Dx Blame ST together Dx Super A

[destroyer X]

using simulator and i have finished to adjust listening....it is playing and i do think anything can be better.... to make it better i have to replace the band that is playing, the audio source.

How can something be better than the perfection?

Soon you gonna have pcboard layout...maybe Zimmer will offer from Brasil some pcboards, but for sure you can make it using the pcboards you already have from the Dx Blame ST and the Supercharged multimodel blue boards.

regards,

Carlos

[destroyer X]

This bothers a lot when we go adjusting as capacitors opposes to voltage variations, it charges and discharges and disturb a lot.

Remove it or install a 100n in place as low values will not bother and will procceed the suck out action too.

regards,

Carlos

[AndrewT]

That capacitor is usually there to minimise the AC impedance between the bases.

That minimised impedance then maintains a very low AC voltage between the bases and that allows the output transistors to perform their function.

yes the cap can be varied over a wide range of values.
Some optimisation will probably benefit the amplifier.

But your statement,

capacitors opposes to voltage variations, it charges and discharges and disturb a lot.

is nonsense. The capacitor holds a charge set by the dc current through the 180r. The AC signal adds, or subtracts, to that charge when the capacitor is too small to pass the AC signal. When the capacitor has low AC impedance for the frequency being processed the variation in charge across the capacitor is the AC current times the AC impedance. That AC voltage must be low, otherwise the capacitor is too small for the signal being processed.

Remove it

cannot be the correct solution.

Measure the voltage across the resistor/capacitor combination, when the amplifier has no input signal.
Measure the DC voltage and the AC voltage across the resistor/capacitor combination, when a signal is passing.
A scope would be better for measuring this but neither end of the combination is at Earth Potential and this makes for skilled scope use to get a result.

[magnoman]

Andrew

One purpose of that capacitor is provide an ac low impedance path from say the top output transistor base (npn) to the bottom driver transistors emitter (pnp). The necessity from needing to quickly turn on the upper transistor (sucking out excess base charge) following a large signal positive excursion, by using the active bottom driver.
This action will result in changing the voltage across this capacitor which will then change the effective bias and idle current across the output transistors.
The delta voltage and time required to return to the nominal bias is dependant on the capacitor value, so you end up balancing cross-over distortion (not enough capacitance to turn-off the upper transistor fast enough) and resulting idle current change (to small a capacitor) and the recover to nominal idle current (too large a capcitor).
Same effect on capacitors which shunt any vbe multiplier except for the difference in how these are charged or discharged during large signals.

These two effects made it really hard for me to measure the heating effects of the output transistors following a short large signal response (I was trying to measure the idle current as an indicator of output transistor temperature change).


Thanks
-Antonio

[destroyer X]

I use to check things into the workbench, with soldering iron in my hands..and there these effects comes true.

Simulator said it was lovely there...that reduced THD..but real life it was driving me crazy... i am returning to my traditional, standard, 100n suck out capacitor.

regards,

Carlos

[vargasmongo3435]

Mister Carlos so it will be back to 100nF or 220nF ?

Regards
Juan

[AndrewT]

Originally Posted by magnoman

................. The necessity from needing to quickly turn on the upper transistor (sucking out excess base charge) following a large signal positive excursion, by using the active bottom driver.
This action will result in changing the voltage across this capacitor which will then change the effective bias and idle current across the output transistors.
The delta voltage and time required to return to the nominal bias is dependant on the capacitor value, so you end up balancing cross-over distortion (not enough capacitance to turn-off the upper transistor fast enough) and resulting idle current change (to small a capacitor) and the recover to nominal idle current (too large a capcitor).
Same effect on capacitors which shunt any vbe multiplier except for the difference in how these are charged or discharged during large signals.

These two effects made it really hard for me to measure the heating effects of the output transistors following a short large signal response (I was trying to measure the idle current as an indicator of output transistor temperature change)..............

I understand the mechanism you describe.
How does one test for the optimum capacitor value?

The optimum is >> 0pF
How much greater?
Presumably it varies with output devices and with output topology and with stage stability. Roender damped the Cap with 0r82 with good reason, not because he thought it looked pretty. I think it confirms that stability does come into the optimisation.

[PETR NICONOV]

Respected vargasmongo3435
Doubt on the connection 680 ohm resistor (R32) in your design PCB layout, which reported 156.
One end of the resistor 680 ohm is connected to collector (C Q8), but on the scheme necessary to the base (B Q8).
Regards PETR NICONOV

[destroyer X]

these last years.

I am listening the unit... having fun.

Dx Super A - Magnificent. - YouTube

regards,

Carlos

[destroyer X]

Yes, it is...but when you connect to speaker it does not look flat anymore...the sonic signature appears.

It looks that has a gain above 10 kilohertz, and it is not a small gain, in a graphic equalizer it is alike to increase 6 dB all frequencies above 10 kilohertz.

What gonna happens?

People not that skilled will say it has lack of bass...negative!..bass is there, and very powerfull, and moving the speaker and pumping huge ammounts of air, but the high end treble looks so strong that we feel the amplifier has not bass...in the reality, when connected to speakers, it reproduces treble very well an this looks, by comparison that we have no bass.

This is a matter of our reference point.... we can say has not bass or we can say has a lot of treble... depends on your mind precision.

regards,

Carlos