F5 Listening Impressions & Discussion

[CBRworm]

I should not comment as I seem to have reached the limits of my speakers and sources, but since not many others are, I will.

I have built two sets of F5's, one with Fairchild parts as designed and one with two pairs of IRF parts and slightly higher rail voltage, slighlty lower bias and a generally larger power supply. And a little more attention to detail.

I honestly can not hear a difference between them except at the limits, they both sound slightly better than my vintage SS gear, but doing blind A/B tests between either of them and my Adcom GFA-555ii - I can't tell a difference, unless I go beyond the limits of the F5. Anytime I think I can hear a difference I can switch and not hear a difference. I have the impression that the single output Fairchild F5 is sounds a little bit better, but I can not quantify it and again, any time I swap the other one sounds just as good. I can identify which of my preamps is in use pretty quickly and accurately, but not these amps. It seems like I should be able to hear a pretty big difference between a class A mosfet and a class AB BJT - but . . .



I have tried a number of different speakers, but nothing really high dollar - I suspect I need to change my speakers and/or my room before any more amp tweaking. My daily use speakers they are connected to are a set of Infinity QLS 5's (F5), and a set of Infinity RSIIIa's(F5+). I have also tried using my ZaphAudio SR71's that are used downstairs and a set of Infinity Beta 20's. My guess is that none of these speakers are revealing enough for me to hear the difference between the different output mosfets. Each speaker set has a very distinct signature, but the amps all sound very much the same through them.

The one thing that I have learned from this is that a good low power amplifier is totally sufficient for daily listening and higher than expected SPL. Unfortunately with my larger F5 I am drawing close to 300 watts at idle which kind of cancels out the typical advantages of a low power amp - ie low power draw and low A/C bill.

Another problem I have found is trying to find really well recorded music.

[westend]

F5 built with Cviller boards, Tech DIY kits with Fairchild outputs, Antek 400va torroid, Vishay rectifiers, PSU-Panasonic TS caps & ERG resistors, wired with 14ga. copper and double shielded input cable, Vampirewire chassis connectors.

Listening with Yamaha C-2a preamp, Yamaha DVD/SACD as primary sources, Fostex 126e BLH, others.

This is one fine amp.
Listening to a 30bit Gold Master Disc of "Sketches of Spain"-Miles Davis, on the first track, I could hear sheet music falling to the floor when someone made an inept page turn. I hadn't heard that, previously.
Listening, initially, was hung up a bit by a previously unnoticed peak in the Fostex mid-range but these horns never fail to change. Getting past that, I noticed that neither bass or treble was missing or imbalanced in most any way. Resolution, speed, and all other attributes used to judge amplifiers seem kind of paltry. The F5 goes beyond all of that because, it seems, it does all of it so well.

[fishski13]

Originally Posted by westend

F5 built with Cviller boards, Tech DIY kits with Fairchild outputs, Antek 400va torroid, Vishay rectifiers, PSU-Panasonic TS caps & ERG resistors, wired with 14ga. copper and double shielded input cable, Vampirewire chassis connectors.

what specifically is the 14ga connecting? i'm trying to figure out what gauge wire is needed in the PS for the F5 to perform at it's best - i'm talking about current. i have 12ga coming off my TX into block rectifers, and 14ga connecting the rectifiers to the PS. all wiring in the PS is done with solder-soaked stranded 18ga. i will probably use dry 18ga to connect the PS to the amp PCBs.

[bobodioulasso]

Hi reviewers,
I have built 3 x F5. Doubles monos, 2 x47000uF per side, no limiters.
For me F5 is "The" amp.
I will only mention i have noticed an improvement when using shottkies instead of common bridges.
Nex thing worth to try, i think, is a capacitance multiplier as PSU.(Lower ESR).

[AR2]

To add to the discussion on bandwidth:

Today I just completed F5 and here is the measurement I did on frequency response. The measurement was done on my HP 3577 network analyzer and is pretty same as Nelson's published specs DC to -0.25Hz at 200 KHz. I would just say this is awesome - flat to DC and -1 db at 252KHz in my measurement! What a amp. Only one thing I wish for - little bit more power.

[alazira]

Originally Posted by TerribleT

Alazira,
Thanks for sharing your F5 impressions. If it is not too much bother, could you post some pictures of your build? I'd be interested in your PCB and PS component layout.
Thanks,
David

Hi David,

You can see pics here

F5 version2 pictures by Charlestonhood - Photobucket

or in the F5 thread, Post 5553

F5 power amplifier

No PCB used, although for the second monoblock channel I did use a teflon sheet. The only wire used on the circuit was 16g silver between the drains for the output. Check the signal path of 1.0 vs 2.0:

F5 v1.0 - RCA - solder point - wire - solder point - PCB trace - solder point - 1k resistor - solder point - PCB trace - solder point - jFet.

F5 v2.0 - RCA - solder point - jFet

Respected members Zen Mod & Magura have said a CLC power supply with an L >= 2mH offers a benefit as long as you stay in class A. Something I will try in the future (after trying Hitachi/Rensasis, Toshiba, and IRF cascode versions) endless fun with the F5

[Netlist]

There’s nothing special about the build, in fact its ugly as hell. Peter Daniel boards, K170/J74 and IRF Mosfets. This amp is by far the best I had in my system for years. Beats my AlephX hands down. There’s rolling and shaking bass, there’s high’s to die for and there’s silk sweet mids to make one drool. My 800's are –unlike other B&W’s- not demanding or power hungry. Like AR2, the only thing I could whish for is a bit more power.

[galop]

F5-sounds better than my phase linear 700b and luxman M02 and 2 x 52 w tube amplifier.Thank you for this design mr pass.

[TerribleT]

[QUOTE=alazira;1994833]



Deleted thermal compensation and protection circuit portions. Also left out R9 and soldered the jFets directly to the female RCA, although do this with caution as leaving out R9 may cause oscillations depending on your source.

Hello Akazira,
Thanks for the reply and great pictures. Could you or someone else explain the purpose of R9? According Pass' F5 article, R9 is there to suppress parasitic oscillation of the input JFETs. How does R9 do this? I like the idea of elimination of parts if the result is better sound, but if the cost is possible failure then maybe not. What is the nature of the source that would cause oscillations or what should the source look like to ensure that oscillation will not occur?
Regards,
David

[alazira]

Originally Posted by TerribleT

Hello Akazira,
Thanks for the reply and great pictures. Could you or someone else explain the purpose of R9? According Pass' F5 article, R9 is there to suppress parasitic oscillation of the input JFETs. How does R9 do this? I like the idea of elimination of parts if the result is better sound, but if the cost is possible failure then maybe not. What is the nature of the source that would cause oscillations or what should the source look like to ensure that oscillation will not occur?
Regards,
David

Hi David,

As the F5 article says, the gate resistors for the jFets (R9) and mosfets (R13&R14) are there to prevent parasitic oscillations. I can't tell you why or how as I don't know myself.

I too like deleting things so I decided to try it sans R9 because I use a lightspeed attenuator, which is basically a variable resistor, so putting R9 in series with it seemed redundant as long as I keep the LSA from dipping below 1K (presumably).

For testing purposes you might try a 1K trim pot (shorting the middle leg to either side) for R9 and see at what resistance, if any, oscillations occur in your system.