Sony vFET Illustrated build guide

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Paul M

Member
Joined 2005
metallicus69 said:
I see the bridged amp very similar to a balanced amp , the difference being the way feedback is applied. In your case the resistive inputs to the common ground will create two equal but out of phase signals for the two halves without the necessity to bring pin 1 to the boards. That's asking for trouble. But I think this is easy to verify yourself if one channel is ready and working. I would just test this to eliminate all the doubts
Click to expand...

Do you mean if I leave R4 to ground vs connecting both R4 together (like what Gyuri implemented some time ago based on ZM's suggestion)?
If I lift both R4 from ground and connect them together, at that point pin 1 should go directly to chassis?


I read other builders' experience with changing offset and voltages as the Vfet warms up. In my case, after warming up the amp for about an hour, I managed to adjust the output offset to about 40 mV, the FE offsets to <200 mV with the voltage drop across R5 and R6 at about 1.56 V.
The thing is that now, when I turn on the amp (cold), I have an initial output offset of about 20 mV which is fine, but the FE offsets are 0.6 and 1.1 V, (voltage across R5 and R6 is about 1.1 V).
Is it normal to have such high FE offsets when cold, even though they steadily drop to <200 mV as it warms up?
 
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metallicus69

Member
Joined 2005
JoeyDD said:
Do you mean if I leave R4 to ground vs connecting both R4 together (like what Gyuri implemented some time ago based on ZM's suggestion)?
Click to expand...

This is the difference between bridge and balanced and this is what I would do when I will build this project (hopefully in the actual incarnation [emoji4]

If I lift both R4 from ground and connect them together, at that point pin 1 should go directly to chassis?
Click to expand...

I say that I would do this irrelevant if the above modification is made or not. You don’t always control what happens in the stage before the final amp and if you tie the pin 1 to the chassis you prevent hum no matter what. Especially when you have both channels in one chassis.



I read other builders' experience with changing offset and voltages as the Vfet warms up. In my case, after warming up the amp for about an hour, I managed to adjust the output offset to about 40 mV, the FE offsets to <200 mV with the voltage drop across R5 and R6 at about 1.56 V.

The thing is that now, when I turn on the amp (cold), I have an initial output offset of about 20 mV which is fine, but the FE offsets are 0.6 and 1.1 V, (voltage across R5 and R6 is about 1.1 V).

Is it normal to have such high FE offsets when cold, even though they steadily drop to <200 mV as it warms up?
Click to expand...


This is something that should be addressed first. I cannot advice anything here as I am still collecting the parts to build a dual balanced power stage. I think it is not normal and you should always have offsets below 50mV no matter the time since start of the amp.
 
P

Paul M

Member
Joined 2005
Thank you for the help Metallicus!
I've connected pin 1 as you suggested and the amp is completely silent when using a test speaker.
I'll try the R4 connection on the second monoblock after I adjust that one and see if I can hear a difference.

As for the high FE offset when cold, it seems Ben Mah's build had a similar behaviour (Sony Vfet Illustrated build guide). In the end, it looks like it was fine to leave it as is.

Paul
 
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mr_whocares

Member
Joined 2005
re-adjusting bias

hi everybody!
today i was wondering if there is any drift in the bias setting with the time , so i measured the amp after almost 18 months. the result : yes there is some :)

at the left channel T18 was drifted from initially 7mv away up to 270mv but the rest of measurements were within the range.

at the right channel T18 was around 190mv (initially 6mv) , but the output DC offset was 350mv!(initially 19mv).

so to stay on the bright side of life, it is probably good to check the bias at least once per year and re-adjust. ;)

any similar expediences with your vfet amp?

best regards
 
M

mr_whocares

Member
Joined 2005
psu transformer

i am using a 300w trani which seems to be sufficient for this amp. however, i own also a second transformer of the same type , so i was thinking to parallel them (300w+300w = 600w ;) ) to check if there is any impact of a stronger transformer in the psu on the overall sound quality.

do you recommend this approach? is paralleling the transformers almost the same effect as using a bigger one or is there is any drawbacks?

if there is no risk doing that , which case would be the better one, A or B?

thanks and regards
 

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