Discrete Fet Buffer - Build Diary

[lordearl]

After some discussion about how best to attenuate volume between a DAC and a power amp, I have ordered a simple FET buffer PCB from SKA Audio and have decided to post a build diary.

The schematic is attached below. Note that the pcb is just a prototype so does not appear to be commercially available yet.

Here's a bill of materials (in case anyone is also interested in doing a build):

For the pcb:

1 x 10K Stereo Potentiomenter
2 x 47R 1/4W Resistor
2 x 100K 1/4W Resistor
2 x 1uF 50V capacitors (I'm using 0.22uF polypropylene caps)
2 X 100R trimpot

Remaining items:
Chassis
Regulated DC power supply + transformer
IEC socket + fuse socket
RCA jacks (input and output)
2 x 47R 1/4w Resistor (for placing from the output of the pcb to the RCA outputs)
Hookup wire & Solder
Knob for volume pot

I'll post some pictures and further comments as it progresses, but any thoughts on the design, etc, would be great!

[johnferrier]

Originally Posted by lordearl

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1 x 10K Stereo Potentiomenter
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. . . any thoughts on the design, etc, would be great!

Keep in mind, if your source has an output impedance of 10k ohms (like my SACD player), then with a 10k potentiometer, you have - 6 dB attenuation at full volume. (I choose a 10k ohm attenuator, as you and don't worry about the extra - 6 dB.)


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[lordearl]

Thanks John – what is the calculation for working out the dB range? My cd player has an output impedance of 22 Ohms and the power amp an input impedance of 100K ohms.

[Steve Eddy]

Originally Posted by johnferrier

Keep in mind, if your source has an output impedance of 10k ohms (like my SACD player)...

Your SACD player has an output impedance of 10k ohms?

Man, get that thing into the shop and have it fixed, or get a rescue inhaler for whatever asthmatic tube output stage it's running.

se

[Steve Eddy]

Originally Posted by lordearl

Thanks John – what is the calculation for working out the dB range? My cd player has an output impedance of 22 Ohms and the power amp an input impedance of 100K ohms.

If your CD Player has an output impedance of 22 ohms and you're driving it into a 10k attenuator, then at full volume, you'll have a signal attenuation of about 0.02dB. Nothing to worry about.

se

[johnferrier]

It's a Sony player. I can't find the spec. sheet online, but I measured it too--it's 10k. It's probably a series resistor? I should change that to 100 ohms, if I can. Thanks, Steve!


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[Samuel Groner]

I measured it too--it's 10k.

And *how* did you measure this figure? You can't just use the Ohms function of your DMM for this.

Samuel

[johnferrier]

Originally Posted by Samuel Groner

And *how* did you measure this figure? You can't just use the Ohms function of your DMM for this.

Samuel

Well . . . measured 1 Vrms (@ 1khz) out unloaded and 0.5 Vrms loaded with a 10k ohm resistor. (Again, the hardcopy spec sheet indicates 10 k ohm output too.)


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[Steve Eddy]

What's the model number?

se

[lineup]

What JFET you plan to use?
Maybe 2SK170BL
2SK170GR has got lower current, but we prefer high current

http://www.ska-audio.com/Forum/YaBB.pl?num=1244063607/0
Greg says this:

Re: A Simple FET Buffer
Reply #2 - Jun 5th, 2009, 8:33am
Hi Mugsy,

You can use the 2SK170 GR,BL, or V..... or ungraded, I simmed it with the SMD BF862 but can input any 2SK170 variant, or even the BF244B.

The 61R resistor was chosen to set the standing Class A current to 5mA, that will vary with the device. The DC null trimpot is VR1 and a 100R should give enough range for a null.

A 25K Alps pot can simply connect conventionally at the input where V2 (the sim test signal source) is shown, input coming from the wiper.

Greg