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Active or passive monitor controller?
Old 4th June 2012
  #31
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Hi
If the 'Pilot' is getting scratchy it COULD be that the amp following or the source has some DC or even HF signal on it as any 'irregularities' encountered when operating a control can make themselves heard.
@ Roger's piece.
Impedance issues can often be measured.
Some of the 'numbers' when talking about a 10K attenuator are incorrect as they stand but do illustrate a point.
It is the inconsistency that can come about by using a 'simple' pot that cause the problems. If you have a dedicated output from your 'source' and never change it then that is not too bad, but if there is any source switching going on you MUST present a constant load on the source that is being measured, otherwise it will change response.
The attenuator should be constant impedance in and out otherwise again the response will alter depending on settings.
Ideally the 'input' to the attenuator needs to be 'infinitly high' but then the output very low, an instant conumdrum!.
Matt S
Old 4th June 2012
  #32
Maybe I confused my post, but I wanted to try to illustrate how the cable capacitance varies in relation to whatever source impedance the switch presents in the circuit, which differs with rotation...

At CCW, the destination input is "grounded" so the cap has no effect, but when you turn it up, the relationship starts to affect the circuit, and that relationship changes with the pick off point of the R string on the switch.

I am currently using a passive solution here, and I don't hear the effects but I do want to pre-empt the effect by wrapping the switch in an active circuit.
Op amp at the input and Class A line driver on the output.
Old 5th June 2012
  #33
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Hi Roger
I am not 'knocking' your piece which has many relevent observations and highlighting the VARIABILITY of a 'simple, passive' system is most important. The frequency response must stay constant (or as near as practicable) for all positions of the monitor control.
The 'passive' brigage carefully omit the fact that real resistors and cable can have significant effect on what appears to be a 'perfect' solution. In some cases you can get away with a simple 'pot' especially if it is low resistance (say 2K or thereabouts) and use short cables.

Matt S
Old 5th June 2012
  #34
Quote:
Originally Posted by Matt Syson View Post
Hi Roger
I am not 'knocking' your piece which has many relevent observations and highlighting the VARIABILITY of a 'simple, passive' system is most important. The frequency response must stay constant (or as near as practicable) for all positions of the monitor control.
The 'passive' brigage carefully omit the fact that real resistors and cable can have significant effect on what appears to be a 'perfect' solution. In some cases you can get away with a simple 'pot' especially if it is low resistance (say 2K or thereabouts) and use short cables.

Matt S
Thanks Matt,

Never got the feeling you were knocking me, and I agree completely!
I need to draw this out on paper to see the various permutations of the RC network and understand the effects therein.

rf
Old 5th June 2012
  #35
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Hi Roger
The impedance of a 10K attenuator (pot) is 'worst' at 6dB 'down' and is a quater of the total value (being 2K5) plus the output impedance of the 'source' which isn usually quite small, a hundred ohms perhaps.
Work this into your 120pF cable and of course you should see some for the input of the power amp. If it has decent RF filtering, the capacitance here can be pretty large, 1nF perhaps.
Matt S
Old 5th June 2012
  #36
Motown legend
 
Bob Olhsson's Avatar
 

It depends entirely on what is driving the passive attenuator. It it has great current capability, they can sound better because of fewer stages. If not, active sounds better.
Old 5th June 2012
  #37
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AlexK's Avatar
 

RE 'scratchy' TC Level Pilots, a small amount of contact cleaner applied directly to the pot itself should solve these issues.
Old 5th June 2012
  #38
the crookwood is passive with active digitally switched relays, best of both worlds?
Old 5th June 2012
  #39
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Hi
If your pot or attenuator (excluding constant impedance types which are rare in 'pot' form) is 10K Ohms then the output impedance will be 2K5 at it's '6dB down' position. If it is being FED from zero Ohms or 200 Ohms it will only make a small difference. It is the variability of the OUTPUT impedance of the pot that is the issue. If you were to feed a 1K pot (attenuator) with an amplifier stage that is happy with this loading, the output impedance would be 250 Ohms at worst case, rather better to combat the following cable and amplifier capacitance.
Constant impedance (in and out) attenuators have an advantage of frequency respone consistency, but if it is more than about 1K ohms then it will suffer HF loss.
If you chose to switch this low resistance attenuator to different 'sources' the level of the monitored source will fall when it is being monitored.
Matt S
Old 5th June 2012
  #40
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Quote:
Originally Posted by Bob Olhsson View Post
It depends entirely on what is driving the passive attenuator. It it has great current capability, they can sound better because of fewer stages. If not, active sounds better.
I drive my Shallco stepped attenuator with one of my headphone output channels, and the cable run to the power amp behind the attenuator is about three feet. Adverse audio impacts from this system is never something I worry about.

Cheers,

Otto
Old 6th June 2012
  #41
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Firechild's Avatar
Quote:
Originally Posted by AlexK View Post
RE 'scratchy' TC Level Pilots, a small amount of contact cleaner applied directly to the pot itself should solve these issues.
How do you "open" the TC Level Pilot ?
Old 6th June 2012
  #42
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AlexK's Avatar
 

Quote:
Originally Posted by Firechild View Post
How do you "open" the TC Level Pilot ?
Clamp one hand around the base, and gently, but firmly pull off the knob. It's not glued or screwed in (or at least mine isn't). It's just a pressure fit.

You can also access the back of the pot by pulling off the rubber mat underneath, but go careful as you might yank the soldering.
Old 6th June 2012
  #43
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Hi
The pot is supposedly a 'Bourns' unit so should really be OK.
Before doing anything 'drastic' it is worthwhile 'testing' it to see if it is your setup that is encouraging 'scratchiness'. One way would be to connect it between 2 other pieces of gear, say a CD player or another source of 'tone or music' and the input of a mixer or other amplifier. Maybe even use it between insert send and insert return on a mixer. If it is then scratchy in the same way and place the Pilot is faulty.
Your usual signal source or monitor amp inputs could have 'issues' with DC or HF noise that are just being revealed by the simple 'pot' of the pilot.
Matt S
Old 8th June 2012
  #44
Gear Guru
 
u b k's Avatar
 

I'll second the notion that if a Level Pilot is getting 'scratchy', odds are pretty good it's being fed DC.

Fortunately, there's a very easy way to tell:

1) Disconnect the inputs of the Level Pilot, but leave the Outputs connected

2) Turn the pot

If you hear noise, it's a dirty pot.

If you don't hear noise, then the Level Pilot is not the source of the noise per se, it just makes the noise in response to being fed DC by the gear upstream.


Gregory Scott - ubk
Old 8th June 2012
  #45
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Hi
I agree with Gregory, but it is also possibler for some DC to be coming OUT of the monitor amp that it is feeding. It shouldn't of course but it is possible with some designs. Granted this would be quite unusual but don't dismiss it as a possibility.
Matt S
Old 29th July 2012
  #46
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Firechild's Avatar
Quote:
Originally Posted by AlexK View Post
RE 'scratchy' TC Level Pilots, a small amount of contact cleaner applied directly to the pot itself should solve these issues.
Worked!
Old 29th July 2012
  #47
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Arksun's Avatar
Quote:
Originally Posted by RogerFoote View Post
In a 10K passive attenuator, you are inserting up to 10,000 Ohms of source impedance to the equation. Matt Syson's post mentions having the attenuator right on the monitor amp. which is fine since you would not have all those pFs of cable capacitance across the downstream end of the 10K... Especially since the wiring would not be STP cable inside the amp case, but individual wires or copper traces which have far less parasitic C than foil covered twisted pair.

So, if the cable is 12pF per foot and you have 10 feet of cable, you have a 120pF cap across the downstream end of a 10K resistor between your DA and monitors at some settings. This RC's effect is partly dependent on source impedance of the DA/Mixer... And worse yet, majorly dependent on attenuator position. In any case, it is a variable first order low pass filter at low settings. At high settings, like fully clockwise, the 120pF cap is in parallel with the 10K.

Not good!
That doesn't take into account passive units that use a stepped attenuator with individual resistors that keep the resistance constant level at ALL attenuator positions.
Old 29th July 2012
  #48
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Quote:
Originally Posted by Fezzle View Post
Isnt it, sorry i guess i mean active electronics. I dont think passive systems interfere with the sound like actives do, . Everybody isnt using passive designs because stepped attenuators are very expensive and I believe they are the only way to reach true transparency without shifting phase etc. VCA's in active systems introduce distortion and that is science. However they are cheaper to implement in a design so thats why active systems are often more accessible. I had a lengthy discussion with a tech before buying mine.. and whilst your not wrong about taste being a factor in what works for you. Passive circuitry (in theory)wins in retaining sonic purity despite other options available unit to unit.. and when monitoring that is of course important. Yet like you say there are exceptions. The Avocet is supposed to be transparent as hell, and is active.. so is a little more versatile, yet that design is pretty much high end when you look into it, using class A buffer amps etc.
I can't agree. In case of monitor controller, passive is usually cheaper to build. The thing is that a passive controller changes the impedance which in combination with capacitance from the cable can affect the sound way more than modern op-amps do.
Old 29th July 2012
  #49
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Hi
Having a constant impedance attenuator using switched resistors solves part of the problem in that the frequency response will stay constant at all settings however it can't solve the conumdrum that you want to maintain a very low output impedance, say 100 Ohms or less AND provide a high impedance (say 3K Ohms or preferably greater) impedance at the 'input' of the attenuator.
Incidentally Roger's 'sums' are a little awry, but it does illustrate the point. The 'source' impedance seen by the output cabling from the pot is at -6dB down position a quarter of the pot resistance (2K5 for a 10K pot).
Matt S
Old 29th July 2012
  #50
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Arksun's Avatar
Quote:
Originally Posted by Matt Syson View Post
Hi
Having a constant impedance attenuator using switched resistors solves part of the problem in that the frequency response will stay constant at all settings however it can't solve the conumdrum that you want to maintain a very low output impedance, say 100 Ohms or less AND provide a high impedance (say 3K Ohms or preferably greater) impedance at the 'input' of the attenuator.
Incidentally Roger's 'sums' are a little awry, but it does illustrate the point. The 'source' impedance seen by the output cabling from the pot is at -6dB down position a quarter of the pot resistance (2K5 for a 10K pot).
Matt S
Good points, but in my setup using a passive stepped SA1X rated at 10K with just 2m length cables going to my monitors (with 10K input) I find it to be incredibly transparent. It certainly sounds a lot better compared to going direct to the audio interfaces output and turning down the digital mixer level in software. Maybe the fact the monitors have some pretty advanced adaptive output impedance tech going on in the amps helps too I dunno.

In the end there's good and bad in both the passive and active routes. If you can afford quality components and are aware of the issues, both ways can achieve very high transparency imho.

My main reason for choosing passive, aside from being able to use shorter cables runs to nearfields, is just the peace of mind of something causing a reduction to the signal passively without fear of an active circuit design suddenly having a catastrophic failure causing some kind of surge to the monitors. Given I completely overloaded my previous pair of monitors with a massive signal and damaged them this affected my decision.
Old 29th July 2012
  #51
Quote:
Originally Posted by Arksun View Post
That doesn't take into account passive units that use a stepped attenuator with individual resistors that keep the resistance constant level at ALL attenuator positions.
Oh, I did take that into account. The difference you refer to is the difference between a "potentiometer" and bridged T or H type attenuator, the latter 2 keep constant impedance but usually don't have as many steps...

I do have a pair of 20 step Daven bridged T attenuators with 20- 1 dB steps, and I may end up using them but what keeps me from doing that is it doesn't go completely off... 20 dB max.

I have a pair of H attenuators (my own design) that will do 30 dB max, still not complete shut off though.

OTOH, potentiometers (resistor string) have full shut off but presents the changing impedance, which theoretically can be an issue.

Roger
Old 30th July 2012
  #52
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Big_Bang's Avatar
 

Well, I must retract my previous post.

Mine got scratchy, strangely enough after not being in my studio for 2 months (and no one else operating it).

It got significantly worse within weeks, but I usually barely touch it until final mix stages.

I thought it could be dirt and had no contact cleaner at hand, so I did the 'ol dirty "wank the knob" trick - for a minute or so, until my wrist got tired. I did agressively so!

The Pilot started squirting clean audio. With a slight scratchiness here or there.

Dirty contacts for me, so off to buy cleaner spray
Old 30th July 2012
  #53
Quote:
Originally Posted by Matt Syson View Post
Hi
Roger's 'sums' are a little awry, but it does illustrate the point. The 'source' impedance seen by the output cabling from the pot is at -6dB down position a quarter of the pot resistance (2K5 for a 10K pot).
Matt S
OK, I see what you are doing, you are speaking of a linear pot...
My "pot", a 4 deck Elma switch, is log taper, not linear as you have posted.
Aprox 1/4 rotation is 202.8R (first six resistors of 23 total/ position seven)
That is around -34 dB in a perfect impedance world...

rf
Old 16th July 2017
  #54
rds
Gear Addict
 

Quote:
Originally Posted by Matt Syson View Post
The important thing with a 'passive' control is that there must be an absolute minimum of capacitive loading (length of cable) attached the the OUTPUT of the control.
In your opinion what is the maximum length that should not be exceeded in this case ?
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