Attenuating DAC output?

[HighFidelityGuy]

I need to reduce the output level of my Caiman a little and I want to do this as an internal mod. I used to use an external 10dB attenuator but since performing the passive output mod on the Caiman I no longer need this much attenuation. So I'm thinking that 6dB might be about right. I've tried Googling this subject but I just keep coming up with ready made external products or L-pad info for speaker level attenuation. I'm guessing the L-pad approach would still apply here but I'm too shite at maths to figure out all the calculations and the online calculators are tailored for speaker applications. So I'm hoping someone more knowledgeable than me can provide some assistance.

So I know I want to reduce the signal by 6dB and I know my amp has a 50K Ohm input impedance. Is that enough to work out what resistor values are needed?

Thanks. :)

[Ali Tait]

I think it's reduced by about this much with the passive mod.Do you require a further 6dB?

[Clive]

Dave, if you need to reduce the Caiman output and the passive mod isn't sufficient, then you should implement this in your preamp. If you place an attenuator in you Caiman you run the risk of creating a filter in conjunction with your interconnect. Place the attenuator at the input of you preamp gets around this. For my 100k preamp I needed 100k to reduce 6db, the resistor being placed in line with the signal "hot" wires. Possibly you need 50k? Or is it 200k?

[HighFidelityGuy]

I think it's reduced by about this much with the passive mod.Do you require a further 6dB?

Yep, the passive output mod doesn't reduce the output enough, so I can't get my volume control to 12 o'clock without it being too loud.


Dave, if you need to reduce the Caiman output and the passive mod isn't sufficient, then you should implement this in your preamp. If you place an attenuator in you Caiman you run the risk of creating a filter in conjunction with your interconnect. Place the attenuator at the input of you preamp gets around this. For my 100k preamp I needed 100k to reduce 6db, the resistor being placed in line with the signal "hot" wires. Possibly you need 50k? Or is it 200k?

I was worried that that might be an issue. The problem I've got is that my "preamp" is passive, so I'm not sure that it actually has an input impedance as such. Basically it's just an alps blue velvet pot in a box with input selection. The 50K Ohm input impedance I referred to is that of my power amps. If at all possible I'd prefer not to modify the "preamp" or power amps as they were quite expensive. So I guess If I absolutely had to do the attenuation outside of the DAC I'd have to make some in-line attenuators. That would be an acceptable compromise.

If it makes any difference; as it stands I have an extremely sort interconnect between the DAC and preamp (about 20cm). This is hard wired directly to the output cap in the DAC and the cable it's self is 5N solid silver in teflon. So it shouldn't be adding much resistance or capacitance I wouldn't have thought.

I'm open to suggestion though, so what would you recommending as being the best approach based on the above?

Cheers. :)

[Ali Tait]

The passive pre will have an input impedance set by the pot in it I think.What value is the pot?

[Clive]

What value is the ALPS pot?

I believe the reason that passive pre's tend to use values such as 10k to 50k is to reduce filter effects with the cable to power amps. Putting a resistor in line with the preamp input may result in high frequencies being filtered, you won't know unless you try it. You need to think of the output from the Caiman via your preamp and to the power amp as one complete path.

[Clive]

I see that CI specify 10k.

[Ali Tait]

Snap Clive!

[Stratmangler]

Yep, the passive output mod doesn't reduce the output enough, so I can't get my volume control to 12 o'clock without it being too loud.

Sounds like you've got a logarithmic pot in there.
A linear pot of the same value would give you more fine control over level over a wider sweep of the pot.

[Clive]

Dave, I don't suppose you want to replace the pot and I believe motorized pots are getting hard to find so finding a linear could be tough but it was a good suggestion by Chris.

I see the amps are 2.23V for full output, usually this would be fine but.....as full output is 550W I can see why you have a problem. Not sure what to suggest!

Ali, we'll have to be more efficient and not do the same searches at the same time!

[Ali Tait]

:) I was lazy and didn't do any searching.What about doing the shunt mod on the pot? That lowers gain does it not?

[Ali Tait]

....and will improve the sound.

[Clive]

worth doing if you are ok attenuating all inputs.

[Mike]

I'd heard that RF Attenuators are good for this, so I got one out of the van and decided to 'have at it' to see what was inside...

What did I find?

Not much TBH, just a 'network' (I use the word loosely) of SMD resistors. All three of them! :lol:

[HighFidelityGuy]

Thanks for all the replies. :)
I've been mulling this over for a couple of days and I've decided that if I can't really modify the DAC to produce less voltage without adversely affecting the performance then I'll have to implement something externally as I really don't want to modify my pre or power amps. So I could look at making some in-line attenuators.
However, as I've now dicovered that my passive "preamp" does have an input impedance and that it's only 10k Ohm, perhaps I'm experiencing an impedance mismatch issue. In which case a buffer stage between the DAC and preamp could be a better option.

Does anyone have an opinion as to which option would be better?

Thanks. :)

[Stratmangler]

Why not try the variable outs on the Caiman ?
The volume is controlled in the analogue domain. Just set it up to the required level and leave it alone.
Control the overall volume with your front end source switcher/rc volume attenuator.

[Reid Malenfant]

Most audio components will have an output impedance of probably 100 ohms or less, it's pretty rare to find something that contains active electronic components that will have an output impedance of more than 100 ohms. Saying that a certain pre amp i can think of has a 1000 ohm output impedance.

Your DAC will have absolutely no problem driving a 10K variable resistor ;) If you happen to have a selector switch so you can switch to other equipment & you only connect the DAC via one set of connectors all you need to do to attenuate the DAC output is connect a 10K resistor between the centre connector of each phono socket inside the pre amp with the other end of each resistor connecting to the selector switch. You'll need to either cut a PCB track or splice the resistors into any wires.

That should drop the input to the volume pot by 6Db :)

E2A:- If there are capacitors on the input for DC isolation make sure that the resistors are placed after the capacitors & you'll actually have a slightly lower -3Db point not a raised one.

[HighFidelityGuy]

Why not try the variable outs on the Caiman ?
The volume is controlled in the analogue domain. Just set it up to the required level and leave it alone.
Control the overall volume with your front end source switcher/rc volume attenuator.

I've implemented the passive output mod on my Caiman, so the variable output is unusable. :)


Most audio components will have an output impedance of probably 100 ohms or less, it's pretty rare to find something that contains active electronic components that will have an output impedance of more than 100 ohms. Saying that a certain pre amp i can think of has a 1000 ohm output impedance.

Your DAC will have absolutely no problem driving a 10K variable resistor ;) If you happen to have a selector switch so you can switch to other equipment & you only connect the DAC via one set of connectors all you need to do to attenuate the DAC output is connect a 10K resistor between the centre connector of each phono socket inside the pre amp with the other end of each resistor connecting to the selector switch. You'll need to either cut a PCB track or splice the resistors into any wires.

That should drop the input to the volume pot by 6Db :)

E2A:- If there are capacitors on the input for DC isolation make sure that the resistors are placed after the capacitors & you'll actually have a slightly lower -3Db point not a raised one.

Thanks Mark.
I don't want to modify anything inside my preamp, so providing there aren't any caps on it's inputs to block DC, can I just put a 10K resistor in-line (in series) on each signal wire between the DAC and preamp?

Thanks. :)

[Reid Malenfant]

Thanks Mark.
I don't want to modify anything inside my preamp, so providing there aren't any caps on it's inputs to block DC, can I just put a 10K resistor in-line (in series) on each signal wire between the DAC and preamp?

Thanks. :)
Not a problem & yes :) I was going to suggest that you could even place it inside one of the interconnect cable phono plugs if you had the room & felt confident doing it.

Just find yourself some decent 1% tolerance resistors, or if you feel like going mad get a couple of 10K 0.1% Holco or Vishay resistors. After all you might as well get the best that you can afford, it's just two resistors when all is said & done :lol:

[Mike]

You can have one of these if you want? :)

It might well be possible to fit it inside a reasonably sized RCA plug.

http://img819.imageshack.us/img819/4476/dscf0304e.jpg


I've got them in various values: 1, 2, 3, 6, 10, 12, and 20dB

Cheers...

[Primalsea]

Personally I wouldnt use 2 passive attenuators directly inline which is what you will effectively have.

In the past I have used a passive L pad to reduce gain and then followed it with a buffer stage made from high quality opamps. This set up tends to be fairly transparent and as the potput impedance is very low it can drive cables with no problem.

The caiman only have a positive voltage rail I believe so you will have to configure the opamp for single rail operation if you go this way.

[Peter Galbavy]

I've implemented the passive output mod on my Caiman, so the variable output is unusable. :)

If you'd done the mod as "remove the op-amp board and plug caps directly in" then the variable output still works fine. Just no headphone amp.