Preamp ideas for F5

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#442
I've also used some Kiwame carbons with the Rhopoint 'econister' for the low power resistors, and the Isabellenhutte PBH Manganin power resistors to achieve a rather smooth and detailed sound and yet still retains that enormous 'grunt' - the power supply also contributed it's share of the 'sound' too - unfortunately, parts like these aren't cheap.

With Juma's version with the smaller k2013/j313s and pretty much the same type of components, this circuit's variation adds a certain 'velvet tone' to the sound - a 'touch of magic' from what is still a rather simple circuit.
 
#443
When somebody else suggested that we use carbon resistors instead of metal film in the F5, here's what Nelson Pass advised:

Original Post #3731

JC951t: "I know I'll get shot for this. If sound (of F5) is too clinical or clean, stay away from metal film resistors."

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Nelson Pass: "Alternatively, you can increase the value of the Source resistors to 1 ohms,
and increase the value of the feedback resistors from 2 X 100 ohms and 10
ohms to 1 X 100 ohms and 22 ohms.

The lower amount of feedback will warm the sound up, and you can keep
increasing proportional values in the feedback loop if you wish."

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rob lenk: "Just thought I'd post my initial impressions of having modified the feedback network relative to Nelson's suggestion( R11 & 12 to 1 ohm/
R5 & R8 to 100 ohms and R1 & R2 to 22ohms-pass site)
Yep,Yep amp runs a little cooler(perhaps 8-10 degree farenheit) and the "tight fist" now has a velvet glove!
I found the original spec a bit dry and etched in sound relative to my dynaco st70- accurate and detailed but lacking a bit of body or warmth. The new feedback seems to be a definite step in the right direction."

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Pillendreher: "Hello there,
i also tried the alternative feedback a´la Nelson's suggestions.
But now i cannot set the bias to more than 1.1A! If i set the dc offset to 0 the bias levels at 0.9A.
Is it correct to increase the value of P1/P2 to 10k?"

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bobodioulasso: "It should be simpler to increase R3 and R4 to 2K7 or 3k3"

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My friend Hamilton also performed this mod, and felt that it warmed things up nicely sound wise, and also kept his sinks cooler! I had bought two sets of resistors to do this mod, but one set got lost, so I haven't done mine yet.
 
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#444
Personally, I love that a proper EE can achieve using precision resistors, by changing values, what a hifi buff attempts to achieve with changes to either expensive or antiquated components. It suggests that component changes are often a poor substitute for really knowing your onions, like Nelson Pass. It's sometimes like comparing curing with snake oil to visiting an experienced doctor. Still, I'm sure carbon resistors do achieve a desired result too... 🙂
 
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#445
I havnt had the guts to admit it in public, but by accident i popped a pair of 1,2 ohm source resistors in, and I sounded so good that it didnt care to change them to 0.47 ohm.

That is the reason that my F5 runs cooler, not the separation of the sinks from the cabinet.

I now run it with a lower gain BA3 and a DDDAC1794 and and optimzed PC source - it sounds wonderful.


Best regards
Arthur.
 
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#446
Hi, I built the preamp juma (post #53-#54) for my F5 and it works great. I've got 25-30% more SPL. I have ridden with cheap metal film resistors from a local store. Accuracy 5%
I would like to upgrade and change all the resistors for something better. Any recommendations?
Thanks
AL
 
#447
albertob said:
Hi, I built the preamp juma (post #53-#54) for my F5 and it works great. I've got 25-30% more SPL. I have ridden with cheap metal film resistors from a local store. Accuracy 5%
I would like to upgrade and change all the resistors for something better. Any recommendations?
Thanks
AL
Click to expand...

PRP 1/4w are fabulous. They're metal film, 1% (but usually much closer), sound great and are 30p each. They were designed for audio use and I use them for pretty much everything up to 1w now (oh yes they do 1/2 watt and 1 watt versions too). They are all red, and have the value printed on them, not stripes, which I also find saves me time and energy. Hificollective.co.uk in the UK stock them, so no import tax to pay (they're made in USA).
 
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#448
You might be able to access something else called TRW or IRC metal film resistors also but it's possibly easier to get those copper wire ones from Roedestein that I thing, used to be known as the 'Resista' range - excellent sound

Mind you, the 'red' RPRs (PRP 9372) are pretty good too, and especially the price - another good source for these is 'handmade audio' in the US at about U$0.42 for the 0.5W ones

Thanks Lukas for the info about the 1/4 waters at the 'Collective'. The colour stripes are getting harder to pick out these days - that 'age problem'!
 
#450
Juma, I seem to remember you saying in this thread that you prefer to put the attenuator before the pre-amp, and couldn't be bothered to get into a discussion of the merits of putting it before or after the pre-amp, but would you mind sharing with me the benefit of placing it before the preamp please.

We know that the main reason to place the attenuator after the preamp is for improved s/n ratio, but I'm sure you have a good reason or two to put the pot first. Please share!

Many thanks
Lucas
 
#451
generally a vol pot needs a buffer after it to drive the interconnects.

If you have a pre-amp then it provides the buffering effect and gives some gain.

The natural place would seem to be before the pre-amp.

For best noise performance the vol pot should be after all the gain stages. This is impractical after the Power Amp.
So we put the vol pot before the Power Amp, then it needs a Buffer stage to drive the cables.
We end up with a pre-amp AND and Buffer stage for lowest noise.
 
#452
LucasAdamson said:
Juma, I seem to remember you saying in this thread that you prefer to put the attenuator before the pre-amp, and couldn't be bothered to get into a discussion of the merits of putting it before or after the pre-amp, but would you mind sharing with me the benefit of placing it before the preamp please.

We know that the main reason to place the attenuator after the preamp is for improved s/n ratio, but I'm sure you have a good reason or two to put the pot first. Please share!

Many thanks
Lucas
Click to expand...

Noise is an issue with line level signals in correctly constructed preamp only on academical level.
I wrote few times why I put the pot before the preamp - without it, the preamp will get on its input full input signal and probably be driven into clipping (depending on input signal amplitude, preamp's gain and output swing capability). Using the pot on the preamp's output is like keeping the accelerator pedal always to the floor (max RPM) and regulating the speed of the car with a clutch.
 
#453
...which creates the need for a buffer, right? What you say makes perfect sense. I agree with the concept of making the preamp gain stage specific to the power amp, in order to drive it to a more acceptable level, and incorporating into the power amp, in a sense as though it was part of the same circuit.

Although, my hesitancy in building one of your suggested gain stages is that I may achieve similar results by raising the F5 voltage from 24vDC to maybe 30vDC with a new transformer, beefing up a few resistors and biasing the amp a bit lower to keep the output devices safe, as described by NP at the start of the F5 Turbo pdf.
 
#457
juma said:
Actually, since my life has been very chaotic in last few months, now I'm not sure what preamp we are talking about, LSK or something else - there's been a lot of them... :scratch:
Click to expand...

The one at the start of this thread, but with a few resistor value changes you specified a bit later on. Anyway, I am going with the voltage increase, bias lowering option to get more power I think.
 
#458
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#460
I look at the latest schematic for the F5 and see that now the Zin going from 100Kohm down too 47.5Kohm.

Maybe the input/output jfet version need some recalculation for the Zin 47.5K then ?

Vds 10 V
Ids 7.3 mA
Vgs 0 V

My prototype mono boards has DC drift below 1mV...nice 🙂

KM
 
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