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Hi!
I am experiencing what I believe is a measurement issue which has me a bit perplexed, so figured I'd come to the measurements experts
I recently completed a tube / FET hybrid phono stage, schematic below. It is being fed from the balanced outputs of my turntable, balanced input to a Lundahl LL9226XL 1:10 MC SUT transformer, through a triode-strapped D3a gain stage, then an all-in-one RIAA EQ, then a triode-strapped EF86 gain stage, followed by a FET output buffer.
The issue I am having is taking measurements to determine adherence to the RIAA curve such that I can make necessary adjustments. The heart of my measurements setup is a MOTU M4 audio interface. I have approached the measurements in two different ways:
1) Using a inverse RIAA filter with built-in voltage dividers to provide either MM or MC level voltages via an on-board switch
2) Running an unfiltered signal into the phono pre across a voltage divider, then applying the inverse RIAA curve via soundcard calibration in REW
In both instances, I am seeing a significant LF rolloff below 500Hz with the inverse RIAA applied. This would result in a very tinny sound which subjectively is not there, the bass is very full, I suspect it is a measurement anomaly. Below is the DSP applied inverse RIAA result in REW
Just to be sure, I did the following to rule out in-circuit high-pass points without a change in the LF rolloff:
-Bypassed the input transformers and fed MM level signal to the input of the D3a
-Added parallel capacitance to the 0.1uF coupling cap
-Added parallel capacitance to the 1.5uF output cap
I am using the line-level balanced output of the MOTU M4. Loopback at line level is flat as can be. So it seems there is some other high-pass being applied inadvertently. At this point, I need a second set of eyes, apologies if the answer is obvious, this is my first phono design. I appreciate the help.
I am experiencing what I believe is a measurement issue which has me a bit perplexed, so figured I'd come to the measurements experts
I recently completed a tube / FET hybrid phono stage, schematic below. It is being fed from the balanced outputs of my turntable, balanced input to a Lundahl LL9226XL 1:10 MC SUT transformer, through a triode-strapped D3a gain stage, then an all-in-one RIAA EQ, then a triode-strapped EF86 gain stage, followed by a FET output buffer.
The issue I am having is taking measurements to determine adherence to the RIAA curve such that I can make necessary adjustments. The heart of my measurements setup is a MOTU M4 audio interface. I have approached the measurements in two different ways:
1) Using a inverse RIAA filter with built-in voltage dividers to provide either MM or MC level voltages via an on-board switch
2) Running an unfiltered signal into the phono pre across a voltage divider, then applying the inverse RIAA curve via soundcard calibration in REW
In both instances, I am seeing a significant LF rolloff below 500Hz with the inverse RIAA applied. This would result in a very tinny sound which subjectively is not there, the bass is very full, I suspect it is a measurement anomaly. Below is the DSP applied inverse RIAA result in REW
Just to be sure, I did the following to rule out in-circuit high-pass points without a change in the LF rolloff:
-Bypassed the input transformers and fed MM level signal to the input of the D3a
-Added parallel capacitance to the 0.1uF coupling cap
-Added parallel capacitance to the 1.5uF output cap
I am using the line-level balanced output of the MOTU M4. Loopback at line level is flat as can be. So it seems there is some other high-pass being applied inadvertently. At this point, I need a second set of eyes, apologies if the answer is obvious, this is my first phono design. I appreciate the help.
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