ZenV4 with differential pair input

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Hi,

In the middle of the process of making my ZenV4 I managed to blow a ZVP3310. I tried to buy one locally but they are not available. So now its running with a ZVP3310 in one channel and an IRF9610 in the other. Surprisingly or not, sound is good.

But I want to get this fixed. So I though of using a differential pair in the input, like an Aleph, with IRF9610’s. I am a bit unsure of how to implement it. I simulated the following circuit but it clips at very low levels on the input, so something is wrong. I don’t know also if the feedback is in the correct phase.

Please give me a hand here. What do you think about the sound – will it improve or not comparing with the original ZenV.4?

Thanks

Miguel
 

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Hi,

Thanks for the answers. Nelson, I really took a look at the Aleph circuit first and saw some similarities. But I want to keep the Zen output stage and the PSU, so I am looking only at the input stage.
I have to say that I still don't catch exactly how the differential pair works (I am reading now the A75 article to get it -excellent to learn).
I did some more simulation and replaced the zen feedback circuit with the Aleph feedback network and got a workable circuit that is in the figure. Problem is that R64 has a maximum value of 250 ohms before clipping with 0.75 Vin. Is this related with that short you mention Uli?
 

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The one and only
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Well then, you want to reference your input stage to a +DC
value above ground. This means that the + input has to be cap coupled, and the Gate of the + input will have a resistor to this v+ value instead of ground.

V+ should be about 10 volts or so, not a critical thing, and can be made from a resistor added to the resistor which biases your current source Zener, and then to ground, cap coupling the junction to ground to clean it up and taking your reference off that.
 
problem is that the 9610 or similar has about -3.5 - 4 v between
gate and source when conducting. this means if you have a
grounded input (ref to gnd by input res) your source voltage is
about +3.5 to 4 v above ground. this is the amount you need to drive the output thus you have no headroom left between source and drain. as the drain is 180° phase you vcannot drive the fet.
example:
source to drain say 20mV dc. gate voltage 200mV ac gain = 10.
if the gate goes up (+200mV) the source goes up at +200mV rel.
ok this means drain goes down 2V. ok.
B U T :
gate down 200mV, source too, drain up 2V - not possible theres
no place (picturesque) to go up! -> distortion!

Uli
 
feedbackloop

the feedback loop is wrong!
it´s necessary to reference both ends of the fb to the same dc
level as the input gate. In your circuit you reference it to gnd and
the input gate to some dc value, thats the fault.
Try putting the fb from BEFORE the output caps and make
the dc bias for the input trimmable.

Uli / -who hates caps in the signal path:devilr:
 
Thanks Uli, I think I got the idea. I took the feedback loop before the output caps, put a 10 uF cap in series with it and put the feedback resistors. Then I connected a 47.5K resistor from the the reference voltage to the minus input of the differential pair. Now everything is balanced and giving good smulation results. But there is one more cap there:dead:.

But then I realized that we are talking about 23 mA of current for the differential pair. So a small transformer with a nice regulation can do the trick. I can make a different PSU for the input pair (if I find a transformer for the right price: < 0 euro), with plus and minus voltages, and then feed the Zen. This circuit gives fine simulation results. But there is still the cap on the input of the IRF044 that does the amplification - M3. I saw that the voltage between the drain of M5 and R64 is -20. At the gate of M3 we have 4.6V. So it still needs that cap :cannotbe:.
 

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This weekend I built the differential pair with the current source, just like in Aleph 3, and connected it to the input cap of the Zen. It sounded ok but not better than the original. The PSU I used was a crappy 10V - 0 - -12V one and current draw through the differential pair was only 10 mA. So I need to get a decent PSU to test this better.

I used the feedback scheme of Aleph 3 (10K - 1K and 220 uF to ground) and gain was too low. I guess this feedback needs some adjustment.

One question arose when looking at Aleph 3 schematic. It says that the output MOSFETS idle at 1 A bias. As the Zen has a bias of 2 A, it is more linear than the Aleph, as the higher the bias the more linear is the fet. Am I right?
 
The one and only
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miguel2 said:
One question arose when looking at Aleph 3 schematic. It says that the output MOSFETS idle at 1 A bias. As the Zen has a bias of 2 A, it is more linear than the Aleph, as the higher the bias the more linear is the fet.
It comes out about the same, as the Aleph 3 has two devices in parallel, each working 1/2 as hard, and with 1/2 the bias.
 
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