Line level attenuator

[Davide]

I am considering getting a power amp to plug directly into the DAC, but as good as the digital volume control is, I have a terrible fear that it will accidentally fry my speakers and ears

Balanced preamps like the Topping Pre90 honestly cost too much in relation to their function, so I'm looking for a cheaper solution.

My current idea is to use a line attenuator H-pad style (on the market there are several such as Rothwell or Shure) that reduces the level to reach, let's say, about 90 db SPL from the speakers, and then I do the remaining attenuation with digital control. So I avoid the aforementioned risk ... and by the way I also gain something in SINAD compared to the complete digital control.

Such a system should be similar to that of the RME ADI-2 or the Gustard P26.

But now my perplexity, not being an expert on the subject, is how to calculate the attenuation necessary to have the desired SPL.
Maybe it would also be nice to size the H-pad directly so I only buy the resistors instead of the commercial attenuator (which still has its cost).

Could anyone kindly help me?

Thanks!

 

[MakeMineVinyl]

Couldn't you find a box with a passive volume control which is situated right before the input to your power amplifier and adjust it to your requirements? That's essentially what I do with the LF amps in my system.

 

[Ron Texas]

Harrison Labs.

 

[Davide]

Honestly I don't find good XLR passive volume control at reasonable cost (in Europe).
For this I am thinking of fixed attenuator + digital control.
I did some calculations anyway and I think I need about 20 db of attenuation...

 

[Slayer]

Honestly I don't find good XLR passive volume control at reasonable cost (in Europe).
For this I am thinking of fixed attenuator + digital control.
I did some calculations anyway and I think I need about 20 db of attenuation...

If you need 20db inline attenuators, just get the Rothwell XLR inline attenuators. They come in 20db attenuation.

 

[MakeMineVinyl]

Honestly I don't find good XLR passive volume control at reasonable cost (in Europe).
For this I am thinking of fixed attenuator + digital control.
I did some calculations anyway and I think I need about 20 db of attenuation...

If you know how to build an attenuator and can get the parts, there's nothing wrong with that. Just make sure that it is as close to the input of the power amplifier as possible.

 

[Davide]

Thanks for all previous reply.

I have found several calculators for H pad online, so I could design the attenuator. Assembly is relative simply.
But using online calculators does not square something for me... inserting the relative output impedance of my DAC (Gustard A18, from the manual it should be 100ohm) and that of the amplifier input (NC502MP, 47kohm buffered) and an attenuation of 20 db, some resistance values obtained are negative.

Maybe I'm doing something wrong...

At this point I wonder though, regarding commercial products, does an attenuator declared at -20 db always work with this attenuation or is it indicative and in fact it depends on the input and output loads?

 

[Marc v E]

When I use my dac with digital volume control it is very clear it is doing this and not the full volume dac function. You see the db clearly in the display. I have the dx7pro btw. If you only have digital sources, this should be all you need (No need for a preamp with the d90se according to Amir and John Yang in the d90se thread). The dx7pro sounds fine to me too. This should save you some money.

 

[Marc v E]

 

[Davide]

Yeah, you're right, but as I said previously, I want to protect my ears and speakers from accidentally peak power due to wrong attenuation settings (it's digital way so I could happens). Furthermore, I would like to make better use of the digital attenuation range based on my actual listening, considering that it is not a linear SPL adjustment (therefore impractical and too much sensitive when increasing the volume).

 

[Grumpish]

Passive pre-amp - I use a dual mono Khozmo in just this way, setting the levels to something comfortable and leaving them there and then using the digital control on the DAC (I also listen to vinyl and it works very well as conventional control there).
https://www.khozmo.com/index.html

 

[Davide]

Yes I saw Khozmo attenuators but a bit too pricey. Also if I have to leave them at a fixed value I might as well use the XLR attenuators that cost 1/10.
However I found two Rothwell XLR -20db attenuators used for 50 euros, so I guess I'll try those, hoping my calculations are correct.

 

[Davide]

Just make sure that it is as close to the input of the power amplifier as possible.

Sorry I only saw this advice now. What is the exact reason? Avoid electromagnetic interference as much as possible?

 

[MakeMineVinyl]

Sorry I only saw this advice now. What is the exact reason? Avoid electromagnetic interference as much as possible?

The output of the attenuator is generally going to be a higher impedance than the output of the upstream component. If the attenuator is at the output device and you have a long cable, that cable's capacitance will form a low pass filter who's frequency is determined by the output impedance (resistance) of the pad and the cable capacitance. This will roll off the highest frequencies. If your cable is only 6' or so, its not going to be an issue - 25' and it could be more of an issue. If the shunt resistance of the pad is lower, like 1k or lower, then it also won't be much of an issue.

 

[Davide]

The output of the attenuator is generally going to be a higher impedance than the output of the upstream component. If the attenuator is at the output device and you have a long cable, that cable's capacitance will form a low pass filter who's frequency is determined by the output impedance (resistance) of the pad and the cable capacitance. This will roll off the highest frequencies. If your cable is only 6' or so, its not going to be an issue - 25' and it could be more of an issue. If the shunt resistance of the pad is lower, like 1k or lower, then it also won't be much of an issue.

Interesting...
In my case I use a dedicated 30 cm cable from Ghent that on one side connects directly to the HYPEX NC502MP module and on the other side I put a female XLR connector that connects to the DAC (no intermediate interface connector therefore).
So the only point where I can connect the attenuators is between the DAC and the aforementioned cable.

 

[Slayer]

Interesting...
In my case I use a dedicated 30 cm cable from Ghent that on one side connects directly to the HYPEX NC502MP module and on the other side I put a female XLR connector that connects to the DAC (no intermediate interface connector therefore).
So the only point where I can connect the attenuators is between the DAC and the aforementioned cable.

DAC to XLR cable
XLR cable then to XLR attenuator
XLR attenuator plugs directly into amp

 

[Davide]

DAC to XLR cable
XLR cable then to XLR attenuator
XLR attenuator plugs directly into amp

Sorry but it would make no sense to introduce an additional XLR cable if there are still those 30 cm between the attenuator and the amplifier (the dedicated cable for the HYPEX module is necessary, can't connect the attenuators directly on the module).
Or maybe I'm missing something?

 

[Slayer]

Sorry but it would make no sense to introduce an additional XLR cable if there are still those 30 cm between the attenuator and the amplifier (the dedicated cable for the HYPEX module is necessary, can't connect the attenuators directly on the module).
Or maybe I'm missing something?

Yes, you are missing something. You are Not adding another cable.
You connect the XLR cable to the DAC's output, then you take the other end of the XLR cable and attach the Rothwell XLR attenuator, then the Attenuator plugs into the amplifier.

 

[Davide]

Yes, you are missing something. You are Not adding another cable.
You connect the XLR cable to the DAC's output, then you take the other end of the XLR cable and attach the Rothwell XLR attenuator, then the Attenuator plugs into the amplifier.

No I don't use that cable.
I have only the 30cm cable that goes from DAC (XLR) to HYPEX module (don't know the type of connector, but it is dedicated one).

 

[Slayer]

No I don't use that cable.
I have only the 30cm cable that goes from DAC (XLR) to HYPEX module (don't know the type of connector, but it is dedicated one).

Ok, so if you are using your DAC as a DAC and Preamp, then what you want to do is.
Rothwell XLR attenuator plugs into the DAC's XLR output, then your cable's XLR into the attenuator, then the other end of cable to your amp.
With a short cable like you are using it will work fine. You can read about it here.
Attenuators (rothwellaudioproducts.co.uk)