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Old 14th January 2005, 05:06 PM   #21
Jennice is offline Jennice  Denmark
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Correction: Maybe I should have written that it's not a dumb, constant DC source, but one which adjusts with temperature, but that is beside the point here. In terms of audio frequencies it's to be regarded as a DC source, as the frequencies / time-constants of the thermal system are way below anything audible, unless you have too small (or no) heat sinks. If so, you have other concerns than the resonance frequency of the thermal system.

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Old 14th January 2005, 05:13 PM   #22
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by AndrewT
Hi,
Mr evil, what effect (better or worse) does the shiklai or darlington Vbe have on the sound quality?
Regards Andrew T.
The difference is too small to be audible (by me at least). The lower impedance will reduce distortions caused by the finite, nonlinear input impedance of the output devices. You may be able to observe the effects of this on a 'scope at high frequencies. It will maintain a low impedance across a wider bandwidth so you can use a smaller capacitor in parallel with it. Also, the temperature coefficient will go up, which may help or not, depending on the design. I just have a thing for using Sziklai pairs wherever possible
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Old 14th January 2005, 06:22 PM   #23
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Jennice, you have missed the point slightly The issue is that the leads for the Vbe transistor induce additional, unwanted, capacitance into the VAS, potentially mucking up it's frequency/gain response. Just have a quick look at the Leach website, it's explained in clear detail
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Old 14th January 2005, 06:35 PM   #24
Jennice is offline Jennice  Denmark
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I can't find what I think you want me to find. Can you provide a link / reference which shows Leach's web page and description of this issue?

Jennice
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Old 14th January 2005, 07:30 PM   #25
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See "The VBE Multiplier Bias Circuit"

http://users.ece.gatech.edu/~mleach/.../2ndstage.html
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Old 15th January 2005, 09:38 AM   #26
Jennice is offline Jennice  Denmark
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Quote:
Originally posted by richie00boy
Jennice, you have missed the point slightly The issue is that the leads for the Vbe transistor induce additional, unwanted, capacitance into the VAS, potentially mucking up it's frequency/gain response. Just have a quick look at the Leach website, it's explained in clear detail
Richie,

No, I don't think I have missed the point.
On the contrary, someone else has missed the sense of realism.

I have read the referenced page, but I do not agree with Leach on this. If he had been brave enough to give some values of the resistors, it would be even clearer. However, I hope I can illustrate my point for you regardless:

The current through the resistors on figure 10 (the referenced link) gives a practical top limit for the resistor values.

Also, P1+R27 need a reasonable value relative to R25+R26.

Also, a high resistance level lovers the possible base current, and the circuit becomes more sensitive to RF sources, as high-impedance circuits have less immunity to radiated fields.

This part of the circuit (which uses 4 diodes) is not protected by the capacitor to stabilise the Vbe voltage, but it can pick up noise.
This noise on the bias-side of the resistor can, at worst, modulate Vbe with the picked-up noise! Does that sound like a tempting concept to you?

Good isolation of the circuit-to-ground capacity can only be obtained with high resistor values. This raises noise sensitivity from fields. This requires a high-gain transistor to compensate for this lower bias current, too!

Isolating the capacitance effect can only happen if the resistors "absorb" the error voltage. This means that he potentially modulates the base current even further.

Also, please explain to me, how acuate tracking / temperature compensation can be obtained with his circuit.

Leach writes:
>>>The wires which run from the circuit board to the transistor exhibit capacitance to ground which can affect the high-frequency response of the second stage.

At worst, this could cause oscillation problems. With the diodes on the heat sink, resistors on the circuit board can be used in series with the wires to isolate this capacitance from the second stage.<<<

Comments:
So, Leach decides to trade-in noise-sensitivity for the placement of the transistor. Reason: capacity from the Vbe circuit to ground, caused by the cables.

I wonder if he ever took time co calculate this capacity?`I am beginning to doubt it!

A length of 10 cm wire, 1 mm diameter, run in parallel with, and 1 cm from reference ground, creates about 0.27pF of capacity.

There are A LOT of issues that cause more concern for me than a 0.27pF stray capacity. Leach writes that this, at worst, could cause oscillation problems.
Maybe it's time for a reality check !!!
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Old 15th January 2005, 02:14 PM   #27
sajti is offline sajti  Hungary
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I planned bias regulator circuit attached, which keeps the bias constant.
The R1 and R2 is the emitter resistor of the amplifier. The collector and the emitter of the optocoupler should be connected to the existing bias multiplier.
Set the maximum bias current on the existing Vbe multiplier, and the circuit will reduce it, and keep it on the value ,You set by R10.
Sorry for the quality of the schematic, I just put it together right now.

Any comments?

sajti
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Old 15th January 2005, 03:43 PM   #28
Jennice is offline Jennice  Denmark
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Unless you have high currents through R1 and R2, my main concern is the off-set error voltage in the op-amps. If the voltages over R1 and R2 are low, the off-set error voltages of the op-amps can become significant.

Jennice
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Old 15th January 2005, 05:13 PM   #29
sajti is offline sajti  Hungary
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Quote:
Originally posted by Jennice
Unless you have high currents through R1 and R2, my main concern is the off-set error voltage in the op-amps. If the voltages over R1 and R2 are low, the off-set error voltages of the op-amps can become significant.

Jennice
Yes! I draw TL074, because it was easy to find in the Eagle database. But with low offset opamp, and some gain in the first stage too can helps to avoid this problem.
I will plan all the details for this circuit.

sajti
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Old 15th January 2005, 08:25 PM   #30
sajti is offline sajti  Hungary
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Quote:
Originally posted by sajti
I planned bias regulator circuit attached, which keeps the bias constant.
The R1 and R2 is the emitter resistor of the amplifier. The collector and the emitter of the optocoupler should be connected to the existing bias multiplier.
Set the maximum bias current on the existing Vbe multiplier, and the circuit will reduce it, and keep it on the value ,You set by R10.
Sorry for the quality of the schematic, I just put it together right now.

Any comments?

sajti

I made mistake in the schematic. Mixed the + and the - input on the 3rd opamp.
I will post the new and simplified schematic!

sajti
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