SSA120

A 120 watt Solid State AmpIntroduction Again an amplifier at a time when nice class-D amps (Hypex) are available? After the absolute success of the SSA35 (it measures and sounds so good) I present its big brother. So I repeat: Against all HiFi-religions I offer a class B amplifier (Douglas Self, others call it class AB) with an op amp and overall feed back which performs excellent (< 0.01 % distortion) into a 4 Ω resistive load. Already in a description of the SSA35 in 'Another 35 watt Solid State Amplifier' on this site, some hints have been dropped about how to build an amp for more than 35 watt, but there are different solutions. Sometimes more power (it is to say: a higher voltage source for reactive loads) is needed, eg. with ESL's.... What I learned from others.. Recently I used a 120 watt clone of a Threshold amplifier to drive my ESL's (+ MFB). Together with Pieter Meijer, I came to the conclusion that the SSA35 sounds better. The question is: why? I decided to do an investigation. Threshold is proud of their feed back: on the front of their amplifiers is stated that the power stages have been left outside the feed back loop! Stability is not ensured with an overall feed back within this amplifier concept indeed. The three stage output stage is the problem. Cross over distortion is only 70 to 80 dB down if the amp is loaded with 4 Ω in spite of five power transistors parallel in CFP (complementary feedback pair). On the site of Douglas Self (www.douglas-self.com/ampins/dipa.htm#1) one could find a lot of information and investigations of the distinguishable three stages in a power amplifier: the input pair, the VAS and the output stage. At the end he comes to a 'blameless class-B amplifier'. Obviously this is to difficult for me. What to do? To avoid the problems of the input stage and the VAS, I do use an op amp. Look at my SSA35 for explanations of this choice. Douglas Self tells in his Audio Power Amplifier Design Handbook that the output stage should be controlled by a voltage source, so my idea to replace the input stage and the VAS with an op amp is not that bad. For high power there is at least one problem: the op amp should operate at voltages larger than +/-18 volt. To make 120 watt into 4 Ω, a power supply of more than +/- 35 volt is needed. To stay away from hum and distortion at low frequencies with high power, I choose for +/- 45 volt. Bear in mind that a power stage is 'a unity-gain output stage, specifically a voltage-follower' as Self states. He discourages strongly to build an output stage with gain: “… so making things worse by seeking voltage gain is not the way forward.... ….my advice would be that you probably do not want to go this way.” (See page 151 of his book: AUDIO POWER AMPLIFIER DESIGN HANDBOOK, Fifth Edition). Bootstrapping? With the SSA35 (on this website) I suggest to bootstrap the op amp to allow a higher power supply voltage. Here I would go a different way….. one is handed down to the idiosyncrasies of it. simulations predict a critical circuit. 2SC5200. but we come to the diagram below: D1 and D4 together with the 1000 µF elco’s avoid the power line of the op amp and the quiescent current from swinging during heavy load of the power stage.000/25x35 = 11. Its open loop gain is 100 dB at 20 Hz and 40 dB at 20 kHz. The OPA445 can only supply 10 mAAC so R5 and R6 should be as high as possible. so….Accoutrement: More simple Looking after op amps with high power supply voltages I stumbled across the LTC6090 which can handle 10 mA with a supply voltage of +/-70 V. Replacing the OPA134 with an OPA445 is very attractive indeed. If a smaller diode is chosen. Using an unknown op amp.5 mA at a maximum output current of 10 A. 1000 µF charged with 45 volt in a low power environment however is inadmissible because of damage of small traces on the PCB in case of a short circuit during experiments. Load of op amp The hfe of the BD230/231 is at least 25 and those of the 2SC5200/2SA1943: 35 so that the peak base current of the BD’s will not exceed 10. DC stability Douglas Self predicts more DC stability with 0. this current will be smaller and .. It could be wise to add a small resistor (47 Ω) between the 1000 µF elco and the rest of the circuit. Without emitter resistors with the BD’s. coming to about the same diagram as that of the SSA35 (see above). but I have excellent results with power stages with 2SA1943. Is this a candidate for the pre amplifier so that no bootstrap is needed? It’s open loop output impedance is 220 Ω. For biasing the BD’s the 1N4148’s (D2 & D3) need some 5 mA. and an OPA445 that can handle >15 mA with a supply voltage of +/-45 V. BD230 and BD231 which are not too complicated so that I will give it a try.1 Ω resistors in series with ‘the big transistors’ (R14/R15 below). This was an inheritance of the SSA35…. R10 Douglas Self states on page 183 of his book: Audio Power Amplifiers Design Handbook. During power on the voltage across C2 could rise to an unknown value..consequently R5 and R6 will become larger (I hope…. cheap) it should be protected towards voltage overload with two diodes anti-parallel. Without R9. The Iq in the power transistors should be 15. that the voltage across the two bases of the BD’s should be 1297 mV with R14 = R15 = 0. With a glass diode this is less simple. Both are in TO-92 version.3V capacitor so that the gain is 1 at subsonic frequencies. C3 (220 pF) is no longer needed. There is another reason to look for a different diode: the diode best should be fixed on top of the BD’s for temperature stability (D. so that they simply could be glued on the BD’s with the flat side down. By the way. Protection of C2 To keep the output offset voltage low.1 Ω.3 mA. C2 has been planned: a 1000 µF / 6. Because the elco is of a low voltage type (small.). To get an idea of the behaviour of TO-92 transistors (BC549 & BC559) in our biasing circuit: . The 1N4148’s are replaced by transistors. fifth edition. Self). Oscillations MicroSim does not ‘detect’ oscillations but as a precaution the RC-combination across the output terminals has been changed to 4 Ω and 330 nF. To which BD should the transistor be fixed? Moreover MicroSim predicts me much less results as with two transistors as diodes. a so called ‘Vbe-doubler’ is out of the question. an NPN (BC549C) and a PNP (BC559C). . to keep the load of the op amp small! Line voltage of the OPA445 The OPA445 has been designed for +/. to match to the attenuator and to avoid this ‘nljc-distortion’. Current limiter If shortcut protection should be served by a transistor. R2 and C1 should be changed to 10 Ω. 10 kΩ and 560 pF respectively.. Non-linear junction C distortion If a ladder attenuator (also on this website) is used at the inlet.45 volt supply.1 Ω resistor in series with the collectors of the power transistors (R14 & R15) and the 1 Ω resistors at the emitters of the BD’s (R9 & R10) a simple current limiter circuit will satisfy. . the addition of R9 & R10 (1 Ω) and R24 & R25 (15 Ω) are proven to be necessary for a large R5 and R6. To avoid a too high voltage on the op amp a resistor between D1 and D4 and the corresponding 1000 µF elco’s could be of help. somewhere in the circuit a voltage of more than 0.. The rates of R27/R30 and R28/R29 define the current limit. Mind that the Vce and the Vcb of these transistors will meet nearly 75 volt (see to the left) so that BC639/640 is a must! Erroneously the types in the picture left are called BC546 and BC556…. We make the statement: the highest output current will be 7 ARMS or 10 Apeak for ≥120 watt into 4 Ω.7 volt should arise when the output current becomes too large. the input circuit R1.For temperature stability R7 & R8 should be replaced by a parallel circuit of a resistor and an NTC-resistor (thermistor) as in the diagram above inherited from the SSA35. Mind that the numbering of some components have been changed meanwhile…. After careful investigations with MicroSim8. R31 & D9 (BAT83) as R32 & D10 (BAT83) isolate the bases of the BD’s from the non-linear parasitic Ccollector of the BC639 and BC640 respectively. In the last decades the mains voltage has been risen from 220 to 230 volt AC in Europe and is planned to rise to 240 volt. the 6 A fuses should serve. With the 0. For longterm shortcuts. the four copper traces at the top are light green and the solder tags are gold-coloured. T12. Both channels have their own PCB with the power transistors positioned at one side. Current limiting printed circuit board The ‘protection circuit’ has been fixed on a separate small PCB which is assembled on top of the main board. The main printed circuit board Below the top view is shown. R24 and R25 so that these current mirrors are not needed any more. The main PCB’s became 239 x 89 mm.5 A is required which is 7. The Realization The amplifier will be built into the cabinet of my Threshold clone which has two nice 45 volt power supplies with big elco’s.8 Apeak so that three transistors would satisfy. Meanwhile the circuit has been changed: C8 (10 nF) is no longer needed and replaced by resistors of 100 Ω to get 60 Ω for R15 and R16. All power transistors are assembled on aluminium angels which in turn are fixed on heat sinks. R15.With R14. However with some more transistors in parallel the cross over distortion decreases. The most components on top are in white. The 100 Ω resistors (R18 & R19) are fixed directly to the emitters of the BD’s. Mind that initially two BC640’s and BC639’s had been planned for a current source to the bias diodes to replace R5 and R6 enlarging the impedance at the output of the op amp. Number of power transistors The β of the power transistors 2SA1943 and 2SC5200 remain constant until a collector current of at least 3 A. The diodes however have been replaced by T11. the copper traces at the bottom are dark green (the board is transparent here). The trace between the 100 Ω resistors and the connectors c and e should be erased. MicroSim tells us that R5 and R6 should be enlarged to 15 kΩ (with R24 = R25 = 15 Ω) which will relieve the op amp! We’ll see…. R11 and R12. For 120 watt into a 4 Ω load an output current of 5. T17 & T18 (ahead T15 & T16) must be BC639 and BC640 because of the high voltages they will meet during large audio periods. Too many power transistors will undoubtedly limit the bandwidth of the power amp so that much feedback will become difficult. R9 and R10 some feed back will be added which enhances the temperature stability and enlarges the input impedance. . R31 & R32 became 39 kΩ. I take four transistors in each leg. which effect is not yet clarified. ‘a’ and ‘g’ are connected with thin cord.8 mm mounting thread. Main board and heat sink . The light green traces are on top.Implementation hints: Above a detail of the main board. Initially two current mirrors (BC640 and BC639) had been planned which are replaced by R5 and R6 (R5 and R8 on the board). The connections are made with 0. these connection could be made with wires. The red encircled connections ‘b’ – ‘f’ correspond with those of the current limiting board to mount it on top. If one could prefer a one sided implementation. Mind that the collectors and bases of the BC’s are soldered to the bases of the BD’s as short as possible to help the temperature transmission. but 330 nF with 4 Ω in series has been installed. .The op amp. It became clear that the biasing of the final stage is much more complicated than with the SSA35! Oscillations MicroSim does not always detect oscillations. As soon as the amplifier had been loaded (with 4 Ω). the protection board and ‘the changes’ are not applied jet. The 1Ω emitter resistors (R9 & R10) and those between T11 and T12 are not fixed yet! Implementation Measurements and tuning Hitherto all values of the components has been designed by simulation. it oscillates in the peaks of an output voltage of a few milli volts already! 100 nF in series with 10 Ω across the output connectors helps. Detail of T11 (BC549) and T12 (BC559) fixed on top of T1 (BD230) and T2 (BD231) respectively. 5 hours rest with power on: Iq of the BD’s is 41 mA and Iq of the 2SC/2SA’s is 26/28 mA. There is one objection: the input voltages (common mode in this case) become too large for the op amp in question so the gain should be changed to.5 kΩ (10k//56k) should be used. R10. After 1 minute: Iq of the BD’s is 42 mA and Iq of the 2SC/2SA’s is 25 mA. After 7.Output power Without current limiter the unloaded output voltage is 80 Vtt.5 hours: Iq of the BD’s is 41 mA and Iq of the 2SC/2SA’s is 24/25 mA. This will decrease the distortion! . Now it will become 95dB@20Hz and 35dB@20kHz. so R4 became 758 Ω (820//10k). The great surprise…… The amplifier is unity gain stable! If the feedback-resistor R4 (= 10kΩ) is replaced by 100 Ω.5 hours rest with power on: Iq of the BD’s is 41 mA and Iq of the 2SC/2SA’s is 24/26 mA. The tuned diagram The collector resistors of the BD’s should be 15 Ω indeed. 4. NB. This counts for all frequencies between 1 Hz and 16 kHz (the used LF-generator stops there). not even when the amp is extremely overdriven.4 dB. R24 and R25 the currents in the NPN and PNP-branch are equal within 3%. the output voltage is 64 Vtt because of the voltage drop of the power supply. There is no need for an NTC resistor to keep the quiescent current in the power transistors in the order of 25 mA independent of their temperature. This results in >120 watt output. One of the greatest enhancements is the increase of the poor feedback at 20 kHz. Loaded with 4 Ω. After 5 minutes full power (70˚C): Iq of the BD’s is 34 mA and Iq of the 2SC/2SA’s is ~9 mA. the amplification is nearly 0 dB without tendency to oscillation. R5 and R6: MicroSim predicted 15 kΩ but in practice 8. This means that the op amp is loaded with ~ 4 kΩ which points out to be no problem. After 1.: because of R9. not even during heavy overdriving. With the diagram above the quiescent current in the BD’s and the power transistors has been measured: Immediately after power on (‘cold’): Iq of the BD’s is 43 mA and Iq of the 2SC/2SA’s is 35 mA. After 1. After 25 minutes: Iq of the BD’s is 41 mA and Iq of the 2SC/2SA’s is 24/25 mA. say. There are no oscillations. PCM1792 and balanced I/V-converter} of -3 dB imply full power.01 % * Distortion at 10 kHz 60 watt into 4 Ω: -72 dB = 0. Mind that R1 on the main board has been changed from 10 Ω to 1 kΩ again! C1 stays 560 pF. So the pre amp should amplify 15.6 kΩ: [27k//15k] (R3 = 1 kΩ) for the right amplification. To ensure full output with -6 dB on the CD. The 330 pF capacitor is a mica cap because of their stability and good sound. Right: -77 dB Unloaded gain: 28. To produce 32 Vtop at the loudspeaker connection.log(32/3. Measurements with the feedback resistor R4 = 758 Ω and with pre amp: Maximum output power into 4 Ω: 118 W Maximum output power into 8 Ω: 80 W Power bandwidth with source impedance of 10 kΩ: 0 . Moreover I will use the ladder attenuator which initially attenuates 6 dB. The small board contains the two pre amps for both channels. My modified players output 3.The needed pre-amplifier Of course 4.4 dB gain is insufficient for a CD-player.1” experiment board with ground plane.05 dB Unloaded maximum output voltage: 80 Vtt Loaded maximum output voltage across 4 Ω: 61 Vtt The internal resistance Ri: < 0. so that the pre amp should account for 26 dB = 21x. Overall. it has been shown in two different ways. R4 must be 9.5 dB at 20 kHz Channels equality: within 0.6 dB. This means that a signal on CD {in my modified CD624 with FPGA-digifi.6 dB or 10x again.5 dB ! Dynamic range: 97 dB Channel separation: 80 dB Frequency characteristic: -0.2 Vtop at 0 dB.50 kHz Open output offset voltage: < 5 mV Distortion at 1 kHz 110 watt into 4 Ω: -80 dB = 0.01 Ω Feedback: 95dB@20Hz and 35dB@20kHz * only 2nd and 3rd harmonics.05 dB Ladder attenuator 3 dB-steps: within 0.5 dB Loaded gain: 28. They are implemented on 0.001 %. . the gain will become 21. All higher harmonics <0. For better readability of the layout. the total gain should be at least 20.2) = 20 dB.025 % * Double tone test with 18 and 20 kHz 110 watt into 4 Ω: Left: -80 dB. A beautiful performance of the 15 young men. Don’t ask me why……. Sometimes a 4 could be noticed on the software spectrum analyzer TrueRTA. The distortion of the unloaded amplifier was < 95 dB at any time at any frequency.74 mm Listening tests The final test is of course a number of listening tests. With this new amplifier I could count them apart for the first time! Used equipment The source was the forceful updated Philips CD624 with digifi on FPGA as described in ‘Ombouw van een CD624 met FPGA en PCM1792’ in Dutch. Listen to them. Moreover a fully driven shorted amp would be overheated. The figures in ‘Measurements’ are those of the worst channel. here R7 and R8) consist of 18 Ω//100 Ω NTCresistors.76 x 88. because the BD230 & BD231 have been paired.Distortion The distortion in the left channel is some 10 dB lower than in the right channel. R5 and R6 could be taken large enough as to the load of the op amp! MicroSim promised me 15 kΩ but in practice they became 8. even when listening with the ear on the speakers. Third: the basses seemed to be weaker. Sorry for the listening hints in Dutch.WAV-files! First burn them on disk. but just with the advent of the digifilter-onFPGA in my CD624 (see elsewhere) I could distinguish most instruments of the ensemble. In the SSA35 the quiescent currents still enhances a bit with temperature! Douglas Self promised sufficient DC-feedback from R14 and R15 (0. All higher ones were unnoticeable (<100 dB). What me stroke first is the total absence of any background noise. Ordered from EuroCircuits: The two boards have been ordered from EuroCircuits in Belgium. even the ‘Netherlands wind ensemble’ with Edo de Waart recorded in 1968 on Philips 420 711-2.74 x 38. Secondly: the ‘rest’ in the performance and the details in the stereo image. Anytime the distortion of the loaded amp became >10 dB worse. every simulation and every practice became handy. which means < 100 dB (= < 0. his or hum. R5 and R6 (there. Moreover a simple current limiter came in sight! The collector resistors of the BD’s (R7 = R8 = 15 Ω) satisfy without any NTC resistor! nd rd The distortion consists mainly of 2 harmonics due to the unmatched NPN/PNP transistors.9 mm and the limiter board became 38.1 Ω) but at first it took me days to find the right combination of the ‘diodes’(T11 & T12). Observances With the SSA35 (also on this website) R14 and R15 are not implemented and the BD139 and BD140 (in this case) do have an emitter-resistor of 0. I’m even not sure if this figure is not due to the signal source: the CD-player and/or the disk.en Geluidsfragmenten).5 kΩ (10k//56k)! Only when I did put a 1 Ω resistor in the emitter duct of the BD’s. 8 in the list) do not move any more in the first minutes if they play stronger and stronger! This is for the first time! Everything sounds better. The speakers are as described in: ‘(Gebogen) Luidspreker Array als Quasi Dipool’ with the MFB-box from ‘ESL + MFB = the best of 2 worlds’ . With a BC546B and BC556B this voltage is about 635/645 mV! That makes a lot! Only with 30 Ω between the biasing ‘diodes’ (R24 & R25). except at 10 kHz in the right channel. Obviously the cross over distortion must be very low because the higher harmonics had not been noticeable on the analyzer. but….5 Ω. Mind that the current limiter presented here is not a real shortcut protection indeed! The heat sink is too small to output continually 120 watt into 4 Ω for longer than about ten minutes. Ultimately the two piano’s in ‘Sonata for 2 pianos and percussion’ of Bartok (nr. It are all . For these I invited two younger men to help my old ears! On this website I gathered a number of recordings for this purpose (Muziek. R5/R6 and R7/R8……. The ‘main board’ is: 238.001 %). The voltage across the diode replacement BC559C and BC549C is about 605 mV with 1 mA as with the 1N4148. The 3 harth monics are always smaller often more than 10 dB. which is often a good sign. Accommodated in: This cabinet has been ‘designed’ for a Threshold clone from the 1980s. which sounded worse than the SSA35…. . .A part of the left and right channel PCB’s. The small PCB with the two pre amps in the middle and the two current limiter boards left end right over the BD’s (seen from top). the PCB’s are very suitable as is! Therefore I will add the Gerber files: http://by-rutgers. Appendix The layout of the PCB’s should have been matched to the final diagram of course but I have built only one amplifier: the prototype! Nevertheless these boards could very well be a good start for DIY’ers to built the amplifier as well. With an PCB-editor program changes could be made (and have been made partly already!).nl/Pictures/SSA120%20boards. 2014 updated: 26-11-2014 .zip March 8.The power supplies in the basement …. Anny way. 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