Output signal

[ccoolliinn]

Looking over the Crack wiring I understand most of it but two things are nagging me 

Why does the signal come off the cathode and not the plate in the 6080 

Is the job of the output capacitor just to stop dc voltage hitting the headphones and how is its value sized as I'm keen to upgrade

 

[Grainger49]

It is an amplifying circuit called a "cathode follower."  It has a low output impedance.  It has low gain, often less than one.  It does not invert the signal. 

In the Foreplay preamps the second stage was always a cathode follower so it could "drive" cables longer than 3 feet without a frequency loss.  It allows them to skip the output transformer, thus lowering the price and making the signal path purer.

However it makes Crack less than optimal driving low impedance headphones like Planar headphones and the Grado headphones.

 

[Paul Birkeland]

Why does the signal come off the cathode and not the plate in the 6080 

The cathode follower output of the Crack trades voltage gain for lower output impedance.  You could rework the circuit to take the signal off the plate, but you'd end up with more gain (something the Crack doesn't need) and higher output impedance (also something the Crack doesn't need). 

Is the job of the output capacitor just to stop dc voltage hitting the headphones and how is its value sized as I'm keen to upgrade

Yes, that is the purpose of the 100uF caps at the output. Here's a calculator for the cap value: Coupling Cap Calculator  I like to be -3dB at 10Hz if possible.

 

[ccoolliinn]

Thanks for the replies. 

Does this mean I can use a lower value cap like Mundford MKP 68up or 82uf  if I'm using Bayer T90 250 ohm and T1 600 ohm.

Would there be a improvement in sound with a lower value or just a change with volume control.

 

[Grainger49]

The output cap and the attached load form an R-C, high pass network. The lower value will roll off the bass.  I wouldn't do that.

The Mundorf cap will sound different, but not change the volume except for the loss of bass.

 

[ccoolliinn]

If I keep to a 100uf 250 v Mundforf will I lose bass also.

I was hoping this would be a upgrade over the original caps. This is the only upgrade I plan to do except for the Speedball which I already have but not yet fitted.

 

[Grainger49]

No, if you keep the same value as the design you lose no bass.

Yes, a Mundorf will sound better than the stock cap, IMHO.  I haven't done this in a Crack but have upgraded the output and interstage (not in Crack) caps in everything I have.

Here is eighteen pages of Crack output cap information:

http://bottlehead.com/smf/index.php?topic=1723.0

 

[ccoolliinn]

Thanks for the advice and guidance . I have ordered a pair of Mandorf caps today will posts pics once fitted ( under English crack)