Hi Guys n Gals.
Friend has a late model P3 pre.
He's wondering what and how the trimpots TVR1 TVR2 TVR3 TVR4 should be adjusted.
His balance is of to one side.
Thanks.
PS it seems Parasound have been bought out, When did that happen, and who owns them now ??
Friend has a late model P3 pre.
He's wondering what and how the trimpots TVR1 TVR2 TVR3 TVR4 should be adjusted.
His balance is of to one side.
Thanks.
PS it seems Parasound have been bought out, When did that happen, and who owns them now ??
Well, you have the schematic, so why not just have a look at it?
TVR1 and TVR2 set the common-mode output voltage for the XLR output driver:
TVR3 and TVR4 adjust the common-mode rejection on the XLR inputs:
You'll notice that it looks a lot like Figure 6 here: https://en.wikipedia.org/wiki/Differential_amplifier
Tom
TVR1 and TVR2 set the common-mode output voltage for the XLR output driver:
TVR3 and TVR4 adjust the common-mode rejection on the XLR inputs:
You'll notice that it looks a lot like Figure 6 here: https://en.wikipedia.org/wiki/Differential_amplifier
Tom
Oh... And the second part of your question: I would leave those pots alone. If your friend has already F...utzed with them you need to realign the circuit.
For TVR1 and TVR2, apply a, say, 1 kHz sine wave to any of the inputs, select that input, and crank the volume control to around unity gain. Measure the output of XLR pin 2 and XLR pin 3 with an oscilloscope and adjust the trimpot until the two waveforms are symmetric around 0 V. You may be able to do even better if you use the scope to invert one channel (say invert pin 3) and adjust until the two signals overlap.
I don't know for sure that this approach will work, but that's what I'd try to start.
TVR3 and TVR4 are easier: Apply a common-mode signal, i.e., connect XLR pins 2 and 3 to the same signal source. Apply a 10 kHz square wave with, say, 10 Vpp amplitude and adjust the trimpot for minimum signal at the output of the diff amp.
The best scenario is if your friend hasn't touched those trimpots. Then you just leave them alone.
Tom
For TVR1 and TVR2, apply a, say, 1 kHz sine wave to any of the inputs, select that input, and crank the volume control to around unity gain. Measure the output of XLR pin 2 and XLR pin 3 with an oscilloscope and adjust the trimpot until the two waveforms are symmetric around 0 V. You may be able to do even better if you use the scope to invert one channel (say invert pin 3) and adjust until the two signals overlap.
I don't know for sure that this approach will work, but that's what I'd try to start.
TVR3 and TVR4 are easier: Apply a common-mode signal, i.e., connect XLR pins 2 and 3 to the same signal source. Apply a 10 kHz square wave with, say, 10 Vpp amplitude and adjust the trimpot for minimum signal at the output of the diff amp.
The best scenario is if your friend hasn't touched those trimpots. Then you just leave them alone.
Tom
I truly appreciate your reply.
As far as I know he has not touched them, So I will advise Not to touch them.
Short version, I'm working three jobs, get ~6 hours sleep when I'm lucky, and am bordering on mental fatigue.
That doesn't include the work I have been turning away.
House etc is all me, I am me, there is no one else.
The list of things I need to do for myself is, extensive, and keeps growing.
If I could I would outsource my life 🙂
So, Thank you, I honestly really do appreciate your reply 👍
As far as I know he has not touched them, So I will advise Not to touch them.
Short version, I'm working three jobs, get ~6 hours sleep when I'm lucky, and am bordering on mental fatigue.
That doesn't include the work I have been turning away.
House etc is all me, I am me, there is no one else.
The list of things I need to do for myself is, extensive, and keeps growing.
If I could I would outsource my life 🙂
So, Thank you, I honestly really do appreciate your reply 👍
Update.................
Short version.
new fault - xlr outputs fine, rca outputs dead.
repair - replacing u15 restored rca outputs, also fixed his original issue with off center levels.
Full version.
p3 owner has previously used its rca outputs with no issues, but normally only uses the xlr outputs.
friend visited p3 owner, plugged in an rca driven home built amplifier, zero action, no music, no noise, nothing.
they tried three different rca leads, three different amps, zero zilch nudda.
xlr outs into the two amps that have xlr inputs worked perfectly.
visitor went home, plugged his rca driven amp back into his system, and it works 100% no issues at all.
p3 owner handed p3 back to me for testing etc.
i ran a 1khz 1.2v sinewave into the direct 1 rca input.
chased it through the pre with a dmm using ac hz (fluke 179, dont have the room to setup my dso)
u15 has the 1khz signal on its outputs.
xlr outputs have the 1khz signal.
loop out, loop in, rca outputs have either a very low 50hz (not audible on any device plugged into the rca outs) or zero signal at all.
all grounds and earths tested and are 100% solid.
i replaced u15 thinking maybe the home brew amp spiked it.
but this is where im confused.
looking at the output side of the preamp.
signal goes into u15, signal comes out of u15, gets split into two stereo output feeds.
one pair feeds 100ohm resistors that feed the 4 opamps for the xlr output.
second pair feeds another 100ohm resistors that feeds the rca outputs.
replacing u15 has fixed the dead rca outputs.
the xlrs were and still are perfectly fine.
but, why and or how, can the xlrs be fine, and the rcas dead, when the two both get their signal feed from the same pins on the u15 opamp ??????
please help me understand 🙂
relevant section attached, a bit blurry, for a way better view, refer to the pdf in the first post.
Short version.
new fault - xlr outputs fine, rca outputs dead.
repair - replacing u15 restored rca outputs, also fixed his original issue with off center levels.
Full version.
p3 owner has previously used its rca outputs with no issues, but normally only uses the xlr outputs.
friend visited p3 owner, plugged in an rca driven home built amplifier, zero action, no music, no noise, nothing.
they tried three different rca leads, three different amps, zero zilch nudda.
xlr outs into the two amps that have xlr inputs worked perfectly.
visitor went home, plugged his rca driven amp back into his system, and it works 100% no issues at all.
p3 owner handed p3 back to me for testing etc.
i ran a 1khz 1.2v sinewave into the direct 1 rca input.
chased it through the pre with a dmm using ac hz (fluke 179, dont have the room to setup my dso)
u15 has the 1khz signal on its outputs.
xlr outputs have the 1khz signal.
loop out, loop in, rca outputs have either a very low 50hz (not audible on any device plugged into the rca outs) or zero signal at all.
all grounds and earths tested and are 100% solid.
i replaced u15 thinking maybe the home brew amp spiked it.
but this is where im confused.
looking at the output side of the preamp.
signal goes into u15, signal comes out of u15, gets split into two stereo output feeds.
one pair feeds 100ohm resistors that feed the 4 opamps for the xlr output.
second pair feeds another 100ohm resistors that feeds the rca outputs.
replacing u15 has fixed the dead rca outputs.
the xlrs were and still are perfectly fine.
but, why and or how, can the xlrs be fine, and the rcas dead, when the two both get their signal feed from the same pins on the u15 opamp ??????
please help me understand 🙂
relevant section attached, a bit blurry, for a way better view, refer to the pdf in the first post.
That's indeed odd. Unless by swapping U15 (which was providing the correct output according to your writeup) you "fixed" something else. Could the solder joints on the 100 Ω resistors be going bad?
It kinda smells like a bad connection that got jostled when the amp was moved and re-jostled when you "fixed" it. I would run a signal into the amp and monitor the outputs on the RCAs while pushing at the circuit board with the plastic handle of a screwdriver.
Tom
It kinda smells like a bad connection that got jostled when the amp was moved and re-jostled when you "fixed" it. I would run a signal into the amp and monitor the outputs on the RCAs while pushing at the circuit board with the plastic handle of a screwdriver.
Tom
It has me scratching my head thats for sure.
Im no genius by any stretch of the imagination, but i like to think i can understand some things.
I was sure it would be the resistors that feed the rcas that were damaged, or a blown earth to the rcas.
Initial testing had me poking and pushing on things while monitoring the output. nothing changed.
All four 100 ohm resistors and solder look fine and measure bang on 100ohms.
hes using it now and will return it at a later date for me to have a good long hard look at.
maybe when i get it back i will replace those resistors as a precautionary 'shotgun' against intermittent issues.
Im no genius by any stretch of the imagination, but i like to think i can understand some things.
I was sure it would be the resistors that feed the rcas that were damaged, or a blown earth to the rcas.
Initial testing had me poking and pushing on things while monitoring the output. nothing changed.
All four 100 ohm resistors and solder look fine and measure bang on 100ohms.
hes using it now and will return it at a later date for me to have a good long hard look at.
maybe when i get it back i will replace those resistors as a precautionary 'shotgun' against intermittent issues.
I shall do that when it returns.
No idea right now when that will be though.
I will report back (eventually) as to what I find.
No idea right now when that will be though.
I will report back (eventually) as to what I find.
Update.
I examined under a mag lamp, all good, heated and froze things, no change at all.
I put the old opamp in, and yup, xlr good, rca dead.
So a new opamp for u15, redid the resistor joints, and a few others in the area even though they looked good, and a few vias in that area.
And reports are its perfectly fine.
Small miracle I guess 🙂
Case closed.
I examined under a mag lamp, all good, heated and froze things, no change at all.
I put the old opamp in, and yup, xlr good, rca dead.
So a new opamp for u15, redid the resistor joints, and a few others in the area even though they looked good, and a few vias in that area.
And reports are its perfectly fine.
Small miracle I guess 🙂
Case closed.