help with zen waveform

[jcayer]

Hi all,

The building of my zen amp goes slowly but surely. I finally found some suitable heatsinks and I could do my tests yesterday and adjust the bias for symetrical clipping.

However, with a sinusoidal signal at the input, the output signal looks good until it reaches about 7 Vrms, afterwards there seems to be some distorsion in the peak of the minimum portion of the signal (as seen in the picture below).

The same thing happens for both channel. So either this behaviour is normal or there is some part of the circuit that is common to both channel that is defective.

Any clues ?

Thanks and best regards,

[jcayer]

Seems that the picture is not showing


OK, the pics just appeared

It seems that there is a small offset on the scope screen but in fact the position of the trace wasn't centered.

This is 9Vrms = 10Wrms in 8 ohms

[azira]

First question: rails and bias?

My Zen is only good to about 8Watts before it can be expected to clip. That is because I used lower rails and bias due to the trafo I chose.
--
Danny

[jcayer]

Hi,

thanks for taking the time to reply

Rails = 33.4 Volts
Bias = 16.8 Volts

[GRollins]

I'd say you're running right about to spec. The Zen is not a huge amplifier. 7Vrms is about 20Vp-p, which is a fairly realistic estimate of what to expect from a single-ended 33V rail using MOSFETs for outputs.
If you want more voltage swing (i.e. more power), you can always increase the rail, but keep an eye on the heat dissipation or you'll fry your output devices. Beyond that, you'll need to consider other options, such as parallel channels or using a different circuit topology...

Grey

[jcayer]

Originally posted by GRollins
I'd say you're running right about to spec. The Zen is not a huge amplifier. 7Vrms is about 20Vp-p, which is a fairly realistic estimate of what to expect from a single-ended 33V rail using MOSFETs for outputs.
Grey

Well I'm a bit confused,

Judging from the original zen schematics, I use exactly the same setup. 2 x 25V transfo output etc... So 7Vrms is only about 6W in 8 ohms. Why would they specify the zen at 10W ?

The only thing that is different are the MPSA92. I used MPSW92, which according to the specs are the exact same thing.

I have made some research through the forum (pass labs) One thing I will try as soon as I can, will be to put a resistor (220 ohms) in serie with the gate of Q1 on the transistor end.

[azira]

Originally posted by jcayer
Hi,

thanks for taking the time to reply

Rails = 33.4 Volts
Bias = 16.8 Volts

Sorry, I meant, bias current. What size is R1?

I guess, while I'm at it. What kind of load are you testing with, and what frequency are you testing at?
--
Danny

[jcayer]

Originally posted by azira



Sorry, I meant, bias current. What size is R1?

I guess, while I'm at it. What kind of load are you testing with, and what frequency are you testing at?
--
Danny

Danny,

R1 is .33 2W and the IBias is 2 amps (very close)...
I'm testing with some 8 ohms power resistors (50 W)
And the picture was taken with 1KHz signal.

[azira]

Ok... I'm no where near as expert on these things as some people around here but I bet you're RIGHT on the edge of some asymmetric clipping.

Firstly, to deliver 10W into 8-ohms you only need about 1.25 Amps of current and around a 20 Volt rail + some headroom. So your amp is capable of delivering that in it's present state.

But, your image shown is about 9Vrms = 12.7Vpeak would get the negative cycle down to around 4 Volts. The .pdf from passdiy doesn't mention when clipping should occur and I honestly haven't measured my own yet. But from experience, I would say that getting lower than 3 Volts DS accross a FET is getting into clipping.

I would say that you should go ahead and increase your input signal a little more so that you'll see pronounced clipping, adjust P1 to make it symmetrical and then back off the input and see if the negative "bump" is still there and assymetrical.

Sorry, it's not super helpful advice..
--
Danny

[jcayer]

Originally posted by azira
Ok... I'm no where near as expert on these things as some people around here but I bet you're RIGHT on the edge of some asymmetric clipping.

...

I would say that you should go ahead and increase your input signal a little more so that you'll see pronounced clipping, adjust P1 to make it symmetrical and then back off the input and see if the negative "bump" is still there and assymetrical.

Sorry, it's not super helpful advice..
--
Danny

Danny,

P1 is adjusted, the clipping looks like it is symetrical, The peaks just flatten simultanously. But this "strange" bump appears around 7Vrms and stays on.

I will try to change the pots (along with adding the resistors at the gate of Q1) for one turn pots. When I mounted this board, I had only some multiturns and I hate playing with these pots

Thanks for your help,