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Old 8th January 2012, 09:15 PM   #11
DF96 is offline DF96  England
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Many amps have a direct-coupled output so no resistor is necessary. Older books may have been written back in the days before DMM when the meter itself provided the resistance.
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Old 8th January 2012, 09:36 PM   #12
hameay is offline hameay  United Kingdom
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I'll use my Avometer Eight Mk.7 next time
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Old 8th January 2012, 10:08 PM   #13
system7 is offline system7  United Kingdom
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Electrolytic capacitors are really odd components. Made of blotting paper soaked in an jelly-like electrolyte and a couple of sheets of aluminium rolled up...

The capacitance is formed by a thin oxide layer that partially disappears in months of storage, but rebuilds/repairs itself when you polarise. They work OK with a hefty polarising voltage across them as in your application, but floating in a signal path, I do not like.

Very non-linear. Very leaky.
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Old 8th January 2012, 10:54 PM   #14
hameay is offline hameay  United Kingdom
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Yeah. From reading around it seems like lots of people avoid electrolytics in the signal path. Since this is my first diy build I thought I'd avoid tweaking it too much to begin with. Once I've got a better handle on the sound of the amp (which I think sounds great as is), I'll consider swapping out the lytics for MKPs or something (massive threads here on the "best capacitors" I see and see if I can tell the difference.
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Old 8th January 2012, 11:10 PM   #15
system7 is offline system7  United Kingdom
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It's an interesting amp you've built there. Ought to sound very good. I like Class A. Everything is easier.

I think I said that electrolytics are OK when polarised. That's what you're doing. All 12V of it! Component tolerances really only affect filters around the crossover point not in the pass band. Your speakers are seeing the Capacitors as a short circuit over the midrange, so really electrolytics are fine there. Anyway they are bypassed by 10uF polypropylene above 2kHz, so the treble should be sweet too.

You really have no alternative to electrolytics at the bass end. The use of electrolytics I really don't like is in the negative feedback circuit (and input) of an amp using balanced twin rail supply. No polarisation, so very non-linear. Actually works better with single-rail supply.

Last edited by system7; 8th January 2012 at 11:34 PM.
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Old 8th January 2012, 11:21 PM   #16
Bigun is online now Bigun  Canada
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There have been measurements of signal distortion made from electrolytics that shows distortion is not helped by having a dc polarizing voltage - in fact the contrary. It seems to be an old wives tale. Unfortunately, I don't have a copy of the web link anymore.

From what I have read, there are two things you can do to reduce the impact of having an electrolytic in the signal path

a) use a high quality one (expensive) because some of them are reported by DIY'ers who've been doing this for many years, to produce less distortion and less high order harmonic distortion. And there have been some measurements to support this. As an example, Elna Cerafine and Mundorf are consistently reported as being superior to most everything else, although I'm too stingy to have tried either yet.


b) use a large valued one (expensive), because measurements do show that the distortion from the cap rises very quickly once you get a significant a.c. voltage across it. For most of the audio range the capacitor will have a very low impedance and so will not develop much of an a.c. voltage. But at low frequencies the capacitors impedance rises and so does the a.c. voltage drop across it. Using a large valued capacitor will keep it's impedance low at low frequencies. Although I haven't seen people do this by design, it might be worth limiting the low freq. bandwidth of the amp earlier, say at the input where a non-electrolytic cap is used, so the output cap never experiences frequencies which might produce distortion.
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Old 8th January 2012, 11:43 PM   #17
system7 is offline system7  United Kingdom
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IIRC, some of the respected budget Creek amplifiers used output electrolytic capacitors on a single rail supply. I think Michael paralleled them too.

We probably don't want to digress into a capacitor debate really, but I remember measuring DC resistance across a capacitor correctly polarised and one reverse polarised, and they are almost diodes in that respect!

Not too sure about this, but I think the non-polarised types used in cheap loudspeaker crossovers are two regular ones back to back. Bit of a bodge really! Give me fat yellow polypropylenes any day.
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Old 9th January 2012, 01:21 AM   #18
djk is offline djk
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DMM has a 10MΩ input impedance, a leakage of a couple of A will show a high DC value.
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Old 9th January 2012, 02:07 AM   #19
system7 is offline system7  United Kingdom
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Ooh! I've just noticed something...

That zero feedback MOSFET amp is 8 ohm output impedance. Or 15 ohms if MOSFET drains are very high output impedance. Been a while since I used them, so I can't remember...but, unless I am very much mistaken, that means low amplifier damping factor.

VERY INTERESTING...Arpeggio design speakers beckon:
Arpeggio Loudspeaker

Last edited by system7; 9th January 2012 at 02:25 AM.
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Old 9th January 2012, 08:14 AM   #20
hameay is offline hameay  United Kingdom
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Sorry guys - meant to reply sooner but my network connection went down!

system7: Yes, I really like the minimalism of Class A too. And I'm very happy with the sound of the amp through my old Celestion Ditton 15XR speakers. It's warm and detailed, reminiscent of a valve amp so my friends tell me who have heard one. I can also turn my radiator down a bit too

I checked the specs on my DMM (a cheap Uni-T UT60A) and it has an input impedance of >= 10MΩ. I didn't know about leakage current before so it's interesting to learn about this.

The discussion on lytics is interesting too. So much to learn! Your suggestions for reducing the impact of the lytic are useful Bigun. For reference, the cap is a cheap Nichicon 4700uF (mouser #647-UVR1H472MRD). I have noticed some bass distortion at the top end of the volume setting (too loud for my listening space anyway at this level), but not sure if that's the amp or the speakers which have an ABR driver. I'm planning on doing some detailed distortion measurements in the future so that may help to pin things down further.

Those Arpeggio speakers look very interesting too...
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