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Old 29th January 2015, 09:53 AM   #1
skyto is offline skyto  Finland
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Default Parallel caps, wire resistance

Hello,
being new to amps this may seem a stupid question. Searched thru older threads to no avail or I just didn't get it.

I'm trying to improve a SE amp that has an irritating hum. Due to cramped layout, additional caps will be distanced from the 1st cap that sits on the pcb. Wire resistance is 0.0013 ohms. Here's the PSUD chart in this case

Click the image to open in full size.

..however, adding a 1ohm resistor in between calms the situation

Click the image to open in full size.

I guess it would be a "good thing" to have the first caps parallelled to have a decent reservoir with as little ESR a possible. However, the connecting wiring seems to give trouble. Is this a PSUD flaw or should I just accept the results - and add resistance in between the first caps?
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Old 29th January 2015, 11:03 AM   #2
DF96 is offline DF96  England
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It could just be that the PSUD2 engine is having trouble converging on a solution.

Why do you think that adding capacitance to the reservoir will reduce hum? It could increase hum. Have you got your grounding right? Most people don't.
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Old 29th January 2015, 11:37 AM   #3
skyto is offline skyto  Finland
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Hi thanks for the reply.

My adding a parallel cap to the first one is mostly ESR related. The rest of the chain would take care of the hum (C2, L1, C3, C4 added to the standard layout).

As of now the 22uf reservoir is a poor chinese electrolyte installed on pcb. Grounding is as is, I'm reluctant to cut pcb traces. As I'm no pro I'm willing to keep these mods within "bolt on" limits.
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Old 29th January 2015, 11:50 AM   #4
DF96 is offline DF96  England
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Don't assume that hum on the output always comes from ripple on the supply rail. It may, but there are plenty of other sources of hum in an amplifier. If you tell us which amp you have, someone on here may know to improve it.
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Old 29th January 2015, 12:18 PM   #5
skyto is offline skyto  Finland
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It's a Gretsch 5220 aka Fender Champion 600. I found it last week during my last landfill pillage. Near mint condition, price tag still attached. The power valve was blown along with the cathode resistor (before that I picked up a near perfect pair of Sonabs).

It's true the Champion 600 hum problems are widely covered. It's just that while I have it open - why not just improve the power supply department with all the surplus I have.

Anyways, these PSUD results are puzzling. Am I in for weird sounds if I keep the arrangement as in the first picture?
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Old 29th January 2015, 04:00 PM   #6
DF96 is offline DF96  England
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The best way to improve a circuit is to correct the things, if any, which are wrong. 'Upgrading' things which are OK is merely a way to introduce new faults.
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Old 29th January 2015, 04:24 PM   #7
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I would suspect the resistor value and cap ESR value being so close is causing PSUD to do odd things.

Personally i would take a step back and look at the stock circuits calculated ripple at I1 and I2 and see if they are enough to cause an audible hum, as above its just as likely the choke is picking up mains noise, or the heater noise from the tubes etc. Also i would set the reporting delay to 2-3 seconds to see what things look like in steady state, especially for the ripple.
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Old 29th January 2015, 05:47 PM   #8
skyto is offline skyto  Finland
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Quote:
Originally Posted by DF96 View Post
The best way to improve a circuit is to correct the things, if any, which are wrong. 'Upgrading' things which are OK is merely a way to introduce new faults.
Ok, guilty . With this chinese-made entry level amp its hard to find those "OK" things, which has made it a popular target for overkill modifications.
  1. all resistors specified as 2 watts were suspiciously small in physical size, replaced them all. Two of them looking overheated, one blown.
  2. chinese tubes, a notoriously bad 6v6
  3. from what I've gathered too much B+ for the 6V6
  4. filament wiring all over the input area
  5. output transformer being placed right beside power transformer
Flat cable - the inputs:


Click the image to open in full size.

Quote:
Originally Posted by mcandmar View Post
I would suspect the resistor value and cap ESR value being so close is causing PSUD to do odd things.

Personally i would take a step back and look at the stock circuits calculated ripple at I1 and I2 and see if they are enough to cause an audible hum, as above its just as likely the choke is picking up mains noise, or the heater noise from the tubes etc. Also i would set the reporting delay to 2-3 seconds to see what things look like in steady state, especially for the ripple.
The choke is one of my additions, not tested yet. My first PSUD guesstimate showed 20V ripple at I1 (OT, 6V6) for the original circuit. I think I'll just carry on and see what happens, if things go for the worse I'll recheck my input data and PSUD results.
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Old 30th January 2015, 11:41 AM   #9
DF96 is offline DF96  England
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The Chinese have learnt that Westerners like buying cheapish junk, so they produce lots of it. They know we buy things on the basis of appearance and a good story so that is what they provide.
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