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PostPosted: 19 Jun 2020, 10:40 
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Is there a relatively simple schematic for a preamp that would have low output impedance? I'm looking for something to put before an icepower module and they like really low input impedance


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PostPosted: 19 Jun 2020, 11:01 
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invaderzim wrote:
Is there a relatively simple schematic for a preamp that would have low output impedance?
Do you mean like this one?

I also looked at the ICEpower 50ASX2SE, just looking for an example. For this unit the DC input impedance is 270kΩ and the midband AC input impedance is about 40kΩ. There are a lot of tube circuits that would drive this amplifier. However, the maximum input voltage is 3.3V meaning that any preamp you put in front of it should be relatively low gain so you don't start clipping the input.

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PostPosted: 19 Jun 2020, 12:33 
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From what I've read these are quirky amps when it comes to input impedance. From another forum:

"it turns out that the input impedance of these ICEPower amps drops significantly with rising frequency. At DC, the input impedance is 270K, at low frequencies, it drops to about 27K, and at 20KHz it's below 6K. This means that the amp needs a low output impedance source to drive it The beauty of these amps as I saw it was that they were complete with only connectors and an enclosure needed. But it turns out that a simple volume potentiometer put in front of the amp messes up the frequency response significantly."


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PostPosted: 19 Jun 2020, 14:21 
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Hi pretty much any tube cathode follower output stage can drive it. One using something like a 12AU7 of any of its variants would be fine. An output impedance of a few hundred ohms is typical.

Good listening
Bruce

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PostPosted: 19 Jun 2020, 15:47 
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I can agree with the statement that adding a volume control pot in front of one of these inputs could negatively affect the high end frequency response. But, at least for the 50ASX2SE, thee manufacturer specs the minimum input impedance of 8kΩ across the audio band. So all you need is a driver with less than 2kΩ of output impedance.

As Bruce said, the 12AU7 would be your buffer of choice. The one I referenced has a 1.4kΩ output impedance, but I have another design with an output impedance of only 575Ω, -0.63dB in gain, and a distortion factor of only 0.16%. They aren't hard to come by. Just put one behind the tube preamp of your choosing. But as I said before, watch that overall gain.

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