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Subsonic Filter

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GrahamD View Drop Down
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    Posted: 19 Aug 2016 at 12:02am
I know Graham Slee may never forgive me but I have added a DIY subsonic filter to the output of my Gram Amp 2 !!!!!
I have 3 or 4 records in my collection that produce excessive cone pumping on the first 2 or 3 tracks which while not audible are not good for the speakers. ( Monitor Audio 6 ).
I have whipped up a little circuit which is plugged in between the preamp and power amp.
It is nothing more than a 0.1uF cap in series and a 75k ohm resistor across the output of each channel which provides attenuation below 20k hz. A switch allows for it to be bypassed when not needed.
It works fine and reduces the cone pumping to a slight tremble with no ill effect to the sound.
Cheers
Graham

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 9:43am
Why should I be upset with you for doing this?

The values give a high pass filter which turns over at 21.22Hz (-3dB) , but when the load of the amp it is driving is taken into consideration, the 75k will not be 75k, it will be less. By how much depends on the input impedance of the amp (or preamp etc).

If a 1Meg valve amp input the parallel combination will be 70k, and the turnover frequency only slightly higher at 22.74Hz.

At the other extreme of say a 22k input solid state, the parallel combination will be 17k, and the turnover frequency much higher at 93.6Hz.

Another much more complex way of doing it would be to use the channel subtract and sum method which has been doing the rounds since 1979 (see: http://www.douglas-self.com/ampins/wwarchive/wwarchive.htm).

This cross feeds the bass from around 150Hz and below and cancels the lateral differences from the cartridge where the rumble, we are told, comes from.

Purists may argue that it makes the lows mono, which isn't a good thing, but we are told that records are cut with mono bass below 300Hz anyway. I beg to differ on that, because some early stereo records have the bassists over to one side or another.

Anyway, as bass is less directional and the "cross feed" circuit doesn't result in phase differences within audibility (which your filter does), then it looks like a better idea.

However, to get the cross feed a large number of op-amp stages are required to do the subtraction and summing and so the resulting sound might not be as good.

Cone flap should not damage well made speakers, and if so it would only be of visual annoyance. Replacing the grilles would remove that annoyance, and not cost anything.

Just my take.
Not simple enough for Google to understand...
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GrahamD View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GrahamD Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 10:31am
Hi Graham.
Just kidding about you not forgiving me, but I know you are not keen on subsonic filters.
The amp is a Yarland Pro 88 SE valve amp with a stated input impedance of 100k ohms @ 1kHz.
Not sure what that does to the crossover frequency, maybe I could use higher value resistors to fine tune it lower. I didn't want a complicated circuit, just something simple to use when required to stop me worrying about the cone movement on the few records that seem to have low frequency rubbish recorded on them.
Cheers
Graham
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Chris Firth View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Chris Firth Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 11:15am
Originally posted by GrahamD GrahamD wrote:

The amp is a Yarland Pro 88 SE valve amp


... and therein lies your problem.
Cone flap occurs with bass reflex speakers, and isn't as much of an issue if your amplifier has a high electrical damping factor.
MA tend to use big solid state power amps with high damping factors when they design their speaker models.

Your amp effectively has sod all electrical damping factor, which in turn means that the back emf from your speaker drivers will have an effect on the signal.

The cure is to get a loudspeaker that is mechanically well damped, and has a relatively benign load.

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BackinBlack View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BackinBlack Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 2:05pm
Another cure - 2 x Proprius to keep them cones under control Wink
Just listen, if it sounds good to you, enjoy it.
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Graham Slee View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 3:12pm
-3dB turnover frequency 37Hz, 0.1u into 43k (75k || 100k). Slope of filter 6dB/octave.

Agreed, damping factor (or should we call it driving impedance?) isn't much from an output transformer, and as such "sod all" is appropriate wording.

Reflex "loading" allows the driver to resonate (pump) at its resonant frequency and a complete lack of damping will allow large cone excursions, but I would have thought the manufacturer would have taken that into consideration so as not to "bottom out" on its suspension. Then again, Chris says these are designed using amps with a stiff driving impedance.

It's always a good idea to do a lot of research before buying, but some manufacturers don't give the full details these days.

And therefore I should remark that the Proprius also has sod-all damping factor Embarrassed

Based on common knowledge it is around 4 at 20Hz. I say common knowledge because some rubbish the whole idea of damping factor. Back emf is the release of inductor energy once the drive has been removed, but as the amplifier is now driving in the opposite direction the maths will be a little more complex, and a factor of 4 might be a long way off.

I once connected a DC coupled amp to a drive unit and tapped the cone and then repeated it after disconnecting the amp. My ears heard no difference. No change in cone excursion. Which suggests that simple calculation is not indicative of the amplifier's control over the cone.

Also being old enough to have only had analogue tape or vinyl sources, cone flap was not considered out of the ordinary. In fact one expected to see it and when not there we thought something must be wrong Wink
Not simple enough for Google to understand...
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Richardl60 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richardl60 Quote  Post ReplyReply Direct Link To This Post Posted: 19 Aug 2016 at 3:13pm
Another angle. If this relates to a small number of records are they warped?   If they are can they be flattened to reduce this issue?
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