"The sole objective of the audio industry as a whole is to sell products..."
| Today, many signal sources such as CD players have sufficient output level to drive
most power amplifiers directly. It means that no additional amplification is required,
so for level purposes there is no need for a preamplifier. With a passive volume
control in between, the signal from the source may be fed directly to the power
In many cases, a passive preamplifier sounds as good or better than the best active
preamplifiers. In other cases, it is our experience that a good line stage following
the volume control may sound more dynamic than a passive volume control. It seems
to depend on many different circumstances, which solution is the better. For instance,
the type and length of cable connected to the output of a passive volume control
stepped attenuator (24 steps with Vishay/Dale military grade resistors)
An active preamplifier usually has high input impedance and low output impedance.
This fact makes the choice of interconnect cables relatively uncritical.
When the preamplifier is replaced by a passive volume control, the situation is
different. The input impedance of a passive volume control is usually not as high
as that of a power amplifier and the output impedance is usually high compared to
a preamplifier's output impedance.
If care is taken in dealing with this issue it is not a problem in most Hi-Fi systems.
The most important matter is to take care of the impedance of the interconnect cables.
The output impedance of the source (for instance a CD player) is usually relatively
low. Therefore, the interconnect cables between the source and the passive volume
control are not as critical as the interconnect cables between the passive volume
control and the power amplifier. The key factor to look out for is the loading capacitance
on the volume control's output. In order to maintain a large bandwidth, the loading
capacitance should be as small as possible. Otherwise, the load capacitance combined
with the volume control output resistance makes a low pass filter that cuts away
treble in the audible frequency range.
Calculate Stepped Attenuator
final stepped attenuator [ 10kOhms ]
Source: Danish Audio ConnecT
Click here to download
PDF version (129KB) of this Passive Preamp Technical Paper from DACT
Here you can buy a stepped attenuator for a goed price
ELMA Rotary Switch Type 04
||Some important point:
- Use as short interconnect cables as possible
- Use cables with low capacitance
- Use low attenuator resistance values (recommended total resistance: 10kOhms)
Your sound system is only as good as the weakest link.
Volume control is one of them.
Calculate Stepped Attenuator
My Passive PreAmplifier
The front panel is designed with the Front Panel Designer from Schaeffer AG
You design your front panel and if you are finished, it is possible to check the
price and to order this panel!
You should try this application, it is free!
In rare cases a DACT switch (attenuator or input selector) may start making noises
in the speakers when operated. The necessity of cleaning will depend on the environment
in which the switch is used (air pollution, humidity, temperature, dust).
The phenomenon has to do with particles being attracted and attached to areas on
the gold plated contacts on the PCB, rivets or sliding contact.
It shows up as noises in the speakers when the switch is operated. It often only
occurs at a single or a few of the switch positions.
Unless any unusual noises are heard there is absolutely no reason for carrying out
the contact cleaning procedure - we recommend never to make any cleaning unless
DACT only warrants one cleaning agent for this purpose: Tuner 600 manufactured
by Kontakt Chemie (CRC Industries Europe NV), Belgium. Tuner 600 is a quick-drying
solvent cleaner, leaves no residue and is neutral on materials. It does not contain
aromatic and chlorinated hydrocarbons.
The product is commonly available from main electronics distributors.
Examples of such are:
RS Components (http://www.rscomponents.com)
In case of any sourcing problems we recommend visiting the manufacturers web site
Source: Danish Audio ConnecT
Price for this Supermalloy Transformer Attenuator STA-522: about $340 USD
Benefit of Transformer Attenuator:
The benefit of replacing plastic type volume control to stepped attenuator is obviously
the accuracy of tracking and the quality of selector switch plus low noise resistors
can reduce the signal loss to some degrees. However the regular stepped attenuator
featuring resistor type fails to offer better impedance matching over plastic volume
control. That's why at high attenuation level (low signal out, low listening level)
the bandwidth of such volume control is very poor and the bandwidth is not constant
over any switch position.
Transformer offers much lower signal loss and much lower output impedance (10 times
lower than regular stepped attenuator) thus it will process very wide frequency
response at any attenuation setting.
Comming weeks i will change the Stepped (resistor) Attuneator in my Passive Pre-Amplifier
with this new Transformer Attenuator...
This transformer can be used in replacement of any existing volume control device
in pre-amp or integrated amp, but it can also be great used as stand alone Passive
Since it has very low output impedance, it can drive long cable better than regular
Passive Pre-amplifier; moreover, its ability to make +6dB signal makes it very versatile
in environment where source level is low or load input sensitivity is low.
You can get such a transformer at:
The transformer i use is from SACThailand
240V Passive Mains Filter
|240V Passive Mains Filter
Considering the quality of the mains voltage with dishwashers, vacuum cleanders,
dimmers, computers and halogen lumination, it is a miracle that most HiFi equipment
seems to be immune to most of the mains polution anyway.
After investing so much money in your HiFi set you probably want to minimize your
chances on humm, clicks, HF noise etc. that could spoil the fun after all.
move cursor over picture for larger version
What is the purpose of filtering?
A filter is a device that lets mains frequencies pass and blocks other before they
enter the audio equipment. In case of a mains filter, it's task is to let the 50Hz
'signal' pass the filter and use it to power our equipment and at the same time
block all other frequencies since these frequencies are not designed to be used
by power supplies of hifi equipment.
Don't forget to use a shielded mains cable and use high quality mains cable to connecting
your equipment as well. Don't let mobile phones or other HF pollute your mains connections
on the last meters.
Schematic 240V mains filter
Used Main Cable: ÖLFLEX CLASSIC 100 CY