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Need help with console modifications.
Old 25th February 2009 | Show parent
  #31
Gear Head
 
🎧 10 years
Ulysses, do you have a rule of thumb for increasing the uF, or do you agree with Matt that is wouldn't hurt it at all increasing it 10 times the original? The reason I ask is mainly the EQ section caps. Most are different uF and it would mean I would be replacing them all with panasonic fc 1uF/35 volt caps. I am wondering if this would change the EQ for the worse doing this. There are 0.68uF, 0.47uF, 0.33uF, 0.15uF, and a 1.5uf that would get replaced with a 2uf panasonic fc.

On the BP note, via the schem a BP 47/25 cap is being used on the low EQ. I don't really understand this as it seems on lead is tide directly to ground with a 50k resistor in line. Seems kind of odd to me as all the caps I just mentioned are in the low mid EQ and the ground leads are tide directly to the same ground as the BP. I do believe I could just drop a polarized cap in its place and be just fine.

Thanks



Quote:
Originally Posted by Matt Syson ➑️
hi
The 22uF would be fine in replacement of the 10uF. If you are interested in lowering LF phase shift you could increase them further. You can be selective or go for a blanket 47uF or 100uF.
Matt S
Old 25th February 2009 | Show parent
  #32
Lives for gear
 
ulysses's Avatar
 
🎧 15 years
Quote:
Originally Posted by John Kent ➑️
Ulysses, do you have a rule of thumb for increasing the uF, or do you agree with Matt that is wouldn't hurt it at all increasing it 10 times the original? The reason I ask is mainly the EQ section caps. Most are different uF and it would mean I would be replacing them all with panasonic fc 1uF/35 volt caps. I am wondering if this would change the EQ for the worse doing this. There are 0.68uF, 0.47uF, 0.33uF, 0.15uF, and a 1.5uf that would get replaced with a 2uf panasonic fc.

On the BP note, via the schem a BP 47/25 cap is being used on the low EQ. I don't really understand this as it seems on lead is tide directly to ground with a 50k resistor in line. Seems kind of odd to me as all the caps I just mentioned are in the low mid EQ and the ground leads are tide directly to the same ground as the BP. I do believe I could just drop a polarized cap in its place and be just fine.
I don't do rules of thumb. You have to know what you're looking at. Some designs have plenty of capacitance to begin with. In 20 years when you're recapping a Great River or Roll Music Systems product, you won't need to increase the capacitance. Some older stuff might really WANT an increase in capacitance, but maybe the power transformer, mains fuse, or rectifier won't be able to handle the increased inrush current at power-up. Those are things you have to look at and consider on a case-by-case basis.

Capacitors in the EQ section should not be electrolytic, and as such probably don't need to be replaced. There may be some electrolytic coupling caps (which pass signal from one stage to another) or decoupling caps (which remove noise from the power supply lines) but I would hope the caps you mentioned (0.68uF, 0.47uF, 0.33uF, 0.15uF, and a 1.5uf) would all be film caps. These would be the caps that determine the frequencies at which the EQ operates, and changing their values would mess up the way the EQ works. They should remain the same values. If they ARE electrolytic caps, you should be able to replace them with polyester film caps of the same value. Electrolytics typically have a tolerance of 20% if you're lucky - sometimes as wide as +100/-50. They are almost never used in frequency-specific applications, and with good reason.

Remember that the main reason we use electrolytics at all is because of the high capacitance they can fit in a given volume. If you don't need a high capacitance, there's not much reason to use one. The small values you're looking at in the EQ circuit are readily available in polyester film, or if space provides, polypropylene film.

I can't comment on the bipolar cap you're describing because I can't see the schematic clearly. Can you post a close-up of the section in question?
Old 25th February 2009 | Show parent
  #33
Lives for gear
 
🎧 15 years
Hi
Woah boy hang on a minute!
Replacing all the 'small' caps in the EQ will definately mess it up!
You are only replacing ELECTROLYTIC caps, not the polyester, mica, or polystyrene types which are used especially in the EQ to set the frequencies of the various 'bands'.
If you wanted to get 'esoteric' at a later date you could swap 'brands' of these small non polarised caps but at present you should be concentrating on the electrolytics which do 'dry' out. The others go not degrade over time as such but some makes / types are 'better sounding' according to some people. Whether you would actually hear any difference in a unit as complicated as a mixer is a moot point.
Matt S
Old 25th February 2009 | Show parent
  #34
Lives for gear
 
🎧 15 years
Hi
As a general note, a flat response from DC to light is not always a good thing. Desks intended for live performance for example would cause havoc when a mic gets dropped when attached to a mega powered PA rig.
Broadcast desks in the past were deliberately rolled off to prevent sub bass from upsetting transmitters and extreme HF which would get cut off anyway. Trying to force HF down a line to the transmitter can cause strange problems.
A well designed piece of gear WILL roll smoothly at LF and HF and use non polarised (high tolerance) components to define this at the input and posibly output of the gear with wider bandwidth being achieved throughout the internal sub circuits. Sometimes roll off would occur in the sub circuits and worse, may change when selecting things like phase reverse. These instances should be attended to.
Matt S
Old 27th February 2009 | Show parent
  #35
Gear Head
 
🎧 10 years
Thanks for the quick replies. I've attached a close up of the low eq schem. As you can see, the caps I'm talking about are electrolitic caps. You can also see the BP I was talking about.
Attached Thumbnails
Need help with console modifications.-channel-strip-low-eq.jpg  
Old 27th February 2009 | Show parent
  #36
Gear Head
 
🎧 10 years
I should also note that those are the only electrolitic caps located in the EQ section.

Thanks
Old 27th February 2009 | Show parent
  #37
Lives for gear
 
🎧 15 years
Hi
The main one to replace is the 47uF in the Low EQ. It is bipolar so use the same or 2 off 100uF wired back to back in series.
The parts in the mid may be electros or more likely Poly something. If electroltyic, you could work out their value and replace with poly somethings (either caps in series or parallel to suit space and sizes available). Whatever their value is crucial to the operation of this part of the EQ.
Matt S
Old 27th February 2009 | Show parent
  #38
Lives for gear
 
JohnRoberts's Avatar
 
🎧 10 years
Replace all the small ones (<1uf) with film, like polyester or whatever you can source. Note the two caps in series can be replaced by one film cap of the appropriate (smaller) value. The equations for series capacitors should be easy to find. It's similar to resistors in parallel.

JR
Old 27th February 2009 | Show parent
  #39
Gear Guru
Use Wima MKS-2 mylars in the electrolytics place for the low EQ. They are available up to 10 uf in a 5mm size.

As to that 47 uf bipolar, if you use either a low bias current bipolar opamp or a pico amp bias FET input opamp, you can eliminate it. I would try a LME49720NA at 7 ua bias current or a FET amp like an AD823 or a pair of OPA627/OPA827 S0-8 opamps on a dual Brown Dog header.

Jim Williams
Audio Upgrades
Old 27th February 2009 | Show parent
  #40
Gear Head
 
🎧 10 years
I do see that the caps in question are in series and can be replaced with one. Thanks for reinforcing my thoughts on that. I'm learning alot. Thanks for all the help.

Let me make sure I'm doing this correct. When caps are wired in series the capacitance is reduced and the voltage is added up. So lets say I'm replacing the .68uf/50volt and the 1.5uf/50volt. I could use a 0.47uf/100volt polypropolene or polyester cap in in place and use a jumper to connect the trace where the other cap was.
Old 27th February 2009 | Show parent
  #41
Gear Head
 
🎧 10 years
Thanks for the input Jim. I'll check out the MKS-2 mylars. As for the rest of it, I do appritiate the input, but it was so far over my head I can't even begin to understand what you just said. My knowledge isn't that far along yet, Im still reading quite a bit on the subject.

Quote:
Originally Posted by Jim Williams ➑️
Use Wima MKS-2 mylars in the electrolytics place for the low EQ. They are available up to 10 uf in a 5mm size.

As to that 47 uf bipolar, if you use either a low bias current bipolar opamp or a pico amp bias FET input opamp, you can eliminate it. I would try a LME49720NA at 7 ua bias current or a FET amp like an AD823 or a pair of OPA627/OPA827 S0-8 opamps on a dual Brown Dog header.

Jim Williams
Audio Upgrades
Old 27th February 2009 | Show parent
  #42
Gear Head
 
🎧 10 years
Seems I can get the Wima MKS-2 from Mouser.com, but they have a 13 week lead time and I the minimum is 1200, so I don't think i'll be goin that route..
Old 27th February 2009 | Show parent
  #43
Gear Head
 
🎧 10 years
Anybody else notice that the caps in question aren't wired in series? They are wired negative to negative, not negative to positive? Now I'm really smackin my head against the wall.... What does that mean.
Old 27th February 2009 | Show parent
  #44
Lives for gear
 
🎧 15 years
Hi
Wired as neg to neg (or pos to pos) would make them equivalent to a bipolar cap.
Strictly speaking the capacitance values OUGHT to be equal but since this application is only handling millivolts of DC offset it does not matter.
You do not need 100 Volt rating but that is not important here because most poly something caps are at least 100 Volt. 50 would be plenty.
As Jim pointed out if you replaced the op amp with a very low offset and bias current type thaen you would not need the 47 uF electro. In fact you could try without it, the only effect you might notice would be a slight 'scratchiness' when operating the particular EQ pot, which of course you are not doing all the time.
Matt S
Old 27th February 2009 | Show parent
  #45
Lives for gear
 
JohnRoberts's Avatar
 
🎧 10 years
Quote:
Originally Posted by John Kent ➑️
I do see that the caps in question are in series and can be replaced with one. Thanks for reinforcing my thoughts on that. I'm learning alot. Thanks for all the help.

Let me make sure I'm doing this correct. When caps are wired in series the capacitance is reduced and the voltage is added up. So lets say I'm replacing the .68uf/50volt and the 1.5uf/50volt. I could use a 0.47uf/100volt polypropolene or polyester cap in in place and use a jumper to connect the trace where the other cap was.

Yes, a single .47uF 50V film should be fine and could easily be an audible improvement if the rest of the path is clean.

Since film caps are non-polar disregard polarity. The order of polarity in series connection is perhaps to mitigate against reverse biases but does not alter the basic series relationship wrt reactance.

Note: voltage in series connections for polar capacitors is not simple addition. Only if same polarity and some biasing network to insure voltage drop is shared. For this application the breakdown voltage could even be lower that 50V with no problem anticipated.

JR
Old 13th August 2009 | Show parent
  #46
Gear Head
 
🎧 10 years
Hope somone sees this new post on my old thread.

So I've recapped the console, sounds great, thanks to everytone for the help.. I learned alot.

To my question at hand. Really the only proble I have with the console is its gain. It seems most of the time it is either to much or not enough. I have found that driving the pres in this thing on drums really adds flaver, but I really would like more controll over the gain. Would it be posible in this desk to replace the the selector pot that it has with a normal resistance pot.

Thoughts?
Old 13th August 2009 | Show parent
  #47
Lives for gear
 
🎧 15 years
Hi
If you are referring to the 'gain' pot then the basic answer is 'no'.
The 'law' of the pot is very extreme and far from ideal at the 'high gain' end and simply put it is almost impossible to manufacture pots with sufficient accuracy.
It is usually a reverse log type (a log pot that operates 'back to front'.
This is why the more expensive gear has a switch, where you can fit resistors to give you predictable amount of gain.
A 12 position switch with steps of 5 or 6 dB is the 'norm'.
Some go for a fancier 'dual control' (coarse and fine).
Matt S
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