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 Hi-Fi
Preamp - Preamp FAQ:
Introduction:
I get a lot of emails about the preamp, and I try to answer every single
one. But, to save a bit of time, I've tried to compile the most commonly-asked
details 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 you can email
me or fill out the form at the bottom
of the page...
FAQ Index:
Section 1: General preamp questions:
Section 2: Component questions and sourcing information:
Section 3: Construction questions:
Section 4: Design questions:
Section 5: Feedback:
Section 1: General Preamp Questions:
Q1.1: I really like your preamp! Can I build one?
A: 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. Currently, PCB layouts, the parts list and
source code are not available.
Q1.2: Why are the parts list and PCB layouts unavailable?
A: 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 at this time.
Q1.3: Why can't I find the source code on your site?
A: At the moment, I'm not prepared to release the code. Partly 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 some effort
in, this should help considerably.
There is a possible option if I don't decide to make the source code public-domain
- I could consider programming 16F877's for people who wanted to build this
project. This is something that I will think about once the code is sufficiently
complete.
UPDATE - 25th October 2006: Since 2002, the position hasn't
changed. Basically, I've been too busy with work, moving house, and having
a baby to look after - the source code remains resolutely unfinished. I do
feel slightly bad about this, but am heartened by the number of people who
have contacted me to tell me that they are developing their own versions.
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 662-343) for £1.69+vat.
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 filled the recess with car-body filler! This worked quite
well. 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:
 These
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 678-077 (£0.45) and 678-041 (£1.94)
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 (£2.46), 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 - these are just £0.88
(part number 674-825). 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 £35
(part number 412-8795), and is highly recommended. Check here
for more info...
2.5: Red relays:
 Made
by NEC and stocked by Farnell (part number 309-3190). 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 feature low-power relay coils - the 12V versions I used
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 Monocor, but CPC don't
give the name of the manufacturer in the catalogue. They're used by lots
of companies, and are reasonably good quality. The CPC part numbers for the
single and dual pair versions are CN04159 (£0.67) and CN04160 (£0.86)
respectively...
 2.7:
Headphone socket:
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 IC's:
I got mine from Burr-Brown as samples. For ages, they weren't easily available
in the UK via any other route 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 843-4905 (DIL package). They
are a bit pricey - £11.77 for small quantities, but I think that's
reasonable value - cheaper than a motorised potentiometer, for example.
2.9: The UCN5821A relay driver IC's:
These are made by Allegro, and Farnell sell them (410-068). Unfortunately,
they're rather expensive - £5.53 in small quantities. If you buy 10,
the price falls to £2.69, which is more like it. These IC's are so
useful that I think I'll do that next time I need one.
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
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
IC's and not even had a reply, let alone the IC's!
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.
Example Farnell part numbers and prices:
14-way female socket with strain relief: 316-6867 (£0.227)
14-way male PCB mounting box header: 316-6946 (£0.361)
34-way male PCB mounting box header with latches: 316-6510 (£0.639)
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 and enabled the
compact layout of the RIAA PCB.
Farnell stock these, but they're not cheap. For example the 4n7 (106-070)
is £0.70 - 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 328-2338, and at the time of writing, it costs £30.07 plus 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 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, using a milling machine.
This 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 using an old tin of Halfords gloss-black paint intended
for car bodywork, I sprayed on some paint.
After lots of very thin coats, the result was a heavily textured matt finish.
I liked this finish, 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 so 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 close-up 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 considerable current, 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!
Section 5: Feedback: Was your question answered
by this FAQ?
Here you can tell me what you think about this FAQ - was your question
answered, or would you like to suggest a question for inclusion in the FAQ?
I did have a feedback form here, but it's been taken down. Thank the spammers.
If you still have a question about the preamp, please contact me from my
usual Contact page.
This FAQ was last updated on July 25th, 2006.
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