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My "Salas hotrodded DCB1" build

Posted 27th January 2011 at 08:06 AM by Rodeodave

I built the 10R version for now, maybe I'll up the current later. The power resistors I used are non-inductive planar 10R 5w thick film resistors (BI Technologies BPC5100J). The noise figures are comparable with metal film it seems. I just thought I'd give these a try, and since I'm using many fancy flavoured parts (at least for my taste) it fits the bill. I had to remove the original legs, which were some clamp-on soldered types, and fit them with bent copper leads since the original spacing was too small for the board. If you look close enough you can see it in the second picture.

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The MOSFETs are IRFP140N/9140N. The 5-LED chain is matched to 9.30V/9.29V (values measured in circuit), using 12VDC and 1k in series with it. The 3-LED chain was matched to 5.59V/5.58V (measured in circuit). The 5-LED chains are fed by 2SK170 that I once measured to have an I_dss of 9.42mA/9.54mA. The 3-LED chain is fed by 9.29mA/9.36mA JFETs. The other two JFETs in the shunt section measured 8.49mA/8.39mA.
For the BC550/560 I used ON semi parts since they have the smallest noise figure according to their datasheet.

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The resistors at the IRFPs' gates are 470R 5% 0.5W Allen Bradley carbon comp resistors. All other non-audio resistors are MRS25 1% 50ppm 0.6W metal film resistors, except for the 1R resistors which are MF50 1R 1% 50ppm 0.5W metal film.

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In the buffer itself I used the following parts: The 2SK170 pairs are matched to 8.02mA/8.00mA (upper/lower) and 7.97mA/7.95mA. The 221R resistors are Vishay-Dale CMF55D and the 1M/221k resistors are Vishay-Dale RN55D.
The relay is a Nais TQ2-5V.

The PSU caps are Nichicon FW 4700uF/35V types, the other electrolytic caps are Nichicon Muse 100uF/50V. The diodes in the PSU are BYF-29F types.
The bypass caps for the 5-LED chain are NCC (Nippon Chemi-Con) FNX-HS 0.47uF/100V film and foil polypropylene. I am using them as the input coupling caps in my headphone amp and I believe they sound right. I left the leads long so they can be easily changed, should I once fell the urge to see if a different cap in this place makes an audible difference.

Now for some numbers I pulled from the circuit (in the test phase I powered the circuit from a benchtop DC PSU; numbers in brackets are other channel/polarity; voltages in reference to ground unless otherwise noted):

V_input = 19.85V(-19.93V) before rectifier
V_before10R = 19.18V(-19.26V)
V_after10R = 17.85V(-17.67V)

Therefore V_drop10R = 1.33V(1.59V)
and assuming the resistors are R=10Ohm we have a current of
I_through10R = 133mA(159mA)

The first IRFP140 measures -13.66V/-9.95V/-17.67 (pin 1/2/3 G/D/S) and the second one -5.96V/GND/-9.95V

The other polarity's first IRFP9140N measures 13.61V/9.94V/17.58V and the second one 6.02V/GND/9.94V (again pin 1/2/3 G/D/S)

The supply voltage for the buffer is 9.94V(-9.95V) accordingly.

Over the LED chains (not to ground, mind you) I measured 9.30V(9.29V) and 5.59V(5.58V).

After 20 minutes the output offset measures -1mV(-2mV) and seems to stay there; the inputs were not grounded taking this measurement. The heatsinks, salvaged from a SMPS, get warm (maybe 40C/104F) but not hot.

One problem I had to solve was emerging from that I didn't have a BC517 on hands and used a BC547 instead. It seemd that with the BC517 the relay briefly oscillated before it latched. Not good.
The problem apparently was solved by a 470nF cap between C and B (pin 1 and 2) of the BC547 that I am using instead of the BC517. I can't see the relay doing any funny stuff on my scope anymore, and neither anything audibly strange during power-up happens when the buffer is hooked up to my amp.

Click the image to open in full size.

And since I'm using the DCB1 purely as a buffer without a volume control I paralleled the 221k resistors with 20k underneath the board to get the input impedance within the desirable range.
The orange blob you can see in the picture of the underside of the pcb is a 100nF cap at the output of the 7812 (and it has been there before the final mods so it's not related to the relay stopping to oscillate).
Oh, and for interconnects I'm using RG316/U if someone wants to know.

I also made a quick PSU for the DCB1. It's simply a metal box housing the transformer, a fuse and a thermistor, plus an X2 cap. The transformer is a traditional E-I type with two secondaries with several taps (0-10-12-15V, twice). It's rated 25VA (0.8A per 15V winding) and, if you believe the stamp on it's bottom, was made on August 23rd 1982.
It's dead quiet, gets slightly warm and I have several of them, which is good since I plan on making several DCB1 buffers.

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