non-inverting preamp with emitter follower ...

RE: non-inverting preamp with emitter follower ..., posted on June 28, 2017 at 20:12:17
Tre' Industry Professional Posts: 17302 Location: So. Cal. Joined: February 9, 2002	You have me wondering, are the 4.7k isolation resistors needed? Tre' Have Fun and Enjoy the Music "Still Working the Problem"

RE: non-inverting preamp with emitter follower ..., posted on June 29, 2017 at 03:22:55
deathtube 667	Bottlehead recommended using isolation resistors when adding a subwoofer output on the ver. 1 Foreplay preamp, a basic cathode follower. Not sure if I can lower the value of these resistors in my application.

RE: non-inverting preamp with emitter follower ..., posted on June 29, 2017 at 07:06:59
Tre' Industry Professional Posts: 17302 Location: So. Cal. Joined: February 9, 2002	What's your application? Tre' Have Fun and Enjoy the Music "Still Working the Problem"

RE: non-inverting preamp with emitter follower ..., posted on June 29, 2017 at 07:51:15

PakProtector
Audiophile

Posts: 12364
Joined: May 14, 2002

Ultimately, you'd be better putting a pair of the 2nd and 3rd stages to load the first stage plate. The capacitance is huge by pre-amp standards, and it would allow at least an order of magnitude drop in output Z. The transistor ought to be delivering more gm than a tube, and so drop its output Z into a few tens of Ohms...

I am not going to go to that level of complexity in a linestage when I can just reverse the plate connections in the amplifier and re-invert there...assuming I'd pay this any attention in the first place( beyond making sure both amps delivered the same phase).

When multi-amping with an active X-O, the final polarity stuff gets done at the speaker terminals anyway.
cheers,
Douglas

Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

Do you really need all that gain? nt, posted on June 29, 2017 at 09:45:38
Ralph Manufacturer Posts: 4772 Location: Minnesota Joined: April 24, 2002	-

RE: yes..., posted on June 29, 2017 at 10:44:48
deathtube 667	The amp is a SE 6CB5 in triode mode . CCS 6SN7 driver, cap coupled stages. I needed the extra gain to drive the finals near clipping @ 5W. dt 667

RE: application ..., posted on June 29, 2017 at 10:51:42
deathtube 667	It is just a linestage with enough gain to drive a SE 6CB5 amp and an active subwoofer.

RE: application ..., posted on June 29, 2017 at 19:34:39
Tre' Industry Professional Posts: 17302 Location: So. Cal. Joined: February 9, 2002	OK, but what function do the isolation resistors serve? What do they isolate against? Tre' Have Fun and Enjoy the Music "Still Working the Problem"

RE: application ..., posted on June 30, 2017 at 05:30:59
PakProtector Audiophile Posts: 12364 Joined: May 14, 2002	Those 'isolation' resistors are making up the Lion's share of your output impedance. cheers, Douglas Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

RE: the idea is borrowed from BH Foreplay ver.1 ..., posted on June 30, 2017 at 06:46:37

deathtube 667

Their manual says add 4.7k resistors in series with the RCA output jacks when adding extra outputs for a subwoofer.

There was a reasoning behind this but I can't recall why it is done that way.

RE: the idea is borrowed from BH Foreplay ver.1 ..., posted on June 30, 2017 at 07:20:59

Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002

I have two outputs per channel on my DIY preamp.

One feeds the input of my SE 300b amp and the other feeds the input to a Opamp based low pass filter that feeds the SS amp driving my woofers.

I don't have isolating resistors and see no need for them.

I use a cap at the input of the 300b amp to create a 6db per octave high pass filter but even if I wasn't using a cap and the signal was just seeing the grid resistor of the input tube I see no need to isolate.

The Opamp low pass filter input is a Opamp followed by the filter parts followed by an opamp. The first opamp isolates the input from the filter so there is no need to further isolate the preamp output.

I asked the question about your application thinking that the isolation resistors might really be needed.

If you are just "doing it because BH did it" without knowing why you might want to re-think.

If there is a reason that those resistors need to be there then by all means use them, if there is no reason then I say get rid of them as they are serving no useful purpose.

Tre'

Have Fun and Enjoy the Music
"Still Working the Problem"

RE: isolation resistors ..., posted on June 30, 2017 at 08:59:59

deathtube 667

The 4.7k isolation resistors seem to be a VoltSecond Foreplay mod idea, but he said that if both output are being run at the same time, the 4.7k value can be dropped down to 100 ohms.

The basic idea is to provide a bit of isolation between the two power amp inputs.

My powered sub has a 3rd order low pass filter built in, so no extra filtering is needed.

I'll probably try lower ohmic value isolation resistors and hear if anything changes.

https://www.audioasylum.com/cgi/t.mpl?f=bottlehead&m=79199

RE: isolation resistors ..., posted on June 30, 2017 at 09:18:06
Tre' Industry Professional Posts: 17302 Location: So. Cal. Joined: February 9, 2002	Try it without the isolation resistors. They should not be needed. Tre' Have Fun and Enjoy the Music "Still Working the Problem"

RE: tried 100 ohm iso resistors ..., posted on July 6, 2017 at 06:10:59
deathtube 667	For LTSpice to simulate both channels, there needs to be some resistance. I didn't notice any subjective difference using 100 ohm vs. 4.7k iso resistors. dt 667

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non-inverting preamp with emitter follower ...

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