optimizing the VBE multiplier

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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.

Jennice
 
AndrewT said:
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:)
 
richie00boy said:
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 !!!
 
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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Jennice said:
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
 
sajti said:
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


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

sajti
 
Point of interest: The Vbe multiplier affects amplifier sonics, not so much of itself as it's a DC voltage source, but via its bypassing capacitor, the size and quality of which is very important, particularly in the ubiquitous Class AB amplifiers most of us use.

Too small a cap will cost strong, tight bass. The wrong cap type will confer a 'grainy' sound on the amp, as the top and bottom halves of the output stage are out of step with one another. You want something with good frequency response and very low dielectric absorption, less than 0.05.

Essentially, all caps in an audio amplifier will have huge effect on the sonics. Use the best you can find everywhere and LISTEN HARD! Even the feedback shunt electro has strong influence on the sonics. Differences in amps all too often can be traced to capacitor choices.

Cheers,

Hugh
 
AKSA,

That sounds reasonable...
You say that :
>>>Even the feedback shunt electro has strong influence on the sonics.<<<

I would think that especially the feed-back capacitor has influence. After all, the main circuits may create errors, but the feedback path is the only part present to correct these errors.

I know that one should try to avoid errors in the first place, but since audio circuits aren't 100% linear, special attention should be given to the feed-back part of the circuit. Obviously, this does not count for non-feedback amplifiers, but then again, how many amps do not have some sort of local or global feedback?


Jennice
 
Sajti,

Have you considered the turn-on and turn-off situations?
What hapens until the circtut has stabilized? Shouldn't you have some diodes or other means of creating a max limit of bias current?

The way I see it, the circuit starts with the opto-coupler totally off, which would destroy the output stage, unless you make some sort of limiter for bias current, or a max Vbe (same issue).

Jennice
 
Jennice said:
Sajti,

Have you considered the turn-on and turn-off situations?
What hapens until the circtut has stabilized? Shouldn't you have some diodes or other means of creating a max limit of bias current?

The way I see it, the circuit starts with the opto-coupler totally off, which would destroy the output stage, unless you make some sort of limiter for bias current, or a max Vbe (same issue).

Jennice

That is good comment!
This circuit is planned to apply in existing amplifier. The amplifier contains the original Vbe multiplier, which helps to avoid the switch on overcurrent. Just set the Vbe multiplier for higher current (400-500mA), and this circuit will reduce it to the level, set by R10.
If You want to apply into brand new amplifier just put some resistor, or diode chain for switch on limitation. But use 5 diode for double darlington, and 7 for triple. This limits the switch on current on high level for the first few seconds.

sajti
 
Point of interest: The Vbe multiplier affects amplifier sonics, not so much of itself as it's a DC voltage source, but via its bypassing capacitor, the size and quality of which is very important, particularly in the ubiquitous Class AB amplifiers most of us use.

Too small a cap will cost strong, tight bass. The wrong cap type will confer a 'grainy' sound on the amp, as the top and bottom halves of the output stage are out of step with one another. You want something with good frequency response and very low dielectric absorption, less than 0.05.

Essentially, all caps in an audio amplifier will have huge effect on the sonics. Use the best you can find everywhere and LISTEN HARD! Even the feedback shunt electro has strong influence on the sonics. Differences in amps all too often can be traced to capacitor choices.

Cheers,

Hugh


:D

this time your talking....:D

AKSA:

if you don't mind, do you have any recommendations on what type of caps best suite for this stage? or maybe an exact value
of the caps your using too with your kits.


regards,
:D


macweb
 
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