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Old 8th January 2004, 05:05 AM   #1
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Default Preamp - Massive DC Offset (OPA2134)

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/data.../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!
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Old 8th January 2004, 07:03 AM   #2
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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
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Old 8th January 2004, 07:20 AM   #3
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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.
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Old 8th January 2004, 07:24 AM   #4
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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!
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Old 8th January 2004, 08:33 AM   #5
Nuuk is offline Nuuk  United Kingdom
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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.
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Old 8th January 2004, 09:53 AM   #6
matjans is offline matjans  Netherlands
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yup, same goes for me. op627 gives a higher voltage than op275 btw.
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Old 8th January 2004, 10:08 AM   #7
miguel2 is offline miguel2  Portugal
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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
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Old 8th January 2004, 02:05 PM   #8
GregGC is offline GregGC  Canada
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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
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Old 8th January 2004, 02:46 PM   #9
gfiandy is offline gfiandy  United Kingdom
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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.
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Old 8th January 2004, 08:59 PM   #10
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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 ; )
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