F4 in Burning Amp Chassis – Wondering What To Do Next?

[kurtbauer]

Hi Everyone,

At long last I got around but finished the build of an F4 in one of my Burning Amp chassis. I actually started right after Nelson gave them away in, what was that, 2008? Kinda slow, huh? As it stands now, it works but I’m at a loss on where to take it from here. Here’s the story…

I had decided to build an F4 variant in a 4-deep, IRF240/9240 Burning Amp Chassis. I built it as follows:

1. Dual mono power supply.

• a. 2 Avel 18+18V 250VA transformers
• b. 2 pairs of Peter Daniel’s Universal PS boards (CRC)
• c. 44,000 µF/rail, 176,000 µF total (8 22,000µF Panasonic TS-HA)
• d. 4 discrete diode rectifiers, 16A/600V diodes

2. Peter Daniel’s F4 clone boards (using driver portion and wiring to MOSFET panels.
3. Replaced Source resistors from 1.8Ω to 0.47Ω.
4. Replaced Gate resistors from 90.9Ω to 150Ω.

As you can see from the pix, the driver boards are mounted on a cool-dude piece of aluminum-backed phenolic that’s attached to cooler-dude transformer pedestal – all courtesy Bill Berndt. The wire routing is dictated by the limited space in the short chassis. Despite the close quarters, the amp is extremely quiet with the only noise being a little bit of mechanical hum from the trafos. The sound is amazingly detailed with good imaging and what I would call a gigantic sound stage. You can see it in action installed in Bill’s rig in the pic.

What’s missing is that the bass is pretty flimsy, lacking punch, and the amp feels like it gets bogged down in complex, high-volume passages. Overall, it seems slow. it makes the me think that the PS isn't up to par. Also, even with the volume control near to the top of the range, it doesn’t get very loud. I’m not sure what the gains are for Bill’s or my test preamps are but I guess they just don’t generate enough voltage swing.

After things get warmed up, it’s getting about +/- 24V on the rails and the deepest bias I can get without burning my fingers with the 5-second rule is about ~490mA.

So I’m wondering what I might be able to do about the sound. Or is this just the way this amp sounds? That seems unlikely but what to do?

Any help would be appreciated.

Kurt

[berndt]

Fwiw, the speakers we are driving are the audio kinesis jazz modules, that I brought to BA this year. We are comparing Kurts f4 build to my 6v6 pp amp that is my daily driver. The 6v6 amp is less than 10w. It has better control in the bass region. The f4 has a wider deeper more resolving image and would wipe the floor with the tube amp except for the timid bass presentation. The speakers are 93db efficient and have an impedance balancing XO that rises to 16 ohm in the lower registers, being designed for atmasphere otls.

[Zen Mod]

- I think you have less than optimal capacity in PSU , considering 4 deep meaning 490mA x 4 Iq ?

- are those signal electrolyts bypassed with anything ?

- are these PSU caps bypassed with - say- solid 4u7 ?

- with F4 you really need proper pre

try - even if just for laugh - your proper tube amp , loaded with 8-16R dummy load - as pre for F4 ; I can bet that - if tube amp have proper bass - F4 ( if properly made ) will not ruin that

then you'll have point from which you can build further

[Nelson Pass]

Originally Posted by kurtbauer

After things get warmed up, it’s getting about +/- 24V on the rails and the deepest bias I can get without burning my fingers with the 5-second rule is about ~490mA.

Is that 490mA per channel?

[Zen Mod]

sometimes I have impression that Papa is following me around ........

[kurtbauer]

And who could blame him, Mr. Mod?!?!

Nelson, yes, that's ~230mV across each 0.47Ω resistor.

ZM, I was thinking the same thing about the capacity of the PS. The PS has two 22,000 µF filter caps separated by 4 0.47Ω resistors per rail (dual mono style). The second filter cap is bypassed by a 10 µF Panasonic film cap. What is confusing to me is that the PS parts list for the F4 clone that I've found here on diyaudio is 8x 15,000µF caps for 12 MOSFETS v 8x 22,000µF caps for 12 MOSFETS that I have. I thought that should put me in the zone though they are configured differently.

The signal electrolytics are Silmic IIs and are not bypassed.

Roger on the proper preamp thing. But it seems like the base shouldn't be weak at lower volumes, right? Also, we did try to 6V6 as a in put to the F4. Unfortunately, it the tube amp motorboated so we stopped the proceedings.

Whadyathink?

Cheers!

K

[Zen Mod]

sorry - blame my eyes , or blame my speed - I didn't saw that you have two levels of caps

that's fine , certainly ;

you didn't wrote what preamps you tried as source for F4 ;

in occasion when you tried tube amp as feed for booster F4 amp - what were dummy load resistors for tube amp ?

regarding low level bass quality - take this as comparison - this test is same as you are trying various headphone amps - doesn't matter what level is - bad headamp always have mediocre sound ; good one always have good sound - in whisper and in thunder state .

in any case - my crystal ball is somewhat muddy lately , so I'm operating strictly with data you laid here . I'm resuming that your F4 is made by the book ; what's current through input Jfets (voltage across R3 and R4 ) ?

[kurtbauer]

Yes, the F4 is by the book with the only differences being that I have 4 output devices per side instead of the original 3 and the PS is different as described above.

The voltages for R3 and R4 (10Ω resistors) are as follows:

Right Channel:

R3: 7.6mA

R4: 7.6mA

Left Channel:

R3: 6.7mA

R4: 6.7mA

For preamps, my test preamp is an old Audio Alchemy DLC with a fancy PS. The specs for its gain are enormous. Bill's pre is a deluxe DIY EF804 (or maybe EF86) preamp. The gain hasn't been measured to my knowledge but my calculated gain is 17dB. I do not, nor should anyone else, trust this number. He drives the 6V6 with it as noted above but has also successfully driven at least one other SS amp with it. Bill, please correct me as required. Bill's sounds better than the DLC, of course, but the DLC is ok. For each of them, bass not a problem with other equipment.

I'm saving the best for last...
We didn't add the dummy load on the 6V6 on the output. We'll run that one again soon.

[Zen Mod]

Originally Posted by kurtbauer

......
I'm saving the best for last...
We didn't add the dummy load on the 6V6 on the output. We'll run that one again soon.

please - do that .....

everything is already written in Papa's F4 user manual

you can even connect toaster to bloody thing

[Nelson Pass]

A couple of thoughts,

1 490 mA is light on bias and will slightly alter the bass

2 If you are using it as a follower for a flea-watt tube amp
then some resistive loading for the tube amp is appropriate,
usually 8 to 22 ohms.

3 It is possible that the flea-watt's output impedance is
somewhere between 1 and 4 ohms, in which case you might
want to put some resistance in series with the output of the
F4 to duplicate that for your speaker.