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1970s Design Indulgence

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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: 22 Jul 2021 at 10:27pm
There's always another tweak, or so it would seem.

It's just my opinion, but -3dB should be accompanied by a 45 degree phase shift, because many agree it sounds natural. It results in "flat phase" at 10 times the cut frequency (at the bass end - the top end is 1/10).

Stack a number of cuts at the same frequency and it sounds as natural as an original CD brick wall filter. The world moved on from that, and so at the bass end I thought I needed to revisit the frequencies and rates.

Assuming an 8 ohm load, the output caps do their cut at approx. 4Hz. It would be nice if the VAS would cut at 2Hz, the IPS at 1Hz and the input at 0.5Hz. The bootstrap is adjusted higher up to take it out of the way at about 16Hz, so it doesn't cause bass peaking.

A double the frequency spread as in a Sallen Key filter offers best two pole flatness, so we can do the same here, but with three orders.

One problem sticks out, and that's the VAS grounding capacitor, which is already large. I had tried to manipulate the lows using a 470u instead of a 1000u, but realise it was like putting the cart before the horse, and sounded so.

I think the required 4700u to obtain 2Hz is OTT as most speakers won't go anywhere near, and neither will our ears. I therefore chose to try 2200u.

The cut coinciding with the output capacitor, will result in 60 degrees, not 45 degrees, but it's closer than what I had.

By spacing the other frequencies at 1:2 or wider, so the IPS decoupling is 2Hz and input high pass is 1Hz, then interaction is minimised.

Only two components (per channel) needed changing. The 470u in the VAS emitter circuit becomes 2200u, and the input cap increases from 220n to 1u.

The resulting bass (so far) seems more natural. The -3dB frequency is around 6.5Hz with +55 degrees phase according to SPICE. It has also shifted the +10 degrees bass phase nearer to 35Hz from 50Hz, so it should be better on big floor standers.

Not simple enough for Google-Bot to understand...
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Sylvain View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jul 2021 at 2:49pm
''...The resulting bass (so far) seems more natural. The -3dB frequency is around 6.5Hz with +55 degrees phase according to SPICE. It has also shifted the +10 degrees bass phase nearer to 35Hz from 50Hz, so it should be better on big floor standers.''

And thank YOU 
......acoustic double bass '' Contre basse '' is still being heard in remastered CD's and Veuve Vinyl. 
Sub woofer in a box produce a good bass effect But do not do justice to a French Contre-basse operating between 36-42 hz. 

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