S-PRE-2

[John Bourke]

A few questions about the S-PRE-2 kit.  Hopefully Kevin can help here.

I'm stuffing the board, and I have ended up with two capacitors that I'm not 100% on.
   

Markings are (as far as I can tell) 471J on the blue green one, and 0.47 J100 on the red one.  The board has spots for a 470p (C7) and a 470n (C9).  My assumption is that the 0.47 on the red one is 0.47 microfarads, which means 470Nanofarads, and it is C9, which leaves the blue green one for C7.

Also - I assume there's nothing keeping me from doing the tube sockets off board?  Is 20AWG stranded good enough for that?

[K O'Connor]

Hi John

Why assume anything? ... although you have made the correct inferences.

Standard prefixes are given in RSG (Ready, Set, Go) and across the web. In electronics, it is extremely bad form to begin a value with a zero or a decimal as the value may be interpreted an order of magnitude high or low if the decimal is not obvious, as with photocopying, for example.

The desgnator for the prefix is used as the decimal holder in values with fractions, such as 2k2 for a 2,200-ohm resistor.

Also note that the prefixes are case-sensitive. For example 'm' is milli and 'M' is mega. Using an ASCII keyboard, 'u' is used for micro as the Greek mu is not available on such a keyboard.

RSG also lists standard values and how values are depicted on different components. cap values when printed numerically use picofarads as the unit, then three digits to express the value. The first two digits are the numeric portion; the third digit is the number of zeros after the numbers. "471' is then 47 followed by one zero, as 470p, so the green cap marked 471J is 470p

[John Bourke]

A brief update...

Everything works.  Have had it for now just sitting between my bass and my Scarlett/computer, which is my practice setup, playing around with tone.  So far very very happy with it.  Of course haven't gigged with it yet, or even played with other people, but I expect good things.

Next question:

I have now purchased an ICEpower 125ASX2SE, which is a 2X125W digital amplifier which I'm going to pair this with to make myself a hybrid amp head.

Looking at the documentation for the amp, it actually looks like it can also be configured as a 1X450W output, which is an intriguing option BUT it needs a balanced signal.  I can, of course, just convert the unbalanced output from the S-PRE-2 to a balanced output using the normal kind of methods (like a transformer) but I wondered if there was a way to mod the preamp to output a balanced signal without the conversion.

That all being said, now I think about it, I was going to be adding a Jensen output transformer to give me a DI out anyway, so I could just tap off that to drive the amp if I wanted to drive it mono.

[K O'Connor]

Hi John

Using the Jensen transformer to drive the PA deferentially only works if the winding has a centre-tap OR if you create a faux-CT using resistors. If the transformer has only the one secondary, then you cannot use it for a line out at the same time.

On S-PRE, you can create a "balanced" out by slashing the plate connection for the CF(V2B) and inserting 47k5 in the break, then use a 470n-400V to couple the signal to the PA. Add 475k to ground off the output end of the cap. This turns the CF into a concertina, which has two outputs in opposite phase. The gain and frequency response of the outputs are different but this will help to offset the low-fidelity of the class-D amp - it is NOT "digital". In this wiring, you can still add a transformer from the S-PRE main output but the level is dependent on how hard the PA is driven.

Have fun