Hi guys.
I'm building an amplifer (preamp and poweramp), and at the output of the preamp stage, I get a very big DC offset when the power rails are turned off.
The opamp I am using is the OPA2134, and a link to the datasheet is given below.
http://www.datasheetcatalog.com/datasheet/O/OPA2134.shtml
I am using 12V rails, and I am getting a -4V DC offset exactly when the rails deplete themselves to 0V (I checked this using a CRO ). This negative offset then slowly makes it way back to 0V over time.
I have no idea what is causing this.... any help with be of great assistance.
The preamp is shown below....
the only inclusion that I stuffed up in the scan is the fact that a 43K resistor is connected to ground at the inverting input.
Thank you in advance guys!
I'm building an amplifer (preamp and poweramp), and at the output of the preamp stage, I get a very big DC offset when the power rails are turned off.
The opamp I am using is the OPA2134, and a link to the datasheet is given below.
http://www.datasheetcatalog.com/datasheet/O/OPA2134.shtml
I am using 12V rails, and I am getting a -4V DC offset exactly when the rails deplete themselves to 0V (I checked this using a CRO ). This negative offset then slowly makes it way back to 0V over time.
I have no idea what is causing this.... any help with be of great assistance.
The preamp is shown below....
the only inclusion that I stuffed up in the scan is the fact that a 43K resistor is connected to ground at the inverting input.
Thank you in advance guys!
Attachments
Hi,
It is difficult to say exactly what causes this, but it is a common thing in both power and preamps.
When you turn off power, and the supply voltages decrease, there comes a point where the bias currents and voltages in the amp are no longer sufficient to keep it at the design point, and that gives you the offset. The actual offset depends on a lot of things, the amp design, the speed with which the supplies decrease, one supply polarity may go down faster than the other, etc.
There is really no easy cure for this, other than perhaps a mute relay at the output activating immediately at switch off and deactivating delayed after switch on to give the amp time to stabilize.
Jan Didden
It is difficult to say exactly what causes this, but it is a common thing in both power and preamps.
When you turn off power, and the supply voltages decrease, there comes a point where the bias currents and voltages in the amp are no longer sufficient to keep it at the design point, and that gives you the offset. The actual offset depends on a lot of things, the amp design, the speed with which the supplies decrease, one supply polarity may go down faster than the other, etc.
There is really no easy cure for this, other than perhaps a mute relay at the output activating immediately at switch off and deactivating delayed after switch on to give the amp time to stabilize.
Jan Didden
Thanks a lot for your help.
I was thinking of using a relay circuit, but was just hoping that there was a simpler fix to the problem.
I've also checked the 12V supply rails, and they both fall at the same rate.
Guess it'll be a relay after all.
Does anyone know of any relay circuits available on the net?
I'm pretty new to relays, so it'd be helpful if there was a site that explained how to calculate resistor/cap values to enable a relay to be activated/deactivated upon power on and power off.
I was thinking of using a relay circuit, but was just hoping that there was a simpler fix to the problem.
I've also checked the 12V supply rails, and they both fall at the same rate.
Guess it'll be a relay after all.
Does anyone know of any relay circuits available on the net?
I'm pretty new to relays, so it'd be helpful if there was a site that explained how to calculate resistor/cap values to enable a relay to be activated/deactivated upon power on and power off.
Oh yeah, just one thing, if I do use a relay to disconnect the speakers from the amp(my preferred relay switching point), how much punishment can relays take?
Enough for me to push 200W rms through?
Thanks again!
Enough for me to push 200W rms through?
Thanks again!
I have a similar situation with the OPA627 buffers on my Gainclones so have to power them up before turning on power to the amp sections.
What I wonder though is how is this problem solved with commercial products. My friend has a comercial preamp using opamps and he can turn that on and off without damaging his speakers even though the power amps are on at the time.
What I wonder though is how is this problem solved with commercial products. My friend has a comercial preamp using opamps and he can turn that on and off without damaging his speakers even though the power amps are on at the time.
How do you connect the pre and the power? I have a OP627/BUF634 pre connected to a GC and if I put a cap in between I get some nasty effects - high offset and turn on/off sound. Without the cap (DC coupled) things are much better but I still have some sound from the speakers on turning on, but nothing to worry about. If you can DC couple the amps. BTW, with the GC alone and no pre, offset is quite low (~5mV) and no turn on/off sounds.
Miguel
Miguel
It looks to me that the +In of the opamp is in the air (as far as DC is concerned). It has to be connected to GND. Without it I'm not sure that thing can work and if you don't have it that might be the problem. Put 47k ( or something similar) from +in to gnd. You can't guarantie that the source will provide you with a good (low enough empedance) DC path to gnd.
Greg
Greg
In reply to your question how do comercial products solve this problem. We usually use relays configured to short the output of the opamp to ground. A small resistor is put on the output of the opamp to prevent damage to the opamp. Normally 47 - 100R is OK here and this also helps to isolate the opamp from cable capacitance, significantly reducing the posibility of instability in the opamp.
Cheaper products sometimes use relays in the signal path, JFETs to ground or transistors that mute to ground. You need to watch the reverse bais on the transistors if you use them.
Andy.
Cheaper products sometimes use relays in the signal path, JFETs to ground or transistors that mute to ground. You need to watch the reverse bais on the transistors if you use them.
Andy.
Sorry, I didn't make it clear, but the opamp is running in inverting mode.
The negative input is grounded through 43K, and the positive input is where the signal is fed.
I spose the main concern for me now is whether what rating relay I need to disconnect the speaker lines if I require to pump 200w rms through them....
Thanks ; )
The negative input is grounded through 43K, and the positive input is where the signal is fed.
I spose the main concern for me now is whether what rating relay I need to disconnect the speaker lines if I require to pump 200w rms through them....
Thanks ; )
There needs to be an earth path from the non-inverting input to ground; through a 10-100kohm resistor, i.e.
Your schema doesn't show it, but is there another 43kohm resistor that connects between the inverting input and ground?
jD
Your schema doesn't show it, but is there another 43kohm resistor that connects between the inverting input and ground?
jD
you really don't push power, it's a concept similar to voltage, not current. basically, 5A may flow through your wires, and if you have an 8ohm speaker you will get 200 watts to the speaker, but 200 watts isn't "flowing" through the wires, and the wires will only dissipate a small amount of power, probably a quarter of a watt.
so really, what you want is a relay with sufficient closed-circuit current and open-circut voltage. and the coil should be a voltage that you can use.
for a preamp this will be almost anything, for a power amp this will be higher.
so really, what you want is a relay with sufficient closed-circuit current and open-circut voltage. and the coil should be a voltage that you can use.
for a preamp this will be almost anything, for a power amp this will be higher.
Yes, there is a 43K resistor connecting the inverting input to ground.
However, at the non-inverting input, this is simply connected directly to the signal input.
So what your saying is that I should have the signal connected to the non-inverting input, whilst having it also connected to ground through a 10K resistor or something?
Thanks for your help...
However, at the non-inverting input, this is simply connected directly to the signal input.
So what your saying is that I should have the signal connected to the non-inverting input, whilst having it also connected to ground through a 10K resistor or something?
Thanks for your help...
electrode10101 said:
theChris said:
Please before you do anything else, PLEASE read again what jD (above) pointed out about +IN of the OPamp and having resistor of 10k-100k from +IN to GND.
EXTREAMLY IMPORTANT!!!! .
Do you have that resistor in place? If not, do that first PLEASE. It'll save you and everybody else so much time and frustration.
DrSplodge said:
It means it's a Non-Inverting amp the way you describe it.
If you have a 43K/43K resistors your gain will be 1. I'm not sure why you need the preamp at all. What kind of power amp do you have?
Lastly it's interesting to play with opamps but it'll help you a lot to read just a little bit about them and how they work. You really don't need much details.
Try this:
http://www.williamson-labs.com/480_opam.htm
Or the following one. (it's way more than you need, but study it. It's the basics of OPAMPS)
http://www.national.com/appinfo/amps/files/Opamp_Trivia_WEBCAST_FINAL.pdf
You can add bleeder resistors across the supply rails, so the discharge quicker. Also, if this bother you then add a blocking cap on the output, place the cut off pont of the filter at about 15-20hz.
Are just shut you amp off first.
Are just shut you amp off first.
Op amp behaviour.
I have never come across an opamp whose output jumps up or down by several volts when powered down . This kind of situation would however occur if it is dc coupled to some other stage which causes a dc shift at the input of the opamp.
It does happen in some discrete circuits.
In the small pic you posted , you said that you have no resistor from the -ve input to the output. I don't know how the rest of the circuit is configured , but with one cap as shown and no resistor to the output the input dc offset will be multiplied several times and appear at the output. Additionally we don't see any path from the +ve input to ground. This is generally mandatory but we can't see the rest of the circuit.
Without knowing how the opamp fits into the rest of the circuit it is hard to make any meaningful suggestion. If this is a stand alone opamp and not in a feed back loop , you should have - as a minimum - a path from the + input to ground ( a large resistor) and a path from the -ve input to output . A resistor from the -ve input to ground will increase the dc and ac gain of this stage. Potentially more problematic.
Post the full circuit to get a proper solution.
Cheers,
Ashok.
I have never come across an opamp whose output jumps up or down by several volts when powered down . This kind of situation would however occur if it is dc coupled to some other stage which causes a dc shift at the input of the opamp.
It does happen in some discrete circuits.
In the small pic you posted , you said that you have no resistor from the -ve input to the output. I don't know how the rest of the circuit is configured , but with one cap as shown and no resistor to the output the input dc offset will be multiplied several times and appear at the output. Additionally we don't see any path from the +ve input to ground. This is generally mandatory but we can't see the rest of the circuit.
Without knowing how the opamp fits into the rest of the circuit it is hard to make any meaningful suggestion. If this is a stand alone opamp and not in a feed back loop , you should have - as a minimum - a path from the + input to ground ( a large resistor) and a path from the -ve input to output . A resistor from the -ve input to ground will increase the dc and ac gain of this stage. Potentially more problematic.
Post the full circuit to get a proper solution.
Cheers,
Ashok.
Thanks guys!
Haven't solved, but I've fixed the problem and the amp now works.
I grounded the OPA using the ground from the shielding of the input, and introduced relays to connect and disconnect the speaker output terminals.
Works well : )
Haven't solved, but I've fixed the problem and the amp now works.
I grounded the OPA using the ground from the shielding of the input, and introduced relays to connect and disconnect the speaker output terminals.
Works well : )
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