[Joe Garfield]

I want to make inline attenuators for my Modi/Vali stack as there is channel imbalance below 8:00 and my max listening volume is 9:00 with either gain setting.

I don't know if I should use an L or Pi network, and don't really understand how to match resistors to impedances or why direction/location matters here. I tried a couple different online calcs and some say to use two resistor values (R1 + (2x) R2 ) and others say to use three values (R1 + R2 + R3).

Modi output = 75 ohm
Vali input = 50k ohm

Attenuation: calcs say 35dB is minimum I can do? The pre-made inline attenuators come in 12dB and supposedly work OK for this stack. I might prefer closer to 24dB.

Any help is appreciated. Thank you.

 

[MindsMirror]

I would make it an L which is just a simple voltage divider. You don't really need to worry about transmission line effects or impedance matching in audio signals which are relatively low frequency. Just make sure the value of the resistor network is greater than the source and less than the load. I would put it geometrically in the middle which is R1+R2 = 2K.

24dB is ~16x attenuation, so R1+R2 = 16*R2, you get values of R1 = 1875 and R2 = 125. That would give you 16x attenuation assuming you have zero source impedance and infinite load impedance. The source and load will change the attenuation slightly, but you don't need it to be exactly 24dB so that's perfectly fine. You won't get resistors with those exact values either which is also fine.

I think 24dB is probably more than you need. You could use a software volume control which shows values in dB to find out how much attenuation would work best before buying any physical components. The Vali gain switch changes the gain by 12dB, so a 12dB attenuation will make high gain like low gain was, and low gain would be another step down. 24dB attenuation would be another step down from that.

 

[Joe Garfield]

Thank you!

Yeah I tried using gain switch and I was thinking it’s about half the attenuation I’m looking for, so I’ll try ~18dB/8x.

 

[Tjj226 Angel]

If you are using a vali, you probably just have an issue with your tube.

Before you go developing some volume control, I would try to go on ebay and hunt down a cheap replacement tube with matched channel balance and slap that in the vali.

 

[Joe Garfield]

If you are using a vali, you probably just have an issue with your tube.

Before you go developing some volume control, I would try to go on ebay and hunt down a cheap replacement tube with matched channel balance and slap that in the vali.

Do you say that because I mentioned channel imbalance, or does tube channel balancing actually affect overall volume output on this amp? The channel imbalance below 8:00 is normal and expected for the Vali.

 

[Tjj226 Angel]

In 99% of tube amps and hybrids that I know of, the tubes do actually amplify the sound. If the tube isn't well balanced, then it doesn't amplify the signal equally and you end up with channel imbalance. The higher the volume, the more apparent the difference in side to side balance should be,

I can't say for absolute certainty that the vali works the same way. For all I know the tubes could be used as a untiy gain buffer to feed the BJTs in which case the tube's internal balance shouldn't have that much of an audible impact.

Now replacement tubes are literally a few bucks. You can get a decent 6bq7 for pennies on the dollar. It might be worth it to get an internally matched tube and test it out before you go make a channel balance attenuator. Worst case, you might have a decent backup tube. Best case, it fixes your problems.