and now for something completely different part 3

[mwl168]

That sounds right to me - similar experience I had first time powering up the completed board!. 

[Helium]

While thinking over building this amp, I'm considering to replace 546B/556B with 550C/560C.

I've got a lot of them matched for B22 projects. Could anyone advise if any problems with them (Vceo lower 45V vs 65V, but typ. hfe is higher)? PS will be 30VDC.

[kevin gilmore]

probably a good idea to use 60V transistors for 30V power supply

[Pars]

Tossed together an LTSpice model for this (warning: I'm a Spice newbie/hack)

 

CFP spice.zip

[Pars]

What would the sonic effect be of running a 25K pot with this? It is what I have (TKD) in my DynaHi/DynaFET and I was planning on running these boards in that. I know you usually spec a 10K.

Thanks!

[kevin gilmore]

Slight DC offset unless you match input parts. So a whoosh as you turn the knob

 

[Helium]

What's the input impedance of cfa3?

And same question for krell ksa5 klone please.

My guess is 10K and 50K respectively, but want to reconfirm.

[kevin gilmore]

that is probably close.  ksa5 has a fet input stage.

[Pars]

Thanks. Like Michael has experienced, even with matching (hFe and resistors), the DC offset without the servo is 66mV (for me), consistent in both boards. The servo brings it down to ~1-2mV. Biasing is at 200mA across the 1 ohm resistors. Q21 (the MJF15030 driven by the 7K resistor and the pot) seems to be running rather warm, but I'll need to recheck that. Overall, the amp doesn't run as warm as the Dynafet does. Running it at +/-30V currently, GRLV.

Sound wise, from a short listen, this didn't sound as "fast" as I was expecting. It still sounded very good. The highs perhaps sounded a little recessed. This is all with the 25K pot. I'll try it with a 10K Alps today and see what difference that makes.

Damn easy to build and get running though; pretty much came right up. As long as you don't make any errors stuffing the board, it works. Yes, I'm looking at you Dynafet

Edited November 25, 2017 by Pars

[mwl168]

Got a shipment in and replaced the 4-pin XLR on my CFP. And, yes, it does look better to my eyes than with the black socket.

 

[Pars]

Looks nice Michael, and agreed on the socket. Having it machined to recess into the front panel might look better as well, but looks great for a prefab case!

Here are a couple of pics of mine so far (the usual not even close to being done )

I took Kevin's suggestion and wired the pass devices off to the heatsink for the GRLV.

Edited November 25, 2017 by Pars

[Helium]

Are you using 1 Ohm wirewound type resistor?

I would stay away from wirewound type in sigal path due to their high capacitance and inductance.

[Pars]

On 4/5/2015 at 5:51 AM, kevin gilmore said:

i will see if i can cram 5 watt resistors in that place.

re: q27,q28 probably better to put 1 watt transistors in there,
trouble is finding them.

 

Those transistors are rated at 500mw, and they are running at 436mw

 

new board will take 5 watt resistors, these

http://www.mouser.com/ProductDetail/Ohmite/WHE1R0FET/?qs=sGAEpiMZZMtlubZbdhIBIFwx82bjKdTOEgG%252bjKeDnyQ%3d

 

but seriously, that power supply should not be able to supply more than 1 amp, so the 1 watt resistors should have been sufficient.

Yes, the 1 ohms are wirewound, from this post. I was thinking about using Mills MRA5s here.

Edited November 25, 2017 by Pars

[Helium]

Well, maybe Kevin knows something I don't know)

Anyway, I would use metal-oxide or metal film insted. To be specific, I'm ordering 71-CPF3-D-1.

Also I remember AMB clearly discouraged against using wirewound in similar locations in his beta.

[kevin gilmore]

The ones I used were metal film, don't remember which ones

inductive things on outputs are a bad idea

[mwl168]

On 11/17/2017 at 9:49 AM, Pars said:

Mouser is pissing me off... placed an order on Sunday night. It didn’t ship until late Tuesday, using that UPS Innovations crap. Supposedly in town being transferred to USPS, but don’t know what’s up with it or when it will be delivered. I was hoping to get the CFP boards at least in a testable state this weekend. Fuckers...
 

I am PISSED OFF at Mouser too. Same story, placed my order last Sunday and paid extra for express shipping. Order was confirmed within a minutes or so but was not shipped till Wednesday.  I have not received the order which means it'll be Monday at the earliest.

This is the same thing that happened to me twice in less than 3 weeks. I called and complained last time, Mouser could not figure out what was wrong but refunded my shipping charge. At least I did receive the shipment on Saturday.

This is too much to be a coincidence. Like Pars, I've always had good experience with Mouser in the past. Wonder what's up with that company.   

Edited November 25, 2017 by mwl168

[Helium]

@Pars,

What p/n you used for 47uF cap? Can you please measure voltage across it (at 30VDC power supply)? I have difficulties in finding that rated over 25V.

[ti5002000]

You have tantalum ones at 35v. I bought some of these to use in this  amp and also in the Grlv. 

[Helium]

Yes, but it's better to know voltage first.

If it's close to 30V, I personally wouldn't use tantalum cap rated 35V. They will be at boarderline of specified voltage and tend to explode, unlike MLCC.

[ti5002000]

9 minutes ago, Helium said:

Yes, but it's better to know voltage first.

If it's close to 30V, I personally wouldn't use tantalum cap rated 35V. They will be at boarderline of specified voltage and tend to explode, unlike MLCC.

Yes, you are right. 

But also as you said before MLCC with more than 25v is hard to find... 

[Pars]

1 hour ago, Helium said:

@Pars,

What p/n you used for 47uF cap? Can you please measure voltage across it (at 30VDC power supply)? I have difficulties in finding that rated over 25V.

The one I used was 81-RDEC71E476MWK1H3B. From the spice model I built, I only see 5Vdc across this cap. It isn't marked polarized on the board, so I used ceramic instead of tantalum here. I don't think I ever saw a BOM here, so don't know what Kevin had in mind. From the silk markings, it would seem ceramic.

[Pars]

Changed out to the 10K TKD from my ss dynalo. Huge difference (for the better). Will need to do some more listening and comparing to the Dynafet.

[kevin gilmore]

newest version of the current feedback amplifier with the servo not in the audio circuit so that you can use cheaper opamps. But if you run 30v, you still need 30v capable opamp. Like opa551.  what the noise difference if any, I don't know yet.

 

 

 

cfp2hmt.zip

Edited November 30, 2017 by kevin gilmore

[Pars]

That was quick Kevin


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[UFN]

FFS Kevin, I just got my "normal" CFA2 boards delivered earlier this week and now you come up with a new version?