Happy new year everyone.
I've built Mark's ZCA MOSFET amp (my first amp build). Had it running open and working fine on an MDF board for a couple of months but finally got round to building a case for it. Everything's in the case now but I'm getting a 12V DC offset (same as MOSFET bias voltage) on the speaker terminals. I'm very dubious about hooking the speakers up to it as, from the way I understand it, a DC offset on the speaker terminals is bad, right?
p.s. ZCA schematics here: DIY Class-A 2SK1058 MOSFET Amplifier Project
I've built Mark's ZCA MOSFET amp (my first amp build). Had it running open and working fine on an MDF board for a couple of months but finally got round to building a case for it. Everything's in the case now but I'm getting a 12V DC offset (same as MOSFET bias voltage) on the speaker terminals. I'm very dubious about hooking the speakers up to it as, from the way I understand it, a DC offset on the speaker terminals is bad, right?
p.s. ZCA schematics here: DIY Class-A 2SK1058 MOSFET Amplifier Project
Cheers - thought so. Glad I contained my enthusiasm and didn't hook it straight up to the speakers!
I've tested the output caps with my DMM and they seem to be working OK (shorted caps, then DMM set to resistance across caps, resistance starts near zero and increases steadily).
Could it be the way I've wired the 2SK1058 MOSFETs? Wired as in schematic, but the source has two routes to ground, one via a connection from source pin to ground, and the other via the MOSFET flange to the heatsink and chassis (since I haven't insulated the interface between MOSFET and heatsink).
I've tested the output caps with my DMM and they seem to be working OK (shorted caps, then DMM set to resistance across caps, resistance starts near zero and increases steadily).
Could it be the way I've wired the 2SK1058 MOSFETs? Wired as in schematic, but the source has two routes to ground, one via a connection from source pin to ground, and the other via the MOSFET flange to the heatsink and chassis (since I haven't insulated the interface between MOSFET and heatsink).
I'd confirm the output capacitor is connected with the right polarity, which is +ve to the +12V FET output if I follow the circuit, and connect a 1K 1/2W resistor across the speaker terminals and remeasure the offset. It might fall away to nothing if it's just a tiny leakage current affecting a high impedance probe.
Electrolytics will allow a considerable current to flow if wired with wrong polarity though, and not last very long.
Always possible the circuit is oscillating with no load too.
Electrolytics will allow a considerable current to flow if wired with wrong polarity though, and not last very long.
Always possible the circuit is oscillating with no load too.
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I'm not familiar with that device, but grounding the source twice is unlikely to do any harm unless it provokes instability.
Did you connect the output electrolytic capacitor the right way round?
Are you just measuring leakage current? A big electrolytic will pass a little current, plenty enough to drive a DMM. Try putting a low value resistor (say, 100-1K or whatever you have in stock) in place of the speaker and measure the voltage again. If now low, nothing to worry about.
system7 has same thoughts!
Did you connect the output electrolytic capacitor the right way round?
Are you just measuring leakage current? A big electrolytic will pass a little current, plenty enough to drive a DMM. Try putting a low value resistor (say, 100-1K or whatever you have in stock) in place of the speaker and measure the voltage again. If now low, nothing to worry about.
system7 has same thoughts!
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I've quadruple (!) checked the polarity of the output caps and they're wired correctly.
As suggested, I wired a 1K 1/2W resistor across the speaker terminals and measured the offset voltage (i.e. connected my DMM across the resistor). The DC voltage quickly (a few seconds) drops to a few mV.
Perhaps I'm misunderstanding offset voltage. Originally, I put my DMM across the speaker terminals (no resistor) and got 12V DC...
So, since with the resistor the offset voltage is only a few mV, is it safe to connect up the speakers?
As suggested, I wired a 1K 1/2W resistor across the speaker terminals and measured the offset voltage (i.e. connected my DMM across the resistor). The DC voltage quickly (a few seconds) drops to a few mV.
Perhaps I'm misunderstanding offset voltage. Originally, I put my DMM across the speaker terminals (no resistor) and got 12V DC...
So, since with the resistor the offset voltage is only a few mV, is it safe to connect up the speakers?
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.
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.
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.
and see if I can tell the difference.
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.
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.
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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.
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.
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.
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.
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:
http://www.diyaudio.com/forums/diyaudio-com-articles/158899-arpeggio-loudspeaker.html
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:
http://www.diyaudio.com/forums/diyaudio-com-articles/158899-arpeggio-loudspeaker.html
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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...
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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