KISS

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karatestu
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Re: KISS

Unread post by karatestu »

I think my next foray into the discarding of components will be trying a home made passive pre. I do love Les'821A preamp boards but there are two dc blocking, phase altering caps on there. My Cdp has no dc output and i assume the phono 1 has none either so i will be ok just putting a volume pot in. Martin T did say when removing the input coupling cap on my power amp that dc may get back to the vol pot and make it noisy. Dont know about that unless i give it a go.

Stu
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Dr Bunsen Honeydew
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Re: KISS

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karatestu
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Re: KISS

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Thanks for the link Doc.

Which one should i go for ? :think:

I presume i need a 10k, mono or stereo (2x mono for a balance control) , shunt version with tantalum resistors. I think i should avoid the ladder types and smd resistors ?

Stu
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eagwok33
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Re: KISS

Unread post by eagwok33 »

Stu, great that you are about to start this passive preamp project! 8-)
The Doc is clearly the expert on passive preamps, so the following are just my humble thoughts / two cents as someone who loves to DIY a bit:

It's funny, I finished a little DIY passive pre last weekend with two switchable input pairs and one output pair :-) as I had some spare parts laying around. I used an Alps blue stereo pot 10K linear, some good quality RCA sockets, a 2-pole silver contact switch and some silver solid core wire and built everything into an open chassis out of plywood. And I like the results a lot (maybe because I built it? :think: ). This little pre has detail, bass, nice mids and a sweet top end. :dance:

Maybe one day, I will go for one of the quality NVA pre's Richard builds, to get the best out of my system! :epopc:
Norbert

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Dr Bunsen Honeydew
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Re: KISS

Unread post by Dr Bunsen Honeydew »

karatestu wrote: Sat Feb 17, 2018 11:06 am Thanks for the link Doc.

Which one should i go for ? :think:

I presume i need a 10k, mono or stereo (2x mono for a balance control) , shunt version with tantalum resistors. I think i should avoid the ladder types and smd resistors ?

Stu
You choose, more expensive better, he builds them, he doesn't do bad ones. Lots of bad ones on ebay, surface mount ones are crap.

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Dr Bunsen Honeydew
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Re: KISS

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eagwok33 wrote: Sat Feb 17, 2018 1:31 pm I used an Alps blue stereo pot 10K linear,
YUK!

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karatestu
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Re: KISS

Unread post by karatestu »

Cheers Doc. There are some really expensive ones. I will have to give them a good look. Tantalum resistors seem to be good. You can even use charcroft z foils for the series resistor. :dance:

Naim used alps pots, maybe still do. I have a noble pot at the moment but its 50k :roll: and no good for a passive .
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eagwok33
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Re: KISS

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Dr Bunsen Honeydew wrote: Sat Feb 17, 2018 1:46 pm
eagwok33 wrote: Sat Feb 17, 2018 1:31 pm I used an Alps blue stereo pot 10K linear,
YUK!
For "YUK!" it sounds astonishingly good I have to say. :-) And the good thing is, to swap the poti for a better one is a quite easy task... 8-)
Norbert

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Dr Bunsen Honeydew
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Re: KISS

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I hate the Blue it is muddled and unmusical (relative to what I use). The ones I use cost me 80p each.

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Re: KISS

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karatestu wrote: Sat Feb 17, 2018 7:56 am

Hey Alfi how's it hangin',

What you need to do is this;

Remove input stage coupling capacitor
Remove wiring between zobel and cap bank 0V
Remove wiring between output spade and positive output on back of amp
Solder a wire from the big pcb area where the ballast resistors (0R22) are joined to the output inductor, to the positive speaker output on back of amp.
There are a few unused holes in the pcb in this plane which were put on to accommodate different component sizes. Use one of those.

This will improve the sound considerably. I found the result to be more satisfying than giving the front end its own regulated power supply. I cant wait to combine the two modifications :grin:

But none of this will fully remove the "rip" as Les calls it. To do that you need to convert the board to a fully complimentary output stage. Remove the baxendall diode and a resistor, change the value of 6 resistors and swap the output transistor on the negative side to a pnp.

When i have found out the exact details and tested it myself you can just send me the amp modules and i will do it for you - free of charge of course :dance:

Stu
Hello Stu.

All fine here Sir, thankyou! :clap:

That's a very kind offer and one I will repay by carrying out the work on your LP12 gratis when the time comes. :epopc:

Best wishes.

Alfi.
I am in the hi-fi trade.
Status: Manufacturer.
Company Name: Analogue innovation.

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