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Salas DCG-3 Preamplifier and DCSTB Power Supply

Posted 10th April 2016 at 12:26 PM by Tea-Bag
Updated 29th December 2016 at 07:37 PM by Tea-Bag

This blog is concerning ordering, parts, assembly and some brief sound review of the two new circuits designed by Salas.

The DCG-3 is a new preamplifier design. It offers 10-15db of gain, and can use several different types of input JFETs.
Here is the preliminary values in the schematic.

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Here is the prototype board

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Here is the Actual Production layout
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The DCSTB is Salas first offering of a series regulated supply. It is very easy to assemble and includes two board assemblies off one power supply. This is a somewhat 'greener' approach and requires less waste current than say it's SSLV1.1 or BIB board equivelant. This board requires no load to test. The output current is fairly easy to set with two resistors.

Here is the simplified schematics
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Here is the actual board layout. It is V-scored and can be broken to be run as a single board or in two different areas if space is tight.

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In this blog, I am using this in a dual mono fashion to feed each channel of the DCG-3 independently.

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Here is the working preamplifier, being fed by two 17V Hammond transformers, and a 12V transformer for the In-Select board for volume and selector duties.

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The hole here represents where a headphone output can be used, because, yes it can do that to. Here is the HP out is used for.

Assembly of DCSTB
First off, look at it and realized it is a mirrored board, so some objects may not be in expected place.

The PF5102 JFETs should be matched within 6-8ma. With these values, the IDSS current remains fixed at 5.5ma. This will give a uniform output voltage. If the voltage appears a little low, increasing the Rx and RyJ resistors from jumper or 1R to adjust to give a uniform voltage. I found if they were a little off, by a ~.3v, then 56R did the trick. With my 17V AC transformers, I am getting out of the unit about 17.65V on all for Voltage outputs.

Here is a picture of Rx and RyJ installed on bottom side of the board, as well as showing LED packs soldered in.

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I do not recommend soldering in the heatsinks initially. connect the MOSFET to the Heatsink and then solder the MOSFET into place. If the there is a problem, removing the heatsink quickly makes accessing the smaller transistors much easier. Also, for ease of tweaking, I recommend soldering the RxJ and RyJ resistors to upside down on the board. This way they can be desoldered with relative ease.

The LED packs have a angled edge on one side that will match up with the angled edge of the art work. Make sure to get this right.

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Note the orientation of the fuse holder as well. It is hard to solder then in place unless done from the top down. I recommend installing as you see it, and solder at least one leg of it from the top. Then flip and solder both of them under.

If using one DCSTB, you will need to use 750ma 5mmx20mm fuses. You will then need to jumper from V+ G and V- from one side to the next.

Otherwise in dual mono 400-500ma slow blow fuses are recommended. 400ma are included in the kit unless otherwise requested.

DCG-3 Setup

Unpacking your Kits
For the transistor kits, the following will be in your kits seperated from the rest of the transistors.

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This represents J1-J2 uPA68 NEC transistors that are matched. M3 is also matched in a seperate bag. M3 are matched so current is close to same between channels.

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The BC560c transistors are in matched quads, represented on the left. On the Right, J3 (BF256) have been measured with a 221R R3 (different than original) resistor to make sure they run at proper IDSS in circuit.

In the full DCG-3 kit you will see with the capacitors that look the same with the exception of a red or black stripe on them.

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The red represents 10pf, the black 33pf
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Resistor changes from original schematic.

I suggest you use a magnifying glass, or a loupe to make note of the resistor values. Since some of them are 10x more or less in value, you want to make sure your inserting the right one in the right place.
R3 is 221R in this kit.
R10 has two options, lower current use will be 12R CMF-1, higher current will be 7.5R. The later only recommended for headphone kits to get proper current for some difficult loads, but otherwise not necessary.

Note Placement of cb, ca capacitors I soldered then in after I had soldered in the 8pin op amp socket.
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Assembly of the DCG-3.

Start with Resistors, then socket, then small transistors.
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Bottom Side LED orientation
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Top side LED orientation
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Note Orientation of U1 socket and relay.
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The most difficult part of assembling this unit is making sure the 6 MOSFETs are drilled and insulated properly from the chassis. Do a continuity test tab of the mosfet to some exposed part of the chassis to make sure it does not conduct. Once assembled. Voltage across R10 should be about 1V for 100ma bias. The LED's will take a second or so to light up, and the relay will click. At this point, you will want to set the DC offset of the unit using VR1.
Try to get this as low as possible, short the inputs. If the BC560C's are well matched, then this should not be a problem. Make sure the input's are shorted and your measuring LINE and G.

After the offset is done, you can insert your OPAMP.

I initially run basic music through an ipod, ipad, laptop Headphone output into the Line IN sections for test. you want to make sure that you are getting AC Voltages out of the Line Out portion.

Grounding. This is the purgatory of all DIYAudio members. All setups are not going to have the same experience grounding, but here is what I did. After some small buzz. I got out some jumpers, and found a 0v reference on the In-Select board and but that on the LineOut-Ground output right near the RCA Outs. From this same place, I also had a wire to a lug to chassis ground, and also from IEC ground lug to chassis as well. Not fancy, but not what I did with my DCB1 either.

I jumpered signal ground between both sides of the DCG3 directly to each other, not to the output grounding point.

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Approximate BOMs for working circuits.
DCG-3 BOM Two channels

DCSTB BOM One board only
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