which is better debate? I am very green at all of this and not trained but a lot of light bulbs are going on today.

[AmCan]

Originally Posted by bbggg

He must have been joking then. There are a gazillion filters inherent in analogue signal processing, including mechanical ones (eg the cartridge/tonearm resonance). Fretting about a capacitor is rather rich in this context.

I am not joking. I am asking questions in order to learn from other people's expertise. I am not an expert at all. I know very little and take a position mostly based upon conjecture and an incomplete understanding of what is occurring. This whole hobby is built around fretting over parts and components and circuit designs so I don't know what you're talking about. Is it your position that an analog signal is just as incomplete as a digital one. That a CD is of equal quality as an LP? or that by the time a digital or analog signal is processed they are all the same? I am opened to being educated so please enlighten me rather than just making quips.