Marshall Valvestate 8008 Question: capacitor C22, channel A.

[Kaz Kylheku]

In the schematic of this amp (easily Google-able), at the very end of the output chain, the output is shunted to ground through a 0.33 ohm resistor R59 in series with a 0.22 uF capacitor.

Is this really just a low pass filter or is something else going on? That cap value seems quite aggressive. Wouldn't this kill the sparkle from the cleans?

[phatt]

There is no freaky super teckno trick going on with the *I* Drive trick.
If you care to take some comparisons it just alters the tone a little.

Quite frankly
In My Limited and humble Experience (but adding ,,,I have been there done all that) It's over rated but that said some teck geeks RAVE ON about the ***MAGICAL ** benifits of I FBack.

It really depends on what else you have to compare it to. 8|

If you have never breadboraded circuits then **AT First hearing*** I FB (Current FeedBack) might sound good to you (You may Even think you found the holy grail or the missing link) until you try a few other tricks which are far simpler and far cheaper to implement.

It's all just tone control of the power amp itself which is no big deal.

Now go check the tone response curves against a REAL Power Tube driven transformer coupled output stages and you may get a massive big fright as to how different they compare in response to SS I drive.

Cheer's Phil.

[Kaz Kylheku]

There is no freaky super teckno trick going on with the *I* Drive trick.

Hi Phil, wrong thread? 

[teemuk]

Is this really just a low pass filter or is something else going on?

Yes there is. That "something" is......








....a drawing error in the schematic. 





Guess you didn't see that coming, right? Refer to channel 2 and corresponding resistor R64. The right value for the resistor is 10-ohms.

[Kaz Kylheku]

Is this really just a low pass filter or is something else going on?

Yes there is. That "something" is......








....a drawing error in the schematic. 





Guess you didn't see that coming, right? Refer to channel 2 and corresponding resistor R64. The right value for the resistor is 10-ohms.

I was comparing the channels, but didn't pick up on that difference. Thanks.

I think I now understand the circuit.

The 10 ohm resistor makes sense. It ensures that the impedance of the shunt is never less than 10 ohms, regardless of frequency.  It doesn't make much of a difference to the treble cut at 10 Khz. The capacitive reactances at 10 kHz, 1 kHz and 100 Hz of the 0.22 uF cap, respectively, are -72.3, -723 and -7230. They all swamp the resistance value.  At 10 Khz, if the resistor were not there, there would only be a 0.8% difference in impedance: smaller than the tolerance on the cap.  We need the 10 ohm series to make sure that at high frequencies far above 10Khz, we are not short-circuiting the amp's output.

I now see that since the amplifier's load is small, even at 10 Khz, the high frequency cut will not be all that aggressive. My intuition for capacitor values is "biased" toward high impedance, small signal circuits. Not used to thinking about tone controls in power sections. A 0.22 uF parallel cap in a small signal, high impedance situation would roll of treble above 1Khz very badly, but so much here.

[Kaz Kylheku]

In addition to the above, I failed to recognize that resistor and capacitor as a Zobel network. It does cut frequencies, but that may be just to offsets the rise in high frequency response due to the inductance of the speaker. (Which will be all the more pronounced when that amp is switched to its mixed-mode feedback guitar mode.)  Also, this arrangement helps prevent high frequency oscillation from developing, like if there are long speaker cables that are capacitive.

I think I'm satisfied with that answer.