A preamp for the pass f5

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pre65
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#16 Re: A preamp for the pass f5

Post by pre65 »

Ant, what about this ?

See section 8 and figure 5.

http://sound.whsites.net/valves/preamps.html
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Mike H
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#17 Re: A preamp for the pass f5

Post by Mike H »

Nick wrote: Fri Feb 03, 2017 10:40 pm Replace the valve with a 5687, replace anode (v1) and cathode load (v2) with depletion mosfet CCS and direct couple the two stages. That would be where I would take it. Or just use a passive :-). The F5 is a easy load, but it has got a pair of jfets at the front so it wont like DC on its input when that pre starts up, so you need a start up delay and mute. Starts getting less simple.

Yes BIG Voltage surge from out of C4 when valves start up. In any case 'output' needs a pull down resistor across it, if not, as it stands, and output is not connected to anything when turned on, C4 will charge up giving same high Voltage pulse when the output IS connected.

Direct coupling did occur to me too, is what I would do, junks 2 resistors and 1 cap. (R4, R6, C2)

The CCS's would keep it more linear.

I wonder if who drew it got the values wrong, take a zero off the end of each of the resistor values, suddenly it makes more sense if a 12AU7 (ECC82).


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#18 Re: A preamp for the pass f5

Post by Mike H »

pre65 wrote: Sat Feb 04, 2017 10:38 am Ant, what about this ?

See section 8 and figure 5.

http://sound.whsites.net/valves/preamps.html

Yes right hand half of Figure 2 is more like it. Bung a DC-coupled CF between Rp and Cc, proper job.

Oh look, just like Figure 4.

:D

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#19 Re: A preamp for the pass f5

Post by Mike H »

Ah Figure 5, bootstrapping!

And with NFB, yet, might be interesting what that's like.

Altho a bootstrap is trying to do what a CCS can do, i.e. keep anode current constant. (Or very nearly.)


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#20 Re: A preamp for the pass f5

Post by cressy »

ta phil, just had a look at that link, fig 5 looks like it ticks the boxes. quite like the feedback to cut distortion down on that, and id bet the 240k value for the fb can be tweaked to clean things up a little more. build it, stick a scope on it and see. ive got a 500k stepped attenuator here (got sent in error instead of a 50k one and the seller just told me to keep it) that could be employed to prat about with the level of feedback

the passive option is what im using now

might just build that as is as a 2 boxer, transformers and ht smoothing in one, reg board and last cap in the circuit box, dc heater module in the psu box to avoid there being dc and ac going through the 4 core cable i have for an umbilical

or something like that
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#21 Re: A preamp for the pass f5

Post by Nick »

If can see distortion on a scope you are up in the percents.
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#22 Re: A preamp for the pass f5

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Ill just stop talking
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#23 Re: A preamp for the pass f5

Post by Mike H »

Nick wrote: Sat Feb 04, 2017 6:30 pm If can see distortion on a scope you are up in the percents.
Unfortunately - but useful for finding where it starts clipping. :D


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#24 Re: A preamp for the pass f5

Post by Mike H »

Ant, note input impedance will be more like 33k than 1 meg, because it's an inverting amplifier with NFB coming back to input grid. The 33k could be increased tho. Say if a gain of X2 was good enough, it could be 100k for example.


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#25 Re: A preamp for the pass f5

Post by Nick »

Mike H wrote: Sat Feb 04, 2017 10:20 pm Ant, note input impedance will be more like 33k than 1 meg, because it's an inverting amplifier with NFB coming back to input grid. The 33k could be increased tho. Say if a gain of X2 was good enough, it could be 100k for example.
Just be careful, it looks like any value will be fine, but I have played with this sort of circuit in the past, its basically a anode follower with a cathode follower buffer. the feedback makes the first valve grid a virtual earth so the impedance looking into the valve is certainly the resistor feeding it, but don't forget Miller, thats still there so you have 100k into the Miller capacitance of the first grid, so you start to lose HF response.

Its basically the circuit Glenn Croft uses for his preamps, with the second valve often replaced with a mosfet source follower. It can sound very good with the above gotchas.

http://www.tubecad.com/october99/page8.html
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#26 Re: A preamp for the pass f5

Post by Mike H »

I forgot about the Miller effect.
 
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#27 Re: A preamp for the pass f5

Post by Nick »

Mike H wrote: Sun Feb 05, 2017 1:17 am I forgot about the Miller effect.
So did I when I build that sort of thing the first time.
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#28 Re: A preamp for the pass f5

Post by Mudshark »

Cheers......
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#29 Re: A preamp for the pass f5

Post by cressy »

Got hold of some rather nice oak for the fronts and backs of the 2 chassis today, went to get some for a customer build and got a large section that was only slightly more expensive than a piece the size I needed for that job.

Question for nick, when the valves are inside the casework are there there any special considerations re heat for other components?

The aluminium extrusions are 4mm thick, 200mm wide by 250mm deep by 100mm tall so there is plenty of room, i'll obviously drill out ventilation holes in the base and top for convection cooling, but usually caps are rated at 105 which i presume is the max temp they should be operated at.

Never even considered it before, and there are only the 2 valves in it, but they do get rather hot. :D

Cheers ant
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#30 Re: A preamp for the pass f5

Post by Nick »

Trying hard not to question the sense of choosing a case before trying out what is going inside it.

As to temp, it depends on the valve in question and any ventilation/dissipation from the casework. The temperature on a cap is the temperature that the rated hours are given for, so a 1000H 105C cap would be good for 1000 hours at 105C, if the temperature is lower the life expectancy increases. A rule of thumb is every 10C lower temperature the lifetime of a capacitors doubles. So your 1000H 105C would last 64000 hours (7 years) at 45C
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