Ways to improve the sound of low frequency amplifiers. Refinement of ULF computer speakers Influence of amplifier design on sound quality
U101 radio upgrade So let's get started! 1. Power supply. Power supply circuits may vary slightly! (transformer connection). To get a decent output power, you need to have a decent power supply. Let's use the gift of the transformer manufacturers: the entire secondary winding is made with one thick wire (0.8 mm in my opinion). Therefore, it is quite possible to switch the power supply of the powerful rectifier VD5 ... VD8 from contacts 4 - 4 * to 3 - 3 *, which will increase the voltage from +/-26V to +/-31V. In this case, the low-current rectifier VD1 ... VD4 becomes unnecessary and is removed along with the wires, and its storage capacitors C2 and C7 are connected in parallel with the corresponding capacitors of a powerful rectifier. But all connections with contacts 5,6 and 9,10 should be preserved. I left this circuit and powered it from 25 volts to power the pre-stabilizer, indicator, protection. The pre stabilizer is made on the LM 7815-7915 on the pre board, 220 microfarad capacitors are added to smooth out ripples.
Then the witchcraft begins. We determine the geometric mean between the ground terminals of the capacitors C2, C3, C4 and C7, C8, C9 on the board of the filter capacitors, clean and tin it. We assign this point as the main common point of the entire amplifier. From it we start up 2 thick wires to the minuses of the output connectors. From it we start up common wires on the UM and UE. From it we start up 2 wires to contacts 6 and 6 * of the transformer, removing the jumper between them. At the same time, we remove the connection between the rectifier board and the case. We organize the connection of the common wire with the case at the input connectors of the amplifier. And check - so that there are no more contacts of the common wire with the case anywhere else. And finally, we connect a 0.047x630V capacitor in parallel with the primary winding of the transformer to suppress impulse noise from the mains. 2. Power amplifier. If KT315D is in place of VT1, then it must be replaced with KT315G to reduce the noise level. If there is KT361D, as in the attached diagram, then you do not need to touch it. The essence of the proposed changes: to pick out the "raisins" of the developers and insert your own. After removing VT6 and VT7, installing a jumper, replacing R10 with diode D7 and shorting R15, the D7-VT5-R11 circuit turns into a diode regulator for the current source on VT8, which is already powered by the swinging transistor VT10. To reduce non-linear distortion, the swinging transistor VT10 must be high-voltage, powerful and with a high gain. KT961A just meets these requirements, so we replace the original transistor with a more suitable one. Classic scheme. The idyll is broken only by the resistor R42. It is soldered from the print side into the notch of the printed conductor near the VT2 collector. The introduction of this resistor increases the stability of the entire PA and allows you to get rid of compensating capacitors C4, C5, C9, C10, as well as resistors R20, R21. Side effects of introducing R42 will appear when listening. For normal operation electrolytic capacitor, he needs a charging potential of 0.6V, but on the C3 plate it is not. Therefore, there should be a non-polar capacitor here, limiting the bandwidth around 5 Hz. Hence the nominal value of 22 microns NP. The setup is normal: connect an ammeter to the power break and set the idle current to about 40 mA. Then restore contact and start working. Newly introduced or modified components and jumpers are highlighted in red, and removed components are highlighted in blue. The numbering corresponds to the standard scheme.
3. Pre-amplifier three-chip option. Chip DA1 introduced into the preamplifier solely for coordination with the piezoceramic pickup. I think that now this is no longer relevant, but it adds noise, and therefore we boldly throw out the DA1 chip along with all the strapping and throw a jumper using the freed holes on the printed circuit board. Newly introduced or modified components and jumpers are highlighted in red, and removed components are highlighted in blue. The numbering corresponds to the regular scheme. This figure shows the DA1 chip and the components that should be removed along with it on the U5 ULF-P board. Next, we more accurately adjust the loudness circuits to the volume control. Then we expand the bandwidth of amplifiers DA2.1 and DA3.1 both in HF and LF and adjust the parameters of the tone block. To return the supply voltage of the DA2 and DA3 microcircuits to an acceptable direction, it is necessary to correct R47 and R48. The preamplifier contains trimmers R24 and R26 to adjust the gain of the entire amplifier. Setting conditions: at the input - 0.5V 1kHz; volume control - to the maximum; at the output - 14V without load, set with resistors R24 and R26. Newly introduced or modified components and jumpers are highlighted in red, and removed components are highlighted in blue. The numbering corresponds to the regular scheme. This diagram shows the ULF-P refinement circuit, the DA1 chip is not shown.
In a preamplifier with 1 microcircuit, nothing needs to be changed. The only thing to completely get confused is to add another microcircuit on top of the first one in order to separate the channels by cutting off the legs of the corresponding channel. Practice and experience have shown that in this opera the channels strongly influence each other. (crosstalk) 4. Indicator. The display unit has built-in parametric stabilizers +15V and -15V. As the voltage increases from 26V to 31V, resistors R3 and R13 start to get very hot. Therefore, they must be replaced with 680 ohms with a power of 0.5 watts. If you want to reduce the brightness of the indicator, then you need to reduce the filament current by increasing R14 and R15 to 20 - 30 ohms. 5. Corrector UPZ-15. To date, all known moving magnet magnetic pickups operate with a correction capacitance of 470pF. Accordingly, the capacitance of capacitors C1 and C2 is changed to 470pF. 6. Input board. To expand the bandwidth down from 20 to 7 Hz, you can increase the capacitance of capacitors C4, C5, C14, C15 to 0.33 microns. This is at the end of the work as the cheeks puff up. In the course of improvements, I never came across an amplifier with a background. Perhaps just lucky, the background is easily eliminated with the appropriate wiring of the ground and screening of the loop from the input switch to the switching board. To do this, remove the connector from the switching board, disassemble it and put a screen on a cable of a suitable size. Of course, "Marantze", "Rotel", "Sherwood" probably will not work out of it, but with such as "Dual", "Grundig", "Tandberg", "Technics", "Panasonic" of that time of release and in the same weight category in terms of sound quality, the modified "U-101 Radio Engineering" may well argue. It is absolutely necessary to remove the bastard voltage multiplier to power the corrector and the stabilizer on the microcircuit and the KT815 transistor (input board). The point is that it comes alternating current from the output 5 of the transformer, then the reverse alternating current goes through the ground. Such a scheme was apparently developed for the EP-101 player, where there are no higher voltages. (in some circuits, most of which I came across, the power to the stub gets after the filter) you can put a stabilizer on KR142EN9 with any letter) It is replaced by a 200 ... 300 ohm resistor (from the +31 bus) and a 24v zener diode. Even when the voltage rises to + -31, the relay (!) On the protection board heats up. You need to either put another relay, or pick up a resistor. In my amplifier, I left a low-power rectifier, switching it to + -26v for protection, pre-and indicator. A powerful + -31 on the mind. Regarding electrolytic capacitors: In any case, change them. What can be recommended here? And be sure to throw away all the clay flags, replacing them with a film, and from the MBM power supply, replacing them with the corresponding film or ceramics. It makes sense to increase all food capacities by 40-100 times. (in practice it is better to put 2 to 4700) Power supply: instead of cans of 2000 microfarads, you can safely put 10,000 microfarads. the dimensions are almost the same. It would not be superfluous to shunt them with 1 microfarad ULF films: Capacitances C14, C15, C16, C17 - 220 microfarads ULF-P: C37, C38 - 470 - 1000 microfarads Protection, display unit: leave the ratings there as they are. I can add the following to the above: 1. It is a good idea to replace the input board with a relay switch. 2. It makes sense to remove the UPZ-15 corrector, because it introduces distortions into the signal, including at neighboring inputs. 3. The Stereo/Mono switch can easily be turned into a tone switch. The thing is useful. The signal "before the tone block" is taken from the points where C23 and C24 used to be. The signal "after the tone block", respectively, from points "4" and "6". Further, the signal is fed to the input of the MIND. I (Pavel) have repeatedly checked this alteration. Measurements on the oscilloscope showed an increase in power up to 42 watts per channel. The analyzer checked the frequency response of the PA, it became more uniform, before after the alteration expanded bandwidth. In practice, we are not making big changes - but believe me, the amplifier sang!!!
Cinematic Sound Enhancement tube amplifier 90U-2 LOMO
At the moment, you can still buy inexpensively, 90U-2 tube mono amplifiers, from the Ukraine cinema installation. This amplifier is not difficult to upgrade and get a very decent sound, high specifications, which allows him to easily beat many transistor amplifiers, both Russian and imported.
Note: I personally believe that old equipment should be cherished and cherished, because there is less and less of it :-(Therefore, it is better to create an amplifier from scratch than to break a factory receiver or amplifier for the sake of experiment, break - do not build!
Konstantin
Factory circuit without alteration
Specification
Transformer winding diagram
| Rated input voltage | 0.3 V |
Rated output power into 4 ohms (without reworking the OUT transformer) |
10 W |
Rated output power at 16 ohms (without altering OUT transformer) |
10 W |
| Nominal frequency response (-3 dB) | 15 Hz - 35 kHz |
| Harmonic coefficient at an output power of 1 W in the nominal frequency range at Rн = 16 Ohm | 0,6 % |
Relative noise level no more |
80 dB |
Appearance of the converted amplifier 90U-2.
For questions about modernization, please send an e-mail to: [email protected]
When using the materials of the article, a link is required.
Various computer speaker systems of the middle price segment (in particular Microlab PRO2 and Thonet & Vander Dass) had one common and very unpleasant drawback - when you plug something into a nearby outlet, loud annoying clicks are heard in the speakers. Which is especially disappointing at night. It is convenient to unscrew the volume knob on computer speakers to a value close to the maximum in order to adjust it in the full range from the computer in the future. Which does not affect the loudness of crackles in the best way. The speakers clicked especially loudly when the soldering air purifier was turned off, but the reaction of the speakers to all sorts of small switching power supplies / chargers turned on and off from a nearby (and not only) outlet was also unpleasant. The indicated problem is a consequence of the total savings by the Chinese on everything in design and production. The solution to the problem is to add to the circuit what was saved.
When examining the internals of the acoustics, it was noticed that there was no filter for interference from the mains voltage. The amplifiers themselves in such devices are traditionally made on microcircuits with a built-in stabilizer, i.e., their entire power supply consists of a transformer, a diode bridge and a pair of electrolytic capacitors (in my amplifiers their capacity is 4700 microfarads per arm).
To begin with, it was decided to install a network filter. It will not solve the problem of clicks when turning on / off the fan in a neighboring outlet (you can verify this by connecting the speakers to a high-quality external surge protector - the clicks do not completely disappear), but it will definitely not be superfluous, given the abundance of impulse noise in the outlet. I didn’t bother much with the filter and ordered it in China, like this (on all aliexpress, such filters are searched search query"EMI power amplifier filter").
The filter is soldered into the cut of the power wires. I didn’t fasten it inside, I just placed it in a small box printed on a 3D printer so that it wouldn’t short out where and wouldn’t electrocute anyone ..
The next simple and obvious way to improve the quality of nutrition is to increase the capacity of "electrolytes" to at least 10,000 - 15,000 microfarads. At the same time, it should be taken into account that starting currents when charging, such capacities will also increase, and the diode bridge must have a good current margin so that it does not get sick when turned on. Also, for better filtering, I added a throttle to each arm (getting a T-shaped LC filter). As a result, the following scheme was drawn:
And the boards are ordered:
Here, up to five electrolytic capacitors with a capacity from 2200 uF to 4700 uF (with an operating voltage of 25 ... 63V) and a pair of non-polar capacitors can be installed in each arm. As the last ones, I used Chinese film ones at 0.22 uF, like this:
The input and output connectors are displayed on the board, moreover, the input can be applied as AC voltage(then a diode bridge is installed), and already rectified (if you plan to use the bridge already in the amplifier).
The resulting fee is as follows:
Next, I removed the rectifier diodes from the amplifier board. In general, they can be left, if it is not critical that a couple more volts of power will fall on them. Instead of diodes, I put a couple more chokes at 100 μH - they definitely won’t get worse from them. I fixed the capacitor board in the case, the wires from the step-down transformer go to its input, the filter output - to power the amplifier board. I also installed another FR157 diode at the filter outputs for shunting impulse noise (with cathodes to the plus), they made a significant contribution to blocking clicks.
The result - clicks when turning off the neighbor-fan began to occur less often, and their volume became noticeably less, they no longer irritated as they were originally. Loud sharp sound when turning on / off is no longer observed at all. Increasing the capacitances in the PSU filter gives smaller voltage drops and at high volume there should no longer be a feeling that the sound falls through.
I bring to your attention a variant of reworking the ULF-50-8 power amplifier module (Radiotehika U101, S70). There are at least two variants of the module. You can distinguish them like this: for the older VT1 - KT315, for the one that is newer - KT361. This description true for the newer version. If you have an older module, and you are going to remake it - adapt the remake for it yourself (it's not difficult at all).
Circuit changes have affected the voltage amplifier, the new version produces less harmonic distortion. In the input and output stages, the transistors are replaced by imported ones.
After the alteration, the sound is clean and clear, does not tire, listening to music is pleasant.
Now the details.
The input transistors KT3107 are replaced by transistors BC560 (pins match). Instead of a pair of MJE350 / 340, I installed SA1358 / SC3421 (other good driver pairs can be used). For the output transistors MJE15032/15033, the emitter and base are "rearranged" in places relative to the previous KT805/837, so I had to cut several tracks and apply MGTF.
"Native" scheme ULF-50-8
diagram showing changes 
The quality of the boards is not too high, so each part can be carefully soldered no more than twice. We do everything carefully.
I removed the load short circuit protection unit. You can leave it, but it is advisable to check its operation and configure it.
The node consists of VT11,12; VD3-6; C6.7; R19,21,24,25,28,29.
VT 9, 13-20 - You can leave it to check the health of the scheme, and replace it with import last.
Cut tracks: R15 to R17; from B VT10; from K VT1.
Jumpers (bottom): R17 to the "-" bus (according to the diagram); K-B VT1. Jumpers MGTFom: B VT3 - B VT8; E VT7 - E VT10. (top): Rx2 - B VT10 (according to the diagram).
CX4 not installed 
C3 - In view of the low direct voltage on it, install a non-polar electrolytic at 100 microfarads.
C8 - 0.33 uF film polypropylene.
C11.12 - 0.47uF 63V polypropylene film.
Cx1 - parallel to R5 in its holes. polypropylene film.
Diodes Dx1-6 1N4148.
R6 - you can solder a 1.2k resistor, but it’s better to put a multi-turn trimmer (2-2.2k) on the VT1 seat instead (it’s done in the photo). On the trimmer it is necessary to "wind" 1.2k. They set 0V at the output of the amplifier.
R9 - 820 Ohm - slightly increase the coefficient. amplification.
R12 - multi-turn trimmer for 2-2.2k (wind to 0). It serves to establish the quiescent current of the output stage (50mA in my version)
R30, 31, 38, 39 - 0.22 Ohm 5W (white crackers).
The shown switching of the windings of the power transformer slightly increases the supply voltage of the amplifier. The output level indicator is best powered from a separate source.
It was very widely used by many music lovers and many still have it.
However, even a superficial analysis of the circuit shows the ULF-50-8 module as the weakest link in the amplifier. The module does not fully realize the possibilities preamplifier assembled on K157UD2 microcircuits.
I propose rework option full amplifier Radio engineering U-101 into a really high-quality amplifier for a household audio equipment complex. Distinctive features: high technical characteristics and reliability with minimal intervention in the converted apparatus, when all functionality original amplifier.
To replace the ULF-50-8 module, UMZCH with low non-linear distortion was selected. Its main technical characteristics:
rated output power at a load with a resistance of 8 ohms, 25 W;
harmonic coefficient in the frequency range 20-20000 Hz 0.03% (0.3% for ULF-50-8);
output voltage slew rate 40 V/µs.
Amplifier circuit for reworking Radio Engineering U101
UMZCH consists of two-stage amplifier voltage (op amp DA1, DA2) and the power amplifier itself (VT1-VT4). Cascades on the op amp DA1, DA2 are powered by identical sources formed by the elements VD1, VD2, R6, R7, C6, C7 and VD3, VD4, R14, R15, C13, C14. The midpoints of these power supplies are connected to a low-resistance voltage divider R5R12R20 connected to the UMZCH output, which ensures that tracking potentials are supplied to the voltage amplifier stages. The R16C8 and R19C10 circuits filter the voltages supplying the first stages from non-linear ripples generated by the signal in the power supply circuits of the output stage.
There were no problems with mounting the UMZCH output transistors on the Radiotekhnika radiator in accordance with the recommendations in. The heatsink has four metal plates insulated with mica spacers. There is enough space for three transistors on each plate, no heatsink modifications are required.
The only problem is the need to reduce the UMZCH printed circuit boards, since there is not enough space in Radio Engineering. Two UMZCH should be assembled on two printed circuit boards, the width of which should not exceed 60 mm. The drawing of the voltage amplifier board (Fig. 3, a c) must be compacted across the width to this size. This is not difficult to do if the K50-6 capacitors are replaced with K50-35 or other small ones. The drawing of the final stage board (Fig. 3.6 c) will fit on a board 60 mm wide without changes (the figures are shown).
The first board is made 240 mm long and one voltage amplifier and two final stages are placed on it. Another voltage amplifier is placed on the second board.
A long board is attached to the radiator of "Radio Engineering" on racks 15 mm long so that it is placed vertically in the amplifier case. ULF-50-8 boards are preliminarily dismantled. The second voltage amplifier board is attached to the long board on racks 20 mm long from the side wall of the amplifier housing.
UMZCH is connected to a power supply of ± 26 V "Radio Engineering". Supply voltage ±30 V is not used. The UMZCH output is connected to the Radio Engineering protection board. The signal wires do not need to be confused (the same applies to the input wires).
Properly assembled and connected UMZCH start working immediately after turning on the power and do not require adjustment. I also recommend replacing the capacitors C3, C4, C8, C9 on the Radio Engineering rectifier board. They have probably already lost part of their capacitance (dry), so it is better to replace them with new ones with a capacity of 4000-5600 microfarads.
All the functionality of "Radio Engineering" after the alteration is preserved. The sound of the converted amplifier can be characterized by epithets: clean, transparent, juicy with a clear localization of sound sources. It's much better than the original amp and noticeably better than the AKAI FD-1 I own.
When the converted amplifier is running on sound speakers with low-frequency heads like 10GD-30 or 25GD-26, characteristic clicks are heard at high volume. This is due to the insufficient rigidity of the caps covering the magnetic gaps of the heads. The caps should be replaced with more rigid ones. The original amplifier has much worse performance, so no clicks were observed.
Ya. M. Kogut, Lviv region.
Literature
1. Amplifier "Radio engineering U-101 stereo". Manual.
2. Ageev A. UMZCH with small non-linear distortions//Radio.-1987. -No.2.-S.26-29.
Here are the drawings of printed circuit boards and the placement of elements published in the Radio magazine, to which the author of the article refers.
RADIOAMTOR №10, 2001
