Preamp FAQ

I get a lot of emails about the preamp, and I try to answer every single one. But, to save me time, I've compiled some of the most commonly asked questions here. If you have a question about the project, please take the time to check here before asking, and if you can't find an answer feel free to contact me...

Section 1: General preamp questions

Section 2: Component questions and sourcing information

Section 3: Construction questions

Section 4: Design questions


Section 1: General Preamp Questions:

Q1.1: I really like your preamp! Can I build one?

Thanks! This preamp is not intended to be a "kit"; rather the information presented here is to help, inspire and guide people who might be planning their own project. PCB layouts, the parts list and source code are not available.

Q1.2: Why can't I find the parts list and PCB layouts?

I haven't had a chance to finalise the parts list, mainly because it is still a "work-in-progress". There isn't really much of an incentive for me to do this, as I've already got the parts!

I used an old DOS version of EasyPC from the mid-80's, so the PCB layouts are unlikely to be compatible with your PCB software. For this and other reasons, I have no plans to release the layouts.

Q1.3: Where is the source code for the PIC processor?

The source code is not available. I'm not prepared to release it for a number of reasons, not least because it is far from finished, but the main reason is just about every line of code is original and entirely my own work.

In the future, when I get a chance to work on the programming guide in the PIC applications section of this site, a lot of the routines that I have written for this project will be detailed there (things like time delays, LCD stuff - writing words, scrolling text, defining UDC's, making bargraphs, etc). For those willing to put in some effort, this should help considerably.

Section 2: Component questions and sourcing information:

2.1: Case

The preamp is built into a 1U rack-case, made by IPK. It features extruded front and rear panels which results in no visible fixings on the front panel. The top and bottom panels are made from 1.5mm clear anodised aluminium. You can buy the case from a number of suppliers, including RS and Electrospeed.

2.2: Volume control knob

This was a tricky part to find. I required basically similar dimensions to the knob that you can see in the development pictures, but as I was using a continuously rotating rotary encoder, I didn't want any sort of pointer. The knob you can see is available from any of the standard suppliers - it's a solid aluminium 28mm knob with a brushed black anodised finish and grub-screw fixing. It has an indicator in the form of a deep recess cut into the knob. Farnell sell these (part number 143-9978) for £1.89+VAT.

In the end, I gave up trying to find a reasonably-priced off-the-shelf control knob that fitted the bill, and decided to use the knob I already had. To remove the pointer, I simply filled the recess with car-body filler! Next, I finely sanded it with 1000-grit wet-and-dry emery cloth to remove the brushed effect, and spray-painted it using the same process as the front panel (see below).

2.3: Illuminated switches

Close-up picture of the switchesThese are made by Mec - I bought mine from Farnell. You buy the switch body and caps separately. There is a choice of LED colour and cap style - the Farnell part numbers and prices for the cap and switch are 113-2905 (£0.48) and 113-2902 (£2.15) respectively. These aren't the cheapest things around, but I think that they're rather nice...

I couldn't find switch bodies with yellow LED's, so I simply replaced the LED. I found that this is relatively easy to do, so rather than pay the extra for switch bodies with blue LED's, I simply bought switch bodies with red LED's and installed blue LED's instead. As you can see, they are just standard 3mm LED's. One extra moneysaving idea that I will try soon - you can buy the switch bodies with no LED, and I'm convinced that you can add LED's to these bodies. Obviously, saving money like this is only viable if it is your hobby and you don't put a price on your time!

The latest Farnell catalogue shows a range of new accessories and styles for these switches - well worth a look...

2.4: VFD display

This is manufactured by Noritake Itron, and stocked by Farnell. The GU140X16G-7806 is an LCD-compatible dot-matrix graphic display that costs around £40 (part number 121-6682), and is highly recommended. Check here for more info...

2.5: Red relays

NEC RelayMade by NEC and stocked by Farnell (part number 109-4028, but are "available until stocks exhausted"). These are signal relays that have the standard BT footprint, so there should be no problems obtaining equivalents in the future if necessary.

The MR82 range features low-power relay coils - the 12V versions used here have 720 ohm coils (that's less than 17mA). You can read more about these relays on the NEC Tokin website. This is the PDF datasheet for the MR82 relay.

2.6: PCB-mount phono sockets

These came from CPC - I believe that they're made by "Pro Signal". They're used by lots of companies, and are reasonably good quality. The CPC part numbers for the single and dual pair versions are AV15008 (£0.78+VAT) and AV15009 (£1.01+VAT) respectively...

2.7: Headphone socket

Headphone socket PCB (6KB)This was 'recycled' from an old bit of kit - I haven't found a supplier yet. They're common enough in consumer equipment, so you could probably buy one as a manufacturers spare. I'm keeping an eye on this, as I'd like one with gold-plated trim...

2.8: The PGA2310 volume control ICs

I got mine from Burr-Brown as samples. For ages, they weren't easily available in the UK via any other means if you were just after small quantities - the only route I found that was open to individuals was Digikey, but the delivery costs are very high.

Luckily, Farnell now stock them - part number 121-2339 (DIL package). They are rather pricey - £13.98 for small quantities, but I think that's reasonable value - cheaper than a good motorised potentiometer, for example.

2.9: The UCN5821A relay driver ICs

These are made by Allegro, and Farnell used to sell them. Unfortunately, they're not made any more, but there is an equivalent. I have some at work, and will report back once I've tested them.

I got the two I used from Allegro as samples. However, I don't recommend that you try to do that - unlike most of the big companies, Allegro have (had?) a terrible sample programme! It took countless emails to get my examples.. This was a while ago, but since then I've tried requesting samples for different ICs and not even had a reply, let alone the ICs!

2.10: Ribbon cable connectors

These are just standard 0.05" IDC (insulation displacement connector) systems. Look in any catalogue and you'll see a range of connectors from manufactures like 3M, Harting and Fujitsu. Prices vary enormously, but I found that Multicomp are the cheapest - they're also perfectly good quality.

To assemble them, I just used a small vice to clamp the two halves together, using thick card to protect the plastic from the rough jaws of the vice. Mark across the width of the ribbon cable using an engineering square to ensure the connectors are at exact right-angles to the cable...

2.11: Gold/blue electrolytic capacitors

These are the Panasonic FC series - low ESR, 105 degree capacitors. Farnell stock them, amongst others. Electrolytic capacitors are a necessary evil, so make sure you fit decent ones!

2.12: Dark grey polystyrene capacitors

The RIAA preamp calls for many 1% tolerance capacitors (20 in all). Axial polystyrene are the normal choice here, but they are physically rather large. The EXFS/HR series are encapsulated in a 7.5mm square case which enabled the compact layout of the RIAA PCB. Farnell stock them, but they're not cheap. For example the 4n7 (952-0260) is £0.85+VAT - however, that's cheaper than the axial versions...

2.13: Rotary Encoder

This item was very expensive, but worth it! The Farnell part number is 114-4747, and at the time of writing costs £41.04+VAT. It's a Bourns 120EN-128-CBL, and is a high-quality, long-life device - it's rated for 10 million revolutions! This is possible because of the optical nature of operation - it works just like the sensors in a mouse, there are no contacts to wear out.

There are cheaper options here - contacting encoders typically cost less than a tenth of the price. However, these need de-bouncing, either in hardware or software. They also "feel" cheaper when operated. Personally, I felt the project was worth the expense!

Section 3: Construction questions

Q3.1: How did you machine and finish the front panel?

The openings in the front panel were done for me. Use of a milling machine resulted in a perfect display window opening, and ensured that all the buttons were in a perfectly straight line. Thanks are due to Malcolm Rowney for that!

The perspex display window was hard work. It would have been easy to just mount the perspex behind the opening in the front panel, but this was cheating! I started by cutting a piece of perspex to roughly the right shape, and gradually filed and sanded it until it fitted. Making the radiused corners fit exactly with the machined front panel was hard work, but the final results were quite good. It is a good friction-fit, and is perfectly flush with the front panel.

The panel was originally finished with a matt black powder-coat finish, which inevitably didn't survive the machining process, despite all the care taken. I was unsure about the best plan, and for a long time considered removing all the paint and going with a silver front panel. Eventually I decided to try a paint finish, and reached for an old tin of Halfords gloss-black paint intended for car bodywork...

After lots of very thin coats, the result was a heavily textured matt finish. I liked this, and decided it would probably do, despite the texture being rather too heavy. But, rather than stopping there, I wondered what would happen if I put some more paint on... Unfortunately it started to look like the shiny black gloss finish that you might expect...

This rather ruined the effect that I'd accidentally found before, so I decided to take drastic measures. Once the paint had hardened, I wet-sanded it using 1000-grit wet-and-dry emery cloth. This smoothed out the rough texture and left a dark-grey matt finish. Then, applying a few more very light coats resulted in a matt-black, lightly textured finish that was similar to before, but much better quality...

This was a learning-curve for me, but was good fun. I will definitely try this technique again!

The front panel labels are rub-down transfers that came from Maplin. Unfortunately, as they are a useful product, they don't sell them anymore! I was using some old stocks. This was extremely time-consuming - it took all afternoon to label 12 items! I tried to get everything level and evenly spaced, and the results are ok. While not as good as a professionally produced product, the results are much better than my previous attempts. In fact, it's only close-up digital camera photos that show up the errors, which explains why there are no such images here!

Section 4: Design questions

Q4.1: Why did you use 12V relays - why not use 5V to match the PIC and VFD?

The 12V trigger outputs are the main reason for having a 12V rail in the control circuit. These are capable of sourcing reasonable currents, so it made sense to regard the 12V rail as the main source of power. So, the next consideration is the power dissipation in the 5V regulator. The VFD takes almost all of the power here. Using 5V relays would have needlessly increased the dissipation in the 5V regulator. Especially as 5V versions of the relays would consume rather more current...

Your mileage may vary - there are no fixed rules...

Q4.2: Why use the expensive UCN5821A's instead of a 74HC595 and ULN2803?

This is a fair question. The 74HC595 (latching shift-register) and ULN2803 are substantially cheaper than a UCN5821A. But, they take up more space, and this was important in my application. Also, my UCN5821A's were free samples...

I had to use this approach for the 12V trigger outputs because I couldn't find a high-side driver that had an integral shift-register. To help with space I used a surface-mounted 74HC595 - this obviously saved having to drill the extra holes for it!


This FAQ was last updated on September 28th, 2009.

Part numbers and prices were updated from the relevant websites.