{"id":495,"date":"2020-03-04T23:25:40","date_gmt":"2020-03-04T20:25:40","guid":{"rendered":"https:\/\/tomorrow82.ru\/?page_id=495"},"modified":"2020-03-04T23:25:40","modified_gmt":"2020-03-04T20:25:40","slug":"the-leach-amp-faq","status":"publish","type":"page","link":"https:\/\/tomorrow82.ru\/?page_id=495","title":{"rendered":"The Leach Amp FAQ"},"content":{"rendered":"\n

Concerning the Bias Current<\/strong><\/p>\n\n\n\n

From a correspondent on 2\/7\/03<\/em> — I have built your low TIM \namp and am completely satisfied with its performance. Just one question.\n The quiescent current increases proportionally to the 8 ohm loaded \noutput amplitude. Neither the frequency nor the waveform affects Iq, \nonly the output amplitude on the 8 ohm load. Without the load, there is \nno Iq change. The following things have I checked: the VBE multiplier \nmaintains its set DC voltage, the DC voltage difference between the \npower BJT’s bases remains unhanged. Still Iq increases. For the \nmeasurements I used low pass filtering and ground-independent DMMs.\n\nI noticed this Iq change inside the amp across the 0R33 resistors and \nalso outside, across a serial 0R22 resistor in the +58V supply rail.\n\nThe amp anyway works perfectly, it has swept my old class-A JLH away, no\n other amp could do that so far.\n\nPlease help me with some info on this Iq phenomenon.\n\n<\/p>\n\n\n\n

With a signal on the load, you are measuring the bias current \nplus the load current. It sounds like your amp is doing what it is \nsupposed to do. The peak load current can get quite large when driving a\n load.<\/em><\/p>\n\n\n\n

\n\n\n\n

Concerning Transistors and Sources<\/strong><\/p>\n\n\n\n

From a correspondent on 2\/8\/00<\/em> — I built your amp and it \nsounds great. I read in your FAQ-section, that one person asked you if \nhe can use the European types MPSA06\/56. I used the BC550\/BC560 types \nand it sounds great. I also used BD243\/BD244 instead of the \nMJE15030\/MJE15031 and it works perfectly. But the Power of my amp is not\n 240 W, it is 310 W!!! Thank you for your perfect amp!!<\/p>\n\n\n\n

From a correspondent on 1\/31\/00<\/em> — This will be my second \nLeach Amp, the first is fantastic! The first time around I made my own \nboards, no problems but once was enough fun for me. This one will be for\n a sub and a center channel. For your information, Allied Electronics \nproved to be MUCH, MUCH cheaper than MCM Electronics for almost all the \necessary transistors. A very good company. <\/p>\n\n\n\n

How does the amplifier sound?<\/strong><\/p>\n\n\n\n

From a correspondent on 6\/27\/00<\/em> — First Excuse me for my bad\n English! I am from Bulgaria. I am 31 years old. Audio is my hobby all \nmy life, especially constructing amplifiers. I have QUAD 303, 405MK1, \nPASS Labs ZEN, Douglas Self Class B amplifier\nall built by myself. Before 3 months I started building your Low TIM \namplifier, and now the work is done. I can say one: this amplifier is \nthe real GREAT sounding amplifier I ever listen. I make comparison with \nall other types I have, but Low TIM is the best one. I use good \ntransformer which gives +\/- 62 volts after rectifying and filtering. I \nuse original components (it is not so easy to find original components \nhere in Bulgaria, but I find it). And again: EXCUSE me for my bad \nEnglish!<\/p>\n\n\n\n

From a correspondent on 12\/15\/99<\/em> — I came across your web \nsite a while ago and noticed that your Low TIM amplifier is still in \nproduction. I built two in about 1976 (immediately after the article in \nAudio ) have been using them ever since. I must admit it was the first \ntime that I heard a very significant difference in sound quality by \nchanging amplifying components.<\/p>\n\n\n\n

From a correspondent on 12\/8\/99<\/em> — Hello! Now my amp works like a dream!! Thanks again for a great design. :-))<\/p>\n\n\n\n

From a correspondent on 11\/29\/99<\/em> — Thank you.. the amp sounds truely wonderful. It sounds so good, I’ll have to go through ALL my CD collection again.<\/p>\n\n\n\n

From a former student on 11\/2\/99<\/em> — I was a student of yours \nin 1978, and I built the amp and set of speakers, which have lasted 21 \nyears now. I work with a tube amp audiophile who used to swear that no \nIC amp could touch his tubes. I used the Leach Amp to drive his Thiel \nspeakers and now he wants to build one. Please tell us how to order sets\n of the circuit boards, as I am ready to build a second amp for myself, \njust for the fun of it. Thanks.<\/p>\n\n\n\n

From a correspondent on 7\/11\/99<\/em> — I have completed, with \ngreat success, two 3-channel Low TIM v4.4 amplifiers. I wanted to let \nyou know how pleased I am with the sound of these amps — I have Carver \namplifiers made in 1990 and 1996 and the Low TIM sounds so much better! \nThank-you for a most stellar design and your efforts in keeping the Low \nTIM web page so informative and current.<\/p>\n\n\n\n

From a correspondent on 7\/25\/99<\/em> — «My husband has been in touch with you regarding parts to build me a pair of Leach amps.<\/p>\n\n\n\n

He tells you true, I did have a pair of your amps many, many moons \nago. I was so bummed when I separated from my first husband. I have to \ntell you losing them amps really hurt…<\/p>\n\n\n\n

BUT, I ended up with an amp, the Bedini 100\/100 classic that I really\n liked. It is strong as a mule with a sincere attempt at reproducing \nmusical reality, teamed up with my Ohm\/Walsh speakers (I am a stickler \nfor certain details such as Thiel alignment) and my R.G. Dynamics \npre-amp (don’t laugh now, I had Mr. Grodinsky assemble this piece of \nequipment for me long after he stopped making them as I had lost the \noriginal with the first set of Leach amps. It was the best pairing for \nthe Leach’s and my Revox turntable that I came across). I had a \nsatisfactory set up. THEN I stumbled upon your current plans online. I \ncopied them and sent my husband to work with them to look them over on \nhis break. When he came home he downloaded all the necessary info and \noff he went, getting everything together to build a pair (YEAH!)<\/p>\n\n\n\n

The Bedini set a standard some years ago for it’s ease of handling of\n any and all sorts of music. I worked at a high end stereo store that \nallowed me to set the Bedini against any and all possible competitors. \nThere was no comparison available on a retail level. Then I ran across a\n pair of your amps. I must say that when I A-B’d amps long ago against \nan original pair of your amps I was amazed at how fast the Leach’s were \nin comparison.\nThey do not ‘stagger’, or drag their ‘feet’ at all. It is something I \nthink that is missing from all the top end amps I have heard to date, \nspeed. When listening to a pair of Leach amps\nI truly felt like I was listening to music, not the electronic \nreproduction of it. So, if I must speak of a comparison, though the \nBedini is a great, no a GREAT amp I will gladly set it aside (sorry John\n B.) to be able to enjoy what music should sound like.<\/p>\n\n\n\n

I just want to thank you personally. If I never again, heaven forbid,\n get to listen to your equipment I have still spent many, many hours \nenjoying music thanks to your design. It is nice to know that there is \nsomeone still around who must understands music from the listeners point\n of view, not just from an engineering standpoint, making such great \nequipment available to the real music lovers.»<\/p>\n\n\n\n

From a correspondent on 7\/15\/99<\/em> — «I’ll e-mail a photo back \nwhen it’s done. My wife is the audiophile of the bunch—she had a pair \nof one of your earlier revisions some years back, but lost them when she\n separated from her first husband. She’s using a Bedini 100\/100 we \nbought used from a friend in Cal City (IL) right now, but claims yours \nsounded much better and has been looking for plans to rebuild a set for \nsome time. I’m building the unit for her as a Christmas (99) gift.»<\/p>\n\n\n\n

From a correspondent on 5\/10\/99<\/em> — «Executive Summary: Wow!!!\n Details: It’s been a while since I started this project. I built both \nchannels, but I had no output when I connected a load. I ended up \ntabling the project for a _long_ time. It turns out I had interchanged \nQ10 and Q11, the protection transistors, and yes, I had made the same \nmistake in both channels. Boy, do I feel stupid. The first thing I \nplayed was the overture from «The Magnificent Seven.» Oh my. Your amp \nalso does well on Bach organ works. I expect the neighbors are going to \ncall the cops on me pretty soon, if I don’t destroy my speakers first. I\n had been using a 14W class A amp before this one, and I thought it was \ngreat, but I think this is better. It has its own sound, but overall \nit’s a neutral sound, and it handles difficult passages effortlessly. \nNow I’m curious _why_ it’s better. My listening room is small, and I \nusually listen at low to moderate levels, but maybe the class A amp was \nclipping. Or maybe yours is better because it’s full-discrete vs. an \nOPA2604 in the front end. Or BJT vs. MOSFET. Or higher rail voltages \ngiving a better small-signal approximation. Further listening and \nexperimentation are required! Thank you.»<\/p>\n\n\n\n

From a correspondent on 5\/10\/99<\/em> — The first thing I played \nwas the overture from «The Magnificent Seven». Oh my. Your amp also does\n well on Bach organ works. I expect the neighbors are going to call the \ncops on me pretty soon, if I don’t destroy my speakers first. I had been\n using a 14W class A amp before this one, and I thought it was great, \nbut I think this is better. It has its own sound, but overall it’s a \nneutral sound, and it handles difficult passages effortlessly. Now I’m \ncurious _why_ it’s better. My listening room is small, and I usually \nlisten at low to moderate levels, but maybe the class A amp was \nclipping. Or maybe yours is better because it’s full-discrete vs. an \nOPA2604 in the front end. Or BJT vs. MOSFET. Or higher rail voltages \ngiving a better small-signal approximation. Further listening and \nexperimentation are required!<\/p>\n\n\n\n

From a correspondent on 5\/4\/99<\/em> — «I would like\nto express my appreciation and thank you for the information in\nthe «Leach amplifier web site.» I have built two\n3-channel units and enjoyed every moment of going through the\nphases of the project.»<\/p>\n\n\n\n

From a correspondent on 3\/11\/99<\/em> — «The amp is\nabout 19 years old now. The speaker fuses blow occasionally\nbecause of dynamic range expansion processors. I want to thank\nyou for creating such a unique amplifier. I’ve heard Amzilla,\nwhich I wanted really bad, and I could not afford it at the time.\nI did get to compare your amp to a couple of Dynaco tube amps and\nI think it compared favorably. I love the amp. I still do. I\nlearned to fix amplifiers by your article and building this\nproject. Thanks again.»<\/p>\n\n\n\n

From a correspondent on 3\/8\/99<\/em> — «This is a\nsuperb design. If you remove one power supply rail, the speaker\nterminals stay at zero volts. I don’t know of any other design\nthat would pass this test! Thanks again professor.»<\/p>\n\n\n\n

From a correspondent on 9\/11\/99<\/em> — «The Leach Amp\nwas successfully built early this year without any problem, and\nit turned out to be better than the one I bought. It is not only\nmy comment, but my friend also said the same thing. He is a DIY\nspeaker builder. Three months ago, he borrowed my Leach Amp to\ntest his Scan-Speak speaker system, he came back with very high\nrating on the Amp. Now, he wanted me to build one for him, and I\nmisplaced your mailing address for ordering the circuit board.\nPlease send me your address again.»<\/p>\n\n\n\n

From a correspondent on 8\/23\/98<\/em> — «I got your\ncircuit diagram for the Leach amp off the web and have been\nmeaning to write for a long time just to say thank you. I built\none and have been listening to the amp for about two months now\n(very sharp reproduction). I decided to build another two amps to\nhear my speakers bi-amped.»<\/p>\n\n\n\n

From a correspondent on 7\/20\/98<\/em> — «I finally\nfinished my first amplifier. They worked perfectly on power up.\nNo smoke. These are very high quality amplifiers, IMHO. Thanks\nfor designing a terrific amplifier.»<\/p>\n\n\n\n

From a correspondent on 1\/13\/98<\/em> — «I built one\nof your Low TIM amplifiers back in ’79 when the article first\nappeared in Audio magazine. I have had a great amplifier for all\nthese years. I had a Crown DC300 I sold in favor of your design\nand my construction. Of course I could be a bit biased but, I’m\nsure your design sounds better.<\/p>\n\n\n\n

\n\n\n\n

There are two connections for central ground on the board.\nThe one on the input side is on the same trace as the signal\nground. What is the purpose of second central ground connection\nif you are isolating the input jacks from the chassis and have\nR51 (82ohm) between input ground and central ground? It seems as\nthough you worked on isolating the two grounds from each other\nand then shorted them with the second central ground connection.<\/em><\/p>\n\n\n\n

For audio frequency equipment, a central ground is used and\nall ground wires run in parallel to this point. This prevents\nwhat are called «longitudinal voltages» in the ground\nsystem from coupling between different stages in the circuit.\nWith parallel ground wires, there is the possibility of what are\ncalled «ground loops» if the ground wires connect\ntogether at either end. Thus the input cable ground is isolated\nfrom the chassis because it is grounded through the circuit board\nground. There are 2 ground tracess on the circuit board, one in\nwhich signal currents flow and one in which ripple currents from\nthe electrolytic decoupling capacitors flow. Each of these ground\ntraces has its own wire that connects to central ground. A single\nground should not be used because the ripple currents could\ninduce hum in the signal part of the circuit. The 2 grounds are\nconnected together with a 82 ohm resistor which is large enough\nto prevent the current in one ground trace from flowing into the\nother ground trace, but small enough to keep both traces at\napproximately the same ac voltage at higher frequencies where the\ninductance of the ground wires causes the ground system impedance\nto increase. The theory of this is covered books on\nelectromagnetic compatibility.<\/p>\n\n\n\n

Can I mount the main heat sinks inside the chassis with the\ntops of the power transistors facing up?<\/em><\/p>\n\n\n\n

If you put the heat sinks inside the chassis, they should be\nmounted vertically with adequate vent holes beneath and above\nthem. If you mount them horizonatlly, you should install a fan to\nkeep them cool. I much prefer mounting the heat sinks vertically\non the back of the chassis.<\/p>\n\n\n\n

\n\n\n\n

Does the amplifier put out a dc voltage on the loudspeaker\nif one of the power supply fuses blow?<\/em><\/p>\n\n\n\n

No. If a power supply fuse blows, the amp goes dead and the\noutput voltage is zero.<\/p>\n\n\n\n

\n\n\n\n

I have compared your circuit diagram to your parts layout\nfor the circuit board, and I think that I have found an error.\nThe order of R30 and D5 is reversed when you compare the diagram\nand the layout, and similarly for R31 and D6.<\/em><\/p>\n\n\n\n

These elements are in series. The order of series elements\ndoesn’t matter. A math analog is 2 + 3 = 3 + 2. It doesn’t matter\nwhich comes first. I have redrawn the circuit diagram to reverse\nthe order of the resistors and diodes.<\/p>\n\n\n\n

\n\n\n\n

I do not need the 120 W power and I am going to remove\ntransistors Q20 and Q21. Should I change any other elements, e.g.\nR36, R37\/R38, or R41\/R42? What about connecting C19 and C20 to\nR37 and R38?<\/em><\/p>\n\n\n\n

Removing 1\/2 the output transistors will not reduce the output\npower. It reduces the maximum output current by a factor of 1\/2.\nFor an 8 ohm load, the power output would not change. For lower\nload impedances, the protection circuits would kick in at a lower\noutput current than with all 4 output transistors. If you want to\noperate the amplifier with 1\/2 the output transistors, Q18 and\nQ19 are the ones to omit. You can also omit R37, R38, R41, R42,\nR45, and R46. These changes will reduce the maximum output\ncurrent by a factor of 1\/2. If you wish to decrease the output\npower, you must reduce the rail voltages. I do not recommend\nreducing them to less than +50 V and -50 V.<\/p>\n\n\n\n

\n\n\n\n

I have a transformer which gives dc rail voltages of +66 V\nand -66 V after rectifying and filtering. Should I change any\nelements, e.g. R34, R35, or R36?<\/em><\/p>\n\n\n\n

The +66 and -66 V rail voltages are too high for the\namplifier. Do not use this transformer. Some of the transistors\ncould break down.<\/p>\n\n\n\n

\n\n\n\n

May I substitute European type transistors for the ones you\nspecify, e.g. BC546\/556 instead of MPSA06\/56?<\/em><\/p>\n\n\n\n

I am sorry, but I do not have any experience with these\ntransistors. If they have similar specifications, they may (or\nmay not) be suitable. I have included specifications for all\ntransistors on the amplifier page. <\/p>\n\n\n\n

\n\n\n\n

Why do you use the 2N3439\/2N5415? These transistors are\ndesignated for switching applications and may have bad linearity.<\/em><\/p>\n\n\n\n

My Motorola data book says that these are «one ampere\ncomplementary silicon high-voltage power transistors.» To my\nknowledge, these are some of the most commonly used complementary\ntransistors for the gain stages and pre-driver stages in power\namplifiers. I am not aware of linearity problems with transistors\nspecified for switching applications. Some of these are very low\ncapacitance and low noise transistors. For example, the\n2N4401\/2N4403 complementary pair are specified as switching\ntransistors. They are some of the lowest noise transistors\navailable for linear audio applications.<\/p>\n\n\n\n

\n\n\n\n

I now have a 40 W amplifier that was published in about\n1985 in «Wireless World» or some similar magazine and\nwas called a «Low TIM Amplifier.» It has a slew rate of\n40 V\/us. The author specified the use of transistors in the\noutput stage with a minimum gain-bandwidth product of 5 MHz to\navoid distortions produced by the charging and discharging of the\nbase-to-emitter junction capacitance. The MJ15003\/15004 have a\ngain-bandwidth product of 3 MHz. What about that?<\/em><\/p>\n\n\n\n

The higher the current rating of a transistor, the larger the\njunction area and the larger the junction capacitance. The driver\ntransistors in the T-circuit output stage have a very low output\nimpedance which can charge and discharge the junction capacitance\nof the MJ15003 and MJ15004 with no problems. The speed of the\namplifier is set by the Q12\/Q13 stage and not the output stage.<\/p>\n\n\n\n

\n\n\n\n

Why didn’t you use current source tail supplies for the\ndiff amps?<\/em><\/p>\n\n\n\n

I had intended to use current sources in the beginning.\nHowever, when I started laying out the circuit board, I found\nthat they added a great deal of complexity to an already dense\npart of the board layout. Therefore, I opted for resistive\nsupplies to simplify the layout. I never had any problems with\nthe resistive supplies. The amp seemed to work perfectly with\nthem. An additional bonus is that the matching of the tail\ncurrents in the two diff amps is not a function of the matching\nof transistor parameters.<\/p>\n\n\n\n

The turn-on characteristics of the diff amps determine the\nturn-on characteristics of the amplifier. With resistive tail\nsupplies, the diff amps turn on very gracefully at a rate set by\nthe RC time constants in the tail supply circuits. This leads to\na thump free turn on, eliminating the need for output relays to\nprotect the speakers. If transistor current source tail supplies\nwere used, the current source transistors tend to turn on\nabruptly as the power supply voltages come up, which can produce\nthumps in the loudspeaker. This problem is even worse if one tail\nsupply transistor turns on before the other.<\/p>\n\n\n\n

\n\n\n\n

Can I use separate power supplies for the two channels?<\/em><\/p>\n\n\n\n

Yes. This will require separate transformers, rectifiers, and\nfilter capacitors. An alternate is to use one transformer with\nseparate rectifiers and filter capacitors for the two channels.\nMy laboratory amplifier uses this configuration.<\/p>\n\n\n\n

\n\n\n\n

Can I mount each power transistor on separate heat sinks?<\/em><\/p>\n\n\n\n

Don’t do this. The amplifier will not be thermally stable. The\nfour bias diodes should see the average temperature of all four\noutput transistors. This occurs only when all transistors and all\nbias diodes are on the same heat sink. There is an exception. You\ncan mount the two NPN output transistors for one channel on one\nheat sink and the two PNP output transistors on another. Either\nput all four bias diodes on one of the heat sinks or put two bias\ndiodes on each.<\/p>\n\n\n\n

I once had a student who used 2 heat sinks per channel with\ntwo output transistors on each, a NPN and a PNP. He put the bias\ndiodes on only one of the heat sinks. His amp was thermally\nunstable. The heat sinks without the diodes overheated. Putting\nboth NPNs on one heat sink and both PNPs on the other solved the\nproblem.<\/p>\n\n\n\n

\n\n\n\n

Can I put the output transistors for both channels on the\nsame heat sink?<\/em><\/p>\n\n\n\n

Yes, but I prefer separate heat sinks. Put the output\ntransistors for the two channels on opposite ends of the heat\nsink channel. The bias diodes for each channel should be in the\ncenter of the output transistors for that channel.<\/p>\n\n\n\n

\n\n\n\n

Can I power more than two channels from a single power\nsupply?<\/em><\/p>\n\n\n\n

Yes. However, the maximum power output per channel with all\nchannels driven simultaneously will be decreased. The power\noutput with only one channel driven will not be affected.<\/p>\n\n\n\n

\n\n\n\n

What is the difference between a toroidal core transformer\nand an E-I core transformer?<\/em><\/p>\n\n\n\n

For a given power rating (or VA rating), the toroid\ntransformer will be lighter. However, I have found that the\nvoltage output of a toroid seems to drop more under load than\nthat of an E-I core transformer with the same power rating. In\naddition, a toroid seems to draw much more current at turn-on,\nthus requiring a larger AC line fuse. I tend to believe that\nweight is an indicator of transformer quality, whether the\ntransformer has a toroid core or an E-I core. The heavier the\ntransformer, the better it is.<\/p>\n\n\n\n

\n\n\n\n

Do I have to match the transistors?<\/em><\/p>\n\n\n\n

No. I never matched the transistors in any of the amplifiers I\nbuilt and I never had any problems with them. If an amplifier has\nmore than 50 mV DC offset at the output with the inputs\ndisconnected, you may wish to check the matching of the\ntransistors in the input diff amps.<\/p>\n\n\n\n

\n\n\n\n

One aspect of your design that’s most curious is putting\nthe 10 ohm — 0.1 uF series output compensation after the output\ninductor. In a normal network, the hf load is before the\ninductor. As I understand it, the whole purpose is to keep a low\nimpedance load on the amp at high frequencies when it’s driving\nan inductive load to help prevent spurious oscillation and other\ninstabilities. Putting this network after the inductor reduces\nits ability to do this.<\/em><\/p>\n\n\n\n

The first amp I built had the R\/C network on the circuit board\nbefore the R\/L network. The amp was subject to random\noverheating. I found it was oscillating. I moved the R\/C network\nto the speaker output terminals and the problem was solved. The\nproblem was caused by the high frequency currents in the R\/C\nnetwork flowing back through the circuit board ground wire,\ncausing positive feedback into the circuit. Putting the network\non the speaker outputs solves this problem, for the currents flow\nin the speaker ground wire instead of the circuit board ground\nwire. With the R\/L network before the R\/C network, the amp is not\ndriving an inductive load at high frequencies because the R is in\nparallel with the L, not in series with it.<\/p>\n\n\n\n

\n\n\n\n

You do not explain the logic of using a fully complimentary\ninput stage and voltage amplifer stage. Is it because of slew\nrate symmetry? Most designs, even high-end ones, use a single\ndifferential pair and a single voltage amp transistor. If the\ndesigner adds additional transistors, normally they’re used for\ncurrent mirrors, cascode stages or as a buffer between the\nvoltage amp and the first emitter follower. All of these have the\npotential for increasing linearity while your extra parts only\nprovide additional (and perhaps cosmetic?) symmetry.<\/em><\/p>\n\n\n\n

My original intent was to design a fully complementary\namplifier because the push pull action of such circuits generates\nless distortion. Therefore, I used complementary diff amps.\nIncidentally, I have seen these used in some very high\nperformance Harris op amps. With the complementary diff amps, the\npositive and negtive sides of the amp turn on at the same rate,\nso there are no thumps in the loudspeaker, and the slew rate is\nperfectly symmetrical. Both diff amps are very linear. They will\nput out undistorted sine waves with a peak-to-peak voltage of\nover 4 V before clipping. This is far more voltage than what is\nrequired to drive the second stage into clipping.<\/p>\n\n\n\n

The complementary or push pull voltage gain stage is superior\nto a single voltage amp transistor, i.e. a single ended stage,\nbecause the push pull action cancels even order distortion\ncomponents. In vacuum tube days, push pull symmetry was always\nconsidered superior to single ended designs, although single\nended output stages seem to be making a sentimental comeback in\nsome tube amps.<\/p>\n\n\n\n

If only a single diff amp is used, the only way to obtain a\ncomplementary second stage is to take differential outputs from\nthe diff amp and to use a current mirror to drive the other side\nof the second stage. Frequency compensation of these circuits is\ntricky because the two signals from the outputs of the diff amp\nto the output of the second stage travel through paths with\ndifferent amplifier configurations. Matching the gain and phase\ncharacteristics of these paths is difficult. For good stability,\nyou usually end up with a lower open loop bandwidth and slew rate\nthan you could obtain with complementary diff amps at the input.<\/p>\n\n\n\n

In addition, I don’t like to use a current mirror in a gain\npath. With discrete transistors, it is difficult to match the two\ntransistors in the mirror. Even when they are well matched, the\nEarly effect and temperature effects cause output current to be\ngreater than the input current. Indeed, the current ratio varies\nwith voltage across the second transistor, which varies with the\nsignal. Series emitter resistors in the mirror are a partial fix\nfor this problem.<\/p>\n\n\n\n

\n\n\n\n

You evaluated a number of output stages using SPICE and I’m\nrather surprised you found the triple Darlington to be the best.\nBest in what way? With 3 base\/emitter junctions in series, triple\nDarlingtons tend to exhibit all sorts of non-linear behavior. Due\nto extremely high current gain, they’re also prone to spurious\noscillations (which you apparently have seen your share of). Yes\nthey do have a high input impedance and provide high current, but\nyou’ve tightly limited the current capability of the amp with the\nprotection circuits anyway. Are you familiar with a Sziklai-Pair\n(Complimentary Feedback Pair)? They tend to be far more linear\nalthough they do present a somewhat lower input impedance but you\nalso save the expense and non-linearity of a set of transistors.<\/em><\/p>\n\n\n\n

You are assuming that the transistors in the driver stage operate\nclass-AB. In the configuration that I used, only the output transistors\noperate class-AB. The four driver transistors operate class-A. In the\nSziklai connection, both the driver transistors and the output transistors\noperate class-AB. This increases crossover distortion and leads to\nproblems with parasitic oscillations.<\/p>\n\n\n\n

My criterion in comparing the output stages was output resistance. A\nperfect voltage amplifier has zero output resistance. The T circuit\nversion of the triple Darlington had the lowest resistance of the stages I\ncompared, including the Sziklai connection.<\/p>\n\n\n\n

I have never really experienced non-linearity problems with\nthe triple Darlington that I could identify. The Low TIM Amp\nnever had spurious or parasitic oscillations. However, I did\nexperience them in the Double Barrelled Amp. Adding series base\nresistors on the output transistors solved the problem. When I\ndiscovered that, I added the resistors to the Low TIM Amp for\ngood measure.<\/p>\n\n\n\n

The Sziklai connection is an interesting one, and at one time\nI thought it would make a better output stage. It has local\nseries-shunt feedback which I thought would give it the lowest\noutput resistance. However, it was not as low as the triple\nDarlington. This is because the Sziklai connection has a very\nhigh output resistance without its local feedback. The local\nfeedback does not lower it to what you can achieve with the\nDarlington. On a further note, the conventional Sziklai\nconnection has only two transistors in it. To make it work in a\nhigh current output stage, an emitter follower must be added\nbetween the two transistors to drive the output transistor. You\nend up with the same number of transistors as with the triple\nDarlington.<\/p>\n\n\n\n

Even though the Sizklai connection operates at unity gain, it\nhas voltatge gain that is reduced to unity by local series shunt\nfeedback. I really dislike operating the output transistors in\nanything but a unity gain configuration, for that is the only way\nyou can get respectable bandwidth out of them. In the Sziklai\nconnection, high frequency load current must come from the driver\nand pre-driver transistors, making these transistors subject to\nfailure. With the triple Darlington, all load current comes from\nthe final output transistors, regardless of frequency.<\/p>\n\n\n\n

I never did a comparison of the different output stages until\nwell after the original amp was designed. A student did a special\nproject where he simulated the stages with SPICE for me and found\nthe T circuit version of the triple Darlington to have the lowest\noutput resistance over the widest frequency band.<\/p>\n\n\n\n

I really never had any of the problems you mentioned with the\ntriple Darlington. The only times I have had problems are when\nstudents design and build an amp with a different output stage\nleading to problems with asymmetrical clipping and parasitic\noscillations. I have seen them all.<\/p>\n\n\n\n

\n\n\n\n

I’m curious why you chose to use four diodes for the\nthermal compensation instead of just mounting the Vbe multiplier\n(Q7) to the heatsink? It’s quite easy to achieve good thermal\nstability with a triple Darlington this way and it makes for very\nsimple construction if you use a small power device (like a\nTO-126 or TO-220) for Q7 that can be readily mounted to the\nheatsink.<\/em><\/p>\n\n\n\n

I like the idea of having the Vbe multiplier on the circuit\nboard to minimize stray capacitance to ground from the output of\nthe second stage. This leads to better stability. Remember, the\nsecond stage sets the dominant pole in the open-loop transfer\nfunction. I don’t want that transfer function to be a function of\nunpredictable wiring capacitance to a Vbe multiplier on the heat\nsink. Therefore, I opted for the diodes on the heat sink. The\nstray capacitance of the leads to the diodes can be isolated with\nseries resistors. You can’t use the series resistors if you put\nthe Vbe multiplier on the heat sink.<\/p>\n\n\n\n

The original amp used 2 diodes. I changed to 3 to after\nmeasuring the bias current as a function of heat sink temperature\nand found the amp to be thermally undercompensated. But with 3\ndiodes, the two leads from the diodes to the circuit board come\nout on opposite sides of the heat sink. I added a 4th to recitfy\nthis problem and found that the amp didn’t seem to be thermally\novercompensated.<\/p>\n\n\n\n

\n\n\n\n

Did you run any open-loop measurements (or even\nsimulations) on the amp? I’m curious what the open loop\nperformance was like and how much feedback you’re running at say\n1khz and 20khz. You call it a «low feedback» design but\nyou don’t specify the parameters. While the input degeneration\nprobably helps, I think the open-loop linearity is severely hurt\nby the lack of current sources for both the input and voltage\ngain stages and by using a triple output stage.<\/em><\/p>\n\n\n\n

With complementary diff amps, it is impossible to use current\nsource loads on the inut stage. The bias currents in the two diff\namps are not stable. You must use resistive loads. For\nfrequencies above the first pole frequency in the open loop\namplifier, most of the signal output current from each diff amp\nflows in the compensation capacitor in the second stage, so\ncurrent source loads in the diff amps would not result in an\nimprovement above the first pole frequency. If current source\nloads were added, the first pole frequency would move down with\nan attendant increase in open loop gain. This would increase the\namount of feedback at the lower frequencies. With the resistive\nloads, it is already high enough at the low frequencies so that I\ndo not feel that any improvement would result. This is a moot\npoint, however, because current source loads cannot be used on\nthe diff amps in a fully complementary amplifier.<\/p>\n\n\n\n

You don’t seem to understand that each transistor in the\nsecond stage does see a current source load. With the fully\ncomplementary second stage circuit, each side of the stage sees a\ncurrent source load which has both ac and dc currents in it. The\ndc currents are common mode. The ac currents are differential.\nWhen one is increasing, the other is decreasing, and vice versa.\nThis push pull action cancels even order distortion components.\nThis cancellation would not occur if only one side of the second\nstage is driven by the signal and the other side is a constant\ncurrent load as you suggest.<\/p>\n\n\n\n

My original article in Audio described the open loop\nmeasurements. It seems I had a 40 kHz open-loop bandwidth and\n0.5% THD just below clipping at 1 kHz with an 8 ohm load. Later,\nI decreased the bandwidth while holding the gain bandwidth\nproduct constant so that the closed-loop bandwidth did not\nchange. Since then I decreased the gain bandwidth product a\nlittle which reduced the closed-loop bandwidth. This caused a\nsmall increase in distortion because it decreased the amount of\nfeedback. However, the stability margin is improved and I think\nthat leads to a better sounding amp.<\/p>\n\n\n\n

The first two transistors in the triple Darlingtons do not\nsupply load current so that the signal currents in these\ntransistors are the base input currents for the following stage.\nThis configuration was called the T circuit by Bart\nLocanthi<\/a> when he published it back in the 1960s. It differs\nfrom the conventional Darlington in that the resistors which set\nthe bias currents in the first two transistors connect to the\nopposite transistor rather than to the amplifier output. This\nfeature does two things. First, the driver transistors operate\nclass A, not class AB, so that no non-linearity is caused by the\nswitching on and off of driver transistors. Second, the output\nresistance is lower. I do not think that this circuit suffers\nfrom any of the problems you associate with the triple\nDarlington.<\/p>\n\n\n\n

In comparing the amount of feedback in amplifiers, the\ngain-bandwidth product is a parameter that is more important than\nthe gain. If you take an amplifier with an open loop gain of 40\ndB and a bandwidth of 40 kHz, it has a gain-bandwidth product of\n4 MHz. You reduce the gain to 20 dB with feedback and you have a\nclosed-loop bandwidth of 400 kHz. With only 20 dB of feedback, it\ncan be called a low feedback amplifier.<\/p>\n\n\n\n

Now, suppose you increase the open-loop gain to 80 dB while\nsimultaneously reducing the open-loop bandwidth to 400 Hz. With a\nclosed-loop gain of 20 dB, you will still have a closed-loop\nbandwidth of 400 kHz. The amplifier is still a low feedback\namplifier. In contrast, if you increase the open-loop gain to 80\ndB while keeping the open-loop bandwidth at 40 kHz, you would\nhave a closed-loop bandwidth of 4 MHz with a closed-loop gain of\n20 dB. The amp is now a high feedback design and would surely\noscillate like crazy.<\/p>\n\n\n\n

I guess what I am saying here is that you should look at the\nopen loop gain above the first pole frequency when you are\ncomparing the amount of feedback. If you increase the open loop\ngain while simultaneously decreasing the first pole frequency,\nthe gain above the former first pole frequency does not change.<\/p>\n\n\n\n

\n\n\n\n

I only have the information that’s currently on your\nwebsite so I’m sure I’m missing some of the background of your\ndesign. It’s still intriguing, however. I learn something from\nnearly every design I look at (even if it’s how to save money!).\nI appreciate your taking the time to publish yours (both in Audio\nand on the web).<\/em><\/p>\n\n\n\n

I was pretty lucky in the original design of the amp because\nit worked so well. Every time I made changes, I tried to keep it\nas close to the original as I could. The feedforward split was\nadded only recently. I did it simply because it made sense. I\nhave supervised more students than I could count who built the\namp, and I have experienced just about every problem that could\noccur. One of the worst to diagnose was when a student last fall\ngot the lead to one of the bias diodes reversed with a lead to an\noutput transistor. The bias in that channel would not set right.\nBefore he found the error, he replaced the bias diodes and so\nmany parts on the circuit board that pads started peeling up. You\nname it, I have seen it.<\/p>\n\n\n\n

\n\n\n\n

Can I use a regulated power supply with the amplifier?<\/em><\/p>\n\n\n\n

Yes. I have had one student who used regulated power supplies\nbecause the bargain transformer that he bought had too high a\nsecondary voltage rating. He designed regulators to drop the\nvoltage down to the required values. I have no recommendations on\nthe design of the regulators. They must be able to pass the full\namplifier current and the pass transistors in them must be heat\nsinked. In effect, each regulator is another power amplifier that\nsupplies voltage and current to the amplifier.<\/p>\n\n\n\n

\n\n\n\n

Can I strap either the Low TIM amplifier or the Double\nBarreled Amplifier?<\/em><\/p>\n\n\n\n

I have never done this. A stereo amp can be converted into a\nmono strapped amplifier by driving both channels with the same\nsignal. However, the signal to one channel must be inverted, i.e.\nmultiplied by -1, with an op amp inverter. Strapping an amplifier\ndoubles the available drive voltage to the loudspeaker, thus\nquadrupling the available power. However, each channel of the\nstrapped amplifier sees one-half the loudspeaker impedance. I\nknow of no reason why either amplifier can’t be strapped, but I\nhave never tried it. I would not recommend strapping any\namplifier to drive low impedance loudspeakers.<\/p>\n","protected":false},"excerpt":{"rendered":"

Concerning the Bias Current From a correspondent on 2\/7\/03 — I have built your low TIM amp and am completely satisfied with its performance. Just one question. The quiescent current increases proportionally to the 8 ohm loaded output amplitude. Neither the frequency nor the waveform affects Iq, only the output amplitude on the 8 ohm… <\/p>\n

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