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Old 17th May 2013, 02:17 PM   #21
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by minifly View Post
..even a small capacitor between the + and - signal inputs...
Yes, that's generally a bad idea but he's not doing that (according to the schematics he showed earlier).

Quote:
Originally Posted by AndrewT View Post
castigated for interfering (commenting) in Peter's/Audiosector's Threads.
Yes, that will happen when someone who knows something about electronics gets too close to someone who thinks bronze heatsinks sound better.

Quote:
Originally Posted by glenv6 View Post
This happened a second time to another pair of speakers...
How about checking for DC before hooking up the speakers next time?

Last edited by godfrey; 17th May 2013 at 02:26 PM.
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Old 17th May 2013, 02:36 PM   #22
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Quote:
Originally Posted by AndrewT View Post
...I did quite a few experiments with that input cap RC and found that very approximately when near 90ms I could not hear any further changes.
This was for a couple of amplifiers that had a very wide pass band. Soon after I did this, I discovered on THIS FORUM that the amplifier pass band should ALWAYS be wider than the signal being applied. The ratio suggested was at least sqrt(2).
Combining the 90ms input filter and the sqrt(2) factor gave the NFB RC >=130ms.
...
For newbies and those in North America, we tend to describe filters using frequency rather than time constant. The relationship is simply a reciprocal:

frequency = 1 / time

So:

frequency = 1 / 90 milliseconds = 11 Hz
frequency = 1 / 130 milliseconds = 7.7 Hz

So Andrew's saying to set your input RC filter to 11 Hz and the negative feedback filter (NFB RC) to 7.7 Hz.
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Old 17th May 2013, 02:41 PM   #23
godfrey is offline godfrey  South Africa
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Nope, corner frequency = 1 / (2 * pi * time constant)
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Old 17th May 2013, 02:52 PM   #24
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AudioLapDance View Post
For newbies ..........

frequency = 1 / 90 milliseconds = 11 Hz
frequency = 1 / 130 milliseconds = 7.7 Hz......
As God correctly states 1/[2pi * RC time constant]
and 2pi ~= 6.283
giving F-3dB of the passive input filter ~ 1.6Hz (for my personal taste in what I think sounds right).

The amp must be lower than the passive filter and the filtering effect of the smoothing caps lower still.
This is where a bottleneck most often occurs.
The Smoothing cap RC >= sqrt(2)*NFB RC
and
The NFB RC >= sqrt(2)*Input RC

That puts the smoothing caps > +-20mF per 8ohms channel.
Skimp on the PSU filter and you should increase the frequency of the Passive input filter to match
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Last edited by AndrewT; 17th May 2013 at 03:04 PM.
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Old 17th May 2013, 05:20 PM   #25
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Quote:
Originally Posted by godfrey View Post
Nope, corner frequency = 1 / (2 * pi * time constant)
Sigh ... thanks for the correction. As if to prove my point--which I hope was not missed. When talking to electronics newbies (esp. N.A.) avoid/explain time constants.

Input filter (Input RC) corner frequency = 1 / (2 * pi * 90 milliseconds) = 1.77 Hz

Feedback (NFB RC) corner frequency = 1 / (2 * pi * 130 milliseconds) = 1.22 Hz


(I thought 11 and 7 seemed high)

Andrew:

The Smoothing cap RC >= sqrt(2)*NFB RC
and
The NFB RC >= sqrt(2)*Input RC


Andrew is succinctly giving you the "big picture" here. Before we unpack the dense math (math's beauty) recall one of Andrew's key design philosophies:

Don't give a circuit a signal that it can't handle!

No frequencies that are too high or too low, here we're focusing on too low.

So your input filter sets a high pass filter at 90ms.

1.8 Hz (90ms)

The next circuit (NFB RC) has to handle 1.8 Hz well so set it to at least 1.414 [square root of 2, or sqrt(2) ] times 90ms. 127 ms (Andrew's 130 ms).

127ms is 1.25 Hz.

So the negative feedback circuit is set to easily handle the 1.8 Hz passed on to it by the input filter.


Now here's Andrew's really cool point! The 'big picture', the amp as an interconnected system, and connected to other systems (source, speakers, ...) ...


Can the actual amp handle 1.25 Hz well? The amp's ability to produce low frequencies interacts with the load (speaker, 8 ohm) to form a ... high pass filter!

This amp-speaker 'filter' should be set to at least 1.414 times 127 ms.
= 180 ms (0.88 Hz)

Amazing and elegant but ... what does it mean? Well, to have a filter you need a active element (cap or inductor) and for the amp this ultimately comes down to the big power supply caps (smoothing cap)! I bet you never saw your ps caps as such a simple filter before!

Now working backwards with our formula:

corner frequency = 1 / (2 * pi * time constant)

time constant = R * C

so

f= 1 / (2 * pi * R * C)

rearrange for C

C= 1 / (2 * pi * R * f)

C= 1 / (2 * pi * 8 ohm * 0.88 Hz)
= 22,600 uF (=20 mF)

On both the positive rail and the negative rail so Andrew's: smoothing caps > +-20mF per 8ohms channel

Now you know how much ps capacitance you theoretically need instead of just 'bigger is better'.

A more level-appropriate explanation will sink in more!

Cheers,
Jeff


PS Careful with Andrew's cool easy formula. It is for time constants not freq! Then use
f = 1 / (2 * pi * time constant)
to convert to freq.

PSS (Oh, please let my math be right or I'll get blasted! I got 2 hrs sleep last night ... for the 7th night in a row ...)

Last edited by AudioLapDance; 17th May 2013 at 05:28 PM.
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Old 17th May 2013, 05:32 PM   #26
AndrewT is offline AndrewT  Scotland
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I and many who do the arithmetic find working with familiar RC time constants far easier than working with frequencies and having to convert to component values, repeatedly.

If you know that the Low Pass input filter is 0.7us then you can almost instantly work out the unknown component value from the known one.

Assume the Rs 1k ohms.
The filtering cap to attenuate the RF will be 0.7nF to give that 0.7us RC
680pF is close enough.
If you have Rs=200ohms and you insert an extra 1800ohms in series the total Rs forming the RF filter is 2kohms. The required cap is 350pF for that 0.7us. Use 330pF.

If you prefer 1us or 0.3us, the calculation for the component values is just as easy.
Convert your required F-3dB frequency to an RC time constant ONCE. Then use RC for all further work with that filter.

BTW,
Audio's explanation is really good, better than I can manage.
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Old 17th May 2013, 08:08 PM   #27
glenv6 is offline glenv6  United States
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Quote:
Originally Posted by godfrey View Post
Yes, that's generally a bad idea but he's not doing that (according to the schematics he showed earlier).


Yes, that will happen when someone who knows something about electronics gets too close to someone who thinks bronze heatsinks sound better.


How about checking for DC before hooking up the speakers next time?
Oh, I checked and tested everything other than DC offset before hooking my first pair of test speakers up. But my issue didn't happen the first time I turned it on, it happened more like the 20th time I turned it on. After reading info AndrewT posted, I changed some things with the power supply, tested everything including DC offset, and again the amp didn't send anything other than sweet sounds to the speakers through many on-off clycles before the horrible hum shot through attempting to foil my listening efforts for a second time.

If I cannot get to the point where I know I don't have to check my amp for DC every time I want to turn it on I will scrap any idea I had around living with a Gainclone. Is that possible or is this Gainclone stuff a never ending nightmare of potentially blown speakers?
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Old 17th May 2013, 10:27 PM   #28
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glenv6, just unplug any running chipamps at this time.

What you describe is strange, we should trouble shoot it. I think a separate thread with a title like "Chipamp: strange, intermittent DC offset!!11!" would be appropriate and help others with similar issues too (that's more that just 'bloated' !! )

It could be something strange like grounding, or turn-on order, quick on/off off/on, ... We need info: schematics, pictures, measurements ... we'll get to the bottom of it.

Oh, and no, chipamps are awesome!!! I properly designed chipamp is very safe and rugged. Remember, chipamps are in TVs, small stereos, gaming systems, small Home Theater systems, .... because they are robust and cheap ... sorry: cheap and robust!

I figured there was more going on once you mentioned the second blown speakers and didn't assume the worse like some posters.

...

Getting back to the OP:

after comparing my GC with quality commercial amps I've confirmed that my build sounds "bloated". By that I mean that there is more than enough bass and mid-bass, but mids and highs seem recessed. The amp obviously has more dynamics compared to others, lacks in HF transparency, but it sounds "bloated"

Build a Andrew approved chipamp and see if it sounds unbalanced.

Cheers,
Jeff

PS
Andrew: "BTW, Audio's explanation is really good, better than I can manage." That made my day!
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Old 18th May 2013, 01:38 AM   #29
glenv6 is offline glenv6  United States
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Quote:
Originally Posted by AudioLapDance View Post
What you describe is strange, we should trouble shoot it. I think a separate thread with a title like "Chipamp: strange, intermittent DC offset!!11!" would be appropriate and help others with similar issues too (that's more that just 'bloated' !! )
AudioLapDance - Good advice! I'll start a separate thread... Thanks!

-Glen
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Old 18th May 2013, 09:06 AM   #30
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by AudioLapDance View Post
.............. "BTW, Audio's explanation is really good, better than I can manage." That made my day!
I'm really not a bad guy, at least not all the time.
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