Saturday, October 23, 2010

Pass B1 in anodised Hammond Al case

The Pass B1 in a Hammond Al case. Nice enclosure even better sound. After assembling a vero-brd. ESP P37a for long term customer I was enspired to tackle other simple SS preamps on vero-brd. See the cct. brd. below.

As you can see above the front panel is simple with just a mini on/off switch, 4oz solid brass black chrome kbob on an Alps blue velvet pot and an aqua led. The back holds two sets of gold RCA inputs and one set off outputs. The rear switch switches between the two inputs. The 19V DC power plug is also on the rear.

I power the preamp with a 19V SMPS. Others have done similar with no complaints of noise or switch mode hash. Under test the preamp -3db points are 5hz -> 780khz. 18V p2p into a 10k load and 8V p2p into a 1k load. Impressive!

Though this preamp is more of a driver, being a source follower, than a preamp its ability to drive a reasonable voltage swing into a heavy load is obvious. The sound from this preamp is extremely raw and natural. The B1's ability to dig out absolutely the finest, earthiest detail is striking obvious the second music is passed through it.

I team this blatantly natural sounding preamp with a mid-range rich SET amp for a beautiful combination. Each time a play a different track I am struck by the rawness of the sound and the depth of the detail. The preamp stays on the warm side of clinical without taking any sonic prisoners. Build it if you can take the truth about what you are listening to.

Friday, October 22, 2010

Pass B1 driver stage

After construction of the ESP P37a I was fired up to build another vero-brd. wonder. This is the Pass B1 preamp/driver cct. brd. which went into the black anodized Hammond case. Two jFETs do all the work in this preamp. One jFET is just a constant current source (CCS) for the other. So really only one jFET sees the signal. Simple and effective. The problem is because this is a source follow there is no gain. In fact only 85% of what goes in comes out. But it comes out very well.

Two 10,000uf caps provide power filter and storage. A 50K Alps pot controls input level and large 1uf PIO Russian military caps de-couples the preamp. a 10uf poly provides output. 2SK170 (BL) jFETs are used.

Placement of components on vero-brd. is critical unless you want span the brd. with links. For me there is a lot of thought which goes into component placement to ensure links are at a minimum.

ESP P37a discrete component class A preamp

I was asked by a long standing customer to construct this ESP P37a on vero-brd. He would provide the case and PS. Basic polypropelene capacitors and metal film resistors were used. A bi-polar capacitor was used on the output stage. Low ESR caps were used in the power filtering stage. Silver based solder used throughout.

Though a simple preamp building on vero-brd. is always time consuming. I was very pleased with the build and had a short listen to ensure the cct. brd. performed well. I was pleased with the sound which appeared flat across the frequency spectrum and very natural.

I charged the customer $AU100 for the finish and tested cct. brd. and this included shipping to Malaysia. I would recommend the preamp but purchase the cct. brd. for it.