Double rectification and smoothing - Linn LK1/Dirak...

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DSJR
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#1 Double rectification and smoothing - Linn LK1/Dirak...

Post by DSJR »

Possibly silly and OTT Linn stuff (usual for them I gather) but I wanted to ask a question please?

To start with... After some persuasive friendly bullying, I got a cheap non-running LK1 preamp as it appears to make (in working condition) a passable cheap MC phono stage if used from the tape outputs (the variable line outs constrict the sound a litytkle too much for any interested parties around these parts and I moved on from this as a full blown product thirty years ago!).

Indeed, the thing does work quite well as a compact sounding phono stage and I had fun restoring the control board (Christmas lights syndrome plus leaky backup battery which nearly killed the board and thus the preamp). Anyway, it's working as well as it ever will and this one lacks the heat damage which occurs around the three hot running regulators on the main board.

Now to the question - The internal transformer is purely that, a small diameter but rather tall custom jobbie with what seems like twin sub-20V secondary's. The main board carries rectification and some form of regulation (plus 15V and a slaved? negative mirror or similar - forgive my obvious ignorance, plus a third regulator for the control board I believe). It's the once upgrade addition of a DIRAK supply that threw me..

Apparently the Dirak has it's own rectification and extensive smoothing (64,000uF over four caps I believe plus bypasses) Linn don't provide circuit details as yet for this unit) PLUS regulation to around 18V + and - it seems, this being used instead of the internal transformer which used to be removed and replaced with a cover plate with dedicated socket and wiring to the board...

With me so far? It *used* to make a small improvement to the thing (the circuit band limiting holds it back fully in subjective terms I feel), but the twin rectification and regulation has totally thrown me! It looks cack handed to me and classic (Klassik?) Linn ballzups BUT it obviously doesn't do any harm long term.

If any of you can understand the cack above, does the above scenario make any sense at all? I may not keep the thing long enough to attempt a DIY Dirak and Linn don't provide any but the basic details on this add-on from yesteryear. I just wanted to try to get my head round why they did this and whether there are any possible advantages to doubling things up after the transformer?

Any thoughts here?
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Nick
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#2 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by Nick »

Yes, I do the same thing in the MDAC power supply I make. The standard MDAC supply is a AC unit, the MDAC itself contains rectifiers and regulators. But if you get the polarity right you can replace the AC with DC, the second rectification just becomes a inline diode and the internal regulation has a lot less to do as its fed low noise DC.
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#3 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by DSJR »

Thanks muchly. :D
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#4 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by Mike H »

*slowly sinking in* Image

Yeah .. yeah .. OK ...
 
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#5 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by pre65 »

So, an inline diode after the rectification stage is a benefit.

Does the type of diode matter ? (assuming it meets the voltage/current ratings)
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#6 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by Mike H »

Don't think it's as simple as that, what's going on is a rectifier, reservoir and regulator stages are given rectified (and reservoir cap) DC instead of the 'normal AC' input.
 
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#7 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by Mike H »

pre65 wrote: Sun Oct 21, 2018 12:29 pm Does the type of diode matter ? (assuming it meets the voltage/current ratings)
Yes that's about it as far as I can see. :thumbleft:
 
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#8 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by Nick »

Mike H wrote: Sun Oct 21, 2018 2:22 pm Don't think it's as simple as that, what's going on is a rectifier, reservoir and regulator stages are given rectified (and reservoir cap) DC instead of the 'normal AC' input.
Yes, that’s exactly it.
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#9 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by pre65 »

So, if I use a second bridge rectifier between the DC output of the original bridge rectifier (inc 1uf cap) and the first choke that will bring ripple down even further ?

EDIT

Drawing out how it's wired has confused me, with regard to the two negative terminals on the second bridge. Are they both to circuit ground ?

This is on the PSU for my single ended 50W Pass amp if it makes a difference.
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#10 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by Nick »

pre65 wrote: Mon Oct 22, 2018 10:56 am So, if I use a second bridge rectifier between the DC output of the original bridge rectifier (inc 1uf cap) and the first choke that will bring ripple down even further ?
No why would you think that?

This is about adding a DC supply to a device that normally takes AC. Its not some general rule of whats better or not.
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#11 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by pre65 »

Nick wrote: Mon Oct 22, 2018 11:30 am
pre65 wrote: Mon Oct 22, 2018 10:56 am So, if I use a second bridge rectifier between the DC output of the original bridge rectifier (inc 1uf cap) and the first choke that will bring ripple down even further ?
No why would you think that?

This is about adding a DC supply to a device that normally takes AC. Its not some general rule of whats better or not.
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#12 Re: Double rectification and smoothing - Linn LK1/Dirak...

Post by DSJR »

It was about me trying to get my head round something Linn did as an 'upgrade' to the complex looking LK1 preamp and also the active Tunebox unit (my cheapo LK1 still tetchy if left for a couple of weeks unpowered and it ain't the back-up battery or the control board caps that needed replacing, so it's all but unsellable really :evil: - my regular phono stage is in a different and higher plane of existence anyway). A power supply concept I haven't seen used anywhere else before Nick's reply. As part of a DIY learning curve, a regulated ~18V dual rail DC supply could be made up as long as I get the polarity right on the connector.
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