How do I build one?
Introduction
This page gives an overview
to building an audio preamp system using these designs and sets out the skills
and facilities which will be needed to undertake this project. A design example
is given for a stereo preamplifier configuration and practical issues for
connecting source equipment and setting the audio levels are
discussed.
The details for each of the
cards are available on their own pages.
Resources Available From
This Website
This website provides
designs which when used together can form an audio preamplifier with a number
of audio input sources and outputs. Bare PCBs are available (see the
Ordering page) which can be used to build a system to
this design. Electronic components or built/tested circuit boards are not
currently available from this site, so the user will need to be able to source
the components, construct and test the individual PCBs - and then test the
complete system.
As the requirements for
power supplies, cases, cables and the control system will be specific for each
user, comments are made on this website to assist the user in designing their
own appropriate solutions.
Full detailed connector
pinout schedules and component schedules (including part numbers for a supplier
- Farnell Electronic Components) are
available on the Photos/Downloads page.
Skills and Facilities
Needed
Do read through all of the
pages and make sure you really wish to build one of these systems before you
start! The investment in time and money will be significant, especially for a
large system - constructing/testing all of the cards, engineering the power
supply, case, control system, etc.
Do check that your costing
is complete before you start.
To build a system, you will
need standard electronic construction equipment including:
- fine tipped soldering iron, preferably
temperature controlled (there are no surface mount
components)
- fine cutters, pliers, and ideally a
crop/forming tool to speed up the preparation of the large number of
resistors
- a PCB assembly jig (a rotatable frame
with a removable foam lid) will also help if a large number of cards are made,
allowing components to be inserted together, before soldering
- test equipment (such as oscillator,
oscilloscope and/or an audio test set).
Design
Example
Overview
A design example is given
for a large stereo configuration, with all of the input and output cards housed
in a single rack chassis frame - which represents a common configuration using
the available cards. This can be used as a guide by users wishing to construct
other system arrangements (e.g. 6 or 8 channel surround
sound).
Design
Scaling
The design example shows the
maximum number of cards which can be fitted into a single 19" subrack frame.
The Motherboard design allows up to eight Dual Input Cards (16 stereo balanced
inputs) and four Dual Output Cards (8 stereo balanced outputs) to be installed
- although other configurations are possible.
This example assumes 16
balanced stereo inputs, two balanced stereo outputs (for amps/loudspeakers in
two rooms), four balanced stereo recording outputs, and one set of transformer
balanced distribution outputs (left, right and mono to feed a multi-room system
in a house). Decide how many inputs you will actually need (a simple
calculation for stereo sources), remembering that there are two stereo inputs
on each Dual Input Card. The number of outputs will be decided by
considering:
- how many loudspeakers/amplifiers you
will wish to connect directly to the main preamp subrack
- if you need to have independent
selection of busses (two stereo busses on the Motherboard) and audio levels to
a number of separate record outputs. It should be possible in most cases to use
one stereo output to feed a number of record inputs in parallel - but remember
to separate any balanced and unbalanced record inputs.
- if you need a line-level distribution
(eg. to other preamplifiers around the house) and if you wish to include a mono
feed or fit transformer outputs (creating a mono output as well as a stereo
output uses a complete Dual Output Card)
remembering that each Dual
Output Card has two independent stereo outputs (so a single card could be used
to feed one power amplifier/loudspeaker pair and a record output).
This system can be built
(and used) in stages as time/money permits by first building the Motherboard
and some/all of the Dual Input Cards and initially using it with a simple
volume control arrangement plus a rotary switch (DC control signals) to select
the input channel required. A more comprehensive control system can be added
later with the Dual Output Cards, allowing remote controlled source selection,
volume and balance etc.
Costing
Parts lists are available as
Microsoft Excel files, which give part codes for
Farnell Electronic Components, but there
are many other suppliers for most components. Each Excel spreadsheet can be
used to calculate the total number of each component needed from the number of
cards the user requires.
The total costing for the
project should include:
- Dual Input Card PCBs and
components
- Motherboard PCB and
components
- Dual Output Card PCBs and components
(or a simple passive volume control equivalent)
- Audio cables (eg. XLRs to XLRs, XLRs
to phono plugs, etc)
- 19" subrack or an equivalent case for
the audio cards, together with top/base and rear screening plates
- Power supply for the audio cards and
for the control system (and case, DC power cables, etc)
- Control system (either microprocessor
design, simple logic or rotary switches) together with a control panel/display
(do not underestimate the cost of making an attractive control panel....if
needed) and connection cables.
Configuring And Using A
System
Overview
This section discusses the
practical issues of connecting and aligning a system after it has been built.
The compromise between noise floor and headroom is also discussed as some users
may wish to adjust the operating level to have the lowest possible noise
floor.
Audio Cables and
Earthing
One of the advantages of
this preamp design is the balanced input stage, which works to reject
common-mode interference (hum etc) from balanced and unbalanced sources.
However, in some circumstances, it is still possible for hum to be present and
so become just audible during quiet passages of music, if played very loudly!
To help overcome these potential problems, a few examples of connection cables
between sources and the input card are given below.
For a balanced source (e.g.
a professional CD player) a conventional 3 pin XLR plug to socket cable can be
used - where the signal wires will be a twisted pair (or "quad" - two twisted
pairs arranged to further reduce hum) with an overall screen (or
shield).
If both the source and the
preamp are connected to the same earth intentionally, or inadvertently through
other cable connections, it is likely that earth currents will flow through the
cable screen. If these currents are large enough compared to the audio signal
level, then this will induce additional signals into the twisted pairs. A
technique to avoid this is to only connect the cable screen at one end of the
cable, as shown below.
It is good practice to
always earth the same end of the cables - probably in this case to connect the
screens at the preamp input end of the cable. This technique is best for
rejecting low frequency interference which is most common, but is not as
effective with high power RF signals.
Where an unbalanced source
is being used (e.g. a domestic CD player), the two twisted pair signal wires
from the input stage should be connected to the two pins of a phono plug (RCA
jack) with the cable screen only connected at the XLR socket, as shown
below.
Note that most commercially
available XLR to phono plug cables do not do this - the cable screen is often
connected to the cold pin of the phono plug together with the cold signal wire.
This will remove any benefit of the balanced input stage!
If the source equipment is
fully floating (clearly a battery operated, plastic cased CD player will be...)
there may be hum problems connecting it to the balanced input, due to very
large common mode hum signals. If this happens, then use the earth link plug on
one of the channels on the Dual Input Card PCB (e.g. left channel only) as this
will provide an earth reference to the source equipment from one point. Note
that only one earth link plug should be needed, as virtually all domestic
equipment has a common "cold" signal connection between both/all channels. (If
no sound is audible when using the earth link plug...this may be due to the
source equipment being earthed through another path and the hot/cold wires in
the cable inadvertently swapped, so causing the earth link plug to short out
the audio signal...!)
There are similar connection
issues for connecting the balanced outputs from the preamp to unbalanced
equipment (such as recording inputs on VCR/DAT/MD and power amplifier inputs).
These will be covered here when the Dual Output Card design is
finalised.
Remember: do not
disconnect mains safety earths from equipment to reduce hum/interference, but
find the cause of the hum instead.
Aligning The Audio
Levels
The input levels onto the
Motherboard busses are adjusted using the gain link plugs on the Dual Input
Cards. To do this, a device which can show true peak audio levels will be
needed, such as a test meter, oscilloscope (a DAT/MD/PC Sound card input meter
can be used to compare the levels between sources, without knowing the absolute
levels). If the user chooses the signal levels suggested here (see the
following section) then the design will carry "peak" (i.e. typical loudest
signals) onto the busses at -2dBu (equivalent to 0.616V rms or 1.74V peak to
peak). The maximum signal on the buss using these card designs is +20.8dBu,
thus there is at least 22dB of headroom above the nominal loudest level (which
may be useful for unexpected very loud signals such as scratch clicks from a
turntable).
With a test meter or an
oscilloscope, connect it to a buss or the output of the Dual Input Card and
play typical loudest items from each source such as a modern pop music CD, or
pop music radio station as these frequently use the full available dynamic
range. Then adjust the gain link plugs until the level is close to the -2dBu
nominal value (with the input card configured for 0dB of gain overall, then
most mass-produced domestic CD players will produce peaks to -2dBu). The
following order of setting the plugs is suggested to maximise headroom and
minimise noise:
- remove link plug SWx01, to give 0dB
gain on the first stage, and set SWx02 and SWx03 to their 0dB
positions
- if the signal is too loud, then use
the attenuation plugs SWx02 and SWx03 in parallel (6dB steps)
- if the signal is too quiet use the
SWx01 gain plug (3dB steps)
- as attenuating signals with the 6dB
step sizes is a "coarse" adjustment, the SWx01 gain plug in 3dB steps can be
used to make the final "fine" adjustment.
If the Dual Input Cards are mounted in a
rack, it may be easier to connect the sources one at a time to one input card,
where the link plugs can be accessed without needing to pull the card in/out of
the rack each time before making an adjustment. Then when the gain settings
have been found for each source, the gain needed on each channel card can be
set. Remember to set the gain plugs in the left and right channels to the same
values...!
To confirm that the gain
settings on each card are correct, play similarly loud music on all sources and
switch between them - the audio levels from all sources should sound
similar.
Compromising Noise for
Headroom - A Discussion
In the days of analogue
turntables when there were no digital sources...having a significant headroom
margin above the nominal "loudest expected" signal level in the electronic
stage before the volume control was very important. This was because the audio
could be swamped by very loud (but brief) scratch clicks which could distort
the rest of the audio signal, and (with some designs) cause ringing/instability
in the amplifier as it recovered from "overload".
A high headroom margin is
also important in designs where the input sources are passively switched (eg.
by relays) to the first stage of electronics, as there is no control over the
input source gain until the volume control. So, the first stage must be able to
cope with widely varying levels from different source equipment - which this
design overcomes by having gain individually set for each channel.
Modern digital audio devices
have a defined signal level range, as it is set by the voltage swing available
from the D to A converters. Most CDs these days have audio levels which reach
the maximum level at some point on the disc. Some headroom is still needed to
allow for "clipped" peaks being "restored" by filtering after the D to A
converter - but only 2 to 5dB is probably needed. [Measurements were made with
different types of D-A converter/output filter to see how much larger the
output analogue signals could be when fed with "clipped" (flat-topped)
waveforms, compared to a digital full scale sine wave. Figures of 2-3dB were
typical, which is similar to measurements made with "electronic" pop music. The
extreme case with a digital square wave whose frequency was very close to half
the sampling frequency produced an analogue signal 8.5dB higher than the
equivalent full scale sine wave.]
This design (including the
component values and power supply/voltage regulator operating voltages) offers
22dB of headroom above the typical loudest signals, once the signal is
on the buss. (Note that headroom is often quoted above a "nominal" operating
level - but headroom above the "loudest expected" level is more meaningful.)
Where the highest possible input signals are fed in to a Dual Input Card (ie.
with 18dB attenuation set), then the input stage headroom reduces to 9dB - but
no more is needed as the source equipment will also have run out of supply
voltage range to generate the signal, even taking into account flat-topped
waveforms from a D-A converter/filter. (The design will accept +26dBu as the
maximum input signal, which is typically the highest signal level found in a
domestic environment, and in most professional installations too!)
(Full details and graphs of
the Dual Input Card's performance are available on the Dual
Input Card - Performance page.)
The only reason to reduce
the headroom (ie. have louder signals on the buss) would be to increase the
effective signal to noise floor ratio. This would only be necessary if the
listener could perceive noise from the preamp, power amp and loudspeaker
combination, when set up for typical listening. (The headroom can be reduced by
changing the resistor values in the Dual Input Cards once the user has decided
on the signal levels etc needed.)
A discussion below shows why
the signal levels/headroom used were chosen for this design and the effect of
this in a typical listening room with a very low noise (good quality) power
amplifier and loudspeakers.
In a typical (medium sized)
listening room, a pair of high quality loudspeakers (eg. 85dB SPL for 1W input
efficiency) standing 3 metres away from the listener, can produce 105dB SPL at
the listening position (above 100Hz, so ignoring room mode effects below 100Hz)
when driven with a 300W amplifier. As a comparison, a full symphony orchestra
playing in a concert hall at fff (very loud) can produce 100dB SPL - so, if you
wish to recreate the full volume experience in your own home (and have no near
neighbours...) this is possible, leaving a further 5dB of "headroom"
!
A very low noise power
amplifier (such as a Bryston) has a signal/noise ratio of 113dB, so the noise
produced (solely) by the loudspeakers/amplifier would be -8dB SPL. Most
domestic listening rooms will have a background noise level of at least 10dB
SPL, due to traffic noise from nearby roads, heating/cooling systems, power
transformer acoustic buzzes etc, which will clearly mask any noise from the
loudspeakers/amplifier. Although 0dB SPL is the average "best" threshold of
human hearing there is a wide spread, so this is another factor which is likely
to prevent people from hearing the system noise even in a room with an
extremely low background noise.
When this preamp design is
used, the signal to noise ratio at the buss is 100dB, and around 97dB when
measured at a Dual Output Card (with 0dB gain set in the volume control - ie.
real use). If used with the power amplifier etc discussed above with its input
gain control set to produce 300W with the "loudest" expected signals, then the
noise at the listening position would be 8dB SPL which again should be
inaudible in most situations.
The best signal/noise ratio
which can be obtained from the design by choosing to have no headroom is
approximately 123dB (which would then be masked by the noise from the power
amp, being 10 dB worse...).
In summary, the design
offers a noise performance which will be inaudible in most circumstances whilst
still reserving a sensible headroom level to cope with "unexpected" high signal
levels from digital audio converters, vinyl disc clicks etc.
Note that the discussion
above ignores the effects of the ear's frequency response weighting at
different sound pressure levels and the noise spectrum, peak to mean ratio etc
of amplifiers, which will all contribute (in some way) to the perceived
audibility of any system noise.
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