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So Good... I built it twice!

I reckon it's FB cap forming. When I built a high gain low noise amp (for measuring etc) this used very low FB resistors: 1k5 and 4R7 for the high gain with low noise so you need a lot of capacitance to get a low corner F.
I initially had erratic results and a higher than expected lower F cut off. When I got the caps I was going to fit, I formed them at a few volts prior to fitment and run them with a small DC bias. These then operated as expected so it could be a small leakage current in the FB cap as this affects bottom end the most and will take quite some time to form in circuit with low dc bias.

I think you may well be right. OM2.7 pair #2 is now sounding very good!!

I did experiment with moving the speaker returns to the PCB and back to the power star. In the past I've preferred to use the power star. It seems to make minimal difference on this amp, maybe because the connections are very short in this particlar setup.
 
I did experiment with moving the speaker returns to the PCB and back to the power star. In the past I've preferred to use the power star. It seems to make minimal difference on this amp, maybe because the connections are very short in this particlar setup.
The decent (1000uF) bypass capacitance on the pcb directly links the fat 0v connection track where the AC speaker current returns to the output stage collectors, keeping Mr Kirchoff happy and with a nice small loop-area.
 
So, I managed to get the paralleled output transistor mods done this weekend. I used matched pairs to get best current sharing, although i found all the devices I had were very similar anyway so it probably wasn't needed. I was only impressed by how closely the NPN and PNP output devices HFEs are matched, all in the mid 80's, these are very good complementary pairs.

Also I've been following the "Wolverine" build thread on DIY Audio (I actually have a set of bare boards I may build at some point) and they have a very similar schematic to these OM2.7s, almost identical input stage and also the Locanthi T output stage. In their BoM they are fitting a 1uF speedup cap for the second driver stage but for the first driver stage the cap is omitted, despite showing 0.1uF on the schematic. Interestingly they are running a higher bias current for the first driver stage, so I've followed suit, dropping R29 to 560R.
Another point of note is that the input cap C2 is quite small on the OM2.7 schematic at 1uF, and I used 4.7uF to make sure there is no potential low end roll off.

I'm not sure if it was my imagination but I do think the sound of my boards had filled out somewhat over the last week or so, though I did also move my speakers back a couple of inches to get a little extra bass reinforcement. They are still perhaps a touch mid forward but generally sounding more balanced, if anything the extra output transistors seemed to have helped giving a bit more bottom end substance.

The extra output transistors have improved the performance quite a lot across the spectrum, better space and dynamics, even more detail but also a fuller and more effortless sound. These are sounding really quite good now.

DSC-0174-0000100002.jpg


DSC-0175-0000100002.jpg
 
A Wolverine is also in my future I feel. Its specs are mind bogglingly good, even better than the Benchmark AHB. If your reaction to that is so what, then you’ve probably never heard such an amp eg a Neurochrome amp. FWIW I also love class A and the whole dominant 2nd harmonic thing, but EXTREME transparency and neutrality has its own charms as well. It’s a classic dichotomy :cool:
 
I have translated the OM2.7 web page from Russian to English (thanks Google!) including the BoM.

If anyone wants a copy, let me know.
 
As an experiment this evening I tried moving the gnd return point from the OM2.7's from the "output" end of the Hackercaps to the "input"/rectifier end. To be honest I wasn't expecting much, if any, difference but to my surprise I was wrong...

The sound changed quite significantly and was noticeably less bright and forward (actually verging on dull sounding) but the timing and coherence was all over the place and very muddled sounding. Not obviously "broken" sounding but definitely worse overall.
I'm not sure I can explain why this would be as the hackercap is star grounded so the "output" ground point isn't sharing any common impedance with the smoothing caps as far as I can see...

..anyway this has got me thinking about grounding and whether there might be some improvement in the upper mid forwardness I'm hearing by running separate grounds for the front end of the OM2.7 back to the power board along ( and also moving the speaker return to there) and then leaving the existing ground connection to the OM2.7 to just carry the returns for the decoupling caps and zobel network.

I must admit this is where my understanding runs out a bit - the OM2.7 has a 1ohm resistor R3 that provides a local ground connection for the front end and feedback to the main ground point on the board. This would appear to be common practice as the Wolverine amp does this also, but using 2.2R. With all that said the Wolverine amp also runs a 0.1uF bypass cap across that resistor as well as the feedback cap... another possible point of experimentation!
 
Thanks for the heads up, have ordered 10 OM2.7's and also the 3 to try. Not that I need more amp PCB's to build, test and use!
 
Had a bit of an epic listening sesh this afternoon when a pal came over to give a 2nd opinion.

We compared the OM2.7 with:
Leach Low TIM
JLH Mosfet (from WW 1982)
Grant GL50 with KT88s

SInce I originally built the 2 pairs of OM2.7 I've built my DiscoCAT speakers and these were used along with LS50 Metas (both with my bass extenders).

The preferred amp did vary a bit depending on the recording, however one of the OM2.7s came out on top overall - the 2nd set, with the yellow caps.

Prior to today I've been trying to get the OM2.7s to sound the same. The grounding in set #1 was very slightly different and changing it brought the 2 pairs closer in sound. Set #1 uses 2SC5200/2SA1943. Set #2 uses NJW0281/0301. I'm beginning to wonder if this the cause of difference.
If I can get them to sound indestinguishable then I can start on mods to see what affects what (daft hobby, really!).
 
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Very daft hobby!...

I have the LS50Ms here until the weekend so I'm trying to optimise the system to get the best out of them. With the OM2.7 the results were very good but I wanted to tone the KEFs down just a smidge. My Mooly Mosfet amp is a bit more mellow sounding (bordering on bland with my DIY speakers), however I didn't have high hopes for it due to its high o/p impedance - courtesy of a Naim style 0.22R output resistor. The LS50s don't play well with my valve amp, we assumed this is due to the wild impedance swings and the amp's lowish damping factor pushing the 3KHz region forward.

I think it was Benny Hill who said "don't ass-u-me". He was right! The LS50Ms sound wonderful on the end of the Mooly amp. Much more tuneful than with any other amp I've tried, with sweet upper-mids and treble, which is a first for the LS50s for me.
 
Oh - Isn't this a daft hobby : )
Nice finding though - yes, simply everything matters ;)

What was that but 'daft' again ..? ATB!
 
@OldSkool and @S-Man did you guys manage to dial the mid range forwardness in your builds

Curious to know what you both think about the sound quality. You both mention Avondale NCC builds. How do these amps compare?
 
@OldSkool and @S-Man did you guys manage to dial the mid range forwardness in your builds

Curious to know what you both think about the sound quality. You both mention Avondale NCC builds. How do these amps compare?

I'm not sure I can really say if the mid forwardness is dialed down. It depends on the music, sometimes it seems very clear and linear sounding i.e. it treats all frequencies and SPLs the same. On other occasions, perhaps with less good recordings it can sound a bit too much.

I don't have any NCCs to do a direct comparison. I have also buiilt a new pair of speakers since I had them and seem to be favouring different amps with the new speakers.

Sorry - not a helpful answer!
 
I'm not sure I can really say if the mid forwardness is dialed down. It depends on the music, sometimes it seems very clear and linear sounding i.e. it treats all frequencies and SPLs the same. On other occasions, perhaps with less good recordings it can sound a bit too much.

I don't have any NCCs to do a direct comparison. I have also buiilt a new pair of speakers since I had them and seem to be favouring different amps with the new speakers.

Sorry - not a helpful answer!
Have you tried changing the feedback cap. From your photo it looks like you're using a standard electrolytic. Try changing to a bead tantalum or wet tantalum to see what difference that makes to sound signature?
 
I have translated the OM2.7 web page from Russian to English (thanks Google!) including the BoM.

If anyone wants a copy, let me know.
Hi hope you are well, i am bulding the only music amps and if you can send copy of the translated info.
Many thanks
George
 
Hi,

All good here thanks. I hope you are well too.

Here is the text. If you want the file with photos please message me your email address.

Good luck.

Barry

Prologue. The idea of creating this amplifier dates back to the development of the Only Music 3 (OM3) amplifier. Then the plans were to make, in addition to the main version of OM3, also a simplified version of it. I abandoned this idea, but after a while I still decided to return to it. This is due to the excessive complexity of repeating OM3, which caused many difficulties for those wishing to repeat the amplifier. This was the reason for the creation of a new amplifier and a return to the original concept of the series - maximum quality with minimal complexity and dimensions.

Concept. As mentioned above, the new amplifier pursues the old concept, laid down in the "slap in the face of mikruham 2" (OM2). When developing the amplifier, the goal was to achieve maximum repeatability, reliability and stability of the circuit, as well as to minimize the diversity of the used element base and exclude from the list of used radio components, less common names. The qualitative parameters of the amplifier also did not go unnoticed - the new amplifier is not inferior to OM3, and even surpasses it in most parameters. Thanks to the emergence of a huge number of PCB manufacturers and their active competition among themselves, there was a huge number of profitable offers for the manufacture of PCBs in small batches, so I was betting on a double-sided printed circuit board designed for prefabrication. However, I did not refuse a single-sided printed circuit board, so I also made a version of the printed circuit board designed for home production using "laser-ironing technology" (LUT). The dimensions of the factory printed circuit board are exactly the same as those of the author's printed circuit board for OM2 - 100x67mm, and the dimensions of the single-sided board are slightly larger - 100x73mm.

Name. You are probably wondering: why 2.7 and not 3.5 for example? In fact, everything is extremely simple here - in this way I wanted to show that this amplifier is closer in concept to OM2 than to OM3.

Scheme. Once again, I decided not to change the traditions and use the classical Linov topology. The idea of the voltage amplifier circuit (VU), which has undergone significant refinement and improvement in this amplifier, was borrowed from the well-proven BackBen amplifier, which I developed. The differential cascade (DC) was supplemented with a cascode, which made it possible to unload the input pair of transistors in terms of power and voltage. This made it possible to use low-voltage, low-noise, super-beta transistors at the input, which, together with a decrease in the collector-emitter voltage of the input pair and the power dissipated by them, made it possible to significantly reduce the noise level at the amplifier output. The ability to adjust the "zero" that was available in the OM2 amplifier was returned, which allows you to accurately balance the differential stage. Stable current generators (GST) for the differential stage and the voltage amplifier stage (KUN) are independent of each other and have independent reference voltage sources. Capacitor C12 increases the stability of the HTS and significantly reduces the ripple of their reference voltage. A fundamentally different method of frequency correction of the amplifier was used, relative to those used in OM2 and OM3, which made it possible to obtain much greater gain with an open loop of general negative feedback than in OM3 and much greater stability than in OM2. During the development of OM2.7, it was decided to abandon one solution used in the BlackBen amplifier - a composite transistor in the voltage amplifier stage, due to the fact that this solution did not provide any noticeable advantages, but reduced the repeatability of the device due to not the highest prevalence in the sale of compound transistors MPSA13. The output stage, for the first time in the OM series, is a three-stage "triple", which made it possible to improve the buffering of the voltage amplifier from the amplifier load, to reduce the dependence of non-linear and intermodulation distortions on the output power and the nature of the load. This amplifier, like OM3, has protection against short circuits in the load.

Specifications. The circuit parameters were taken using the RMAA 6.2.5 computer program. An ASUS Xonar Essence ST sound card was used as a measuring DAC and ADC. As a load for the amplifier, two powerful 10Ω resistors connected in parallel were used (which gave a resulting resistance of 5Ω). To measure the rise/fall rate of the output voltage of the amplifier, a rectangular pulse generator and a UNI-T UTD2025CL oscilloscope were used. As a laboratory power supply, a classic, non-stabilized power supply was used, consisting of: a transformer, a diode bridge and smoothing capacitors with a capacity of 15000 uF in each arm. The voltage on the amplifier power buses in the absence of an input signal is +/- 42V, the quiescent current of the output transistors is 80mA. Parameters marked with an asterisk (*) - obtained using computer simulation of the operation of the circuit, marked with two asterisks (**) - by recalculating the actually obtained parameters for other measurement conditions (different load, different signal frequency, and so on).

Output power (1kHz):

Output power (4Ω) = 140W**

Output power (5Ω) = 110W

Output power (8Ω) = 70W**

Noise level (A-weighted) = not worse than -100 dB

The lower limit of the frequency range (at the level of -1.5dB re 1kHz) = 18 Hz

(Note: the blockage on the right side of the graph is due solely to the sound card parameter and does not depend on the presence or absence of the amplifier under test in the signal circuit).

Upper frequency limit (-1.5dB re 1kHz) = 177 kHz*

Rate of change of output voltage = 25 V/µS

Transient response (1, 10, 20 kHz):

The given technical characteristics correspond to the author's version of the amplifier assembled on the factory printed circuit board, they may differ slightly, for better or worse, for each specific instance of the amplifier. The parameters of the amplifier assembled on a homemade board differ by no more than the measurement error. When comparing the parameters of amplifiers assembled on factory and home-made printed circuit boards, absolutely identical radio components and even wires were used. The first amplifier was assembled and measured on the factory board. Then all the radio components were soldered from the factory board and installed on a home-made board, after which the amplifier parameters on this board were measured. Comparative characteristics of the parameters of amplifiers assembled on different printed circuit boards and photos of finished amplifiers:

Element base. In this part of the article, I will simply list the main points related to the element base, which should be taken into account and strictly adhered to in order to successfully repeat and run this amplifier:

  • all elements used in the assembly of the amplifier, before installing them on the board, must be checked for operability and compliance with the necessary parameters (resistance, capacitance, ESR, gain, and so on);
  • it is not allowed to install elements focusing solely on their marking, without checking for compliance with the actual denomination of the marking;
  • it is better to abandon the use of used radio components and use only new radio components during assembly;
  • it is preferable to use metal film resistors (MF) and refuse to use carbon resistors (CF) due to their high self-noise;
  • the use of metal-film resistors produced in the USSR is allowed;
  • when using resistors with a tolerance of 5%, it is highly desirable to select in pairs, resistors R6 and R11, R7 and R12, with an accuracy of no worse than 1%;
  • to achieve equal gain and, accordingly, equal volume of both stereo amplifier channels, it is recommended to pre-select two pairs of resistors 15kΩ and 470Ω (for the left and right amplifier channels), with an accuracy of no worse than 1%, in order to use them as - R1 and R19 , R2 and R8;
  • when using resistors with a tolerance of 1% - there is no need to select;
  • use of wirewound resistors as R35 and R36 is not recommended;
  • it is strictly forbidden to arbitrarily replace any resistors with resistors of a different rating that differs from the rating indicated in the circuit;
  • as trimmer resistors R9 and R20, it is allowed to use only multi-turn resistors of the 3296W type, it is allowed to use trimmer resistors with a resistance of 200 to 470 ohms;
  • when installed on the board, the engine of the tuning resistor R9 should be in the middle position (the resistance between each of the extreme and central terminals should be the same), and the resistance of the tuning resistor R20 should be maximum;
all types of capacitors used in the circuit and their minimum required operating voltage are indicated in the list of radio elements for this article;

it is allowed to use capacitors with a higher operating voltage than indicated in the list of radio elements;

  • do not overpay for audiophile series capacitors - using them in an amplifier circuit will not lead to any improvement in the characteristics of the amplifier or its sound;
  • each capacitor, before installing it on the board, must be checked for compliance with its marking of the real capacitance value, checked for the ESR value, checked for the absence of excessive leakage. This procedure is performed for EVERY capacitor installed on the amplifier board;
  • it is not allowed to use electrolytic capacitors manufactured in the USSR, as well as used electrolytic capacitors, capacitors with visible defects in the form of dents or swelling, electrolyte smudges;
  • it is allowed to use zener diodes of other models, but with the same nominal stabilization voltage and power;
  • it is allowed to use only those models of transistors that are indicated in the diagram, or their analogues indicated in the list of radio elements;
  • before installing each of their transistors on the board, you need to make sure that they are working and that the parameters specified in the datasheet for this transistor are correct;
  • transistors VT18, VT19, VT20, VT21 and VT12 must be installed on a common radiator. The area of the radiator, very approximately, can be chosen at the rate of 10-15cm2 for each Watt of the output power of the amplifier (1000-1500cm2 for an amplifier with an output power of 100W);
  • transistors VT9 and VT13 can be installed on small heat sinks (there are places on printed circuit boards for this), however, the amplifier can be operated without installing transistors VT9 and VT13 on radiators;
  • transistors VT2 and VT6, VT3 and VT7, VT4 and VT8, must be matched in pairs by gain with an accuracy of at least 1%;
Printed circuit boards. Photos of "clean" printed circuit boards, as well as photos of these boards in the process of manufacturing, installation and testing:

Outputs on the printed circuit board. The name and purpose of the pins on the single-sided and double-sided boards are the same, so the following information is relevant for both boards.

+ U - power plus;

-U - power minus;

GND - main power ground;

oGND - output ground (negative output terminal);

sGND - signal ground;

IN - signal input;

OUT - signal output (positive output terminal).

Switching amplifier circuits. The correct way to connect the blocks and grounds of a stereo amplifier is:

Operation of the amplifier without a loudspeaker protector is not safe and is strongly discouraged. The operation of the amplifier without protection devices is allowed only for the first start-up and adjustment of the amplifier. As speaker protection for this amplifier, I recommend using a protection device - DEF 2017.

Setting. After a successful first turn on of the amplifier, it is necessary to adjust the "zero" and the quiescent current. To adjust the "zero", it is necessary to close the amplifier input (close the IN and sGND pins on the amplifier board), connect a millivoltmeter or multimeter to the amplifier output (OUT and oGND pins on the board), and then rotate the trimmer resistor R9 to achieve the minimum constant voltage at the output of the amplifier (the result can be considered good when the constant voltage at the output of the amplifier does not exceed +/-5mV). At the next stage of tuning, it is necessary to set some value of the quiescent current in order to warm up the amplifier before the final adjustment of the quiescent current. The probes of the multimeter (millivoltmeter) must be connected to the emitters of the output transistors (VT20 and VT21), as shown in the illustration:

After that, by rotating the engine of the tuning resistor R20, set a small quiescent current (approximately 40-50mA, which corresponds to the readings of an 18-22mV millivoltmeter connected to the output) and leave the amplifier in this state to warm up for about ten minutes. Adjusting the quiescent current without warming up the amplifier is not recommended, because after the amplifier warms up, the value of the quiescent current will change relative to the value set on the cold amplifier. When the amplifier warms up (the temperature of the output transistors stabilizes at one value and stops growing), you can proceed directly to the quiescent current adjustment itself. To do this, in the same way, by rotating the R20 slider, we set the value of the quiescent current you need. I recommend setting the value of the quiescent current in the range from 70 to 100mA (which corresponds to the readings of a millivoltmeter connected to the amplifier output - 30-44mV). A higher value of the quiescent current will not positively affect either the characteristics of the amplifier or its sound, but it will significantly increase the heating of the output transistors and reduce the efficiency. There is no need to connect the load to the amplifier output to adjust the "zero" and quiescent current. At this point, the setup can be considered completed, and the amplifier is ready for use. Now you can start listening.

Thank you for your attention!

P.S. Under the list of used radioelements, you can find and download printed circuit board files: one-sided - designed for home-made LUT (in .lay format), double-sided - designed for factory production (in Gerber format). The archive with Gerber files is already prepared for factory production, it does not need any modifications and can be immediately sent to any PCB manufacturer. Do not feed intermediaries - order factory boards directly from the manufacturer. Now that's all for sure!

List of radio elements for Only Music 2.7

Designation
Type
Rating
Value
Qty
Note
R3, R31, R32Resistor0.25W1 ohm3
R23, R24, R25, R26Resistor0.25W22 ohm4
R6, R7, R11, R12, R16, R17, R30Resistor0.25W100 ohm7
R5Resistor0.25W200 ohm1
R2, R8, R21Resistor0.25W470 ohm3
R14, R29, R33, R34Resistor0.25W1K Ohm
4
R13, R18Resistor0.25W2.4k Ohm2
R1, R4, R10, R15, R19, R22, R27, R28Resistor0.25W15K Ohm
8
R38, R39Resistor1W3.6 ohm2SMD 2512
R37Resistor1W10 ohm1SMD 2512
R35, R36Resistor2W0.22 ohm2
R9, R20ResistorTrimmer200 ohm23296W
C17, C18CapacitorCeramic47 pF 50V2MLCC (NP0) 50V or CT81 (Y5P) 1000V
C11CapacitorCeramic4.7nF 1000V1CT81 (Y5P) 1000V or MLCC (X7R) 50V
C15CapacitorCeramic0.1uF 50V1MLCC (X7R) 50V
C7, C19, C20CapacitorFilm1 nF 100V3CL11 100V or MLCC (X7R) 50V
C26CapacitorFilm47 nF 100V1CL11 100V
C1, C3, C9, C16, C25, C27, C28, C29CapacitorFilm0.1uF 100V8CL11 100V
C2, C13, C21, C22CapacitorFilm1uF 63V4CL21 63V
C8CapacitorElectrolytic100uF 16V1Non-polar (NP) 16V
C12CapacitorElectrolytic100uF 50V1
C4CapacitorElectrolytic220uF 25V1
C5, C6CapacitorElectrolytic470uF 50V2
C23, C24CapacitorElectrolytic1000uF 50V2
VD1Zener Diode0.5WBZX55-C15115V
VD2Zener Diode0.5WBZX55-C3V313.3V
VD3, VD4, VD5, VD6Rectifier Diode1N41484
VT10TransistorBipolarBC550
1BC550B or BC550C
VT2, VT3, VT6, VT7TransistorBipolarBC5604BC560B or BC560C
VT4, VT8, VT14, VT16TransistorBipolar2N55514
VT1, VT5, VT11, VT15, VT17TransistorBipolar2N5401
5
VT12TransistorBipolarBD1351or BD137, BD139, KT815, KT817
VT9, VT19TransistorBipolar2SD6692or HSD669
VT13, VT18TransistorBipolar2SB6492or HSB649
VT20TransistorBipolarNJW02811or NJW3281, NJW21194, 2SC5198, 2SC5200
VT21TransistorBipolarNJW03021or NJW1302, NJW21193, 2SA1941, 2SA1943
L1Inductor1 μH112-15 turns, wire d1.0-1.2mm, diameter 8.0-8.5mm
 


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