Crack 75k and 33k attenuation / pad

Banzaiburrito · 3466

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Offline Banzaiburrito

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on: November 13, 2018, 01:17:23 AM
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


Bottlehead Crack
Musical Fidelity M1 Dac
Sennheiser HD650
Kimber cables
Russ Andrews mains leads and conditioning


Offline Deluk

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Reply #1 on: November 13, 2018, 01:35:09 AM
Aren't these fitted across the headphone socket connections?



Offline adydula

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Reply #2 on: November 13, 2018, 03:18:31 AM

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.



Offline Paul Birkeland

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Reply #3 on: November 13, 2018, 04:30:38 AM
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. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Banzaiburrito

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Reply #4 on: November 13, 2018, 08:36:50 AM
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!

Bottlehead Crack
Musical Fidelity M1 Dac
Sennheiser HD650
Kimber cables
Russ Andrews mains leads and conditioning


Offline Paul Birkeland

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Reply #5 on: November 13, 2018, 08:46:09 AM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Banzaiburrito

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Reply #6 on: November 13, 2018, 09:02:44 AM
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. 

Bottlehead Crack
Musical Fidelity M1 Dac
Sennheiser HD650
Kimber cables
Russ Andrews mains leads and conditioning


Offline Paul Birkeland

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Reply #7 on: November 13, 2018, 09:04:05 AM
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

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Banzaiburrito

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Reply #8 on: November 14, 2018, 12:05:12 AM
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.

Bottlehead Crack
Musical Fidelity M1 Dac
Sennheiser HD650
Kimber cables
Russ Andrews mains leads and conditioning


Offline larcenasb

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Reply #9 on: October 19, 2022, 02:59:44 PM
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!

Lowell B.


Offline Paul Birkeland

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Reply #10 on: October 20, 2022, 06:24:08 AM
A photo is available on the FAQ page here:
https://forum.bottlehead.com/index.php?topic=4295.0

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline larcenasb

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Reply #11 on: October 20, 2022, 06:51:18 AM
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.

Lowell B.


Offline Paul Birkeland

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Reply #12 on: October 20, 2022, 06:52:26 AM
No, I do not have photos of that.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline larcenasb

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Reply #13 on: October 20, 2022, 06:54:50 AM
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.

Lowell B.


Offline Paul Birkeland

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Reply #14 on: October 20, 2022, 07:04:54 AM
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.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man