Gustard A18 Balanced Desktop DAC Review

[Davide]

I studied a bit and I was able to do the math. EVAL1 has 14.6 dB gain in buffer stage and 1ET400A requires 9.6V for 1% THD at 65V supply.
Therefore, it is reasonable to use up to 1.8V at the EVAL1 input.
In the case of our A18, considering the maximum 6.35V measured by Wolf, 11 dB of attenuation are needed to get 1.8V at its XLR output (SINAD lowering is negligible as it is always higher than EVAL1).
Alternatively, however, I have seen that we can modify the gain of the EVAL1, and if I did the math correctly it would be enough to replace the 1.5k Ohm "R17" with a 2.7k Ohm one.
Unfortunately it is an SMD component so soldering is delicate to perform ...
This should be the point of having a perfectly compatible system .. if I have something wrong, please correct me.

 

[Morla]

Please have a look at

https://www.audiophonics.fr/en/blog...-bypass-on-purifi-amplifier-module-ampli.html

"The bypass mode is relevant in cases where the Purifi amplifier is used with a strong preamplifier supporting a load of 2 to 4kOhm with a voltage ~ 10V RMS."

Maybe that helps?

 

[Davide]

Please have a look at

https://www.audiophonics.fr/en/blog...-bypass-on-purifi-amplifier-module-ampli.html

"The bypass mode is relevant in cases where the Purifi amplifier is used with a strong preamplifier supporting a load of 2 to 4kOhm with a voltage ~ 10V RMS."

Maybe that helps?

Yeah, it's not good to use the DAC directly without gain stage.

 

[Morla]

Yes it is?

 

[Davide]

Yes it is?

Sorry but I think I don't understand your comment.
I just wanted to say that according to the technicalities it is not the ideal solution to use the A18 to directly drive the 1ET400 modules.

 

[Morla]

It's probably good enough if u used that audiophonics board, no? Then again.. you surely spent enough time getting the math right. It just questions the way I am running it currentoy lol.

 

[Davide]

It's probably good enough if u used that audiophonics board, no? Then again.. you surely spent enough time getting the math right. It just questions the way I am running it currentoy lol.

It would be nice to understand how the A18 performs with a low impedance load like the Purifi modules ... but for lack of measures one can only consider the fact that being an atypical load it would be better to avoid (then it is clear that if you use it like this and it sounds good it means that there are no relevant problems!).
The fact remains that the output voltage of the A18 does not allow you to exploit all the power of the Purifi, then the question of whether this is necessary or not is another.

 

[JohnYang1997]

It would be nice to understand how the A18 performs with a low impedance load like the Purifi modules ... but for lack of measures one can only consider the fact that being an atypical load it would be better to avoid (then it is clear that if you use it like this and it sounds good it means that there are no relevant problems!).
The fact remains that the output voltage of the A18 does not allow you to exploit all the power of the Purifi, then the question of whether this is necessary or not is another.

Sigh... Those purifi modules are not low impedance load. Opamps can drive 600ohm with minimal distortion since like forever. One potential issue rather is whether the source impedance can affect the control loop/circuit operation on the purifi end. But driving the purifi module isn't a issue.
My designs have mostly 2k-2.5kohm input impedance but no issue has been found except when paring with high output impedance tube outputs.

 

[Davide]

Sigh... Those purifi modules are not low impedance load. Opamps can drive 600ohm with minimal distortion since like forever. One potential issue rather is whether the source impedance can affect the control loop/circuit operation on the purifi end. But driving the purifi module isn't a issue.
My designs have mostly 2k-2.5kohm input impedance but no issue has been found except when paring with high output impedance tube outputs.

I wrote this by simply quoting tomchr on Diyaudio forum.

The input impedance of the Hypex NC500 is 3.6 kΩ differential (1.8 kΩ from each signal pin to ground). The input impedance of the Purifi 1ET400A is 4.4 kΩ (2.2 kΩ from each signal pin to ground). That is a very heavy (low impedance) load to drive for most preamps. It's not until within the past decade or so that we've had opamps available that could drive (i.e. source enough current) 2 kΩ cleanly (i.e. with low distortion). Many audio opamps of years past were specified to 10 kΩ load. The same goes for many preamps.
So job #1 of the buffer is to source enough current to produce a clean signal at the input of the amplifier module.

Now, I understand that that load is theoretically sustainable for an opamp like the 49860, but as an ignoramus I wonder, in the way it is implemented in the A18 is it able to provide the necessary current?
I only understand that it cannot supply the necessary voltage because the A18 reaches about 6V RMS, while the Purifi requires about 10V for full power (so economically it's not a logical match for me).

 

[sq225917]

On 15v rails it'll happily drive 10k load to pretty much its measured specs, at lower voltages, not so much..

 

[Davide]

On 15v rails it'll happily drive 10k load to pretty much its measured specs, at lower voltages, not so much..

I suppose A18 has 15v rails, or not?

 

[JohnYang1997]

I suppose A18 has 15v rails, or not?

Don't worry about it. There's no such thing.

 

[sq225917]

The spec sheets disagree with you.

 

[JohnYang1997]

The spec sheets disagree with you.

Really? +-12V 600ohm.
6V BAL output is 3Vrms each branch exactly at the lowest THD+N point. And for the range of 1-6Vrms is still better than 120dB.

 

[sq225917]

Take a look at the 2.5v into 2k, thats what I'm talking about.

 

[Powell2199]

Does A18 has a decent inbuilt preamp or an external pre-amp is still needed?

 

[sq225917]

It has no analogue throughput option, it's a dac only with internal volume control. No need for a preamp if it's your only source. It provides more than enough output for almost any power amp.

 

[the_hamster 2]

Does A18 has a decent inbuilt preamp or an external pre-amp is still needed?

Been using the A18 as preamp while matched with Schiit SYS volume control, and pleased with result…just set DAC to 0.0 dB, and all good.

 

[Yasser 06]

It has no analogue throughput option, it's a dac only with internal volume control. No need for a preamp if it's your only source. It provides more than enough output for almost any power amp.

It lack inputs. many people would need 3-4 digital in.

Example : Streamer (raspbeery + hat), Computer (Vidéos, You tube etc , Music that is not yet on your NAS ..), Second computer (your laptop), BT receiver (such as a Fiio BTA30 but cheaper better options exists as well as "audiophile"ones) .

A digital bypass can also be usefull : BT transmitter, second system etc .....

I don t get why they don't include that by default, the extra cost would not be high.

Of course you can use an optical switch, -- i cannot find cheap coax one -- but this is MESSY.

In our days i don't really see many people with a single optical digital source , coaxial is like ubiquitous ....

And i forgot about Playstations, and TVs since i have none of these ...

 

[the_hamster 2]

It lack inputs. many people would need 3-4 digital in.

Example : Streamer (raspbeery + hat), Computer (Vidéos, You tube etc , Music that is not yet on your NAS ..), Second computer (your laptop), BT receiver (such as a Fiio BTA30 but cheaper better options exists as well as "audiophile"ones) .

A digital bypass can also be usefull : BT transmitter, second system etc .....

I don t get why they don't include that by default, the extra cost would not be high.

Of course you can use an optical switch, -- i cannot find cheap coax one -- but this is MESSY.

In our days i don't really see many people with a single optical digital source , coaxial is like ubiquitous ....

And i forgot about Playstations, and TVs since i have none of these ...

As part of a complete system I use an E30 DAC paired with a CCA (opt.) and a (rarely used) DVD player (coax) paired to ADCOM preamp (RCA) ; the A18 is paired with CD (coax), Smart TV (opt.), and iPad or PC (USB), also > ADCOM preamp (XLR). So, for a relatively inexpensive outlay, 5 devices can be handled conveniently…YMMV.