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Old 6th November 2014, 09:19 PM   #1
narmex is offline narmex  United States
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Default JLH 10W 1996 Update Questions

hi guys,

i found the TCAAS site at ESP, it's really a great site, a lot of great information collected together in one place, especially what JLH himself originally said about it, just great, thank you Geoff and ESP!

so i threw it together with a quick and dirty job on breadboard, just curious, just wanted to get a feel of it. it's sort of a mutant though, a lower supply voltage, lower wattage, mono, built with junk parts, driving a small junk desktop speaker as well, version of it.

the sound is clear, a lot of musical nuances showing through, explosive during eruptive passages, bass is alive with shades and colors instead of dead thumps, the musical space is very open, spacious, and relaxed, very lyrical, and all is done with ease, no impression of contrivedness at all, no impression of being cold, flat, mechanical, or overly analytical. very human, music breathes with it, vocals are very immediate, singing right in front of you sort of thing, absolutely a pleasure to listen to.

some questions on things i don't understand:-

1) what's the use of
R8, 39k/22k at the top,
C1, just above the feedback path,
and R9, 1k0, at the output to speaker?

2) in order to have it as a true Class A amp, the voltage waveforms at the bases, and the current waveforms at the emitter/collector of Tr1/Tr2 must be full waveforms, otherwise it'd be a Class B amp, is that right?

3) i have these current numbers
collector of Tr4, about 200uA,
collector of Tr3, 5.5mA,
collector/emitter of Tr1/Tr2, about 140mA.

are these numbers ok in terms of their relationship? what do these numbers tell us, and how does one judge them? i just wanted to turn the output current down, because JLH has provide "an elegant way of adjusting the output current", and i don't have large heat sinks.

simulation says each output bjt dissipates about 830mW, so that makes it a 1.7W power amp? more than loud enough for me, for desktop use.

4) THD at
collector of Tr4 is 15.771096%(15.771096%),
collector of Tr3 is 0.323791%(0.323790%),
output to speaker is 0.009462%(0.009441%.

why is the THD at the input stage is much higher, is this pattern normal, is it because of negative feedback, or is there something wrong about it that needs to be fixed?

thank you for all helps/comments!
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Last edited by narmex; 6th November 2014 at 09:28 PM.
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Old 6th November 2014, 10:01 PM   #2
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Tr4 is a transconductance stage, meaning it is converting the input voltage to an output current, to drive Tr3. This can be seen by there being more AC voltage at it's emitter than it's collector. A distortion measurement based on the voltage of it's collector is pretty much meaningless. The output power is based on idle current of the output stage. The JLH is a quasi push-pull affair and can supply about 1.3 times the idle current to the load. Your idle current being 140mA equates to about 260mW output into 8 ohms (idle current times 1.3 squared times load). R8 and C5 form a filter to reduce power supply noise at the input. R9 is there to charge the output capacitor when no speaker is attached to avoid a large thump when connecting one.

Last edited by jerluwoo; 6th November 2014 at 10:16 PM.
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Old 7th November 2014, 11:49 AM   #3
AndrewT is offline AndrewT  Scotland
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The time constant of Rg C2 is 2.2 seconds. 2200uF = 0.0022F, Rg 1k = 1000ohms, RC = 0.0022*1000 = 2.2 seconds.
The capacitor will be substantially charged after about 10 RC periods, after Vx reaches operating voltage.


i.e. If Vx reaches operating voltage 1second after power is switched on then the speaker should have a time delayed relay of >23seconds.
And that relay should have instant off, if power ever drops out.
But if you have a slow ramp up of supply voltage taking, say 10 seconds, then the minimum speaker delay is 32seconds.
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Last edited by AndrewT; 7th November 2014 at 11:53 AM.
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Old 7th November 2014, 11:54 AM   #4
Vostro is offline Vostro  South Africa
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Iv'e always seen 5RC as close and makes no difference to Vm in conventional circuits
How necessary is it to wait extra time after the cap reached 99.3%.Vm ?

Regards

Last edited by Vostro; 7th November 2014 at 11:57 AM.
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Old 7th November 2014, 12:41 PM   #5
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"1) what's the use of
R8, 39k/22k at the top,"

It's a voltage drop resistor, but in your schematic a 100µF electrolytic is missing. It must be connected from the junction of R8 and R6 to ground!

R9/ 1K to ground is a bleeder resistor to discharge the large output coupling capacitor. It discharges the output electrolytic when the amplifier is turned off.
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Old 7th November 2014, 01:13 PM   #6
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Can you tell me the supply voltage you are using? A few criticisms...

1. I don't like the way the power rail can couple onto the output so easily via R1 and C1, looks like you might have some trouble with power supply hum there.

2. Also you can significantly reduce distortion in capacitor coupled outputs by taking your negative feedback from the other side of the output capacitor. This can be done fairly easily.

3. The bias network on the input is a function of the supply rails, it could easily start producing rather large amounts of subsonic distortion at higher power levels.

My two cents at least .
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Old 7th November 2014, 05:48 PM   #7
narmex is offline narmex  United States
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Quote:
Originally Posted by jerluwoo View Post
Tr4 is a transconductance stage, meaning it is converting the input voltage to an output current, to drive Tr3. This can be seen by there being more AC voltage at it's emitter than it's collector. A distortion measurement based on the voltage of it's collector is pretty much meaningless. The output power is based on idle current of the output stage. The JLH is a quasi push-pull affair and can supply about 1.3 times the idle current to the load. Your idle current being 140mA equates to about 260mW output into 8 ohms (idle current times 1.3 squared times load). R8 and C5 form a filter to reduce power supply noise at the input. R9 is there to charge the output capacitor when no speaker is attached to avoid a large thump when connecting one.
thank you jerluwoo for your help!

i ran a THD calculation on Tr4's emitter, instead of collector, and the number is 0.006749%(0.006718%). wow, the 2 terminals can be that different! is phase a factor-in-play here?

also, since the output is taken from Tr4's collector, i also did the THD thing on the current at Tr4's collector, got 4.569551%(4.569550%. although still high comparing to 0.009% something, but a lot lower from 15% something. what i'm wondering is, is it valid to directly compare a current-THD to a voltage-THD? what's a good way of gaining some idea about the THD of this Tr4 stage? i just imagined that maybe i should look at the THD of each stage, in order to get the whole circuit's THD to be as low as possible.

1.3 times, alright! very useful bit of information, thank you! i had no idea of how lound 260mW can be, now i know, loud enough. 140mA is what my multimeter says when adjusting the trim pot, and i Am using an 8ohm speaker, so 260mW should be correct. one annoying thing is that i have no equipment other than a multimeter, so i can not measure the built circuit, and don't really know it's working state, too bad.
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Old 7th November 2014, 05:52 PM   #8
narmex is offline narmex  United States
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Quote:
Originally Posted by AndrewT View Post
The time constant of Rg C2 is 2.2 seconds. 2200uF = 0.0022F, Rg 1k = 1000ohms, RC = 0.0022*1000 = 2.2 seconds.
The capacitor will be substantially charged after about 10 RC periods, after Vx reaches operating voltage.

i.e. If Vx reaches operating voltage 1second after power is switched on then the speaker should have a time delayed relay of >23seconds.
And that relay should have instant off, if power ever drops out.
But if you have a slow ramp up of supply voltage taking, say 10 seconds, then the minimum speaker delay is 32seconds.
thank you AndrewT!
now i know, and can take advantage of such design features, nice.
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Old 7th November 2014, 05:56 PM   #9
narmex is offline narmex  United States
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Quote:
Originally Posted by Klarskov View Post
"1) what's the use of
R8, 39k/22k at the top,"

It's a voltage drop resistor, but in your schematic a 100µF electrolytic is missing. It must be connected from the junction of R8 and R6 to ground!

R9/ 1K to ground is a bleeder resistor to discharge the large output coupling capacitor. It discharges the output electrolytic when the amplifier is turned off.
thank you Klarskov for the help!

just a side note, this circuit is not mine, i wish , it's taken directly from JLH's 1996 update article, and there is a C5, which i think is what you meant. but that's not important, now i got it, thanks!
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Old 7th November 2014, 06:21 PM   #10
narmex is offline narmex  United States
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Quote:
Originally Posted by monty78pig View Post
Can you tell me the supply voltage you are using? A few criticisms...
very cool monty, please teach me about these stuff, about which i'd very much like to know and understand.

Quote:
1. I don't like the way the power rail can couple onto the output so easily via R1 and C1, looks like you might have some trouble with power supply hum there.
the circuit picture is taken directly from JLH's WW article. i'm only using a 12V DC wall adapter as power supply, and there is no hum. of course, i have no equipment to measure with, so can't say how smooth the power supply is. besides, it's alway good to learn, even if there is no hum here and now.

Quote:
2. Also you can significantly reduce distortion in capacitor coupled outputs by taking your negative feedback from the other side of the output capacitor. This can be done fairly easily.
this is most interesting! could you explain a little about how/why such a move reduces distortion? i did that in simulation, THD went to 2.739451%(2.739468%) from 0.009462%(0.009441%). of course this is the result of simply moving the connection to the other side, and the emitter of Tr4 doesn't seem to be getting any bias now, so this number deosn't really mean anything. haven't worked this out yet, still trying, but in any case, techniques/experiences in controlling THD is most interesting to me!

Quote:
3. The bias network on the input is a function of the supply rails, it could easily start producing rather large amounts of subsonic distortion at higher power levels.
again, this is most interesting. could you expand on that a little, plus some comments on what can be done about that?

thanks a lot!

Last edited by narmex; 7th November 2014 at 06:24 PM.
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