Hi!
I recently built Rod Elliot's p88 preamplifier. https://sound-au.com/project88.htm
I love his designs and boards, the thing is, one of the channels of the preamp does not output the preamplified signal.
To troubleshoot this I injected a 1Khz sin signal in R In and L In
The following first gain stage works fine for both channels and I see the 1Khz sin signal both in AL and AR (measured with my oscilloscope).
The balance and volume controls also work great, and I get an adjustable 1Khz sin signal in BL or BR , depending on the channel where I inject the input signal:
The problem starts in the second stage. U2B (OPA2134) works great and I get the preamplified R Out signal, whose gain is adjustable by Gain R switches.
However, I can't get a preamplified signal out of U2A pin 1.
Pin 3 shows the input 1khz singal, pin 8 and 4 show +15v and -15v, but pin 2 and pin 1 read 0v, no signal present.
What could cause this? I checked the board and all components are placed in the right place. Can one of OPA2134's opamps be faulty, while the other is working fine?
Any ideas of how to further troubleshoot this, in circuit? I soldered U2 in place without a socket, and would love to hear some of your advice before desoldering and soldering a new one.
Your help is strongly appreciated!
Thank you so much!!
Fran
I recently built Rod Elliot's p88 preamplifier. https://sound-au.com/project88.htm
I love his designs and boards, the thing is, one of the channels of the preamp does not output the preamplified signal.
To troubleshoot this I injected a 1Khz sin signal in R In and L In
The following first gain stage works fine for both channels and I see the 1Khz sin signal both in AL and AR (measured with my oscilloscope).
The balance and volume controls also work great, and I get an adjustable 1Khz sin signal in BL or BR , depending on the channel where I inject the input signal:
The problem starts in the second stage. U2B (OPA2134) works great and I get the preamplified R Out signal, whose gain is adjustable by Gain R switches.
However, I can't get a preamplified signal out of U2A pin 1.
Pin 3 shows the input 1khz singal, pin 8 and 4 show +15v and -15v, but pin 2 and pin 1 read 0v, no signal present.
What could cause this? I checked the board and all components are placed in the right place. Can one of OPA2134's opamps be faulty, while the other is working fine?
Any ideas of how to further troubleshoot this, in circuit? I soldered U2 in place without a socket, and would love to hear some of your advice before desoldering and soldering a new one.
Your help is strongly appreciated!
Thank you so much!!
Fran
I can't see anything that could cause it other than a faulty opamp. If the other half of the opamp works, then it is not soldered upside down. A correct opamp without a signal would show some small DC offset, 1-2mV. Do not run away from sockets for opamps, they always allow easy replacement. Tomorrow you'll want a better opamp, I'm almost convinced.
If pin 2 is connected to ground, then you will get this issue.
Check those R7 resistors and their connections.
Check those R7 resistors and their connections.
Thank you guys, I'll recheck all R7 resistors and the connections around Pin2. I reckon I already checked it and was not connected to ground, but will double check!
Then I don't know what to say. If you have sockets, I would say switch the opamps, it's awkward like this when they are soldered. When I try anything, I have several (fake but working) TL071 and TL072, and check everything with that. Even if it is damaged, it is not expensive.
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absolutely!
nor does it cause sadness that a "good" op-amp has died....current limiting when testing should be mandatory.
Thank you guys, it was a bad opa2134, I desoldered it, soldered a socket in place and placed a ne5532 and both channels now work just fine !! It sounds fine, no noise or hum, so I guess that’s it!
One thing I noticed now, however is that my oscilloscope screen shows the amplified input signal at the preamp outputs, but it flickers a little, and vertical pikes fast appear and disappear on screen. This did not happen with the working channel of the opa2134. A steady sin wave appeared at the preamp output. Does this have something to do with ne5532?
I’m quite a newbie at all this, and just wanted to make sure .
Thank you!!
One thing I noticed now, however is that my oscilloscope screen shows the amplified input signal at the preamp outputs, but it flickers a little, and vertical pikes fast appear and disappear on screen. This did not happen with the working channel of the opa2134. A steady sin wave appeared at the preamp output. Does this have something to do with ne5532?
I’m quite a newbie at all this, and just wanted to make sure .
Thank you!!
I think that has to do with oscilloscope. They flicker sometimes.
Think your amplifier works correctly.
Glad you solved your issue.
Think your amplifier works correctly.
Glad you solved your issue.