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The TNT Pressive®

A DIY passive preamplifier

[Italian version]

Product: TNT Pressive - DIY passive preamp
Manufacturer: TNT-Audio DIY design - NOT for sale!
Approx. cost: variable, depending on configuration/components
Designer: Giorgio Pozzoli
Published: August, 2001 (originally published in 1997)

The preamplifier is the device that has always attracted me, from a "circuitry" point of view.
The main duty of a preamplifier is to raise the signal from a source's output level to an amplifier's input level.

A "high level" source's (that is, excluding microphones and phono cartridges, with their very low output levels and other problems) output level is typically in the hundreds of millivolts range, while an amplifier usually sports an input sensitivity in the volts range.
Thus, a preamplifier's typical gain must be chosen in accommodation of those levels.
On the other hand, it's true that in some particular cases the amplifier can sport a very high sensitivity and input impedance; yet it still is not an integrated amp, because it lacks all the control facilities usually found in the preamplifier section, such as input selector, tape output and even a volume knob.

In such cases, a passive preamplifier can be used - a preamplifier without active amplification stages.
Obvious pluses are a strong reduction in the number of components along the signal path and the absence of active stages that can possibly pollute a signal with power-related disturbances.
Minuses are due to the intrinsic nature of passive devices (unless coupling transformers are used, output signal levels can only be lower than the input one), which limit their use to systems with sources sporting outputs higher than amplifier inputs, something that is not so common.

It's worth noting that, in this case, the amplifier's input actually presents an integrated-amplifier-like sensitivity, and the passive preamplifier's role is only to supply  that part of the circuitry that allows it to transform the amplifier into an integrated amplifier.
It must be stated clearly that, in the case of DIY amplifiers, the right solution is to place the input selector, volume and balance controls into the amplifier's chassis, making it a real integrated amp: it's completely wrong to add a couple of meters of signal cable and four RCA sockets, if it can be avoided.
Yet, since I happened to be in the aforementioned situation, a passive preamplifier was a pretty correct solution.
In my particular case, I had to drive an amplifier with a vacuum-tubed input stage (EF86, a very good signal penthode for audio purposes), with an input impedance of  220 kohm and an adequate sensitivity - to prove I'm right, the preamplifier with which it is normally matched is an active device, but with unity-gain.

What do we need a unity-gain preamplifier for? To have a pretty low output impedance ... since it is a tubed device, too, it presents an output impedance of about 220 ohm, and that allows for an easy drive of any amplifier sporting a sane input.
It must be said that it is an economically justified device, since it has a phono stage, so all power supply circuitry is shared with the preamplifier, and the cost of a the unity-gain line stage alone is probably lower than the connectors of its inputs...
To design and build a unity-gain line-only tubed preamplifier would probably not be so cost effective!

Now, back to the passive preamplifier. Clearly stated that it is not an ordinary device, and it is to be used only when the above concepts apply, let's take a look at its fundamental parts.

I wanted to make an extremely... weird (somewhat) line level preamplifier, since the device is to be used in two different ways.

It's a preamplifier that has the following characteristics

[Circuit schematic]

To be able to switch off the tape output you just have to make accessible an unused position of the tape out selector.
The choice of two independent potentiometers allows you to avoid one component in the signal path, while still performing a balance control; although, this tradeoff is paid for by a remarkable lack of operating comfort (you have two knobs for the two channel to be separately regulated...).
It's not an easy choice to make - if you want to adopt it, be sure to check it well before starting the assemblage of a configuration that does not allow you to switch back to a more usual configuration with two double pots, one for the balance and one for the volume.

Tape output regulation is rather anomalous. Actually, this second output was not conceived for a recorder: it was so this preamplifier could also be used as a switching box to test  line stages or signal processors, comparing their output with the "direct" signal. In this case, it has to be used according to the following scheme.

[Connection schematic]

In such cases, accurate (and objectively correct: instrumentationally checked!!!) regulation of the listening level is fundamental; a slight difference in level is enough to significantly alter the soundstage, making musical instruments appear closer or farther when switching between the inputs.
Under this point of view, and if you eventually want to compare two different configuration of the same device applying each of them to only one channel, the independent channel regulation again becomes important.

This preamplifier doesn't solve all the problems - for instance, "direct" signal passes through only one potentiometer, while the one from the D.U.T. passes through two pots.
Yet, in the second case, it must be taken into account also that the signal passes through the usual four RCA sockets and interconnects. In other words, you cannot think that you can have absolutely "exact" comparisons, because the internal cabling in the preamplifier is different for the diffenent paths, and even the internal cabling for the left and right channels are not exactly the same, and furthermore, the active components met by the signal are different, and may not have exactly the same characteristics (what about two valves with a gain difference of several dbs?...) etcetera, etcetera, etcetera.

There are also other, very serious, problems - for instance, output impedance of a passive preamplifier depends on volume or output level regulation, and that could alter frequency response in one of the two different configuration.
Moreover, output impedance depends also on the precedent device's output impedance; thus, comparisons between the two configurations depends on the relative impedance of both devices (source and D.U.T.), and that could as well be part of the test, as an undesired phenomenon.
What I want to clarify is that this preamplifier is useful only to have a stable and repeatable scheme with which one can perform comparison tests, with correct level regulation, something that in lots of cases is (wrongly!!!) forgotten.
This is such a simple kit that its assembling is pleasant and its cost is modest, particularly if you point also to a DIY cabinet ... (here is the link to its Italian page).

About assembling, below you can find a parts list to realize a good level of achievement.
You can also choose other solutions (printed circuits or other), but point-to-point wiring is certainly fast and pleasant.
An elegant alternative could be to substitute pots with stepped attenuators made of rotating selectors and resistances.
This would lead to interesting results, but at prohibitive costs. Keep in mind that we need four independent single pots.
Anyway, keep all the connections as short as you can, connect the ground to the inputs, and the cabinet to the ground near the inputs. Connect the outputs' grounds to the the corresponding pots' grounds.

I personally mounted all the components near the back side, extending the controls for input selector, rec out selector and left and right volume to the front side with internal extensions.
Tape output level pots are mounted backward, looking toward and running through the back side, so that they can be adjusted using a screwdriver (luckily, I found components with the appropriate slots: if your doesn't have it, gently saw it...).

The preamplifier, obviously, doesn't need any power supply.
Anyway, take into account all the usual precautions, because if other devices shouldn't work well, a voltage could be carried in by signal cables.
In that case, there could be voltage from the AC wall outlet or, in case of tubed equipment, even much higher voltages (even DEADLY!!!). Do not underrate safety rules (here is the link).

An extreme solution could be a power cord, whose live and neutral could be eliminated by hard wiring them into a terminal block strip, and whose ground wire should be connected to the ground of the preamplifier (check your local electric code).
BEWARE: this is a solution to maximize safety, but it certainly creates ground loops (for instance, a loop is made between the grounding of the preamplifier, connection between the ground of its cabinet and signal's ground, connection from here to source's signal ground, via a ground wire or the signal cable shield, the link between source's signal ground to source's cabinet ground, and to AC ground via the source's power cord) that can create buzzes and other problems.
The PressiveŠ works not with the lowest level signals, so this phenomenon is not very probable, yet it can happen!

If, in particular, you connect old tubed equipment, keep well in mind that they normally had their chassis connected to the AC outlet.

What's worse, if you normally use non-grounded devices, nothing happens (if you are well insulated, you can touch the devices' ground  without even thinking you have a problem), but if you link it to a properly grounded device via an RCA interconnect that, unlike the old habits, doesn't lead to the output transformer, you will at least have your general AC breaker or fuse to shut down... and probably the device to which you connected the preamplifier destroyed!

Here we have the components' list:




RCA sockets 10 (for 3 inputs) gold-plated and teflon-insulated
Input selectors 2 2 ways, 6 blockable positions, gold-plated contacts, blinded
100Kohm B potentiometers (logaritmic) 4 blinded; eventually plastic-film type
cable depending on layout teflon-insulated silver copper

© Copyright 1997-2001 Giorgio Pozzoli - http://www.tnt-audio.com

Translation: Carlo Iaccarino - Supervisor: Michael McDonald

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