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-   -   Preamp disappointment and passive mod??? (http://www.diyaudio.com/forums/analog-line-level/187388-preamp-disappointment-passive-mod.html)

KanedaK 19th April 2011 03:05 PM

Preamp disappointment and passive mod???
 
Hello everybody,

last week I updated the 30 years old crossovers in my speakers with new crossovers (for those who are familiar with it, the speakers are Klipsch LaScalas modded with Beyma cP25 tweeters and EV SM120A midrange horn lenses, old type AA networks replaced with new ALK).

After the few usual days spent being amazed with the greatly improved clarity these new crossovers bring, I'm now facing a problem I didn't expect to happen: there's something "weird" with the way treble sounds. That is, I can now hear something in my electronics isn't quite right. I've always been thinking they were up to the task but obviously something in the link isn't, something that was unnoticed with the old, more "rounded off" crossovers.

It's like the treble lacks purity and speed, like it's "dirty", and sometimes it feels like it's playing on its own.

So the first thing I tried today is to get rid of the preamplifier (a DIY Kaneda line stage) and connect the CD player output directly to the power amp (CD player has variable output).

I must say I'm totally bluffed :o/ even with the CD player output level set at -39dB, wich is not supposed to sound good, it still sounds way better than with the preamp! treble is pure and extended, imaging follows exactly what's on the recording, sometimes very wide and enveloping, sometimes centered right between the speakers, bass is faster and punchier (but a bit less "warm" and "cuddly"), midrange is not grainy anymore (something I was afraid was coming from my power amp, but obviously not, this power amp is as clean and sweet as one can wish for)
...plus with the preamp there's always been some noise problem wich I already posted about in this forum.

so now i'm thinking: the gain stage in my preamp is AFTER the volume control. The latter is a stepped attenuator, 50K.
What if I just bypass the active stage from the signal, and connect the output of the attenuator directly to the output of the preamp? that way I could still use the input selector (4 inputs, relays, very handy) and use my preamp until I buy / build something better (maybe a passive, maybe a tube pre).
Now would this work? assuming for practical reasons I would leave the active gain stage and the PSU in the box, would this cause any problem to the sound and/or to the gain stage board? I don't wanna destroy the Kaneda line stage obviously, maybe someday I know better and I'm able to make it sound better, at the moment I just want an easy fix to make my system sound good.
I'm using mainly CD's and also FLAC files from my mac, and my speakers are 104dB sensitive so I don't need any gainstage in the preamp, I realise...

If for some reason it's not a good idea to just bypass the active gain stage in my preamp, could anybody recommand me a good quality and CHEAP passive preamp, possibly to be found second-hand? I'm on a low budget.

Power amp is technics SE-A5, input sensitivity/impedance 1V / 47Kohms

thanks!!

François

lineup 19th April 2011 03:58 PM

For CD output many times we need no gain=preamp.
If CD player and similar is the only sound source,
we can attach the signal direct to power amplifier.

A passive preamp can be very simple. A few resistors and one potentiometer, perhaps.
The less components, resistors, the better.
Few components keep the signal clean.

KanedaK 20th April 2011 01:10 AM

tonight it got worse
 
I heard tonight for the first time some "distant thunder" noise trough the right channel

i'm afraid the caps might be bad and that's what causes the not-so-good sound, the noise and hum that I posted about a few weeks ago, and now this sound wich is, i guess, what people call "motorboating"?

i disconnected the preamp totally from the system to make sure the problem doesnt come from the amp but i don't hear any strange noise so far and it sounds great...

so i guess i'm in for a servicing of the preamp before I decide anything.
problem is those caps are HUGE and I don't know how much it's gonna cost me -i suspect the ones inside are cheap quality as I don't even see any label or anything printed on them... but they're the size of beer cans...

cbdb 20th April 2011 03:10 AM

Your pre amp has caps the size of beer cans??? Kind of unusual. What kind?

Conrad Hoffman 20th April 2011 04:20 AM

I've never needed or used a gain stage in my preamps. Whenever I've installed one, no matter how good I think it is, the result is too much gain and poorer sound. Admittedly I drive short cables into amps that have a flat input impedance. Read the gain staging article!

KanedaK 20th April 2011 11:30 AM

Quote:

Originally Posted by cbdb (Post 2544605)
Your pre amp has caps the size of beer cans??? Kind of unusual. What kind?

The preamp is a Kaneda line stage.
The previous owner (who built the preamp) told me the filter caps are 169000uF total (2 caps), but I suspect he used cheap brand as there is nothing printed on the cans, no references, brandname, ratings, nothing wich is weird when you condider even the smallest caps have a whole litterature on them...

OTOH such big caps from a GOOD audiophile reliable brand must be quite expensive...

Itsmee 20th April 2011 11:52 AM

You could cannibalize your Kaneda and make a Pass B1 (the DC coupled version), a couple of sets of jfets wouldn't cost much.

KanedaK 21st April 2011 09:16 AM

Quote:

Originally Posted by Itsmee (Post 2544901)
You could cannibalize your Kaneda and make a Pass B1 (the DC coupled version), a couple of sets of jfets wouldn't cost much.

Interesting indeed; I found a website selling pre-built B1 modules in Germany. Good quality parts and very cheap.
funny enough the same website sells high quality, pre-built PSU boards for... Kaneda linestages. with Roederstein and wima caps and 6x15000uF.

So I guess the best would be to get a Kaneda PSU, see how it makes my preamp sound; then if it's not to my liking (but performs as it should) I can sell it and try and build a Pass B1 kit.

What do you guys think?

Diy-Audio-World Onlineshop: a la Kaneda Preamp

Diy-Audio-World Onlineshop: B1 Fet Booster

Itsmee 21st April 2011 09:41 AM

If you buy a B1 kit, I would go for the DCB1, boards are available from diyAudio member Teabag, you can buy the original from papa, available from the First Watt web site.

toprepairman 21st April 2011 12:56 PM

Forget the b1 kit and just make it entirely paasive as you suggested in your original thread.


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