Bottlehead Forum

Bottlehead Kits => Crack => Topic started by: sharpi31 on August 06, 2010, 08:03:57 AM

Title: Crack as preamplifier
Post by: sharpi31 on August 06, 2010, 08:03:57 AM

Just grabbed a cheap 3.5mm stereo jack to 2x RCA cable, and a 6.35mm to 3.5mm stereo adapter, and plugged the output of the Crack into my Mini Aleph power amp (10K input impedance). Been using a Nelson Pass B1 buffer preamp until now.

Awesome....

;-)

Title: Re: Crack as preamplifier
Post by: Doc B. on August 06, 2010, 08:34:30 AM

We have been taking this idea a step or two further with very good results. Will let folks know when I have something I'm ready to talk about in more detail.

Title: Re: Crack as preamplifier
Post by: sharpi31 on August 07, 2010, 08:22:31 AM

Thanks Doc.

Word of warning - I tried the Crack as a preamp feeding a UCD180 power amp with coupling caps removed. UCD didn't like it - kept switching the protection on. Suspect a grounding issue - the mini Aleph has Lundahl input transformers so would be immune to ground problems.

Going to install input transformers in the UCD to see if that makes it happy.

Don't assume the Crack will work with any power-amp - use a pair of crappy speakers for testing first.

Title: Re: Crack as preamplifier
Post by: Paul Birkeland on August 07, 2010, 05:26:47 PM

Quote from: sharpi31 on August 07, 2010, 08:22:31 AM

Thanks Doc.

Word of warning - I tried the Crack as a preamp feeding a UCD180 power amp with coupling caps removed.

Which coupling caps did you pull?

Title: Re: Crack as preamplifier
Post by: sharpi31 on August 07, 2010, 10:59:46 PM

The one's on the input to the UCD, not the ones on the output of the crack ;-) That would have been exciting - 117V DC into UCD line level input.

The reason my UCD wasn't happy with the crack was because the lighting film caps (2x50uF in parallel each channel) I had used for output coupling had 0.5Mohm bleeder resistors in parallel with the cap. With two in parallel I had made a voltage divider with 250Kohm in the caps and 2.5Kohm (in parallel with the headphone or amp input impedance) to ground. With no headphone or amp attached, I had about 1V DC offset on each output. No wonder the UCD kept going into protection.

I risked damaging both my amp and HD800s. Fortunately the worst I have to deal with is a couple of Lundahl transformers at my mini aleph input that should be demagnetized (easy).

Needless to say if I'd used the parts Doc supplied and measured all voltages and resitances religiously as he rightly recommends, this wouldn't have happened.