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-   -   Easy tweak for Zen V4. (http://www.diyaudio.com/forums/pass-labs/18000-easy-tweak-zen-v4.html)

Panelhead 20th July 2003 10:01 PM

Easy tweak for Zen V4.
 
My Zen V4 has been finished for a while now. It has a couple changes from the suggested construction.
Listening to it versus a good SET and a Aleph X show up a couple of weakness areas. The dynamics, clarity, and frequency extremes come to kind. The overall sound is pleasant and has long term listenability.
One surprising item was that is requires a strong source driving it. The 47K input impedance and normal sensistivity do not match this. Changing preamps makes a big change in the problem areas even at very low power levels.
Looking to tweak something I started looking at the input buffer. Nelson mentions cranking up the current running through the ZVP3310.
My amp is using a pair of tail to tail connected 47 ufd Silmic electrolytic to couple to the IRF044. Thinking about the warning that this cap needs to be at least 10 ufd to maintain low frequency response made me think.
The current through the buffer is 7 - 8 ma. The regulated voltage is 42 volts, drop 4 across the buffer and the 4.75K dropping resistor gives about 8 at best.
I placed another resistor across the 4.75K 1/2 WATT carbon comp to make a 2.8K 1 watt resistor. This bumped the current up to 13.5 ma.
Now the lows are more exteneded and the highs are crisper. The dynamics seem a little better also. The easiest to identify change is the improved clarity. Some of the colorations are gone.
Overall adding the two resistors took 15 minutes to do and upgraded the sound a bunch.
This may not be the optimum value. My power demands are minimal, 1 watt is plenty. Using normal sensitivity speakers may respond better to running the buffer at 20 -30 ma. Also the two electrolytics used for coupling in my amp may require more current than a single large value film cap.
But whatever the reason, bumping up the idling current through the buffer may help fix some of the problems people have posted about no bass or dynamics from the Zen V4.
I may try 20 ma in mine to see if it helps the sonics even more. The ZVP3310 is rated for 100 ma so it should be fine at any sane level.
I had already replaced the filter cap above the buffer with a 470 ufd cap. I have bumped up all the electrolytics to try and add some slam to the sound. It the dropping resistor is lowered a lot, it might work better if the capacitance is increased to maintain the same level of filtering on the buffer.

George

Fred Dieckmann 20th July 2003 11:11 PM

Not so hot of an idea
 
The maximum power dissapation for the ZVP3310 is about 0.6 watts at 25 degrees c ambient. The inside of the amp is probably more likely over 40 degrees C and even less power can be dissapated at higher temp. You can run into dissapation problems quite fast at higher currents. The disspation is the voltage across the device times the current. The maximum power dissipation for the ZVP3310 is about 0.6 watts at 25 degrees C ambient. The inside of the amp is probably more likely over 40 or 50 degrees C and even less power can be dissipated at this temperature. I don't see the power vs. temperature derating numbers in the data sheet the datasheet but would not put more than than about a quarter of a watt across it at 40 to 50 degrees ambient in my estimation. You can run into dissipation problems quite fast at higher bias currents. The power dissipation is the voltage across the device times the current through it.

fdegrove 20th July 2003 11:40 PM

ECHOES?....
 
Hi,

Is someone going into thermal runaway here or is it just George B.s' propaganda machine at work?

Fred's right.Fred's quite often right.Fred's almost always right.
Fred's right.Fred's quite often right.Fred's almost always right.
Fred's right.Fred's quite often right.Fred's almost always right.
Fred's right.Fred's quite often right.Fred's almost always right.

All in all Fred must be right...;)

jh6you 21st July 2003 01:13 AM

The original ZV4 is designed with the Vds of about 4Vdc across ZVP3310, and the article says, For the ZVP3310 we will be using, you could run it as high as 50 mA, which is a lot.

If with the 50mA, the power dissipation would be approximated as about 0.2W, which is at 1/3 level of the maximum power rating of 0.6W. And, I assume that the 0.2W is a safe value derated considering the operational temperature higher than 25 degC.

Therefore, it is deemed that the current is applicable up to the max 50mA, isnít it, Jaws?

:yinyang:

Fred Dieckmann 21st July 2003 01:47 AM

Who the hell is Jaws?
 
From the The Zen Variations - Part 4
article.

"Second, it is self-adjusting in the sense that the Source will
operate at +Vgs where Vgs is the Gate to Source voltage for the
Mosfet, or about 3.5 volts. The amount of current supplied by
the resistor is not critical, and can be quite high if desired, as the
Mosfet will only dissipate that current times the Vgs value. For
the ZVP3310 we will be using, you could run it as high as 50 mA,
which is a lot.
Third, the P channel non-linearity will tend to operate in opposition
to the N channel distortion of the gain Mosfet, giving some
distortion reduction due to cancellation. You can adjust this
cancellation a bit by adjusting the bias current and loading of the
P channel device."


You might want to use a current source
in parallel with the load resistor for biasing this mosfet since loading with a small enough resistor to approach 50mA will mostly increase distortion since you are driving more signal current from the ZVP3310 which increasies distortion. I appears that Mr. Pass has optimized this distortion cancelation with the current design. Mosfets do sound and measure better for a given load impedance with an increase in bias current I have used the Zetex ( N and P) devices as followers with about 15 volts drain to source (where the device capacitances really drop) and biased with CCS's and they sound very good.

jh6you 21st July 2003 02:37 AM

I think that your successful experience as in Post#5 must be a good info for the ZV4 tweakers, who are tackling only the experiment with different load resistor values. I wrote it down on my memo note. Thanks.

Who the hell is Jaws?

:yinyang:

Panelhead 21st July 2003 10:34 AM

Re: Who the hell is Jaws?
 
Quote:

Originally posted by Fred Dieckmann



You might want to use a current source
in parallel with the load resistor for biasing this mosfet since loading with a small enough resistor to approach 50mA will mostly increase distortion since you are driving more signal current from the ZVP3310 which increasies distortion. I appears that Mr. Pass has optimized this distortion cancelation with the current design. Mosfets do sound and measure better for a given load impedance with an increase in bias current I have used the Zetex ( N and P) devices as followers with about 15 volts drain to source (where the device capacitances really drop) and biased with CCS's and they sound very good.

Fred,
You are right in bringing up the power dissapation in lowering the value of the resistor. With the voltage constant at 38 volts in is very easy to calculate the the power.
At 50ma the resistor will be running almost 2 watts!. I suspect the original value of 8 ma was chosen to allow a 1/4 watt resistor to be used.
I thought the improvement in sonics was mainly just more current available when bias was increased from 8 ma to 13.5. But it may be more the P channel - N channel distortion cancelation and the fact that the fet sounds better at higher bias.
Either way it is easy to do. Adding another resistor in my unit not only saved time, two hours to remove and reinstall boards, but allwed bumping up the power rating on the resistor. The holes in the factory PC boards will not accept 2 watt resistor leads. I know I tried when building. Even 1 watt AB leads did not go in well.

George

Nelson Pass 21st July 2003 06:06 PM

I have no doubt that bumping up the current on the ZVP3310
will improve the performance, which is why I mentioned the
possibility. It''s much more fun for me to watch you gild a lily
than do it myself.

:cool:


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