I have built a prototype implementation of the PFF circuit using the TPA3126 chip. Just to remind you, the preamp stage is tube-based, utilizing nuvistors produced in the former USSR—specifically, 6S51N-V triodes (the "V" suffix indicates a military-grade version rather than a standard civilian type). In my opinion, I have achieved the intended goals. The PCB design will undergo minor adjustments.
During measurements, I used a load consisting of four 33 ohm, 20 W cermet resistors connected in parallel, to which I added a voltage divider composed of two 47 ohm, 5 W resistors and one 27 ohm, 5 W resistor. The measured output voltage was taken across the 27 ohm resistor. This 27-ohm resistor is paralleled with a 33 nF MKT capacitor, forming a first-order filter with a cutoff frequency of approximately 51 kHz.
The measuring device used was the QuantAsylum QA-403.
First, the frequency response. The drive level was set to -3 dB (with 0 dB being the threshold at which distortion begins to occur). The frequency response measurement uses a signal known as expo chirp (it provides a slightly sharper response line in the upper part of the audio band compared to the method based on white noise):
FFT analysis at a frequency of 1 kHz, signal level at -3 dB. Frequency scale shown in both linear and logarithmic formats:
FFT analysis at frequencies of 125 Hz and 8 kHz:
FFT analysis at 1 kHz and a drive level of -3 dB, using A-weighting:
The SNR value was 95.2 dB, and 105.3 dB when using A-weighting. These values were measured with a 1 kHz sine wave input signal at a drive level of -3 dB.
FFT analysis for a multitone test at a drive level of -20 dB:
Time for THD values broken down by harmonic components, at a drive level of -3 dB and for the following frequencies: 125 Hz, 1 kHz, and 8 kHz:
The slight "notch" visible just below 1 kHz in the FFT analysis is, in my opinion, a remnant of a minor instability in the voltage regulation loop of the boost converter that supplies 54 V to the tube-based input stages.
I find it difficult to interpret the results of the multitone test conclusively.
As for the THD levels and the distribution of harmonics, a clear increase can be observed with rising frequency. However, the distortion is lower than the values specified in the TPA3126 datasheet (although the curve needs to be extrapolated as a parabola, since the sharp 24 kHz low-pass filter suppresses the higher-order harmonics).
Once again, I’d like to remind that the input stage is tube-based, which contributes additional distortion — notably, a significant second harmonic component is evident, which aligns with the design goals.
Now I'm waiting for the proverbial cold shower — or perhaps even a bucket of scorn poured over my head. I welcome all comments and feedback.
Best regards, Tomasz.
During measurements, I used a load consisting of four 33 ohm, 20 W cermet resistors connected in parallel, to which I added a voltage divider composed of two 47 ohm, 5 W resistors and one 27 ohm, 5 W resistor. The measured output voltage was taken across the 27 ohm resistor. This 27-ohm resistor is paralleled with a 33 nF MKT capacitor, forming a first-order filter with a cutoff frequency of approximately 51 kHz.
The measuring device used was the QuantAsylum QA-403.
First, the frequency response. The drive level was set to -3 dB (with 0 dB being the threshold at which distortion begins to occur). The frequency response measurement uses a signal known as expo chirp (it provides a slightly sharper response line in the upper part of the audio band compared to the method based on white noise):
FFT analysis at a frequency of 1 kHz, signal level at -3 dB. Frequency scale shown in both linear and logarithmic formats:
FFT analysis at frequencies of 125 Hz and 8 kHz:
FFT analysis at 1 kHz and a drive level of -3 dB, using A-weighting:
The SNR value was 95.2 dB, and 105.3 dB when using A-weighting. These values were measured with a 1 kHz sine wave input signal at a drive level of -3 dB.
FFT analysis for a multitone test at a drive level of -20 dB:
Time for THD values broken down by harmonic components, at a drive level of -3 dB and for the following frequencies: 125 Hz, 1 kHz, and 8 kHz:
The slight "notch" visible just below 1 kHz in the FFT analysis is, in my opinion, a remnant of a minor instability in the voltage regulation loop of the boost converter that supplies 54 V to the tube-based input stages.
I find it difficult to interpret the results of the multitone test conclusively.
As for the THD levels and the distribution of harmonics, a clear increase can be observed with rising frequency. However, the distortion is lower than the values specified in the TPA3126 datasheet (although the curve needs to be extrapolated as a parabola, since the sharp 24 kHz low-pass filter suppresses the higher-order harmonics).
Once again, I’d like to remind that the input stage is tube-based, which contributes additional distortion — notably, a significant second harmonic component is evident, which aligns with the design goals.
Now I'm waiting for the proverbial cold shower — or perhaps even a bucket of scorn poured over my head. I welcome all comments and feedback.
Best regards, Tomasz.
Your distortion spectra do not look that impressive. Maybe your coupling network to the analyzer is the culprit.
As the bridged output is symmetrical, your attenuator should be designed strictly symmetrical.
Otherwise you measure additional THD due to common mode distortion -
which is nullifyed in the bridge configuration.
As the bridged output is symmetrical, your attenuator should be designed strictly symmetrical.
Otherwise you measure additional THD due to common mode distortion -
which is nullifyed in the bridge configuration.
Schema of probe between balanced outputs of power amplifier and balanced inputs of meter - analyser.
The distortion spectrum is a combination of distortions introduced by the Class D power stage with the PFF loop, and those contributed by the tube-based preamp stage.
An additional note: this is not at 1W output power, but rather at around 16W (i.e., -3 dB from maximum output).
The tube stage’s signature is visible in the relatively strong second harmonic, as well as in the third.
The power stage contributes primarily to the 5th and higher-order harmonics.
There is a clear pattern—odd-order harmonics dominate, which I believe result from the influence of the filter inductor cores.
In the simulations, ideal inductors were used.
The higher odd-order harmonics of 1kHz are at around -90 dB, corresponding to approximately 0.003%. For 8kHz are at around -60 dB corresponding to approximately 0.003%.
According to the TPA3126 datasheet, for 16W output power, the THD is 0.02% at 1 kHz and about 0.2% at 6 kHz (contains only 2, 3 harmonic!), based on measurements from the evaluation board.
Furthermore, the higher even-order harmonics are about 12 dB lower, i.e., below 0.0008%.
I would need to modify the test circuit in order to bypass the tube stage entirely.
Alternatively, I might experiment with changing the modulation scheme.
The distortion spectrum is a combination of distortions introduced by the Class D power stage with the PFF loop, and those contributed by the tube-based preamp stage.
An additional note: this is not at 1W output power, but rather at around 16W (i.e., -3 dB from maximum output).
The tube stage’s signature is visible in the relatively strong second harmonic, as well as in the third.
The power stage contributes primarily to the 5th and higher-order harmonics.
There is a clear pattern—odd-order harmonics dominate, which I believe result from the influence of the filter inductor cores.
In the simulations, ideal inductors were used.
The higher odd-order harmonics of 1kHz are at around -90 dB, corresponding to approximately 0.003%. For 8kHz are at around -60 dB corresponding to approximately 0.003%.
According to the TPA3126 datasheet, for 16W output power, the THD is 0.02% at 1 kHz and about 0.2% at 6 kHz (contains only 2, 3 harmonic!), based on measurements from the evaluation board.
Furthermore, the higher even-order harmonics are about 12 dB lower, i.e., below 0.0008%.
I would need to modify the test circuit in order to bypass the tube stage entirely.
Alternatively, I might experiment with changing the modulation scheme.
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By omitting the second harmonic (and assuming its value equals the arithmetic average of the remaining even-order harmonics, i.e., -105 dB), we obtain a THD level of 0.005%.
This is four times lower than the value given in the datasheet, which is 0.02%.
The depth of the PFF loop corresponds to a reduction factor of 4.9.
In my opinion, this result shows good agreement with expectations.
This is four times lower than the value given in the datasheet, which is 0.02%.
The depth of the PFF loop corresponds to a reduction factor of 4.9.
In my opinion, this result shows good agreement with expectations.
Yes, because the analyzer—not only containing an ADC for measuring the amplifier's output signal—also includes a DAC that provides the input signal to the amplifier. In this case, the signal is unbalanced (single-ended), meaning the analyzer’s ground is connected to the amplifier.
If the generator (DAC) and the analyzer (ADC) were separate, galvanically isolated devices, then both resistors wouldn’t necessarily have to be identical.
Of course, I used commercial cermet resistors, which don’t have ultra-tight tolerance (e.g., 1%).
Nevertheless, we see that the higher even-order harmonics are approximately 12 dB lower than the higher odd-order harmonics.
For a quick and reasonably simplified assessment, in my opinion, this approach is sufficient.
If I manage to get a QA-451 (load and 7th-order 67 kHz low-pass filter with a balanced measurement probe), I plan to deliberately build a test setup without the tube preamp stage (using OPA862 and TPA3126) and measure it.
In the post https://www.diyaudio.com/community/...timizing-pffb-for-tpa3245.415655/post-8018330, I mistakenly pasted the FFT spectrum for 8 kHz instead of 1 kHz.
Apologies for that.
Here is the correct plot, now with a logarithmic frequency scale.
Apologies for that.
Here is the correct plot, now with a logarithmic frequency scale.
Considering your output level being close to clipping results including tube pre look not bad at all.
Higher harmonics are typical for TPA3118, they will decrease with lower output levels.
Getting lower THD than TI datasheet is to be expected, as your inductors are better than the ones on te EVA board.
And keep in mind that THD spec is a guaranteed limit, not a typical lab measurement value.
Higher harmonics are typical for TPA3118, they will decrease with lower output levels.
Getting lower THD than TI datasheet is to be expected, as your inductors are better than the ones on te EVA board.
And keep in mind that THD spec is a guaranteed limit, not a typical lab measurement value.
I should add that the graphs shown in the datasheet are actually described as EVM measurements.
However, I have a question regarding the multitone test result (performed at –20 dB drive level). I received a comment in response to the graph and would be interested to hear your evaluation of it.
Multitone Test Summary
A 51-tone multitone signal was used to evaluate the linearity and distortion performance of the amplifier under test, which includes a vacuum tube input stage and a TPA3126 Class D output stage with PFFB (Post-Filter Feedback). The drive level was set to –20 dB relative to full-scale output.
- Fundamental peaks: ~–25 dBFS
- Noise floor:
- At low frequencies (~100 Hz): ranges from –92 dBFS to –120 dBFS
- Above 2 kHz: significantly lower floor, reaching –145 dBFS
- Spurious tones: Present between harmonics, typically no higher than –85 dBFS
- FFT window used: Hann (fixed, not user-selectable)
- SNR at –3 dBFS drive:
- Unweighted: 95.2 dB
- A-weighted: 105.3 dB
The noise floor shape above 2 kHz closely resembles the amplifier’s intrinsic noise spectrum, indicating a clean signal path. The slight rise of the noise floor at low frequencies is consistent with 1/f noise and the influence of the vacuum tube stage. The presence of low-level intermodulation products between tones is acceptable and expected given the windowing limitations and test configuration.
Overall, the multitone test confirms very low distortion, high dynamic range, and a clean spectral response, especially in the mid and high-frequency ranges. The performance aligns well with expectations for a properly implemented PFFB topology and high-SNR design.
That is a lot of data - it would be helpful to see the data with tube input stage bypassed and using a reference output level closer to listening volumes rather than -3dB away from max (clip). Say the usual 2.83vrms into 8ohm load (1W). I see the 2nd harmonic at -30dB or -40dB and that is quite high (1%) and I wonder if that is dominated by the tube. I generally like to see THD about 0.06% max. The decreasing higher orders is good though.
Hi y'all, i have to second xrk - these are not very impressive thd numbers.
What i suspect: second harmonic is coming from the tube pre. Tubes are said the produce a significant amount of even order thd, that is what makes their sound.
The third order harmonic can stem from the actual measurement system, especially the load resistors are sometimes tricky due to their voltage/resistance nonlinearity.
Last time i did amp measurements i tried three different load resistors with three different results. The worst ones were the non-inductive ceramic types, some cheap chinese ones did actually perform quite well. (Green wirewound types with 25W)
In the end i did settle for 6 150W types https://www.widap.com/en/products/wire-wound-resistors-wd/
The Multitone Test looks quite ok on the other hand. Did you do dualtone tests?
What i suspect: second harmonic is coming from the tube pre. Tubes are said the produce a significant amount of even order thd, that is what makes their sound.
The third order harmonic can stem from the actual measurement system, especially the load resistors are sometimes tricky due to their voltage/resistance nonlinearity.
Last time i did amp measurements i tried three different load resistors with three different results. The worst ones were the non-inductive ceramic types, some cheap chinese ones did actually perform quite well. (Green wirewound types with 25W)
In the end i did settle for 6 150W types https://www.widap.com/en/products/wire-wound-resistors-wd/
The Multitone Test looks quite ok on the other hand. Did you do dualtone tests?
I had a different experience with the green wire wound resistors - big 100w ones were terrible and added significant THD to measurement. Best resistor is a flat thin film with heatsink capability made by EBC. These are 300W and can be found in 10ohm values on eBay for a reasonable price. They are absolutely the best - I get the lowest THD with them and coupled with a CPU cooler heatsink fins with fan, lets you drive them hard.
EBC UXP-300 is the model I think. They look like this:
EBC UXP-300 is the model I think. They look like this:
The test board is not exactly modification-friendly. On top of that, there are issues with MKS2 Wima capacitors (zero stock availability and even restrictions on sales to Poland, where I live). As for testing, unfortunately the QA-403 software does not support typical dual-tone IMD tests with freely selectable frequencies. Only the multitone test is available. Yes, it is possible to generate signals externally and feed them into the QA-403, but that adds complexity.
There is also one more thing to investigate: the influence of signals outside the DAC's sampling frequency range, as they can potentially introduce spurious tones in the FFT spectrum within the audio band. So, one would need to ground the inputs of the OPA862 and capture the FFT spectrum both with and without power supplied to check for any residual switching artifacts affecting the DAC.
I understand that you are interested in results without the tube preamp stage, so I’ll need to prepare modifications to the test board accordingly.
Regarding the distortion level introduced by the tube: around 1%, mostly second harmonic, with the third harmonic at around 0.1%, followed by a rapid drop-off to levels below 0.01% — in my opinion, this adds a desirable “coloration” that is not perceived negatively by our auditory system. The second harmonic level drops roughly linearly with the decrease in signal level. The output power at which odd-order harmonics begin to appear is around 33W into 8 ohms. The proposed 1W measurement would correspond to roughly 1/33 of that power, or about 17% of the voltage drive. I estimate that the second harmonic level — and thus the resulting THD — would be around 0.2%.
If I may add a personal opinion: very low levels of second harmonic distortion hardly affect the sound character by adding a “tube signature.”
In the meantime, I’ve been working on a revised amplifier concept using a different tube – the 6N28B-V, a frame-grid subminiature dual triode with characteristics similar to rod tubes. This change allows a new topology: one gain stage, a split-load phase inverter, and op-amp-based buffers. Due to the relatively low B+ supply (around 50–55 V), tube-based cathode followers would not provide low enough output impedance for proper PFF symmetry when driving the TPA3126.
The updated design includes a minor frequency response correction in the preamp stage and slight tuning of the PFF loop components. Since loudspeakers are complex, non-standardized loads (composed of various R, L, and C elements), I simulated the frequency response over a wide range of RL combinations as a practical compromise.
The simulation used R values from 3.9 Ω to 68 Ω (E12 series) and inductances of: 1 nH, 1 µH, 2.2 µH, 4.7 µH, 6.8 µH, and from 10 µH up to 6.8 mH (also E12 series). This results in a wide tolerance band for frequency response. The worst-case -0.5 dB bandwidth occurs with L ≈ 30 µH and low series resistance, falling to ~16 kHz. For higher inductance values, the -0.5 dB bandwidth may improve, but a gradual gain drop can already be seen from 1 kHz upward. Things look better when considering the -3 dB point, which gives a more relaxed margin. See the attached graph and simplified schematic used for the simulation.
I’m fully aware that modeling the load as a simple RL network does not fully reflect the behavior of a real loudspeaker system. However, generating a meaningful tolerance field for frequency response based on actual loudspeaker realizations — even for a hypothetical "average" batch of typical two-way or three-way crossover designs — is not feasible within a reasonable time and effort. After all, where would one even obtain a comprehensive dataset covering the construction variants and crossover topologies used in commercial loudspeakers?
So, at some point, a compromise has to be made — even if it's not ideal, it’s a pragmatic approach that still provides useful insights into how the amplifier behaves under a range of reactive loads.
There is also one more thing to investigate: the influence of signals outside the DAC's sampling frequency range, as they can potentially introduce spurious tones in the FFT spectrum within the audio band. So, one would need to ground the inputs of the OPA862 and capture the FFT spectrum both with and without power supplied to check for any residual switching artifacts affecting the DAC.
I understand that you are interested in results without the tube preamp stage, so I’ll need to prepare modifications to the test board accordingly.
Regarding the distortion level introduced by the tube: around 1%, mostly second harmonic, with the third harmonic at around 0.1%, followed by a rapid drop-off to levels below 0.01% — in my opinion, this adds a desirable “coloration” that is not perceived negatively by our auditory system. The second harmonic level drops roughly linearly with the decrease in signal level. The output power at which odd-order harmonics begin to appear is around 33W into 8 ohms. The proposed 1W measurement would correspond to roughly 1/33 of that power, or about 17% of the voltage drive. I estimate that the second harmonic level — and thus the resulting THD — would be around 0.2%.
If I may add a personal opinion: very low levels of second harmonic distortion hardly affect the sound character by adding a “tube signature.”
In the meantime, I’ve been working on a revised amplifier concept using a different tube – the 6N28B-V, a frame-grid subminiature dual triode with characteristics similar to rod tubes. This change allows a new topology: one gain stage, a split-load phase inverter, and op-amp-based buffers. Due to the relatively low B+ supply (around 50–55 V), tube-based cathode followers would not provide low enough output impedance for proper PFF symmetry when driving the TPA3126.
The updated design includes a minor frequency response correction in the preamp stage and slight tuning of the PFF loop components. Since loudspeakers are complex, non-standardized loads (composed of various R, L, and C elements), I simulated the frequency response over a wide range of RL combinations as a practical compromise.
The simulation used R values from 3.9 Ω to 68 Ω (E12 series) and inductances of: 1 nH, 1 µH, 2.2 µH, 4.7 µH, 6.8 µH, and from 10 µH up to 6.8 mH (also E12 series). This results in a wide tolerance band for frequency response. The worst-case -0.5 dB bandwidth occurs with L ≈ 30 µH and low series resistance, falling to ~16 kHz. For higher inductance values, the -0.5 dB bandwidth may improve, but a gradual gain drop can already be seen from 1 kHz upward. Things look better when considering the -3 dB point, which gives a more relaxed margin. See the attached graph and simplified schematic used for the simulation.
I’m fully aware that modeling the load as a simple RL network does not fully reflect the behavior of a real loudspeaker system. However, generating a meaningful tolerance field for frequency response based on actual loudspeaker realizations — even for a hypothetical "average" batch of typical two-way or three-way crossover designs — is not feasible within a reasonable time and effort. After all, where would one even obtain a comprehensive dataset covering the construction variants and crossover topologies used in commercial loudspeakers?
So, at some point, a compromise has to be made — even if it's not ideal, it’s a pragmatic approach that still provides useful insights into how the amplifier behaves under a range of reactive loads.