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Old 9th August 2012, 12:45 AM   #491
diyAudio Member
Join Date: Feb 2005
Location: Melbourne
Ahh Bob, I went a bit crazy about this resistor thing after getting the big improvements with all the caps "circus". (I'm still a bit surprised at the combination of the Rifa peg124, with it's K73-16 bipass, for C4 - they're not the easiest caps to use anywhere - quite possibly, could be compensating for a "mistake" somewhere else)

The BHCs I used were the Slit Foil ones and very impressive mids but a bit thin on the bass for my system, but when used only for the HP, definitely the cleanest mids - I did add a 220uF bipass that helped a bit too (10 of those K73-16 caps)

The first thing was the power resistors and tried the Caddock 930s, then onto the Isabellenhutte PBH ones - I got Hannes ('ha' here) to organise some direct - otherwise from Sweden) - just a softer, more 'analogue' sound for me.

- then, the 9k1 input resistor was first Z-foils, then Rhopoint 8E16D
- then, all the gate stoppers became TRW (IRCs) then changed to the homemade Manganin wire Rs for all the gate stoppers, including the rail Cmultiplier, that I imagined made a difference! (there's quite a lot of space for imaginary effects here! expensive indulgence sometimes!)

- then, the pot feeding C3 got an IRC 2kR resistor across it and the pot adjusted for the same impedance (about 1kR total, I think)

- then, the Feedback resistor became IRC, I think and the rest ended up as the pink PRPs.

In fact, the only Dales are those 0.1R 3W surface mount ones in the R-C-R-C power supply filter - soldered the wires directly to them.

You might keep mind that I have a Coral betta8 FR driver (>140Hz) for the speakers and they're big on detail and dynamics - a reasonable compromise between say, a Lowther and the wrinkley cone Fostex - had to get the cone resonance under control, and the resistor choice, chamber, wire, etc helped this a lot but the biggest factor is the amps current output characteristic - IMO, naturally!.
A frend of mine tried it with his W series Scan based speakers and it seemed a bit dull, but with his Sabre chip dac and his AudioQuest interconnects (the retail price of these things is extraordinary!), it came together very nicely.

- so the choices of the components are particular for my system here that does include the NOS 1541A dac chip, and a Lightspeed Vol control.

I'm collecting bits to build a Moskido to see what it sound like as a comparison - I did try a couple of 300B, 2A3, etc valve amps with this "setup" and with an "Aikido" is very nice indeed (impressive unit!) but the F3 is a better overall combination for the system and my wide choice of music.

Hope this rambling 'yarn' is of some use to you....
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Old 9th August 2012, 01:14 AM   #492
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Join Date: Feb 2004
Location: Davis,CA

Thanks for the information
It is really informative and very useful

I am using the F3 with BHC slits and Obbligatos to drive the PHL 1130s - Really nice. My Raven tweeters still are driven by a Aleph 30. I have no complaint about the Aleph on highs.

However I am going to experiment with resistors in my F4 headphone amp. I will eventually try it on the Ravens. Also thinking about Nelsons F6. I like the sound of transformer interstage.


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Old 9th August 2012, 01:50 AM   #493
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Location: Melbourne
That sound like a nice combination Bob,

I quite like some of the PHL
drivers, the older ones particularly - one guy here had some trouble with
the 8" field coil mids recently - wanted to try Cathy Fertin's 7" FR driver
(MSpeaker, I think) but couldn't get a reply to email queries - Aleph into
Raven should be, as you say, sweet and detailed.

Yeah, I couldn't get over this business about changing resistors for sound
variations - capacitors, chokes, transistors, wire no problems, but
resistors were a long way down the list! Just goes to show, eh!

That F6 looks quite tempting - nothing much to it also.

The F5 is quite a contrary bugger in that if you breath on it on a cold day,
it changes it's sound! Well, not quite but it irritated me enough that I
added a copper strip from the heatsink across the pcb to the 2 input jfets
and altho it's said that the transconduction of the jfets is effected by
temp, I found it actually sounded better when the jfets were hotter -
possibly because of the further playing with components, and possible a bit
of that imagination, too!

Interesting things, these "simple" amps!
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Old 9th August 2012, 02:44 AM   #494
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Location: Davis,CA
Thanks again James

The PHL 1120s are I think what Nelson used in the Rushmores. To me those are great speakers !!!!

Looking for sources of the Isabellenhutte PBH and Rhopoints.Not something Mouser or Digikey carry


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Old 19th December 2012, 10:17 PM   #495
fork is offline fork  United States
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Location: Bellevue, WA
I wonder if anyone can help with a question about DC offset on my F3. A friend measured my amp and found there was about 4-6 volts of DC offset when the amp is turned on or off. It seems to settle down quickly. However, I wonder if this is unusual and could do any damage to the speakers?

I thought the thermistor at the IEC was to correct this sort of thing. If this needs to be corrected I would appreciate any tips on where to focus. thanks
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Old 19th December 2012, 10:31 PM   #496
6L6 is offline 6L6  United States
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The thermistor at the IEC is to provide a small resistance from the safety earth to the circuit ground, keeping it quiet from hum but still able to flow lots of current to ground if there is a genuine fault.

It has nothing to do with the DC offset of the amp.
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Old 19th December 2012, 10:34 PM   #497
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Location: Melbourne
The F3 has a dc blocking cap on it's o/p so don't know how you're getting any dc there unless something has gone wrong.

The only things that come to mind is if the 100 ohm resistor (from O/P terminal to ground) generally known as R10 on diagrams hasn't been included/blown/etc in your amp, or if there are problems with the o/p dc blocking caps.

The thermistor is just a device that limits the mains turn-on current surge - the CL-60 that many people use has about 10 ohms impedance when you turn the amp on and quickly drops to low value when operating as normal - should have nothing to do with any dc offset on the O/P terminals.

Are you sure you're not talking about an F5 amp?

... my 2 cents, hope it helps.
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Old 19th December 2012, 10:37 PM   #498
fork is offline fork  United States
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Thank you for the clarification 6L6.

Everyone else please disregard the 2nd portion of my question about the thermistor and just let me know if you have any advice about the DC Offset issue (if it is one).
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Old 20th December 2012, 12:31 AM   #499
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I need some help! My f3 is almost finish, but I have a problem with one channel. When I adjust P1 to adjust the output voltage, it don't move. It's stuck to about 8.75V. Increasing or decreasing P1 won't affect the voltage. I got about 1.45V at R5, abot 39V at the drain of Q3. I take out P1 and it is working.
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Old 1st December 2013, 03:52 AM   #500
coils is offline coils  United States
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Location: Los Angeles
Default Hot Heatsinks!

Hi everyone,

I have a question. I made an F3 clone a few years back, it has always been hot, and works fine though, sounds good too. In order to tame the heat, and extend the life of the parts, can decrease the Iq by removing R6 or R7, just one of them. And can it be done without other circuit changes?

Thanks in advance for your thoughts
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