Hi andrew,
I am using the original value : 0R33, any thing particular on your mind?
Siince you brought it up, I might as well ask : I check almost every component before I put it on the board except these power resistors. All I can do is to serial 5 of them and check the resistance (but usually not accurate), knowing that none is open but never knowing the exact value of each individual. Any method rather than the wheatstone bridge?
Any suggestion on the heatsink? I will use a 8cm fan to blow the heatsink during testing, just to be safe. What I might be doing is to turn the fan off and then watch closely (probably without taking my hand off for about 15 mins ) to see if it is getting too hot. That's all I can do.
I am using the original value : 0R33, any thing particular on your mind?
Siince you brought it up, I might as well ask : I check almost every component before I put it on the board except these power resistors. All I can do is to serial 5 of them and check the resistance (but usually not accurate), knowing that none is open but never knowing the exact value of each individual. Any method rather than the wheatstone bridge?
Any suggestion on the heatsink? I will use a 8cm fan to blow the heatsink during testing, just to be safe. What I might be doing is to turn the fan off and then watch closely (probably without taking my hand off for about 15 mins ) to see if it is getting too hot. That's all I can do.
andrew,
in the article, Leach used 0.22ohms/5watt resistors.....
bigpanda,
the caps are 3in diam. and about 5inches tall...
if you are interested, Apex Jr. sell 68,000ufd/100v caps at U$each..www.apexjr.com
in the article, Leach used 0.22ohms/5watt resistors.....
bigpanda,
the caps are 3in diam. and about 5inches tall...
if you are interested, Apex Jr. sell 68,000ufd/100v caps at U$each..www.apexjr.com
Hi Bigpanda,
your 0r33 would need 45mA (Vre=15mV) to 76mV (Vre=25mV) for each output pair.
A two pair, or in the super Leach case, two quad, would need 90mA to 150mA of quiescent current. Well below 250mA.
I just series connect my power resistors (10 to 15 at a time) and then pass an appropriate current from my lab supply to bring the DMM Vdc (across each) upto a reading which gives good resolution. 1A still only dissipates 0.33W in a 3W resistor. It hardly feels warm, 2A if you prefer.
Matching can be better than 0.5%. A good DMM might get you better than 0.1% without having to use a bridge.
your 0r33 would need 45mA (Vre=15mV) to 76mV (Vre=25mV) for each output pair.
A two pair, or in the super Leach case, two quad, would need 90mA to 150mA of quiescent current. Well below 250mA.
I just series connect my power resistors (10 to 15 at a time) and then pass an appropriate current from my lab supply to bring the DMM Vdc (across each) upto a reading which gives good resolution. 1A still only dissipates 0.33W in a 3W resistor. It hardly feels warm, 2A if you prefer.
Matching can be better than 0.5%. A good DMM might get you better than 0.1% without having to use a bridge.
rolandong said:I was about to purchase a bunch of precision/low inductance LVR series from farnell but it costs 123php/pc and to mention they were made by which i'm employed at - couldn't even get some samples
ouch, that's too much...sorry about that...will you please give my regards to mr. manny laxa, he knows me...
@bigpanda,
when i built my first leach super amp in 1984, i mounted thermal cut-off's on the heatsinks so that the speaker relays i used will be disengaged once temps of 85*C was reached...using fans on them like what you contemplate, prevented relays from disengaging..
Hi Tony,
I am getting a bit confused. According to here :
1. http://users.ece.gatech.edu/~mleach/superamp/
2. http://users.ece.gatech.edu/~mleach/lowtim/part3.html
Where 1 is for superleach and 2 is for lowtim leach.
There seems to have no mentioning of 0R22 resistors.
I do have a speaker protection circuit that cuts off the speaker at 70 C, would 85 C be any better?
As I was discussing with a friend of mine about the heatsink size, would 2pcs of 200 x 150 do as good as 1pcs of 400 x 150 ? Referring to picture I had, if I don't change the orientation of the board, I would have 1 square bar on each heatsink. Or should I rotate the board 90 deg. so that each of the square bar is on both heatsink (that can also give mechanical support on the whole setup too). With this orientation, would a longer square bar (and also a long plate) help the dissipation?
Andrew,
Problem is that I had to get myself a lab power supply.
I am getting a bit confused. According to here :
1. http://users.ece.gatech.edu/~mleach/superamp/
2. http://users.ece.gatech.edu/~mleach/lowtim/part3.html
Where 1 is for superleach and 2 is for lowtim leach.
There seems to have no mentioning of 0R22 resistors.
I do have a speaker protection circuit that cuts off the speaker at 70 C, would 85 C be any better?
As I was discussing with a friend of mine about the heatsink size, would 2pcs of 200 x 150 do as good as 1pcs of 400 x 150 ? Referring to picture I had, if I don't change the orientation of the board, I would have 1 square bar on each heatsink. Or should I rotate the board 90 deg. so that each of the square bar is on both heatsink (that can also give mechanical support on the whole setup too). With this orientation, would a longer square bar (and also a long plate) help the dissipation?
Andrew,
Problem is that I had to get myself a lab power supply.
Leach must have updated it on his website, but on page 60 of the article published in 1980, 0.22/5watts was called for.
i do not see any problem in using 0.33ohm instead...
you can see that page here:http://s20.photobucket.com/albums/b201/tecson/?action=view¤t=leachsuperamp19.jpg
yes, i have both part1 and part2, i would like to share them to all, except that i am on dial up now...if there is someone willing to host them i am more than glad to share...the change from 0.22 to 0.33ohm gives more stability to the amps' bias points.
hey andrewT,
this is the page that says 250mA.......http://s20.photobucket.com/albums/b201/tecson/?action=view¤t=leachsuperamp17.jpg
P1 in the current BOM is labeled R28 in this article and is a 5K instead of 2K. Several other parts value changes. The higher bias number (250) in this blast from the past accounts for the little known and even lesser applied method to improve not only the bass response but also clarity in the treble. Some caution tho, inadequate heat sinks lead to letting out the magic smoke.
Note the current recommended value is 120mA for the .33R's.
Note the current recommended value is 120mA for the .33R's.
hey bigpanda,
here is how my last leach amp, circa1992, looked like:
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpFront.jpg
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpRSide.jpg
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpBack2.jpg
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpTop.jpg
here is how my last leach amp, circa1992, looked like:
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpFront.jpg
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpRSide.jpg
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpBack2.jpg
http://i20.photobucket.com/albums/b201/tecson/leach amp/AmpTop.jpg
another option i've seen is that there were thermal resistors used on amps in the '80's that were mounted to the heatsink, and reduced the audio input if the heatsink got above 80C. that way, the amp would still work (at reduced wattage) without a sudden speaker cutout. the temp/resistance curve was somewhat nonlinear, so if you compensated by turning up the volume, and the temp kept rising, it would just reduce the volume more until you ran out of volume control, and eventually the heatsink would cool down enough to get full wattage...... kind of a self limiting thing...... it was quite effective, as i have never seen an amp with those devices overheat. the limiting effect was slow enough that it wasn't really noticeable.
Hi,
there is some confusion on my part so please bear with me while I sumarise what I think you & Leach are telling me.
Iq can be in the range 120mA to 250mA.
The 250mA figure includes the voltage amp currents.
Re can be 0r22 or 0r33.
Leach has advised caution with heatsink size.
Leach has a misprint in the 1980 article;- 0.25A total rail current gives a peak ClassA current of less than 0.5Apk, probably about .45Apk ~=0.8W ClassA
Assuming all the above is correct, let's do some numbers.
Taking 15mV to 25mV as the range of Vre for an optimum bias set-up for a ClassAB amplifier.
minimum Iq = 68mA/output pair for Re=0r22 and 46mA, Re=0r33.
Maximum Iq=114mA, Re=0r22 and 76mA, Re=0r33.
For a two quad assembly, that gives a range of 92mA to 228mA and if 18mA is added for the voltage amp +ve rail draw then the fuse current will be 110mA to 246mA. The amplifier is going to run at very different temperatures within this bias range and that in turn will vary the SOAR for the outputs (and drivers if attached) and for the variation of Iq with junction temperature, both static (from cold to hot) and dynamic (before and after a power output pulse). To reduce the risk of under biasing due to dynamic effects some will recommend that bias should be set towards the upper limit of 15 to 25mV for Vre. Design your heatsink requirements accordingly.
It is on the basis of heatsink temperature that I suspect that the emitter resistor has been increased to 0r33 (to improve static bias stability) and again for reduced heatsink temperature (to improve SOAR limitations) that the total current is reduced from 250mA to 120mA.
Both these changes could be good if it were NOT for inadequate heatsinking temperatures and solely due to sound quality. If sound quality is the aim I suggest that the heatsink be made sufficiently large that bias stability be removed from the equation and then one can set Iq and Re for best sound and not the other way around (just a guess if I have read between the lines correctly).
there is some confusion on my part so please bear with me while I sumarise what I think you & Leach are telling me.
Iq can be in the range 120mA to 250mA.
The 250mA figure includes the voltage amp currents.
Re can be 0r22 or 0r33.
Leach has advised caution with heatsink size.
Leach has a misprint in the 1980 article;- 0.25A total rail current gives a peak ClassA current of less than 0.5Apk, probably about .45Apk ~=0.8W ClassA
Assuming all the above is correct, let's do some numbers.
Taking 15mV to 25mV as the range of Vre for an optimum bias set-up for a ClassAB amplifier.
minimum Iq = 68mA/output pair for Re=0r22 and 46mA, Re=0r33.
Maximum Iq=114mA, Re=0r22 and 76mA, Re=0r33.
For a two quad assembly, that gives a range of 92mA to 228mA and if 18mA is added for the voltage amp +ve rail draw then the fuse current will be 110mA to 246mA. The amplifier is going to run at very different temperatures within this bias range and that in turn will vary the SOAR for the outputs (and drivers if attached) and for the variation of Iq with junction temperature, both static (from cold to hot) and dynamic (before and after a power output pulse). To reduce the risk of under biasing due to dynamic effects some will recommend that bias should be set towards the upper limit of 15 to 25mV for Vre. Design your heatsink requirements accordingly.
It is on the basis of heatsink temperature that I suspect that the emitter resistor has been increased to 0r33 (to improve static bias stability) and again for reduced heatsink temperature (to improve SOAR limitations) that the total current is reduced from 250mA to 120mA.
Both these changes could be good if it were NOT for inadequate heatsinking temperatures and solely due to sound quality. If sound quality is the aim I suggest that the heatsink be made sufficiently large that bias stability be removed from the equation and then one can set Iq and Re for best sound and not the other way around (just a guess if I have read between the lines correctly).
hi Andrew,
i do not recall leach as having put forth an optimum value for Vre, IIRC it was john curl who did simulations and found out that there is an optimum value of Vre.
Leach used a very robust heatsink in the 1980 article that is why the 250mA rail current, since i know i used smaller heatsinks, it is but natural that i used lower values. btw i was also repairing japanese amps at the time i was building leach amps in the 80's till the middle of 1990. and i know that these japnaese amps were biased at around 20 to 50mA per collector/transistors....
i am not confused at all, since the super leach amp at his website used a much smaller heatsink, that is why upping to 0.33 and lowering the idle current makes a lot of sense...
i do not recall leach as having put forth an optimum value for Vre, IIRC it was john curl who did simulations and found out that there is an optimum value of Vre.
Leach used a very robust heatsink in the 1980 article that is why the 250mA rail current, since i know i used smaller heatsinks, it is but natural that i used lower values. btw i was also repairing japanese amps at the time i was building leach amps in the 80's till the middle of 1990. and i know that these japnaese amps were biased at around 20 to 50mA per collector/transistors....
i am not confused at all, since the super leach amp at his website used a much smaller heatsink, that is why upping to 0.33 and lowering the idle current makes a lot of sense...
Howdy Andrew,
Reading between the lines or not, you got it right.
Sound quality is reported by many to be improved both in the bass and treble when using the upper limits of bias values. Howsomever, caution when using those Wakefield sinks from the BOM if you do this.
They are not big enough for a bias set at 250 and then driven hard for long periods of time.
The SuperAmp is not really intended for the band on the road. But I've heard from folks who use it just for that.
As with all solid state amps, big heat sinks are your friend. As a practical matter, at usual living room listening levels the amp is below 2 watts output. If the heat sinks are blood temperature, you are probably in good shape.
HTH
Reading between the lines or not, you got it right.
Sound quality is reported by many to be improved both in the bass and treble when using the upper limits of bias values. Howsomever, caution when using those Wakefield sinks from the BOM if you do this.
They are not big enough for a bias set at 250 and then driven hard for long periods of time.
The SuperAmp is not really intended for the band on the road. But I've heard from folks who use it just for that.
As with all solid state amps, big heat sinks are your friend. As a practical matter, at usual living room listening levels the amp is below 2 watts output. If the heat sinks are blood temperature, you are probably in good shape.
HTH
Hi,
I just dug the board out of its grave a few days ago and after cleaning them up, conntected to the PS for testing as the article suggested. 100ohm's from both ends of R36 to output, jumper across C12, and 100ohm current limiters installed.
I had the DC offset voltage at 1.28 with
rail voltage = +-67.8v
+ve current = 30mA
-ve currnet = 28mA
voltage across D13,D14 = 39V
voltage across D16,D15 = 38.4V
All components have been checked before they are soldered (of course I might blow them during soldering). Where can I take TP voltages to trace the error? Do I have to matched exactly the voltages across D13D14 and D16D15? (I have assumed what I get should be OK).
Tks for giving a hand.
I just dug the board out of its grave a few days ago and after cleaning them up, conntected to the PS for testing as the article suggested. 100ohm's from both ends of R36 to output, jumper across C12, and 100ohm current limiters installed.
I had the DC offset voltage at 1.28 with
rail voltage = +-67.8v
+ve current = 30mA
-ve currnet = 28mA
voltage across D13,D14 = 39V
voltage across D16,D15 = 38.4V
All components have been checked before they are soldered (of course I might blow them during soldering). Where can I take TP voltages to trace the error? Do I have to matched exactly the voltages across D13D14 and D16D15? (I have assumed what I get should be OK).
Tks for giving a hand.
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