Crack 75k and 33k attenuation / pad

[Banzaiburrito]

Hi

I note the recommendation on the Crack FAQ page to install 75k and 33k resistors on the pot to attenuate the signal to overcome channel imbalance.

What typical dB attenuation would this create?  I have used RCA attenuators with good results on my previous headphone amp but the -10db seems to strangle the dynamics on the Crack so would like to try something a little less aggressive. My only recollection is that 3db increase is a 100% volume increase so presumably -3db would halve  it?

Kind regards

 

[Deluk]

Aren't these fitted across the headphone socket connections?

 

[adydula]

Disconnect the white wire at the pot that originated at the left RCA jack.  Install a 75K resistor between this wire and where it connected on the pot. 
Repeat this step for the red wire coming from the right RCA jack.
Attach and solder one 33K resistor between each outer pair of lugs on each level of the volume pot and the ground lugs on the pot.
Resistor wattage is unimportant, 1/10 Watt or greater will work nicely.

 

[Paul Birkeland]

The in-line attenuators that you can plug into the RCA jacks are usually 10K devices, so that might be a little low for your sources and be a large part of what you're hearing.

The 75K/33K combination gives -12dB of attenuation. 

 

[Banzaiburrito]

Disconnect the white wire at the pot that originated at the left RCA jack.  Install a 75K resistor between this wire and where it connected on the pot. 
Repeat this step for the red wire coming from the right RCA jack.
Attach and solder one 33K resistor between each outer pair of lugs on each level of the volume pot and the ground lugs on the pot.
Resistor wattage is unimportant, 1/10 Watt or greater will work nicely.

Many thanks, got that but I was hoping to go with the RCA attenuators approach, the pot tested my patience during construction so the thought of undoing it and reworking it is not appealing!

 

[Paul Birkeland]

You could also build the pad onto the RCA jacks, though from a technical perspective it is a bit better on the pot.

Another mod I have done occasionally is to drill another hole behind the volume pot and put in a rotary switch to have switchable -6, -12, and -24dB pads.

 

[Banzaiburrito]

The in-line attenuators that you can plug into the RCA jacks are usually 10K devices, so that might be a little low for your sources and be a large part of what you're hearing.

The 75K/33K combination gives -12dB of attenuation.

Thanks Paul, I’m not sure I fully understand. When you say 10k devices is this the resistance?  I have two sets of attenuators -10 and -20dB.  On my Musical Fidelity the -20s really made the sound muddy, the -10s were an acceptable but perhaps not perfect balance of; less sharpness and harshness but some muddiness.

With the speedball installed on the Crack,(omg by the way) I don’t need the volume as loud and I’m falling into the territory of pot imbalance. To overcome this I tried the -10s and it seemed as if they were over attenuating and killing the dynamics much as the -20 on the MF did.  So I was thinking to go less than -10db rather than nearer -13dB. 

 

[Paul Birkeland]

Are you plugging these into the Crack or into your source?

Yes, 10K is the load that these devices generally present.  If you add the resistors to the pot instead, the load your source sees stays at 100K.

-PB

 

[Banzaiburrito]

Are you plugging these into the Crack or into your source?

Yes, 10K is the load that these devices generally present.  If you add the resistors to the pot instead, the load your source sees stays at 100K.

-PB

I’m using the -10dbs at the amp. The -20dbs are designed for source only.

 

[larcenasb]

Another mod I have done occasionally is to drill another hole behind the volume pot and put in a rotary switch to have switchable -6, -12, and -24dB pads.

Hi PB, or anyone else that has done this, do you have a picture of this mod and/or could you explain how it can be done? I’d like to try this on a basic Crack. Thanks!

 

[Paul Birkeland]

A photo is available on the FAQ page here:
https://forum.bottlehead.com/index.php?topic=4295.0

 

[larcenasb]

Sorry PB, I meant your mod with a rotary knob to select between -6 db, -12 db, and -24 db of attenuation. How would that be done? Is there a picture of that? Thanks.

 

[Paul Birkeland]

No, I do not have photos of that.

 

[larcenasb]

When you have time could you please explain how that would be wired up and what parts are needed. Thanks so much for your time and help.

 

[Paul Birkeland]

I would recommend using the 75K and 33K resistors.  If more or less attenuation is needed, it's easy enough to just adjust those later.

 

[larcenasb]

Alright, I did notice before with those resistors it was perfect for my 600-ohm AKGs, probably wouldn’t use anything harder to drive than those. It just seemed like a really cool mod to have the rotary options. Thanks.