I am working on a vintage stereo receiver that has a rather poor phono pre-amp.
As a result, I built my own from Rod Elliots site. Works great.
Now, I would like to install this phono pre-amp board inside the stereo (don't want any external extra boxes, etc). Problem is: the NE5532 is +/- 15 and the only voltages I have inside the unit are +/- 26 volts and +/- 35 volts.
I got a couple DC buck/boost converter (PWM controlled) all-in-one boards I could use, but rather keep this linear to ensure no extra noise is introduced. Thus, I scraped around online and came across the classic "Zener/Transistor" regulator diagrams..
Here is one I am looking at:
Now, I think I understand how this works and - please correct me if I am wrong here - but given MY supply (input) voltage, I will not be able to use a general 2n2222 or similar transistor, correct?
BASED ON THE VOLTAGES REFERENCED ABOVE...
My understanding is: once powered up, as long as the voltage across RL is 5.05 volts, the effective voltage between the base and emiter is essentially becomes 0 - and the transistor turns off.. hence "following" the voltage of the zener. BUT... at the moment the circuit is powered on, the voltage between base and emiter becoes the full regulated potential, thus..
In MY OWN scenario - where input voltage is around 26-28 volts, and regulated voltage would be 15... the E-B junction will see the full 15 volts when power is first applied - correct?
Meaning... I would not be able to use a simple general purpose NPN for this, correct?
I also have the "negative version" of this circuit using the zener in reverse and a PNP transistor. So my plan is to "mirror" this for the negative rail.
Thanks!
As a result, I built my own from Rod Elliots site. Works great.
Now, I would like to install this phono pre-amp board inside the stereo (don't want any external extra boxes, etc). Problem is: the NE5532 is +/- 15 and the only voltages I have inside the unit are +/- 26 volts and +/- 35 volts.
I got a couple DC buck/boost converter (PWM controlled) all-in-one boards I could use, but rather keep this linear to ensure no extra noise is introduced. Thus, I scraped around online and came across the classic "Zener/Transistor" regulator diagrams..
Here is one I am looking at:
Now, I think I understand how this works and - please correct me if I am wrong here - but given MY supply (input) voltage, I will not be able to use a general 2n2222 or similar transistor, correct?
BASED ON THE VOLTAGES REFERENCED ABOVE...
My understanding is: once powered up, as long as the voltage across RL is 5.05 volts, the effective voltage between the base and emiter is essentially becomes 0 - and the transistor turns off.. hence "following" the voltage of the zener. BUT... at the moment the circuit is powered on, the voltage between base and emiter becoes the full regulated potential, thus..
In MY OWN scenario - where input voltage is around 26-28 volts, and regulated voltage would be 15... the E-B junction will see the full 15 volts when power is first applied - correct?
Meaning... I would not be able to use a simple general purpose NPN for this, correct?
I also have the "negative version" of this circuit using the zener in reverse and a PNP transistor. So my plan is to "mirror" this for the negative rail.
Thanks!
On power up, the Zener is open, and the capacitor exponentially charges up from 0V through the 150R.
The voltage across a capacitor must be continuous, since the charge on its plates is continuous, and V = Q/C.
As soon as the capacitor voltage reaches the rated Zener voltage, the Zener clamps the base voltage
to the Zener voltage. The Vbe will never be more than 0.6V. The 2N2222A should be ok for this.
The voltage across a capacitor must be continuous, since the charge on its plates is continuous, and V = Q/C.
As soon as the capacitor voltage reaches the rated Zener voltage, the Zener clamps the base voltage
to the Zener voltage. The Vbe will never be more than 0.6V. The 2N2222A should be ok for this.
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When the transistor is working, the Vbe is alwyas about 0.65V. It varies depending on the output current, between say 0.6V and 0.7V.
That means this is not a regulator - it is an emitter follower, sometimes fancifully named a capactance multiplier.
The output always follows the base voltage with a ~0.65V loss to keep it operating.
As @rayma mentioned, this circuit stabilizes the base voltage with the zener, which reasonbly stabilises the output voltage at about 0.65V lower.
Jan
That means this is not a regulator - it is an emitter follower, sometimes fancifully named a capactance multiplier.
The output always follows the base voltage with a ~0.65V loss to keep it operating.
As @rayma mentioned, this circuit stabilizes the base voltage with the zener, which reasonbly stabilises the output voltage at about 0.65V lower.
Jan
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The circuit you designed is fine, but let me suggest replace the zener with a TL431 easily converting it into regulated and with much less internal impedance and better PSRR at the expense of some cents more in components.
BTW, you asked about a 2N2222. That would work fine in principle, but may get too hot.
If your use the +/-26V as input for +/-15V as output, there will be 26-15=11V across the transistor.
If as shown the load is 100mA, that flows through the transistor.
Since power P = V x I, that works out to 1.1W dissipated in the transistor.
The 2N2222 can handle that assuming it has adequate ventilation or a small clip-on heatsink.
It would be better to use a small TO126 transistor with a small heatsink like the BD135 etc series. But there are many that would fit.
Jan
If your use the +/-26V as input for +/-15V as output, there will be 26-15=11V across the transistor.
If as shown the load is 100mA, that flows through the transistor.
Since power P = V x I, that works out to 1.1W dissipated in the transistor.
The 2N2222 can handle that assuming it has adequate ventilation or a small clip-on heatsink.
It would be better to use a small TO126 transistor with a small heatsink like the BD135 etc series. But there are many that would fit.
Jan
Thanks to all for the help!
So.. the RL follows the zener voltage, minus the transistor voltage drop. And while the capacitor is coming up, so is the voltage of RL.
I am going to bread-board the circuit and test it out this afternoon.
Osvaldo de Banfield - thanks for the heads-up. I just took a look and - while I could order this from Mouser, I was trying to go the route of what-I-have-on-hand to get my stereo back together. Just got a new Audio-Technica MicroLine in for the turntable and really want to spin some records.
If it was a simple matter of ordering something, I would have just got a couple 12 or 15 volt LDO's and called it a day. But... I live in Mexico and it generally takes 2-3 weeks for anything I order from Mouser to come in. I will certainly take a closer look at this for a future revision.
I really need to find a good local source of parts here in central MX.
Thanks again, I will report back how it goes!
So.. the RL follows the zener voltage, minus the transistor voltage drop. And while the capacitor is coming up, so is the voltage of RL.
I am going to bread-board the circuit and test it out this afternoon.
Osvaldo de Banfield - thanks for the heads-up. I just took a look and - while I could order this from Mouser, I was trying to go the route of what-I-have-on-hand to get my stereo back together. Just got a new Audio-Technica MicroLine in for the turntable and really want to spin some records.
If it was a simple matter of ordering something, I would have just got a couple 12 or 15 volt LDO's and called it a day. But... I live in Mexico and it generally takes 2-3 weeks for anything I order from Mouser to come in. I will certainly take a closer look at this for a future revision.
I really need to find a good local source of parts here in central MX.
Thanks again, I will report back how it goes!
The picture shown was from another site online.
My load will be 2 NE5532 OP-Amps in the phono pre-amp. I doubt it will ever see more than 20-30mA.
But I DO have a decent selection of some other higher-power transistors in my trays (including a few dozen various TO126 style). Lemme take a look and see what I have. Always better to be safer than sorry. I should be able to find a compliment pair.
The circuit this is powering is Rod Elliots HiFi phono pre-amp:
https://sound-au.com/project06.htm
No LED's, etc. Very low-power. And the output of this is driving a rather high-ish impedance input to the stereos tone board.
This is how you can use TL431.
It keeps the voltage stable and the result is rather clean approximate 15.0 Volt.
Attachments
Given high enough input voltage, replace the BJT by a MOSFET. This minimizes looses inside the regulator. Also add a 470pF to 0.001uF across anode and adjusting terminal of 431 to prevent oscillation.
The higher the beta of the pass transistor at the operating current, the better the regulation.
So a smaller transistor should work better in this circuit, given its power limitations.
I just measured the circuit while playing some Men At Work 🙂
In operation, the circuit only draws 4 mA.
Even if I doubled this to 8mA.. With 28 volts input, 15 out. That is a 13 volt drop. At DOUBLE the operational current, I am looking at 100mW.
This seems EXTREMELY well within the limits of a simple 2n2222 and 2907 as far as I can tell.
Gotta run and take care of a few things... gonna whip up the final version and test later tonight.
In operation, the circuit only draws 4 mA.
Even if I doubled this to 8mA.. With 28 volts input, 15 out. That is a 13 volt drop. At DOUBLE the operational current, I am looking at 100mW.
This seems EXTREMELY well within the limits of a simple 2n2222 and 2907 as far as I can tell.
Gotta run and take care of a few things... gonna whip up the final version and test later tonight.
The circuit is fine except a Zener current 10x the base current would be excessive, and 28x way too much. I recommend Rs = (26-15)(load current/beta *~2) = ~5K or more, maybe 10K. This circuit is a regulator, just not highly regulated. Super regulators are mostly useless in an application like this. But this app is a perfect job for the standard 3 terminal 15V regulator chips 7815 and 7915. If you know the current is 4mA, you could just use a 2.2K dropping resistor and a filter ~100uF capacitor. Putting Zeners directly on the op-amp supply is typical, with no pass transistor, just dropping resistors. I have used op-amps on a raw supply with no sign of noise in the output because op-amps have very good power supply rejection ratio. Blasphemy to some, I know. Supply noise on the output is more likely due to bad grounding, which a supply filter will mask. So I would use 1K resistors and 15V Zeners, and a 100uF caps for up to 10mA op-amp current, ie no pass transistor required, or 7815 and 7915.
You can use 1k resistors to the 25v supplies and 15v zener plus 100uf caps on each 5532 rail.
This will provide a max of 10 ma to the 5532s.
If it needs more use 680 ohm.
This will provide a max of 10 ma to the 5532s.
If it needs more use 680 ohm.
Updated my design as mentioned by stocktrader2000 and integrated the entire zener/resistor directly onto my phono pre-amp board since I already had the bypass caps soldered to the board.
Ran it for a couple hours last night and monitored current and temps.. all sounds great and absolutely dead-silent.
-Dean
Ran it for a couple hours last night and monitored current and temps.. all sounds great and absolutely dead-silent.
-Dean