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Old 19th August 2007, 01:58 AM   #1
Bigred is offline Bigred  Canada
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Default Upgrades and Restoring

With respect to restoring and/or upgrading of Solid State Audio Power Amps and Pre-Amps. ANY THOUGHTS AND OPINIONS for the following??

1. Replace all carbon film R's with metal film. Besides having tighter tolerances and less drift with time and heat what other advantages? Necessary? Do they really sound better?

2. Should I go out of my way to use higher wattage rating providing space isn't a problem i.e. 1/4watt to 1/2 watt. What is to benefit? I have "heard" this has an effect on sound.

3. ceramic caps in the audio circuit.... change to polystyrene? Where do YOU source from?

4. Electrolytics.... Realising any older unit is gonna benefit with newer electrolytics, what is the consensus on brands and quality. Where do YOU source from?

5. How common is it that polar electrolytics are used where no polarizing voltage is across it? Should the effort be made to replaced with a non-polar unit?

6. Thoughts on replacing caps with a higher voltage. Can you over do it in this regard?

Without dealing with specific designs and schematics are there any other general tweaks, upgrades, or improvements anyone can add? Although I am MAINLY concerned with sound any ideas are welcomed. For example, I have replace cheap VR's with bourns multi-turn trimmers in the past.
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Old 19th August 2007, 03:19 AM   #2
mrbubbs is offline mrbubbs  United States
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What a good topic. I posted another thread about figuring out some of the components in my amp, which will hopefully lead me to kind of freshening up my older amp.

I'm interested in the replies in this thread! I think someone recommended upgrade critical caps to Black Gate.
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Old 19th August 2007, 04:43 AM   #3
ppfred is offline ppfred  Canada
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Quote:
Thoughts on replacing caps with a higher voltage. Can you over do it in this regard?
Yes. If you OVER do it then in time they could dry out.

Here's a posting that you should consider:
http://www.tnt-audio.com/clinica/bias_e.html
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Old 19th August 2007, 04:57 AM   #4
Bigred is offline Bigred  Canada
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Quote:
Originally posted by ppfred


Yes. If you OVER do it then in time they could dry out.

Here's a posting that you should consider:
http://www.tnt-audio.com/clinica/bias_e.html
thanks ppfred, I figured excessive rating would pose a problem with regards to the lifespan of the cap. I'm thinking its best to keep the working voltage within 70-80% of the caps rating.
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Old 19th August 2007, 06:17 AM   #5
mfratus is offline mfratus  United States
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There are some changes you can make that won't make much difference, but will add reliability. There are some you need to do anyway unless you just like working on the same amp over and over.

Per your questions:
1. Metal film resistors are quieter than carbon film, but unless they are in a critical position you won't hear any difference. They are more reliable, though. Do it.

2. Higher wattage resistors would add reliability, but no difference in sound. If you want, do it, but I wouldn't.

3. Sometimes you want a ceramic cap. They make excellent snubbers for high frequency feedback, but they are too wide-band for audio signal path. Polypropylene are probably the most reliable of the film caps. Metal foil is better than metalized film. Oil & paper caps are the longest-lasting, but do you want an amp for 50 years? I use the Mouser catalog.

4. Electrolytics: I look for caps with low-ESR. Basically, there's more "cap" to them. Some cheaper ones with high ESR (equivalent series resistance) work like they have a resistor in series inside the cap. Not always good. But, on some classic tube amps, you actually can ruin the sound by going with too good a cap... but not on transistor amps. I use a lot of Sprague caps, but there's a lot of good brands. Look at the specs. Again, Mouser has specs on-line.

5. There is almost always SOME voltage across a cap. Exceptions are in cross-overs, tone control circuits, and other passive networks or AC circuits. Even at the inputs, if the input source is at ground, the bias of the input stage is probably at some positive voltage. But I would change caps in the audio signal path to film caps rather than electrolytic of any sort. Solen is good for film caps, but there's a lot of other good brands. See the Mouser catalog. See also http://electrochem.cwru.edu/ed/encyc...4-appguide.pdf for "too much" cap information from Cornell-Dubilier, a good cap manufacturer.

6. No problem with, say, a 63 volt in place of a 50 volt cap, but more voltage carries with it other problems, like higher ESR and larger size caps. Newer caps are generally smaller anyway, but I would not use 500 volt caps throughout a solid-state amp. Aim at 50% over maximum voltage the cap sees, for safety. Never go under what the OEM put in there.

Lead dress and shielding is important. All inputs should be shielded. Very-high-gain inputs probably could use better shielding. Most of it is only 70% shielded.
Run wires along the metal chassis to prevent them from radiating or picking up signals.
Good grounding is lost from bare-metal grounds getting oxidized. Use star washers to bite through the paint, oxide, etc.
Ventilation on some amps seems to be limited to the heat-sinked output devices. The inside of an amp can get REAL warm. You can string two 12v (computer) fans in series and they are real quiet.
I'd use also 105 degree caps, touch up the soldering on all the boards, and check stuff like big resistors heating up electrolytics next to them. Dried out caps are no good for anyone.

I repair amps for a living, so I see a lot of things that most often need done.
Mouser is here in Texas, so I get my parts in two days max, and pay for UPS ground. And their prices are good.

Mike.
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Old 20th August 2007, 04:24 AM   #6
Bigred is offline Bigred  Canada
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Thanks for the cap guide Mike. I presume you were pointing me towards voltage derating.
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Old 20th August 2007, 04:54 AM   #7
anatech is offline anatech  Canada
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Default Slow down now fellas!

Firstly, I hate that darn article that ppfred posted. It has more emotion than fact. Please ignore it because I don't want to dissect it line by line. Use common sense - please!

One thing I will say. There is nothing wrong, or cheap about most of the 270 trim pots used for adjustments. Multiturn controls may actually be worse in these locations.

Metal Oxide resistors are more reliable than some film types. I have measured some fairly inductive metal film parts over the years.

So, unless you have the schematic (service manual is much better), you should not be swapping out resistors. Beware of the maximum voltage ratings for some resistors.

So, with that in mind ....
1. Don't waste your time with this. There are a couple that might improve the S/N ratio. I doubt it though.
2. There is no effect on sound. If you see some overheating, use a higher wattage that still fits and space them up some.
3. Avoid oil and paper caps. They can be very unreliable. Most mica and film caps will be better than ceramic types. NPO's may be okay.
4. Some older units used great capacitors. On average, new current electrolytics are better than older types. The larger cans are usually better than the smaller can sizes.
5. Often there may only be a few mV across some electrolytics. Use polarized in those cases. A non-polarized cap will be too large.
6. No problem. I often use higher voltage caps because the leads will fit in the holes. The size is closer to the original part. Higher voltage rated caps will last longer. They will not "dry out" any faster and may actually last much longer.

Use common sense.

-Chris
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Old 20th August 2007, 05:29 AM   #8
mfratus is offline mfratus  United States
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"Aim at 50% over maximum voltage the cap sees, for safety."

I guess I should have said, "Use a cap rated at around 150% of the applied voltage. If the circuit is 42 volts, use a 63 volt cap."

Mike
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Old 20th August 2007, 06:46 AM   #9
Bigred is offline Bigred  Canada
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Default Re: Slow down now fellas!

Quote:
Originally posted by anatech
One thing I will say. There is nothing wrong, or cheap about most of the 270 trim pots used for adjustments. Multiturn controls may actually be worse in these locations.
AS always your wisdom is appreciated Chris. What would be your concerns regarding the trimmers? I might be missing something here but I thought a variable resistor is just that, a variable resistor. I fail to see why replacing single turn trimmers with sealed multi-turn of the same values being of any concern. I see nothing but benefits like no more dust/dirt problems with less chance of drifting and less sensitivity with precise adjustment. The only thing I can think of is the initial setting of the new VR. If for example I'm changing out a bias adj. pot I make sure its set correctly and then measure the pot resistance. The new multi-turn is then preset and replaced and tweaked if needed.
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Old 20th August 2007, 06:52 PM   #10
anatech is offline anatech  Canada
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Hi Bigred,
Well, for one, the element and wiper are smaller on multiturns. That means less contact area and possibly less pressure. When you turn the screw, you are actuating a geared down wiper. This means that it is easier to set a value using an incremental change, but this is of little use for bias level setting. A false sense of improvement.

The problem is that a multiturn control is not designed to pass current on average and the contact will not put up with any oxide at all. The 270 types are easy to set if the design engineer did his job properly. They can easily dissipate 1/4 watt and pass some current as the moving contact is larger and is under more pressure. I think the 270 control is well suited to the job at hand. Even if the wipers and wattages were the same in comparison, a multiturn type sells you things like a reduction gear drive the is of no advantage to you.

Keep in mind that they all oxidize over time. I've had to replace 10 turn trimmers as well in test gear. I think someone who doesn't understand electronics liked the finer control and higher price tag of the 10 turn types and decided arbitrarily that the 270 type was used to reduce costs. This is pure silliness of course, the 10 turn control is not well suited to these jobs to begin with.

-Chris

Edit: If you have a dust and dirt problem, there are covered 270 types, but this would not be your biggest problem. So dust and dirt is really a non-issue.
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