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RME User Forum → ADI-2 & 2/4 Pro series, ADI-2 DAC series → Driving 1ET400A with ADI-2 Pro
I am building a power amp based on the Purifi 1ET400A.
I was wondering if I could drive it directly with the ADI-2 Pro's balanced output, given that the input impedance is 4.4 kohm, or should I put a buffer in between.
Anyone tried it?
I can't answer your question regarding input impendance.
I would put it this way .. You can use the ADi-2 Pro as Preamp, if the power amp has line level inputs.
The ADI-2 Pro offers a wide range of output levels (4 ref levels) which can adapt to any scenario.
Buffers are not needed.
Buffers are not needed.
Great, thanks.
I was also thinking of attenuating the ADI-2 Pro input to match its auto level in a sweet spot that works best for me and my setup.
As I don't need the buffers, could I realise this with an H-pad?
My reasoning:
Power amp module has a fixed 12.8 dB gain.
I would like to match the +13 dBu @0dBFS ADI-2 Pro level with 1W into my 8 ohm speakers, which requires driving them with 2.83V or 11.25 dBu.
In order to do so, I need to attenuate the ADI-2 Pro XLR balanced line output by 13 + 12.8 - 11.25 = 14.55 dB.
Should I calculate the resistances in the -14.55 dB H-pad to match the ADI-2 Pro's output impedance (200 ohm), or the power amp module's input impedance (4.4 kohm)?
11.25 dBu you get with auto reflevel enabled and output level at -13
Then you are 2dB lower than full reflevel of +13 dBu, then SNR is 120-2=118 dB / 122 dB(A).
I was also thinking of attenuating the ADI-2 Pro input to match its auto level in a sweet spot that works best for me and my setup.
As I don't need the buffers, could I realise this with an H-pad?My reasoning:
Power amp module has a fixed 12.8 dB gain.
I would like to match the +13 dBu @0dBFS ADI-2 Pro level with 1W into my 8 ohm speakers, which requires driving them with 2.83V or 11.25 dBu.
In order to do so, I need to attenuate the ADI-2 Pro XLR balanced line output by 13 + 12.8 - 11.25 = 14.55 dB.Should I calculate the resistances in the -14.55 dB H-pad to match the ADI-2 Pro's output impedance (200 ohm), or the power amp module's input impedance (4.4 kohm)?
Neither.
1. The amp has a nominal 425 W into 4 Ohm power output.
This equals 41.2 V or 34.5 dBu
Gain is 12.8 dB from the amps balanced input.
34.5 dBu - 12.8 dB (gain) = 21.7 dBu for full drive.
The amps spec’s even say 11.2 V input for full drive, this would be 23.2 dBu.
This almost perfectly fits to ADI-2‘s 24 / 27.5 dBu full scale balanced output, including a little headroom.
ADI-2 Pro’s “Auto Ref Level” could care for the attenuation at lower listening levels.
A passive attenuation of more than 4 dB would limit the max. power output possible.
2. If, for whatever reason, you really want to go for ADI-2 out 13 dBu = 1 W into 8 Ohm, a 3 resistor balanced voltage divider (no H-pad) would be the way to go.
The dividers output resistance / shunting resistor should be quite low to terminate the amp’s input for lowest noise.
• A 330 Ohm shunt resistor, R3 about preserves the value ADI-2 Pro offers.
• The 2 series resistors R1, R2 can be 560 Ohm each, which do not over-stress ADI-2 Pro’s output chips.
The resulting damping, including:
• ADI-2 Pro’s 200 Ohm output resistance,
• 4 Ohm for the cable,
• 4,400 Ohm amps input resistance is:
-14.51 dB, a close match to the 14.55 dB needed.
This voltage divider’s output resistance is 265 Ohm, low enough to avoid amp noise.
Thanks Kai!
The amps spec’s even say 11.2 V input for full drive, this would be 23.2 dBu.
This almost perfectly fits to ADI-2‘s 24 / 27.5 dBu full scale balanced output, including a little headroom.
ADI-2 Pro’s “Auto Ref Level” could care for the attenuation at lower listening levels.
The above is true if you want to drive the amp to its full power.
The speakers that I am driving have the sensitivity of 94 dB SPL @ 1m, 1W into 8 ohm (2.83V = 11.25 dBu). Driving them at more than 5W in my listening environment is very rare (read: unbearable).
If I were to drive the power amp directly from ADI-2 Pro unattenuated, 5W would correspond to 5.4 dBu of ADI-2 Pro's output, or -18.5 dBr of volume in auto ref level mode, and I would go almost exclusively lower than that, hence biting into the pure digital volume reduction after -20 dBr. In that case, I would not benefit much from the four reference levels.
This was the motivation for using an attenuator, and bring the sweet spot up on the volume dBr scale, and leverage auto ref level better for my listening conditions.
Thanks ramses!
I love your chart and blog :-), it provided great info before I bought my RME's (although ADI-2 DAC has a bug and 'eats' up 0.5 dB upon transition between ref levels - see my post in this forum about it).
11.25 dBu you get with auto reflevel enabled and output level at -13
Then you are 2dB lower than full reflevel of +13 dBu, then SNR is 120-2=118 dB / 122 dB(A).
To clarify, I was talking about getting 11.25 dBu out of the power amp, not ADI-2 Pro directly.
RME User Forum → ADI-2 & 2/4 Pro series, ADI-2 DAC series → Driving 1ET400A with ADI-2 Pro
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