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Old 9th October 2018, 03:24 PM   #41
Guerilla is offline Guerilla  Denmark
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I dont have technical insight, but seems interesting. I wonder why there are no replies. Nor positive or negative What are the needs that a compound amp that a VSA or TCA cant cover alone?
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Old 9th October 2018, 03:28 PM   #42
Zen Mod is offline Zen Mod  Serbia
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Old 9th October 2018, 11:54 PM   #43
Antoinel is offline Antoinel  United States
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Compound Power Amplifiers
Default A Stereo Compound Amp using Positive Current Feedback

Thank you Zen Mod and Guerilla for your comments. My compound amps are experimental curiosities wherein I search for an improved subjective performance in my listening room. Compound amps are common. For example; the STASIS power output stage of a THRESHOLD amp is a compound amp. It is [a marriage of] a powerful bilateral current source amp which bootstraps a low power bilateral voltage source amp. Others exist.

The bench prototype which I already described in my previous post has been upgraded from mono to stereo operation. Please print the attached schematic and note the following explanations:

1. The STASIS amp [U1] and its OPAmp circuits to the left of the $ sign vertical barrier is considered/taken to be the Left Channel. It is fed with the Left output signal from a CD player.

2. The STASIS amp [U2] and its OpAmp circuits to the right of the $ vertical barrier is the Right Channel of this compound amp. It is fed with the Right
output signal from the same CD player.

3. Positive Current Feedback [PCF] has a strong effect on the low [<200 Hz] audio frequencies which drive the woofer. PCF enhances the damping of the loudspeaker's woofer. Such low frequencies are roughly mono in nature. Thus, this amp readily played in stereo; such that the Right and Left images were readily discerned.

Stereo and/or dual channel power amps are plentiful! They can be commercial or DIY. For example; U1 and U2 can be diyF5, diy DEF etc. diyF5 in this application may further be loosely named a stereo diyF7 clone; albeit a powerful one [75 W/ch across 8 Ohm] and not a sissy!.

I refer to this amp as as a compound; because of the power transformer which enables crosstalk between U1 and U2. Its presence or not [remove it] in this circuit and the resultant sonic value will be investigated. Is it needed?

I will need to understand the influence of direct PCF inside the same amp versus the cross PCF which I show in this schematic.

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File Type: png StereoComboAmpWithPCF.png (41.4 KB, 84 views)
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Old 11th October 2018, 10:00 PM   #44
Antoinel is offline Antoinel  United States
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Compound Power Amplifiers
Default THRESHOLD S/150 Using Positive Current Feedback

The attached is a simple schematic for the subject amp. I no longer call it a compound amp. Because it lacks the power transformer [see past 2 posts] which enabled cross-talk between the two power amps, and further embodies Intra PCF instead of Inter PCF.

Here is the method to determine the value of the encircled resistors [5.6 K]. This resistor determines the extent of acceptable/safe PCF; meaning to prevent each amp from oscilllating. A resistor value of 3.3 K caused oscillation!

1. I replaced the 5 Amp Fast Blow output fuse in each channel with a 1/2 Amp Fast Blow.

2. I progressively lowered the value of the encircled resistors starting with 100 K. I used the standard values of [82K, 68K, 56K, 47K etc ] down to 3.3K. At this point the 1/2 Amp fuses blew.

3. Each amp was stable with an encircled resistor value of 4.7K. I chose 5.6K for added precaution when I restored the original 5 Amp output fuse.

This is a great sounding system.

I experimented with the following variables:

1. No transformer, a 115Vac:115Vac transformer [VPT230-110], and a 24Vac:24Vac transformer [VPT48-520]. I used VPT48 normal secondaries for this function. Bass loudness followed the trend VPT48>VPT230>no transformer.

2. Intra PCF versus Inter PCF with the variables of #1. I favored Intra PCF

3. PCF versus none. I favored Intra PCF or Inter PCF over no PCF. Bass was tight and detailed with either PCF.

With the exception of the subjective results of #3, the circuit variations due to #1 and #2 gave fully acceptable subjectives.

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File Type: png THRESHOLD S150WithPCF.png (43.0 KB, 57 views)
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