I bought pcm58 boards and I am changing the opamps in one of them for opa1611 output.
I planned to use a trimpot of 100k to attenuate the signal. So far so good...
I traced the path of pin6 of output opamp (which is in buffer mode /filter).
The signal goes to the ground,
Then the + of the RCA has a 100R to ground.
Maybe it is late, I cannot figure it out. It must be simple.
It has a relay switches, I tried on and off, no difference.
On the schematic it says 100R in series from Pin6 to + of rca with 10K to ground.
There are the two resistors which both measure 100R, one of them could be the 10K and they are reading in // no clue
I planned to use a trimpot of 100k to attenuate the signal. So far so good...
I traced the path of pin6 of output opamp (which is in buffer mode /filter).
The signal goes to the ground,
Then the + of the RCA has a 100R to ground.
Maybe it is late, I cannot figure it out. It must be simple.
It has a relay switches, I tried on and off, no difference.
On the schematic it says 100R in series from Pin6 to + of rca with 10K to ground.
There are the two resistors which both measure 100R, one of them could be the 10K and they are reading in // no clue
Last edited:
You can't read R values in circuit. 100R reading to ground most likely due to reading through the opamp etc or perhaps through a mute relay that shunts the output to ground.
100k trim pot! Feedback or used as a conventional attenuator? If the latter then the value sounds way to high as it will give poor noise and HF performance. Probably not great as feedback either as it will pick up and add stray noise.
We need to see the circuit.
100k trim pot! Feedback or used as a conventional attenuator? If the latter then the value sounds way to high as it will give poor noise and HF performance. Probably not great as feedback either as it will pick up and add stray noise.
We need to see the circuit.
That is the schematic, however it doesn't match reality. The ground is floating.
I have other values of trimpot to use. I would like to place opa1611 to replace Ne5534 and directly drive my hd650, I know they should be sufficient.
I am planning to place a 15R resistance in series, and a 3K trimpot to ground. Thanks for pointing me the 100k was absurd.
Attachments
I did another set of readings on the dac with opamps removed.
The output of the dac OFF is sent to ground. There is a 200R reading between L/R + s .
And a 100R reading from R+ to Gnd, L+ to Gnd, 100 as well.
The output of the dac OFF is sent to ground. There is a 200R reading between L/R + s .
And a 100R reading from R+ to Gnd, L+ to Gnd, 100 as well.
The circuit you show looks conventional enough but the grounds have to be referenced to 'something'. Digital gear often has separate grounds for digital stuff, analogue stuff and noisy high current servo's and so on. Ultimately though, the audio grounds have to be referenced to the analogue ground part of the DAC chip.
When you connect the outputs to an amp the L and R grounds also connect together via the interconnect leads.
If you use the OPA chip it would be wise to remove the 27pF compensation cap even though the data sheets says no internal connections to those pins.
15 ohm where? and where would the trimmer go? The wiper of the preset should only be loaded with a highish impedance.
The 100 ohm and 1000pF cap form a low pass filter although the -3db point is over 1.5MHz. It will be some form of RF filter to remove extremely high frequency noise and hash.
The final output from the op-amps is shunted to ground when off. That is normal practice and the best way to achieve muting rather than a relay in series with the audio.
When you connect the outputs to an amp the L and R grounds also connect together via the interconnect leads.
If you use the OPA chip it would be wise to remove the 27pF compensation cap even though the data sheets says no internal connections to those pins.
15 ohm where? and where would the trimmer go? The wiper of the preset should only be loaded with a highish impedance.
The 100 ohm and 1000pF cap form a low pass filter although the -3db point is over 1.5MHz. It will be some form of RF filter to remove extremely high frequency noise and hash.
The final output from the op-amps is shunted to ground when off. That is normal practice and the best way to achieve muting rather than a relay in series with the audio.
Hi Mooly, I removed the resistors on the board and they are indeed 100R and 10K.
The relay ground the opamp output until voltages are sufficient.
------------------------------------------
Could the output Z of the opamp which is near 0R explains the readings????
------------------------------------------
After reading your posts:
I plan to remove the 1nf
I plan to reduce 100R to 15R to drive directly the headphones or main amp.
The 10K could be replaced with 100K trimpot? which would be in // with the amp trimpot which is also 100K I believe.
My other amp is a tube amp with 100K trimpot then input capacitor to tube grid.
My main amp has a 100k or 50k wiper with 700khz input filter.
Ideally a 3K trimpot would get some volume down for the headphones directly but would be too low and cut bass for the main amps.
The relay ground the opamp output until voltages are sufficient.
------------------------------------------
Could the output Z of the opamp which is near 0R explains the readings????
------------------------------------------
After reading your posts:
I plan to remove the 1nf
I plan to reduce 100R to 15R to drive directly the headphones or main amp.
The 10K could be replaced with 100K trimpot? which would be in // with the amp trimpot which is also 100K I believe.
My other amp is a tube amp with 100K trimpot then input capacitor to tube grid.
My main amp has a 100k or 50k wiper with 700khz input filter.
Ideally a 3K trimpot would get some volume down for the headphones directly but would be too low and cut bass for the main amps.