Samuel Groner
Well-known member
Hi
I implemented design B from the "Mic Preamp Schematic Collection" thread (www.groupdiy.com/index.php?topic=20038) as I was interested in its performance and because I got asked for a layout from different people.
Here are the important files:
Schematic: monte_generoso_r1.pdf
Layout: monte_generoso_r1_PCB.pdf
Gain switch resistor values: monte_generoso_r1_gain_switch.pdf
The topology used allows complete freedom from electrolytic capacitors and DC servos, at the cost of not perfectly zero output offset. It turned out that DC precision was better than expected--the trimmer was needed for less than 100 uV input related offset correction. It drifts a bit but the output seems to stay within 10 mV offset all the time, which is perfectly fine I'd say.
Gain is set with a 24 position switch from 7.5 dB to 65 dB in 2.5 dB steps. Polarity is switched at the output with a relays.
And now a few measurements:
Normalised frequency response: monte_generoso_r1_freq.pdf
THD+N: monte_generoso_r1_thdn.pdf
CMRR: monte_generoso_r1_cmrr.pdf
Note that the frequency response is almost perfectly equal for all gain settings. Only at the highest frequency shown there is some indication that the first stage runs on somewhat reduced loop gain, although the -3 dB bandwidth of the first stage is still above 1 MHz. The bass response is very pretty--a direct result of having only one AC coupling capacitor pair in the signal path.
The THD+N plots show good performance as well. The residual consists mostly of noise, except for the highest gain where a faint of third harmonic starts to become visible above 5 kHz. If you want to compare this data to other preamps please note that the measurements were done at high bandwidth, somewhat higher-than-usual maximum gain (65 dB instead of 60 dB) and on the bench without any shielding. This will easily increase the THD+N reading by a factor of 4 or more at the maximum gain in contrast to other published data.
Slew rate is typically 110 V/us and EIN has been measured at about -129 dBu for a 150 Ohm source (20 kHz bandwidth, 65 dB gain) and -135 dBu with shorted input.
CMRR at low frequencies is the only thing I'm not absolutely happy with though it surely is enough for real world use and (way) better than many other preamps. The reason for the decreasing CMRR towards the lower range is mismatch in the input coupling capacitors--I didn't realise that this is that critical up to almost the upper end of the audio range. This can be figthed by tightly matching the capacitors C105 and C106 (e.g. by the addition of small compensation capacitors in order to avoid the need for buying a large batch of expensive 33 uF film capacitors). If I'd do another revision (not to be expected soon) I think I'd use the common-mode bootstrap method invented by Bill Whitlock to avoide the matching need.
On the other hand the input RFI filter is very effective indeed (at least as far as I can tell). Above the shown range CMRR very rapidly increases. The onset of this is just visible at 1 MHz. In addition to this I noticed that common-mode signals show very low waveform distortion even at the highest frequencies (not something I usually observe with transformerless designs). That's a result of having high slew-rate amplifiers and will account for low intermodulation distortion with spurious HF products.
Overall very pleasing results, I'd say. I was not able to give it a detailed listen yet (this will--for my convenience--have to wait until it got a case), but I would be surprised if this preamp wouldn't be very clean and transparent.
[Edit--some pictures: monte_generoso.html.]
Samuel
I implemented design B from the "Mic Preamp Schematic Collection" thread (www.groupdiy.com/index.php?topic=20038) as I was interested in its performance and because I got asked for a layout from different people.
Here are the important files:
Schematic: monte_generoso_r1.pdf
Layout: monte_generoso_r1_PCB.pdf
Gain switch resistor values: monte_generoso_r1_gain_switch.pdf
The topology used allows complete freedom from electrolytic capacitors and DC servos, at the cost of not perfectly zero output offset. It turned out that DC precision was better than expected--the trimmer was needed for less than 100 uV input related offset correction. It drifts a bit but the output seems to stay within 10 mV offset all the time, which is perfectly fine I'd say.
Gain is set with a 24 position switch from 7.5 dB to 65 dB in 2.5 dB steps. Polarity is switched at the output with a relays.
And now a few measurements:
Normalised frequency response: monte_generoso_r1_freq.pdf
THD+N: monte_generoso_r1_thdn.pdf
CMRR: monte_generoso_r1_cmrr.pdf
Note that the frequency response is almost perfectly equal for all gain settings. Only at the highest frequency shown there is some indication that the first stage runs on somewhat reduced loop gain, although the -3 dB bandwidth of the first stage is still above 1 MHz. The bass response is very pretty--a direct result of having only one AC coupling capacitor pair in the signal path.
The THD+N plots show good performance as well. The residual consists mostly of noise, except for the highest gain where a faint of third harmonic starts to become visible above 5 kHz. If you want to compare this data to other preamps please note that the measurements were done at high bandwidth, somewhat higher-than-usual maximum gain (65 dB instead of 60 dB) and on the bench without any shielding. This will easily increase the THD+N reading by a factor of 4 or more at the maximum gain in contrast to other published data.
Slew rate is typically 110 V/us and EIN has been measured at about -129 dBu for a 150 Ohm source (20 kHz bandwidth, 65 dB gain) and -135 dBu with shorted input.
CMRR at low frequencies is the only thing I'm not absolutely happy with though it surely is enough for real world use and (way) better than many other preamps. The reason for the decreasing CMRR towards the lower range is mismatch in the input coupling capacitors--I didn't realise that this is that critical up to almost the upper end of the audio range. This can be figthed by tightly matching the capacitors C105 and C106 (e.g. by the addition of small compensation capacitors in order to avoid the need for buying a large batch of expensive 33 uF film capacitors). If I'd do another revision (not to be expected soon) I think I'd use the common-mode bootstrap method invented by Bill Whitlock to avoide the matching need.
On the other hand the input RFI filter is very effective indeed (at least as far as I can tell). Above the shown range CMRR very rapidly increases. The onset of this is just visible at 1 MHz. In addition to this I noticed that common-mode signals show very low waveform distortion even at the highest frequencies (not something I usually observe with transformerless designs). That's a result of having high slew-rate amplifiers and will account for low intermodulation distortion with spurious HF products.
Overall very pleasing results, I'd say. I was not able to give it a detailed listen yet (this will--for my convenience--have to wait until it got a case), but I would be surprised if this preamp wouldn't be very clean and transparent.
[Edit--some pictures: monte_generoso.html.]
Samuel