[Whitebread]

Hello folks,
I'm trying to purchase components in order to build an AMB Delta1 attenuator. However, I'm not certain what I should set the Zatt value to in the calculator. I'll have three inputs within the amp. The first will be the Gamma1/Gamma2 stack. The second is a Phono pre-amp. I've reached out for the output impedance value from the manufacturer and I'm waiting on that. The Gamma1/Gamma2 stack doesn't seem to have any information about it's output impedance value on amb.org. So, I'm wondering what others have used as their input impedance value when building the Delta1 for use with this DAC. Is the default value of ~25k ohms reasonable for most applications? (USB DACs, Phono preamps, CD/SACD players, etc)?

Thanks

 

[MindsMirror]

From the AMB website regarding the Gamma2:

Analog output stage
The Wolfson DAC chip provides differential analog voltage outputs for each channel, which is sent to the analog output stage comprised of U7, a low-noise rail-to-rail precision dual opamp, and its surrounding parts. Two opamp options have been chosen for this stage, the OPA2365 or the AD8656.

U7 serves triple duty as balanced-to-unbalanced converter, analog low pass filter, and output buffer. Rather than simply using the the non-inverting signal for each channel (and discarding the inverting one), γ2's balanced-to-unbalanced converter retains high common-mode rejection (CMR), ensuring that the full S/N ratio and distortion performance of the DAC to be realized. It also allows complete differential output voltage swing.

The second-order (12dB per octave) analog low-pass filter with corner frequency of 100KHz removes out-of-band noise and artifacts. This is important if the DAC is to be used with a wideband, high-speed amplifier. Polypropylene film capacitors are used for the filters. The output buffer provides low output impedance and high current drive capability. If the AD8656 opamp and 470µF output coupling capacitors are used, γ2 could drive low-impedance headphones directly.

The DAC is direct-coupled to this stage, thus the opamp is biased to VADD/2 to each of its inputs. Since the opamp is also powered from the single analog supply, its output DC offset is also at VADD/2 at idle, and requires output coupling capacitors (C20, C26) prior to the output jacks. Several types of audio-grade capacitors are selected as options for these, as well as film bypass capacitors (C19, C25). Due to the low analog supply voltage, the "gain" of this stage is set to 0.7, to ensure that the signal will never clip at 0dBFS.

I'd guess low output impedance means a few Ohms to tens of Ohms at most. It should have no trouble driving 25K.

 

[Whitebread]

Thanks.

 

[tomb]

Ask your questions on AMB's forum. He'll respond.