Firstwatt F5: Why some people should not DIY

[spritzer]

I think Kevin was referring to the F1 which is very picky indeed.

[Jon L]

any specifics on the F5 build? what speakers? i think the F5 has much better groove and dynamics compared to the LM3875 Gainclone driving FR Fostex.

F5 definitely had a more defined, taut, dynamic presentation, but it had almost ZERO bloom ("distortion" I'm sure some will call it), less than what I hear with live music. This effect was similar to what I heard when I increased negative feedback on a VAC amp I used to own.

This was driving AKG K1000 directly, various 'stats via transformer, and my 95dB sensitive speakers with ribbon tweeters.

[luvdunhill]

I think Kevin was referring to the F1 which is very picky indeed.

I figured that and totally agree that 99% of people use the F1 where it shouldn't be used. Now, I'd have to reevaluate that assertion if we're talking about the F1J.

Kevin, the F5 I think would be fine with the most inductive speaker load you can through at them... even if you remove some of the protections in the circuit.

[kevin gilmore]

Yep F1. At least i think its the F1. I have a real nelson pass unit around here somewhere.

Same as Recstar has. All of my speakers are way to hungry to run with something like that,

so its really just a balanced output headphone amp. And i like the dynafet better for that purpose.

[recstar24]

Yup the f1 is super picky. But when mated with the right speaker aka Lowther the results are amazing. Still pondering the f1j upgrade but damn NP is asking for $500 and several weeks (which I acknowledge is commensurate with the labor probably involved)

[FallenAngel]

Oh yeah, that DIY job makes me feel better about taking on an F5 build for Christmas

[ujamerstand]

Now you need a good preamp for it too. Which speakers are you planning on using with them?

Edited December 3, 2010 by ujamerstand
bad grammar *palm

[Sherwood]

Great to see the F5 alive. Mine is a keeper, for sure.

[spritzer]

Oh yeah, that DIY job makes me feel better about taking on an F5 build for Christmas

The F5 isn't that much harder to build then for instance a gainclone. I've already started on a second one which will probably be a fully integrated unit.

Now you need a good preamp for it too. Which speakers are you planning on using with them?

Already started a Dynalo woth switchable gain. It will probably relay based switching so when a headphone is plugged in the amp it switches off the preout and raises the gain. Remove the plug and it drops back to 1-2x. It will also have remote power for the F5 (I'll just loop it's power line through the preamp), a standby function and perhaps a remote control (via a motorized TKD2511). It will also feature only carbon film resistors in which made me come up with this madness:

The 51R resistors were far cheaper than the 24R so why not just double up. All matched naturally...

The speakers are Spendor S3/5SE.

[fishski13]

Now you need a good preamp for it too. Which speakers are you planning on using with them?

i'm using a B1 and y2 DAC upstream, driving small 89dB Fostex FF85K speakers. for near-field listening in a 12"x12" room, i have enough gain for most albums, but would want more gain if in a larger room.

[ujamerstand]

Already started a Dynalo woth switchable gain. It will probably relay based switching so when a headphone is plugged in the amp it switches off the preout and raises the gain. Remove the plug and it drops back to 1-2x.

Interesting, how are you going to implement jack sensing? Got a connector with a sense pin?

[livewire]

I would think he is using a switched jack.

Something like this:

CLIFF ELECTRONIC COMPONENTS|S2BBB|SOCKET, 1/4" JACK, SWITCHED, 3POLE | element14 Australia

[ujamerstand]

You would need extra circuitry to get sensing to work in that case though, and you would be sharing the signal line with the sensing function. I'm not sure if I'd want that.

[spritzer]

There are jacks with separate switches available but after talking to Justin I'm leaning towards just having an external switch plus a second one for low/high gain on the headphone output. This would be really simple as the switches just control power to the relays so no external circuitry is needed. Call me lazy but I do like simplicity sometimes...

[FallenAngel]

Yay, my F5 is almost done, doing final testing and biasing it tonight!

No preamp yet, not until I finish building the Buffalo2 with digital volume control.

Hoping the amp will nicely drive my Harbeth HL-P3ES-2 : 6 Ohms, 83dB 1W/1m in a small room.

[spritzer]

It drives my Spendor S3/5SE's with ease but almost imploded when driving a Stax transformer box. I clearly need to build a bridged unit...

[FallenAngel]

Considering building EUVL's F5X?

[spritzer]

Nahh, I need moar powah. I'm going to build another F5 just because I have all the parts (integrated this time with a buffer and some gain) but the next large project will be what ever power amp KG comes up with.

[fishski13]

Yay, my F5 is almost done, doing final testing and biasing it tonight!

No preamp yet, not until I finish building the Buffalo2 with digital volume control.

Hoping the amp will nicely drive my Harbeth HL-P3ES-2 : 6 Ohms, 83dB 1W/1m in a small room.

congrats!

i don't think you'll have an issue but would recommend a preamp some modest gain.

[FallenAngel]

Oh, I'm certainly having an issue Reversed V +/- polarity from PSU and burned the 0R47 resistors, replaced them but something else must be fried, probably 450/550s.

[spritzer]

That sucks... The circuit is pretty robust but I'd check everything, especially the 450/550. Did you burn both boards?

[FallenAngel]

Just one board, but the other had other issue, unlucky build, will be going through it double checking every part after weekend.

[FallenAngel]

Yay, 1 channel running. Noticed some very funky instructions in the Pass guide to this amp - it says to start with both trimpots at minimum - that resulted in about 8V across R12, had to very quickly adjust it to get a more reasonable voltage, was scared I burned something.

Second channel is getting some funky bias settings (where I reversed PSU polarity), replaced R11/R12 and 450/550s and the input jfets, but can't get above 0.4V across R11/R12. Could the mosfets be fried? It sounds strange, as I'd expect to get 0 output then. This board was getting 0V across the resistors before I replaced 450/550s. No change after replacing the jfets. Any ideas? I'm a little lost, not really sure what to replace next, except the mosfets as all the resistors look fine (not burned).

[spritzer]

It's strange you got such a high voltage across R12. I don't remember something like that on either of the two channels in mine.

Have you measured the pots? They could be damaged and it would be well worth the time to do a diode test on the mosfets, just to see of they have shorted.

One more thing, I don't recommend prolonged use of the F5 with a low ratio Stax transformer at loud levels. It got, very, very toasty...

[FallenAngel]

Getting a VERY funky reading across R3/R4 - it's a dead short! I've removed the trimpots and checked them, they're good at 5K. The 2K2 resistors don't look burned at all, but I'll remove them now and check them. Nothing else should be able to short that unless the MOSFETs are fried. Just found that Digikey started stocking IRFP240 & IRFP9240, just ordered a pair last night, getting them delivered tomorrow (oh how I love Digikey Canada - $8 for overnight shipping).