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jFET phono preamp - boozhoundlabs
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Author:  mwhouston [ 18 Jan 2011, 01:45 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

CrazzyAbtTubes wrote:
Is it possible to post the schematic directly to this thread, please, I don't have any way of viewing data sheets at the moment. :(

Looks like I have another small project on my hands though. :twisted:

Can't you open the schematic on an earlier post?

I had some Golden ears from the Melbourne Audio Club visit today and help select which of my BozSE or BoZoaC sounded better. And to comment on the new member of the family the Boozhoundlabs phono preamp. Firstly the preamp got the thumbs up and after a bit of A/Bing BoZSE got the vote. BoZoaC is just a hint brigher where BoZSE is smooth and balanced. But my Golden eared friend said either one was very good with his preference going to BoZSE.

We played every thing through the Boozie phono pre from Mahler to Rober Plant's "Band of Joy". At every point the phono preamp absolutely shone. I still think the best improvement to my analogue section I could make.

A technical comment about Boozie phono preamp: it draws 13.5mA at 24V. Now don't you feel better for knowing that??

Author:  cheap-Jack [ 18 Jan 2011, 14:36 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

mwhouston wrote:
this is the best I have made compared to OPAMP and JAN6418 based units I have made in the past.


No wonder you find yr jFET phono amp sound better than the Opamps you used before.

How can it sound good when the complex music signals got to go thru tons of P/N junctions of the many bipolars inside the chip with loop NFB all over the place? This is physics.

With the very few P/N junctions to go thru, the music surely sound better in yr jFET amp. This is physics.

But but the best biplars can do is to be remotely matching a pentode as both got the similar kinked transfer curve. Triode beat any pentode in terms of linearity & odd order of harmonic distortion.
That is why triode is the best sounding active audio device. Period. This is physics.

Don't get me wrong. I don't mean costly brandnames always sound better. NOT at all. Just I am fortunate enough to have auditioned so many very very expensive brandname SS phono pre-amps but still beaten by triode phono preamps of good designs.

A good sounding triode phonostage can be DIYed very simple & cheap, but sound like a chime.
The key of the game is to keep the number of active stages to minimum. Less active devices to go thru by the music signals, better will it sound. This is again physics. My picky ears tell me so thru my DIYed 1-stage triode passive RIAA phonostage.

I can see you enjoy headphones (not for me, sorry!). I can supply you schematic of triode headphone of very simple OTL designs. By far simpler than yr Jan 6418 hybrid design & should sound better.


Author:  mwhouston [ 18 Jan 2011, 18:40 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

Cheap-Jack: Got a link to your single triode phono stage? I may just give it a go.

Author:  cheap-Jack [ 19 Jan 2011, 10:40 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

mwhouston wrote:
single triode phono stage?


Disclaimer: I don't think know posting too much on tube amp will be deemed O.T. in this SS forum.

Very briefly, it is a 1/2 ECC83 with B+=440V & super simple RIAA passive EQ & battery cathode biased.. It is still in on a breadboard for half a year now sitting on the metal cage of my Dyanco ST-70 power amp. I don't want to rebuild it properly as I don't want to miss its sound for one day!

This is supposed to be fed into a linestage to get 'adeqate' gain to drive any power amp. But being a "less electronics deliver more musc" die-hard, I have it hooked up to drive my super-upgraded Dynaco ST-70 power amp direct.

With volume control (500KR) set to max for low level classical reocordings & 4/5 full volume for pop/light music, I'm amazed by the extreme transparency, details, precise soundstaging & imaging (my first priority consideration in sizing up any audio system) & vocal interpretation (e.g. the forceful voice of Pavarotti, the Hi-C king) delivered by this breadboard !!!.

How can a one-stage phonostage breadboard can deliver such superb sound despite being a bit too low gain to drive a power amp direct?? Sonically, it beats any SS & tube phonostages, DIY, brandnames I have auditioned.

All textbooks on this planet preach us a phonostage 'MUST' get 40dB gain, blah blah blah...Now I have proven this is not neccessariy all true IF if one gets a pair of ears that go for music quality rather than music quantities.

The logic I can gather is the music goes thru minimum active electronics with minimum 'condamination' & therefore sounds superb. It's that simple.

While I am still working on 440V regulated PS (which is imperial, IMO, for such a simple phonostage), & so far not much luck with the LR8 chip. Once I completed the regulated PS, I think it should sound even better.

Other option is a chip CCS to replace the 100K Rp so that regulated PS is not that crtical.

Anyway, once I finsihed the whole thing to my own design satisfaction (though it already sounds very good), I''ll post it.


Author:  mwhouston [ 19 Jan 2011, 17:22 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

Cheap-Jack: Agree with minimalist approach that is why I have single FET (Pass style) preamp and a two tube (not counting rec. tube) SET power amp. Now with a two stage jFET phono pre. there are only five active components in the signal path. Also there are only four (4) capacitors in the signal path. My preamps phono pre has no input cap, my BoZSE preamp has no input cap and my SET no input cap.

Five active components and four caps!

Author:  cheap-Jack [ 20 Jan 2011, 16:28 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

mwhouston wrote:
my BoZSE preamp has no input cap



Any tube I/P pre-amps & power amps don't need I/P caps because the electrodes are isolated by the vaccuum inside the bottle.

BUT but any bipolar, in unbalanced I/P mode like the BoZSE, should get an I/P coupling cap to block any DC offset to be enable it to work stably as the electrodes are all interconnected.

jFET is not a tube !!!


Author:  mwhouston [ 20 Jan 2011, 17:53 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

It seem odd to me that the phono preamp doesn't have an input cap. It appeared to work alright over the last few days. When I went to use it last night it sounded like the RIAA equqlization stopped working. How can this be there are only Rs and Cs passively doing the work?

It is boosting the signal but the sound is tinny, bright and bass is very low. Any clues how this can happen? If I swap it out for one of my others it sounds perfect. So it is in the preamp.

Author:  cheap-Jack [ 20 Jan 2011, 19:26 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

mwhouston wrote:
but the sound is tinny, bright and bass is very low.


It sounds like the jFET(s) is going.

Check the PS voltage OK. If OK, measure of bi-polars electrode voltages OK or not.

One suggestion is to add an I/P cap to the first stage jFET to see if it works.


Author:  mwhouston [ 20 Jan 2011, 22:50 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

cheap-Jack wrote:
One suggestion is to add an I/P cap to the first stage jFET to see if it works.

Trying a 10uf poly now.

Author:  mwhouston [ 21 Jan 2011, 06:28 ]
Post subject:  Re: jFET phono preamp - boozhoundlabs

Silly me:

It appears the material I was listening to earlier did not show that there was a problem with the phono preamp. The material I played later sounded wrong. I decided to trace the voltages through the preamp, comparing one channel to the other. They tested OK. I then started measuring resistors to find the load resistor in one channel was the correct 47K while the other channel 100ohms. The cartridge was having one channel heavily loaded causing the cartridge to perform poorly.

The offending R has been replaced and the newly named Mongrel Dod Audio Pacific phono preamp now performs beatifully. I must remark though I is quite bright. If your system is already bright this may not be the phono preamp for you.

The OPAMP based one I have compared it with is much more mellow. Pacific high lights mids and highs. I can now say it does it very well.

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