Hi all,
Just finished building Jan Dupont's Lynx amp found at http://www.audio-circuit.dk. i used a +/-35V test power supply and measured 3V at the output. Idle starts at 75mA and cannot be lowered anymore. If I connect the Zobel network, the DC at the output becomes 5-6V and the current goes 300mA+.
I've checked everything, even the resistors. They all passed. All transistors are ok, OPS have no short to heatsink.
Am I missing something? Please help me.
Regards,
JojoD
Just finished building Jan Dupont's Lynx amp found at http://www.audio-circuit.dk. i used a +/-35V test power supply and measured 3V at the output. Idle starts at 75mA and cannot be lowered anymore. If I connect the Zobel network, the DC at the output becomes 5-6V and the current goes 300mA+.
I've checked everything, even the resistors. They all passed. All transistors are ok, OPS have no short to heatsink.
Am I missing something? Please help me.
Regards,
JojoD
I've been checking the copy of the PCB layout you emailed me.
Can you tell which make the MPSA42 and 92 are...
Collector and Emitter swaps places in different brands!
Your layout fitts Philips brands, but if you are using transistors
from ON Semi, then you have to turn them 180 degrees....
Can you tell which make the MPSA42 and 92 are...
Collector and Emitter swaps places in different brands!
Your layout fitts Philips brands, but if you are using transistors
from ON Semi, then you have to turn them 180 degrees....
Still checking...
Still here... 🙂
sajti,
I don't have a scope, so I cannot verify if the amps oscillates.
Jan,
I'm using the one from On Semi
, can you be more specific with the pins for the mpsa42/92 transistors?
Thanks,
JojoD
Still here... 🙂
sajti,
I don't have a scope, so I cannot verify if the amps oscillates.
Jan,
I'm using the one from On Semi
, can you be more specific with the pins for the mpsa42/92 transistors?Thanks,
JojoD
Jan,
I've tried disconecting the power transistors already, the dc at the output only decreases to 1.4-1.8V dc.
When the power transistors are connected, I cannot connect the zobel network or even a speaker because the dc only gets higher!
Surprisingly, at +/-12V i can connect a speaker and hear a hum when I touch the input which is normal right.
I'm out of ideas and any suggestion would be helpful...
JojoD
I've tried disconecting the power transistors already, the dc at the output only decreases to 1.4-1.8V dc.
When the power transistors are connected, I cannot connect the zobel network or even a speaker because the dc only gets higher!
Surprisingly, at +/-12V i can connect a speaker and hear a hum when I touch the input which is normal right.
I'm out of ideas and any suggestion would be helpful...
JojoD
From the schematic, the transistors that gets hot are Q103, Q104, Q110, and Q111. Resistors that are hot are, R118, R119, R111, and R112. Output transistors are not even warm. Opamp used is an OPA134 which I tested (again) on a mic preamp I use.
JojoD
JojoD
Please examine your pin connections to Bias transistor Q105.
Try placing the decoupling cap C104 directly on transistor pins instead of on the PCB.... If those transistors/resistors (q103/104 and R111/112) get really hot, it can be caused by that Q105 almost acts like an "short circuit"... What are the Voltage readings across R111 and R112??
R118/119 will normally not be very hot, however if some reason the OpAmp oscillating they will get hot...............
Try putting a 100-500 pF cap from pin 2 of opamp to GND.
Try placing the decoupling cap C104 directly on transistor pins instead of on the PCB.... If those transistors/resistors (q103/104 and R111/112) get really hot, it can be caused by that Q105 almost acts like an "short circuit"... What are the Voltage readings across R111 and R112??
R118/119 will normally not be very hot, however if some reason the OpAmp oscillating they will get hot...............
Try putting a 100-500 pF cap from pin 2 of opamp to GND.
Jan,
I already have a 100pf cap across the 2M resistor which is from pin2 to ground. I will try moving the decoupling cap directly to the bias transistor. I'll post my findings asap.
Thanks,
JojoD
I already have a 100pf cap across the 2M resistor which is from pin2 to ground. I will try moving the decoupling cap directly to the bias transistor. I'll post my findings asap.
Thanks,
JojoD
Wrong component designation!
Do not use OPA620 as stated in the schematics!
The correct designation in the schematic should have been OPA627!
Do not use OPA620 as stated in the schematics!
The correct designation in the schematic should have been OPA627!
It's working!
Jan,
You were right, the pins for the MPSA42/92 were indeed reversed.
I followed datasheet from ON Semi, don't know what went wrong though. The amp is now working, with 0.03mV dc offset and idle at 100mA. The MJE340/350 and 330Ohms/2W are very HOT. 
Maybe they got damaged or something during the time I thought I got the MPSA42/92 correctly placed? Just have to replace them tomorrow just to be sure.
Thank you so much, I'll post voltage readings you requested tomorrow.
JojoD 😎
Jan,
You were right, the pins for the MPSA42/92 were indeed reversed.
I followed datasheet from ON Semi, don't know what went wrong though. The amp is now working, with 0.03mV dc offset and idle at 100mA. The MJE340/350 and 330Ohms/2W are very HOT. Maybe they got damaged or something during the time I thought I got the MPSA42/92 correctly placed? Just have to replace them tomorrow just to be sure.Thank you so much, I'll post voltage readings you requested tomorrow.
JojoD 😎
Jojo
I'm glad that you found the problem 🙂
However it's "just" electronics, so there are always a logical explaination (even it can be hard to find sometimes)
Looking forward to see your Voltage readings.
I'm glad that you found the problem 🙂
However it's "just" electronics, so there are always a logical explaination (even it can be hard to find sometimes)
Looking forward to see your Voltage readings.
Jojo - Your email
Hello Jojo,
I have tried to email you several times, but my emails are returned as undelivered?????
Hello Jojo,
I have tried to email you several times, but my emails are returned as undelivered?????
ACD,
I printed the schematics from Your site yesterday. I have few questions about the amp:
1, I use MJE15030/31 in my amp. But I found that the B of this trannies falls rapidly under 100mA. Did You checked it? Because Your predriver stage use them with small current. (MJE15032/33 has same characteristic). Maybe MJE340/350 would be better?
2, Why do You use separated emitter resistors for the drivers (180 for pre, and 5.6 for drivers), instead of single resistor between the emitters?
3, Did You ever try to use output stage as in the ETI amplifiers? (CFP with voltage gain) It can be connected directly to the output of the opamp.
Sajti
I printed the schematics from Your site yesterday. I have few questions about the amp:
1, I use MJE15030/31 in my amp. But I found that the B of this trannies falls rapidly under 100mA. Did You checked it? Because Your predriver stage use them with small current. (MJE15032/33 has same characteristic). Maybe MJE340/350 would be better?
2, Why do You use separated emitter resistors for the drivers (180 for pre, and 5.6 for drivers), instead of single resistor between the emitters?
3, Did You ever try to use output stage as in the ETI amplifiers? (CFP with voltage gain) It can be connected directly to the output of the opamp.
Sajti
Hello sajti,
I appreciate your interest in my amplifier, however as stated on my website, this amplifier is not entended to be a "State of the art" product, but more a tribute to the classic power amplifiers...
Saving said that, I am aware of the problems with MJE15032/33.
Personally I have only used them in 1 of the 11 amps I have build sofare..... I prefer MJE340/350 however after receiving app. 30 emails a day (from people who didn't get that this amp was ment as a replica) regarding using these "old" transistors as drivers, I've changed the schematic to MJE15032/33🙂
I've have used seperate predriver and driver resistors because I've for the last 25 years have had very good experience with this topology. There are no problems in using a single local feedback resistor (Emitter to Emitter), however I can't see the meaning of that in this type of circuit designed for qverall feedback🙂
I'm aware of the circuit in the ETI amps, and it may be incorporated when I decide to make a real "High End" amp.
For now, I'm building several variants of current design to different peoples and companies.
But please keep the comments coming.............
New ideas are always welcome
I appreciate your interest in my amplifier, however as stated on my website, this amplifier is not entended to be a "State of the art" product, but more a tribute to the classic power amplifiers...
Saving said that, I am aware of the problems with MJE15032/33.
Personally I have only used them in 1 of the 11 amps I have build sofare..... I prefer MJE340/350 however after receiving app. 30 emails a day (from people who didn't get that this amp was ment as a replica) regarding using these "old" transistors as drivers, I've changed the schematic to MJE15032/33🙂
I've have used seperate predriver and driver resistors because I've for the last 25 years have had very good experience with this topology. There are no problems in using a single local feedback resistor (Emitter to Emitter), however I can't see the meaning of that in this type of circuit designed for qverall feedback🙂
I'm aware of the circuit in the ETI amps, and it may be incorporated when I decide to make a real "High End" amp.
For now, I'm building several variants of current design to different peoples and companies.
But please keep the comments coming.............
New ideas are always welcome
Emails
Jan,
I got your emails, thank you. 🙂
Before I send you any voltage readings, I would first like to replace ALL transistors in the pcb as well as the bias transistor so that I have a fresh start before measuring anything. I'll send them data to you asap.
This sure sounds very good for a "replica" of an old amp. Sounds like an old SAE my father and I used to listen. 😎
BTW, what idle current are you using?
Regards,
JojoD
Jan,
I got your emails, thank you. 🙂
Before I send you any voltage readings, I would first like to replace ALL transistors in the pcb as well as the bias transistor so that I have a fresh start before measuring anything. I'll send them data to you asap.
This sure sounds very good for a "replica" of an old amp. Sounds like an old SAE my father and I used to listen. 😎
BTW, what idle current are you using?
Regards,
JojoD
Hi Jojo,
Glad you got the emails🙂
I'm also happy to hear, that you think I managed to "recreate" this amazing sound from the old amps🙂
The idle we have found most suitable is app. 315mV across each of R122 and R123, making the Drivers running class A while output transistors are running class B.
I know that many will comment this, however it work just fine, and most amps from the old days used this technique....
You can raise the bias if you want to... The design allows you to run class A all the way, however you will need some pretty big heatsink and some fans🙂
I'm looking forward to see your voltage readings.
Glad you got the emails🙂
I'm also happy to hear, that you think I managed to "recreate" this amazing sound from the old amps🙂
The idle we have found most suitable is app. 315mV across each of R122 and R123, making the Drivers running class A while output transistors are running class B.
I know that many will comment this, however it work just fine, and most amps from the old days used this technique....
You can raise the bias if you want to... The design allows you to run class A all the way, however you will need some pretty big heatsink and some fans🙂
I'm looking forward to see your voltage readings.
Distorted
Jan,
The amp sounds good at low volumes, but when I increase the volume, the sound (bass especially) becomes distorted . The 330ohms/2W and MJE350/340 are still very hot even after I replaced all the transistors in the pcb. Any suggestions?
JojoD
Jan,
The amp sounds good at low volumes, but when I increase the volume, the sound (bass especially) becomes distorted
. The 330ohms/2W and MJE350/340 are still very hot even after I replaced all the transistors in the pcb. Any suggestions?JojoD
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