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JLH 10 Watt class A amplifier
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Old 22nd April 2012, 06:33 AM   #2421
ashok is offline ashok
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JLH 10 Watt class A amplifier
Thanks for that explanation Mike. You are right about me not liking caps in the signal path. This is because I do hear differences between different types of caps and find they don't always behave exactly the same way in different circuits. Meaning that if X cap sounds better in circuit A , it's possible that X cap doesn't sound so good in circuit B ! I don't always simulate or measure these things but just try out the caps practically. Saves time and does involve the final listening test. Maybe some simulation can help identify a trend.

In fact some very simple inexpensive caps have produced some very good sounding amps. Sound so close to some botique caps that makes the expensive cap usage in that circuit redundant.

Cheers.
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Old 22nd April 2012, 06:47 AM   #2422
PMA is offline PMA  Europe
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Quote:
Originally Posted by mikelm View Post

I recently added 1.0uF as bypass cap for an o/p stage, everything becomes much clearer but with strange character to the sound . When I add 0.1R in series with the 1.0uF then the sound is clean again.

In spice the effect ( and the cure ) can be clearly seen. Not sure how much this applies to an o/p cap but the laws of physics cannot be escaped - if you add C & L with low R you have high Q at resonant frequency.
Hi Mike,

could you kindly post a realistic spice simulation of this. Such simulation should cover the amp under test (for the reason that a realistic output impedance must be taken into account), models of both electrolytic cap and foil cap (for foil cap, please use Roederstein 3.3uF MKT with axial leads, MKT1813 type) and a cable + speaker models. None of the components should be omitted for the reason to get a kind of reliable result.

Thanks,

Last edited by PMA; 22nd April 2012 at 06:50 AM.
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Old 22nd April 2012, 08:59 AM   #2423
mikelm is offline mikelm  England
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Hi Pavel,

When I chose a bypass cap as an example of spurious resonance I was not thinking of your o/p arrangement - only after I post I realise this may seem like a pointed comment but actually, as I mentioned earlier, I was thinking of an o/p stage bypass cap - very common even in simple amps.

I cannot do the research you mention - don't have the models, the time, or the inclination. It may be you do not have a problem and if you don't that's great.

I show below the general spice research I did to explain to myself ( again ) the cause of the roughness I heard with 1uF bypassing 470uF ( with other caps upstream ) The findings shown match what my ears clearly hear

This graph shows AC sweeps. The 1uF bypass cap with R sweeping between .005R & .205R.

I have the feeling this is not news to you - but may be to others

Whoops - I post 4700uF but actually it makes little difference - the principal is clearly shown
Attached Images
File Type: jpg bypass cap resonance.jpg (109.5 KB, 643 views)

Last edited by mikelm; 22nd April 2012 at 09:07 AM.
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Old 22nd April 2012, 10:06 AM   #2424
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Mike,
could you zoom in on the resonant peaks and label the traces with their corresponding damping resistor values?
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Old 22nd April 2012, 10:13 AM   #2425
mikelm is offline mikelm  England
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Andrew,

If you hit the "actual size" button bottom left, you will see the info you require at the bottom of the graph.

I can do a zoom later if u still need it - let me know

mike
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Old 22nd April 2012, 10:19 AM   #2426
AndrewT is offline AndrewT  Scotland
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I did.
I think I can see 5 traces, but no labels. But, now I see the legend along the bottom.
Thanks.

I still think a zoomed version would help.

What happens when rs>0r205
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Old 22nd April 2012, 10:32 AM   #2427
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Mike,

a resonance of a capacitor with inductance of lead conductors is normal, standard phenomenon.

But, your simulation may be pretty misleading for unexperienced diyers, for the reason that there is a speaker impedance in series with output capacitors.

Your simulation fits in case that you put a capacitor across amplifier output terminals (directly to ground). Then, as every amp designer knows, we see damped resonance (ringing) caused by wire inductance and capacitor capacitance.

I am sure that ESR and ESL of the output capacitors do not cause any ringing, when driven from real amplifier output and loaded by real electrodynamic speaker. Please note that I do not speak about AMPLIFIER instability and oscillations, but just about RLC.
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Old 22nd April 2012, 10:36 AM   #2428
mikelm is offline mikelm  England
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Hi Pavel,

I think you did not read my posts - because I clearly state that this does not refer to your o/p cap arrangement . . .

This refers only to o/p stage power rail bypass caps which I now state for the 3rd time !

But sorry for any confusion

mike

Last edited by mikelm; 22nd April 2012 at 10:39 AM.
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Old 22nd April 2012, 10:46 AM   #2429
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Hi Mike,

I am sorry if I confused the issue, but what I refer to is
Quote:
But the ears can hear.

I recently added 1.0uF as bypass cap for an o/p stage, everything becomes much clearer but with strange character to the sound . When I add 0.1R in series with the 1.0uF then the sound is clean again.
I understood that it was related to output capacitor (bypass for an o/p stage). I am sorry if I was wrong.

Regarding supply rails decoupling, I agree, this is a real science.
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Old 22nd April 2012, 10:47 AM   #2430
mikelm is offline mikelm  England
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We have resolution
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