How to Build Audio DIY
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On: 06 Agu, 2020
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My (first) version of Class-A
Some time ago, I ran across the net in the creatures of Jean Hiraga. In particular, my attention was captured by the apparent simplicity of two of his class A power amplifier designs: Monster and Class-A. One ...
My (first) version of Class-A
Some time ago, I ran across the net in the creatures of Jean Hiraga. In particular, my attention was captured by the apparent simplicity of two of his class A power amplifier designs: Monster and Class-A. One of the problems faced today by those who want to try their hand at making these amplifiers is the difficulty in finding the transistors used in the original designs, which are no longer produced.
Below I propose, to those interested, a reinterpretation of Class-A. In addition to using transistors that are readily available today, the diagram I present differs from the original one, in particular in the power supply section of the input stage. In fact, I present two different versions which, from a technical point of view, differ in the choice of pilot transitors. The first scheme is the one that is more "faithful" to the original design, since, in the foreseen configuration, the amplifier absorbs a current of about 800 mA per branch at rest. In the configuration envisaged in the second diagram, however, the amplifier absorbs a current of about 400 mA per branch at rest.
As for the power supply, I suggest using the following scheme, recovered from the network.
The schematic in version 2 (which I do not report) is identical in all to the schematic version 1, except for the fact that the transistors TIP2955 and TIP3055 are replaced by BDX54 and BDX53, respectively. The final transistors heat up a lot, especially in version 1. They must therefore be mounted on a heatsink of considerable size.
From the point of view of musical reproduction, the amplifier in version 1 is characterized by a warmer tone and richer in harmonics. More "dry", but not poor, the reproduction of the finale in version 2.
Finally, here is a photo of the prototype I made.
A low-gain (revised) hybrid preamp
This is a reinterpretation of the first version. The preamplifier schematic (shown below for one channel only) has undergone minor changes.
The power supply section has been changed, which is now based on a transformer with a secondary 55/0/55 volts. In this version, when the circuit is connected (in stereo configuration), about 85 volts must be present at point B +. The output capacitor (6 uF, polyester) must be 100 V.
I had the opportunity to try this preamp as a pair (with a 6922 Electro Harmonix) and the class A power amp (in the more "power hungry" version) with the Sonus Faber Cremona (first series): a pleasant surprise!
..The solid state version
That it was a fairly versatile project I said in the initial presentation. To confirm, here is the full solid state version of the preamp.
The power supply stage remains unchanged (apart from the 1k resistance which becomes 1.5k). In this case, when the circuit is connected (in stereo configuration), a little more than 90 volts must be present at point B +. The output capacitor can also be chosen with a higher capacity (9-10 uF).
One output stage for DAC
Inspired by a famous Nelson Pass line preamp -the Bride of Zen-, the diagram below is related to a simple amplifier stage that I used as a (quality) audio output stage in a DVD player.
The diagram refers to the single channel and to the power supply section. At the input (it must be connected to the output of the dac -in my case a pcm1742 Burr Brown-) there is a low-pass filter that helps to cut unwanted high frequencies. The calibration of the stage is simple and consists in adjusting the 4.7k trimmer so that 14-15 volts are present on the drain of the mosfet (referred to ground). The stage gain can be increased by reducing the resistor value on the source to 270 or 180 ohms.
Here are some photos of the circuit inserted in the dvd player (Pioneer DV-350).
Editing Dac-01A Xiang Sheng
The Dac-01A Xiang Sheng is one of the many "made in China" products that can be purchased today, generally online, at a decidedly competitive price. In reality, it is a "curious" contraption, which encloses 3 distinct instruments in a single cabinet: a digital / analog converter with resolution up to 192 khz 24 bit with three inputs (coaxial, optical and usb), a preamplifier with two line inputs (plus 3 digital) and a dedicated headphone amplifier. As if that were not enough, at the output (line) it is possible to choose between a signal processed only by solid state components and a signal that also "passes" through a valve buffer stage (6N3 or 5670), both of variable or fixed level (a choice).
Certainly, the heart of the machine is the conversion circuit, based on the CS4398 chip, often used on even quite expensive players. This fact, together with its versatility, make this device an ideal object for those who want to have fun in an attempt to improve its performance and functionality.
The modification operations that can be carried out are many and the overall "volume" of possible interventions depends on a fundamental choice: to change the cabinet or not.
Personally, I bought the item on ebay some time ago. Having no intention of changing cabinets, I only made a few possible interventions. The changes made, which I describe in order of "importance", are however more than enough to improve the musical performance of the dac in a clear way.
The photo above (recovered here, where you can find a review) shows the inside of the machine (in the version that came to me, instead of the 2 green 2.2mF capacitors on the tube output there were 2 8mF electrolytics ). There are 3 operational amps in the circuit. Two LF353 and one OPA2604 Burr Brown. One of the LF353s (shown on the right in the photo) is used as a pre stage in the headphone amplifier. The other ( in the photo) acts as a (line) preamplifier for all analog signals, ie the signals relating to the 2 line inputs and the analog output signal from the dac chip. The latter, before reaching the LF353 , is pre-amplified and filtered by the other operational ( in the photo), the OPA2604. The operational  is the main flaw of the device. The first thing to do, therefore, to improve its performance, is to get rid of the LF353 . The operation is not very simple because the operational has no base. It must be unsoldered. To do this, perhaps the least dangerous thing is to cut the "legs" with a pair of cutters and then free the holes on the printed page with the help of tweezers. At this point you can insert a socket and, if you are interested in using the device as a preamplifier, replace the LF353 with a higher quality opamp, such as another OPA2604 or an OP2134, to name a couple. If you want to use the device only as a dac, it would be better to completely bypass this stage, bridging pins 1 - 3 and 5 - 7 of the socket inserted. I had two single LT1115 opamps, credited with very low noise and low distortion. I thought they might fit the output of the dac chip, instead of the OPA2604, which I could eventually use instead of the unsoldered LF353. I therefore built a small circuit to "embed" the two LT1115s in the socket previously occupied by the OPA2604. The photo below shows the detail of the two LT1115s at the output of the dac and of the socket inserted with the two jumpers that bypass the pre stage originally designed around the LF353. Note the detail of the jumpers ( in the first photo) set to exclude the volume control on the line outputs.
The one just described is the first (most important) intervention to do. Then, you can also:
a) increase the capacitance of the capacitors  in the first photo. They are 0.47mF Wima capacitors. I thought of soldering in parallel to each capacitor another of the same capacity (and quality).
b) change the output capacitors on the tube stage ( in the first photo). In my case 8mF electrolytics which I replaced with 10mf tantalum electrolytics.
c) change the other electrolytic capacitors on the signal path. In particular, I replaced the 10 mF capacitors ( in the first photo) and the 3.3 mF capacitors ( in the first photo), with 10 mF and 4.7 mF capacitance tantalum capacitors respectively.
d) increase the filter capacities in 3 sections of the power supply. In particular, the electrolytic filter of the anode voltage of the valve ( in the first photo), from 100 mf -100 V the small and 150 mF - 200 V the large, have been replaced by capacitors of 470 mF - 100 V and 330 MF - 200 V, respectively. Furthermore, the two 3300 mf - 25 V electrolytics ( in the first photo), used to filter the dual power supply voltage of the opamps and the final stage of the headphone amplifier (unstabilized voltage!), Were "side by side" (ie put in parallel) two other "big" electrolytics of 22000 mF, one for each. Finally, another 2200 MF capacitor was added to the filter capacitor of the digital department supply voltage ( in the first photo), capacitor of 3300 mF - 25 V. I have also inserted, on each of the supply branches mentioned above, a 0.1 mF polyester capacitor (of adequate working voltage) in parallel with the filter electrolytic.
The following photo shows all the changes made. The two 0.47mF capacitors and the 0.1mF capacitors are not visible because they are soldered directly to the back of the board. The positive effect of the changes on the sound performance is very clear!
One last note. At the end of the modification operations, once everything had been reassembled, the unit suffered from a humming output, light but clear. I didn't understand what its origin was. However, the problem disappeared by completely isolating the circuit from the cabinet. It was necessary to interrupt a track of the circuit, in the section indicated by  in the first photo.
If the vinyl still runs ..
... then we may need a good phono preamp, maybe a little vintage. And here is the diagram (only one channel) of a pre for MM heads, needless to say with valves, but at low voltage. The circuit, which adopts the double ECC86 triodes (two valves per channel), is powered at an (anode) voltage of only 24 Vdc. For the filaments you need 6.3 Vdc, with a total current (for the stereo version) of about 1.4 A. The ECC86s were produced for use in the first car radios.
The preamplifier consists of three blocks, two active and one passive. The first block (the first two triodes) amplifies the signal coming from the head. The amplification factor of each triode (theoretical maximum) is 14. The second block consists of the RIAA equalization network. Another amplification block follows, similar to the first one.
A prototype (pictured) of the preamplifier is currently being tested with very good results. For the anodic power supply I used a simple circuit based on the usual 7824, not skimping (indeed exaggerating) on the filter capacity. To give voltage to the filaments I resorted to a switching power supply recovered from an old playstation 1. The output is not stabilized and the 7.5 volts supplied empty drop to 6.1 with the load connected (4 valves). The current supplied is sufficient (but we are at the limit). Of course, at present, it is to be considered as a basic project to work on and improve on.
UPDATE: Amplification may be too high. If the power supply voltage is reduced (by not changing anything else), a smaller amplitude of the output signal is obtained (but the timbre also changes). After some trial and error, I chose about 21 V power as a compromise solution.
A line preamp with 12B4 tubes
On the net I have read several positive comments on the use of the 12B4 in line preamps. Intrigued, I wanted to try. And, in fact, the results have gone beyond all expectations.
The 12B4 tube is a (low) power triode, used essentially as a vertical deflection amplifier in older CRT TVs. This triode, however, has some good characteristics that make it, at least in principle, a good candidate to perform the task of amplifier in an audio preamplifier: low amplification factor (about 6), low plate resistance (about 1000 ohms) and good linearity .
The circuit shown in the diagram below is very simple and relatively easy to make.
The voltages necessary for the filaments (12V alternating) and for the anode (about 150V at point B + with 2 valves - one per channel - powered) are obtained by two toroidal transformers with 12Vac secondary, of those, to understand each other, used for halogen spotlights. For the filaments, a total of 600mA is required. The resistance R in the diagram must be chosen based on the light intensity required of the power LED. At the ends of the 470 uF capacitor it is possible to recover (as I did) the direct voltage to power an anti bump circuit.
Here is the prototype mounted in a container (of those once sold by Nuova Elettronica). The valves used are two NOS GE purchased on Ebay.
The tests I carried out confirm what we read on the net: the performance does not seem that of a "classic" tube preamplifier, but neutrality, transparency and amplitude of the scene are appreciated. Truly an excellent preamp!
UPDATE: The capacitance of the input capacitor has been increased (from 0.22mF) to 2mF.
Minimum: A minimalist line
Always convinced of the sensibility of the "low mu" approach in the construction of line preamps, the results obtained with the 12B4 valve preamp prompted me to insist in this direction. Unfortunately, if you exclude the medium-high power tubes, there are not many triodes or double triodes with low amplification factor (<= 10). In addition to the 12B4, there is the 6AH4, the 2C50 and little else. However, there are valid alternatives (at least in principle) if we consider the pentodes, in the triode configuration. For example, among the pentodes commonly used in past years in amplifications there is the EL86. This tube is very interesting because, in the triode configuration (i.e. with grid 2 connected to the anode), it has a low amplification factor (8), a relatively low plate resistance (around 1200-1500 ohms) and a voltage operating anode also relatively low. The first two characteristics bring the EL86 closer to the 12B4 and therefore make it a potential active element of a valid amplifier stage. In fact, I was even more curious about the UL84 which is identical to the EL86 except for the filament voltage, which turns out to be 45 volts. Hence, UL84 in triode configuration has low amplification factor, low plate resistance, low anode operating voltage, relatively high filament voltage (but with low current (only 100 mA)). It appeared to me as the ideal valve to build a truly minimalist line preamplifier!
Here is the wiring diagram of the circuit born after some work and tests. Salient feature of the circuit is that it uses the same voltage (45 V) for filament and anode. This makes the preamp very simple, even to build. A transformer with a 36 V secondary, capable of delivering 250-300 mA, is enough to supply current. It is a very common transformer. The 2.2k trimmer present in the diagram must be adjusted to have, at point B1 +, the required voltage of 45 V. The power supply supplies current to the two circuits, relative to the left and right channels (the diagram shows the amplifier circuit relative to a single channel). The working point chosen for each valve provides that the grid is at approximately -1.6 V with respect to the cathode and that the anode current at rest is approximately 7.5 mA.
Here is a photo of the prototype built, which mounts two (old) Philips UL84s, a very "robust" transformer that has shown surprising musicality qualities.
This kit, with its low cost, would have given me the opportunity to try circuit diagrams that I thought were interesting. So, I bought it on ebay, directly in Europe (Germany).
I immediately say that I have never mounted the kit in its original form; so i don't know what that version sounds like. I wanted to test a circuit with no feedback, with a low voltage gain (around 7-8), which used pentodes in a triode configuration. Of course, I wanted to do this by trying to recover, where possible, the components that arrived with the kit. My attention therefore turned to the 6AK6 and 6G6G tubes. These are two pentodes with different bases (7 and 8 pins, respectively - as for the 6J4 and 6P6P-) but electrically equivalent, with a gain of 9.5 in triode configuration and with a low filament current of only 150 mA . I therefore used, for each channel, a 6AK6 as an amplifier tube and a 6G6G as a buffer. Here is the wiring diagram of the circuit I arrived at.
As you can see, some care has required the feeding section, in which the 6Z5P grinding machine supplied in the kit is still used. The 3.75K resistor is obtained by paralleling the two 7.5K 10W resistors supplied. The 22uF and 330uF electrolytics are also those in the kit. With 4 valves powered (2 6AK6 + 2 6G6G), at point B + there are about 170 volts (referred to ground), while at point B1 + there are about 153 volts. The power supply of the filaments remains alternating. This places fewer constraints when trying other tubes.
It is clear that different configurations are possible. For example, you might decide to use the 6G6Gs as amplifiers and the 6AK6s as buffers. Or, with the 6AK6 as amplifiers you could use the supplied 6P6P as a buffer. Basically 6G6G and 6P6P are interchangeable here. The power supply will find itself supplying the higher current required by the 6P6P filaments and, of course, the voltages read at points B + and B1 + will also be different. Again, as an alternative to 6G6G, 6K6G (or GT) could be used as a buffer. Finally, as an alternative to the 6AK6, the EL91 could be used as amplifying valves. These require a little more current for the filaments (200 mA) and have a slightly higher gain (12). But be careful, because in this case the pinout is different.
Here are the photos of the assembled preamp. The tubes used are two 6AK6 RCA and two 6G6G National Union. As you can see, I also added a small mains filter.
Transparency, amplitude and depth of scene are the qualities that strike this version of the preamplifier. Those who bought the kit and are not completely satisfied with the original preamplifier version can try this route. I believe he will not be disappointed.
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