"Le Mono Bloc"

MonoBloc Amplifier

These amplifiers are not in production at the moment they are available 
on demand and only by quantities).
Your demands are wellcome.

Technical leader, the Mono Bloc is integrating all the J.-C. Verdier know-how in the hi fi power amplifier field.

Its distributed output stage has 10 EL 84 tubes in push-pull, parallel biased in pure A class. The input stage, composed by of symmetric structure of bipolar transistors, gives a harmonic range of sound, almost perfect.

The Mono Bloc amplifiers are always delivered in pairs, carefully selected. Their unconditional stability and their distinguished aptitude to suit to most difficult loads, makes them ideal for driving low efficiency speakers and big static or isodynamic panels.

The MONO BLOC amplifier is the fruit of a long study.

Its original circuit has been realized for 2 year before leading to a first commercial realization in 1989; it was the 250/275 amplifier built on a unique chassis. In 1992, this model was replaced by the Mono Bloc, taking the same scheme but with more advantages : more powerful, from 50 to 70 Watts per channel and complete division of the two stereophonic canals by the use of distinct chassis which give the name of the apparatus.

Technical analysis

The input stage of the amplifier is constituted of a differential pair of PNP transistors attacking the tension amplifiers stages constituted of 2 NPN transistors coupled directly with the PNP Darlingtons to ensure the very low output impedance.

The whole of these circuits is subject to a double internal feed-back which it is possible to modify the rate of one of the loops. (adjustment of gradation)

The Darlingtons are followed by the tube output stage inflecting the power throughout excellent Millerioux transformers
The power supply is ensured by a transformer widely calculated of 250 VA.
Integrated regulators guarantees the perfect stability of the power supply on input stages.
All the stages are in pure A class.

Adjustable Harmonic Spectrum

It is now highly showed that a very low distortion rate don't forcedly give the best auditory result, on the other hand, a regular gradation of the harmonic distortion spectrum is the guarantee of a natural reproduction and devoid of aggressiveness.

The virtues of the mono triode amplification are well-known in this domain. The particularities of the Mono Bloc amplifier allow for the first time, the modifying of the harmonic distortion spectrum shape.

By the mean of an adjustable potentiometer, it is possible to have a rich impairs harmonic spectrum (pentodes push-pull characteristics, often qualified as "hard to hear") or a smashing gradation worthy of the best direct heating triodes.

Linking without capacitor

Power tubes are fit directly to the phase inverter/pre-amplification board without linking capacitor.

At the time of the modulation peaks, the linking capacitors of a classical amplifier discharge under the effect of grid courant and modify for a long time, the effective polarity of the power stages. A pure A class amplifier can then be in a B Class during more than a second after a surcharge bringing about all kind of temporary distortions.

This disagreeable phenomenon is radically removed by the direct link.

Incidentally, this plan of action has the advantage to allow, by the mean of a complete modulation of the output tubes to extract much more power without been forced to rise the power supply tension in proportions, dangerous for the length of living of the tubes . So, the Mono Bloc amplifier is said 70 Watts with a plate tension of 270 Volts while the EL 84 tube can run safely with 300 Volts.

Distributed Power Stage :

The power stage is constituted with a quintuple push-pull of EL84 tubes.

The putting in parallel of the tubes in the purpose to modulate a superior power is back to the radio techniques beginning. And don't constitute an originality. For the Mono Bloc amplifier, the technique of distributed amplifier has been retained. It consists in the introduction of impedances which value is carefully determined between the grids and the plates of each tube in the aim to constitute delay lines. The tube equivalent to five tubes fitted following this principle sees its transconductance multiplied by 5 and its output impedance divided by 5 while its input capacity stays still, all important characteristics for a decreasing phasing rotation is an extension of the bandwidth to the high.


All the electronic circuits are organized on a single printed board. The links to the other parts are made with a cabling with high quality wires and the cables are gathered.
The chassis is made in stainless steel of 15/10 carefully polished. A cover with the same appearance protects the electronic part. The whole, is assemble on an removable underside painted in a black satin leather grain color. The paint is furnaced.

The transformers are particularly manufactured very carefully. The output transformer (Milleroux FH47B) is coiled up in 17 sections gathered on a magnetic circuit in steel sheet with oriented grain anneal.

Impregnated with the wax, the transformer is closed in a steel partition. The power supply transformer of 250VA is vacuum impregnated. Finally, the 2 transformers are included in steel cases with leather look, a coating fullfiled with resin charged of marble powder to avoid any risk of vibration.

Moreover, the cleverness conception of a circuit is nothing if an hasty and disordered realization comes to compromise it.

Mono Bloc Amplifiers are settled and built according to the rule book, in the aim to ensure a troubleless functioning for your greatest satisfaction.


The following sheets have been realized on a PC compatible with analyses application of measure CLIO.
All these curves are made automatically and are printed without any retouch. Discontinuities that appears, shows the limits of the computer possibilities but not the amplifier failures. To interpret correctly these curves, we have to smooth the lines. For a maximum objectivity, this is left to the reader of these documents

Analyses Curves

To see a curve, CLICK on the link below

Power requirement

220 Volts

Power consumption

250 Watts

Power output

70 Watts at 8 Ohms

Input Sensitivity

1 Volt

Power bandwidth (-3 dB)

20 Hz 80 kHz

Total harmonic distortion

0,2 %


Power :

Three terminals sockets CEE Standard

Input :

RCA socket insulated from chasse

Output :

Terminal posts for 4 mm banana plug or tripped wire.

Dimensions :

300 (W) x 170 (H) x 390 (D)

Net Weight:

14 Kg

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