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Old 31st March 2014, 11:55 PM   #1
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Default DAC output attenuation advice

Hi,
I have a modded Eastern Electric DAC (ES9018) which had its volume control and input selection switch bypassed. These bypasses appear to have increased the output gain a little (too much?).
I tried placing some borrowed Rothwell inline attenuators on my preampís input to hopefully make the sound a little less ďhotĒ and the volume control a bit more gradual.
This works OK except I find it dulls the sound. I would like to know how I could slightly attenuate the gain from my DACís output. I believe this involves inserting one or two resistors from the board to the DACís RCA output jacks.
Could anyone provide some advice as to how this is done and what value resistor(s) would be a good starting point to achieve a -10db attenuation.
Thanks
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Old 1st April 2014, 01:16 AM   #2
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Quote:
Originally Posted by McCrackers View Post
Hi,
I have a modded Eastern Electric DAC (ES9018) which had its volume control and input selection switch bypassed. These bypasses appear to have increased the output gain a little (too much?).
I tried placing some borrowed Rothwell inline attenuators on my preampís input to hopefully make the sound a little less ďhotĒ and the volume control a bit more gradual.
This works OK except I find it dulls the sound. I would like to know how I could slightly attenuate the gain from my DACís output. I believe this involves inserting one or two resistors from the board to the DACís RCA output jacks.
Could anyone provide some advice as to how this is done and what value resistor(s) would be a good starting point to achieve a -10db attenuation.
Thanks
Hi you need to use an L pad, which is 4 resistors, 2 for each channel. Using a circuit breadboard arrange a series resistor of approx 3.3k for each channel and a shunt ( to ground ) resistor of approx 12k for each channel. Alternatively an LDR based attenuator will give excellent audio and allow you to vary volume to suit.

Cheers / Chris
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Old 1st April 2014, 02:05 AM   #3
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Quote:
Originally Posted by Chris Daly View Post
Hi you need to use an L pad, which is 4 resistors, 2 for each channel. Using a circuit breadboard arrange a series resistor of approx 3.3k for each channel and a shunt ( to ground ) resistor of approx 12k for each channel. Alternatively an LDR based attenuator will give excellent audio and allow you to vary volume to suit.

Cheers / Chris
Chris,

With my very limited DIY experience I understand the first option but not sure why a breadboard is needed.

You lost me with the LDR based option.

Thanks again,
Mike
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Old 1st April 2014, 02:23 AM   #4
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Hi Mike
A breadboard might be needed if you are cutting the RCA cable, there is then ability to use 1/4 - 1/2 watt resistors. Of course on the RCA socket internally inside the casework is a bit neater ( but was unsure if you could get to that, hence suggestion of breadboard ).

As you may have observed already even resistors impart a sonic signature. Further up the path not to take, is placing resistors or variable means of attenuation like potentiometers or switched attenuators that involve switching of contacts, hence if you needed to vary the attenuation LDR's are a nice option.

Fixed resistors are simple and should work well for the purpose required.

Cheers / Chris
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Old 1st April 2014, 02:30 AM   #5
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Attenuators tend to dull the sound because if put in-line they use a noisy 0V reference - the screen of the connecting cable. To get an attenuator which doesn't squash the dynamics you'll need to use a clean 0V reference and this means opening up the DAC to do the mod. For a person with limited DIY skills I'd not recommend this route.

Alternatively, use a transformer with a 3:1 turns ratio. This provides isolation and accomplishes the attenuation without introducing noise.
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Old 1st April 2014, 08:40 PM   #6
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Thanks guys.
I should be able to squeeze an L pad between the board and the RCAs from the inside. The best resistors I know of for this application would probably be naked Vishay or the Texas Instrument equivalents. They are very small and are the closest thing to a piece of wire sound wise. The downside is their price.
I’ve experimented with output transformers in the past to replace opamps on a CS4398 board.

At the time I had a very helpful forum member walk me through the steps. I wouldn’t feel comfortable doing it on my own and unfortunately the fellow (Bill Fuss for those who knew him) passed away a while back.

Last edited by McCrackers; 1st April 2014 at 08:42 PM.
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Old 1st April 2014, 10:02 PM   #7
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Sorry I meant Texas Components and not Texas Instruments resistors
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Old 2nd April 2014, 11:52 AM   #8
DF96 is offline DF96  England
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Quote:
Originally Posted by McCrackers
Could anyone provide some advice as to how this is done and what value resistor(s) would be a good starting point to achieve a -10db attenuation.
Quote:
Originally Posted by Chris Daly
Using a circuit breadboard arrange a series resistor of approx 3.3k for each channel and a shunt ( to ground ) resistor of approx 12k for each channel.
Before you get too carried away with choosing resistor brand etc. it would pay to get the values correct. 3.3k series and 12k shunt would give an attenuation of -2.1dB (assuming a low impedance source and high impedance load).

Swapping them over gives -13.3dB. Try 6.8k series and 3.3k shunt - this gives -9.7dB.

Note that the best place for an attenuator is at the receiving end of the cable. At the sending end it has to drive the cable capacitance so some HF loss can occur.

LDR attenuators are fine if you want adjustment and can cope with a little distortion (LDRs are slightly non-linear - ordinary resistors are much better).
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Old 3rd April 2014, 04:50 AM   #9
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Hi McCrackers
It would pay to measure with DAC switched off with your multimeter what resistance the DAC already has from its RCA output to Ground, as adding further shunt resistance as suggested with an L pad will then be in parallel. From that figure you can then work out correct shunt resistance you are adding, to attain the 10db required.

Cheers / Chris
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Old 3rd April 2014, 10:29 AM   #10
DF96 is offline DF96  England
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Measuring 'cold' resistance, as Chris Daly suggests, will tell you precisely nothing. The actual 'hot' impedance with the device switched on could be much higher or much lower; it is very unlikely to be the same, circuits don't work like that.

Whatever impedance there is will be in series with the L pad, not parallel as he says.
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