the 2 small boards are for the thermal compensation diodes - pick one, toss the other. Jens' site has a nice assembly procedure for his 10 transistor version, that shows how the compensation board connects. Although not complete, it will give you a good idea what you need to do. see http://www.delta-audio.com/Leach-Clone.htm
Yes, Terry, this board will handle your 62 volt rails. Probably better to use it here than on your Krell clone space heater. 
You may need to adjust a couple of resistors (R13 and R14 on Dr. Leach's schematic, I am not sure what Jens' designator is.) But for this amp a couple of volts either way won't amount to much.
You may need to adjust a couple of resistors (R13 and R14 on Dr. Leach's schematic, I am not sure what Jens' designator is.) But for this amp a couple of volts either way won't amount to much.
BrianDonegan said:
Dissipation = (Vrails-Vout) Iout= (Vrails-Vout)Vout/ R
This is maximal when the derivative d(Vrails-Vout)Vout/R =0.
This is when Vout = Vrails/2.
that is the peak value, the average is sqrt2 lower, so Vac/2
For the worst case, power surge is 10% higher.
55Vdc ~40Vac, +10% = 44, divided 2= 22 volts.
60Vdc ~43Vac, +10% = 47.3, divided 2 = 23.65 volts
65Vdc ~46.6Vac, +10% = 51.2, divided 2 = 25.6 volts.
Total dissipation is sqr(V)/ R
Thermal resistance Rjc for 200 watt output device: 0.625
Thermal resistance rcs for TO247 mica : 0.500
For 3 ohms use:
55V=> 162 watts Rsink= 0.45 C/W
60V=> 186 watts Rsink= 0.35 C/W
65V=> 218 watts Rsink= 0.27 C/W
For 4 ohms use:
55V=> 121 watts Rsink= 0.65 C/W
60V=> 140 watts Rsink= 0.53 C/W
65V=> 164 watts Rsink= 0.43 C/W
Received my board today. These are really nice. Very heavy duty. Thanks for all the effort that went into packing and shipping these, and to Jens for the effort to design the layout. Looking forward to getting them operational. Regarding that, someone mentioned a thread or wiki about doing just that. I think it is a good idea to discuss such topics as parts selection, layout, parts sourcing, etc. But I think a thread would be better, as I find the wiki editing a pain for normal use.
I'll start a thread "Building a Leach Amp", and begin with the following:
Having built two Leach amps in the past, I have not been a fan of biasing the way Leach recommends. Pulling those fuses in and out, etc., to measure bias amps, to me, was always a cause of concern for, well, danger. Layout became an issue to make that marginally convenient. Also, the current seemed to always be drifting around a great deal, making it hard to pin down. Further, it has become apparent that biasing by measuring current may not be the best way to do it.
Self states that an optimum bias is about twice what Leach recommends. I found I like the higher bias. Andy_C, in another thread, determined that Self's optimum bias equated to the lowest variation in output impedance, which was also expressed by Oliver in the Feb '71 issue of the Hewlett Packard Journal. Self states that the optimum bias is really a function of the voltage across the output emitter resistors, also mentioned by Oliver.
Self's optimum Vq bias for .33 ohm emittor resistors is 47.6 mv across the emitter series pair, for about 75 ma current. For .47 ohm resistors, it is 54.8mv for 59ma current. Again, this is across the series combination. Therefore, to make biasing easy without opening the enclosure, connect the ends of a series pair of emitter resistors to connectors on the outside of the case, so you can insert probe tips of a volt meter into to measure the Vq voltage for biasing. Also, get a multi-turn trim pot that can be mounted on the enclosure so that a screwdriver can access it from the outside of the box, or layout the boards such that a hole drilled in the enclosure can provide access to the trim screw.
I found that the voltage Vq was less variable than measuring bias current, once warmed up.
I'll start a thread "Building a Leach Amp", and begin with the following:
Having built two Leach amps in the past, I have not been a fan of biasing the way Leach recommends. Pulling those fuses in and out, etc., to measure bias amps, to me, was always a cause of concern for, well, danger. Layout became an issue to make that marginally convenient. Also, the current seemed to always be drifting around a great deal, making it hard to pin down. Further, it has become apparent that biasing by measuring current may not be the best way to do it.
Self states that an optimum bias is about twice what Leach recommends. I found I like the higher bias. Andy_C, in another thread, determined that Self's optimum bias equated to the lowest variation in output impedance, which was also expressed by Oliver in the Feb '71 issue of the Hewlett Packard Journal. Self states that the optimum bias is really a function of the voltage across the output emitter resistors, also mentioned by Oliver.
Self's optimum Vq bias for .33 ohm emittor resistors is 47.6 mv across the emitter series pair, for about 75 ma current. For .47 ohm resistors, it is 54.8mv for 59ma current. Again, this is across the series combination. Therefore, to make biasing easy without opening the enclosure, connect the ends of a series pair of emitter resistors to connectors on the outside of the case, so you can insert probe tips of a volt meter into to measure the Vq voltage for biasing. Also, get a multi-turn trim pot that can be mounted on the enclosure so that a screwdriver can access it from the outside of the box, or layout the boards such that a hole drilled in the enclosure can provide access to the trim screw.
I found that the voltage Vq was less variable than measuring bias current, once warmed up.
Got mine today. Wiki updated.
Wiki Link bump: http://www.diyaudio.com/wiki/index.php?page=+Leach+Amp+PCB+Group+Buy
Wiki Link bump: http://www.diyaudio.com/wiki/index.php?page=+Leach+Amp+PCB+Group+Buy
Jens,
I've been pouring over your schematic and parts selection and noticed you left off the resistor to one side of the diode string, which is R25 in Leach's schematic, and combined its value with Leach's R26 to make your R29. This makes sense, value-wise. However, I think Leach put in R25 for a reason. My guess is to decouple any capacitance from the leads of the diode string. Since your leads don't go as far to the heat sinks, perhaps it's no problem.
I've been pouring over your schematic and parts selection and noticed you left off the resistor to one side of the diode string, which is R25 in Leach's schematic, and combined its value with Leach's R26 to make your R29. This makes sense, value-wise. However, I think Leach put in R25 for a reason. My guess is to decouple any capacitance from the leads of the diode string. Since your leads don't go as far to the heat sinks, perhaps it's no problem.
pooge said:
Pooge,
I'm not really sure what you mean by "decouple any capacitance from the leads of the diode string" I think Mr. Leach placed the extra resistor in order to make the layout job easier.... The extra resistor can be used to bridge some tracks on the original board.
Updated parts list (Only minor changes)
\Jens
Isn't it a very good idea of mine (see below) to have full info over the group buy and write down the status of each shipping with invoice/paid/recieved etc?BrianDonegan said:
This group buy seems to run as it should be, full info to everybody and professional admistration and most import of all a good "product". It will be very interesting when 20-50 amplifiers are built and the conclusion of Jens' efforts is made.
JensRasmussen said:
I doubt Leach would have specified a resistor where a jumper could have been used. If you look at where the diode string attaches, it is at the same node as the Miller cap. Since long wiring was needed with Leach's design to reach the diodes on the heat sink, the capacitance of the string may affect the stability of the amp. This is my guess. I think I may have read about it somewhere. With Leach's design, he probably couldn't be sure what various constructors would do with the length of the wires. It would depend on which heat sinks were selected, etc. But your design has a much shorter path to the diodes, so it could no way be in the same degree of problem as Leach's. If in doubt, the wire to the diode board could be made from the missing resistor and the other transistor scaled back to Leach's. You did get a slight overshoot in the square waves of your prototype. I was going to suggest raising the value of the Miller caps to see if that helped. You've used different transistors than Leach, so it's possible that the compensation would not be exactly the same.
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