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Old 19th June 2004, 04:21 AM   #11
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Quote:
No RLC network can help.
As you all can see, the output is only secondary coil. This secondary coil is directly connected to the speaker. I've tried putting RLC in this, no effect at all. How about putting this RLC between drain and +Vcc (crossing primary coils). Will it help?

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Feedback can be helpful, and it lowers the output impedance too, wich is now very high. Additionally, you should mach the transistors by transfer admittance
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I think the simplest way to cure that problem is to add some feedback to linearise the circuit and reduce the DC error seen by the transformer
How can I make feedback with the existing topology? I cannot put some wire into the core.

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An idea: if I were You, I would connect speaker directly to drains. Smaller transformator, better coupling at high freq.
That would be ZEN V-7R. But right now I want to troubleshoot the existing problem.

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BTW: this is a common source class AB amplifier. Without feedback, it must distort.
Mind telling me why is that? And how to apply feedback in the existing topology?

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The only way to get this topology to work properly is to ensure no DC at all is applied to the transformers
As in the schematic, the primary+final mosfets are designed TO HAVE bias about 60mA. Why is this so contrary to the fact that transformer do not handle DC? If I put blocking cap between drains and transformer, I dont think the whold system would work. Any suggestion on how to block DC, but still have bias flowing at the mosfets?

ZEN-V7R also have big bias with this primary. But the output is different, they are from the drain, like Pafi suggested. This one has output from secondary. Is that different?

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I think this kind of behavior is exactly what you should expect from such a nonsense design.
MOSFETs show pretty non-linear transfer characteristics and when they are operated open-loop in such a flawed topology, they will add lots of asymmetric distortion to any asymmetric signal and this is potentially a source of DC applied to the transformer, that would require a *huge* gap to handle it
You are right, EVA. The design is non feedback. The NJM386 is just a 15V power audio IC. It has its feeback loop inside, about 20x. But from there, there is no feedback at all from the outputs, as you can see, the output is just secondary windings floating.
Plus the nonlinearity of the mosfet, operating in non feedback.
Making more bias will help or make it alot worse due to DC? Mosfets needs high bias to be linear, dont they? But the Core doesnt like DC at all. This is confusing situation.

How about this. I put gate resistor about 1k, and put feedback from drain to gate about 10k (gain =10x), like Mr.Pass uses on Zen. That is to make sure the mosfets only working in 10x gain, not more. Will this help?
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Old 19th June 2004, 08:35 AM   #12
Eva is offline Eva  Spain
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I'm still thinking about how to add feedback without changing the topology . The main problem arises because the circuit shows non-balanced transfer characteristics and this means that the error-corrected gate driving signal required to get a balanced output will be itself unbalanced, so it can't be applied through a drive transformer. How about driving the gates from op-amps?

I think that adding local feedback to the MOSFETs will help curing the problem so I recommend trying it. It may also require to add more turns to the secondaries of the drive transformer so more drive voltage is available

About transformers and DC : The transformer only sees the sum of the DC components applied to both primeries. It sees no net DC at all as long as equal DC currents flow through both primaries but in opposite directions [they 'cancel out'], so increased biasing by itself shouldn't cause the transformer to saturate. But when DC currents aren't of the same value or don't flow in opposite directions, the net difference [bias imbalance or unbalanced distortion products] is effectively applied to the transformer. A gapped design may withstand some tens of mA of imbalance without saturating. An ungapped design may saturate witn imbalances as low as 1mA
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Old 19th June 2004, 10:16 AM   #13
Pafi is offline Pafi  Hungary
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Eva!

I agree with you at almost every points exept saturation current:

I(u,l,n)=l*B/u/n where u is permeability, l is the average length of magnetic circle.

Let's have a 200 turns primer, and a relative permeability of 10000:

Isat=0.1m*1T/10000/1,25*10^-6/200=40 mA

If You use a single 0,1mm air gap, current will be approximately

Isat=0.0001*1T/1,25*10^-6/200=400 mA (the field concentrated in the gap.)

But You can use more air gap. At 1,6mm (2*0,8mm) gap the inductance will be 30mH (assuming 10cm^2 cross-section), wich sets 20 Hz lower limit at 4 ohm.

With this gap Isat will be 6.4A, wich must be enough.

And one more thing: DC could be arises even if transfer charasteristics was perfectly balanced. Integral(-pi/2..pi/2,(cos(t)+cos(2*t))^2) is greater than integral(pi/2..3*pi/2,(cos(t)+cos(2*t))^2).

In english: let's have a specific signal with zero DC! We apply a quadratic transfer function on it, than we get a signal with DC component. (Average voltage is greater in the positive half of period, than the negative one.)

lumanauw!

The transfer function in this circuit is something like this: I=v^2 at positive, and I=-(v^2) in negative. There is no linear domain.

"How about this. I put gate resistor about 1k, and put feedback from drain to gate about 10k (gain =10x), like Mr.Pass uses on Zen. That is to make sure the mosfets only working in 10x gain, not more. Will this help?"

This can make it much better, but I'm afraid it won't be enough at higher levels.

I'm planning a similar amp, but at Class-A operating point. This makes the transfer charasteristics absolutely linear (theoretically ). It will have local feedback, and drain-coupled. You said, it's ZEN V-7R? Well, then I will build a ZEN V-7R.
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Old 19th June 2004, 12:41 PM   #14
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"But You can use more air gap. At 1,6mm (2*0,8mm) gap the inductance will be 30mH (assuming 10cm^2 cross-section), wich sets 20 Hz lower limit at 4 ohm."

Listening to a resistor are we?

A ported speaker with a big magnet may have impedance peaks of 100 ohms or more, 30mH is totally inadequate.

Cheap tube amps have similar problems for similar reasons.
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Old 19th June 2004, 02:23 PM   #15
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Hi, all,

Thanks for the opinions. All of you seems to direct to one point, that is the non-linearity of the transformer when driven by open loop mosfet (while mosfet itself is non linear).

Putting the 10k+1k resistors in drain and gate will limit the AC loop of the driving mosfet about 10x. My question is, will this lower the gain of the overall power amp? Or it will stay the same gain as original (I dont want the power amp sound tobe weak)

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The main problem arises because the circuit shows non-balanced transfer characteristics and this means that the error-corrected gate driving signal required to get a balanced output will be itself unbalanced, so it can't be applied through a drive transformer
EVA, I dont understand this. Care to explain more?

Quote:
The transfer function in this circuit is something like this: I=v^2 at positive, and I=-(v^2) in negative. There is no linear domain.
Also dont understand this. Quadratic comes from mosfet transfer function? If we replace the final transistor with bipolars, will it more linear?

Pafi, sorry for the wrong writings. It should be ZEN V7-T, not ZEN V7-R. T is the one who uses transformer primaries for CT inductance, R is still using hi-watt resistor.

Wondering, if anyone has the same problem with ZEN V7-T? Distorted piano sound?

Pafi, you seems to understand much about tranformer. How can I calculate minimum requirement (minimum inductance or resistance) for the ZEN V7-T? Mr Pass himself does not touch this in his article (can be seen in Passdiy). He just use ordinary 120V+120V of 300VA primaries. If I want to DIY this load CT inductance, not using power transformer primaries, how can I calculate the requirement (toward voltage and speaker load)? What will be the number of turns and sizes of the magnet wire?
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Old 20th June 2004, 11:28 AM   #16
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Audio transformer usually use metal iron for its core. SMPS uses ferrites. IS ferrites suitable for audio transformer (works fine up to 20hz?)
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Old 20th June 2004, 12:39 PM   #17
Pafi is offline Pafi  Hungary
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Ferrites are suitable, but you must use much bigger frerrite core then iron core, so it's absolutely not economical.

Sorry, I can't explain simply the calculation method.

I don't know what does "CT inductance" mean.

Power transformer is not a bad idea.
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Old 20th June 2004, 03:00 PM   #18
Steven is offline Steven  Netherlands
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This is a very common topology for (cheap) Public Address amplifiers that have a 100V output (Europe/Asia) or 70V output (USA). Most of them use an output transformer with center tapped primary connected to a relatively low DC supply. Often just 12/24/48V because the amplifier must also run on batteries for use in cars, boats or with emergency power supplies. The output transformer is a requirement in order to get such a high voltage output, but also for safety. If the amplifier is grounded the output is still floating, so you can safely touch ONE output terminal.
Most of these amplifiers use BJTs in the output and have feedback applied from a separate transformer winding that is grounded on one end. The transformer is gapped and in most cases that is good enough to not even bother about DC balancing. They only have a preset for bias current.

Steven
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Old 20th June 2004, 03:30 PM   #19
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There are some small but very important differences between this topology and classic PA amplifiers topology :

- PA ones use feedback so unbalance due to bad linearity and asymmetric signals is corrected to prevent transformer saturation

- PA ones drive the output transistors directly from op-amps or other transistors [DC coupled], so the unbalanced base drive currents required to correct device non-linearities with asymmetric signals are allowed

Anyway, without any DC balancing mechanism, transformer saturation is almost unavoidable when those amplifiers are driven into hard clipping with asymmetric signals
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Old 20th June 2004, 03:42 PM   #20
Steven is offline Steven  Netherlands
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Eva,

I agree with you on these issues.

Steven
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