"Now, some people may prefer the (slightly) distorted sound". This is along the lines of what I'm getting at. There is no one THD figure. Real distortion profiles are a 3D graph of level, frequency, and spectrum. The graph shape varies with the distortion mechanism. The rate of distortion change is highly variable from a very gradual slope to a flat brick wall. There's just no way to discuss THD audibility simply.
However, when low enough, and in the presence of other audio signals that are 50+ dB higher in level, audibility literally becomes a function of masking.
However, when low enough, and in the presence of other audio signals that are 50+ dB higher in level, audibility literally becomes a function of masking.
Since there has been some questions on the availability of these boards. I am starting a group buy to see if there is enough interest to warrant a manufacturing run. If interested, please sign up. The group buy will close on Saturday, September 30, so be sure to sign up soon.
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Interesting exercise, but you are aware that many popular tweeters have H2 of only -30 dB H2, perhaps even as "bad" as -25 dB H2, at the low end of their passband?
The part list a.k.a. BOM for one balanced line receiver board (see post #6 for the schematic and board outline):
With the part numbers shown in the right column and today's prices on Mouser.com, the total cost of this BOM with 1% 1kOhm resistors is just shy of $11.
Just a reminder: the GB for these boards is closing tomorrow.
| Qty | Value | Device | Package | Names | Example P/N |
|---|---|---|---|---|---|
| 2 | LM4562 | Dual opamp | DIP8 | U1, U2 | LM4562NA/NOPB |
| 1 | 2M2 | Metal film resistor | Axial 0204 | R1 | MFR-12FTF52-2M2 |
| 2 | 47k | Metal film resistor | Axial 0204 | R2, R3 | MFR-12FTF52-47k |
| 2 | 12k | Metal film resistor | Axial 0204 | R4, R5 | MFR-12FTF52-12k |
| 4 | 1k | Precision metal film resistor | Axial 0204 | R6, R7, R14, R15/R16 | MFR-12FTF52-1k (1%) or CMF501K0000BEEB (0.1%) |
| 4 | 10k | Metal film resistor | Axial 0204 | R8, R9, R12, R13 | MFR-12FTF52-10k |
| 2 | 220k | Metal film resistor | Axial 0204 | R10, R11 | MFR-12FTF52-220k |
| 2 | 22R | Metal film resistor | Axial 0204 | R17, R18 | MFR-12FTF52-22R |
| 2 | 100R | Metal film resistor | Axial 0204 | R19, R20 | MFR-12FTF52-100R |
| 4 | 47u | Electrolytic capacitor | Radial D=5mm | C1, C2, C3, C4 | EEU-FC1C470B |
| 2 | 470p | NP0/C0G ceramic capacitor | Radial LS=5mm | C5, C6 | FG28C0G1H471JNT06 |
| 4 | 10u | Non-polar electrolytic capacitor | Radial LS=5mm | C7, C8, C9, C10 | ECE-A1VN100U |
With the part numbers shown in the right column and today's prices on Mouser.com, the total cost of this BOM with 1% 1kOhm resistors is just shy of $11.
Just a reminder: the GB for these boards is closing tomorrow.
Hi Alex,
I'm enclosing measurement results obtained with your PCB in two configurations: 2xLM4562 and OPA1656/OPA2156.
The measurement setup consisted of the Victor's oscillator with a 1656 parallel buffer, Hall topology notch filter (Groner PCB), Groner's 60dB LNA (0.39nV/rtHz) and a Cosmos ADC. The PSU was Jan Diddens's SilentSwitcher, itself supplied by a Power Bank.
The filter represents a load of about 1.8k, so at an output voltage of 2Vrms my measurement results should be comparable to yours at the first page of this thread.
The first measurement, made with 2xLM4562 on the PCB, produced the 2nd at -135dBc (-85.5 - 50 for the filter attenuation), which is the same as you obtained. The 3rd is at -158 in my case, some 18dB lower than your result.
The combination of OPA1656 at the input and OPA2156 as the diff. stage resulted in excellent figures for the 2nd and 3rd harmonics: -156 and -162dBc, respectively. Since the harmonics were just visible with the arithmetic averaging in REW, I had to resort to the coherent averaging (s. the 2nd att).
Now I'm fully aware of the fact that at these distortion levels I must take the results obtained with a grain of salt, but since TI measured the 2nd and 3rd harmonics of less than -160dBc at Vout of 3Vrms (s. Fig. 12 in the OPA1656 DS), the figures I obtained are at least not impossible.
It's a nice PCB you designed, Alex!
I'm enclosing measurement results obtained with your PCB in two configurations: 2xLM4562 and OPA1656/OPA2156.
The measurement setup consisted of the Victor's oscillator with a 1656 parallel buffer, Hall topology notch filter (Groner PCB), Groner's 60dB LNA (0.39nV/rtHz) and a Cosmos ADC. The PSU was Jan Diddens's SilentSwitcher, itself supplied by a Power Bank.
The filter represents a load of about 1.8k, so at an output voltage of 2Vrms my measurement results should be comparable to yours at the first page of this thread.
The first measurement, made with 2xLM4562 on the PCB, produced the 2nd at -135dBc (-85.5 - 50 for the filter attenuation), which is the same as you obtained. The 3rd is at -158 in my case, some 18dB lower than your result.
The combination of OPA1656 at the input and OPA2156 as the diff. stage resulted in excellent figures for the 2nd and 3rd harmonics: -156 and -162dBc, respectively. Since the harmonics were just visible with the arithmetic averaging in REW, I had to resort to the coherent averaging (s. the 2nd att).
Now I'm fully aware of the fact that at these distortion levels I must take the results obtained with a grain of salt, but since TI measured the 2nd and 3rd harmonics of less than -160dBc at Vout of 3Vrms (s. Fig. 12 in the OPA1656 DS), the figures I obtained are at least not impossible.
It's a nice PCB you designed, Alex!
Attachments
I guess I'm a bit late to this particular party, but just wanted to mention that you can use a T-type topology for input capacitors much like with resistors R1-3 (you could even put them directly in parallel). Doing so with values of e.g. 47p/470p/470p would have pushed high-frequency CMRR degradation substantially further out while making capacitor matching a lot less critical for CMRR performance up there.Here is the full schematic:
Except that caps arranged as a T wouldn't protect the input opamps from RF ingress, which is why the caps are there in the first place. (Instead, each of the two input opamos would see an identical RF input, and that's would not help their linearity.)
A better idea would be to bootstrap the middle point of both resistors and capacitors, as proposed by Bill Whitlock of Jensen Transformers - see the attached patent.
A better idea would be to bootstrap the middle point of both resistors and capacitors, as proposed by Bill Whitlock of Jensen Transformers - see the attached patent.
THAT1200 is (unfortunately) a not-so-linear implementation of the above patent, referred to as InGenius in the datasheet. If you don't have linearity to begin with, the input LPF probably doesn't matter as much.
Look at the patent instead, specifically at the "preferred embodiment" in Fig. 12 and its description, particularly page/column 10, lines 30-45. You can choose a larger midpoint-to-ground capacitor (the patient mentions 470pF) to provide better RF immunity and then bootstrap it to keep the CMRR high at low frequencies.
Look at the patent instead, specifically at the "preferred embodiment" in Fig. 12 and its description, particularly page/column 10, lines 30-45. You can choose a larger midpoint-to-ground capacitor (the patient mentions 470pF) to provide better RF immunity and then bootstrap it to keep the CMRR high at low frequencies.