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Re: KISS
Posted: Sat Feb 17, 2018 9:30 am
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
Re: KISS
Posted: Sat Feb 17, 2018 9:42 am
by Dr Bunsen Honeydew
Re: KISS
Posted: Sat Feb 17, 2018 11:06 am
by karatestu
Thanks for the link Doc.
Which one should i go for ?
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
Re: KISS
Posted: Sat Feb 17, 2018 1:31 pm
by eagwok33
Stu, great that you are about to start this passive preamp project!
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?
). This little pre has detail, bass, nice mids and a sweet top end.
Maybe one day, I will go for one of the quality NVA pre's Richard builds, to get the best out of my system!
Re: KISS
Posted: Sat Feb 17, 2018 1:44 pm
by Dr Bunsen Honeydew
karatestu wrote: ↑Sat Feb 17, 2018 11:06 am
Thanks for the link Doc.
Which one should i go for ?
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.
Re: KISS
Posted: Sat Feb 17, 2018 1:46 pm
by Dr Bunsen Honeydew
eagwok33 wrote: ↑Sat Feb 17, 2018 1:31 pm
I used an Alps blue stereo pot 10K linear,
YUK!
Re: KISS
Posted: Sat Feb 17, 2018 2:43 pm
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.
Naim used alps pots, maybe still do. I have a noble pot at the moment but its 50k
and no good for a passive .
Re: KISS
Posted: Sat Feb 17, 2018 3:12 pm
by eagwok33
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...
Re: KISS
Posted: Sat Feb 17, 2018 4:07 pm
by Dr Bunsen Honeydew
I hate the Blue it is muddled and unmusical (relative to what I use). The ones I use cost me 80p each.
Re: KISS
Posted: Sat Feb 17, 2018 4:10 pm
by Alfi
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
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
Stu
Hello Stu.
All fine here Sir, thankyou!
That's a very kind offer and one I will repay by carrying out the work on your LP12 gratis when the time comes.
Best wishes.
Alfi.