Why the treble loss?

[stonerbox]

I get treble loss after the recovery stage, why is that? LTSpice says the impedance at 10kHZ is 1.6M and then gently sloping down to 800k at around 20kHz.

[Rob Strand]

I get treble loss after the recovery stage, why is that? LTSpice says the impedance at 10kHZ is 1.6M (then gently sloping down to 600k at around 20kHz).

The 20k+80k+1n load on the 100k collector resistor will cause a low-pass shelf.  Perhaps that's what you are hearing/seeing.   I suspect that is unintentional as the 1n is probably added to *stop* HF roll-off.

The output impedance of the transistor stage needs to be low compared to the load to prevent the shelf effect.

[fryingpan]

What are you trying to do? I see a HPF up top and a simple boost down below.

[stonerbox]

Rob, thank you! Low it is then. Back to calculations.

Fryingpan, it is a frequency splitter / bass shelf boost + an output recovery stage + 100k pot (R42+R43) with a huge treble bleed (1nF).

[fryingpan]

Gentlemen, I am sorry for not fully explaining what is going on here. The 1N (treble bleed) + 100k Pot (R42+R43) was put in place in order to get some of the highs back on low volumes but I am hoping to remove that when the treble loss is fixed.




No matter if the output i hooked up to a Hi-z interface of an amp it looses a lot of top end. When I tap the audio directly from the filter everything sounds crips and clear.

If you take the signal out from the transistor's output, with just a blocking cap, what do you get?

[merlinb]

LTSpice says the impedance at 10kHZ is 1.6M and then gently sloping down to 800k at around 20kHz.

The impedance of what? Those numbers don't make a whole lot of sense

[PRR]

impedance at 10kHZ is 1.6M and then gently sloping down to 800k at around 20kHz.

The impedance of what? Those numbers don't make a whole lot of sense

I dunno what SPICE is doing, but those numbers say "10pFd" to me.

[antonis]

I dunno what SPICE is doing, but those numbers say "10pFd" to me.

Maybe SPICE simulates a pach cable..

[stonerbox]

impedance at 10kHZ is 1.6M and then gently sloping down to 800k at around 20kHz.

The impedance of what? Those numbers don't make a whole lot of sense

I dunno what SPICE is doing, but those numbers say "10pFd" to me.

Currently sick and stuck in front of the tube a lot, hence the late response.

That is the output impedance of the last stage, measured at BC549c collector leg (R38/C29).
Upon running the same res. values again, LTspice first crashed, then gave me completely new results. First session must have bugged out completely and given me false readings. The fresh impedance sim reads 300k at 100Hz (big hump), 37k 10kHz then down to 9k at 20kHz.



Updated with increased current. 550k at 100Hz (big hump) and 22k at 10kHz then down to 6.2k at 20kHz

[fryingpan]

...that's not how you read output impedance. 

To read output impedance, you need to turn off any voltage sources (LTSpice does this for you when it's DC, but you need to turn Vs off too) and actually feed a signal *after* the last stage to measure the output impedance, and measure the current.

[stonerbox]

GET OUTTA HERE! 



I was extremely suspicious to the results but wanted the jury's opinion before I went any further.

[fryingpan]

GET OUTTA HERE! 



I was extremely suspicious to the results but wanted the jury's opinion before I went any further.

Like so.

[antonis]

No need for precise measurements..

CE amp output impedance is roughly its Collector resistor value..
(actually, a bit less due to slight domination by parallel Collector internal resistance..)

[m4268588]

V(OUT)/I(C29)
It matches with @antonis

https://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws#Kirchhoff%27s_current_law

[m4268588]

I get treble loss after the recovery stage, why is that?

2nd order HPF

[Rob Strand]

To read output impedance, you need to turn off any voltage sources (LTSpice does this for you when it's DC, but you need to turn Vs off too) and actually feed a signal *after* the last stage to measure the output impedance, and measure the current.

I often inject a 1mA AC *current source* into the output then read the voltage off directly in kohm.

Not to detract from antonis's result.  When you have feedback biasing the output impedance is usually much lower than the collector resistor.



Also the output impedance depends what what is at the input:  open, grounded, or a input series resistor (often used to set the gain) will change the output impedance.  A shorted input and no series resistor will make the output impedance equal the collector resistor (well that in parallel with the feedback resistor, but that's usually high).  A series input cap will make the output impedance change with frequency.


Here's an example: (image server was playing up)


[repost]

[antonis]

When you have feedback biasing the output impedance is usually much lower than the collector resistor.

1+ agree..!! 

A shorted input and no series resistor will make the output impedance equal the collector resistor

On the other extreme, an open input makes Base considered virtual ground (well, hardly but let it be..) so output impedance is made equal the equivalent impedance of Collector resistor in parallel with feedback resistor divided with stage's gain plus unity..
(the later due to Miller's theorem)

 

[amptramp]

The feedback resistor will also reduce the input impedance of the amplifier.  It has the opposite effect  to the rise in impedance from bootstrapping.  The input coupling capacitor should be increased accordingly.