Best Options for making F-5 an Integrated Amp

pcb for Juma's little preamp

here comes the pcb for Juma's pre based on bf862 (I added the buffer too). the layout was subject to the following constraint: two pcbs will be stacked up on top of each other (for 4 channels total). this with only an inch long spacers and the top pcb will be flipped so they will be facing each other :rolleyes:. this is why all the tall components like caps etc. are close to the edge so when mirrored they do not collide with those on the other pcb. it was not all that easy but looks Ok right now. I will be making manufacturing files and ordering soon. In the Fall I can probably tell you how they sound with an F5 and with an ACA, (have some speaker work to do first :D).

I did the copper pour for the grounds, all one within a channel, no separate signal ground. I hope that's Ok?

also, Juma: Do you think the gain could be raised by 2-3 dBs? (which would allow use of 2x4 miniDSP with only 0.9Vrms outs). Currently it is 9dBs which mandates using a 2x8 XO which comes with 2Vrms outputs, and I am planning on bi-amping only. Thanks a lot!!
 

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  • bf862_Juma_pcb_v1.1.GIF
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never got to post the pic of my 6 channel bf862 pre with Salas reg PS and Dantimax relay volume control. The only thing I overlooked was how many shielded wires one has to deal with in a multi-channel preamp. I started with 8 channels and then decided to save one 2-chan board as I did not really need it and the package was hard to manage. Just the number of shields to ground to chassis was overwhelming.

Question for Juma: I am thinking of making another 4-channels of this great preamp. I checked my stock for parts and I am short of one BC557C. the BC557 is no longer available in C rating so I got some BTF version. Is that significant if I substitute it in one channel? Thx.
 

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Thanks Juma.

Btw it is a very nice, wide bandwidth preamp, a good match for F5.
I wonder why this jfet never received more attention on Pass forum.
I do not believe Papa considered it (maybe when he was looking across jfets for LXMini Xover but I am not sure). Anyways, thanks a lot again.
 
WTB BF862 SOT23

I managed to fry a few channels and have no spares :( of smd version of BF862.215 NXP I bought long time ago from Mouser (now discontinued).

If someone had a few pieces of the part in SOT23 package and was kind enough to part with them I would truly appreciate it

p.s. I ordered some "originals" from China but have no idea if those will be genuine and they will take forever to get over here, so again: if you guys could please check your parts drawers... THX.
 
koja, did you sort yourself out some bf862.215? might take a while to get there from here, although given how tiny, could perhaps be sent as express letter. happy to send you some. you dont need matched sets here, so anything off the reel is fine yeah? if thats the case, you cover postage and I dont need anything for them. depends how many you need. drop me a PM
 
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Hi Koja, I ordered a bunch of the 'AW2' parts several years ago and built XRK971's Class A headphone amp with it but someone posted a few months ago about these being remarked. The telltale sign is that the surface shows sanding marks.

I ordered another batch of AW2 in the last year and when I checked both under a USB microscope, you can clearly see that they have been sanded so I strongly doubt yours will be any different.

I remember testing these for VGS and they were in the expected range but I have no idea what they actually are.

The headamp works just fine and sounds great so I didn't bother looking further into it.
 
Attenuator POPPING NOISES

I fixed the preamp. The things about this bf862 pre that bother me are:

a) the popping noises from Dantimax relay based ladder attenuator at the input (before the preamp circuits) are just atrocious when switching levels. Is there anything that can be done here? I mean I can hear them in my AlephPs where they are at the output but softer and can be tolerated (though I must say with different amps and speakers so not an apple-to-apple comparison!). I will try the bf 862 pre with a different amp at output.

b) sound quality wise: so far I only did a comparison when driving ACAs and here I do not like the preamp in the audio chain because it emphasizes the top to a point where it gets annoying and mid sounds thinner; I prefer the source (4x10hd miniDSP) driving ACAs directly. That was probably to be expected based on the scope measurements where the preamp was good up to almost 200kHz (so kind of F5-like wide bandwidth sound quality I guess). I will try it with F5 to see that killer combo (I need a slight repair on the f5 PS first).
 
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Attenuator POPPING NOISES

I fixed the preamp. The things about this bf862 pre that bother me are:

a) the popping noises from Dantimax relay based ladder attenuator at the input (before the preamp circuits) are just atrocious when switching levels. Is there anything that can be done here? I mean I can hear them in my AlephPs where they are at the output but softer and can be tolerated (though I must say with different amps and speakers so not an apple-to-apple comparison!). I will try the bf 862 pre with a different amp at output.

b) sound quality wise: so far I only did a comparison when driving ACAs and here I do not like the preamp in the audio chain because it emphasizes the top to a point where it gets annoying and mid sounds thinner; I prefer the source (4x10hd miniDSP) driving ACAs directly. That was probably to be expected based on the scope measurements where the preamp was good up to almost 200kHz (so kind of F5-like wide bandwidth sound quality I guess). I will try it with F5 to see that killer combo (I need a slight repair on the f5 PS first).

if you're sure that there is no DC neither on input nor at the output of attenuator, sole reason for popping remains bug in software

meaning - if you have manual mechanical switch for anything in audio chain ( selector or volume) you always need to use Make Before Brake type, to avoid sudden changes in (possible DC) levels and to avoid Mu state of important nodes (point just hanging in the air)

when dealing with software commanded relays, same thing is achieved with simple trick - actuate next relay, and only then release previous one, in whichever direction it is going ( simply delay for Off command)

so, check DC levels at both sides and if you're OK, contact our friend Dantimax ( Mikel, if I remember correctly)

I know he knows his things, but ookoops are ..... possible