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Old 2nd June 2010, 01:48 PM   #71
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Dear Panson,

I see that the Musical Fidelity A1 2008 edition has a cap between the bases as well. Since I've seen this cap in all designs the the SAP and STD devices I think this is a thing you should accept. I have no experience with other darlington's, but with other tripple darlington circuits, and my experience is there is always the danger of oscillation if one not careful compensate and choose a careful layout. If the amplifier was for myself only I would't mind to let it operate on the edge and omit as much as possible capacitors. But if the design is for a commercial/DIY market, you never know what the end user will do with it under what circumstances, and then I think better be safe then sorry at a certain cost of absolute fidelity.

Btw. I forgot to askm which supply you rung the STD03's on? My experience was as well the higher the supply voltage the more risk on oscillation.

I think the high value base stoppers is no go and that should't be the solution. Those resistors highly slow down the system, and even a few ohm already degrade the subjective sound performance in my opinion. If the cap between the bases is the solution I would go for that if I was you. It also improve the switching behavior of the transistor pair.

I wonder why Musical fidelity put the extra diodes in series with the diode string. Must be to reach the Class A level bias? I have no doubt the A1 is a reliable amplifier, but according to the SAP application note, the thermal tracking get less stable with bias idling above 40mA. D If the A is a truly 20 watt Class A amplifier the bias current must be way above 40mA total Re. Do I have your permission to put this schematic online Panson, so others can give their insights as well?

With kind regards,
Bas

Last edited by Sebastiaan; 2nd June 2010 at 01:50 PM.
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Old 3rd June 2010, 01:47 AM   #72
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Hi Bas,

Please feel free to put the schematic online.

The writer of the article measured the bias current of the new A1. It is 680 mA +/- 10 mA. The new A1 is indeed a deeply biased class AB. He also mentioned the original A1 bias current equal to 800 mA.

The supply of my test amp is +/- 38 V.

Panson
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Old 3rd June 2010, 08:03 AM   #73
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Quote:
Originally Posted by Sebastiaan View Post
If the amplifier was for myself only I would't mind to let it operate on the edge and omit as much as possible capacitors. But if the design is for a commercial/DIY market, you never know what the end user will do with it under what circumstances, and then I think better be safe then sorry at a certain cost of absolute fidelity.
Hi Bas,

I definitely agree with you.

Your latest schematic shows no output coil. Is it also true for your hardware? With the output coil and damping resistor, I don't get oscillation problem. I think in practice an output coil is essential for robustness.

Panson
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Old 3rd June 2010, 03:00 PM   #74
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Quote:
Originally Posted by panson_hk View Post
Hi Bas,

I definitely agree with you.

Your latest schematic shows no output coil. Is it also true for your hardware? With the output coil and damping resistor, I don't get oscillation problem. I think in practice an output coil is essential for robustness.

Panson
Dear Panson,

Does this mean all your oscillation problems are solved now with adding the output inductor and resistor?

And I am agree with you, that in any amplifier for general purpose this coil and resistor should be included. However, the last schematic I provided is part of an active 3 way system, where two STD03 amplifiers are used for the tweeter and midrange. Because we know the load, and because there isn't a passive crossover and because of the speaker wires are really short we can omit the output inductor and resistor in this particularly case.


With kind regards,
Bas
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Old 3rd June 2010, 03:23 PM   #75
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Default Bias

Dear Panson and other readers,

Since this is a solid STD03 tread, I think this stuff is also useful for other readers. I've seen many other STD03 DIY and commercial designs where the pre bias isn't right, or get measured the wrong way. This degrade the stability and thermal tracking. You can change the idle bias more forgiving, but the pre bias must be a truth fixed 2.5mA and as precise as possible.

It is hard to measure exact current, so here my way to measure and to be sure the pre bias current is 2.5mA

Who reads the data-sheet carefully knows that the diodes in the STD03N will have a forward voltage of 705mV by a forward current of 2.5mA

The diodes in the STD03P will have a forward voltage of 1540 by a forward current of 2.5mA.

So to set the pre bias on exactly 2.5mA do as follow:

1: Set the pot-meter (the one between the diode string) for the idle bias all the way to 0 ohm
2: Measure the voltage across the two bases of the STD03N/P
3: The reading should be 2.25VDC
4: Now after warming up adjust the idle bias current to 40-60mA.

It is very advisable to make the pre bias adjustable with a potential meter to avoid variations.

With kind regards,
Bas
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Old 4th June 2010, 12:36 AM   #76
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Bas,

Quote:
Originally Posted by Sebastiaan View Post
I see that the Musical Fidelity A1 2008 edition has a cap between the bases as well. Since I've seen this cap in all designs the the SAP and STD devices I think this is a thing you should accept.
The whole point of the diode stack is to apply a constant voltage between the bases. "Constant" is a relative term here as everybody knows (or should know) that the voltage across a diode depends on temperature. But as the Vbe's also vary with temperature, the "constant" voltage across the diodes set up a constant quiescent current in the output transistors.

However, the diodes aren't perfect. They have some dynamic impedance and other non-idealities that prevent them from being perfect, ideal voltage sources. As a result, the audio signal applied at the base of the output transistors will cause a little bit of AC current to flow in the diodes. Recall, that an emitter follower has a voltage gain of slightly less than unity. Thus, the voltage applied at the base is roughly equivalent to the output voltage. This is pretty big compared to the 2.5-ish volt across the diode stack. So... The current through the diode stack varies slightly (both from the diode non-idealities, and from the fact that an increasing load current on the amp output will cause higher base current to flow --> less current available for diode stack) and, hence, the bias voltage across the diode stack varies slightly as function of the input signal voltage vs time. This means the quiescent point for the output devices changes ever so slightly as the input signal changes. It should be fairly intuitive that this has the potential for causing all sorts of harmonic mixing and, thus, harmonic distortion.

The 10 uF cap from base to base (across the diode stack) makes it so that the bias voltage between the bases doesn't change significantly when audio frequencies are applied to the amp input. This does lower the THD. I measured about an order of magnitude reduction in THD with/without cap. I.e. from 0.0x % THD without cap to 0.00x % THD with the cap in place.

As far as I know the cap doesn't do anything for the overall amp stability. It may help a little with bias thermal stability, but that's really a guess. I chose to add the cap because of the 10-fold reduction in THD.

~Tom
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Old 4th June 2010, 12:47 AM   #77
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Quote:
Originally Posted by tomchr View Post
Bas,



The whole point of the diode stack is to apply a constant voltage between the bases. "Constant" is a relative term here as everybody knows (or should know) that the voltage across a diode depends on temperature. But as the Vbe's also vary with temperature, the "constant" voltage across the diodes set up a constant quiescent current in the output transistors.

However, the diodes aren't perfect. They have some dynamic impedance and other non-idealities that prevent them from being perfect, ideal voltage sources. As a result, the audio signal applied at the base of the output transistors will cause a little bit of AC current to flow in the diodes. Recall, that an emitter follower has a voltage gain of slightly less than unity. Thus, the voltage applied at the base is roughly equivalent to the output voltage. This is pretty big compared to the 2.5-ish volt across the diode stack. So... The current through the diode stack varies slightly (both from the diode non-idealities, and from the fact that an increasing load current on the amp output will cause higher base current to flow --> less current available for diode stack) and, hence, the bias voltage across the diode stack varies slightly as function of the input signal voltage vs time. This means the quiescent point for the output devices changes ever so slightly as the input signal changes. It should be fairly intuitive that this has the potential for causing all sorts of harmonic mixing and, thus, harmonic distortion.

The 10 uF cap from base to base (across the diode stack) makes it so that the bias voltage between the bases doesn't change significantly when audio frequencies are applied to the amp input. This does lower the THD. I measured about an order of magnitude reduction in THD with/without cap. I.e. from 0.0x % THD without cap to 0.00x % THD with the cap in place.

As far as I know the cap doesn't do anything for the overall amp stability. It may help a little with bias thermal stability, but that's really a guess. I chose to add the cap because of the 10-fold reduction in THD.

~Tom
Thank you Tom for this good explanation. It makes me understand the function of the cap. Did you also experiment with different values?

With kind regards,
Bas
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Old 4th June 2010, 08:04 AM   #78
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Quote:
Originally Posted by Sebastiaan View Post
Does this mean all your oscillation problems are solved now with adding the output inductor and resistor?
Yes, it does. I can remove all caps (cap across B pins, cap across C and B) now.

Quote:
Originally Posted by tomchr View Post
Recall, that an emitter follower has a voltage gain of slightly less than unity. Thus, the voltage applied at the base is roughly equivalent to the output voltage. This is pretty big compared to the 2.5-ish volt across the diode stack.
We should look at the voltage across the diode stack or differential voltage across base pins instead of the base to ground voltage. The differential voltage across base pins (or across the diode stack) is nearly constant. It can be verified by a DVM in AC mode measuring the base to base AC voltage. The amp input can be 1 kHz generated by a PC sound card. You won't see the level comparable to the output signal. You may see mV over there.

Quote:
Originally Posted by tomchr View Post
However, the diodes aren't perfect. They have some dynamic impedance and other non-idealities that prevent them from being perfect,

Dynamical resistance of a forward bias diode is less than one ohm. For instance, a 1N4148 dynamic resistance at 2.5 mA bias point is about 50 mR. The 03N/P diode dynamic resistance will be more or less the same. This tiny resistance will not cause any bias point variation during operation. Even it does. It will be negligible compared to that caused by the trim pot, thermal tracking error.

A 10 uF cap impedance is 16 Ohm at 1 kHz and 0.796 Ohm at 20 kHz. It is higher than the dynamic resistance of the diode stack.

In my test unit, I don't see any difference between using and not using a cap (470 uF || 0.1 uF) across the base pins (diode stack and trim pot included).
Attached Images
File Type: jpg THD+N vs freq for cap across base pins.JPG (106.2 KB, 661 views)

Last edited by panson_hk; 4th June 2010 at 08:20 AM.
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Old 4th June 2010, 08:56 PM   #79
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Quote:
Originally Posted by Sebastiaan View Post
Thank you Tom for this good explanation. It makes me understand the function of the cap. Did you also experiment with different values?
I did not experiment with different values, I just slapped 10 uF on there and called it good.

Bottom line is that the cap needs to make the voltage across the diode stack constant at audio frequencies. So figure it should set a pole no higher than 1~2 Hz.

The input impedance of the bipolar transistor should be on the order of (beta)*Re. With two pairs of transistors in parallel, this becomes:

8000*0.22*0.5 = 880 ohm

2 Hz cutoff: 1/(2*pi*R*C) < 2 Hz --> C(min) = 1/(2*pi*880*2) = 0.90 uF.

Note that I'm using the high-beta versions of the devices. The minimum beta is about 8000. The factor of 0.5 is because the input impedance of the two pairs in parallel is 0.5x that of just one transistor.

~Tom
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Old 4th June 2010, 09:04 PM   #80
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Quote:
Originally Posted by panson_hk View Post
In my test unit, I don't see any difference between using and not using a cap (470 uF || 0.1 uF) across the base pins (diode stack and trim pot included).
Odd... I included the cap because I wanted the voltage across the diode stack to remain constant. I've also seen commercial designs (Parasound A23) that used the cap. Theory says the voltage should be constant. Good commercial guys put it there... So I put it in there. You don't like it? Take it out... Easy... It's not mission critical.

Thanks for sharing your data, though. Data (usually) doesn't lie.

~Tom
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