Is the need for this circuit already written above? did you read it?
everyone who has a common negative feedback and the output stage operates in a non-linear mode.
Sorry, I don't read stupid reviews...
I was replying to post 92, which you have not seen or you wouldn't have asked the question.
You must be clairvoyant to know that a magazine review is 'stupid' without even reading it.
Pity we aren't all as clever as you....
yeah, in your post, a quote from my post 96 with the answer)))
Have you seen him? otherwise they wouldn't write nonsense ...
the choice is not large, or you write nonsense or a review - choose for yourself ...
what frequency corresponds to the time constant of the LR circuit indicated in the picture in post 92? the next question is what should be the unity gain frequency of the amplifier for this circuit?
Ooh dear, i bought a few of these. But the schematic does not look good.
I'm not surprised it is on the verge of oscillating, with an emitter follower setup like that and such high speed devices you're asking for trouble.
Anyone made a spice model yet? I think adding a 10pf bypass cap on the feedback resistor might help.
I'm not surprised it is on the verge of oscillating, with an emitter follower setup like that and such high speed devices you're asking for trouble.
Anyone made a spice model yet? I think adding a 10pf bypass cap on the feedback resistor might help.
no inductance damping resistors in the driver or output bases either. i also suspect this one will sing like a screeching bird....
Good morning dear friends of this group.
I am new to amplifier assembly, I have a problem and I need your valuable help, I have a toroidal transformer of +-66V already rectified, but the L20SE says that it works up to +-65V, could someone tell me what elements I have to change to What can withstand this voltage?
Greetings and in advance thank you.
I am new to amplifier assembly, I have a problem and I need your valuable help, I have a toroidal transformer of +-66V already rectified, but the L20SE says that it works up to +-65V, could someone tell me what elements I have to change to What can withstand this voltage?
Greetings and in advance thank you.
You don't need to do anything if the amp is receiving + - 66v instead of + - 65v and the power supply capacitors are ideally 80v or more. (63v ones will probably be fine)
I've finally been able to simulate the schematic. I couldn't find any problems with oscillations, but the bias definitely needs some attention. It needs about 15mv per output resistor. Otherwise distortion and noise is significantly higher. Then again, i haven't tested this in practice yet.
Since I have a pair mounted on heat sinks, I'm looking forward to your results. Do you have a speaker protect? Mine will be running at +- 45v. I'm guessing the voltage is too high for a 4 ohm load?
Well? I wired up my heat sinks. One amp works the other doesn't. Hard to tell what it sounds like. I mean it was fine for a short test. Now to troubleshoot. Then some break in.... Dissappointed
OK, found out my passive volume box lost a channel. fixed that and now have both channels. Played it for an hour found that the top end was bright. Bottom was recessed. If you have some dark sounding speakers, it could be the ticket. Absolutely nothing in temp on heat sinks. Also tone controls could help some bottom. Still wondering if +-45v is too high for 4ohm speakers? I used an old Peavey stereo amp for most the big parts needed. Not sure when I'll try it on my main speakers. And I need to put my amp meter across the fuse holders. So that's my story!....
Checked the rail current draw for each amp. #1 -24 &+29mA. #2 -18 & +36 . Would a 1amp fuse per rail per amp be too big? What would be recommended?
Yes this is a LJM amp off of Ebay model L20SE 4 output per board 1943/5200. It sounds good. I fused each rail for safety and for checking current draw. I used all the parts that came with it. Very little heat so far. Fun interesting project .
ola meu amigo Eliseu, tenho dois pares desses mesmo amplificadores aqui em casa, o primeiro par acabou se destruindo porque usei um transformadoe de 40+40, os transistores de saida eram de má procedecencia, comprei o segundo kit porque gostei muito do som desse amplificador, uso ele a mais de um ano e gosto muito, recentemente me dediquei a recuperar as placas do primeiro par de amplidicador, usei compontenes de uma para colocar na outra e consegui colocar pra funcionar, o problema é que não tenho o transistor 669 para ajuste de bias, então coloquei o kse340, esta funcionando, porem, os transisores de saida estão aquecendo muito mesmo sem entrada de sinal, o que eu poderia fazer para resolver esse problema? posso usar o bd139?
hello my friend Eliseu, I have two pairs of these same amplifiers here at home, the first pair ended up being destroyed because I used a 40+40 transformer, the output transistors were of bad origin, I bought the second kit because I really liked the sound of this amplifier , I've been using it for over a year and I like it a lot, I recently dedicated myself to recovering the boards for the first pair of amplifiers, I used components from one to put them in the other and I managed to get them to work, the problem is that I don't have the 669 transistor to adjust bias, so I put the kse340, it's working, however, the output transistors are heating up a lot even without signal input, what could I do to solve this problem? can i use bd139?
Re-posting here the "LJM L20SE Design Notes" that I previously posted elsewhere in diyAudio.
These notes refer to the electric diagram that I am attaching bellow as LJM_L20SE_electric_scheme-XPZMIePKdFlkA.pdf.
LJM L20SE Design Notes:
1. The amplifier is built on a green PCB with approximately 110 x 50 X 40 mm (LxWxH).
2. The audio signal first passes R1/C1 which form a first-order input low-pass filter that keeps out
unwanted radio frequencies. R2 provides a return path for the input bias current of the amplifier’s
input stage (note that the value of R2 is set to equal that of R14 so that voltage drops across these
two resistors balance out any DC offset). The signal then reaches transistor Q2, which forms a
differential amplifier input stage with Q3. It is fed a constant current from the constant current
source (CCS) Q8/Q1/R3. Q4 and Q5 cascode the differential stage. They keep the heat power loss
low and provide for low input capacitance and high bandwidth. Towards the negative supply line
follows a current mirror comprised of Q6/R8/Q7/R9. It guarantees for good symmetry, high
openloop gain and low sensitivity against disturbances on the supply rail (PSRR).
3. Q10 forms the voltage amplifying stage (VAS) that works on the CCS-connected Q9 as load.
Between Q10 and Q9 we find the biasing network Q11/R17/R18/C8 that generates and regulates
the bias voltage for the output stage.
4. R14 and R15/C3 define the amplification factor. The large capacitor C3 improves offset accuracy
by reducing the closed-loop gain to unity at DC. That spares the use of an active DC-servo stage
and functions rather well under most circumstances.
5. C5 is connected across Q10’s collector and base as a compensation cap to limit Q10 gain at high
frequencies (Miller Effect).
6. The driver transistors Q12/Q14 and Q13/15 Darlington drive the power transistors Q16/Q18 and
Q17/Q19. The bases of Q14 and Q15 are connected together by R23. An arrangement that
resembles a Lokanti output stage.
7. The power transistors are connected to the speakers by means of the R24/R26 and R25/R27
damper-resistors.
8. At the output we find a R28/C11 Zobel network (the R28 10 Ohm approximates to the expected
loudspeaker load impedance and the C11 capacitor is invariably 100 nF) to mitigate amplifier high
frequency instability due to loudspeaker voice-coil inductive reactance.
9. The diodes D3 and D4 are “catcher-diodes”, i.e., they have a dual purpose: they protect from
accidental power supply reverse polarity and they also protect the amplifier from loudspeaker
inductive loads that can push energy back into the amplifier output, resulting in a possible rise of
the output voltage above the supply rails and possible destruction of the output transistors due to
polarity reversal. These diodes catch away the overvoltage to their maximum forward voltage
(hence the term catcher-diode).
(*) Based on Calvin’s work: L12-2, powerful, good, low-cost - calvins-audio-pages
J.A.
These notes refer to the electric diagram that I am attaching bellow as LJM_L20SE_electric_scheme-XPZMIePKdFlkA.pdf.
LJM L20SE Design Notes:
1. The amplifier is built on a green PCB with approximately 110 x 50 X 40 mm (LxWxH).
2. The audio signal first passes R1/C1 which form a first-order input low-pass filter that keeps out
unwanted radio frequencies. R2 provides a return path for the input bias current of the amplifier’s
input stage (note that the value of R2 is set to equal that of R14 so that voltage drops across these
two resistors balance out any DC offset). The signal then reaches transistor Q2, which forms a
differential amplifier input stage with Q3. It is fed a constant current from the constant current
source (CCS) Q8/Q1/R3. Q4 and Q5 cascode the differential stage. They keep the heat power loss
low and provide for low input capacitance and high bandwidth. Towards the negative supply line
follows a current mirror comprised of Q6/R8/Q7/R9. It guarantees for good symmetry, high
openloop gain and low sensitivity against disturbances on the supply rail (PSRR).
3. Q10 forms the voltage amplifying stage (VAS) that works on the CCS-connected Q9 as load.
Between Q10 and Q9 we find the biasing network Q11/R17/R18/C8 that generates and regulates
the bias voltage for the output stage.
4. R14 and R15/C3 define the amplification factor. The large capacitor C3 improves offset accuracy
by reducing the closed-loop gain to unity at DC. That spares the use of an active DC-servo stage
and functions rather well under most circumstances.
5. C5 is connected across Q10’s collector and base as a compensation cap to limit Q10 gain at high
frequencies (Miller Effect).
6. The driver transistors Q12/Q14 and Q13/15 Darlington drive the power transistors Q16/Q18 and
Q17/Q19. The bases of Q14 and Q15 are connected together by R23. An arrangement that
resembles a Lokanti output stage.
7. The power transistors are connected to the speakers by means of the R24/R26 and R25/R27
damper-resistors.
8. At the output we find a R28/C11 Zobel network (the R28 10 Ohm approximates to the expected
loudspeaker load impedance and the C11 capacitor is invariably 100 nF) to mitigate amplifier high
frequency instability due to loudspeaker voice-coil inductive reactance.
9. The diodes D3 and D4 are “catcher-diodes”, i.e., they have a dual purpose: they protect from
accidental power supply reverse polarity and they also protect the amplifier from loudspeaker
inductive loads that can push energy back into the amplifier output, resulting in a possible rise of
the output voltage above the supply rails and possible destruction of the output transistors due to
polarity reversal. These diodes catch away the overvoltage to their maximum forward voltage
(hence the term catcher-diode).
(*) Based on Calvin’s work: L12-2, powerful, good, low-cost - calvins-audio-pages
J.A.
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