and now for something completely different part 3

[JoaMat]

On 4/5/2023 at 3:17 PM, pontif_ said:

Can anyone tell me what the single 10k trim pot is for on the CFA3 board?

I’m not sure of the purpose of the 10K trimmer. The two op amps control the outputs very well so I think you can set the trimmer to any position without noticing much.

Anyhow, I omitted the trimmer along with the 10K resistor connected to trimmer center pin in my CFA3_something project.

Edited April 6, 2023 by JoaMat

[simmconn]

I think the 10k trimmer balances the P and N arms of the input stage, hence reduces the input bias current of the amp. Too much input bias current could cause a scratchy volume pot over time. The adjustment is outside of the servo feedback loop therefore needs to be adjusted separately. I would leave the volume pot to max or disconnect the pot, adjust the trimmer to minimize the voltage across the input resistor (100k) or across the volume pot, after the amp is fully warmed up.

[JoaMat]

Thanks simmconn. I think you are absolutely right about that trimmer.

When working with my version of CFA3 I added a small trimmer to the  lower current source (sink) to be able to, as you put it, balance the P and N arms of the input stage.

I don’t remember my reasons to omit the 10K trimmer. But I made some simulations now in LTspice and they indicate that the trimmer changes the voltage cross 100K resistors with  0.1mV or so (but that is my simulations).

[simmconn]

The trimmer should change the input bias current by only a small amount. The real-world devices are not as well-matched as the models in SPICE, which is why a trimmer becomes handy. Also, if you use the global NFB version of the schematic for sim, the feedback resistor will drag the input toward the output (hopefully zero) such that the change of the voltage across the input resistor is less. It would be more pronounced with the non-NFB version. Another way to look at it in sim is to monitor the input bias current directly through the 100 Ohm resistors at the base of the input transistors as the trimmer is adjusted.

[bbest]

Assembled my balanced Dynahi old version of 2004.

Using it with HiFiman HE-6. Sound: entertaining, opened, dancing cheerful. Compared it with beta22.

Have only couple of questions:

* does it make sence to change main capacitors from Panasonic FC 470uF on Nichicon KG 680uF

* DC offset without opamp +-10mV, with opamp OP27 +-2mV. Is it ok, left as is or try to make it better by changing on opa134 or something like this

* does it make sence to make some other mods

What you think?

Edited April 20, 2023 by bbest

[audiostar]

Nice! Suggestion for a mod, or more like a necessity 🙂 with dynamic headphone you should consider putting the KG headphone protector on the output, before the headphone connector. 

Why not a CF2/CFA3, just curious?

[bbest]

I just had 2004 boards from another guy and finished the project. And I am not sure what is the best for isodynamic headphones: CFA3 or Dynahi_2004.

Edited April 20, 2023 by bbest

[audiostar]

I have a CFA3 for my HE-6 and like it a lot. But no Dynahi to compare. 

For me CFA3 was the next evolution step after the Dynahi, but maybe Kevin may chime in. 

[bbest]

Have you heard beta22?

Edited April 20, 2023 by bbest

[audiostar]

No, I don't like the heatsinks mid inside the chassis 🙂

[JoaMat]

Never underestimate good old designs.

[Pars]

I personally prefer the CFP3 to a Dynahi, but I have only compared SE directly. The CFP3 is probably my favorite dynamic amp, but my SE Dynafet is awfully close (and better than an SE Dynahi in my opinion).

I've never built a Beta22, but have heard them at meets (10 or more years ago). I don't recall being particularly impressed then, even though I have a lot of respect for Ti.

Edited April 21, 2023 by Pars

[bbest]

6 hours ago, Pars said:

I personally prefer the CFP3 to a Dynahi, but I have only compared SE directly. The CFP3 is probably my favorite dynamic amp, but my SE Dynafet is awfully close (and better than an SE Dynahi in my opinion).

I've never built a Beta22, but have heard them at meets (10 or more years ago). I don't recall being particularly impressed then, even though I have a lot of respect for Ti.

And what difference between CFA3 and Dynahi? My balanced Dynahi seems more rude, more simpler then beta22. Last one very close to electrostatic sound.

[Pars]

From recall, it seemed that the Dynahi was more clinical than the CFA3. Not that the CFA3 lacks detail at all, I guess your description of rude might be a good way of putting it. They are both great amps, I just prefer the CFA.

[arcanaking]

Hi all, sorry for requiring the pdf/gerber. I am trying to build my CFA3 with the balanced audio protection board. Yet, it seems hard to find the gerber or pdf of the protection board in other threads. Can anyone tell me if it is a must to use the protection board? If yes, where can I find the document?

[audiostar]

6 hours ago, arcanaking said:

Hi all, sorry for requiring the pdf/gerber. I am trying to build my CFA3 with the balanced audio protection board. Yet, it seems hard to find the gerber or pdf of the protection board in other threads. Can anyone tell me if it is a must to use the protection board? If yes, where can I find the document?

You must not use the protection board, it is recommended though.

[JoaMat]

8 hours ago, arcanaking said:

where can I find the document?

Take a look in the balanced headphone protector thread. Just a few pages and I think you might find something useful there.

[Satyrnine]

On 4/22/2023 at 10:18 PM, arcanaking said:

Hi all, sorry for requiring the pdf/gerber. I am trying to build my CFA3 with the balanced audio protection board. Yet, it seems hard to find the gerber or pdf of the protection board in other threads. Can anyone tell me if it is a must to use the protection board? If yes, where can I find the document?

https://drive.google.com/file/d/1wsVGv82qhTbcngFUPGC_LqT3Po8_XJVX/view?usp=share_link

This is the headphone protect gerber you're looking for.

[Satyrnine]

Almost ready to start building my CFA2/3! 

• Dual Mono, 2x GRLV, 4x CFA2 boards, and maybe a bal/unbal down the road. I plan to use this with my LCD-2's (orig), and my desk monitors via added speaker outs. I have all the components on hand but the chassis so far.
• Details: Dissipante 3U chassis. Mounting boards directly to heatsink.
• 2x Antek 100VA 30V trafos with steel covers
• MUR820 Ultra Fast diodes on daughterboards vs standard GBU4 package rectifiers for GRLV boards
• Goldpoint quad/balanced stepped attenuator
• KG Headphone Protector Board (just had a set made at Oshpark)
• Options for Bal and SE in
• Bal and Unbal outs (unbal out only works with se ins in CFA2 mode obv)
• Dale RN60 resistors throughout, Used Pars's BOM

Here's what I have drawn up so far for Front/Rear panels I'm having made at Modushop, as well as internal layout plan thus far. Getting panels engraved/machined, with the "CFA" cut deeply at a bevel. Large circle where vol knob is is just a placeholder for the largest possible size knob. Dotted line around headphone/speaker switch is a rear recess since front panel is 10mm thick.

Many Thanks to the infamous Dukei/Miroslav and Pars for guidance, as well as arteom from diyaudio for generously supplying boards, and of course the man himself for this awesome design.

[Satyrnine]

One side question: My understanding is that Nichicon MUSE/Fine Gold, or SIlmic/etc caps are inferior to power-supply specific caps for rails usage, no? Always thought they were for audio path/coupling use when needed, where an ultra low ESR, high mtbf hrs, and high temp (105c) cap is best for rails. I have just seen MUSE/FG/etc in places I'd usually choose a psu cap for. Am I off here?

[Helium]

4 hours ago, Satyrnine said:

Almost ready to start building my CFA2/3! 

• Dual Mono, 2x GRLV, 4x CFA2 boards, and maybe a bal/unbal down the road. I plan to use this with my LCD-2's (orig), and my desk monitors via added speaker outs. I have all the components on hand but the chassis so far.
• Details: Dissipante 3U chassis. Mounting boards directly to heatsink.
• 2x Antek 100VA 30V trafos with steel covers
• MUR820 Ultra Fast diodes on daughterboards vs standard GBU4 package rectifiers for GRLV boards
• Goldpoint quad/balanced stepped attenuator
• KG Headphone Protector Board (just had a set made at Oshpark)
• Options for Bal and SE in
• Bal and Unbal outs (unbal out only works with se ins in CFA2 mode obv)
• Dale RN60 resistors throughout, Used Pars's BOM

Here's what I have drawn up so far for Front/Rear panels I'm having made at Modushop, as well as internal layout plan thus far. Getting panels engraved/machined, with the "CFA" cut deeply at a bevel. Large circle where vol knob is is just a placeholder for the largest possible size knob. Dotted line around headphone/speaker switch is a rear recess since front panel is 10mm thick.

Many Thanks to the infamous Dukei/Miroslav and Pars for guidance, as well as arteom from diyaudio for generously supplying boards, and of course the man himself for this awesome design.

I used modushop dissipante quite a few times. I doubt that you will be able to mount boards on side heatsinks this way. Have you accounted for steel support brackets that take up significant area of the heatsinks, and use of steel chassis on the floor? Since you are mounting transformers on the floor it's good practice to use steel chassis (sold separately) to get extra rigidity.

[Satyrnine]

5 hours ago, Helium said:

I used modushop dissipante quite a few times. I doubt that you will be able to mount boards on side heatsinks this way. Have you accounted for steel support brackets that take up significant area of the heatsinks, and use of steel chassis on the floor? Since you are mounting transformers on the floor it's good practice to use steel chassis (sold separately) to get extra rigidity.

I haven’t, thanks for the heads up, definitely the reason I posted here before finalizing/ordering. Thank You! That said, Dukei is the one who suggested it, sounding like it was possible. I’ll touch base with him on those concerns and either redesign or report back with a possible solution. 
 

As far as the steel floor chassis add on, this is good to know too. The trafo covers i ordered through antek are nice and beefy and support/clamp to the bottom plate around the edge of the cover as well as via the thicker/wider center support which maybe would prevent/reduce any rigidity related issues vs bare toroid? Def open to the add on support, but prefer not to if possible, if only for aesthetics. Since I’ll be assembling myself I don’t have to worry about shipping with trafos installed at least.

[Helium]

1 hour ago, Satyrnine said:

I haven’t, thanks for the heads up, definitely the reason I posted here before finalizing/ordering. Thank You! That said, Dukei is the one who suggested it, sounding like it was possible. I’ll touch base with him on those concerns and either redesign or report back with a possible solution. 
 

As far as the steel floor chassis add on, this is good to know too. The trafo covers i ordered through antek are nice and beefy and support/clamp to the bottom plate around the edge of the cover as well as via the thicker/wider center support which maybe would prevent/reduce any rigidity related issues vs bare toroid? Def open to the add on support, but prefer not to if possible, if only for aesthetics. Since I’ll be assembling myself I don’t have to worry about shipping with trafos installed at least.

Well, I myself don't like mounting trafos on aluminum plate. Dissipante comes with 3mm anodized top and bottom plates, mounted with 4 small screws m3. For me this structure is not very good to sustain weight of trafos.

Moreover, if you use steel chassis to mount everything inside, you can do without any holes in bottom plate (except those for feet).

[Helium]

This is how 3U heatsink looks like with brackets installed.

[Satyrnine]

This is great, thanks Helium! You don't happen to have dimensions of the bracket do you? Edit: I may be able to determine dimensions from that top view by measuring off it and scaling. I may be able to make it work, considering I'll be cutting off the parts of the CFA boards where the transistors would typically lie, would give me some extra room. As Dukei said "it will be a tight fit, patience will be key" haha!

I mainly wanted a better thermal connection from transistors to heatsink since I'll be driving speakers as well. Coming from IT, thermals are always on my mind, although maybe unneccessary in this situation.

As far as the trafo mounting goes, I might add a plate under them for a bit of stiffening. Maybe drill a few more mounting holes in the bottom plate/brackets to connect it more securely. Only 4 bolts is indeed concerning. I just personally don't care for the looks of the steel bottom plate, although it would be very convenient that's for sure.

Thanks for your help here!