A3 attenuator help for an Aikido line amp - diyAudio
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Old 15th February 2013, 11:21 PM   #1
PJN is offline PJN  United States
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Default A3 attenuator help for an Aikido line amp

Hi Guys,

I recently completed a dual mono Aikido line preamp build using all 6CG7 tubes and dual Janus shunt regulator PSUs and used an A3 mini as an attenuator. The A3 is using 1% metal film resistors kit, 20K with 1.5 dB resolution. The pre is driving a 150 wpc SKA dual mono SS amp. The A3 appears to be working correctly in that the center switch controls the coarse attenuation for both channels and the switches on either side control the finer attenuation for each channel. The fine adjustments are fine, my problem is that even at the lowest coarse attenuator setting things are too loud for comfortable listening. Im doubting that the 2 dB resolution resistors shown in the table in the user guide will provide enough attenuation. What I need is coarse adjustment that is significantly greater than I have now. I do like the ability to do some channel balancing with attenuator design and would like to try and keep it. Can you help point me in the right direction to calculate some more suitable values for the coarse adjustment.

By the way I think that the dual mono version sounds significantly better than the 9 pin all in one that I also built.

Thanks,

PJN
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Old 16th February 2013, 03:39 PM   #2
PJN is offline PJN  United States
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Location: Pa, USA
After thinking about it some more things, it doesn't make sense that something that claims -64 dB attenuation wouldn't be able to control a low gain setup like this current configuration. It's much more likely that the resistor kit that came with the A3 was in error. I'll order some more resistors based on Broskies tables and put them in and see where I land.
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Old 16th February 2013, 04:21 PM   #3
DF96 is offline DF96  England
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Doesn't the Aikido give a voltage gain of a little below valve mu? If so, it is not a 'low gain setup' unless you used something like 12B4.
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Old 16th February 2013, 06:36 PM   #4
PJN is offline PJN  United States
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With the 6CG7s that I used according to the table in the users guide the gain should be around 10, I think that that should be close to a typical preamp. On my 9 pin all in one high gain build with a gain ~ 28 for a F5 amp, I used a 50K 23 step dact type pot and it was just fine.
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Old 17th March 2013, 09:22 PM   #5
PJN is offline PJN  United States
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Here is an update, I redid the attenuator with values of 1% resistors from the manual for a 25K impedance and it did control better, but something was still a little squirrely with the fine adjustment. In the end I ended up replacing it with a 50K 24 step ladder pot using dale resistors, and it was a significant improvement in both sound quality and control. Live and learn. I also noticed that when I replaced a pot on another preamp build I did with an optical based pot some of the dymanics and sound quality seemed to be diminished, I plan to use another ladder type stepped attenuator to replace it in the near future.

PJN
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Old 10th July 2013, 04:39 PM   #6
Mihan is offline Mihan  New Zealand
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Default A3 Stepped Attenuator - Choosing between 1db or 2db resolution

In the manual for the A3 Attenuator are two charts with resistor values for the attenuator. One chart list values for -2db fine & -12db course decrements and the other chart lists -1db fine & -6db course decrements.
I realize that 1 db are finer decrements than 2db, but what should determine one's choice? The attenuator will be installed in an Aikido line preamp.

I will be driving the preamp mainly from high output digital sources into a 100K ohm input tube power amp, so should the attenuator be 100K or would 50K be better?
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