How to figure out which caps are in the signal path?

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honkyjonk
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How to figure out which caps are in the signal path?

Post by honkyjonk » Mon Feb 16, 2015 9:06 am

This might be sort of an obvious thing for more experienced people, but I wonder if someone could help me understand which caps are in the signal path of my console's input strips?

Here is the schematic: http://www.rylandbouchard.com/files/PM2 ... matics.pdf
The input strip circuit for the PM2000 is on the first two pages of that pdf.

Anyway, is it the case that if a cap is in parallel then it isn't in the signal path? So, I'm looking for caps that are in series? Forgive me if this is obvious. . . .

I'm wanting to replace tantalums that are in the signal path, and maybe try bypassing the electrolytics with little wima polypros and seeing if I can't get it sounding a little better.
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Post by Drone » Fri Feb 20, 2015 11:55 am

Caps in parallel with what?

How much do you know of schematics? Just imagine your signal is flowing left to right, those arrow things are IC's your signal flows into one side and out the point end onto the next one.

Now if it flows thru a cap, it flows thru the cap, but also some caps are in the signal path filtering it to ground, they could also be considered part of the signal path, as they have an effect on the signal, but the signal doesn't flow thru them to the endpoint, rather some flows thru them down the drain (to ground).

You also have some caps on the power lines of the IC's (Vcc) and they are for draining the noise from the power line, and no signal flows thru them as such.
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Post by ashcat_lt » Fri Feb 20, 2015 2:31 pm

Drone wrote:...but the signal doesn't flow thru them to the endpoint, rather some flows thru them down the drain (to ground).
Not to be that guy, but it's a little better to say that some of the signal completes its circuit early. Ground is not really a destination, but a reference voltage and/or return path. It's a minor point, and usually doesn't much matter, but it is a bit better way of visualizing things.

I would say that pretty much any cap that connects to the actual signal path will affect the signal, and might be a candidate for upgrade/replacement. That said, any caps across the power supply will have a real effect on noise if nothing else. Electrolytics here should usually have smaller film or ceramic caps in parallel, too.

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Post by The Scum » Fri Feb 20, 2015 2:52 pm

If you pull out a cap and no longer get signal, it was in the signal path.

But it's somewhat complicated by the fact that there are stages of the circuit that the cap that the signal is going through is necessary for the operation of the circuit - the hipass and EQ won't function without the caps. We want to get rid of any RF that comes in.

If you really want to upgrade things, get rid of those dinosaur SIP opamps. A bunch of the caps on that schematic are there to stabilize those opamps - all of the pin5 business. Modern opamps don't need that. A low offset opamp (Those Nat Semi LME series are good candidates) would allow you to toss out the compensation networks and a fair number of the interstage coupling caps.
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honkyjonk
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Post by honkyjonk » Mon Feb 23, 2015 9:05 am

Hey guys,

Thanks a bunch for the info.

Scum,

So, are you talking about using something like this adaptor and some DIP chips? http://cimarrontechnology.com/so8dip8to ... 60301.aspx

I'd definitely be into giving it a shot if I knew exactly which chip/s wouldn't require a power supply upgrade and exactly which caps to remove. Are you talking about the caps that are between pin 5 and pin 1.

So, the 33pF and 330pF in the case of IC2, and the 47pF and 470pF in the case of IC3 and so on. . .
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Post by germaniac » Mon Feb 23, 2015 12:13 pm

The Scum wrote:If you really want to upgrade things, get rid of those dinosaur SIP opamps. A bunch of the caps on that schematic are there to stabilize those opamps - all of the pin5 business. Modern opamps don't need that. A low offset opamp (Those Nat Semi LME series are good candidates) would allow you to toss out the compensation networks and a fair number of the interstage coupling caps.
Not to challenge your advice, but just to add another point of view, I've heard those particular SIP op amps (HA1457) are pretty good. Maybe a lot depends on running them at +24V. At any rate, I wouldn't just replace them without at least A/Bing them against something else. It would certainly save a lot of work and expense if they're already okay.

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Post by The Scum » Mon Feb 23, 2015 5:14 pm

Ahhh...the +/- 24V supplies hose up a lot of opamp swaps.

I don't have any specific recollection that informs my in any particular direction about the HA1457. Just the fact that they're externally compensated informs me that they're ancient, and better replacements can probably be found.

Even if they're fine in terms of noise and distortion, all of those coupling caps tell me their DC behavior probably leaves something to be desired. Taking a buncha poles out of the system by removing a pile of lytics will probably help with low-end clarity - moreso than just replacing them.

These arguments also wander into confirmation bias territory pretty quickly. We'll hear what we want to hear.

Jon, have you seen this article?
https://flyingeyepro.wordpress.com/tag/ha1457/
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honkyjonk
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Post by honkyjonk » Tue Feb 24, 2015 4:03 pm

Awesome,

Thanks for that link! Sounds like he thinks replacing those chips was a worthwhile thing to do. I'll be following that.
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