How to Build DIY hybrid amplifier

How to Build DIY hybrid amplifier

Reviewed by: Lillian

On: 10 Okt, 2020

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Category: Amplifier

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There are projects that take shape over the years, this is one of them. At the beginning, the idea was to put a power amplifier module with Raspberry Pi and DAC in one housing. The whole thing should work very modern with switching power supplies ...

There are projects that take shape over the years, this is one of them. At the beginning, the idea was to put a power amplifier module with Raspberry Pi and DAC in one housing. The whole thing should work very modern with switching power supplies and a Class-D power amplifier with little effort. The result in a simple wooden box looked like this at the time. The wooden box was so well received during a demonstration at a meeting of the DIY HiFi enthusiasts that it soon found a new owner. The new owner has now bought a new, prettier dress for the technology. The structure still works without any problems.

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A short time later, the Class D modules from Anaview were praised in a forum and a collective order found participants quickly enough to make the prices for the modules very cheap. Since the Anaview modules have modern, symmetrical signal inputs, a special board was also designed by a member of this forum in order to balance the old signal sources (such as tuners, CD players or turntable preamplifiers). At this time the idea was born to connect the ALC-0300 modules with a balanced tube preamp. Nils aka Sparky was quickly won over as an ally for the project. Back then, he built the tube preamp, while I housed the Anaview modules and preamp boards in the tried and tested wooden box construction. The crowning glory for the amplifier should be two fidget hands and a circuit board from the Far East. In order to operate the Anaview power amplifiers with classic sources, relays were built in to be able to switch between the symmetrical and asymmetrical inputs.

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The joint handicraft days, on which we tried to marry the tube preamp with the Anaview modules, brought us the following experiences:

The output levels of approx. 15V of the tube preamplifier are too high for the inputs of the Class-D power amplifiers known to us. A level adjustment is therefore absolutely necessary.
In addition to the level, it is also necessary to adjust the impedance.
Classic tube construction with ground and earthing is incompatible with the grounding of the Anaview modules

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The last point caused the project to fail. The Anaview power amplifiers found a new owner and I got an unbalanced tube preamp with "magic scales" from the Sparky. Unfortunately, I had no use for the preamp as my Good Old HiFi collection consists almost entirely of integrated amplifiers.

In the meantime, we have dealt with the practical question: switched-mode power supply or classic transformer with rectifier for Class D. Our measurements and ears have recommended the use of a transformer with rectifier including filtering. (We have documented our experiences here in the forum: Pimp my miniAmp

The topic of Power Class D has not let go of me. Since the first Class-D circuit board from HiFiMeDIY worked well, the thought arose: Why not use something from their range again? I found what I was looking for on the provider's website: The HiFiMeDIY T3 board should be. With a classic transformer. Since this stereo power amplifier has a fixed gain, a preamp is required. I chose a PGA2311 preamplifier, also from China.

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The craft fever grew higher. Calculate the transformer and have a custom-made product made. A 1kW toroidal chunk like that is pretty heavy. Appropriate sieving estimated and a lot of µF obtained. (Here, too, a reference to a forum thread: Unfortunately, I didn't have the time to install the whole thing myself. Thankfully, Sparky took over the construction of the prototype (in return, a fine-pearled hop & barley spritzer was agreed). This was finished in time for the Spacko meeting in Halver.

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After a successful test run, the monster was transplanted into a matching sheet metal dress at Nils.

The idea of ​​the hybrid amplifier was revived! The pre-stage also got a new sheet metal cover and two potentiometers to destroy the output voltage.


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The first test runs with the tube preamp and the output stage direct input of Technics SU-V900 have shown that Nils' preamp plays very detailed and has no problems with dynamics. The "PowerAmp-Direct" input of the Technics SU-V900 gets along well with the tube. For this purpose, the "burned out" pots for the voltage of the tube have proven to be very helpful in setting the optimal balance between the output level of the preamp and the input sensitivity of the Technics power amplifier.

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Since the potentiometers represent an adjustable voltage divider, a small output voltage of the pre-stage has the disadvantage of destroying the fine details as well. Therefore the output level should not be too low and still match the power amplifier.

The do-it-yourself amp made of a PGA2311 and T3 power amplifier has two inputs that go directly to the power amplifier. The tube preamp was connected to one of these direct inputs and the level was set using the potentiometers. Due to the sensitivity of the T3 output stage, the full scale has already been reached at an input level of 1.5V. To do this, almost the entire output voltage of the tube must be destroyed with the voltage dividers. Go. Experiment works. Unfortunately, the sonic experience can be described as barely sufficient, as too many musical details are wasted.

What now? The solution is to use an input transformer (LineIn transformer). On the one hand, this achieves complete galvanic decoupling between the preamp and output stage, and on the other hand, the voltage can be reduced. That’s the theory.

So turn on the soldering iron again, carve out a breadboard and tinker with inexpensive Monacor LTR-110 line transformers. These have an input impedance of 600 ohms and 300 ohms when used 2: 1. This halves the input level in the transformer. This brings you closer to the desired 1.5V.

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First installation, switch on and enjoy 50Hz hum at full volume. Take a shielded cable, because the T3 power amplifier is a bitchy diva. Better.

Turn on the preliminary stage and let's hear what happens when the vinyl is played. Again a 50Hz hum is superimposed on the music. But where does it come from? Preamp and output stage are galvanically separated. After a few attempts and the consultation of forums it is clear: Since the T3 power amplifier can play so fine, it is also able to give back any dirt that collects in the line. Unfortunately, the manufacturer of the Class-D power amplifier does not reveal the optimal impedance for the pre-amplifier that is playing, which also turns the adaptation using the LineIn Transformer into a game of chance.

After some trial and error, it now works with a shielded cable, whereby the shield is connected to the signal ground of the output of the preamp. The hum is gone, but can come back (voodoo stop).

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With the line transformers now working satisfactorily, the level of the preamp can be raised a little. It is not enough for an absolute top result, but the achieved sound quality is already above the PGA2311 China board.

The exciting question is: project completed?

Actually yes and then not.

The current status is an extremely powerful integrated amplifier operated with PGA2311 and HiFiMeDIY, which plays incredibly cleanly. As Sparky once said: Tell the amplifier to deflect the bass of the Grand Duetta by 0.1mm, it will do exactly that.

The potential of the tube preamp is not yet exhausted by the Class-D power amplifier, there is still room for improvement. On the other hand, the result achieved is already very good, just bitchy.
In the case of the tube preamp, the magic balance could be converted from a linear level display to a logarithmic one, if a suitable circuit could be found.

One option to make optimal use of the preamp would be to switch to other LineIn transformers. According to current knowledge, 4: 1 (possibly 8: 1) transformers would be optimal. These are available from the Swedish manufacturer "Jensen Transformers". This is where the financial aspect comes into play. Perhaps the better JT-6110K-BTB transformers have a price of 100U $ per piece (plus a few additional costs such as shipping, VAT, possibly customs fees). The costs are too high for an experiment with an uncertain outcome.

Alternatively, one could still try to reduce the level further after the existing Monacor transformers by means of an OpAmp circuit. But here, too, it means a development effort for a suitable circuit including impedance matching, and it also requires a further symmetrical power supply. And there would be more components in the signal path again, which is not always beneficial.

This project shows that despite initial failures and failures (such as unsoldering measuring resistors using Class-D modules because of a short to ground via the signal path, etc.) it is not impossible to implement something like this. The motto is: Fall down, get up again, straighten the crown and move on. Connecting tube preamps with "classic" transistors is not the big problem. Class-D, at least DIY circuit boards, represent a special challenge here, as the manufacturers usually do not come up with the optimal operating values ​​and you can only approach them with something that a technician lecturer for control engineering, his character Doctor of Physics, sometimes referred to as “approximate approximation”, which describes the word “approach” very nicely. This project is instructive and gives a lot of new knowledge about the interaction of different technologies and their pitfalls. Where we were initially far from a functional prototype, we now have a version that plays on the level of modern hi-fi devices. However, there should still be potential upwards and so we continue to research how this can still be developed without having to recreate the ISS or call in a shaman.

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