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-   -   A bjt Bride of Zen (http://www.diyaudio.com/forums/pass-labs/161203-bjt-bride-zen.html)

ginetto61 13th February 2010 09:28 AM

A bjt Bride of Zen
 
Hello to Everyone !


I have a Bride of Zen in my setup and like it
Nevertheless I would like to understand if a BOZ with a bjt like, for instance, a 2SC2238 would be feasible or not
Which are the most important shortcomings of using a bjt in place of the irf510/610 ?
Are mentioned mosfets more linear than a 2SC2238 ? maybe less distorting ? do they have more bandwidth ?
Thanks a lot and kind regards,

gino

Johnloudb 13th February 2010 02:14 PM

Well, I'm not familiar with that part but power BJTs required a lot of base drive. So, that would load you're source too much and wouldn't perform too well.

You could try a medium power darlington BJT, with high beta. That might work but I don't know how it would sound.

ginetto61 13th February 2010 02:41 PM

Quote:

Originally Posted by Johnloudb (Post 2084118)
Well, I'm not familiar with that part but power BJTs required a lot of base drive.
So, that would load you're source too much and wouldn't perform too well.

Thanks for your kind and helpful reply
The part is quite common as driver in power amp, just before the output devices
It is a good part, I understand
Do you mean that the input impedance would be too low with the bjt ?

Quote:

You could try a medium power darlington BJT, with high beta.
That might work but I don't know how it would sound.
All right. Do you think that mosfet are more linear in comparison with darlington ?
To be clear, I would like to understand better why a mosfet has been selected for a single component gain stage, a solution that intrigues me so much for its excpetional simplicity
I like simple things, in the kiss principle way

Kind regards,
gino

Johnloudb 13th February 2010 04:32 PM

I looked up the part data sheet, and it has a hFE of just over 100 when biased at 40mA. So, it could conceivably work, but probably wouldn't be to linear with a 10k potentiometer at the input.

If you use a volume control at the output like Pass did, then it would probably work okay, given that your source components have a reasonably low output impedance.

But it's not just a plug-in replacement and to get the best performance you'd have to adjust the bias.

The bias adjustment circuit Pass used assumed a the high DC input impedance of a MOSFET. So, you have to lower some resistor values so you get enough current to achieve bias stability, and that would also lower the input impedance of the preamp.

Well, it might work, but I don't see it as a plug in replacement for the IRF610/510. It would require some redesign work.

A Darlington BJT would probably work okay without modification I think. You'd need to adjust the bias to 40mA again. But, I don't know how it would sound.

Geek 14th February 2010 05:22 AM

Hi,

A BJT in this setup may need a DC servo.... BJT's drift in the wrong direction as they warm up ;)

Cheers!

ginetto61 14th February 2010 08:09 AM

Quote:

Originally Posted by Johnloudb (Post 2084268)
I looked up the part data sheet, and it has a hFE of just over 100 when biased at 40mA.
So, it could conceivably work, but probably wouldn't be to linear with a 10k potentiometer at the input.
If you use a volume control at the output like Pass did, then it would probably work okay, given that your source components have a reasonably low output impedance.
But it's not just a plug-in replacement and to get the best performance you'd have to adjust the bias.
The bias adjustment circuit Pass used assumed a the high DC input impedance of a MOSFET. So, you have to lower some resistor values so you get enough current to achieve bias stability, and that would also lower the input impedance of the preamp.
Well, it might work, but I don't see it as a plug in replacement for the IRF610/510. It would require some redesign work.
A Darlington BJT would probably work okay without modification I think. You'd need to adjust the bias to 40mA again.
But, I don't know how it would sound.

Thank you so much for your extremely kind and comprehensive reply
I understand that the mentioned bjt a plug in replacement for the IRF610/510
So basically all the circuit must be re-thought
You say I don't know how it would sound
I would be curious about that
From what I understand the part is of a very high quality, used as a driver in power amps
By the way, thank you sincerely again for your kind and helpful advice
Kind regards,
gino

ginetto61 14th February 2010 08:15 AM

Quote:

Originally Posted by Geek (Post 2084909)
Hi,
A BJT in this setup may need a DC servo.... BJT's drift in the wrong direction as they warm up ;)
Cheers!

This is indeed a poit, thank you for your kind advice
I read about the problem with bjt
Mosfets tend to stabilize if I understand well
Maybe a lower bias could be used
Nevertheless I know of "cold amps" that sound pretty well
In the sense that a high bias current is not a guarantee of good sound, and vice versa
But my very question would be, is the mosfet intrinsically a better part for that duty (i.e. to amplify 5,6 times a line level signal) ?
Is it more linear, does it have a wider bandwidth, is it less distorting, does it have a better slew rate ?

Thanks again and best regards,
gino

revintage 14th February 2010 12:09 PM

If copied straight off from BOZ:

The BJT will have lower THD.

But a lot worse overtonespectrum.

Lousy thermal behaviour.

So probaly not worth trying...

ginetto61 14th February 2010 12:37 PM

Quote:

Originally Posted by revintage (Post 2085096)
If copied straight off from BOZ:
The BJT will have lower THD.
But a lot worse overtonespectrum.
Lousy thermal behaviour.
So probaly not worth trying...

Thank you very much for your very kind and valuable advice
But with other Ibias, other Vce ?
My main regret is to have not a simulation software
I think that an idea of the real behaviour could be acheived with a simple simulation, maybe varying the working parameters to find an optimum

More in general I like the "philosophy" of using the lowest number of active components in their optimal working conditions
But this is very superficial statement

Regarding the BOZ using it I found that its performance is so much dependent on the quality of the power supply
I think that with a PS of exceptional quality it would be fantastic
My own is just very good ... that is not bad indeed
Still I think about a bjt BOZ ...

Kind regards,
gino


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