distortion increase....how so?
Back to Jocko's iv stage. Jocko states that distortion is higher with the ccs load. But how can a resistor (a linear device) without the ccs cause an increase in distortion in a non-feedback iv stage? I am not doubting the results, but just trying to understand why. Seems to me that the resistor may not be linear without the ccs, since you now have the full swing across the resistor. Whereas with the ccs, the resistor is not impacted by the full swing signal.
Any thoughts?
Thanks,
RL
Back to Jocko's iv stage. Jocko states that distortion is higher with the ccs load. But how can a resistor (a linear device) without the ccs cause an increase in distortion in a non-feedback iv stage? I am not doubting the results, but just trying to understand why. Seems to me that the resistor may not be linear without the ccs, since you now have the full swing across the resistor. Whereas with the ccs, the resistor is not impacted by the full swing signal.
Any thoughts?
Thanks,
RL
help please guys.
Ive tried getting this going and its giving grief.
Q1, Q2, Q3 are mpsap2 Q4 is mpsa42.
R2 & R3 are 220 Ohm
R4 ,R5,R6 are 5K
R1 is 3K
C1 is 100uF electrolytic.
Ive just used parts I have to see if I can get it going.
Using +- 24 rails and resistors that Jocko recommended with the higher rails.
I took the opamp out of my Philips CD640 and connected the I/V input to where the opamps inverting input was.
I connected the output of the I/V to the the output connection of the second opamp, buffer I guess.
It has a faint distorted noise that slightly resembles music, but gain has to be maxed out on the preamp to get that much.
Apart from the green leds impressing my wife (me too) its a bit of a dead loss:-(
What can I do to get it going.
Ive tried getting this going and its giving grief.
Q1, Q2, Q3 are mpsap2 Q4 is mpsa42.
R2 & R3 are 220 Ohm
R4 ,R5,R6 are 5K
R1 is 3K
C1 is 100uF electrolytic.
Ive just used parts I have to see if I can get it going.
Using +- 24 rails and resistors that Jocko recommended with the higher rails.
I took the opamp out of my Philips CD640 and connected the I/V input to where the opamps inverting input was.
I connected the output of the I/V to the the output connection of the second opamp, buffer I guess.
It has a faint distorted noise that slightly resembles music, but gain has to be maxed out on the preamp to get that much.
Apart from the green leds impressing my wife (me too) its a bit of a dead loss:-(
What can I do to get it going.
Luke said:
Take the output directly from the end of the I/V and don't connect it to anything else in your CD player. That should work well enough to determine that the I/V is working. After that you can think about how to buffer the output.
Unless you have the schematic for your player you have no idea exactly what is in the circuit. If you have pulled the op-amp that you are sure implements the I/V conversion there is probably nothing hanging around at the input to your new I/V that will cause trouble. The R and C in the feedback loop of the transimpedance amp will still be connected at one end, but not connected to anything at the other.
I have tried this it gave similar results, if anything a little worse.
Prehaps the problem is that I need to buffer the output to get it going?
I forgot to mention I have the schematic, and I know which pin comes from the I/V output of the TDA1541. There is noting between the I/V output of the TDA1541and the input of the new I/V stage. It was late last night when I tried this. Perhaps I should check the wiring and see if Ive made some mistakes.
I assume the values and components selected are ok?
Francis_Vaughan said:
If the DAC circuits in the R2R ladder switch from the reference voltage supply to ground, then the circuit would work perfectly into a resistor. Where you get problems is when the circuit switches from the reference supply to just an open circuit. I discuss this at my page http://pw2.netcom.com/~wa2ise/radios/tubedac.htm with some simulated diagrams.
Also Rotel used a 100 ohm resistance with a PCM63 DAC chip that then feeds a virtual ground. Which tells me that this manufacturer found no problems with a circuit that is quite similar to our I/V resistor circuits. Scroll down a bit at my web page above to see a partial diagram of this circuit.
Re: help please
It really sounds totally inoperative. My first question when faced with any such misbehaving circuit is to measure the Vbe on each transistor. It should be 0.6V (or close, depending on the exact working conditions for any transistor that is actually operating.) If it isn't, you already have problems without going any further. The fact that the LEDs are lit suggests that you won't pass even this first pass check. That would really suggest a fundamental wiring error. The favorite being incorrect transistor connection.
Bad.Luke said:
Power = VI. 80mA times 7 Volts = 0.56 Watts. That is a lot of power for a TO92.
It really sounds totally inoperative. My first question when faced with any such misbehaving circuit is to measure the Vbe on each transistor. It should be 0.6V (or close, depending on the exact working conditions for any transistor that is actually operating.) If it isn't, you already have problems without going any further. The fact that the LEDs are lit suggests that you won't pass even this first pass check. That would really suggest a fundamental wiring error. The favorite being incorrect transistor connection.
Sorry, I was getting ahead of things. The LEDs are a voltage reference, so if lit they are both operating and defining the voltage at the base of the transistor. Since you measured 11 volts across the corresponding resistor this already sounded inoperative.
By using the known voltages, i.e. LED forward drop, Vbe voltage, applying Ohms Law, and Kirkoff's Law, you can pretty well understand the entire circuit, and predict everything you should be able measure. Assuming a red LED you should have about 1.7 volts (maybe a little less) at the base relative to the supply rail. You know the Vbe drop, so now you know the voltage across the resistor, hence the current etc etc.
Perhaps you have the NPN PNP varieties reversed and in backwards. Whatever, it sounds as if the transistors are close to saturated, and hence everything is getting rather toasty, and otherwise is generally not working.
I would be very inclined to get the whole thing working (as in not getting hot, and sensible voltages and currents) before connecting it to anything. Connecting a 24 volt rail directly to the output of your DAC is not a practice conducive to success. And that is certainly one possible failure mode of the converter.
By using the known voltages, i.e. LED forward drop, Vbe voltage, applying Ohms Law, and Kirkoff's Law, you can pretty well understand the entire circuit, and predict everything you should be able measure. Assuming a red LED you should have about 1.7 volts (maybe a little less) at the base relative to the supply rail. You know the Vbe drop, so now you know the voltage across the resistor, hence the current etc etc.
Perhaps you have the NPN PNP varieties reversed and in backwards. Whatever, it sounds as if the transistors are close to saturated, and hence everything is getting rather toasty, and otherwise is generally not working.
I would be very inclined to get the whole thing working (as in not getting hot, and sensible voltages and currents) before connecting it to anything. Connecting a 24 volt rail directly to the output of your DAC is not a practice conducive to success. And that is certainly one possible failure mode of the converter.
yes Im a bit embarrased by that. I was left feeling that the pinouts must be wrong or the types were wrong.
Sad thing is I downloaded the spec sheets of all the transistors before I started so I didnt tool it. Go figure.
thanks for your help Francis, I think I should be able to get it going form here. Will post back and let you know how it goes
Sad thing is I downloaded the spec sheets of all the transistors before I started so I didnt tool it. Go figure.
thanks for your help Francis, I think I should be able to get it going form here. Will post back and let you know how it goes
Hi Francis,
Thanks for your help, I got the transistors round the wrong way, ie PNP for NPN and vice versa.
Thing kinda works now,however distortion is really unaceptable, especially in louder passages of music. I get about 2.7V across the 220 ohm resistors, R2 and R3, this is about 7.7 mA bias. Is this enough for the circuit or should I lower the resistor values and bump up the juice?
cheers Arthur
Thanks for your help, I got the transistors round the wrong way, ie PNP for NPN and vice versa.
Thing kinda works now,however distortion is really unaceptable, especially in louder passages of music. I get about 2.7V across the 220 ohm resistors, R2 and R3, this is about 7.7 mA bias. Is this enough for the circuit or should I lower the resistor values and bump up the juice?
cheers Arthur
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.