Differences between Low and High inputs in theory

[pdf64]

Well, to summarise post #11, the HI input is louder and brighter than the LO input!

[Tom Phillips]

Maybe someone can take a look at this Laney Lionheart schematic and see if there is anything special going on in the inputs? I don't understand all that stuff

The cathode bypass cap, C16, on V1A is grounded through a switching contact in the LO Input jack. When you plug into the LO Input jack the switching contact opens effectively removing C16 from the circuit which lowers the gain of V1A.

[andrius]

I'm sorry, I somehow missed the schematic that pdf64 provided.

So that means that LOW input on the Lionheart is kind of a normal input here and High input adds a treble / mid boost? I'm confused: does it add gain or not? Because it feels to me as if it does...

[Chuck H]

Actually, with the typical Fender input arrangement the Hi input only sees 34k series resistance (two 68k resistors in parallel), the Lo input sees 68k. Same with Marshall.

Actually, the load is also part of the formula. In the typical arrangement the load changes from 1M to 68k.

Besides, as it happens the amp in question doesn't use the typical arrangement.

[Tom Phillips]

...So that means that LOW input on the Lionheart is kind of a normal input here and High input adds a treble / mid boost?...

You could say that but the meaning of “Normal” will vary from person to person.
The low input configures the input triode stage with no cathode bypassing. The gain of the stage will be relatively flat across the frequencies we are concerned with.

... does it add gain or not?...

Yes but not across the whole frequency band. When the 680nF cathode bypass cap is in the circuit the gain of the stage is ~5dB higher above ~110Hz.

[J M Fahey]

Actually, with the typical Fender input arrangement the Hi input only sees 34k series resistance (two 68k resistors in parallel), the Lo input sees 68k. Same with Marshall.

Sorry but no.
The Hi input sees 34 or 68K (depending on jack wiring) *in series* with 1M to ground, so the 1M becomes the dominant part of input impedance.
For the guitar pickup, being loaded with 1034K or 1068K is the same.
Now, the Lo input, shows 136K with 1M in parallel, around 120K.

[andrius]

You could say that but the meaning of “Normal” will vary from person to person.
The low input configures the input triode stage with no cathode bypassing. The gain of the stage will be relatively flat across the frequencies we are concerned with.


Yes but not across the whole frequency band. When the 680nF cathode bypass cap is in the circuit the gain of the stage is ~5dB higher above ~110Hz.

The thing is that I hear a big difference in volume between High and Low on the clean channel. Is that volume difference caused solely by boosting high+mid frequencies?

[Wombaticus]

Sorry but no.
The Hi input sees 34 or 68K (depending on jack wiring) *in series* with 1M to ground, so the 1M becomes the dominant part of input impedance.
For the guitar pickup, being loaded with 1034K or 1068K is the same.
Now, the Lo input, shows 136K with 1M in parallel, around 120K.

Almost right, but not quite...

In the case of the low input, if the 1Meg were on the *other side* of the grid stopper, it would be 68K + (1Meg || 68K) = approx. 132K, as it would be in parallel with only one of the stoppers.

As it actually is though, the 1Meg is on the input side of the stopper, so is effectively shorted in low input mode... low mode input impedance is therefore 136K.

[pdf64]

See calculator Cathode Bypass Capacitor Calculator
With the values from your amp, with the HI input it works out to graph below; so with -3dB about 200Hz, the boost extends down to low mids / high bass.
Using the LO input, the response of that stage would be pretty much flat across the guitar range, at about 27.5dB.

[hasserl]

The Hi input sees 34k...*in series*...

That's what I said.

[Tom Phillips]

The thing is that I hear a big difference in volume between High and Low on the clean channel. Is that volume difference caused solely by boosting high+mid frequencies?

Yes. You've got it. It's been said several different ways above including how you just stated it.

[Chuck H]

That's what I said.

Yep. It is. You said that when you were trying to correct me.

When an input is referred to as "high impedance" it's a reference to the load. Not the series resistance. How did that even become an issue? Since this is standard practice, and the "low" input in the standard type arrangement has a lower load impedance, it's a lower impedance input. Series resistance not withstanding.

[Waster]

Does the difference come from more of the signal going straight to ground instead of the grid with the low impedance input?

[hasserl]

Yep. It is. You said that when you were trying to correct me.

When an input is referred to as "high impedance" it's a reference to the load. Not the series resistance. How did that even become an issue? Since this is standard practice, and the "low" input in the standard type arrangement has a lower load impedance, it's a lower impedance input. Series resistance not withstanding.

I was trying to clarify the truth of the situation. Usually people say thank you for doing that.

The inputs weren't identified as high impedance, they were identified as low and high. The impedance was mentioned during the explanation.

I am a bit surprised at the response by you and Juan, and don't really understand the apparent animosity.

[Chuck H]

It's all good. I was working with the standard nomenclature (which is the load) and you were working with the series impedance. As to the actual circuit impedance niether is correct by itself. We're both equally wrong.