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Old 1st November 2012, 06:09 PM   #1
willamp is offline willamp  United States
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Default output cap bypass?

Hi All,

Tried searching but didn't find anything, sorry if this is covered somewhere.

I just built an Aikido pre-amp from the 9-pin all in one kit. When I listen to it, it sounds very good but I seem to notice that I am hearing more bass than with my passive-pre that I was using. The passive pre is still in my path feeding the aikido. Suspicious that the Aikido is not increasing the bass but attenuating the treble I connected it up to my sound card to do a frequency sweep using Audio Tester software.

I do indeed see a rolloff of high frequency beginning at 3khz and down about 0.9db at 20khz. I verified the setup with a Pass B1 clone I have and it's only down about 0.1db at 20khz so seems like the setup is OK.

My question is if it's possible to add another cap in parallel with my output cap to reduce the high frequency roll-off?

I am currently using a pair of 2.2uf mundorf M Cap Supreme POI.

I don't know if this would have any affect but spice modelling does not show this roll-off. It shows freq resp flat up to ~350khz so I can't really model the situation. Seems to me the model of the output cap is probably not accurate. The other attributes such as gain and THD were modelled very accurately in my spice schem. I do realize my m-audio 2496 has limited bw so I'm not expecting see higher frequencies than in the trace.

Maybe you'll tell me this roll-off is insignificant to the sound and that's not what I'm hearing. The Aikido sounds very good.

Attached is the freq sweep and aikido schem for reference

Thanks in advance for any help/advice
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File Type: jpg aikido sweep.jpg (247.3 KB, 134 views)
File Type: jpg general_aikido_schem.jpg (46.0 KB, 132 views)
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Old 1st November 2012, 06:18 PM   #2
DF96 is offline DF96  England
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Very unlikely to be the output cap. Increasing the output cap would reduce LF rolloff, not HF rolloff.

Stray capacitance in the circuit might do it, especially if it gets multiplied by Miller effect in the first stage. The critical points are the first triode grid and anode circuits, and strays between these two points.
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Old 1st November 2012, 06:46 PM   #3
willamp is offline willamp  United States
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Quote:
Originally Posted by DF96 View Post
Very unlikely to be the output cap. Increasing the output cap would reduce LF rolloff, not HF rolloff.

Stray capacitance in the circuit might do it, especially if it gets multiplied by Miller effect in the first stage. The critical points are the first triode grid and anode circuits, and strays between these two points.
OK, I'll think about what you're saying and try to understand/analize. But wanted to mention I wasn't thinking in the direction of increasing cap value (although anything in parallel would) but instead wondering if some small value cap in the nono or pico farad range in parallel could pass the high frequencies more efficiently than the big 2.2uf PIO. I'm probably just showing my ignorance.
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Old 1st November 2012, 06:50 PM   #4
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A small capacitor across the 430 ohm cathode resistor on the bottom 6N1P might give a slight high frequency boost.
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Old 1st November 2012, 06:58 PM   #5
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Sound cards rarely have flat frequency responses so you are probably recording the intrinsic roll off of the sound card rather than the preamp.

I spent a very frustrating day working this out after I lost my good signal generator and tried to use my computer as a substitute.

Shoog
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Old 1st November 2012, 07:08 PM   #6
willamp is offline willamp  United States
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Quote:
Originally Posted by Shoog View Post
Sound cards rarely have flat frequency responses so you are probably recording the intrinsic roll off of the sound card rather than the preamp.

I spent a very frustrating day working this out after I lost my good signal generator and tried to use my computer as a substitute.

Shoog
OK, my next plan then will be to take some gain measurements at different frequencies with my scope and see If It matches the scan or not. If I do see it I can move to the input of the cap and then the output of the 1st stage to isolate.

So, you guys fell the output cap I mentioned should not be limiting high freq response then?

thanks
Phil
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Old 1st November 2012, 07:15 PM   #7
DF96 is offline DF96  England
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The output cap could only do it if it had very high inductance. Unlikely, unless it has something peculiar in its manufacture. You could check by replacing it with a cheap bog-standard capacitor.
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Old 1st November 2012, 07:21 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Looks consistent with high output impedance driving cable capacitance.

rgds, sreten.
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Old 1st November 2012, 07:22 PM   #9
willamp is offline willamp  United States
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Quote:
Originally Posted by Frank Berry View Post
A small capacitor across the 430 ohm cathode resistor on the bottom 6N1P might give a slight high frequency boost.
Thanks Frank, if I find the 1st stage to be contributing I'll look into that.
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Old 1st November 2012, 07:27 PM   #10
DF96 is offline DF96  England
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The output CF ought to be able to drive any reasonable cable.

Adding a small cathode bypass cap will hide the problem, but not solve it.
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