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Headphone amp - long gestation, successful birth

Posted by Doc B. (M) on March 30, 2004 at 20:34:16

I've spent the evening listening to my new headphone amp, both with headphones and as a line stage. This has been under construction off and on for seven months.

The amp uses dual mono single gain stage spud amps, 6C45s. Of course they are C4S'd, with a new higher voltage rated transistor and Agilent HLMP 6000 LEDs for bias. They are also shunt regulated by triode wired 6BQ5/431 hybrid regulators. The regulators have C4S's on top of them. Dual mono power supplies are quite similar to those in the Paradox prototypes - MQ PGP 8.1 with a cool snap on PC board for the B+ filters and heater DC supply. The boards also contain delay relays for muting the output at startup. The filter is choke input, using an MQ EXO-03 as a filter choke. The DC heater supply also uses filter chokes in both the + and - rails. Grounding is star type.

Single ended parafeed output is through a custom Jupiter beeswax 3.3uF coupling capacitor feeding an MQ TL-404 headphone autoformer with selectable 600/240/60 ohm taps.

The three pairs of inputs are set up for both single ended RCA and balanced XLR. Inputs feed Jensen step down transformers, which feed a 10 step 100dB and 10 step 10dB Daven 600 ohm attenuator wired in series for 110dB of range. Input headroom is a whopping +26dB.

The assembly is in a 3U rack box 17" deep. There are three bulkheads. The rearmost houses the input and ouput jacks, ground lift and phase switches, and power inlet. The next bulkhead houses the input selector, tubes, load and shunt reg boards, autoformers and coupling caps. The forward bulkhead carries the power supplies.

Total weight is 32 lbs. High Dunker factor, output is about 1.25wpc.

The case has been powdercoated in a hammered copper finish. The front panel is veneered with bookmatched lacewood veneer with some nice figure finished with Varathane Diamond. Knobs are vintage Dakaware refitted with Allen head set screws, and the headphone jack is a Neutrik normalling jack (headphone cuts out line output when inserted).

Switches are all top of the line Electroswitch ceramic. Wiring is 22 ga. solid copper Alpha. XLRs are Switchcraft, RCAs are our house gold RCAs. Filter caps are Panasonic TSHA, rectifers are UF4007 with RRSF for B+ and Schottky for heater supply. Control shafts are 303 stainless with cushioned couplings and Rulon panel bushings.

I'll get some pics of the innards tomorrow. No this won't be a kit (don't be silly) and yes, we will custom build these for folks. But right now I'm too busy listening...

This post is made possible by the generous support of people like you and our sponsors:

VH Audio

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