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Old 9th January 2012, 10:05 AM   #21
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Bigun View Post
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.
This is precisely the lesson I learned from the experts on this Forum. I have been promoting that same message ever since, possibly for >5years now.

Set the passband with the two input filters.
Ensure the NFB RC time constant exceeds the input DC blocking RC time constant. The only discussion left is by how big a factor the NFB RC should exceed the Input RC.

Similarly the smoothing capacitance RC must exceed the NFB RC.
In a dual polarity supplied amplifier the caps are in series. In a single polarity supplied amplifier, one of those caps is relocated to be in series with the load.
It is those same two capacitors whose RC must exceed the NFB RC.
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Last edited by AndrewT; 9th January 2012 at 10:10 AM.
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Old 9th January 2012, 10:41 AM   #22
DF96 is offline DF96  England
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Quote:
Originally Posted by Bigun
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.
I assume all competent designers do this, although they might not draw attention to it.
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Old 9th January 2012, 10:41 AM   #23
system7 is offline system7  United Kingdom
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This particular amp is a rather different ZCA MOSFET zero feedback design, AndrewT.
DIY Class-A 2SK1058 MOSFET Amplifier

Any non-linearity in the bass is as likely to be down to the single MOSFET itself, or power supply, as the capacitor. I'd be careful what I conclude here.

I remember the Celestion Ditton 15 with ABR (that passive driver at the bottom, which acts as a reflex) which IIRC had a different tweeter from your 15XR (pictured here):

Click the image to open in full size.

What I was getting at with remarks about output impedance, is that a 15 ohm output amp will be tonally different from a pure voltage amplifier with speakers. The bass will be less tightly damped, and the treble will rise at high frequency as discussed in that Arpeggio article. For all I know, those vintage speakers are designed for valve amps, so you may have got lucky here. But interestingly, you can work round the impedance issue anyway with crossover design.
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Old 9th January 2012, 10:50 AM   #24
AndrewT is offline AndrewT  Scotland
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Originally Posted by system7 View Post
Any non-linearity in the bass is as likely to be down to the single MOSFET itself, or power supply, as the capacitor. I'd be careful what I conclude here.
Is this message implying or accusing me of supplying the wrong information to Bigun and Hameay?
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Old 9th January 2012, 10:59 AM   #25
system7 is offline system7  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Is this message implying or accusing me of supplying the wrong information to Bigun and Hameay?
No criticism implied at all. Just don't want hameay fretting uneccessarily about an output electrolytic capacitor that may well be working just fine!

The devil is often in the detail with these sort of things.
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Old 9th January 2012, 11:09 AM   #26
AndrewT is offline AndrewT  Scotland
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Are you saying that I am misinforming Hameay?
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Old 9th January 2012, 11:19 AM   #27
system7 is offline system7  United Kingdom
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Quote:
Originally Posted by AndrewT View Post
Are you saying that I am misinforming Hameay?
Andrew, if I understand what you are saying here, you are talking about a NFB amp. This isn't a NFB amp. It has an inherent distortion as output rises.

But it certainly makes sense to limit the amplifier input low frequencies too. Everybody is agreed on that.

Let's not go off-topic into a squabble.
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Old 9th January 2012, 11:26 AM   #28
AndrewT is offline AndrewT  Scotland
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Now that you are being specific, I can help you understand what my post was about and why I am giving Hameay the correct information.

I gave three separate conditions for good amplifier passband practice.
1.) set the pass bands limits with input filters.
2.) set the NFB RC > Input RC
3.) set the PSU/Output RC > NFB RC.

Where there is an open loop amplifier, then the NFB RC does not exist.
A combination of conditions 2 & 3 gives
2a.) set the PSU/Output >>Input RC.

What is wrong with that advice? as implied with
Quote:
I'd be careful what I conclude here.
meaning you conclude to something different to what I was recommending.
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Old 9th January 2012, 11:44 AM   #29
system7 is offline system7  United Kingdom
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OK, I think I follow. You want to make the input filter RC of the amp smaller than the output RC on the speaker/output capacitor...I'd hope the designer has done that and haven't worked it out, but here's the circuit:

Click the image to open in full size.
DIY Class-A 2SK1058 MOSFET Amplifier Project

I wasn't planning to redesign the amp, as it goes...
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Old 9th January 2012, 11:45 AM   #30
hameay is offline hameay  United Kingdom
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Thanks for all the information guys. It's really appreciated and definitely broadens my knowledge of electrolytics and their characteristics. Given my current level of electronics expertise it may well take me a few months to digest it And all from a question about DC offset at the speaker outputs I'm looking forward to exploring the issue more with my signal generator and oscilloscope when I get time, and will report back any interesting findings.

system7: yes - those are the speakers. My tweeters "look" the same as those in your picture. They seem to work well with the amp, which the designer recommends sensitive speakers for. I've been looking into redoing the crossovers (still the originals) and will be interested to see how they change the quality of the sound when I finally get round to it.
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