EDIT: I failed at reading comprehension. rayma smacked me on the head and got me back on course. No further reading on your part is require 🙂
I'm building a GlassWare Aikido line stage and am using a GlassWare Attn-2 stepped attenuator board for volume control. I selected my output coupling capacitor values using the formula 159155/C/R, based on an R value of 47k, which is the input impedance of my amplifier.
The manual for the Attn-2 says that it will present an input impedance of 20k to the line stage, and describes how this would cause poor interactions with an older tube line stage that expected a 500k load.
My question is, when using this attenuator, does the input impedance that the line stage sees change to 20k? Does it become the sum of the attenuator impedance and the amplifier impedance? Or something else entirely?
I'm building a GlassWare Aikido line stage and am using a GlassWare Attn-2 stepped attenuator board for volume control. I selected my output coupling capacitor values using the formula 159155/C/R, based on an R value of 47k, which is the input impedance of my amplifier.
The manual for the Attn-2 says that it will present an input impedance of 20k to the line stage, and describes how this would cause poor interactions with an older tube line stage that expected a 500k load.
My question is, when using this attenuator, does the input impedance that the line stage sees change to 20k? Does it become the sum of the attenuator impedance and the amplifier impedance? Or something else entirely?
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Don't you want to have the attenuator before the line stage? Otherwise the line stage
will be amplifying the source at full gain all the time, possibly (likely) overloading and distorting.
will be amplifying the source at full gain all the time, possibly (likely) overloading and distorting.
Ohh, I see where I messed up. He's talking about the source device seeing a different input impedance in the line stage amplifier. So this doesn't bare on the line stage circuit at all, but on the source circuit. Which, in my case, is currently all modern solid state stuff, and later will include another Broskie tube product.
Thanks Rayma! I appreciate you!
Thanks Rayma! I appreciate you!
Yes, however a few line stages have had (ganged) level pots at both the inputs and outputs,
to reduce the output noise of the line stage, and with an output buffer for source low impedance.
But this is uncommon, and is seldom necessary or worthwhile, especially now with only low gain needed.
to reduce the output noise of the line stage, and with an output buffer for source low impedance.
But this is uncommon, and is seldom necessary or worthwhile, especially now with only low gain needed.
Something else. At full volume setting the source sees the attenuator resistance in parallel with the load, at low/zero volume it sees just the attenuator impedance.
If the load's impedance isn't much larger than the attenuator impedance it will react with it to change the response curve somewhat. You can design an attenuator for a specific load impedance if you want to cancel this effect, but in all cases the attenuator input impedance will vary as it is adjusted (well, with standard multi-way switches that is - there are ways of using bunches of individual double-throw switches to maintain constant impedance - one of the reasons relay-switched attenuators exist).
This sensitivity to impedance is one of the reasons active attenuators can actually be superior in many situations as they way less fussy about both input and output impedance than simple volume pots or stepped attenuators.