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John Warren

CL-D Bi and Tri-amping (lots of photos and plots)

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The CL-D discussion got me thinking about buffers so I decided to put a thought to hardware.

 

The idea here is to split the small signal, say from the preamp or CD, into three "sub-channels".  Each split can then go to a LF, MID or HF active filter and a CL-D amplifier "sized" for that driver.  Alternatively, each split can go straight to a sized CL-D amp and then to a passive network connected its respective LF, MID or HF driver.

 

Doug Self designed a buffer to do the same thing but this one has less parts, on board +/-15VDC rails, XLR combo inputs and really nice Mill-Max IC sockets.  It might not be as quiet as Mr. Small's design claims to be but this design is pretty quiet, distortion is very low and it's got a ton of output current capacity (each "split" will provide +/-22mA across a 600Ohm load with +/-14VDC rails!).  

 

Ideally, having each split feeding an active filter for a woofer, mid and tweeter and then outputting the contoured small signal to a CL-D amp sized for the particular driver is where I want to take this idea (the filters will be a separate thread)

 

It's a stacked op-amp configuration, unity gain, voltage follower based on the LME49720 package.  I designed the circuit with RFI immunity built in.  Input impedance is between 9-10kOhms for unbalced and 18-20kOhm for balanced.  The power supply I'm testing it with is something I had from a previous job, a bel +/-24VDC, 600mA, linear supply (HAA24-0.6-AG).  They're made in China but good quality, low ripple and moderately priced at $60.  The +/-24VDC powers the +/15VDC supplies on the board.  Any DC supply capable of up to about 80V can be used.

 

First three photos are of the board, board with hardware and assembled.

 

buffer_5.jpg

 

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Here's the test setup.  The CLIO FW unit is used in FFT mode and is 24-bit, 192kHz sampling rate.

buffer_7.jpg

buffer_3.jpg

buffer_6.jpg

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Power supply transistor temperature after 2h operating all 6 outputs at condition shown above.  Well below max of 150C.

buffer_13.jpg

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Distortion/Output Voltage (7VRMS), 20kHz, across 600Ohm load, all outputs loaded.

Analyzer as 30KHz low-pass filter engaged, this represent %THD in "audible" spectrum (if your a dog).

 

buffer_10.jpg

buffer_9.jpg

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I really like the XLR combo inputs but there a bit of a pain to design around.  Phot shows RCA adapter.

See AoE, 3rd ed. Fig. 8.127 A for Pin 1 wiring.

 

buffer_4.jpg

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Is there any measurable loss or interference when using an rca/balanced adapter?

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50 minutes ago, Schu said:

Is there any measurable loss or interference when using an rca/balanced adapter?

It's not a balanced line using the 1/4" RCA adapter shown above. 

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I'm sorry... I was seeing things in that last photo.

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Final (through hole and socketed) design shown below.  After working with the prototype I revised chassis and circuit ground paths, added RFI/EMI common and differential mode front end filtering, reduced feedback factor to about 2X, added additional filtering to supply rails and re-routed a few of the traces.  For this build bandwidth across RL=10kOhm is -3dB @ 80kHz.  

 

With headphones, a listen with either an LME49720 or NE5532 evidences nice, quiet background.  Buffer/splitter takes balance or unbalanced signal and splits it three-ways for tri-amplifier arrangement.  The board shown is two channel board so six splits in total, three/channel.  Each split can feed a separate amplifier or active filter.  Input impedance is about 18kOhm.  Output impedance is about 600Ohm with the LME package and lower with the 5532.  Resistors are 0.01% (lower will improve circuit CMRR, theoretically).  The plots are with through hole components and socket mounted packages, power supply is unit described above.  Might be a nice kit for builders.  The SMD version is much tougher to solder up.   

R3.0_build_1.jpg

bandwidth_RL=10kOhm.jpg

FFT_RL=10kOhm.jpg

%THD_RL=10kOhm.jpg

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:thumbs up:

Trying to digest all of this is somewhat difficult for me.

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On ‎11‎/‎18‎/‎2017 at 11:55 AM, Schu said:

:thumbs up:

Trying to digest all of this is somewhat difficult for me.

Me too.  So this is a splitter and each channel is identical to the original signal nor do they split into different frequencies?

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9 hours ago, CECAA850 said:

Me too.  So this is a splitter and each channel is identical to the original signal nor do they split into different frequencies?

 

Yup.  Each channel split up into three identical signals (essentially identical, there's always a bit of a difference).   There's some filtering at the front end for RFI but, other than that, no signal conditioning across the audio range.    

 

The webpage isn't complete.  I have some content related to common mode rejection and resistor selection that I'd like to publish.   I'd such a nice job of selecting resistors the CMR isn't easily measured however.  

 

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6 hours ago, Khornukopia said:

It is always interesting to read about your projects.

 

Glad to hear it. 

 

I got an email asking if this module could be used to split the signal out from an electric guitar so let's see where that goes(?).    

 

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15 hours ago, John Warren said:

 

Yup.  Each channel split up into three identical signals (essentially identical, there's always a bit of a difference).   There's some filtering at the front end for RFI but, other than that, no signal conditioning across the audio range.    

 

The webpage isn't complete.  I have some content related to common mode rejection and resistor selection that I'd like to publish.   I'd such a nice job of selecting resistors the CMR isn't easily measured however.  

 

The advantages of using this over, say, a couple of RCA splitters?

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