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Class D Switching Power Amplifiers and Power D/A conversion

Impedance ratio and T-amps
Impedance ratio and T-amps
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Old 15th February 2007, 02:09 AM   #1
Sasquatch is offline Sasquatch  United States
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Join Date: Aug 2004
Location: Alabama
Default Impedance ratio and T-amps

I have been registered with this forum for a while but have only started posting in the last couple of weeks, so forgive me for just jumping into the discussion.

I love the sound of tube amps and bought the Tripath SI on a whim based on the laudatory reviews. I thought the sound was clear and far better than solid state, but thin and without good bass. Being a tweaker, I immediately removed the circuit board from the case and installed it in a different chassis with bigger and more adequate input caps, a switching power supply to provide adequate current, and high quality bypass caps. I added a 50,000 mike Sprague reservoir cap that can run the amp alone for .5-1.0 sec for slam. The sound was better but still a little thin in the midrange. FTR, I like a big, fleshed out "organic" sound with good dynamics.

After looking at the schematic (and confirming this on a multimeter), I realized that the amp has a 20K input impedance but has a 50K (in my case 100k) pot in front of it. Thus, voltage distortion should be unacceptable by common design standards.The optimal impedance ratio ought to be better than 1:10 (first to second stage). So, I built a resistor loaded tube buffer after the pot and before the amp. This is a stupid amount of circuitry in front of that little circuit board. Nonetheless, the sound was superb--better than any tube amp I have built. This has been the greatest pleasure I have had in audio. The true measure of great audio is whether your shoulders slump within 5 seconds of listening. Nothing else matters. If you don't relax, it's not good. This configuration does it with only the slightest hint of improvement needed (nothing's ever perfect)

Well, after looking over reviews of these amps, I noticed a pattern. People who don't like the amp usually don't report having preamps. People who love the amp, do. They turn the volume control to max and then use their preamp to control the volume. This means that they have supplied the board with a good impedance ratio.

I submit that the major factor in the appreciation of this wonderful chip is the impedance ratio driving the chip itself. And adding some tube distortion may help matters.
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Old 14th April 2013, 12:14 PM   #2
hotironsmoke is offline hotironsmoke  United Kingdom
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Default schematic

Could you show us your full final schematic.Thanks
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Old 14th April 2013, 03:40 PM   #3
ATAUDIO is offline ATAUDIO  Austria
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Join Date: May 2011
Location: Wien
Originally Posted by Sasquatch View Post
I realized that the amp has a 20K input impedance but has a 50K (in my case 100k) pot in front of it. Thus, voltage distortion should be unacceptable by common design standards..
What you say *might* have a meaning in subjective/audiophile sense, but it is meaningless in an electronics - scientific POW.

Resistors, by definition are linear components and do not carry distortion with the notable plus of not influencing the phase. And even if you have massively unmatched input iimpedance (that is NOT your case) this would affect power transfer between the two devices, NOT distortion.

In general having a source (including the input pot) with an impedance realtively higher than the input (the T-amp) is NOT a problem when you consider it as a Voltage amplifier. You have just a sub-optimal Power transfer, but since we are speakig about voltage signal, this should not be a problem.

Note that this is valid also when considering the impedance given by capacitive (usually) or inductive components in the input network. In fact inductors and capacitor as referred as linear devices in electronics.
As long you have only linear devices in your signal path (and of course that would be great, but it is not the reality) you have none distortion.

How this rhen applies in general to T amps, it is also something that i cannot understand from your post. For the few that I know. in T-amps (i.e TA2020 stuff), as usual, the input inmpedance is greatly the result of the rf/ri network used to estabilish the closed loop gain.
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Old 14th April 2013, 07:44 PM   #4
Sasquatch is offline Sasquatch  United States
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Join Date: Aug 2004
Location: Alabama
Funny. The original post is six years old, and I have long since gone onto other things. At the time I had a faulty understanding regarding impedance ratios and distortion. I know now that impedance matching is critical for power transfer or insuring adequate current deliver to the second stage, but couldn't affect the signal fidelity as described here.

Still, I have always thought a tube (in this case, I think it was a resistor loaded 6bl7 or something similar) to sound nice following a digital source. No theory as to why. Pleasant harmonic distortion? But it does seem to flesh out the sound and enhance the sense of presence.
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