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Old 9th March 2007, 02:52 PM   #1
SRMcGee is offline SRMcGee  United States
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Default Lowering Gain on an Aikido Preamp

Folks:

I have just completed an Aikido preamplifier using PCBs provided by Bas Hornemen (thanks Bas!). I'm thrilled with the sound of this preamp, but do have a problem. The preamp includes a Dantimax remote control system that provides 63 dB worth of attenuation, but music is still audible at the attenuator's lowest setting (i.e., -63dB). The highest volume (0dB attenuation) is far too loud. I'm using a complement of 6N1P / 6H30N-EP tubes in the Aikido boards and I'd like to lower the volume by another 6 or 12 dB. My question is, is it better to swap out one of the tubes for something with lower gain or should I insert a resistor (or a T-pad) in-line at the preamp's output? Cost is not a concern -- not adversely affecting the quality of the sound is.

Any advice you could offer would be greatly appreciated.

Regards,
Scott
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Old 9th March 2007, 04:15 PM   #2
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Default Re: Lowering Gain on an Aikido Preamp

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Originally posted by SRMcGee
Folks:

I have just completed an Aikido preamplifier using PCBs provided by Bas Hornemen (thanks Bas!). I'm thrilled with the sound of this preamp, but do have a problem. The preamp includes a Dantimax remote control system that provides 63 dB worth of attenuation, but music is still audible at the attenuator's lowest setting (i.e., -63dB). The highest volume (0dB attenuation) is far too loud. I'm using a complement of 6N1P / 6H30N-EP tubes in the Aikido boards and I'd like to lower the volume by another 6 or 12 dB. My question is, is it better to swap out one of the tubes for something with lower gain or should I insert a resistor (or a T-pad) in-line at the preamp's output? Cost is not a concern -- not adversely affecting the quality of the sound is.

Any advice you could offer would be greatly appreciated.

Regards,
Scott

Hi Scott,
Is the remote control unit active? (i.e. does it contribute some gain itself?) If so, then there's where I would start.
My Aikido project uses 6H30s throughout, as I don't need much (any) gain - I'm going for low output impedance to drive long cables/difficult loads. Just doing the chassis at the moment - hope to get it finished sometime next week
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Old 9th March 2007, 04:49 PM   #3
SY is offline SY  United States
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Pad down the input, not the output. When choosing pad resistor values, don't forget about the input capacitance.
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Old 9th March 2007, 05:11 PM   #4
SRMcGee is offline SRMcGee  United States
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SY:

Go easy on me -- I'm a newbie. I understand that impedances are important in determining resistor pad values (how are they determined?), but what should I also know about input capacitance?

Regards,
Scott
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Old 9th March 2007, 05:27 PM   #5
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The input stage will have a capacitance associated with it consisting of various strays, the grid/cathode capacitance (somewhat reduced by the unbypassed cathode resistor), and the grid/plate capacitance The last is modified by the Miller effect, i.e., it is increased by the input tube's gain, and generally is the dominant factor. So if a tube has 5pF of grid to plate capacitance and it's run at a gain of 10, the effective input capacitance is 10 x 5pF = 50pF. I'll leave calculation of that for your particular tube to you, but as a rule of thumb, the Aikido circuit gives a gain of about mu/2.

Now, the input divider presents a source impedance to the input, and the presence of capacitance there causes high frequency losses, since the source impedance and shunt capacitance form an RC filter. So, the lower the source impedance, the less problem that rolloff is, but the more difficult the preamp is as a load.

Let's make a couple of WAGs to see the order of magnitude. Say we have 50pF of input capacitance. Let's also pad down the input by a factor of 2. If we have an input grid leak resistor of 100k, that means we would add a 100k resistor in series with the input. The source impedance presented to the grid is then the two resistances in parallel, or in this case 50k.

OK, let's calculate. With a 50k source and 50pF of input capacitance, we would have a rolloff with the f3 at about 64kHz. That's probably OK but marginal. I'd want to make that at least twice as high, so for this example, I'd change the resistors to 50k each and get the f3 over 100kHz.

I'll emphasize that these are not the numbers appropriate for your preamp, I don't have the datasheets in front of me, they're just for illustrative purposes.
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Old 9th March 2007, 06:39 PM   #6
SRMcGee is offline SRMcGee  United States
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SY:

While it may take some time to digest this information, I now have a starting point for understanding some practical aspects of padding the input. But what I don't understand is the impact on gain. Accepting that your example is just that -- a hypothetical -- what would the attenuation be of a 50k input pad resistor in a preamp circuit with a gain of 10?

Thanks,
Scott
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Old 9th March 2007, 09:08 PM   #7
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Default Input vs Output Padding

Hi SY,

Will you please explain the differences (pros & cons)? No intenion on hijacking the thread, but the reason I am asking is that I'm thinking about Aikido driving two identical power amps through PLLXO. Louspeakers are of different sensitivity, so I will need to adjust the gain per channel. Adjusting at the input will require two Aikido pres, adjusting at the PLLXO (otput) - only one.

So what are the drawbacks of output adjustment?

Thanks!
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Old 10th March 2007, 01:22 AM   #8
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Hi, I think we might be getting a bit too complicated here - why not put a divider on the input (or output, if you are sure you won't be overloading the pre with the input signal). Hell, just put a preset pot (use a high quality panasonic, if you like) in and adjust to suit.
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Old 10th March 2007, 01:37 AM   #9
SY is offline SY  United States
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Beelzebub, that's exactly what I'm suggesting, an input divider. But I'm cautioning that there is a tradeoff when selecting the values.

The idea of a preamp is to have a box of gain that is easy on the input (high impedance) and has a low source impedance to drive cables and amplifiers having significant capacitance. Padding the output defeats the latter. Padding the input needn't defeat the former, but can't be done willy-nilly with zero feedback triode voltage amplifiers like this.
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Old 10th March 2007, 02:09 AM   #10
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Quote:
Originally posted by SY
Beelzebub, that's exactly what I'm suggesting, an input divider. But I'm cautioning that there is a tradeoff when selecting the values.

The idea of a preamp is to have a box of gain that is easy on the input (high impedance) and has a low source impedance to drive cables and amplifiers having significant capacitance. Padding the output defeats the latter. Padding the input needn't defeat the former, but can't be done willy-nilly with zero feedback triode voltage amplifiers like this.
Hi Sy, How would you compensate for a variable pad (volume pot or switched attenuator)?
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