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kevin gilmore

and now for something completely different part 3

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Look at the schematic. The ones opposing are the pairs which should be matched if possible. It isn't critical however.

Reading through the thread oftentimes finds the answers to questions as well...

Edited by Pars
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Gotcha, thank you very much! Do you mean opposing transistors as in complimentary transistors? Google wasn't very helpful in that regard...

Reading through the thread is a little overwhelming x)

Edited by Ariamella

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CFA2 (CFP) - current feedback amp

I still cannot figure out how to connect them into balanced version of CFA2 (CFP) - current feedback amp as they are single end type of pcb.

I know I need four boards, but how do I  connect the negative input rail ?

Your help is much appreciated.

 

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10 hours ago, samsie said:

I know I need four boards, but how do I  connect the negative input rail ?

Source must be balanced (or made balanced as per Pars suggestion above).

Then, for one balanced channel, one board amplifies the positive signal and another board amplifies the negative signal. 

In this context, it might help to think of a balanced source as a four channel device. 

Edited by MLA

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A pair of balanced CFA boards? Sweet!

Happy to see another CFA being built! It is still my favorite dynamic amp of all time!

Keep us posted! 

Edited by mwl168

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Looks good. I don't really need more headphone amps, but do you have any spare boards? 😂

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The PCB shop could only do 5 as a minimum, so yeah, have 3 spares.

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So I just ordered some of the parts to start building my single ended CFA and some decisions need to be made before I continue for which I ask your help:

1. KG recommends +-24V, is there any real benefit in going up to 30V for "regular" cans such as HD650. I do not see myself buying a pair of orthos in the near future although I would like to try them at some point. I am looking to build a single chassis in a somewhat small footprint so having to deal with the extra heat might be a problem.

2. What heatsink size would you recommend for this? I am thinking of placing both boards on a 200x120x40 heatsink from modushop to make a single heatsink enclosure like the one I sketched.  

3. Any suggestions regarding stepped attenuators or volume pots that don't break the bank? I'd like some better quality than a RK27... 

I'll probably have some more doubts before I finish so you'll have to bear with me on this one. 

20200408_221102.jpg

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1. I experimented with different voltage rails PSU with my build. Cannot say I heard a real difference but my CFA is balanced, not single ended. 

2. The heatsink size you are planning for should be fine. You do need to pay attention to heatsinking the PSU as well. This amp draws significant current from the PSU. 

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8 minutes ago, mwl168 said:

1. I experimented with different voltage rails PSU with my build. Cannot say I heard a real difference but my CFA is balanced, not single ended. 

2. The heatsink size you are planning for should be fine. You do need to pay attention to heatsinking the PSU as well. This amp draws significant current from the PSU. 

I plan to use this side panel, it is 10mm thick and is able to cope with a few watts. Should be similar to a bottom panel some others have used I believe. 

sidepanel.jpg

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I settled on this case: 

width 320, height 70, depth 248, plenty of space for a single ended cfa and I might even be able to fit a coilcraft COMOCO choke filter (comon and differential filter in the same choke.)

Transformer will be a 100VA from toroidy.pl, used them in other projects and quality is top notch.

Now on to find a suitable 10K volume control. Would like something a little better than a RK27. stepped attenuator or a relay based one. Any ideas that don't break the bank?

 

s-l500.jpg

s-l500-2.jpg

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So, one channel of the CFA3 is assembled. As expected, it's acting funny!

20200523_030704.thumb.jpg.d8b6baa9a6a0c70e4f8a3f27c9a2a54f.jpg

On the amp board only that LED stays lit. The four LEDs near the input stage only briefly light up when I'm powering on. The fifth outermost LED is dark no matter what.

I have no problem biasing the output stages to 180mA, but have no idea how to do it for the input stage.

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the unbal to bal part of the board had a problem with the current source, this was discussed over at diyaudio

on the negative current source the 10k resistor went to the emitter. it needs to go to the base.

first picture is the fix, second picture was the original with the error.

cfp3rssfix.jpg

cfp3rssbroke.jpg

Edited by kevin gilmore
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Looks great!

One thing, when I was searching for parts, the 600pf ceramic cap seemed to be problematic.

image.png.e3a6ada81d5ea09c7b7558115b1a9601.png

Mouser only had one part and the lead spacing was more similar to what's for the 47uF cap.

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Second channel came up without a fault. For a 2U heatsink 200mA seems to be the highest what's thermally viable, the hottest transistors were above 70C. Once the spare THAT340 arrives, I'll try to do a listening test.

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I presume the front end on these is similar or the same as the SS Dynalo? If so, JFETs sound a lot better than the THAT340 does.

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