Partial cathode bypass to effect a treble boost.

[Shoog]

Hi there all,
My current amp is shown below.
As it is it sounds very good, but I am noticing a slight loss of cymbal sparkle which I associate with a falling response with the interstage and the none linearity of the 6080's.
I can try to compensate for this by adding feedback around the interstage transformer. This would be tricky because any useful value of feedback resistor will unbalance the load on the output transformer with unpredicatable results.
The second option would be to add gNFB. I don't really want to do this because it is liable to introduce instability.
The third option I thought of was to partially bypass the cathode resistors of the 5687 in order to introduce a rise starting at about 15khz, where I suspect the response starts falling (unfortunately my scope is down and I can't test this).

The questions are 1)will it work in practice 2) can anyone suggest a suitable value of cap to start experimenting with, or the formula that I can use to calculate the value. I have seen some formula's - but I'm not certain if they apply to this situation.

All help appreciated.

[dsavitsk]

To find the 3db point, consider Rk in parallel with (Ra + ra)/(mu + 1) and plug this value in for R in (F = 1/(2 * pi * C * R)).

Guitar amps do this all the time to make the amp brighter, but how it will work for hi-fi is anyone's guess.

[aerius]

I tried this with my headphone amp some time ago to compensate for the falling treble response of my headphones, then I fiddled with my headphones some more and got the response straightened out enough so I could put the amp back to flat.

The good news is that the amp tweak does work, the treble was lifted nicely and the cymbal sparkle and so forth was back. The bad news is it did wonky things to the soundstage, the depth got flattened a bit and imaging was a bit fuzzy. It was a trade-off I was never really happy with which was why I went back to fiddling with my headphones.

As for the formula, I used the one on this page as a starting point and tuned it by ear from there. I think I ended up with a larger value than calculated, but I'll have to dig out my notes to confirm it.

[Shoog]

The good news is that the amp tweak does work, the treble was lifted nicely and the cymbal sparkle and so forth was back. The bad news is it did wonky things to the soundstage, the depth got flattened a bit and imaging was a bit fuzzy. It was a trade-off I was never really happy with which was why I went back to fiddling with my headphones.

As for the formula, I used the one on this page as a starting point and tuned it by ear from there. I think I ended up with a larger value than calculated, but I'll have to dig out my notes to confirm it.

Thanks for that. Its just what I was looking for - a simple explanation of how to do the calculations.
I had my suspicions that it might have the effects you described. I imagine its because it introduces some element of phase shift. I will have to experiment and see if i can live with the trade offs. Fortunately I think the roll off is sufficiently high that the adjustment wont be to noticable.

Shoog

[Shoog]

Hi there,
Status report. Got it working. I started out with a knee at 8khz using a 0.1uf cap. Realised I needed to be using the point where the treble lift rolls off. In the end I am using a 5nF cap which just adds sparkle back in without significantly altering the overall response.

Shoog