Hi Mooly
i just remove the 0.22uf capacitor,the dc offset drop into 0.6v,and then i put again those capacitor,but i remove the 47pf caps which place on collector to base q42 & q58,the dc drop again into 0.4v,but when i connect to the loudspeaker,the dc offset is gone,almost 0volt,but as i tell before,when it play music,the sound is very poor,broke,clip on every level,even in low volume.
thank you
i just remove the 0.22uf capacitor,the dc offset drop into 0.6v,and then i put again those capacitor,but i remove the 47pf caps which place on collector to base q42 & q58,the dc drop again into 0.4v,but when i connect to the loudspeaker,the dc offset is gone,almost 0volt,but as i tell before,when it play music,the sound is very poor,broke,clip on every level,even in low volume.
thank you
anyone know?what is the function of two 12k resistors which place between base q50 and q51
thanks
thanks
It sounds like it's oscillating to me.
Tomat... Is the other channel OK ? and are you sure the speaker is OK too ?
Can you answer yes or no to both those questions.
The 12k resistors are part of the bias componenets for the input stage.
Connecting the speaker should not alter the offset... I suspect a problem with the power amp.
Tomat... Is the other channel OK ? and are you sure the speaker is OK too ?
Can you answer yes or no to both those questions.
The 12k resistors are part of the bias componenets for the input stage.
Connecting the speaker should not alter the offset... I suspect a problem with the power amp.
Some tests for you, sorry not very scientific but it may help.
Put all components back apart from the 0.22 cap.
Switch off and make sure all is discharged.
Measure all the low value resistors (these are low enough in value to measure in circuit mostly) in particular the emmiter resistors of Q45 and Q52.
The two diodes look to be drawn the wrong way round unless they are they zeners ?
The lack of component reference numbers makes it hard to explain what to measure 🙂
Put all components back apart from the 0.22 cap.
Switch off and make sure all is discharged.
Measure all the low value resistors (these are low enough in value to measure in circuit mostly) in particular the emmiter resistors of Q45 and Q52.
The two diodes look to be drawn the wrong way round unless they are they zeners ?
The lack of component reference numbers makes it hard to explain what to measure 🙂
Hello Mooly,
thanks,the answer is yes.
I was ever try to reduce the value of 12k resistor,i reduce it 6k8 value,the sound was bit better,but after two or three minutes,the output transistor getting very hot,and...... blown..., but that's already replace now
i'm very sorry due to lack component measure numbers,i will draw again & give the complete number
the emmiter resistors of q45 and q52 are 0.47R,i have measure again,thevalue is still the same.
let me make sure for the two diodes,are they zeners or not,i will inform you immediately 🙂
thanks
As I note by simulation, you have an oscillator through not existing compensation devices (look at the botton pdf files). Additional I miss the boucherot (zobel) network parallel to the loudspeaker terminal (absolutely necessary for checking and measuring without load).
I haven't spice model of your BjT transistor types, but the used types of me are certainly accurate enough to assess this. After introduce the capacitors Cx and Cy the parasitic oscillaton goes away.
Please note the different values of the resistances for the wanted quiescent current and offset (0 volts at output).
Please check first the output buffer without the tube voltage amp. Dont forget to introduce the serial capacity for the input (AC coupling). If this power buffer stage works fine without the parasitic oscillation (connect oscilloscope at the output without and with load) and with stable idle current, then you must check the voltage amp without this buffer. Connect it to an old power amp with integrated volume level control. If this device do his work perfectly, too, then you can connect both stages for checking the functions.
Check also the DC conditions and compare it to that one of simulation.
Last edited:
Hi Tiefbassuebertr
thank you very much,amaze 🙂,i will try with your schematic you give,i will inform immediately
Hi Tiefbassuebertr,
I like this buffer.
Though, i do not understand how thermaltracking is done here.
Do you know if it should be possible to upscale to 90volts rails.
I have a bunch of sankens A1995/C3264
A1294/C3263 (drivers)
A1837/C4793 (pre-drivers)
These triples are used in the Rotel RB990 with 90 volts rails and quads outputs.
What could i use for Q16/17?
Thanks in advance
I like this buffer.
Though, i do not understand how thermaltracking is done here.
Do you know if it should be possible to upscale to 90volts rails.
I have a bunch of sankens A1995/C3264
A1294/C3263 (drivers)
A1837/C4793 (pre-drivers)
These triples are used in the Rotel RB990 with 90 volts rails and quads outputs.
What could i use for Q16/17?
Thanks in advance
Hi Tiefbassuebertr
i have try your schematic,with 5k resistors between q5 and q15 as on your schematic,the output transistors run very hot quickly,no more than two minutes after the amp turn on,is it no problem? and also one channel have 40vdc on speaker output,but if i change the resistor to 10k,the dc is reduce till a few hundred mv,any advice?
thanks very much
i have try your schematic,with 5k resistors between q5 and q15 as on your schematic,the output transistors run very hot quickly,no more than two minutes after the amp turn on,is it no problem? and also one channel have 40vdc on speaker output,but if i change the resistor to 10k,the dc is reduce till a few hundred mv,any advice?
thanks very much
Hi Mooly
good morning 🙂,the component number are the same with the schematic from Mr Tiefbassuebertr
thanks
Tomat,
Without knowing your level of understanding this may be dificult to faultfind on.
You say one channel is working correctly... that has to be your guide.
And don't let the outputs overheat.
Some basic checks,
What is the DC voltage across R1
What is the DC voltage across R2
What is the voltage across R33... 47 ohms
What is the voltage across R3 ....47 ohms
What is the DC voltage across C3 (47mfd) ?
Without knowing your level of understanding this may be dificult to faultfind on.
You say one channel is working correctly... that has to be your guide.
And don't let the outputs overheat.
Some basic checks,
What is the DC voltage across R1
What is the DC voltage across R2
What is the voltage across R33... 47 ohms
What is the voltage across R3 ....47 ohms
What is the DC voltage across C3 (47mfd) ?
If you get nowhere doing that I would suggest removing Q1,Q2,Q4,Q6,Q8 and Q9 so you have JUST the front end transistors in. Make sure all the resistor values are back to the ORIGINAL values.
Make sure you have NO speaker connected.
Switch on and measure the DC volts at the junction of the emmiters of Q5 and Q15 which should be zero.
Again what is the DC volts across R1 and R2. I would expect around 0.75 volts or so.
If that is OK then measure the DC volts across C3.
What is it ?
Make sure you have NO speaker connected.
Switch on and measure the DC volts at the junction of the emmiters of Q5 and Q15 which should be zero.
Again what is the DC volts across R1 and R2. I would expect around 0.75 volts or so.
If that is OK then measure the DC volts across C3.
What is it ?
I'll look in again tommorow 🙂
Doing all that Tomat should tell us a lot.
The circuit splits nicely into the part to the right of C3 and the part to the left. There is no feedback to complicate things.
The voltage that appears across C3 sets the current in the output stage. The lower you make R18 and 19 will have the effect of turning OFF those input transistors which will cause the volts across C3 to rise and the outputs to get hot (to much current)
It's a fine balancing act. If there is any imbalance in the two transistor operating points that will cause an offset voltage.
Doing all that Tomat should tell us a lot.
The circuit splits nicely into the part to the right of C3 and the part to the left. There is no feedback to complicate things.
The voltage that appears across C3 sets the current in the output stage. The lower you make R18 and 19 will have the effect of turning OFF those input transistors which will cause the volts across C3 to rise and the outputs to get hot (to much current)
It's a fine balancing act. If there is any imbalance in the two transistor operating points that will cause an offset voltage.
Hello again Mooly 🙂
here is the measurement result
with q1,q2,q4,q6,q8 and q9 on board:
- dc across R1 = 0.8v
- dc across R2 = -0.9v
- dc across R31 (47R)= -0.1v
- dc across R3 (47R)= 0.1v
- dc across c3 (47uf)= -2.3v
measurement with q1,q2,q4,q6,q8 and q9 remove
- dc across R1= 0.8v
- dc acroos R2= -0.9v
- dc across c3= 2.2v
- dc volt at junction of emmiters q5 and q15= 1.7v
i think the problem is here,voltage at emmiters junction of q5 and q15 is not at zero volt.
Mooly,is this have an effect with bad sound quality from this amp?,clip and distorted sound?
thank you very much and hope you are not bored with this thread 🙂
here is the measurement result
with q1,q2,q4,q6,q8 and q9 on board:
- dc across R1 = 0.8v
- dc across R2 = -0.9v
- dc across R31 (47R)= -0.1v
- dc across R3 (47R)= 0.1v
- dc across c3 (47uf)= -2.3v
measurement with q1,q2,q4,q6,q8 and q9 remove
- dc across R1= 0.8v
- dc acroos R2= -0.9v
- dc across c3= 2.2v
- dc volt at junction of emmiters q5 and q15= 1.7v
i think the problem is here,voltage at emmiters junction of q5 and q15 is not at zero volt.
Mooly,is this have an effect with bad sound quality from this amp?,clip and distorted sound?
thank you very much and hope you are not bored with this thread 🙂
Mooly,is the 1.7v at emmiters junction of q5 and q15 caused with unmatched of this two transistors?
thanks
thanks
First thing is that 0.1 volts across R3 and R31 is not enough to turn the output transistors on.
Are you saying that the outputs get hot with this 0.1 volts present... they should be totally cold at that. If they are not then there is a problem around the outputs.
The voltage across R1 and R2 is correct.
Can you measure also the DC voltage across R14 and R32
Are you saying that the outputs get hot with this 0.1 volts present... they should be totally cold at that. If they are not then there is a problem around the outputs.
The voltage across R1 and R2 is correct.
Can you measure also the DC voltage across R14 and R32
- Status
- Not open for further replies.