Passive pre out from kel84

[jimangus]

Perhaps a little obvious but wish to check. Can i take signal from the kel84 pcb input pins out to a pair of spare phono plugs at the back to create a passive pre out ? Does this create any problems ? And if so how can i correct them?

Regards
Jim

[Richard]

Hi Jim

Yes with a couple of considerations,

1 the extra cabling attached between pcb and new phono output sockets will add extra capacitance to the input so use very low cap cable.

2 any device/cables then attached to the new output will also add to the amp's input capacitance, so keep cabling from the new output phonos to the external amp/whatever to a minimum or disconnect when not in use. Remember that a passive pre using a 50K pot will be OK for short runs of cabling but try to keep capacitance to 100pF or so or you may lose hf.

In practice you'll be able to connect cabling and listen for any difference beween connected and disconnected to see if there's a problem using Kel84 with it connected.

Rich

[KennyK]

what about 1:1 isolation transformers, or even a cathode follower stage hanging off the back?

[jimangus]

Originally Posted by Richard

Hi Jim

Yes with a couple of considerations,

1 the extra cabling attached between pcb and new phono output sockets will add extra capacitance to the input so use very low cap cable.

2 any device/cables then attached to the new output will also add to the amp's input capacitance, so keep cabling from the new output phonos to the external amp/whatever to a minimum or disconnect when not in use. Remember that a passive pre using a 50K pot will be OK for short runs of cabling but try to keep capacitance to 100pF or so or you may lose hf.

In practice you'll be able to connect cabling and listen for any difference beween connected and disconnected to see if there's a problem using Kel84 with it connected.

Rich

Hi rich,
my kel 84 is of the old school using a 100k pot, will this make any difference?

the plan is to take out a passive pre output to a seperate amp to drive the treble of my loudspeakers. I had planned to use twisted pairs of 0.6mm teflon insulated silver cable for both output and interconnect, keeping them as short as possible. Unfortuanatly i do not know the capacitance of this type of cable, have you any idea if it would be suitable ?

Any help would be fantastic

Regards
Jim

[Richard]

Hi Jim

A 100K pot should still be OK for short runs with low cap cables if you keep the Kel input at 1Meg and make your new amp a similar high value so the 2 parallel inputs do not load the pot output too much.

The cap of such a twisted pair will be very low, perhaps 30pF/metre, but I'm not sure I'd use unscreened cables for such a high impedance connection due to possible noise. Perhaps try it and see as there's not much wiring involved inside the amp and the external interconnect will be simply plugged in. If there's a problem try well screened coax of up to 100pf/metre. Also be aware any un-terminated input may pick up noise, ie if using Kel without the other amp connected.

Rich

[jimangus]

Hi evry one,

well after some experimenting i can say that th doubts of its possibility was completly correct. A lot of noise does get input into the signal using the kel as a passive pre.


Another question now arises however if i was to use the kel84 as a power amp from an active preamp what changes would need to be made ? Would it be possible to just remove the vol pot/selector switch ?

I remember some years back wad did a wright up on a kel 84 power amps avialability in hfw. Any one know what was changed if anything ?

Any thoughts ?

Regards
Jim

[david counter]

the only modification is to change R2/3 from 1M to 100K

[jimangus]

Originally Posted by david counter

the only modification is to change R2/3 from 1M to 100K

Thank you david just the info i was looking for.

Now the other amp i was going to use was a single ended el84 amp fpr th treble, would th same tweak apply. Removing the large value resistor on the input and replacing it with the value of the volume pot ? ( I can't remembr what these valus are without looking at the schematic )

Many thanks
jim

[jimangus]

Originally Posted by david counter

the only modification is to change R2/3 from 1M to 100K

Hi all,

i have carried out this change and plugged my MF X-Pre in. It has given me a Hummm.

I have tried diferent ic's lifted shield earth from one end both ends, took flying lead from shield to ground but to no avail, i was wondering if this is related to the input impeadance. Would a higher value than 100k perhaps be neccessary?

An interesting point is that using the pre through the selector and volume pot with the original 1M resistor produced no hum.

Also interesting is that if only one of the ic's are connected the hum goes away.

And the humm is uneffected by volume.

Thanks for any help in advance

Regards
Jim

[david counter]

Help!!
worked ok on mine, I had a 100K pot, don't know if that makes a difference,

hopefully someone will enlighten us