Coupling cap in input stage

[bluefoxicy]

In my guitar amp I've added an input stage filter cap. Starting with the below input stage:

 Input     | Preamp

(in) -----------R(10k)-- 12AX7 grid
             |
             R(1M)
             |
  GND---------

As you can see above, my input stage consists of... an input jack. After this, a 1M resistor references the 12AX7 grid to ground (preamp stage) and then a 10k resistor couples input to the grid.

I modified the input stage by making it the below:

 Input           | Preamp
(in) --------| |------R(10k)-- 12AX7 grid
        |   (1uF)  |
        R(1M)      R(1M)
        |          |
  GND---------------

The input stage above consists of the input jack; an input to ground reference (1M resistor); and a 1uF mylar/film non-polar coupling cap. This then leads into the preamp stage (grid to ground, 10k resistor, etc).

I've got a Crybaby wah pedal plugged into the wall, no real ground reference, and I believe the thing contains an op-amp. I thought the op-amp was messing with my tone for a while, possibly preamping a bit much and making my amp respond strangely.

I hesitate to say it, but this coupling cap seems to have magically given me good results. The amp sounds a little bit cleaner, not very noticeable but it's just not annoying me anymore and "sounds better." It seems completely plausible to me that the pedal had a different idea of "ground level" than the amp, and added a DC offset; so I threw this cap in there to block any DC (effectively, frequency == 0) and only pass signal through. I'm still skeptical, but it sounds good so far.

Thoughts?

[Tubis]

Well, you altered the input impedance in the same time with the extra 1M resistor. Could make a diffence depending on how high the output impedance is from the sound source. A guitar straight into the amp will sound different. I usualy put a 2M2 resisor. The input cap always go there in my builds.

[tubeswell]

The input cap also could also alter the freq passing through to the grid would it not? Maybe a 1uF rolls off a teency bit of bass, (but probably not much - the cap would need to be in the pF to nF range to get a noticable change in freq)?

[Tubis]

The bas rolloff is easely calculated by the formula 1/(pi*2*C*R).
That will make a roll of at -3dB at 0.159Hz. Thats extremely minimul rolloff.
I slapp a 47nF in at a roll off at 3.386Hz. You dont hear any bascut there.
So now you might beginning to question those 22nF coupling caps in the preamp. Way to big most of the times.

[bluefoxicy]

That will make a roll of at -3dB at 0.159Hz.

Truth, it doesn't matter even like the 1000th harmonic isn't audible lol

It blocks DC. If the signal's ground point is +5V above my amp's ground point, it'll center it for me. I think. Something like that.

Well, you altered the input impedance in the same time with the extra 1M resistor.

Yes, I did. The original design used a 68k to ground, but I'd thought about going back and getting a couple 2M resistors to pull this off with.

[Tubis]

Truth, it doesn't matter even like the 1000th harmonic isn't audible lol

It blocks DC. If the signal's ground point is +5V above my amp's ground point, it'll center it for me. I think. Something like that.

One should also ask themselfes how low freq. are coming from the guitar. Those subfreq. must be there in order to create harmonics later in the amp. Blocking stuff under the guitar tone kan make the amp more humfree. I cant hear any loss in harmonic content by using a 47nF/1M at the amp input.

[Bruce / Mission Amps]

One should also ask themselfes how low freq. are coming from the guitar. Those subfreq. must be there in order to create harmonics later in the amp. Blocking stuff under the guitar tone kan make the amp more humfree. I cant hear any loss in harmonic content by using a 47nF/1M at the amp input.

What sub-freq harmonics? Explain please.

[Tubis]

What sub-freq harmonics? Explain please.

Very low freq. but not harmonics just low freq. Then every stage you overdrive the subfreq creates a harmonic content of differnet
types of harmonics like 2, 3 , 4 order and so on. The more even order harmonics the better in my book.

[mooreamps]

Sorry... I'm not so sure the instument can generate a "sub-freq" harmonic below the fundamendal freq. I could be wrong, but typically when I voice a pre-amp for electric guitar, I really choke down the lower frequency responce because I think too much bass responce can make the tone sound too muddy.

But, using a series cap on the input, is a trick that is not well known, but is used by some of the more knowledgeable builders. Most of them just won't tell you and/or say what value works best with either a Strat or a LP....

-g

[bluefoxicy]

Sorry... I'm not so sure the instument can generate a "sub-freq" harmonic below the fundamendal freq. I could be wrong, but typically when I voice a pre-amp for electric guitar, I really choke down the lower frequency responce because I think too much bass responce can make the tone sound too muddy.

-g

Put 0.047 and 0.052 filter caps in after your preamp gain stages. You'll hear a load of crud and blart and junk. Yes, too much base response can and WILL make the tone sound like shit.

[Bruce / Mission Amps]

Sorry... I'm not so sure the instument can generate a "sub-freq" harmonic below the fundamendal freq. I could be wrong, but typically when I voice a pre-amp for electric guitar, I really choke down the lower frequency responce because I think too much bass responce can make the tone sound too muddy.

But, using a series cap on the input, is a trick that is not well known, but is used by some of the more knowledgeable builders. Most of them just won't tell you and/or say what value works best with either a Strat or a LP....

-g

Exactly correct on both counts.
Most pro players who know what they are doing will have an overall "tone" that is quite a bit snappier and brighter with much less bass and woofie drone then 95% of the wankers I hear weekly! Ha ha..
And man do those wankers get upset when you tell them their "live" tone is god awful... but when they hear someone who knows these tone secrets, they think those players sound great!

[bluefoxicy]

Exactly correct on both counts.
Most pro players who know what they are doing will have an overall "tone" that is quite a bit snappier and brighter with much less bass and woofie drone then 95% of the wankers I hear weekly! Ha ha..
And man do those wankers get upset when you tell them their "live" tone is god awful... but when they hear someone who knows these tone secrets, they think those players sound great!

This is why I want a TMB stack in my amp, to clean that up a bit and control the drive. A little low-end growl is great for low notes, but I'm getting too much low end.

It's funny, Tony Iommi plays 0.008 gauge strings (I forget what the low E is, like 0.038) because they have "fewer overtones" ... I put thicker strings on my low end for more tension in Drop D, and the damn things sound like a church bell above the 12th fret and have this thick low-end hum that makes my amp muddy. This is where I wake up and go, "Damn, I want to attenuate the bass a bit more..."

[mooreamps]

And man do those wankers get upset when you tell them their "live" tone is god awful...

Ya, no kidding!! I could name a few, but they already know who they are....

-g

[kg]

Sorry... I'm not so sure the instument can generate a "sub-freq" harmonic below the fundamendal freq.

with a single note played at a time, you are correct.

however, with more than one note being played simultaneously, you have the beat frequency to contend with, and this can extend well below the fundamental range of the instrument.

[mooreamps]

with a single note played at a time, you are correct.

however, with more than one note being played simultaneously, you have the beat frequency to contend with, and this can extend well below the fundamental range of the instrument.

I'd have to see it on an audio spectum analyzer first, before I could believe it.

-g