A preamp for the pass f5

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cressy
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#1 A preamp for the pass f5

Post by cressy »

Ive been musing on a preamp for the pass f5 I got off Ali and came across this.
http://diyaudioprojects.com/Schematics/ ... lifier.htm

I was looking for the rca manual and came across what i was looking for on this site instead

I dont need a great deal of gain so this looks ideal, although I would have preferred a lower ht voltage so i could have used one of nick ds's regulator boards.

Plan is to build up whatever circuit I go for on some matrix board and then have that in one of the extruded aluminium cases I have been working on, and put the psu in another.

I'll change the 500k pot for a 100k alps black I have laying around if i go with this one.

Anyone used this circuit before? I dont want to spoil what the f5 does with passive im using at the mo, and want something rather clean without a shedload of valviness if that makes sense.

Anyone have any recommendations?

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

Post by pre65 »

Have you seen the Blue Velvet preamp ?

HT on that is 204v

http://www.enjoythemusic.com/magazine/e ... velvet.htm
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#3 Re: A preamp for the pass f5

Post by cressy »

Im not a great 6sn7 fan tbh
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#4 Re: A preamp for the pass f5

Post by Mike H »

I'm sure the first one can be made to run from 200V, say if R2 was changed to 100k. I also like that it has a cathode follower output.

.
 
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#5 Re: A preamp for the pass f5

Post by cressy »

That's what i liked about it, it would drive pretty much anything
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#6 Re: A preamp for the pass f5

Post by pre65 »

If you had a lower HT one anode would also have lower HT, cathode follower section.
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#7 Re: A preamp for the pass f5

Post by Mike H »

Well spotted, however A. would that really be a huge problem, only one way to find out; B. if it were, simply reduce R4 value a bit until V2 current is back to more like what it would be at 300V HT.

At a guess, R4 could be reduced to 3.3k say.
 
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#8 Re: A preamp for the pass f5

Post by cressy »

Not sure what r6 is for unless its the grid leak, i'll have a look at reworking the cct for 200v, not sure what current they have set it at, at a guess about 10ma for each part. Although thats irrelevant if im going to rework it. I think the mj nickds reg boards can handle a max of 25ma iirc so i'll have to bear that in mind and see if its viable.
Mind you, I do have 2 boards somewhere
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#9 Re: A preamp for the pass f5

Post by Mike H »

Yes R6 is the grid leak.

Needed to work! :D
 
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#10 Re: A preamp for the pass f5

Post by Mike H »

If R2 = 220k then no not 10mA, a bit under 1 mA more like. Should come out near enough the same if R2 is 100k and HT = 200V.

A bit more current for the CF part, but not much. Hence my thought for decreasing R4 value somewhat, to raise it a bit. Especially as HT is dropped.

HTH ?
 
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#11 Re: A preamp for the pass f5

Post by Nick »

cressy wrote: Fri Feb 03, 2017 5:50 pm That's what i liked about it, it would drive pretty much anything
Hmm, not sure I would say a cathode follower at 1ma will "drive anything", it will have a low output impedance, but not much current to push into that load.
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#12 Re: A preamp for the pass f5

Post by cressy »

1ma?
Is that it?
I hadnt worked out the current
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#13 Re: A preamp for the pass f5

Post by Mike H »

On the basis that R2 = 220k, if across the whole 300V, still only < 1.5 mA.

This is increasingly not making sense, I assumed valve = ECC83, but 12AU7 = ECC82. Impossible with those values.

12AX7 = ECC83.

First stage seems to work OK if 12Ax7, or ECC83. Current around 0.3 - 0.4 mA. Bit low, must be said.

If R2 = 100k, I would want a bit more, say 1 mA, = a shade over -0.5V grid bias, if anode = 100. That makes R3 around 560R instead of 5.6k. Can always be tweaked to alter anode Volts. However means max input is imited to 0.5V peak (500 mV), BUT gain will compensate for that as has got a Vol pot at the input to keep input level at the grid at low level.

I should say R4 should be similar, 560R or so.
 
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#14 Re: A preamp for the pass f5

Post by Mike H »

Nick wrote: Fri Feb 03, 2017 8:14 pm
cressy wrote: Fri Feb 03, 2017 5:50 pm That's what i liked about it, it would drive pretty much anything
Hmm, not sure I would say a cathode follower at 1ma will "drive anything", it will have a low output impedance, but not much current to push into that load.
Yes it's a relative concept. :D

Ideally, the second stage could be a ECC82. Possible since two valves will be needed for stereo, so half ECC83 each for first stage and half ECC82 each for second stage.
 
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#15 Re: A preamp for the pass f5

Post by Nick »

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.
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