Preamp output question (cathode follower)

[bluemonk]

I built a preamp based upon Alembic F2B.
http://www.geofex.com/FX_images/alembpre.gif.
Replaced 1M output resistor with a pot.
Then I added a cathode follower to the output to get ALSO a low impedance output.
(12AU7 / ECC82 Cathode Follower Tube Preamplifier Schematic).

Is there any issue to use both outputs (hi-Z and lo-Z) at the same time?

Thanks

[pdf64]

Any cable capacitence on the hi-z output will roll off the high end.
Why not use the low-z for both outputs?
Another problem from connecting up various mains powered kit might be hum loops.

[bluemonk]

Is Lo-Z output to prefer over Hi-Z also when using a tube power amp?
I believe to have read somewhere that SS's likes Lo-Z at input, tubes like Hi-Z. Is that correct?
Where is the border between Lo-Z and Hi-Z (5k, 10k, 100k...)?
Regards.

[Robert M. Martinelli]

Hi,
the "borders" are not all that perfectly defined ( in preamps ).
A perfect impedance ( Zsource=Zload ) matching is required only for maximum power transfer ( output stages ). In preamps, where we mainly deal with voltages, it's more important not to load a stage too heavily, therefore it's perfectly normal to connect stages having a different impedance ( e.g. a low impedance output going into a high impedance input ) - this practice is often referred to as "impedance bridging".

Hope this helps

Best regards

Bob

[bluemonk]

Thank you everybody

[mooreamps]

I built a preamp based upon Alembic F2B.
http://www.geofex.com/FX_images/alembpre.gif.
Replaced 1M output resistor with a pot.
Then I added a cathode follower to the output to get ALSO a low impedance output.
(12AU7 / ECC82 Cathode Follower Tube Preamplifier Schematic).

Is there any issue to use both outputs (hi-Z and lo-Z) at the same time?

Thanks

No issue that I can think of. It's just that I would never use a CF to directly drive a power tube. That just whacks the tone. Most layouts I've seen, they use the hi-Z for the power tubes and/or PI's, and use the lo-Z out as part of the effects loop.

-g

[bluemonk]

I would never use a CF to directly drive a power tube. That just whacks the tone. Most layouts I've seen, they use the hi-Z for the power tubes and/or PI's, and use the lo-Z out as part of the effects loop.
-g

Well, I was just wondering why branded preamps have only Hi-Z output (Alembic F2B is an example). I think that CF output would help to drive SS stuff.

[Steve Conner]

I think the cathode follower is important to drive the cable capacitance. I can only assume the F2B sounds best with a short cable, or nobody notices the loss of high end because they use it for bass guitar.

[pdf64]

A 1M pot can be a very high impedance source, comparitively - it may get to ~ 250k at the electrical half way point. A 220pF capacitive loading on that (eg regular guitar lead) would create a low pass filter with a break point of 3kHz, which should be very apparant, unless you play with the tone control on minimum.
Also it'll be prone to hum pick up.
It's the 1M output master vol pot causing these issues - put it on the power amp (if you must have it) to avoid them.
The alembic pre amp output Z will be about 40k, which is much more reasonable.

[mooreamps]

it's never an issue if you use an electronic volume control.


-g

[mooreamps]

I think the cathode follower is important to drive the cable capacitance. I can only assume the F2B sounds best with a short cable, or nobody notices the loss of high end because they use it for bass guitar.

It depends on what we are talking about here..... If you mean the cable between the guitar and the amp, then yes that may be a valid point. If it's cables inside the amp, then perhaps not.

I was saying, just having a CF follower drive a power tube, it just does not sound very good. Now, I have seen some layouts where, they have a CF driving a tone stack, and then drive a power tube or a PI.

Now, if you want to drive a 100 foot snake where cable capacitance really becomes a factor, then yes you would want a CF, or better yet , a SF.


-g

[Enzo]

You don't like the SVT? Just my opinion, of course, but I think an SVT sounds pretty good, and the power tubes are driven by 12AU7 cathode followers. The little PV Classic 50/50 stereo amp uses 12AT7 CFs to drive the EL84 power tubes. I think that too is a good sounding amp. Fender Bassman300 (Sunn 300T).

Or am I missing the point?

[JoeM]

Doesn't driving a power tube from a CF help with blocking distortion? (Or a MOSFET source follower? R.G. Keen has an article on this)

[bluemonk]

A 1M pot can be a very high impedance source, comparitively - it may get to ~ 250k at the electrical half way point. A 220pF capacitive loading on that (eg regular guitar lead) would create a low pass filter with a break point of 3kHz, which should be very apparant...
The alembic pre amp output Z will be about 40k, which is much more reasonable.

Does it mean that the stock 1M resistor at output is ok, but 1M pot will tame treble as far as it lowers resistance? In that case should a 2M or 10M pot work better?
(A master volume pot is actually useful in that preamp if you want it to be connected to a guitar amp or mixing console. Not to mention the option to drive tube to crunch).

[mooreamps]

You don't like the SVT? Just my opinion, of course, but I think an SVT sounds pretty good, and the power tubes are driven by 12AU7 cathode followers. The little PV Classic 50/50 stereo amp uses 12AT7 CFs to drive the EL84 power tubes. I think that too is a good sounding amp. Fender Bassman300 (Sunn 300T).

Or am I missing the point?

i've never heard the SVT. I'm sure it's good amp.


-g