John Warren Posted October 7, 2017 Share Posted October 7, 2017 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. Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 Here's the test setup. The CLIO FW unit is used in FFT mode and is 24-bit, 192kHz sampling rate. Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 FFT at 1000 and 10kHz, 600Ohm load, 6VRMS output. Output is dBSPL Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 FFT at 1000Hz, three output levels, 0.1, 1.0, 6.0VRMS. Output is dBV where 0dBV=1VRMS Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 1000 (green) and 20kHz %THD, 10kOhm load on each of the three outputs. Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 HF compensation, 20kHz sq. wave across 10kOhm load. Output is nearly 14Vp-p. Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 Power supply transistor temperature after 2h operating all 6 outputs at condition shown above. Well below max of 150C. Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 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). Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 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. Quote Link to comment Share on other sites More sharing options...
Schu Posted October 7, 2017 Share Posted October 7, 2017 Is there any measurable loss or interference when using an rca/balanced adapter? Quote Link to comment Share on other sites More sharing options...
John Warren Posted October 7, 2017 Author Share Posted October 7, 2017 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. Quote Link to comment Share on other sites More sharing options...
Schu Posted October 7, 2017 Share Posted October 7, 2017 I'm sorry... I was seeing things in that last photo. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 6, 2017 Author Share Posted November 6, 2017 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. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 18, 2017 Author Share Posted November 18, 2017 http://northreadingeng.com/Buffer_R3/buffer.htm Quote Link to comment Share on other sites More sharing options...
Schu Posted November 18, 2017 Share Posted November 18, 2017 :thumbs up: Trying to digest all of this is somewhat difficult for me. Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted November 21, 2017 Share Posted November 21, 2017 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? Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 21, 2017 Author Share Posted November 21, 2017 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. Quote Link to comment Share on other sites More sharing options...
Khornukopia Posted November 22, 2017 Share Posted November 22, 2017 It is always interesting to read about your projects. Quote Link to comment Share on other sites More sharing options...
John Warren Posted November 22, 2017 Author Share Posted November 22, 2017 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(?). Quote Link to comment Share on other sites More sharing options...
CECAA850 Posted November 22, 2017 Share Posted November 22, 2017 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? Quote Link to comment Share on other sites More sharing options...
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