My pre-amp / buffer

Hi,

I previously used op-amps for a pre-amp, in the form of the Rod Elliott Project 88 with his matching power supply project. A friend gave me the boards and parts so I built it. Not bad sound, and I would like to say Mr Elliott's approach to help guides etc. is first-rate for a newbie. It's obviously not an audiophile design and wasn't meant to be.

Another friend later supplied me with a PCB and parts for a discrete buffer. I think it's quite a standard design, but I don't really know electronics too well! I got it from my friend Lee, called Thomo on this forum.

Simon

Parts details

For now the chassis is a 1U steel rack-mount job from Maplin. Not my choice - it came with the first kit. Thanks to my old friend and former house-mate Mike

The transformer is an 80VA toroid that I had lying around. The rectifier is made of some large schottky diodes I found lying around, I'm not sure why I bought them originally. The smoothing caps are 5800uF/40V Sprague Powerlytics, very large for their capacitance. They are peeled and connected via copper foil screwed down - because I can't solder to the screw terminals and they don't hold ordinary wire very well - easiest would have been to source some "tags".

Next up is the regulators, which are Sercal S Power - brilliant devices! Local decoupling is peeled Rubycon ZA 100uF/50V. Power wiring is individual bits of Kimber 4VS I had from offcuts. Ground is the black parts of 4VS and it all joins on one star point, which is made from strip-board, caked in solder.

The other parts on the board are Mundorf MKP caps and Kiwame resistors. Not sure what the FETs and transistors are! Signal wiring is Mundorf copper foil which I insulated with spray PCB acrylic lacquer.

CDP in and the main output sockets are CMC thick silver over solid copper - the best RCAs?! Pot is Alps Blue 20k log. Selector is cheap Maplin job. These are placed near the back courtesy of some shaft extenders purchased from Lee @ DIY-Hifi. These are great but with one small problem - I can't get the knobs to line up perfectly straight.

Simon

Sockets and ground star

Here's a close-up showing the sockets, pot, selector switch and star earth.

Here's the last pic for now, sorry about the poor lighting, the room was dark and my camera is cheap.

The sound is, ignoring any limits elsewhere, superb. I'm tempted to say flawless, but I can't really say that without hearing other designs

The music flows effortlessly and is brimming with detail, tone and stereo information.

Simon

Future plans

I am considering upgrading the chassis, mainly for looks, but also to accommodate a processor-driven stepped attenuator volume control & source selector kit. I also plan to build a 2nd power supply and stereo LM3886 power amp into this, for powering my headphones. As the power requirements are low the filter caps could be exotic, perhaps even film-only. I don't know if this is a good idea but would be fun anyway.

Simon

RCruz said:

Hi Simon

Superb work.

Why are you using copper foil ?

Why the 20k pot ?

Click to expand...

Hi Ricardo,

Thanks! The copper foil sounds good and is cheap compared to other "audiophile" solutions, plus it gives the preamp a unique look inside. The sound is the main thing though.

I first used a 10k linear Vishay pot (that came with the bundle of stuff I first got), but I found a linear pot was terrible as a volume control - hard to get a "just right" volume and it had awful tracking /channel balance at the low volume range. I then tried a cheap 10k log pot, but this also had poor balance at the low range.

So for a double-whammy I moved to 20k (more range at lower levels) and Alps Blue type, as this is known to have exceptional channel balance. I get even L-R sound even at very, very low night time levels now.

Simon

RCruz said:

Hi Simon

I really like the look of the foils... do you prefer them over silver strand ?

I also have trouble with low volume balance with 101´s old carbon wipers.

Is the Alps 20k log type ?

What is your setup like ? Do you have the pot after the input buffer ?

Is there an output buffer after the pot ?

Ricardo

Click to expand...

Log pot, yep.

Simply it is... sockets > selector > pot > buffer > sockets

Another buffer would be pointless for CD listening IMO. It sounds wonderful.

I'm yet to compare the foils to a serious silver interconnect but so far they've shown up:

Kimber Crystal-Cu (£88 i/c)
Enamelled DCT twisted pair with copper plugs
Mundorf silver/gold/teflon twin pairs (on same plugs as foil i/c)

Of those 3 the DCT twisted pair is by far the best in my system, but the foils are another level of detail, air and a completely "correct" and lively sound. Silver foil + silver bullet plugs + C37 lacquer + DCT would be interesting, but too expensive to justify to me at the moment.

Other areas need to be perfected before spending ~£200+ on one DIY wire.

Simon

Nope, it's all discrete

Hmm, well I dunno about balanced myself

Try your silver wire, and later on try foil if you fancy it! It's only wire at the end of the day. hehe

Simon