Adding a buffer to my Zen V3

[Shoog]

Hi there,
I dragged out my old ZenV3 a few days ago. I haven't had a chance to listen to it recently and I want to compare it to my Valve Parafeed amp.
The Zen was my first project and is a bit delicate so I don't want to do much to it, but I feel it needs its low input impedance issues to be addressed before it can go head to head. I really don't want to alter it as much as converting it to the Zen V4, so I have devised a simple gainstage which I can add to the front end to bring up its impedance and gain.
Anyone care to comment on whether it will work.

Thanks Shoog

 

[Nelson Pass]

I would put maybe 100 ohms on the Source to ground of the
transistor, and make the 100K input resistor a pot so that you
can adjust the DC output value.

 

[Shoog]

Thanks for that Nelson. I will certainly add the 100R resistor, and will think about the 100K pot.

Will report back on the final result.

Shoog

 

[Shoog]

Everything seems to work just fine. Getting probably a 3db gain and no major distortion. Only tried it with a test speaker so far.
Managed to short out my capacitance multiplier so now have quite a bit of hum - will work on a "neat" rebuild tonight. Should be in business by tomorrow.

Shoog

 

[Shoog]

Hi all,
Just a status report. Got everything working. After repairing the capacitance multiplier I still had a hum, which I didn't remember from before. Tracked it down to the buffer. Added another stage of filtering 22R followed by 1000uf. That did it - nice and quiet.
Listening now on my main system. Sounds nice. Very soft sound. Unfortunately clarity and punch is less than my parafeed amp, so it will probably go back into storage again. I didn't do an especially good job on my Zen so there is no saying that someone else hasn't got a Zen which could better my other amp.

Thanks all

Shoog

 

[Shoog]

Hi there,
Made a small adjustment to the buffer. Changed the 500R for a 1K resistor. Improved the top end a huge amount.
Closer examination shows that the circuit I used was the V2. Still not as good as my parafeed amp or buffered gainclone, but good enough to have had playing in the main system for the last four days. very enveloping sound which is great on jazz and folk, but when thing get complex with techno or hard rock, things seem to get muddled.
An interesting experiment overall.

Shoog

 

[PMA]

Would not you better use N-MOSFET source follower as a buffer?

 

[Shoog]

Buffer is really the wrong description for what I did. I had the irf610's to hand and I wanted a little gain (to make it fit with my unity gain preamp) as well as impedance increase, so this circuit suits my purposes perfectly. It works well and I am happy.
It certainly may not be the best solution for everyone. I would expect most people would be building ZenV4 by now, which should perform better.

Shoog

 

[PMA]

In case you use source resistor, as per NP's recommendation, it might be OK. The gain would be roughly

G = drain resistor / source resistor

Omitting source resistor would lead to high distortion.

 

[Nelson Pass]

Would not you better use N-MOSFET source follower as a buffer?

A P channel enjoys some cancellation with the N channel, if
that's what you're looking for.

 

[PMA]

A P channel enjoys some cancellation with the N channel, if
that's what you're looking for.

Yes, it does. I was confused by the image in his first post, which was not a buffer at all, but common source amplifying stage, with 500 Ohms as drain load.

 

[Shoog]

As I later stated, a buffer was the wrong description. I did indeed follower Nelson advise and included the 100R resistor, this and the 1K drain resistor works very well, even though it is running at half the current of the 500R drain resistor.
As I said, just an experiment and nothing to get excited about. The main thing is it works and has brought my Zen amp back into the fold of usable amps.

Shoog